Forestry, Faculty of
DSpace
UBCV
Leung, Lukie Hon
Master of Science - MSc
University of British Columbia
Wooden deck-boards are susceptible to surface checking and distortion during weathering. Surface profiling is an effective physical pre-treatment to reduce the checking of boards, but it is less effective at reducing cupping, unless it is also used on the underside of boards. I hypothesize that the checking and cupping of softwood deck-boards exposed outdoors can be significantly reduced by optimizing the geometries of surface and sub-surface profiles. To test this hypothesis, I examine the effect of profile geometry and growth ring orientation on the checking and cupping of western larch deck-boards exposed outdoors. Standard and tall rib profiles were significantly more effective at restricting checking than other profile types, particularly when profiled boards were oriented pith-side-up. However, such growth ring orientation made larch deck-boards more susceptible to shelling, a severe defect caused by the separation of growth rings. Confocal profilometry and macro-photography were used to characterize the projection of separated latewood at deck-board surfaces. A negative correlation was revealed between the heights of protruding latewood and growth ring angle to the weathered surface of flat larch boards oriented pith-side-up. The number of shelled lanceolate-shaped tips at the peaks of profiled boards was more common in boards with rib profiles particularly those oriented pith-side-up. I conclude that profiled larch boards should be oriented bark-side-up rather than pith-side-up. Hence, there was a need to develop different sub-surface profiles to distinguish pith- and bark-sides of boards so that boards can be correctly oriented in decks with their bark-side facing uppermost. Pacific silver fir and western hemlock boards were chosen to test sub-surface grooving as an alternative to sub-surface profiling because previous studies had optimized surface profiles for both species. There was an unexpected effect of number of grooves on the cupping of hem-fir boards. Boards machined with an odd number of grooves that included a groove running down the centre-line of the underside of boards were less prone to cupping compared to boards with an even number of grooves. In conclusion, results demonstrate that checking and cupping of softwood deck-boards can be reduced by a combination of surface profiling and sub-surface grooving.
OPTIMIZING PROFILE GEOMETRY TO REDUCE THE CHECKING, SHELLING AND CUPPING OF PROFILED SOFTWOOD DECK-BOARDS EXPOSED TO NATURAL WEATHERING by Lukie Hon Leung B.Sc. in Wood Products Processing, The University of British Columbia, 2018 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Forestry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) December 2020 ©Lukie Hon Leung, 2020 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: Optimizing Profile Geometry to Reduce the Checking, Shelling and Cupping of Softwood Deck-boards Exposed to Natural Weathering submitted by Lukie Hon Leung in partial fulfillment of the requirements for the degree of Master of Science in Forestry Examining Committee: Professor Philip D. Evans, Wood Science Supervisor Dr Christopher Gaston, Wood Science Supervisory Committee Member Dr Rod Stirling, FPInnovations Supervisory Committee Member Dr Harry Nelson, Natural Resources Management Chair of Examination Committee Dr Adnan Uzunovic, FP Innovations Examiner iii Abstract Wooden deck-boards are susceptible to surface checking and distortion during weathering. Surface profiling is an effective physical pre-treatment to reduce the checking of boards, but it is less effective at reducing cupping, unless it is also used on the underside of boards. I hypothesize that the checking and cupping of softwood deck-boards exposed outdoors can be significantly reduced by optimizing the geometries of surface and sub-surface profiles. To test this hypothesis, I examine the effect of profile geometry and growth ring orientation on the checking and cupping of western larch deck-boards exposed outdoors. Standard and tall rib profiles were significantly more effective at restricting checking than other profile types, particularly when profiled boards were oriented pith-side-up. However, such growth ring orientation made larch deck-boards more susceptible to shelling, a severe defect caused by the separation of growth rings. Confocal profilometry and macro-photography were used to characterize the projection of separated latewood at deck-board surfaces. A negative correlation was revealed between the heights of protruding latewood and growth ring angle to the weathered surface of flat larch boards oriented pith-side-up. The number of shelled lanceolate-shaped tips at the peaks of profiled boards was more common in boards with rib profiles particularly those oriented pith-side-up. I conclude that profiled larch boards should be oriented bark-side-up rather than pith-side-up. Hence, there was a need to develop different sub-surface profiles to distinguish pith- and bark-sides of boards so that boards can be correctly oriented in decks with their bark-side facing uppermost. Pacific silver fir and western hemlock boards were chosen to test sub-surface grooving as an alternative to sub-surface profiling because previous studies had optimized surface profiles for both species. There was an unexpected effect of number of grooves on the cupping of hem-fir boards. Boards machined with an odd number of grooves that included a groove running down the centre-line of the underside of boards were less prone to cupping compared to boards with an even number of grooves. In conclusion, results demonstrate that checking and cupping of softwood deck-boards can be reduced by a combination of surface profiling and sub-surface grooving. iv Lay Summary Surface cracking and distortion (cupping) of softwood deck-boards occur when they are exposed to the weather. In this thesis, I examined whether the performance of softwood deck-boards could be improved by optimizing the geometry of valleys and grooves machined into the surface and underside of boards. Profiled deck-boards with V-shaped grooves checked less than flat boards. Deck-boards with their growth rings curving away from the surface had fewer surface splinters than boards with growth rings curving towards the surface. I created grooves on the underside (sub-surface) of boards to reduce cupping. Boards with a central sub-surface groove cupped less than boards which lacked a central groove. I conclude that employing surface profiling and sub-surface grooving, and orienting deck-boards with growth rings curving away from the upper surface are measures that can significantly improve outdoor performance of softwood deck-boards and make them more appealing. v Preface The research in this thesis is the original work of Lukie Hon Leung. The experimental research and design, statistical analyses and writing of this thesis were done under the supervision of Professor Philip D. Evans. Parts of Chapter 4 are published in the peer-reviewed open access journal, Coatings, which has an impact factor of 2.4. I conducted the experimental research for this paper and wrote the manuscript. My co-author Professor Philip D. Evans assisted with the experimental design, statistical analyses and edited the final manuscript. The citation for the paper is: Leung LH, Evans PD. 2020. Shelling of growth rings at softwood surfaces exposed to natural weathering. Coatings. 10 (862). doi:10.3390/coatings10090862. vi Table of Contents Abstract ........................................................................................................................................... iii Lay Summary ................................................................................................................................... iv Preface ............................................................................................................................................. v Table of Contents ............................................................................................................................ vi List of Tables ................................................................................................................................... xi List of Figures ................................................................................................................................. xii Acknowledgments....................................................................................................................... xviii Chapter 1: General Introduction ..................................................................................................... 1 1.1 Wood decking market ........................................................................................................... 1 1.2 Performance of wooden decks ............................................................................................. 2 1.3 Improving the performance of wooden deck-boards ........................................................... 4 1.4 Aim, hypothesis and significance .......................................................................................... 5 1.5 Outline of thesis .................................................................................................................... 6 Chapter 2: Literature Review .......................................................................................................... 7 2.1 Wood materials used for decking ......................................................................................... 7 2.1.1 Pressure-treated wood ................................................................................................... 7 2.1.1.1 Copper-based preservative treatments ................................................................... 7 2.1.1.2 Wood species commonly used for pressure-treated wood .................................... 9 2.1.2 Larch and other naturally durable wood species ......................................................... 11 2.1.2.1 Larch ....................................................................................................................... 11 2.1.2.2 Naturally durable softwoods ................................................................................. 12 2.1.2.3 Naturally durable hardwoods ................................................................................ 13 2.1.3 Wood plastic composites ............................................................................................. 15 vii 2.2 Factors affecting the performance of wood decking .......................................................... 17 2.2.1 Checking ........................................................................................................................ 17 2.2.1.1 Formation of checks ............................................................................................... 17 2.2.1.2 Check Types ........................................................................................................... 21 2.2.1.3 Wood grain orientation and checking ................................................................... 24 2.2.2 Distortion ...................................................................................................................... 25 2.2.2.1 Types of distortion ................................................................................................. 25 2.2.2.2 Wood structure and distortion .............................................................................. 27 2.2.2.3 Distortion during kiln drying and weathering ........................................................ 29 2.2.3 Other factors (affecting performance of decking) ....................................................... 31 2.2.3.1 Shelling ................................................................................................................... 31 2.2.3.2 Fasteners ................................................................................................................ 34 2.2.3.3 Colour changes ....................................................................................................... 35 2.2.3.4 Decay ...................................................................................................................... 36 2.3 Reducing the negative effects of checking and cupping ..................................................... 38 2.3.1 Surface profiling ............................................................................................................ 38 2.3.2 Surface grooving ........................................................................................................... 45 2.3.3 Kerfing ........................................................................................................................... 46 2.3.4 Incising .......................................................................................................................... 49 2.3.5 Chemical and thermal treatments ............................................................................... 50 2.3.6 Coatings and water-repellent additives ....................................................................... 52 2.3.6.1 Coatings.................................................................................................................. 52 2.3.6.2 Water-repellent additives ...................................................................................... 54 2.4 Concluding Summary .......................................................................................................... 54 viii Chapter 3: Optimizing Profiling to Reduce the Checking and Cupping of Western Larch Deck-boards ........................................................................................................................................... 56 3.1 Introduction ......................................................................................................................... 56 3.2 Material and methods ......................................................................................................... 57 3.2.1 Experimental design ..................................................................................................... 57 3.2.2 Manufacture of western larch deck-boards ................................................................. 57 3.2.3 Natural weathering of deck-board samples ................................................................. 59 3.2.4 Measurement of cupping ............................................................................................. 60 3.2.5 Measurement of surface checking ............................................................................... 62 3.3 Results ................................................................................................................................. 63 3.3.1 Checking of western larch deck-boards after weathering ........................................... 63 3.3.2 Cupping of deck-boards after weathering.................................................................... 68 3.4 Discussion ............................................................................................................................ 69 3.5 Conclusions .......................................................................................................................... 71 Chapter 4: Shelling of Western Larch Deck-boards ...................................................................... 72 4.1 Introduction ......................................................................................................................... 72 4.2 Material and methods ......................................................................................................... 73 4.2.1 Experimental design and statistical analysis ................................................................ 73 4.2.2 Preparation and weathering of western larch deck-boards ........................................ 73 4.2.3 Measurement and imaging of shelling of deck-boards ................................................ 74 4.3 Results ................................................................................................................................. 76 4.3.1 Appearance of shelling in flat western larch boards .................................................... 76 4.3.2 Correlation between the heights of shelled growth rings and growth ring angle ....... 80 4.3.3 Shelling at the surface of profiled western larch boards ............................................. 81 ix 4.3.4 Number of shelled tips in profiled boards .................................................................... 83 4.4 Discussion ............................................................................................................................ 85 4.5 Conclusions .......................................................................................................................... 87 Chapter 5: Reducing the Checking and Cupping of Pacific Silver Fir and Hemlock Deck-boards with Sub-surface Profiles ...................................................................................................................... 88 5.1 Introduction ......................................................................................................................... 88 5.2 Material and methods ......................................................................................................... 89 5.2.1 Experimental design and statistical analysis ................................................................ 89 5.2.2 Sub-surface profiles ...................................................................................................... 90 5.2.2.1 Survey of sub-surface profiles used commercially ................................................ 90 5.2.2.2 Design of sub-surface groove profiles ................................................................... 93 5.2.3 Manufacture of surface profiled and sub-surface grooved deck-boards .................... 94 5.2.4 Natural weathering of deck-board samples ................................................................. 98 5.2.5 Measurement of surface checking and cupping .......................................................... 99 5.3 Results ............................................................................................................................... 100 5.3.1 Cupping of grooved western hemlock and pacific silver fir deck-boards .................. 100 5.3.2 Surface checking of grooved western hemlock and pacific silver fir deck-boards .... 102 5.3.3 Cupping and checking of western hemlock and pacific silver fir deck-boards .......... 103 5.4 Discussion .......................................................................................................................... 105 5.5 Conclusions ........................................................................................................................ 106 Chapter 6: General Discussion, Conclusions and Suggestions for Further Research ................. 107 6.1 General discussion ............................................................................................................ 107 6.2 Conclusions ........................................................................................................................ 109 6.3 Suggestions for further research ....................................................................................... 110 x References .................................................................................................................................. 112 Appendices .................................................................................................................................. 127 Appendix 1: AutoCAD images of grooving knives designed for the experiment described in Chapter 5 ................................................................................................................................. 127 Appendix 2: Statistical analyses for Chapter 3 ........................................................................ 129 Appendix 3: Statistical analyses for Chapter 4 ........................................................................ 129 Appendix 4: Statistical analyses for Chapter 5 ........................................................................ 129 Appendix 5: Paper published in open access journal Coatings ............................................... 129 xi List of Tables Table 2.1 Common preservatives used for decking in the United States ...................................... 8 Table 2.2 Summary of common species of larch (Bergstedt and Lyck 2007) ............................... 11 Table 2.3 Cedar and redwood species commonly used in the United States and their durability rating ............................................................................................................................................. 13 Table 2.4 Origins, basic density and durability rating of commercially imported hardwood species used for wooden decking in the United States ............................................................................ 14 Table 2.5 Wood species and geometries of commercially available profiled deck-boards collected by Cheng and Evans (2016) ........................................................................................................... 40 Table 3.1 Wood properties of six different western larch boards obtained from forests in the interior of British Columbia........................................................................................................... 58 Table 3.2 Geometry of profiles used to manufacture profiled deck-boards ................................ 58 Table 3.3 Weather conditions in Vancouver, British Columbia, Canada during the 7-month outdoor weathering trial (March to October 2019) ..................................................................... 60 Table 4.1 Average heights of shelled latewood and number of shelled growth rings in flat western larch deck-boards oriented pith-side-up and exposed to the weather for 7 months.................. 78 Table 5.1 Description of ten commercially available sub-surface grooved deck-boards ............. 90 Table 5.2 Dimensions of groove types applied to the underside of pacific silver fir and western hemlock deck-boards .................................................................................................................... 93 Table 5.3 Wood properties of the five replicate blocks (boards) of pacific silver fir and western hemlock ......................................................................................................................................... 95 Table 5.4 Weather conditions in Vancouver, British Columbia, Canada during the 6-month outdoor weathering trial (February to August 2019) ................................................................... 99 xii List of Figures Figure 1.1 Illustration showing the main components of an exterior deck: beams, decking, joists, posts and railing .............................................................................................................................. 1 Figure 1.2 Deck-board market composition in 2015 and 2018 (demand in lineal feet). The market share of different decking material was calculated based on the total demand (100%) within the same fiscal year (Freedonia Group 2016a; Freedonia Group 2020a) ............................................. 2 Figure 2.1 Images of wood plastic composite decking: (a) capstock-coated and (b) unocated .. 16 Figure 2.2 Mechanism of check formation of a flat-sawn deck-board during kiln drying (a-e) and outdoor weathering (f-j). Moisture content of wood is indicated by colour (Blue is wet and yellow is dry). This diagram is redrawn from ones in McMillen (1955), Evans (2008) and Cheng (2015)....................................................................................................................................................... 19 Figure 2.3 Different types of wood separations at a board surface as described by Yang and Normand (2012) and Cheng (2015): end check, surface check, internal check, micro-check, split, shake and crack ............................................................................................................................. 22 Figure 2.4 The five common types of distortion in wood described by Simpson (1991): bow, crook, twist, cup/crown and diamonding/oval. The red dashed lines outline the presumed geometry of the board before distortion occurred ........................................................................................... 26 Figure 2.5 Diagram of different forms of distortion due to differential shrinakge in the radial and tangential directions: (a) Flat-sawn board, greater tangential shrinkage and cupping; (b) riftsawn board; (c) square, balanced radial and tangential shrinkage; (d) riftsawn square, diamonding; (e) quartersawn board, greater radial shrinkage; (f) pith-center board; prone to all types of distortion; (g) round dowel, ovaling (Simpson 1991) ..................................................................................... 28 Figure 2.6 Diagram of Norway spruce and Scots pine cladding exposed to accelerated cyclic weathering: (a) surface of cladding during exposure to water showing the moisture-induced crowning of boards; (b) Cupping of boards as a result of drying of the surface of boards (Virta 2005) ............................................................................................................................................. 30 Figure 2.7 Severe shelling of Douglas fir lumber after kiln drying (Simpson 1991) ..................... 32 Figure 2.8 Shelling of a profiled flat-sawn western larch deck-board oriented pith-side-up and exposed to the weather for 2 years (photo, Dr. Philip D. Evans) ................................................. 33 xiii Figure 2.9 Fastener systems used in the studies of (a) radiata pine decking by Evans et al. (2003) and (b) southern pine deck-boards by Urban and Evans (2005) .................................................. 34 Figure 2.10 Dimensional parameters that were extracted from profilometry line scans of profiled decking: radii of the grooves (R1), radii of peaks (R2), heights (H) and widths (W) of peaks (Cheng and Evans 2016) ............................................................................................................................ 38 Figure 2.11 Rib and ripple profiles developed by Morris and McFarling (2008): (a) Rippled-flat edge; (b) rippled to edge; (c) ribbed-eased edge; (d) and ribbed-flat edge and centre .............. 41 Figure 2.12 Profiles that were machined into the surface of pacific silver fir deck-boards in the study by Cheng and Evans (2018): (a) Graph showing the geometry of the different rib, ribble and ripple profiles; (b) appearance of the eleven profiles used in the study; (c) and a sample of a single-sided ribbed deck-board .................................................................................................... 43 Figure 2.13 Groove profiles machined into the surface of pine and spruce roof friezes (Böttcher 1977): (a) big semicircular grooves; (b) big rectangular grooves; (c) small semicircular grooves; (d) small rectangular grooves; (e) wedge-shaped grooves. The boards were 80 mm by 18 mm. Red circled area shows the spot where checks developed ................................................................. 45 Figure 2.14 Bongossi floating pier decking with: (a) narrow-spaced rectangular grooves and (b) wide-spaced rectangular grooves (Shida et al. 1992)................................................................... 46 Figure 2.15 Images of 10 mm deep kerfed southern pine deck-boards used in study by Ratu et al. (2007): (a) one kerf; (b) two kerfs; (c) three kerfs (photos, Dr. Philip D. Evans) .......................... 48 Figure 2.16 Severe scaling of an incised and preservative-treated hem-fir deck-board after outdoor exposure for eight years (Evans 2016) ........................................................................... 49 Figure 3.1 Cross-sectional view of profiled (standard rib) western larch deck-boards oriented: (a) pith-side-up, concave; and (b) bark-side-up, convex ................................................................... 59 Figure 3.2 Photograph of profiled and flat deck-board samples on the six weathering racks on the first day of the weathering trial .................................................................................................... 59 Figure 3.3 Measuring system for assessing the cupping or crowning of boards: (a) Nine transducer heads obtained from a swellometer; (b) overview of the system; (c) layout of the measuring platform (dimensions are in mm) ................................................................................................. 61 Figure 3.4 Tools used to measure the length and width of each check: (a) Transparent Plexiglas ruler and (b) calibrated optical eyepiece graticule ....................................................................... 62 xiv Figure 3.5 Average width of five largest checks in western larch deck-boards oriented pith-side-up (concave) or bark-side-up (convex) and exposed to the weather for 7 months. Results are averaged across boards with different profiles because of the insignificant (p>0.05) interaction of profile type and growth ring orientation on check width ............................................................ 63 Figure 3.6 Profiled (standard ribble) western larch deck-boards from the same parent block oriented (a) pith-side-up versus (b) bark-side-up and exposed to natural weathering in Vancouver 7 months. Scale bar = 50 mm ....................................................................................................... 64 Figure 3.7 Average width of five largest checks in profiled and flat western larch boards exposed to the weather for 7 months. Results are averaged across boards with different growth ring orientations because of the insignificant (p>0.05) interaction of profile type and growth ring orientation on check width ........................................................................................................... 65 Figure 3.8 Interaction of profile type and growth ring orientation on the length of five largest checks in deck-boards exposed to the weather for 7 months ..................................................... 66 Figure 3.9 Interaction of profile type and growth ring orientation on the area of five largest checks in western larch deck-boards exposed to the weather for 7 months .......................................... 67 Figure 3.10 Effect of growth ring orientation (pith-side-up, concave; bark-side-up, convex) on the cupping of western larch deck-boards exposed to the weather for 7 months. Results are averaged across boards with different profiles because of the insignificant (p>0.05) interaction of profile type and orientation on cupping .................................................................................................. 68 Figure 3.11 Cupping of profiled (standard rib) western larch deck-boards from the same parent block oriented (a) pith-side-up versus (b) bark-side-up exposed to natural weathering in Vancouver for 7 months ............................................................................................................... 68 Figure 4.1 Custom profile knives used for double-sided profiling of western larch deck-board samples. Geometries for different profiles are listed in Table 3.1 ............................................... 74 Figure 4.2 Systems used to image the shelling of western larch deck-boards: (a) Altisurf 500 non-contact confocal profilometer; (b) Canon EOS 80D digital single lens reflex camera equipped with Canon EF-S 35 mm f/2.8 macro lens ............................................................................................. 75 Figure 4.3 Flat western larch deck-board oriented pith-side-up; Examples of shallow (α) or steep (β) angle of growth rings to the deck-board surface exposed to natural weathering ................. 75 xv Figure 4.4 Shelled growth rings at the surface of an unprofiled western larch deck-board (Board 6) oriented pith-side-up and exposed to the weather for 7 months: (a) confocal profilometry height map showing five projecting growth rings at the surface; (b) Macro-photographic image of the matching 3D profilometry map in Figure 4.4a; (c) Confocal profilometry line scan showing the heights of shelled growth rings corresponding with the images in Figure 4.4a,b ................. 76 Figure 4.5 Shelling of growth rings at the surface of an unprofiled western larch deck-board (Board 4) oriented pith-side-up and exposed to the weather for 7 months: (a) Confocal profilometry height map showing protruding growth rings labelled 1,2,3,4; (b) Photograph of four shelled growth rings matching the image in Figure 4.5a; (c) Confocal profilometry line scan showing the heights of shelled growth rings corresponding with the images in Figure 4.5a,b ... 77 Figure 4.6 Appearance of a flat western larch deck-board oriented bark-side-up and exposed to the weather for 7 months: (a) Confocal profilometry height map showing surface checking and roughening of the surface of a deck-board; (b) Photograph matching the height map in Figure 4.6a; (c) Confocal profilometry line scan showing the variation in height corresponding with the images in Figure 4.6a,b ................................................................................................................. 79 Figure 4.7 Linear regression (red line) of heights of shelled growth rings versus growth ring angle to the exposed surface of flat western larch deck-boards oriented pith-side-up and exposed to the weather for 7 months; regression includes 95% confidence intervals (blue-lines) ............... 80 Figure 4.8 Shelling of a latewood sliver on the peak of a profiled (rib) western larch deck-board oriented pith-side-up and exposed to the weather for 7 months: (a) Confocal profilometry image of a shelled latewood sliver; (b) Photograph of a shelled latewood sliver matching the image in Figure 4.8a; (c) Confocal profilometry scan of a profile peak showing the height of the protruding latewood tip in Figure 4.8a,b ........................................................................................................ 81 Figure 4.9 Raised tip of a growth ring on the peak of a profiled (short rib) western larch deck-board oriented bark-side-up and exposed to the weather for 7 months: (a) Confocal profilometry image of a raised latewood tip; (b) Photograph matching the image in Figure 4.9a; (c) Confocal profilometry scan of profile peak showing the height of the same latewood tip in Figure 4.9a,b....................................................................................................................................................... 82 Figure 4.10 Effect of growth ring orientation on the number of shelled growth rings (tips) at the surface of profiled western larch deck-boards exposed to the weather for 7 months. Results are averaged across boards with different profile types because of the insignificant (p>0.05) interaction of growth ring orientation and profile type on the number of raised latewood tips. Error bars = Least significant difference, p<0.05 .......................................................................... 83 xvi Figure 4.11 Effect of profile type on the number of shelled growth rings (tips) at the surface of profiled western larch deck-boards exposed to the weather for 7 months. Results are averaged across boards with different growth ring orientations because of the insignificant (p>0.05) interaction of growth ring orientation and profile type on the number of raised latewood tips 84 Figure 4.12 Appearance of shelled latewood tips at the surface of profiled western larch deck-boards oriented pith-side-up: (a) sample board with a rib profile; (b) sample board profiled with a ribble profile ............................................................................................................................... 84 Figure 5.1 Parameters used to design sub-surface grooved profiles ........................................... 93 Figure 5.2 Custom-designed knives for different sub-surface groove types and top profile used to manufacture deck-boards from pacific silver fir and western hemlock ....................................... 94 Figure 5.3 Cross-sectional views of surface profiled (wide rib) and sub-surface grooved pacific silver fir deck-boards. Dashline indicates the centre of boards ................................................... 96 Figure 5.4 Pilot-scale preservative treatment cylinder used to pressure treat pacific silver fir and western hemlock deck-board samples ......................................................................................... 97 Figure 5.5 Photograph of surface profiled and sub-surface grooved western hemlock and pacific silver fir deck-board samples on the five weathering racks on the first day of the weathering trial....................................................................................................................................................... 98 Figure 5.6 Interaction of sub-surface groove type and wood species on the cupping of sample deck-boards exposed to the weather for 6 months. Note that boards with 6 grooves per 15 cm do not contain a groove running along their centre-line ........................................................... 100 Figure 5.7 Cross-sections of pacific silver fir deck-boards after weathering for six months: (a) double-sided profiled wide rib control; (b) Sub-surface groove profile (Tall 5). Note the difference in cupping between the two boards ........................................................................................... 101 Figure 5.8 Interaction of sub-surface groove profile and wood species on the average area of ten largest checks in deck-board samples exposed to the weather for 6 months. Note that boards with 6 grooves per 15 cm do not contain a groove running along their centre-line ................. 102 Figure 5.9 Surface check area and cupping of grooved western hemlock deck-boards exposed outdoors for 6 months. Labels on the x-axis refer to the sub-surface groove types listed in Table 5.2. The y1-axis (left) refers to the average area of ten largest checks at the surface of grooved western hemlock deck-boards. The y-2 axis (right) refers to the cupping of western hemlock deck-board samples ............................................................................................................................. 103 xvii Figure 5.10 Surface check area and cupping of grooved pacific silver fir deck-boards exposed to the weather for 6 months. Labels on the x-axis refer the sub-surface groove types machined into the underside of pacific silver fir deck-boards. The y1-axis (left) refers to the average area of ten largest checks at the surface of grooved pacific silver fir deck-boards. The y-2 axis (right) refers to the cupping of pacific silver fir deck-board samples .............................................................. 104 xviii Acknowledgments I am grateful for the support and supervision from Professor Philip D. Evans throughout my graduate study. His advice on life, career and academia have helped me to develop personally and professionally. I greatly appreciated Professor Evanâs patient guidance during my experimental work and my writing. I am very proud and honoured to have been a student of Professor Evans. I would like to extend my gratitude to my supervisory committee members Dr. Christopher Gaston and Dr. Rod Stirling for their advice and suggestions. I thank all the members of the Evans research group for their friendship and assistance. I am especially thankful to Joseph Doh Wook Kim for his friendship and guidance; Sadegh Mazloomi for his advice on research; Kenneth Cheng for his knowledge on wooden deck-boards; Dengcheng Feng and Farbod Moheb for always being willing to help me. I would also like to acknowledge: NSERC for financial support (CRDPJ485007-15); Accsys Technologies for financial support; Mitacs for awarding me a Mitacs-Accelerate Award; Tolko Industries for in-kind support (donation of western larch boards); FPInnovations for in-kind support (donation of pacific silver fir and western hemlock boards); Daniel Wong of FPInnovations for expert technical assistance. My warmest thanks go to friends and family. I thank my sister Tiffany Leung for her encouragement. Lastly, I thank my parents Idy and Sammy Leung for their love and support which helped me achieve this significant milestone in my life. 1 Chapter 1: General Introduction 1.1 Wood decking market Wood is a very important material for residential construction in North America (Burrows 2013). For example, dimension lumber is the material of choice for the framing that provides structural support for detached homes. Many other wood products are used for homes including ones used for exterior applications such as siding and decking (Burrows 2013). For example, over half of the new houses built in the United States between 1996 and 1998 have an outdoor wooden deck consisting of a structural frame, deck-boards and railing (Shook and Eastin 2001; Hislop 2006) (Figure 1.1). Hence, there is a considerable market for wooden deck-boards, including boards made from preservative-treated wood and naturally durable wood species (Freedonia Group 2016a). Figure 1.1 Illustration showing the main components of an exterior deck: beams, decking, joists, posts and railing The decking market in the United States was estimated to be worth upwards of 5.6 billion USD per annum in 2015 with over 3.3 million lineal feet of deck-boards sold (Freedonia Group 2016a). The demand for deck-boards continued to grow slowly, reaching 3.4 million lineal feet in the United States in 2018 (Gaston and Pahkasalo 2016; Freedonia Group 2020a). However, the market share of different types of decking products has been changing and an increasing number 2 of consumers have been favouring deck-boards made from wood plastic composites rather than solid wood (Freedonia Group 2020a; Freedonia Group 2020b) (Figure 1.2). Figure 1.2 Deck-board market composition in 2015 and 2018 (demand in lineal feet). The market share of different decking material was calculated based on the total demand (100%) within the same fiscal year (Freedonia Group 2016a; Freedonia Group 2020a) 1.2 Performance of wooden decks Consumersâ main concerns when selecting deck-boards are durability, aesthetics and consistent material quality (Shook and Eastin 2001). These concerns often mirror the main reasons for the replacement of decks (Alderman et al. 2003). For example, the top two reasons for replacing deck-boards are biodeterioration (decay) and loss of appearance (aesthetics) due to weathering (greying, discolouration, checking and warping). In addition, consumers seek deck-boards with a homogeneous appearance during service (Høibø and Nyrud 2010). Loss of aesthetics can lead to premature replacement of treated deck-boards even though boards are structurally sound (Alderman et al. 2003; Bolin and Smith 2010). Evidence to support the importance of appearance of deck-boards to consumers are the findings from a study by McQueen and Stevens (1998). They surveyed 191 carpenters in the southeastern region of the 3 United States and found that approximately 57% of deck replacements resulted from decay or insect attack. But the second leading cause was related to the poor appearance of a deck. Solid wood deck-boards are susceptible to decay, surface weathering and warping during their service life. The decay of wood can be prevented with the use of preservatives (Morris and Ingram 2012). Hence, exterior wood products made from non-durable or moderately durable softwood species are invariably treated with preservatives to improve their durability. However, wood preservatives are less effective at restricting the weathering of wood (Nejad and Cooper 2011a). For example, preservative-treated wood often checks (cracks) when exposed outdoors, and some wood species are susceptible to shelling. Checking is the separation of wood cells at the surface of boards (Côté 1968). Large checks adversely affect the appearance of deck-boards and encourage decay (Gellerich et al. 2017; Evans 2016). Treated wood decking is still competitive in the market because it is cheap, derived from a renewable material and has the natural grainy appearance of wood (Freedonia Group 2020c). People are also very familiar and experienced in using wood. Nevertheless, Ganguly and Eastin (2009) anticipated that treated and naturally durable wooden decking will be replaced by wood plastic composites (WPC), which maintain their appearance better than wood when exposed outdoors, and hence require less maintenance. Wood plastic compositesâ durable colourful appearance and resistance to surface checking and warping means that WPC decking is a threat to the wooden decking industry (Green 2005; Markarian 2008). Accordingly, significant effort is required to overcome the deficiencies of solid wood decking, particularly its susceptibility to checking and warping. 4 1.3 Improving the performance of wooden deck-boards There have been many attempts to reduce the checking of deck-boards using either chemical or physical treatments (Zahora 1995; McFarling and Morris 2005). One such physical treatment is surface profiling which can significantly reduce checking at the surface of treated deck-boards (Böttcher 1977; McFarling and Morris 2005; Evans et al. 2010; Cheng and Evans 2018). Surface profiling has only been tested on eight different softwood species: Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), lodgepole pine (Pinus contorta var. latifolia Wats.), pacific silver fir (Abies amabilis Douglas ex J. Forbe), radiata pine (Pinus radiata D. Don), southern pine (Pinus sp.), subalpine fir (Abies lasiocarpa (Hook.) Nutt.), western hemlock (Tsuga heterophylla Raf. Sarg.), and white spruce (Picea glauca (Moench) Voss). The effectiveness of profiling at reducing checking at the surface of deck-boards varies with wood species. For example, Evans et al. (2010) found that a rib profile was more effective at reducing the checking of pacific silver fir whereas a ripple profile was more effective at reducing the checking of southern pine. Hence, different types of profiles need to be tested on the various wood species that have potential to be used for decking. There will be a significant increase in the supply of western larch (Larix occidentalis Nutt.) within the next twenty years because of a forest regeneration program in the interior of British Columbia (Newsome et al. 2016). Western larch is suitable for exterior deck-boards because of its strength and moderate durability against decay (Johnson and Bradner 1932; Morris et al. 2016). Therefore, in this thesis, I test whether profiling is an effective method of reducing the checking and cupping of western larch deck-boards exposed to the weather, to compliment studies done on other species. Wood species like western larch and southern pine have distinct latewood and as a result are susceptible to shelling. Shelling is a surface defect in which latewood separates from earlywood (Koehler 1929). Shelling, cupping and checking are affected by the orientation of growth rings in boards (Koehler 1929; Urban and Evans 2005; Heshmati et al. 2020). Hence, as part of my experiment to optimize profiling to reduce checking and cupping of larch, I also examined the effects of profiling and growth ring orientation on the shelling of larch. Single-sided profiling of deck-boards has been used to restrict the developments of surface checks, but it increases the boardâs tendency to cup (Morris and McFarling 2008; Cheng 5 and Evans 2018). This problem was investigated by Heshmati et al. (2020). They found that double-sided profiling could balance strains in boards, and hence reduce cupping of profiled boards. However, double-sided profiling is not an ideal solution because both sides look the same, making it difficult to fix boards with preferred growth ring orientation. An alternative solution is to profile one side and to use sub-surface grooving on the underside that is not exposed to the weather. 1.4 Aim, hypothesis and significance The objectives of this thesis were to: (1) determine the profile that is most effective at reducing checking and cupping of flat-sawn western larch boards with convex and concave growth ring orientations; (2) examine the effects of growth ring orientation and surface profiling on the shelling of western larch deck-boards; (3) investigate the use of sub-surface grooving as an alternative to double-sided profiling for reducing the cupping and checking of treated pacific silver fir and western hemlock deck-boards. I hypothesize that the effectiveness of profiling at restricting checking and cupping of softwood decking can be improved by optimizing surface and sub-surface profiles. Such improvements are needed because severe checking and cupping of solid wood deck-boards during weathering are disliked by homeowners, and has encouraged many of them to switch to boards made from wood plastic composites. I anticipate that my findings could help to improve the competitive position of solid wood decking, in particular, decking made from western larch, pacific silver fir and western hemlock, by making boards less susceptible to the adverse effects of weathering. Therein lies the significance of my research. 6 1.5 Outline of thesis This chapter (Chapter 1) provides background information, objectives and the general hypothesis for this thesis. Chapter 2 reviews the relevant literature on wooden deck-boards, the adverse effects of weathering (checking, cupping and shelling) on the properties of solid wood decking, and pre-treatments (physical and chemical) that have been used to reduce the unwanted effects of weathering on the properties of wood. The physical pre-treatments that are reviewed include profiling, grooving, kerfing and incising. The effects of wood quality and growth ring orientation within boards on checking, shelling and warping of wood are also reviewed. Chapter 3 describes an experiment that examined the effects of profile geometry and growth ring orientation on the checking and cupping of flat-sawn western larch deck-boards. Chapter 4 is an extension of Chapter 3, characterizing the appearance of shelling of wood at the surface of flat and profiled western larch boards. Research presented in Chapter 4 also examines the effects of growth ring orientation and angle on shelling. Chapter 5 describes an experiment that compared the performance (checking and cupping) of surface profiled and sub-surface grooved hem-fir decking samples with boards that were profiled on both sides (double-sided profiling). Finally, Chapter 6 discusses important findings from the three experimental chapters, draws conclusions and makes suggestions for further research. 7 Chapter 2: Literature Review 2.1 Wood materials used for decking 2.1.1 Pressure-treated wood 2.1.1.1 Copper-based preservative treatments Waterborne copper-based preservatives are commonly used to treat non-durable wood species because they offer long-term protection against biological degradation and are inexpensive to make (Morrell 2018). In the United States, the standardization of wood preservatives and chemical retention levels is governed by the American Wood Protection Association. Depending on the intended hazard class for the pressure-treated wood, different formulations and retentions are required (Lebow 2010). Chromated copper arsenate (CCA), alkaline copper quaternary (ACQ) and micronized copper azole (MCA) are the three main formulations currently used by industry (Morrell 2018). Over 83% of treated wood produced in Canada as of 1999 was treated with waterborne copper preservatives (Morris and Wang 2006). Lebow (2010) and Morrell (2018) summarized the main commercially available preservatives in use in the United States (Table 2.1). As a result of the initiative in 2004 to voluntarily withdraw CCA for most residential uses, new copper-based formulations were introduced as substitutes for CCA (Feldman et al. 2003). After the ban, ACQ emerged as the predominant soluble copper-based preservative for treating wood used for decking (Lebow 2004; Freedonia Group 2016b). Morris et al. (2002) compared the penetration and retention of CCA and ACQ in treated boards made from pacific silver fir, western hemlock, white spruce and Douglas fir. ACQ penetrated deeper than CCA in pacific silver fir and western hemlock boards. Increased penetration of ACQ in white spruce and Douglas fir boards was also observed when the treatment temperature was raised to 60°C. The retention of ACQ was greater than CCA in western hemlock and white spruce boards, but there was no difference in retention in pacific silver fir and Douglas fir boards (Morris et al. 2002). However, ACQ was more susceptible to leaching than CCA and it caused greater corrosion of fasteners (Temiz et al. 2006; Zelinka et al. 2010). 8 Table 2.1 Common preservatives used for decking in the United States Common name Trade name Properties / Applications CCA Chromated copper arsenate One of the oldest soluble copper preservatives on the market. CCA is still used because it is effective against many biodegradation agents. In 2004, EPA and treaters limited CCA to industrial applications and a limited number of non-industrial end uses due to environmental concerns about chromium and arsenic. ACZA Ammoniacal copper zinc arsenate Ammonia in ACZA facilitates better penetration of copper and zinc into refractory species. ACZA treated wood is mainly used in industrial applications. ACQ Alkaline copper quaternary ACQ became widely used when CCA was phased out for residential applications. Formulations of ACQ are free of chromium and arsenic. The presence of an organic biocide (didecyl dimethyl ammonium chloride or carbonate) provides resistance against copper-tolerant fungi. CA Copper azole CA is another CCA replacement and contains copper and tebuconazole as a co-biocide. It is only common in North America. Borates Borax or boric acid Borates can be combined with copper or zinc and are effective against copper-tolerant fungi and insects. MCA / MCQ Micronized copper azole / Micronized copper quaternary A new particulate system where basic copper carbonate is suspended in water. The preservatives also contain an organic biocide (quat or azole). EcolifeTM EL-2 (DCOI) EcolifeTM is an organic (non-metalic) preservative that contains a fungicide 4,5-dichloro-2-octyl-4-isothiazolin-3-one (DCOI), an insectide imidacloprid and water-repellent additives. EcolifeTM-treated wood can only be used for above-ground applications, according to AWPA standards. PTI Organic preservative containing fungicides propiconazole and tebuconazole and the insecticide imidacloprid. A new preservative system was introduced into the market in 2007 that consists of solid copper particles emulsified in water rather than soluble copper (Freeman and McIntyre 2008). This preservative also contains an organic co-biocide. Micronized copper -quaternary and -azole (MCQ and MCA) preservatives have largely replaced ACQ in the marketplace. Micronized copper particles are physically deposited into voids within wood, whereas copper in ACQ is bonded to wood cell walls by chemical reactions (Freeman and McIntyre 2008). Cooper and Ung (2009) found that copper in MCQ was less susceptible to leaching than copper than ACQ, but their 9 organic co-biocides showed similar susceptibility to leaching. Micronized copper-based preservatives are less corrosive to fasteners than ACQ, except for fasteners made from aluminum (Zelinka 2014). However, one negative feature of micronized copper preservatives is that they are less able to penetrate refractory wood species like Norway spruce (Picea abies (L.) Karst.) compared to ACQ (Civardi et al. 2016). It has been suggested that the inability of copper particles to penetrate into refractory wood will lead to premature failure of wood products treated with micronized preservatives (Preston et al. 2008). Nevertheless, Matsunaga et al. (2007) and Stirling et al. (2008) showed micronized copper treatments were able to penetrate permeable species like southern pine; smaller particles were able to penetrate cell walls whereas larger ones were deposited in cell lumens. Feng et al. (2019) recently used X-ray micro-computed tomography to visualize the distribution of copper in ACQ and MCA treated southern pine. Both preservatives were deposited close to rays and resin canals, but ACQ was concentrated near the surface of the treated wood. Evans et al. (2008) pointed out that micronized copper was the largest end use of nanotechnology. 2.1.1.2 Wood species commonly used for pressure-treated wood A variety of softwood species, including Jack pine (Pinus banksiana Lamb.), southern pine, Douglas fir and hem-fir (commonly sold as a mix of western hemlock and various Abies spp.), are used to produce preservative-treated wood products in the United States and Canada (Freedonia Group 2020c). The volume of different wood species that are treated was calculated following a survey in 2007 of 136 treatment plants (Vlosky 2009). The top three wood species used for treated products in United States (by volume) were: southern pine (73%), hem-fir (6%) and Douglas fir (6%). Moreover, the Canadian industry mainly treated spruce-pine-fir (SPF) (68%), red pine (Pinus resinosa Ait.) (11%) and hem-fir (8%) (Morris and Wang 2006). Southern pine and hem-fir are suitable for preservation because they are abundant and inexpensive in the United States and Canada, and can be easily treated (Morris 1995; Groenier 2006). Western hemlock and pacific silver fir are commonly sold together, but they vary in treatability (Morris 1995; Morris et al. 2002). For example, Morris (1995) compared the treatability of western hemlock and pacific silver fir boards following pressure-treatment with a chromated copper arsenate (CCA) preservative. CCA penetrated and was absorbed by pacific 10 silver fir boards 1.8 times more than that of western hemlock boards. Hence, Morris (1995) suggested that treatment variability in hem-fir boards could be reduced by sorting the two species prior to treatment. In a later study, Morris et al. (2002) treated boards from pacific silver fir, western hemlock, white spruce and Douglas fir with alkaline copper quaternary (ACQ) and CCA. As expected, pacific silver fir boards had the highest preservative penetration and retention, followed by western hemlock, white spruce, and Douglas fir (Morris et al. 2002). Hence, Morris (1995) recommended separating pacific silver fir boards from western hemlock boards and marketing the former as a premium treated product. Full sapwood penetration by preservatives is desirable for outdoor applications and hence pacific silver fir could find a niche for treated products such as deck-boards (Morris et al. 2002; Morris and McFarling 2008). SPF lumber is mainly used for framing construction in North America, but there is interest in using it for other applications. Subalpine fir will make an increasing proportion of the SPF mix in the future and there has also been interest in exploring its suitability for decking (McFarling et al. 2009; Morris and Ingram 2015). McFarling et al. (2009) suggested that surface profiling is needed for treated subalpine fir deck-boards because unprofiled subalpine fir boards are susceptible to surface checking and warping. Preservative-treated southern pine deck-boards dominate the North American market for preservative-treated decking, as mentioned above (Ganguly et al. 2010; Freedonia Group 2020c). Another popular choice is ponderosa pine because boards made from this species have grainy appearance and resistance to distortion and splintering. However, southern pine and ponderosa pine decking boards are not available in some parts North America (Civardi et al. 2016). Hence, Douglas fir and hem-fir boards substitute for southern and ponderosa pine particularly in western North America (Ganguly et al. 2010; Freedonia Group 2020c). Small quantities of treated deck-boards are produced from cypress (Taxodium distichum (L.) Rich. and T. ascendens Brogn.), birch (Betula spp.), maple (Acer spp.) and ash (Fraxinus spp.) (Lebow 2010; Freedonia Group 2020c). There is little information in the literature on the different wood species used for treated decking in Europe. Norway spruce (Picea abies (L.) Krast.), Scots pine (Pinus sylvestris L.) and European larch (Larix decidua Mill) are commonly used in Europe due to their availability (Englund 2010; 11 Cheng and Evans 2016; Civardi et al. 2016). However, Norway spruce is a refractory species which requires physical pre-treatment like incising to improve treatability (Civardi et al. 2016). 2.1.2 Larch and other naturally durable wood species 2.1.2.1 Larch Larch (Larix spp.) is a softwood species that grows throughout the Northern hemisphere (Bergstedt and Lyck 2007). The heartwood of larch is reddish-brown whereas its sapwood is pale brown (Bergstedt and Lyck 2007). Another visible characteristic of larch is its prominent and distinct growth rings which reflects the fact that the latewood of larch is much denser and darker in colour than earlywood (Johnson and Bradner 1932; Leavengood 1998). For example, Bergstedt and Lyck (2007) reported that the average density of earlywood and latewood in larch was 362 kg/m3 and 863 kg/m3, respectively. Accordingly, the tangential shrinkage of latewood in larch is twice that of earlywood. The basic density of larch ranges from 470 to 650 kg/m3 depending on species (EN 350-2 1994; Bergstedt and Lyck 2007). Table 2.2 lists the ten identified species in the genus Larix and where they are endemic. Table 2.2 Summary of common species of larch (Bergstedt and Lyck 2007) Scientific name Common name Origin Density (kg/m3) Larix decidua Mill European larch Central Europe 540-6601 Larix sibirica Ledeb. Siberian larch NE Russia, Siberia, Central Asia Larix gmelinii (Rupr.) Larix gmelinii var. olgensis Asian larch Dahurian larch Siberia, China, Mongolia Larix laricina (Du Roi) K. Koch Tamarack North America 5201 Larix kaempferi (Lamb.) Carr. Japanese larch Japan 5101 Larix griffithiana Carr. Sikkim larch Himalayas Larix potaninii Batalin Chinese larch China Larix mastersiana R. te W. Masters larch Western China Larix lyallii Parl. Subalpine larch North America Larix occidentalis Nutt. Western larch North America 6611,2 1Mean density at 12% moisture content; 2Measurements from four different stands of 20-year-old western larch trees in the interior forest of British Columbia (Ratcliffe et al. 2014) European larch and western larch are used for exterior decking in Europe, Canada, and the United States (Englund 2010; Asselin 2014). Larch has a number of characteristics that make it suitable for decking. It is readily available and is strong for a softwood (Johnson and Bradner 1932). Secondly, heartwood of larch timber is moderately durable (Clausen 2010). For example, 12 Morris et al. (2016) tested the field performance of deck-boards made from western larch and five other moderately or naturally durable softwood species. He exposed deck-boards outdoors for 10 years at four sites with different climates. The decking boards were then evaluated based on the degradation of their end-grain using the rating system AWPA E25-08 from American Wood Protection Association (2008). Morris et al. (2016) found the degradation of western larch decking was similar at all four sites to boards made from other moderately durable species such as Douglas fir, tamarack and eastern white cedar (Thuja occidentalis L.). Nevertheless, western larch is generally treated with preservatives to improve its durability for outdoor applications (Clausen 2010). However, treatment is difficult because the heartwood of western larch is impermeable compared to other softwood species (MacLean 1952). As a result, Park et al. (2017) suggested that physical pre-treatments such as boring and kerfing are needed to improve the treatability of dahurian larch lumber prior to impregnation with fire retardant. Western larch boards tend to check more during weathering compared to boards made from wood species with a similar density such as Douglas fir and southern pine according to Johnson and Bradner (1932). They are also more susceptible to greying than other species when exposed outdoors. For example, Oberhofnerová et al. (2017) compared the discolouration of deck-boards made from European larch with eight other commercially important wood species after boards were weathered for 12 months in Prague, Czech Republic. European larch decking had the largest change in colour (ÎE) and the deepest discolouration, extending 1.26 mm below the surface, compared to deck-boards made from the other species. 2.1.2.2 Naturally durable softwoods Most native softwood species in North America are not naturally durable and thus require preservative treatments to increase their decay resistance and suitability for exterior applications, for example wooden deck-boards. However, consumers prefer untreated and naturally durable deck-boards over ones that are treated with preservatives (Nyrud et al. 2008). Hence, the demand for western red cedar and redwood deck-boards is substantial, and predicted to reach 500 million lineal feet by 2023 (Freedonia Group 2020c) (Table 2.3). However, the trade in cedar and redwood decking in the United States is limited by their restricted geographic locations and 13 high transportation costs. Therefore, most of the cedar and redwood deck-boards are sold on the west coast of North America where the two species are abundant (Freedonia Group 2020c). Table 2.3 Cedar and redwood species commonly used in the United States and their durability rating Species Scientific name Basic density (g/cm3) Durability rating1 Yellow cedar Callitropsis nootkatensis (D. Don) Ãrsted 0.43 Durable Incense cedar Calocedrus decurrens 0.39 Moderately durable Port Orford cedar Chamaecyparis lawsoniana (A. Murr.) Parl. 0.47 Durable Atlantic white cedar Chamaecyparis thyoides L. 0.38 Durable Northern white cedar Thuja occidentalis L. 0.35 Durable Western red cedar Thuja plicata D. Don 0.37 Durable Redwood Sequoia sempervirens (D. Don) L. 0.42 Durable 1Durability rating according to Scheffer and Morrell (1998) and Clausen (2010) Morris et al. (2016) found that deck-boards made from western red cedar and yellow cedar showed no significant decay after ten years outdoor exposure at four different sites with continental, temperate, subtropical or tropical climates. Highley (1995) exposed fence cross-brace units made from redwood outdoors and above ground in Wisconsin and Mississippi for 22 years. His observations suggested that heartwood of redwood when used above ground had a service life of 20 to 30 years in Mississippi and Wisconsin, respectively. Conversely, the expected service life of redwood braces that contained sapwood was five years less compared to those consisting only of heartwood. In addition to being naturally durable, redwood deck-boards are also more fire resistant compared to wood plastic composites, which is important for exterior decks located in regions such as California where fire is a feature of the natural environment (Hasburgh et al. 2017; Freedonia Group 2020c). In addition to cedar and redwood, there are other durable softwood species which could be used for decking, such as baldcypress (Taxodium distichum (L.) Rich. var. distichum) and juniper (Juniperus spp.) (Clausen 2010). 2.1.2.3 Naturally durable hardwoods There are only a few naturally durable hardwood species in the United States that are suitable for exterior decking. For examples, species such as white oak (Quercus alba L.) and black walnut (Juglans nigra L.) have been used for decking (Freedonia Group 2020c). However, many 14 tropical hardwoods are durable and are imported from Central and South America, Africa and Asia-Pacific to fulfill the demand in North America for naturally durable deck-boards. Tropical hardwood decking is more expensive than pressure-treated wood and naturally durable softwoods, but many homeowners still select hardwood decking because of its resistance to physical deterioration, pleasant colour and appearance. Table 2.4 lists some of the most common hardwood deck-board species imported into the United States and their durability ratings (Freedonia Group 2020c). Table 2.4 Origins, basic density and durability rating of commercially imported hardwood species used for wooden decking in the United States Species Scientific name Origin Basic density (g/cm3) Durability rating1 American Mahogany Swietenia macrophylla King C. America 0.72 Durable Ipe Handroanthus spp. S. America 1.10 Very durable Tigerwood Lovoa trichilioides Harms S. America 0.91 Moderately durable Cambara Erisma uncinatum Warm S. America 0.65 Moderately durable Garapa Apuleia leiocarpa (Vog.) Macbr S. America 0.82 Durable Cumaru Dipteryx odorata (Abul.) Wild. S. America 1.00 Very durable Teak Tectona grandis L.f Asia-Pacific 0.68 Very durable Balau Shorea spp. Asia-Pacific 0.80 Very durable Dark red meranti (Red lauan) Shorea negrosensis Foxw Asia-Pacific 0.68 Moderately durable 1Durability rating according to Scheffer and Morrell (1998) and Clausen (2010) Very few studies have examined the performance of tropical hardwood decking. Williams et al. (2001) exposed ten different tropical hardwood panels to accelerated weathering for 2400 hours and compared their performance with those of similarly weathered naturally-grown and plantation-grown teak panels (control). Teak panels had fewer checks and less distortion than ones made from the other ten tropical hardwoods. Nevertheless, ipe performed better compared to other hardwood species aside from teak (Williams, Miller and Gangstad 2001). Cheng (2015) examined the performance of hardwood decking exposed to the weather for one year in Vancouver. Most of the species he tested were temperate hardwoods but he included ipe in his 15 trial. His results showed that ipe was one of the best performing species, in accord with the results of Williams et al. (2001). Reinprecht et al. (2018) observed the checking and colour change of boards made from seven different hardwoods exposed to artificial weathering for 2016 hours or natural weathering for three years. As expected, ipe was the most resistant to checking and greying when exposed to the weather (artificial and natural). However, they also noted that cumura boards had the smallest change in lightness compared to other species when exposed to artificial weathering. In other words, camura boards were less susceptible to photobleaching during accelerated weathering (Reinprecht et al. 2018). 2.1.3 Wood plastic composites Wood plastic composites (WPC) are materials derived from wood particles and a thermoplastic polymer (Youngquist 1993). The wood particles act as a filler and reinforcing agent in the composites. Hence, wood can increase the strength of the composite (Youngquist 1993). WPC is manufactured in two steps: compounding and extrusion (Clemons 2002). Compounding is the mixing of wood particles, plastic and various additives. The ratio of wood to plastic depends on the product specifications; the amount of plastic can range between 40 and 70 percent. For wood plastic composite decking, manufacturers often select a 50/50 ratio of wood flour and polymer, usually high density polyethylene (HDPE), but also polypropylene (PP) or polyvinyl chloride (PVC) (Clemons 2002). WPC deck-boards are available in different colours depending on the wood species used as filler and the colourants added to the mix (Green 2005). After compounding, the wood-polymer mix is either pressed into pellets for future applications or extruded into the final product (Clemons 2002). Since WPC contains only 50% wood, its performance against moisture induced swelling is better than those of plywood, MDF and lumber (Defoirdt et al. 2010). Furthermore, it is more resistant to fungal and insect attack than lumber because these micro-organisms cannot utilize plastic as a food source (Morrell et al. 2009; Defoirdt et al. 2010). In addition, WPC weathers evenly, fades uniformly and is less susceptible to checking than lumber (Kim et al. 2008). These characteristics minimize annual maintenance of WPC decking by homeowners (Freedonia Group 2016c). However, one disadvantage of WPC is that it cannot be used for structural purposes because flour-formed composites are not designed for critical load bearing applications (Balatinecz and Woodhams 1993). Thus, WPC has a lower 16 design factor than solid wood, and is only suitable for appearance and moderate load-bearing situations (Carroll et al. 2001). Ganguly and Eastin (2009) observed that professional deck builders in the United States tend to use WPC for the surface decking while constructing the subframe from treated wood. A capstock-coated surface layer is often employed on WPC deck-boards to improve resistance to wear, photodegradation and decay (Freedonia Group 2020b). This layer consists of polymer and it is free of wood (Figure 2.1). Hence, it forms a barrier between the wood material in the core and the natural elements. An increasing number of homeowners are choosing capstock-coated WPC deck-boards (Figure 2.1a) over uncoated ones (Figure 2.1b) because of the improved performance of the former against abrasion and weathering. However, some homeowners dislike the additional resin cap because it makes the decking surface look artificial and lacking the grainy appearance of natural wood (Freedonia Group 2020b). Uncapped WPC deck-boards remain popular in the market because they are cheaper than capstock-coated boards (Freedonia Group 2020b). Figure 2.1 Images of wood plastic composite decking: (a) capstock-coated and (b) unocated Fabiyi et al. (2008) manufactured uncapped HDPE- and PP- based WPC deck-boards and exposed decking samples to natural weathering for two years in Moscow, USA. They found that both types of uncapped WPC deck-boards began to grey after eight months of outdoor weathering. Fabiyi et al. (2008) also examined the surface of weathered WPC deck-boards using scanning electron microscopy and found that micro-cracks formed at the surface of WPC boards as a result of degradation and erosion of wood. Subsequently, Fabiyi and McDonald (2010) compared the colour change of uncapped WPC decking made from five different softwood and hardwood species exposed to accelerated weathering. The severity of discolouration of these 17 weathered WPC deck-boards varied with wood species. The following ranking shows the overall colour change of artificial weathered WPC deck-boards from most to least discoloured: hybrid poplar (Populas deltoides x Populas tremuloides), ponderosa pine, white oak, Douglas fir and honey locust (Gleditsia triacanthos L.) (Fabiyi and McDonald 2010). This finding suggested that WPC decking made from naturally or moderately durable wood species performs better than ones made from non-durable wood species. Furthermore, Ibach et al. (2020) found that adding zinc borate and UV absorbers reduced the decay and photodegradation of uncapped HDPE deck-boards exposed outdoors in Mississippi or Wisconsin, United States. 2.2 Factors affecting the performance of wood decking 2.2.1 Checking 2.2.1.1 Formation of checks A check is a longitudinal separation at the surface of wood resulting from the release of drying stresses. These stresses occur due to the creation of moisture gradients within wood and differential shrinkage (Schniewind 1963; Côté 1968). The mechanisms responsible for checking of wood during kiln drying and when wood is exposed to weathering are similar in principle. Figure 2.2 shows the dimensional movement and formation of checks during kiln drying and outdoor weathering of wood. McMillen (1955) described the checking mechanisms of wood (lumber) during kiln drying. First, the outer surface of flat-sawn lumber is dried by circulating kiln heat, whereas the core of lumber remains saturated with water. Therefore, a moisture gradient is created between the surface and the core of the wood (Figure 2.2a). As drying progresses, the wood tries to cup because of differential shrinkage in radial and tangential directions (Figure 2.2b). However, such distortion is restrained by the wet core because water has not been removed from this area. Therefore, there is no shrinkage or checking in the core (Song and Shida 2010). As a result, tensile and compressive strains develop at the surface and the core of the wood, respectively. These strains cause longitudinal fractures (checks) along the surface of wood (Figure 2.2c). The bark-side of flat-sawn boards, in particular, experiences greater tensile strains and may develop larger checks than those at the pith-side especially during the early stages of kiln drying. This difference in checking of the two board types occurs because growth rings in bark-side boards are more tangential than those in pith-side boards. As moisture gradients inside 18 the wood normalize, the restraining effect of the wet core diminishes. The wood begins to cup towards the bark-side, and checks located on that face close and become narrower (Figure 2.2d). As a result, checks on the pith-side of boards widen to counteract movement on the opposite face. At the same time, the outer shell of boards begins to settle into a configured case-hardened shape while the core continues to dry. The hardened âcaseâ prevents the release of strains in the core in the later stages of drying, which may lead to internal checking (Figure 2.2e) (McMillen 1955). The severity and occurrence of checks increase if kiln operators use a more aggressive drying schedule (Simpson 1991). During the conditioning period towards the end of a kiln drying schedule, checks may close-up and reduce in size to become micro-checks, which are not visible to the naked eye (Yang and Normand 2012). Mackay (1973) suggested that such micro-checks can reopen when kiln dried boards are exposed outdoors. However, Perem (1971) examined the checking of sugar maple (A. saccharum Marshall) railway ties after kiln drying and also after 4 years of in-track service. He found that only 5 to 14% of the total checks that developed during outdoor exposure could be accounted for by the checks that were initially present after kiln drying. Hence, the correlation between checking during kiln drying and during weathering was low, but statistically significant (Perem 1971). Evans (2008) and Ratu (2009) examined the effect of environmental factors such as solar radiation and water on the checking of wood during artificial and natural weathering. Evans (2008) also developed a mechanism to explain the checking of wood during natural weathering. This mechanism was based on the aforementioned mechanism for the checking of wood during kiln drying (McMillen 1955), but also accounted for the fact that fastener systems in wood exposed outdoor often restrict release of strains through cupping. According to Evans (2008), checking of deck-boards during weathering begins when rain wets the surface of the wood. Some of the water slowly diffuses into the sub-surface of boards (Figure 2.2f) (Evans 2008). Subsequently, the surface of deck-boards dries on exposure to sunlight, whereas the sub-surface of boards remains wet. A moisture gradient between the surface and sub-surface of boards develops (Figure 2.2g). As a result, the deck-board tries to cup as the surface shrinks (Figure 2.2h). However, such distortion is restrained by fasteners that fix deck-boards to the sub-frame (Figure 2.2i). Fasteners and the wet sub-surface keep the exposed 19 surface from shrinking, and as a result, surface tensile strains develop. Checks form when such surface tensile strains exceed the tensile strength of wood perpendicular to the grain (Figure 2.2j). Figure 2.2 Mechanism of check formation of a flat-sawn deck-board during kiln drying (a-e) and outdoor weathering (f-j). Moisture content of wood is indicated by colour (Blue is wet and yellow is dry). This diagram is redrawn from ones in McMillen (1955), Evans (2008) and Cheng (2015) In addition to the purely physical mechanism of checking described above, surface chemical degradation also affects the checking of wood. For example, Evans et al. (2008) examined the effect of photodegradation on the checking of lodgepole pine decking exposed under different UV/visible filters to natural weathering for 36 weeks. They found that UV light degraded lignin at the surface of wood and weakened the interface between tracheids. As a result, boards exposed to UV and visible light checked more than those that were exposed under a filter that blocked solar radiation from degrading the wood surface (Evans, Urban and Chowdhury 2008). In a later study, Ratu and Evans (2008) constructed a weatherometer to expose southern pine and western red cedar deck-boards to different environmental conditions. They found that 20 boards exposed to a weathering cycle with UV light exposure had more and longer checks compared to boards exposed to other cycles that did not include UV light. Information from the study by Ratu and Evans (2008) was used by Ribarits and Evans (2010) to develop a finite element model of water diffusion and checking of flat-sawn Norway spruce boards. Their model showed that high tensile strains developed at the surface of boards during initial drying and initiated surface checking (Ribarits and Evans 2010). Subsequently, rays in the core of boards sustained high tensile strains when the surface was exposed to moisture. Repetitive tensile and compressive strains in rays during cyclic weathering encouraged propagation of checks along the rays into the core of the wood. In addition, internal tensile strains developed causing internal checking. Ribarits and Evans (2010) suggested that these internal checks connected with surface checks resulting in the developments of large cracks in deck-boards. Heshmati et al. (2018) developed another related finite element model to simulate the stress and check developments of profiled white spruce decking boards exposed to wetting and drying. Their finite element model showed that boards with shallower grooves (short rib) had more localized areas with high tensile strain compared to boards with deeper grooves (tall rib). Heshmati et al. (2018) concluded that profile geometry influenced the stresses and checking that develops at the surface of profiled deck-boards exposed to natural weathering. Mallet et al. (2018) used digital image correlation to compare strains that developed at the surface of profiled and flat radiata pine deck-boards exposed to wetting and drying. Digital image correlation clearly showed that drying strains were concentrated in the grooves of profiled boards and explained the tendency of checks to form in profile grooves. More recently, Heshmati et al. (2020) suggested that microbial degradation of wood in grooves resulting from the tendency of grooves to trap dust and water might explain the pattern of checking in untreated or poorly treated profiled boards. 21 2.2.1.2 Check Types Check, split, and crack are some of the common terms used to describe the longitudinal separation of wood. These terms are sometimes used interchangeably in the literature, which causes ambiguity because they have different meanings (Bucur 2011). There are only a few papers that define and clarify different terms relating to the longitudinal separation of wood (Flæte et al. 2000; Conners 2008; Bucur 2011; Yang and Normand 2012). Checks are defined as macroscopic separation of wood cells that occur on a single surface of a wooden board, whereas splits propagate through the entire thickness of a board (Conners 2008). Cracks are checks that penetrate more than 75% through a boardâs thickness (Flæte et al. 2000). Bucur (2011) refined the classification of different types of checks occurring in round timber based on their location on a pole and their propagation pattern. Subsequently, Yang and Normand (2012) defined check types that are specific to dimensional lumber (Figure 2.3) as follows: a) End checks: Checks that occur in the cross-section of boards resulting from rapid moisture loss via end-grain of boards during kiln drying. b) Surface checks: Longitudinal separation of wood at the surface of boards. They often develop and propagate via rays in wood. c) Internal checks: Internal voids resulting from the collapse of wood during kiln drying or heat treatment. In addition, case-hardening of wood during kiln drying may increase internal checking because strains in the core cannot be released through dimensional movement. d) Micro-checks: Checks that are not visible to the naked eye, but they can be clearly seen under a microscope at 50x magnification. Micro-checks in wood can open up and develop into larger surface checks when wood is exposed to cyclical weathering. e) Splits: Lengthwise separations that extend through a piece of wood. Splits form when metal teeth in debarking and sawing equipment compress and fracture the wood during handling. Furthermore, other types of checks may develop around a split due to the mechanical weakening of the material. 22 f) Shakes: Separations of growth rings resulting from inherent growth stresses and wind damage. True firs (Abies spp.) and western larch are susceptible to developing shakes (Yang and Normand 2012). Meyer and Leney (1968) found that shakes in coniferous trees usually occur in latewood. Unlike other types of checks (above), shakes cannot be prevented with slower kiln drying schedules and reduced handling. However, shakes become more visible after kiln drying because of the differential shrinkage between earlywood and latewood. The appearance of a shake on a longitudinal surface is similar to that of surface checks. However, there have been no studies that have separated the two types of checks when assessing checking at the surface of deck-boards. Figure 2.3 Different types of wood separations at a board surface as described by Yang and Normand (2012) and Cheng (2015): end check, surface check, internal check, micro-check, split, shake and crack The severity of checking at the surface of deck-boards can be quantified using the following parameters: width, depth, length, area, volume and combinations of these parameters such as length/width ratio (Evans et al. 1997). An accurate method of assessing the width and length of checks at the surface of deck-boards involves using an optical calibrated eyepiece graticule and a transparent Plexiglas ruler, respectively (Evans et al. 1997; 2010). Christy et al. (2005) developed an automated measuring system to quantify surface checks at deck-board surfaces. First, they used a desktop scanner to produce high-contrast greyscale images of deck-board surfaces. Image contrast was adjusted so that checks appeared as black objects against a 23 white background. A computer program identified individual checks and quantified their length and width. There was a high correlation between automated and manual measurements of check length (R2 = 0.99) and width (R2 = 0.95). However, automated detection of micro-checks and horizontal checks that run across the width of boards was limited by the technology available at the time (Christy et al. 2005). Staining of wood makes micro-checks more visible so their dimensions can be more easily assessed (Yang and Normand 2012; Cheng and Evans 2018). Different automated systems have also been used to characterize surface checks, such as stylus profilometry, laser diffraction and acoustic emission. However, these automated systems are not widely employed because they are laborious to setup and operate. Kamdem and Zhang (2000) used a contact stylus profilometer to characterize surface checks in preservative-treated deck-boards exposed to the weather for twenty years. Wahl et al. (2001) developed a check measurement system that analyzed laser intensity variations of a scanned surface of dried Scots pine logs. During surface scanning of checked logs, the laser beams were scattered and produced distinct speckle patterns. Width and depth of surface checks were quantified by these speckle pattern measurements (Wahl et al. 2001). Quarles (1992) used acoustic emissions to measure the propagation of cracks in black oak (Quercus kelloggi Newb.) boards during kiln drying. However, his technique was only capable of measuring the occurrence of such internal and surface checks rather than their dimensions (Quarles 1992). 24 2.2.1.3 Wood grain orientation and checking The orientation of growth rings influences the severity of checking at the surface of wooden boards (Sandberg 1999). For example, flat-sawn boards expose more of the tangential surface of wood to the weather than quarter-sawn boards (Sandberg and Söderström 2006). The tangential shrinkage at the surface of flat-sawn boards is greater than that of boards that are rift-sawn or quarter-sawn. As a result, flat-sawn boards check more than boards which are rift- or quarter-sawn (Sandberg and Söderström 2006). Yata (2001) examined the effect of growth ring orientation (pith-side-up versus bark-side-up) on the checking of flat-sawn Japanese cypress, Japanese cedar and western hemlock boards exposed to natural weathering. Boards that were oriented pith-side-up had shorter checks than boards oriented bark-side-up. Subsequently, Urban and Evans (2005) examined the checking of flat-sawn and unprofiled southern pine deck-boards exposed to the weather. They found that boards oriented pith-side-up had fewer and smaller checks than boards oriented bark-side-up. Morris and McFarling (2008) found that orienting lodgepole pine boards pith-side-up rather than bark-side-up reduced checking of deck-boards exposed to the weather. Conversely, growth ring orientation had on effect on the checking of flat-sawn pacific silver fir deck-boards (Morris and McFarling 2008). More recently, Heshmati et al. (2020) examined the checking of profiled Douglas fir, western hemlock and white spruce deck-boards oriented pith-side-up or bark-side-up and exposed to natural weathering. Double-sided profiled deck-boards oriented bark-side-up checked significantly more than similarly profiled boards that were oriented pith-side-up. However, growth ring orientation had no effect on the checking of single-sided profiled deck-boards made from the three softwood species (Heshmati et al. 2020). 25 2.2.2 Distortion 2.2.2.1 Types of distortion Distortion of wood, commonly known as warping, is the unwanted change in dimension of wood after drying (Simpson 1991; Bergman 2010). Wood with severe distortion suffers strength loss due to its twisted grain orientation, and a low recovery rate as the distorted wood requires machining to convert it into workable square or rectangular shapes. Thus, distorted wood is regarded as inferior for sawn products and is devalued based on the type of warping it suffers from. Warping of timber is classified into five types based on the faces or edges of wood that have distorted (Simpson 1991), (Figure 2.4), as follows: a) Bow: One of the faces forms a curvature along the length of a board. Bowing is measured in the middle of the board, which is often the point of greatest deflection. b) Crook: Crook occurs at the edges of a board. Similar to bowing, the deviation in the middle of a board is used to quantify the severity of crook. c) Twist: Unlike other forms of warping that only measure distortion on a single face or edge, twist is quantified as the difference in degrees of rotation between two ends of a board. However, it is impractical to measure the rotational difference in degrees. A simpler method is to fix three corners of a board on a flat reference surface and measure the gap from the fourth corner to the bed. d) Cup / Crown: Cross-section of a squared board curves and deviates from flatness with reference to a line that spans across the width of a board. The severity of cupping is measured by the distance perpendicularly from the line to the point of greatest deflection. Cup in deck-boards often occurs as curvature towards the exposed surface of the deck. Conversely, crown is the cupping of deck-boards away from the exposed surface. e) Diamonding / Oval: Rectangular cross-section of a board distorts into a diamond-like shape and the four corners on the plane are no longer squared (90°). Square posts and thick lumber are prone to diamonding. Oval describes the geometric distortion of a round wooden piece (dowel), where the wood is longer in the radial direction compared to the tangential direction. 26 Figure 2.4 The five common types of distortion in wood described by Simpson (1991): bow, crook, twist, cup/crown and diamonding/oval. The red dashed lines outline the presumed geometry of the board before distortion occurred The European Commission of Science and Technology E53 Group, quality control for wood and wood products, developed a standardized comprehensive guide to quantifying distortion and to explain the influence of wood properties and drying practices on warping (Grohmann et al. 2010). Furthermore, the guide specifies the maximum allowable limit for distortion for different product types. For example, wall studs are only permitted to have less than 6 mm bowing, whereas panel products can have up to 50 mm of bow. In addition, the focus group introduced a method to express cup and twist as a percentage of board width because the absolute value cannot always reflect the severity of these two types of distortion, with respect to the overall size of a board (Grohmann et al. 2010). Studies on distortion of deck-boards have mainly focused on cupping and twisting as the distorted boards pose a tripping hazard for users, and possibly influence the checking of boards (Piper 2004; Ratu et al. 2007). Akhtari et al. (2014) studied cupping and twisting behaviour of artificially weathered treated southern pine deck-boards. They only reported the mean value for two types of distortion. Their measurements run counter to the suggestion of Hansen and Nicolajsen (1999) who indicated that the mean cupping value does not fully represent the overall 27 dimensional stability of lumber, as sometimes a large positive cupping value for a board is offset by an equally large negative cupping measurement from the same sample. Hence, they suggested that the standard deviations of the dataset should also be specified when assessing dimensional stability (Hansen and Nicolajsen 1999). For example, large standard deviations in cupping measurements can indicate boards that are also distorted by a large degree of twisting. 2.2.2.2 Wood structure and distortion Distortion of wood is mainly related to woodâs structure and differential moisture-induced shrinkage or swelling (Simpson 1991). Wood shrinkage in the tangential direction is approximately twice that of radial shrinkage (Côté 1968). As a result of such anisotropic shrinkage, various forms of distortion occur when wood shrinks (Simpson 1991) (Figure 2.5). In addition, the magnitude of cupping increases if boards are wide and thin (Knight 1961; Simpson 1991). Sandberg (2005) investigated the effect of annual ring orientation on distortion of Norway spruce and Scots pine lumber. He machined boards from round logs; the boards had eight predetermined growth ring orientations. The boards were conditioned for three months after kiln drying. Flat-sawn boards were more prone to cupping compared to rift-sawn and quarter-sawn boards because flat-sawn boards had growth rings oriented tangentially to their wide faces. Furthermore, the severity of cupping of flat-sawn boards increased if boards were cut closer to the pith because boards contained more juvenile wood (Sandberg 2005). The presence of juvenile wood increases the severity of distortion. Juvenile wood shows more anisotropic shrinkage compared to mature wood (Simpson 1991; Glass and Zelinka 2010). Hence, Williams and Knaebe (1995) suggested orienting lumber to minimize the amount of juvenile wood or tangentially oriented growth rings directly exposed to water and sunlight, thereby reducing cupping. Irregular grain caused by knots or reaction wood also increases the distortion of wood (Knight 1961). For example, longitudinal grain that is not parallel to the edge of lumber encourages twisting of wood during drying (Sandberg 2005). Also, wood species like beech (Fagus spp.) and elm (Ulmus spp.), containing spiral or interlocked grain, are prone to cupping and twisting (Côté 1968). Twisting of wood produced from straight and high sweep small diameter Douglas-fir saw logs was examined by Wagner et al. (2002). They found that straight logs contained more spiral grain than high sweep logs, and boards produced from the former were 28 more susceptible to twisting. Wood density is another key attribute that affects the tendency of wood to twist (Williams 2005). Denser wood species like southern yellow pine are more susceptible to twist than lower density wood species such as redwood according to Williams (2005). Belt et al. (2013) observed that asymmetrical density profiles from surface densification affected cupping behaviour of Scots pine boards. There was a correlation between the compaction ratio and the cupping direction. Boards with a low compaction ratio cupped towards the undensified side whereas highly compacted boards cupped in the opposite direction (Belt et al. 2013). Figure 2.5 Diagram of different forms of distortion due to differential shrinakge in the radial and tangential directions: (a) Flat-sawn board, greater tangential shrinkage and cupping; (b) riftsawn board; (c) square, balanced radial and tangential shrinkage; (d) riftsawn square, diamonding; (e) quartersawn board, greater radial shrinkage; (f) pith-center board; prone to all types of distortion; (g) round dowel, ovaling (Simpson 1991) 29 2.2.2.3 Distortion during kiln drying and weathering Distortion of wood occurs both during kiln drying and outdoor exposure because of changes in wood moisture content, below the fibre saturation point. During kiln drying, distortion of lumber is caused by the âcombination of inherent growth stress and drying stressâ according to Grohmann et al. (2010). Simpson (1991) suggested that systematic stacking and sorting practices can reduce distortion such as cupping and twisting. He recommended that lifts of lumber should consist of uniform length pieces, supported with an adequate number of stickers throughout the length of a board. Otherwise, stacked lumber of mixed lengths will warp (twist and cup) as they are not adequately restrained by the weight of lumber above them. Warping further compounds the problem because warped pieces disturb air circulating through the wood pile leading to inconsistent drying and differential shrinkage of some pieces. As a result, some piece will distort when they try to reach a moisture equilibrium with ambient environments. Frühwald (2007) found that using high temperature drying and steaming combined with a high degree of physical restraint (top loaded stacks) reduced twisting of Norway spruce during kiln drying. Exposure of deck-boards to sunlight increases cupping on the exposed face of boards because of greater differential shrinkage of the exposed face compared to the underside (Williams and Knaebe 1995). Virta (2005) studied the cupping of 21 mm and 28 mm thick untreated or heat treated Norway spruce and Scots pine exterior cladding (weatherboards), oriented pith-side-up and exposed to accelerated weathering. The samples were subjected to 14 cycles of 3 hours wetting followed by 21 hours of drying. During the wetting phase, the surface of the cladding became saturated with water, while the center of boards remained dry and, hence, restricted movement of the swelled surface (Virta 2005; Dengyun et al. 2007). This moisture gradient and stress differential caused boards to crown (Williams and Knaebe 1995) (Figure 2.6a). During drying, the surface of cladding shrank more than the center, and boards cupped (Figure 2.6b). During natural weathering, similar anisotropy is caused by rain and the drying effect of sunlight (Williams and Knaebe 1995). 30 Figure 2.6 Diagram of Norway spruce and Scots pine cladding exposed to accelerated cyclic weathering: (a) surface of cladding during exposure to water showing the moisture-induced crowning of boards; (b) Cupping of boards as a result of drying of the surface of boards (Virta 2005) Single-sided profiling increased the tendency of pacific silver fir deck-boards to cup during natural weathering (Morris and McFarling 2008; Cheng and Evans 2018). Similar cupping behaviour was also observed in single-sided profiled Douglas fir plywood cladding and western red cedar shakes (Deskey 1942; Norlander and Knowles 1970). However, Akhtari and Nicholas (2014a) found that single-sided rib profiling reduced cupping of southern pine deck-boards exposed to accelerated weathering. Profiling increased moisture-induced strains that develop at the surface of deck-boards (Mallet et al. 2018). Hence, profiling increased the tendency of boards to cup according to Cheng and Evans (2018). Similarly, Morris and Ingram (2002) also noted that incising increased cupping of CCA-treated decking made from seven different commercially available softwood species. Crowning and cupping of boards can be restricted by fasteners (Koehler 1957). Nevertheless, repetitive moisture-induced movement of boards pushes out fixings from the subframe and weakens their holding power, which allows greater flexing of boards and increases 31 the magnitude of crowning and cupping (Virta 2005). Water-repellent emulsion treatments reduced moisture content fluctuations of chromated copper arsenate treated southern pine deck-boards during weathering (Zahora 1995). Deck-boards treated with water repellents flexed less between wet and dry cycles. As a result, nail pulls were reduced. In contrast, untreated decking flexed and fixings were lifted out from the surface of boards by as much as 5 mm (Zahora 1995). On the other hand, water repellent treated boards can cup more than that of untreated boards because the treatment reduces release of stresses via checking (Plackett et al. 1985; Zahora 1995). 2.2.3 Other factors (affecting performance of decking) 2.2.3.1 Shelling Shelling, sometimes called raised or loosened grain, is a defect resulting from the separation of wood cells at the interface between earlywood and latewood (Paul 1955; Williams and Knaebe 1995). Shelled wood surfaces contain slivers of wood that project beyond the surface and give the wood a corrugated appearance (Figure 2.7) (Koehler 1929). Shelling can be caused by improper rotary machining and excessive pressure exerted by infeed rollers (Koehler 1929; Lamb 1992). For example, rotary cutting machines with dull knives or ones with small clearance angles remove wood fibers unevenly off the surface. Softer earlywood is gouged away and the denser latewood is compressed into the material (Lamb 1992). Subsequently, deformed latewood recovers and separates from earlywood. In addition, cutting with a higher number of knife marks per inch increases shelling as the knives contact the surface more frequently (Lamb 1992). Similarly, moulders and wide-belt sanders that employ over pressurized infeed rollers will press latewood into underlying earlywood. As a result of such compression, there is shearing at the interface between indented latewood and underlying earlywood (Koehler 1929; Simpson 1991). When the wood dries, the detached latewood layer shrinks more transversely and less longitudinally than adjacent earlywood (Koehler 1929). The difference in shrinkage of these wood tissues causes them to delaminate resulting in shelling (Koehler 1929; Lamb 1992). In addition to improper machining, in-service fatigue, sub-optimal kiln drying schedules and lignin degradation can also cause shelling of lumber (Paul 1955; Lamb 1992; Donaldson 2011). Paul (1955) found that repetitive foot traffic caused fatigue of wooden flooring due to continuous compression of 32 latewood. Soft maple and Douglas-fir flooring are prone to fatigue and develop surface undulations as earlywood and latewood separate (Paul 1955). Beaulieu and Barton (1939) observed that shelling during kiln drying is caused by a sudden drop in temperature and relative humidity within the kiln. However, there are not many recorded cases of shelling developing during kiln drying apart from shelling of Hoop pine (Araucaria cunninghami D. Don) noted by Donaldson (2011). There are two forms of wood degradation that seem to be associated with shelling. According to Bariska et al (1988), acidic chromated copper arsenate preservative degrades the interface between earlywood and latewood through hydrolysis of hemicelluloses and promotes shelling. In addition, ultraviolet light breaks down lignin at the surface of wood products used outdoors, which might increase the severity of shelling (Evans, Urban and Chowdhury 2008; Donaldson 2011). Figure 2.7 Severe shelling of Douglas fir lumber after kiln drying (Simpson 1991) Flat-sawn lumber oriented pith-side up and exposed outdoors to the weather can develop pronounced shelling (Koehler 1929). Wood oriented pith-side-up wood has the tips of the latewood running out of the surface, where shelling often begins and then spreads (Koehler 1929). Furthermore, it is evident that wood species with denser latewood like southern pine are more prone to severe shelling (Williams and Knaebe 1995; Morris and McFarling 2008). Several publications have commented on the severity of shelling of flat-sawn western larch deck-boards when they are oriented pith-side-up (Koehler 1929; Johnson and Bradner 1932; Asselin 2014). 33 Asselin (2014) suggested that the density contrast of earlywood and latewood in larch combined with high differential shrinkage in radial and tangential directions were responsible for the susceptibility of flat western larch decking to shell when exposed outdoors. Shelling of profiled western larch deck-boards installed pith-side-up is shown in Figure 2.8. Severe shelling can pose a tripping hazard to people, according to Johnson and Bradner (1932). Hence, it is advised that flat-sawn deck-boards made from species that have distinct growth rings should not be oriented pith-side-up to avoid pronounced shelling, even though orienting deck-boards pith-side-up reduces checking (Williams and Knaebe 1995; Heshmati et al. 2020). Figure 2.8 Shelling of a profiled flat-sawn western larch deck-board oriented pith-side-up and exposed to the weather for 2 years (photo, Dr. Philip D. Evans) There is no standard quantitative method of measuring the severity of shelling. However, many techniques have been used in previous studies to characterize the corrugated appearance of shelled latewood, such as confocal profilometry, digital photography and scanning electron microscopy (Stehr and Ãstlund 2000; Singh et al. 2008). Stehr and Ãstlund (2000) used scanning electron microscopy to image shelled flat-sawn Scots pine boards and found that micro-cracks developed at the interface of latewood tips and adjacent earlywood. Their images also showed that earlywood was compressed by the denser latewood. Stehr and Ãstlund (2000) extended their obervations by developing a finite model for the shelling of Scots pine boards exposed to cyclical drying and wetting. Their model revealed a negative correlation between the severity of shelling and the distance from the pith of boards. In other words, the protrusion of shelled latewood diminished as growth rings were inclined at a higher angle to the surface. In addition, they noted that the pith-side of Scots pine boards was more susceptible to shelling (Stehr and Ãstlund 2000). Subsequently, Singh et al. (2008) examined shelling at the wetted surface of coated flat-sawn radiata pine boards oriented pith-side-up. They found that the protruding area 34 of boards under the paint film corresponded to the moisture-induced rebounding of shelled and detached latewood. 2.2.3.2 Fasteners Fasteners restrict deck-boards from relieving stress via warping (Evans 2008). Face screws and nails are the main method of fastening deck-boards onto subframes (Unick 2011). Evans et al. (2003) examined the effects of self-drilling screws, screw-nails, and nail-gun nails on the checking of CCA treated radiata pine deck-boards exposed outdoors (Figure 2.9a). They found that the three fasteners had little overall effect on the total number of checks on CCA treated deck-boards. However, longer and wider checks developed around the self-drilling screws possibly because the holes they created were larger compared to those associated with the two nail types (Evans et al. 2003). In a follow-up study, Urban and Evans (2005) found southern pine decking affixed with stealth fasteners or screws containing rubber washers developed checks with a higher length to width ratio compared to checks in decking fixed with traditional decking screws (Figure 2.9b). They suggested that the hidden fixings and modified screws allowed deck-boards to release stress during service by cupping rather than checking. They suggested that as a result of such flexing the pattern of stresses at the surface of deck-boards was altered, resulting in the formation of longer and thinner checks (Urban and Evans 2005). Figure 2.9 Fastener systems used in the studies of (a) radiata pine decking by Evans et al. (2003) and (b) southern pine deck-boards by Urban and Evans (2005) The performance of the fastening system is an important factor to consider when installing a deck, especially the holding power and corrosion resistance of the fasteners (Urban and Evans 2005). Corrosion significantly decreases the holding strength of fasteners, which 35 reduces their ability to restrain distortion of wood (Zelinka and Rammer 2012). Zelinka (2014) found that copper ions in water-soluble wood preservatives oxidize and corrode galvanized metal fixings. ACQ and copper azole are regared as being more corrosive to galvanized fasteners than other copper-based preservatives (Groenier 2006; Zelinka et al. 2010). Hence, Zelinka and Passarini (2018) recommended that homeowners should switch to stainless steel fixings as they have better corrosion resistance than galvanized fasteners. However, others still prefer galvanized fasteners because they cost less than stainless steel fixings and may provide protection against decay initiation in fastener holes due to mobile zinc (Stirling 2020, personal communication). There are no studies that have shown a positive correlation between corrosion of fasteners and the checking of deck-boards. 2.2.3.3 Colour changes Manufactured, untreated wood products discolour when they are exposed outdoors (Feist 1983). The discolouration of wood can be categorized into chemical and biological colour changes (Feist 1983; Kreber 1994). Photo-chemical discolouration of wood occurs during weathering. Visible and ultraviolet light penetrate wood surfaces and rapidly degrade lignin and extractives which are leached from wood by water (Feist 1983). As a result, a âwhiteâ cellulose-rich surface is created which is colonized by dark-coloured, melanin-rich, staining fungi that are resistant to UV radiation and able to metabolize lignin-breakdown products (Schoeman and Dickinson 1997; Hernandez and Evans 2015). Hence, weathered wood is grey. The severity of greying depends on wood species and their extractive composition. The grey layer of wood can extend approximately 0.01 to 2 mm into wood. The extent of colour change of wood during weathering is influenced by temperature and humidity (Feist 1983). For example, Liu et al. (2017) compared discolouration of European ash (Fraxinus excelsior L.), Chinese ash (Fraxinus mandshuria Rupr. var. japonica Maxim), European walnut (Juglans regia L.) and Chinese walnut (Juglans mandshurica Maxim.) boards exposed to accelerated weathering for 288 hours. They found that the lightness (L*) of ash and walnut boards decreased during artificial weathering. In other words, the boards became darker. European ash darkened the most, followed by European walnut, Chinese walnut and Chinese ash. The yellowness and redness of boards across all species increased after weathering except for European walnut boards whose yellowness decreased (Liu 36 et al. 2017). Oberhofnerová et al. (2017) examined the change in colour and gloss of boards made from nine different hardwood and softwood species after they were exposed to the weather for twelve months. Their findings of the loss of lightness at the surface of naturally weathered boards accords with those of Liu et al. (2017). However, Oberhofnerová et al. (2017) found that redness and yellowness of their sample boards increased initially and then decreased during their outdoor weathering trial. They also observed a high correlation (R2=0.83) between colour change and the depth of affected wood layers. However, weathering had no significant (p>0.05) effect on the surface gloss of weathered hardwoods and softwoods (Oberhofnerová et al. 2017). Kubovský et al. (2018) observed colour changes of English oak (Quercus robur L.), black locust (Robinia pseudoacacia L.), poplar (Populus sp.), black alder (Alnus glutinosa (L.) Gaertn) and sycamore maple (Acer pseudoplatanus L.). They found that significant surface greying of the hardwood boards occurred after three months of weathering. Discolouration of these hardwoods diminished in the second year of their weathering trial. After two years of outdoor exposure, white oak (naturally durable) boards had the least discolouration out of the five test species. In contrast, the surface colour of sycamore maple (non-durable) boards changed the most (Kubovský et al. 2018). In addition to photodegradation, wood species that are rich in tannins, such as oak and western red cedar, can develop grey and black stains when the surface is contaminated by iron (Kreber 1994). Such discolouration is commonly referred to as iron stain. Aside from chemical discolouration, fungal activities are also involved in the colour changes of wood during weathering, as mentioned above. The grey discoloration of wood during weathering is often caused by the fungus Aureobasidium pullulans (de Bary) Arnaud, commonly referred to as mildew (Feist 1983). Other common fungal species found on the surface of weathered wood include Epicoccum spp., Macrosporium spp., Tetracoccosporium spp., Cladosporium spp., Alternaria spp., Stemphylium spp., Torula spp., and Sclerophoma spp. (Evans 2008). 2.2.3.4 Decay Alderman et al. (2003) suggested that decay was the main factor responsible for homeowners in North America replacing their wood decks, although other studies have found that surface degradation such as checking and greying (aesthetics) ranks higher than decay as a 37 reason for deck replacement (McQueen and Stevens 1998). Decay by basidiomycete fungi can cause rapid and significant strength losses of wood (Wilcox 1978; Clausen 2010; Morrell 2018). Decay of wood decks poses a safety hazard because decayed decks can suddenly fail even though the wood appears to be structurally sound. Significant fungal decay of wood can occur when the following conditions are met: wood is above its fiber saturation point of approximately 30% for prolonged periods of time, a temperature between 10 to 35 degrees Celsius, access to oxygen, and sources of carbohydrates or lignin, and nutrients (nitrogen) (Clausen 2010). Brown-rot and white-rot are the two major decay types. White-rot breaks down all three of woodâs structural components (cellulose, hemicellulose and lignin). As a result of lignin degradation, decayed wood appears more white than normal. White-rot fungi mainly attack hardwood rather than softwood. The adverse effects of white-rot on wood develop more slowly compared to brown-rot. Wood attacked by white-rot does not collapse and deform from its original shape until the wood is severely decayed. Conversely, brown-rotted wood loses most of its mechanical strength in the early stages of decay. Brown-rot extensively breaks down cellulose and hemicelluloses, and modifies lignin (Filley et al. 2002). A visible indication of such decay in later stages is that brown-rotted wood has a reddish-brown colour and contains many cross-grain checks. Softwoods, in particular, are more prone to brown-rot (Clausen 2010). Hence, brown-rot is the main decay type in houses and exterior decks in North America because they are mainly constructed from softwood lumber (Burrows 2013; Morrell 2018). Decay in buildings timbers is mainly due to poor moisture control and inadequate treatment. For example, Brischke et al. (2017) exposed siding made from fifteen wood species to the weather for seventy seven months in Hannover-Herrenhausen, Germany. They found that water traps and lack of ventilation due to poorly designed structures were associated with decay of the wooden siding. Nevertheless, Brischke et al. (2017) suggested that in-service fungal attack of wood is unpredictable and depends on spore germination by either brown-rot or white-rot fungi. Most preservative treatments are effective at restricting decay of wood if the wood is treated to standard specifications. Copper-based preservatives are effective against most fungi, except for a few copper-tolerant fungi, which are controlled by the organic co-biocides added to copper-based preservatives (Nicholas and Schultz 1997). 38 2.3 Reducing the negative effects of checking and cupping 2.3.1 Surface profiling Profiling is a physical treatment in which wavy ridges and grooves are machined into the surface of wooden deck-boards (McFarling and Morris 2005; Cheng and Evans 2018). Profiled deck-boards were commercialized in Europe and Australia over 30 years ago (Böttcher 1977; Shida et al. 1992; McFarling and Morris 2005). The initial goal of profiling deck-boards was to reduce their slipperiness when used for decking, for example, deck-boards used on floating piers (Shida et al. 1992). A positive effect of profiling on the checking of wooden deck-boards was first noted by Böttcher (1977). However, the geometry of profiles in profiled deck-boards manufactured commercially vary significantly across different countries and are not standardized (Cheng and Evans 2016). McFarling and Morris (2005) took the first step towards standardizing profiled deck-boards by classifying them into two types: boards with either rib or ripple profiles. Rib profiles have distinct hemispherical peaks separated by small grooves, whereas ripple profiles are wavy with the same amplitude shown in cross-section (Cheng and Evans 2016). Considerable variation in the geometry of profiles within the rib and ripple classifications exist (McFarling and Morris 2005). Hence, a method of quantifying the topography of different profiles was needed to compare the effects of different profiles on deck-board performance. Cheng and Evans (2016) obtained twenty eight different commercially available profiled deck-boards and characterized the surface profiles using confocal profilometry. Four parameters were extracted from line scan measurements and used to characterize the profiles: the radii of the grooves (R1), radii of peaks (R2), heights (H) and widths (W) of peaks (Figure 2.10). Figure 2.10 Dimensional parameters that were extracted from profilometry line scans of profiled decking: radii of the grooves (R1), radii of peaks (R2), heights (H) and widths (W) of peaks (Cheng and Evans 2016) 39 Cheng and Evans (2016) developed a classification system using the ratio between R1 and R2 of profiles (expressed as a percentage), and categorized profiles into three major types: rib, ribble, and ripple. Profiles with a R1/R2 ratio of less than 30% were identified as a rib, whereas ripple profiles had ratios greater than 60%. The intermediate profile with ratios between 30-60% was classified as a ribble as its geometric characteristics fell between rib and ripple profiles (Cheng and Evans 2016). Within these three categories, there were considerable variations in the height to width ratio of the different profile peaks. Twenty eight commercially available profiled deck-boards were collected from seven different countries and categorized into one of three profile classes according to their R1/R2 ratio (Cheng and Evans 2016). Subsequently, Evans obtained six additional profiled deck-board samples from Norway and characterized their profiles. The R1/R2 ratios of all of the commercially available profiled deck-boards that have been characterized using confocal profilometry are shown in Table 2.5. The collection of commercial profiled deck-board samples contains twenty three rib profiles, six ribble profiles and five ripple profiles. 40 Table 2.5 Wood species and geometries of commercially available profiled deck-boards collected by Cheng and Evans (2016) Profile Species Name Latin Name Height* Width* R1 R2 R1/R2 Rib1 Southern pine Pinus sp. 0.92 5.99 0.15 5.33 2.9% Rib Douglas fir P. menziesii 1.16 4.00 0.09 2.54 3.5% Rib Spruce Picea sp. 2.12 5.99 0.12 3.23 3.7% Rib Merbau Intsia sp. 1.59 4.89 0.10 2.53 4.0% Rib Radiata pine P. radiata 1.64 4.98 0.12 2.56 4.7% Rib1 Douglas fir P. menziesii 1.15 5.01 0.15 3.15 4.9% Rib Merbau Intsia sp. 2.19 5.03 0.12 2.37 5.1% Rib Radiata pine P. radiata 1.33 4.98 0.15 2.94 5.1% Rib Radiata pine P. radiata 2.00 4.92 0.14 2.28 6.1% Rib Luan Shorea sp. 1.74 4.31 0.14 2.09 6.7% Rib Pine Pinus sp. 1.54 4.83 0.17 2.49 6.8% Rib Mangium A. mangium 0.71 2.61 0.10 1.46 6.8% Rib Larch Larix sp. 1.98 4.97 0.16 2.01 8.0% Rib Ipe Handroanthus sp. 2.12 4.79 0.18 2.17 8.3% Rib Scots pine P. sylvestris 2.06 4.53 0.19 2.08 9.1% Rib Radiata pine P. radiata 0.71 5.22 0.47 4.45 10.6% Rib Ipe Handroanthus sp. 1.80 4.76 0.24 2.18 11.0% Rib Scots pine P. sylvestris 1.85 6.99 0.24 2.10 11.4% Rib Radiata pine P. radiata 1.53 5.01 0.29 2.37 12.2% Rib1 Scots pine P. sylvestris 1.56 7.86 0.71 5.13 13.8% Rib Pacific silver fir A. amabilis 1.25 3.89 0.27 1.82 14.8% Rib1 Scots pine P. sylvestris 2.48 7.08 0.61 3.46 17.6% Rib Merbau Intsia sp. 2.29 4.96 0.45 1.83 24.6% Ribble1 Scots pine P. sylvestris 1.46 4.53 0.64 1.94 32.9% Ribble1 Scots pine P. sylvestris 2.54 4.99 0.91 2.31 39.2% Ribble Pacific silver fir A. amabilis 1.50 3.92 0.44 1.07 41.1% Ribble Southern pine Pinus sp. 2.08 4.84 0.73 1.49 49.0% Ribble Cypress pine Callitris sp. 1.82 4.99 0.50 0.97 51.5% Ribble Radiata pine P. radiata 1.68 6.18 1.25 2.38 52.5% Ripple Spruce Picea sp. 1.85 5.87 1.38 1.91 72.3% Ripple Pine Pinus sp. 1.79 4.82 0.97 1.21 80.2% Ripple Grapia Apuleia sp. 2.01 5.84 1.30 1.51 86.1% Ripple Pine Pinus sp. 1.72 4.14 0.59 0.68 86.8% Ripple Hemlock Tsuga sp. 0.85 3.63 1.32 0.99 133.3% 1Profiled deck-boards that were collected by Evans including those that were not documented by Cheng and Evans (2016); *Height and width data in mm 41 McFarling and Morris (2005) examined whether profiling could reduce the checking of softwood species that are susceptible to checking. They found that pressure-treated subalpine fir deck-boards with a rib profile and exposed to weathering had fewer and smaller checks compared to unprofiled boards. They noted that checks developed in profile grooves, which made them difficult to see with the naked eye, especially from a standing height (McFarling and Morris 2005). This observation was later explained by the effect of profiling on concentrating strains in profile grooves (Heshmati et al. 2018; Mallet et al. 2018). Subsequent studies looked at the effect of various profile types on the checking and cupping of boards made from different wood species when they were exposed to natural or artificial weathering (Morris and McFarling 2008; McFarling et al. 2009; Evans et al. 2010; Akhtari and Nicholas 2014a; Morris and Ingram 2015; Cheng and Evans 2018; Heshmati et al. 2018; 2020). Morris and McFarling (2008) machined four different rib and ripple profiles into the surfaces of pacific silver fir and lodgepole pine decking (Figure 2.11), which were then pressure-treated and exposed to the weather for one year. They found that the rippled-flat edge profile was more effective at reducing check length and depth in lodgepole pine deck-boards than the other three profiles. The opposite was observed for pacific silver fir. In this case, the ribbed-eased edge was more effective at reducing checking (Morris and McFarling 2008). Figure 2.11 Rib and ripple profiles developed by Morris and McFarling (2008): (a) Rippled-flat edge; (b) rippled to edge; (c) ribbed-eased edge; (d) and ribbed-flat edge and centre Subsequently, Evans et al. (2010) examined the checking of unprofiled, ribbed and rippled southern pine and pacific silver fir deck-boards exposed to accelerated weathering for five days. They found surface profiling significantly reduced the number and the width of checks in the weathered deck-boards. Profiling improved the overall appearance of deck-boards, but the ribbed profile was better that the ripple profile at reducing the checking of pacific silver fir deck-boards whereas the opposite was the case with southern pine. Hence, Evans et al. (2010) 42 suggested that wood species and profile type could interact to influence the severity of checking in profiled deck-boards exposed to the weather. As a result of the increasing commercial importance of subalpine fir boards in British Columbia, McFarling et al. (2009) and Morris and Ingram (2015) examined whether profiling could reduce subalpine firâs susceptibility to checking and make it more suitable for decking. McFarling et al. (2009) found that ribbed profiling reduced severe checking and dimensional instability of subalpine fir boards, making them more suitable for use as exterior decking (McFarling et al. 2009; Morris and Ingram 2015). Cheng and Evans (2018) made deck-boards with various rib, ribble and ripple profiles to determine which profile type (R1/R2 and H/W ratio) was the most effective at restricting the checking of pacific silver fir deck-boards (Figure 2.12). Their profiles captured the variation in profile geometry observed in the 28 commercial deck-boards obtained from several different countries (Cheng and Evans 2016). They first defined a baseline H/W ratio at 40% across all profile types, and standardized R1/R2 ratios for rib (8.66%), ribble (50%) and ripple (83.3%) profiles. Heights of the baseline profiles were modified by ± 25% to create tall and short versions for each of the three profile types. Since the rib profile is the most common profile type used commercially (Cheng and Evans 2016), two additional variations of rib profiles were developed: narrow rib (- 25% width) and wide rib (+ 25% width). Cheng and Evans (2018) then machined the eleven different profiles into the surfaces of pacific silver fir deck-boards. Flat deck-boards acted as a control. The boards were then exposed outdoors for six months. Rib profiles were generally more effective at reducing the checking of pacific silver fir boards than ribble or ripple profiles. This observation accords with the findings of Morris and McFarling (2008) and Evans et al. (2010). In addition, their standard rib profile was more effective at reducing checking compared to the short rib profile (Cheng and Evans 2018). In a subsequent study, Heshmati et al. (2018) profiled Douglas fir, western hemlock and white spruce boards to produce profiled deck-boards with short rib, standard rib, tall rib, standard ribble and standard ripple profiles (Figure 2.12c). The sample boards were treated with ACQ prior to the six-month weathering trial. The tall rib and standard rib profiles were more effective at restricting checking in all three species compared to short rib, ribble and ripple. Heshmati et al. (2018) also modelled stresses at the surface of profiled (rib) white spruce boards. Results from their numerical model were consistent with observations 43 made in their weathering trial. Ribbed deck-boards with deeper grooves dried more slowly than ones with shallower grooves when exposed to cyclical wetting and drying, and stresses in the former profile were smaller than those in the short rib profile. These modelling results explained why boards with taller rib profiles checked less than boards with a shallow rib profile, according to Heshmati et al. (2018). Figure 2.12 Profiles that were machined into the surface of pacific silver fir deck-boards in the study by Cheng and Evans (2018): (a) Graph showing the geometry of the different rib, ribble and ripple profiles; (b) appearance of the eleven profiles used in the study; (c) and a sample of a single-sided ribbed deck-board One undesirable effect of profiling when it is applied to one side of deck-boards (single-sided profiling) is an increase in the cupping of boards exposed to weather (Morris and McFarling 2008; Cheng and Evans 2018; Heshmati et al. 2018; Mallet et al. 2018). Such physical distortion poses a tripping hazard as the edges of severely cupped boards protrude from the deck surface (Cheng and Evans 2018). According to Mallet et al. (2018), profiling increases strains at the surface of deck-boards which causes cupping of boards. Akhtari and Nicholas (2014a) found that preservative-treated southern pine deck-boards that were profiled and coated with zinc oxide or titanium dioxide cupped less than similarly coated flat boards possibly because the coating reduced the magnitude of moisture induced stresses and strains at the surface of deck-boards. 44 Heshmati et al. (2020) compared the cupping of single-sided and double-sided profiled Douglas fir, western hemlock and white spruce deck-boards exposed to natural weathering for 7 months. Unprofiled boards were also prepared and acted as a control. Both single-sided and double-sided profiling reduced the checking of boards made from all three species in comparison to the unprofiled control, in accord with previous findings for single-sided profiling (Heshmati et al. 2018). Most importantly, doubled-sided profiled deck-boards had the least cupping. In contrast, single-sided profiling increased cupping as observed previously (Cheng and Evans 2018; Heshmati et al. 2018). Hence, Heshmati et al. (2020) concluded that profiling both sides of boards is an effective method of reducing the checking and cupping of Douglas fir, western hemlock and white spruce deck-boards. One potential negative effect of profiling is that grooves of profiles could trap moisture and dirt during outdoor exposure, thus increasing decay. Morris and Ingram (2015) found that surface profiling had minimal effects on the overall decay rating of deck-boards. However, Heshmati et al. (2020) suggested that increased microbial activity could encourage deterioration and checking in the grooves of boards that were poorly treated or inherently susceptible to decay. They further suggested that research was needed to test this hypothesis. 45 2.3.2 Surface grooving Grooving involves the machining of a slot or trough into the upper or underside of a deck-board. Grooves run parallel to the grain and are usually wider than kerfs, which can also be machined into the undersides of deck-boards. There is little information from either accelerated or natural weathering trials on the effectiveness of surface grooving at reducing checking of wood, with the exception of the work by Böttcher (1977). He machined five different grooved profiles into the face of roof friezes made from pine (Pinus sp.) and spruce (Picea sp.) (Figure 2.13). Grooving significantly reduced the checking of spruce panels after one year of natural weathering (Böttcher 1977). However, the opposite effect was observed in pine as grooved pine boards checked more than ungrooved ones. Most of the checks in both pine and spruce boards occurred at the bottom of the semicircular and the flat-edged grooves whereas checks occurred at the corners of the rectangular grooves (Figure 2.13) (Böttcher 1977). Figure 2.13 Groove profiles machined into the surface of pine and spruce roof friezes (Böttcher 1977): (a) big semicircular grooves; (b) big rectangular grooves; (c) small semicircular grooves; (d) small rectangular grooves; (e) wedge-shaped grooves. The boards were 80 mm by 18 mm. Red circled area shows the spot where checks developed Despite the positive effect of grooving on the surface checking of spruce boards, it has not been widely studied as a method of reducing the checking of deck-boards. Instead, grooving is mainly viewed as a method of improving the slip resistance of deck-boards (Hislop 2006). The 46 channels in grooved decking encourage the drainage of standing water, which makes deck-board surfaces less slippery (Hislop 2006). Shida et al. (1992) measured the coefficients of slip resistance, both parallel and perpendicular to the grain, of grooved floating pier decking samples. Samples of Bongossi (Lophira alata Banks ex Gaertn) and western hemlock were machined with rectangular and trapezoidal grooves, respectively (Figure 2.14). They found that the coefficients of flat and grooved Bongossi boards were identical when the surfaces were dry. However, once moisture was introduced, the slip resistance of the flat deck-boards was significantly lower than that of boards containing the two different rectangular grooved profiles. Shida et al. (1992) also observed that western hemlock deck-boards containing trapezoidal grooves were less slippery in both dry and wet environments compared to the flat control. Figure 2.14 Bongossi floating pier decking with: (a) narrow-spaced rectangular grooves and (b) wide-spaced rectangular grooves (Shida et al. 1992) Grooves are often machined into the underside of indoor hardwood flooring and exterior decking to improve their dimensional stability during service (Hislop 2006; Wengert 2007). However, there is little evidence from previous studies to show whether such grooving reduces cupping of deck-boards (Englund 2010). Nevertheless, patents for grooved decking have been filed on the basis that grooving improves the dimensional stability of boards (Nystrom 1995; Swanson 2012). Cheng (2015) characterized the trapezoidal grooves machined into seven commercial deck-boards. He found no consistency in the design of grooved deck-boards made by different manufacturers. The trapezoidal grooves varied from: 11-16 mm wide at the long base, 1-4 mm wide at the short base, and 2-4 mm in depth (Cheng 2015). 2.3.3 Kerfing Kerfing, also known as Sewari in Japan, is a stress relieving and check reducing technique that involves sawing a single or multiple kerfs longitudinally along the surface of wood. 47 Preservative-treated poles and posts often employ kerfing prior to preservative treatment because it localizes the stresses from pressure treatment and drying, which restricts checks from penetrating beyond the treated layer of poles (Morrell and Blake 1991). Therefore, it reduces the probability of internal decay of poles (Helsing and Graham 1976). Accordingly, Morrell and Blake (1991) found that single kerfing prevented checking from reaching the untreated core of large-diameter treated Douglas fir transmission poles, reducing decay seven-fold. Ruddick and Ross (1979) cut a single 4 mm wide kerf into untreated and unincised green Douglas fir poles and weathered samples outdoors for 44 months. They found the widths and depth of checks in kerfed poles were reduced more than two-fold compared to unkerfed poles. Furthermore, only 20% of the kerfed poles had more than one check that propagated beyond 10 mm from the surface, whereas 85% of the unkerfed poles had at least two checks with depths greater than 10 mm. Clearly, kerfing is effective at reducing the number of severe checks in both treated and untreated Douglas fir poles (Ruddick and Ross 1979; Morrell and Blake 1991). Graham (1973) observed that all kerfs in shallow triple kerfed treated Douglas fir posts would initially widen during drying; however, only one of the kerfs would continue to open and deepen into the center of posts. Conversely, single deep kerfs in posts widened, but did not become longer. Hence, Graham (1973) suggested that employing a single deep kerf was better than triple kerfing at controlling checking of Douglas fir posts. Ruddick (1981) also tested single kerfing on ammoniacal copper arsenate treated white spruce poles. Kerfing reduced the average depth and width of the deepest checks in white spruce poles by half. He also recorded a 75% increase in the width of the single kerf, from 8.1 mm to 14.1 mm (Ruddick 1981; 1988). Kerf widening in poles during service is undesirable because it poses a climbing hazard to maintenance lineman; therefore, single kerfing was not advocated by Morrell and Blake (1991). Evans et al. (1997) further investigated the effectiveness of different check reducing techniques such as single kerfing, double kerfing, centre boring, incising and kiln-drying treatments. They measured the checking of CCA-C treated slash pine (Pinus elliotii Engelm) posts after air-drying, pressure treatment and one-year of weathering in Australia. Kerfing was more effective at reducing the size and the number of checks in posts than the other treatments. Double kerfing was preferred because the kerf in single kerfed posts during weathering was much 48 wider than that of kerfs in double kerfed posts. Thus, Evans et al. (1997) concluded that double kerfing was a better check-reducing treatment for round pine posts exposed to the weather. Kurisaki (2004) also conducted a 17-month weathering trial comparing the effectiveness of kerfing and other physical treatments at reducing the checking of ACQ-treated 100 mm diameter posts made from Japanese sugi (Cryptomeria japonica D. Don). He found that kerfing significantly reduced the length, width and depth of checks that developed in posts exposed outdoors on a rooftop. Furthermore, the percentage of checks that penetrated into the untreated core were 6% and 18% in single and double kerfed poles compared to 35% in unmachined posts (Kurisaki 2004). Evans et al. (2000) compared the effects of different kerf dimensions on the checking of ACQ treated posts. Kerfs with total depth of 30, 45 and 60 mm were cut into two different diameter ACQ treated radiata pine and slash pine posts. They found an increase in kerf depth generally resulted in fewer and smaller sized checks. This correlation between kerf depth and checking was more apparent for double kerfing than single kerfing (Evans et al. 2000). Ratu et al. (2007) kerfed the underside of flat southern pine decking as a means of reducing the checking of deck-boards during natural weathering (Figure 2.15). Deck-boards were machined with one, two or three kerfs to depth of 10, 13 or 20 mm. Kerfing was ineffective at reducing checking of deck-boards. Ratu et al. (2007) observed that kerf openings became narrower during weathering. They speculated that flexing on the bottom side of boards created tension on the top surface which could have encouraged surface checking. Nevertheless, Ratu et al. (2007) found that machining an odd number of saw kerfs (Figure 2.15a,c) on the bottom of boards reduced cupping of deck-boards. They speculated that double kerfs were too close to the fasteners, which made them ineffective (Ratu et al. 2007). Their findings were in accord with Hislopâs (2006) suggestion that triple kerfing on the underside of deck-boards would relieve moisture-induced strain in boards and reduce their tendency to distort during weathering. Figure 2.15 Images of 10 mm deep kerfed southern pine deck-boards used in study by Ratu et al. (2007): (a) one kerf; (b) two kerfs; (c) three kerfs (photos, Dr. Philip D. Evans) 49 2.3.4 Incising The effects of incising on the checking of railway ties, poles and glulam and dimension lumber was recently reviewed by Evans (2016). He mentioned two studies that have examined the effects of incising on the checking of decking. Crawford et al. (1999) examined the effects of incising, preservative treatments (CCA or ACA), and wood species on the failure of decks at two different test sites (Amherst, Mississippi and Cumberland, Maine). None of their incised and CCA-treated decks failed at the Amherst site except for an unincised and CCA-treated white pine (Pinus strobus L.) deck. Results for ACA-treated decks were equivocal; an unincised red pine deck at the Amherst site and an incised eastern hemlock (Tsuga canadensis (L.) Carr.) deck at the Cumberland site failed. In addition, incising led to scaling and failure of a pine deck at the Amherst site (Figure 2.16). In contrast, Morris and Ingram (2002) found a stronger effect of incising on the checking of CCA-treated softwood decking (seven species) exposed outdoors for ten years. They observed that incising reduced the adverse effect of checking on the appearance of all of the boards in their trial. Figure 2.16 Severe scaling of an incised and preservative-treated hem-fir deck-board after outdoor exposure for eight years (Evans 2016) 50 2.3.5 Chemical and thermal treatments In this section, I review the chemical, thermal and impregnation modifications that can be employed to improve the dimensional stability, decay and insect resistance of wood. Conventional chemical treatments (preservatives) used to improve the durability of wood were reviewed in Section 2.1.1.1. Chemical modification involves reaction of a chemical reagent to form a covalent bond with wood (Hill 2006). Current chemical modification techniques often replace or cross-link hydroxyl groups in the cell wall, which increases the dimensional stability of wood (Hill 2006). For example, acetylation is a form of chemical modification that reacts wood with acetic anhydride. As a result, the hydroxyl groups in wood are permanently replaced with acetyl groups. These acetyl groups bulk the cell wall and reduce the swelling of wood fibers when they come into contact with water (Hill 2006). Acetylation of plantation-grown radiata pine and Scots pine lumber has been commercialized by Accsys Technologies PLC in London, United Kingdom. These acetylated wood products are collectively branded as Accoya®, and are increasingly used for wet and demanding applications such as exterior decking, cladding, structural beams, windows and doors (Mantanis 2017). Chan and Evans (2008) exposed deck-boards made from, acetylated Scots pine, acetylated radiata pine and the tropical hardwood ipe to accelerated weathering for five days. Both types of acetylated pine deck-boards developed fewer and narrower checks than boards made from naturally durable tropical hardwood ipe. Thermal modification is the use of heat (thermal energy) to alter the hygroscopic properties of wood by degrading hemicellulose (Hill 2006). Thermal treatments use one of the following heating mediums: air, gas, water vapour and oil (Rowell et al. 2009). Virta (2005) found that a hot-air-circulated heat treatment reduced cupping of Norway spruce and Scots pine cladding during accelerated weathering by 90% compared to the untreated control. Furthermore, he suggested in-service cupping of the cladding could be further decreased if a higher temperature and longer heat treatment was employed. Dubey et al. (2010) compared the swelling of untreated and oil-bath heat treated radiata pine boards exposed to accelerated weathering. They found that thermally modifying wood at 210°C was more effective at reducing wood swelling than a thermal treatment at 160°C or 180°C. Furthermore, surface checking at the weathered surface of heat treated pine boards was minor and not visible to the naked eye (Dubey 51 et al. 2010). Bongers and de Meijer (2012) compared the performance of untreated Scots pine, acetylated radiata pine and thermally modified Norway spruce in a water immersion and drying test. Both acetylated pine and heat-treated spruce cupped far less than that of untreated pine (Bongers and de Meijer 2012). Impregnation modification involves filling internal voids in wood and wood cell walls with an inert material to form barriers to water (Hill 2006). An example of impregnation modification is furfurylation, which impregnates wood with a renewable resin, furfuryl alcohol. The polymerization of furfuryl alcohol under heat creates barriers in rays and blocks cell wall voids, which reduces moisture uptake (Hill 2006). Hence, decay resistance and dimensional stability of furfurylated wooden boards are improved. Moghaddam et al. (2017) examined the microstructure of furfurylated southern yellow pine and Norway maple (Acer platanoides) boards using X-ray micro-computed tomography. They found that the cell walls of furfurylated boards were thickened by the cross-linked furfuryl alcohol polymers, and the overall porosity of the boards was reduced (Moghaddam et al. 2017). Furfurylation also changes the colour of pine wood from yellow to a rich dark-brown colour, which is similar to the colour of tropical hardwoods. Accordingly, the commercialized furfurylated wood product is branded as Kebony®, a word-play on the well-known and highly sought-after timber ebony (Diospyros spp.) (Mantanis 2017). Lande et al. (2004) exposed untreated and furfurylated (Kebony®) European beech wood boards to basidiomycetes in pure culture (brown-rot and white-rot) and compared weight losses after 16 weeks exposure. They found that the untreated beech boards lost more weight compared to boards that were furfurylated. In addition to laboratory testing, Lande et al. (2004) also examined the decay of furfurylated pine and preservative-treated pine stakes exposed to the weather for one year in three different sites in Sweden. Furfurylated pine stakes were less decayed than ones made from preservative-treated pine (Lande et al. 2004). Impregnation of wood with the N-methylol compound 1, 3-dimethylol-4, 5-dihydroxyethyleneurea (DMDHEU) also improves the dimensional stability and decay resistance of wood (Emmerich et al. 2019.). It also provides some photo-protection to wood (Xie et al. 2005). In general, most modification techniques improve the dimensional stability of wood and its resistance to biodeterioration, 52 checking and cupping. However, they are less effective at restricting the photodegradation of wood (greying). Hence, coatings are still applied to modified woods exposed to the weather. 2.3.6 Coatings and water-repellent additives 2.3.6.1 Coatings Many homeowners in North America apply an exterior wood finish to further protect newly installed wooden decking from the adverse effects of the weather (Nejad and Cooper 2011a). Feist (2006) categorized exterior wood finishes into two sub-classes. First, penetrating finishes that are absorbed into the wood surface, such as oils, solvent-borne stains, brush-on preservatives and water repellants. Secondly, film-forming finishes that deposit a polymerized coating on top of the surface of wood, such as opaque paints, high solid content varnishes and stains (Feist 2006). However, there are far fewer available finishes today because solvent-based stains have largely disappeared from the market and have been replaced by low build film-forming water-based finishes. Pigments in these film-forming coatings offer more protection against photodegradation compared to penetrating finishes, but they require more maintenance than penetrating finishes. However, semi-transparent stains are a popular choice among homeowners because they are only mildly pigmented, and hence do not hide woodâs attractive natural appearance (Nejad and Cooper 2011a). Furthermore, these stains are porous to water vapour and flexible to accommodate moisture-induced movement of wood. However, semi-transparent finishes are less effective at protecting wood from photodegradation compared to opaque paints. The high pigment content of paints can completely block the transmission of ultraviolet light (Nejad and Cooper 2011a). Therefore, delignification at the surface of wood does not occur (Singh and Dawson 2003). However, paints can become more brittle during outdoor exposure. Cyclical moisture changes induce stresses in aged and brittle painted surfaces, and eventually crack the paint film. As a result, paints can delaminate from wood surfaces when moisture is trapped between the film and the substrate. Therefore, deck-boards are not painted. Feist (2006) recommended semi-transparent finishes for the finishing of wooden deck-boards. Semi-transparent stains are more suitable for wood decking because this type of finish is more effective on rougher surfaces, and penetrates wood to a greater extent than highly pigmented systems. Most importantly, stains are more flexible, and thus suitable for outdoor 53 applications. In addition, Feist (2006) suggested that stains can be easily reapplied over weathered and stained wood surfaces without sanding, which makes deck maintenance easier for homeowners. Nejad and Cooper (2011a) coated the surfaces of treated (CCA, CA and ACQ) and untreated southern pine boards with one of fourteen selected commercially available semi-transparent stains and exposed boards to natural weathering for three years. They found that semi-transparent stains reduced checking of treated and untreated southern pine boards by 30 to 40% compared to uncoated samples. However, the performance of one of the alkyd solvent-based coatings was not statistically significant. Nejad and Cooper (2011a) noted that the poorly performing alkyd solvent-based stain was not viscous and did not form a film at the surface of boards. Hence, they suggested that coatings with a higher viscosity, which form a thicker and flexible film were more effective at reducing the checking of southern pine boards than thinner, less viscous, finishes (Nejad and Cooper 2011a; 2011b). Nejad and Cooper (2011b) also examined the weathered surface of coated pine boards using scanning electron microscopy and found that stains penetrated through rays into the wood. Since surface checking develops mainly via wood rays, their observations provide one explanation for why coatings reduce checking of weathered deck-board surfaces, in addition to their positive effect on dimensional stability (Nejad and Cooper 2011b). Morris and McFarling (2008) conducted a trial which examined the effects of wood species (pacific silver fir and blue-stained lodgepole pine), profile type, preservative type (ACQ and carbon-based) and coating (Natural Deck OilTM) on the checking of deck-boards. After one year of exterior exposure, the coating had a substantial but statistically insignificant effect on the checking of pacific silver fir boards treated with the carbon-based preservative, but it had no effect on the checking of ACQ-treated boards (Morris and McFarling 2008). Akhtari and Nicholas (2014a) coated the surface of profiled southern deck-boards with zinc oxide or titanium dioxide particles and exposed the boards to accelerated weathering. They found that the zinc oxide and titanium dioxide coatings reduced cupping of profiled pine boards by 51% and 33.5%, respectively, compared to the uncoated control. Furthermore, the two coatings were also effective at restricting checking at the surface of the pine deck-boards (Akhtari and Nicholas 2014a). 54 2.3.6.2 Water-repellent additives The ability of coatings to restrict the checking the checking of deck-boards exposed outdoors has been explained, in part by their water-repellency. Therefore, it is not surprising that when water-repellents are added to wood preservatives, the modified preservative is more effective at restricting checking than unmodified preservative. Hence, wax emulsion additives for CCA and ACQ were able to reduce the cupping and checking that developed when treated wood was exposed to artificial accelerated or natural weathering (Zahora 2000; Christy et al. 2005). An oil emulsion additive restricted the checking of CCA-treated wood exposed to natural weathering (Fowlie et al. 1990; Zahora 1992; Evans et al. 2003; 2009). More recently, Akhatri et al. (2014b) found that the water-repellent preservative EcolifeTM reduced the number of checks and warping that developed in profiled southern pine deck-boards exposed to accelerated weathering compared to boards treated with amine copper azole. 2.4 Concluding Summary Preservative-treated wood is the most common material used for decking in the United States (Freedonia Group 2020a). Treated wood decking is susceptible to in-service checking, shelling and cupping during outdoor exposure, which affects the appearance and uniformity of the deck surface (McFarling and Morris 2005; Evans 2008; Cheng and Evans 2018). There are only a few publications in the literature describing the shelling of wood surfaces (Koehler 1957; Williams and Knaebe 1995). Shelled latewood protrudes above adjacent earlywood, which creates a corrugated appearance. Checking, shelling and warping are aesthetic defects that are disliked by homeowners and often lead to premature replacements of decks (Ganguly and Eastin 2009). Conversely, wood plastic composite deck-boards are far less susceptible to such defects during service (Green 2005). Hence, wooden deck-boardsâ market share is under threat from wood plastic composite decking. Nevertheless, wood decking still has many competitive advantages over wood plastic composite deck-boards, such as sustainability, natural grainy appearance, lighter weight, lower price and fire resistance (Freedonia Group 2020c). Furthermore, wooden deck-boards do not get too hot when exposed to direct sunlike, unlike wood plastic composite decking, and they are less susceptible to creep (Ozdemir and Mengeloglu 2008). 55 There are many chemical and physical treatments that can improve the performance and appearance of wooden deck-boards exposed to the weather. Chemical treatments include wood preservatives (mainly), chemically modification and coatings. Deck-boards made from chemically modified (acetylated and furfurylated) wood are dimensionally stable in demanding situations and thus less susceptible to checking and cupping. However, their price and availability make them less widely used by homeowners. Coatings are an easy and moderately effective method of reducing the checking of wooden deck-boards, but they require periodic maintenance (Nejad and Cooper 2011a). One promising physical treatment to reduce the checking of wood is surface profiling (McFarling and Morris 2005; Evans et al. 2010; Akhtari et al. 2014; Cheng and Evans 2018; Heshmati et al. 2018). Cheng and Evans (2016) suggested that the performance of commercially available profiled deck-boards was not consistent across different wood species. For example, previous studies (McFarling and Morris 2005; Evans et al. 2010) found that the effectiveness of profiling at reducing surface checking varied with wood species and profile geometry (valleys and grooves). The geometry of profiles affects water ingress into wood, which influences moisture-induced surface strains and the development of checks (Heshmati et al. 2018; Mallet et al. 2018). To-date, profiling has only been tested on eight different softwood species. Furthermore, most studies on the effectiveness of surface profiling at reducing the checking of softwood deck-boards only employed profiling on one side of boards. Single-sided profiling is effective at reducing the checking of boards, but it increases strain difference between the surface and the underside of boards, which encourages cupping of boards (Mallet et al. 2018). Recently, Heshmati et al. (2020) found that double-sided profiling reduced both the checking and cupping of profiled Douglas fir, white spruce and western hemlock deck-boards. There is some evidence that surface grooving can reduce both checking and cupping of boards (Böttcher 1977; Wengert 2007). The aforementioned findings suggest the checking and cupping of softwood deck-boards could be reduced by optimizing the geometry of both surface and sub-surface profiles (including sub-surface grooving). 56 Chapter 3: Optimizing Profiling to Reduce the Checking and Cupping of Western Larch Deck-boards 3.1 Introduction European larch is used for residential decking in Europe and western larch deck-boards have been shown to be moderately durable during field trials (Englund 2010; Morris et al. 2016; Stirling and Wong 2020). The durability and good strength properties of western larch makes it a potential candidate for use as exterior decking. However, deck-boards made from solid wood are susceptible to checking and cupping when they are exposed to the weather (Knight 1961; Schniewind 1963; Côté 1968). Furthermore, western larch boards tend to develop more checks compared to Douglas fir and southern pine, wood species with similar density to western larch (Johnson and Bradner 1932). The irregular patterns of checks at the surface of deck-boards diminishes their visual appeal, which, as mentioned above, often leads to premature replacement of decks by homeowners (Bolin and Smith 2010). One promising solution to reduce checking of deck-boards, as pointed out in Chapter 2 is to machine profiles into the surface of boards (McFarling and Morris 2005; McFarling et al. 2009; Evans et al. 2010; Morris and Ingram 2015). Furthermore, Heshmati et al. (2018; 2020) found that double-sided profiling could reduce both the checking and the cupping of flat-sawn deck-boards made of Douglas fir, western hemlock and white spruce. Heshmati et al. (2020) also found that profiled deck-boards for the aforementioned three species, which were oriented pith-side-up, checked less than boards oriented bark-side-up. Douglas fir, in particular, is a wood species with similar characteristics to western larch and the two species are not separated during the harvesting of forests in the interior of British Columbia. Hence, I hypothesize that profiling and orientation of deck-boards (pith-side-up vs. bark-side-up) will influence the checking and cupping of western larch deck-boards exposed to natural weathering. The aim of this chapter is to test this hypothesis and to suggest the optimal profile type and deck-board orientation that maintains the appearance and shape of western larch deck-boards exposed to the weather. 57 3.2 Material and methods 3.2.1 Experimental design My experiment was a randomized block design with two fixed factors: profile type [flat, rib, short rib, tall rib, ribble and ripple] and growth ring orientation [pith-side-up, concave; bark-side-up, convex]. Six replicate blocks of twelve samples (6 replicates x 6 profiles x 2 orientations) were prepared to examine the effects of the two aforementioned fixed factors on the checking and cupping of western larch deck-boards exposed to natural weathering. Analysis of variance (ANOVA) for a randomized block design was used to examine the effect of profile type and growth ring orientation on the checking and cupping of weathered deck-boards samples. Tests were carried out on data to determine if they conformed to the underlying assumptions of ANOVA (normality, equal variance and independence). Data on check area and width were transformed (natural logarithm) before the final analysis. Statistical analyses used the software, Genstat (v.19). Significant results (p<0.05) are presented graphically and Fisherâs least significant difference (LSD) bar can be used to determine whether differences between individual means are statistically significant (Williams and Abdi 2012). 3.2.2 Manufacture of western larch deck-boards Western larch was selected because it is moderately durable and has been used in the past for solid wood decking (Asselin 2014). Six kiln-dried and flat-sawn western larch boards measuring ~ 36 x 135 x 3670 mm3 were sourced from forests in the interior of British Columbia and donated to UBC by Tolko Industries Limited (Table 3.1). Each parent board was cross-cut using a pendulum saw (Stromab ps 50/f, Campagnola Emilia, Italy) to produce six 600 mm long samples. Samples from the same parent board were then randomly assigned to the flat profile or one of the five deck-board profiles that have been used in previous studies to reduce the checking of other softwood deck-boards (Cheng and Evans 2018; Heshmati et al. 2018) (Table 3.2). Both faces of the sample boards were profiled using a moulding machine (Weinig Powermat 700, Michael Weinig Inc., Mooresville, NC, USA) equipped with a 125 mm diameter, two-wing, cylindrical rotary cutter head (Great Loc SG Positive Clamping Universal Tool System, Great Lakes Custom Tool Mfg. Inc., Pestigo, WI, USA). Two 150 mm wide knives for the same profile were secured onto the cutter head by tightening nine clamping screws to apply even pressure to the 58 built-in gib. The cutter head was then locked onto the machine spindle and centered using the computerized control panel of the moulder. Table 3.1 Wood properties of six different western larch boards obtained from forests in the interior of British Columbia Board number (Block) Growth rings per cm1 Basic wood density2 (g/cm3) 1 9.0 0.517 2 5.0 0.431 3 6.0 0.456 4 6.0 0.467 5 7.5 0.442 6 8.5 0.488 1Measured with a transparent ruler; 2Measured by Archimedean displacement and oven drying (Evans et al. 2010) Samples from different parent boards were fed into the moulder, using a feed speed of 8 m/min and a spindle speed of 7000 rpm. Sample boards were shorter than the distance between the infeed and outfeed rollers; therefore, a sacrificial spacer board, one meter long, was used to ensure that profiled boards moved through to the outfeed of the moulder. The width of sample boards was also reduced during the profiling of the top surface. The underside of boards was machined using the cutter head for the top profile. Table 3.2 Geometry of profiles used to manufacture profiled deck-boards Profile type Groove depth (mm) Groove radius (mm) Peak radius (mm) Flat - - - Rib (Standard) 2.0 0.16 2.4 Short rib 1.5 0.16 2.4 Tall rib 2.5 0.15 2.2 Ribble 2.0 0.65 1.3 Ripple 2.0 1.0 1.2 *Number of peaks per 150 mm was 30 for all profiles except flat samples After double-sided profiling, the 600 mm long sample boards were cut into two using a pendulum saw, as above. The sub-divided deck-boards were randomly assigned to the factor âorientationâ (pith-side-up or bark-side-up) for the subsequent natural weathering (Figure 3.1). A total of seventy two flat-sawn western larch deck-board samples each with a final dimension of 292 (length) x 130 (width) x 32 mm (thickness) were manufactured. After machining, the end-grain of deck-board samples was sealed with two coats of epoxy resin (Intergard® 740, 59 International Paint Singapore Pty Ltd., Singapore) to prevent end checking of boards during conditioning and weathering. Deck-board samples were stored in a conditioning room at 20 ± 1°C and 65 ± 5 % r.h. for at least 3 months before the outdoor weathering trial. Figure 3.1 Cross-sectional view of profiled (standard rib) western larch deck-boards oriented: (a) pith-side-up, concave; and (b) bark-side-up, convex 3.2.3 Natural weathering of deck-board samples Profiled and flat deck-board samples from each of the six different parent blocks of western larch were secured onto separate racks made from preservative-treated 2 x 4 dimensional lumber (Figure 3.2). Figure 3.2 Photograph of profiled and flat deck-board samples on the six weathering racks on the first day of the weathering trial Each deck-board was spaced 10 mm apart from adjacent boards and fastened at the edges of its four corners onto the supporting frame using hidden galvanized decking screws (CAMO® fastening system, National Nail Corp. Grand Rapids, MI 49519, USA. http://www.camofasteners.com/). In addition, boards made from preservative-treated lumber measuring ~ 300 x 90 x 40 mm3 were screwed onto the two ends of each rack to prevent edges 60 of sample boards at the ends of each rack from weathering (Figure 3.2). Six mini-decks each containing 12 deck-board samples were constructed. Each mini-deck measured 30 cm long, 177 cm wide and 61 cm high. The test decks were then placed outdoors at FPInnovationâs test site in Vancouver for 7 months from March to October 2019 (Table 3.3). Table 3.3 Weather conditions in Vancouver, British Columbia, Canada during the 7-month outdoor weathering trial (March to October 2019) Month Temperature (°C) Total Precipitation (mm) Mean Maximum1 Minimum1 March 6.0 10.6 (17.3) 1.3 (-4.7) 31.2 April 9.5 13.1 (15.9) 5.9 (1.1) 110.8 May 14.2 18.4 (23.9) 9.9 (5.3) 30.4 June 16.5 21.1 (29.9) 11.9 (7.9) 26.2 July 18.5 22.8 (29.2) 14.1 (10.4) 30.8 August 18.7 23.1 (26.0) 14.2 (10.0) 18.7 September 15.6 18.9 (27.6) 12.2 (3.6) 122.2 1Mean minimum and maximum temperatures (with extremes in parentheses). Data are for Vancouver International Airport (49.196, -123.182), obtained from the Government of Canada website http://climate.weather.gc.ca/historical_data/search_historic_data_e.html. The airport is located 9 km from the test site 3.2.4 Measurement of cupping Cupping of sample deck-boards was measured a week after the experimental weathering trial (during a dry period) so that the measurements represented the distortion that occurred during outdoor exposure. Hence, after the weathering trial ended, test decks were kept in an outdoor open covered area rather than a constant climate room. In addition, boards were left fixed to the sub-frame just before the cupping measurements so that boards would not distort further when the restraining effect of fasteners was removed. Deck-board samples from the same parent block were unscrewed from each rack together and cupping measurements occurred immediately thereafter. I developed a purpose-built system to measure cupping or crowning of deck-boards. The system consisted of a brass frame and nine transducer heads spaced 60 mm apart width-wise and 100 or 140 mm apart length-wise (Figure 3.3). Each transducer was mounted approximately 4 mm above the flat plane of the brass frame (Figure 3.3a). A piece of flat glass was used to calibrate the transducers prior to cupping measurements. Sample boards were then placed onto 61 the frame. Displacement measurements from horizontal were recorded for each of the nine heads (Figure 3.3c). Cupping of deck-board samples was then calculated using transducer displacement data from the three transverse lines running across the width of each board. A total of six cupping measurements, three for each face, were recorded per board. Three cupping measurements were carried out per face for each sample. The three measurements on each face were then averaged. Cupping and crowning of boards were expressed as positive and negative displacements (in mm) from the flat reference line to the highest point of distortion, respectively. Figure 3.3 Measuring system for assessing the cupping or crowning of boards: (a) Nine transducer heads obtained from a swellometer; (b) overview of the system; (c) layout of the measuring platform (dimensions are in mm) 62 3.2.5 Measurement of surface checking Visible checks at the surface of the deck-boards were counted except those located 25.4 mm from the ends to exclude checks occurring on end-grain. Also, checks within any knots at the surface of boards were not evaluated. All surface checks were marked with a soft pencil (2B) and labelled with a reference number. The length and width of the identified checks were measured using a transparent Plexiglas ruler and a calibrated optical eyepiece graticule, respectively (Figure 3.4) (Evans et al. 1997). The dimensions of the five largest checks in deck-boards were analyzed because only large checks have a deleterious effect on the visual appearance of deck-boards; most of the small checks were too small to affect the overall appearance of deck-board samples. Figure 3.4 Tools used to measure the length and width of each check: (a) Transparent Plexiglas ruler and (b) calibrated optical eyepiece graticule 63 3.3 Results 3.3.1 Checking of western larch deck-boards after weathering There was a significant (p<0.001) effect of growth ring orientation on the average width of the five largest checks in deck-board samples. The five largest checks on weathered western larch deck-boards oriented pith-side-up were significantly (p<0.05) narrower than those in similarly weathered boards oriented bark-side-up (Figure 3.5). The interaction of growth ring orientation x profile type on check width was not significant (p>0.05). In other words, the effect of growth ring orientation (pith-side-up and bark-side-up) was consistent across samples with different profiles. Figure 3.5 Average width of five largest checks in western larch deck-boards oriented pith-side-up (concave) or bark-side-up (convex) and exposed to the weather for 7 months. Results are averaged across boards with different profiles because of the insignificant (p>0.05) interaction of profile type and growth ring orientation on check width 64 The difference in checking of profiled boards oriented pith-side-up (concave) versus bark-side-up (convex) were not visible to the naked eye because most of the checks were restricted in the grooves of the profile (Figure 3.6). Figure 3.6 Profiled (standard ribble) western larch deck-boards from the same parent block oriented (a) pith-side-up versus (b) bark-side-up and exposed to natural weathering in Vancouver 7 months. Scale bar = 50 mm 65 There was a highly significant (p<0.001) effect of profile type on the average width of the five largest checks in profiled boards. All of the profiled boards, except for those with a ribble profile had significantly (p<0.05) narrower checks than the flat boards (Figure 3.7). The width of large checks in boards with standard and tall rib profiles were significantly different (p<0.05) from those checks in boards with wavy profiles (ribble and ripple). Figure 3.7 Average width of five largest checks in profiled and flat western larch boards exposed to the weather for 7 months. Results are averaged across boards with different growth ring orientations because of the insignificant (p>0.05) interaction of profile type and growth ring orientation on check width There was a highly significant (p<0.001) effect of orientation on the average length of the five longest checks in profiled boards. Boards oriented pith-side-up had significantly (p<0.05) shorter checks than boards oriented bark-side-up. Figure 3.8 shows the length of large checks in flat and profiled boards oriented pith-side-up and bark-side-up and exposed to the weather to 7 months. However, the interaction of growth ring orientation x profile type was also significant (p=0.015), because the difference in check length in boards with the two different orientations was not significant for flat boards or those with a ribble profile, whereas for all other profiled boards, those oriented pith-side-up had significantly (p<0.05) shorter checks than those oriented bark-side-up. Checks were significantly (p<0.05) longer in profiled boards than in flat boards 66 (Figure 3.8). The exceptions were checks in rib and tall rib boards with concave orientation which were significantly (p<0.05) shorter, and checks in tall rib boards with convex orientation which were not significantly (p>0.05) different from those in flat boards. Figure 3.8 Interaction of profile type and growth ring orientation on the length of five largest checks in deck-boards exposed to the weather for 7 months Orientation, profile type and the interaction of orientation x profile type had significant (p<0.001) effects on the area of the five largest checks in boards (Figure 3.9). The area of checks was significantly (p<0.05) smaller in boards oriented pith-side-up than that of boards oriented bark-side-up across different profile types. The interaction occurred because the difference in check area of boards oriented pith-side-up (concave) versus bark-side-up (convex) was much more pronounced in boards with standard and tall rib profiles and less pronounced in boards with wavy profiles (ribble and ripple). Boards oriented pith-side-up and profiled with a rib or tall rib had significantly (p<0.05) smaller checks than those in boards with other orientation and profile geometry combinations. 67 Figure 3.9 Interaction of profile type and growth ring orientation on the area of five largest checks in western larch deck-boards exposed to the weather for 7 months 68 3.3.2 Cupping of deck-boards after weathering There was a significant (p=0.001) effect of growth ring orientation on the cupping of deck-boards. Orienting boards pith-side-up significantly (p<0.05) reduced the cupping of western larch boards compared to orienting boards bark-side-up (Figure 3.10). The effect of profile type on the cupping of boards was not statistically significant (p=0.721). There was no significant (p=0.45) interaction of orientation and profile type on cupping, which indicates that the effect of growth ring orientation was consistent across profile types. Figure 3.10 Effect of growth ring orientation (pith-side-up, concave; bark-side-up, convex) on the cupping of western larch deck-boards exposed to the weather for 7 months. Results are averaged across boards with different profiles because of the insignificant (p>0.05) interaction of profile type and orientation on cupping The difference in cupping of boards oriented pith-side-up versus those oriented bark-side-up was noticeable to the naked eye. Figure 3.11 shows cross-sections of profiled boards oriented either pith-side-up or bark-side-up after exposure to the weather for 7 months. Figure 3.11 Cupping of profiled (standard rib) western larch deck-boards from the same parent block oriented (a) pith-side-up versus (b) bark-side-up exposed to natural weathering in Vancouver for 7 months 69 3.4 Discussion Profiling of deck-boards has been shown to be effective at restricting checking of weathered deck-boards made from softwoods with distinct growth rings (southern pine and Douglas fir) and exposed to artificial or natural weathering (Evans et al. 2010; Akhtari and Nicholas 2014b; Heshmati et al. 2018; 2020). Western larch has distinct growth rings (Johnson and Bradner 1932), but not all the profiles I tested showed the same desired effect of reducing the development of large surface checks at the surface of profiled western larch deck-boards exposed to natural weathering. For example, short rib or wavy (ribble and ripple) profiles at the surface of western larch had little effect on checking. Conversely, profiling with either a standard rib or a tall rib profile reduced checking and improved the appearance of western larch deck-boards exposed outdoors, especially for those boards that were oriented pith-side-up. These findings accord with recent research by Heshmati et al. (2018) on the checking of profiled Douglas fir, western hemlock and white spruce deck-boards oriented pith-side-up or bark-side-up and exposed to natural weathering. In addition, my experimental results on differences in the checking of deck-boards with short rib versus tall rib profiles accord with the predictions of a finite element model of the checking of profiled white spruce deck-boards (Heshmati et al. 2018). During cyclical wetting and drying of deck-boards, which occurs during natural weathering, peaks on the tall rib profile may have dried more slowly compared to peaks in boards with a short rib profile. Such a difference in moisture diffusion properties influences strain development in the grooves of profiled deck-boards (Heshmati et al. 2018) and may explain why tall rib profiles are more effective at reducing checking than short rib profiles. Profiling restricted checks at the surface of weathered western larch deck-boards from becoming wider but had no such effect on check length. These findings on the effects of profiling on the length and the width of checks accord with previous findings of McFarling et al. (2009) and Evans et al. (2010). In summary, checks in profiled western larch deck-boards with ribs were longer and narrower than those in flat boards. This change in the shape of checks makes them more difficult to see from a standing height, especially since checks in profiled boards were mainly located in the grooves of profiles as has been observed previously (McFarling and Morris 2005). The severity of checking at the surface of weathered western larch boards was also 70 influenced by growth ring orientation. Checking was less pronounced in boards oriented pith-side-up versus those oriented the bark-side-up. These observations confirm some findings that flat and profiled deck-boards oriented pith-side-up check less than boards oriented bark-side-up when they are exposed to weathering (Yata 2001; Morris and McFarling 2008). Urban and Evans (2005) observed a similar effect of growth ring orientation on the checking of unprofiled deck-boards made from southern pine, a wood species with distinct growth rings like western larch. However, these beneficial effects of growth ring orientation on checking of profiled deck-boards were absent in Douglas fir, western hemlock and white spruce boards that were profiled on one side and exposed to the weather for 6 months (Heshmati et al. 2020). I found that profile geometry had no effect on the cupping of boards; the cupping of profiled boards was similar to that of flat boards. However, boards oriented pith-side-up cupped less than boards oriented bark-side-up. This finding accords with previous observations of the cupping of weathered profiled decking made from Douglas fir, western hemlock and white spruce (Heshmati et al. 2020). In contrast, Williams and Knaebe (1995) suggested that flat deck-boards should be oriented bark-side-up to reduce the cupping of boards during weathering, because less tangentially oriented growth rings are exposed, resulting in less differential shrinkage and cupping. Their suggestion may be more relevant to flat-sawn boards consisting mainly of mature wood, whereas larch boards used here were sawn close to the pith and may have contained more juvenile wood, which increases shrinkage anisotropy and severity of cupping of boards according to Simpson (1991). Some previous studies have recommended installing deck-boards pith-side-up to reduce the checking and cupping of deck-boards exposed outdoors (Yata 2001; Urban and Evans 2005; Heshmati et al. 2020). However, Williams and Knaebe (1995) commented that orienting deck-boards pith-side-up encourages shelling, a type of oblique checking that results in latewood protruding from the surface of weathered deck-boards. Shelling is a serious defect as described in Chapter 2 (Williams et al. 2000). Accordingly, the occurrence of shelling at the surface of deck-boards oriented pith-side-up overrides the positive effect of such orientation on checking and cupping (Williams and Knaebe 1995; Heshmati et al. 2020). Therefore, current recommendations are to orient deck-boards bark-side-up to avoid shelling (Koehler 1929). The effect of growth ring 71 orientation and profile geometry on the shelling of profiled and flat western larch deck-boards exposed to natural weathering is examined in detail in the next chapter. 3.5 Conclusions In conclusion, the effect of profiling at restricting checking of western larch varied depended on profile geometry, growth ring orientation and the measure used to quantify checking. I found that a standard rib or a tall rib profile was more effective at reducing checks from becoming wider at the surface of western larch boards than wavy profiles (ripple and ripple). Profiling did not restrict checks from becoming longer or reduce cupping of boards. However, orienting boards pith-side-up reduced both the checking and cupping of deck-boards. The related research in the next chapter examines the effects of growth ring orientation and profiling on shelling of deck-boards so as to arrive at a conclusion as to whether profiled western larch deck-boards should be oriented pith-side-up or bark-side-up. 72 Chapter 4: Shelling of Western Larch Deck-boards 4.1 Introduction Shelling is a surface defect in which slivers of latewood project above the surface of wood giving it a corrugated appearance (Koehler 1929). Shelling can develop during the rotary planning of lumber when feed rollers and dull knives compress dense latewood at the surface of boards into softer underlying earlywood. Subsequently, the compressed latewood springs back projecting above the surface of adjacent earlywood. Shelling can also develop when flat-sawn boards are exposed outdoors to natural weathering, as mentioned in Chapter 2. Photodegradation and differential shrinkage of wood causes oblique checking at the interface between latewood and earlywood. The detached latewood shrinks more transversely and less longitudinally than the adjacent earlywood which causes the former to project above the surface of weathered boards (Koehler 1929). Shelling is most pronounced in softwood species that have an abrupt transition from earlywood to latewood, such as larch, Douglas fir and southern pine (Koehler 1929; Williams and Knaebe 1995; Heshmati et al. 2020). Non-contact confocal profilometry has been used to characterize the localized protrusions of growth rings at the wetted surface of painted radiata pine panels (Singh et al. 2010). Deviations in height across the uneven painted surface were extracted from profilometry line scans (Singh et al. 2010). In principle, the same confocal profilometry technique could be employed to quantify the shelling of weathered flat and profiled western larch sample deck-boards tested in Chapter 3. Previous studies suggested that unprofiled deck-boards with growth rings oriented pith-side-up are more susceptible to shelling than boards oriented bark-side-up (Koehler 1929; Williams and Knaebe 1995). However, there are no reports in the literature on the shelling of profiled deck-boards even though, as described in Chapter 2, profiling is commonly used in Australia, Europe, Japan and New Zealand to reduce the checking of deck-boards exposed outdoors (Cheng and Evans 2016). Hence, I hypothesize that growth ring orientation (pith-side-up vs. bark-side-up) and profiling will influence the shelling of flat and profiled western larch boards exposed to the weather. The aim of this chapter is to test this hypothesis and to determine 73 the optimal deck-board orientation and profile geometry to reduce shelling of western larch deck-boards exposed to the weather. 4.2 Material and methods 4.2.1 Experimental design and statistical analysis The sample deck-boards here were part of my experiment in Chapter 3 designed to examine the effects of profile type and growth ring orientation on the checking and cupping of western larch deck-boards. The design of this experiment was described in the previous chapter (Section 3.2.1). Six replicate blocks were prepared to examine the effects of profiling and growth ring orientation on the shelling of flat and profiled western larch deck-boards. Linear regression was used to examine the relationship between growth ring angle to the weathered surface and the height of shelled latewood in flat western larch deck-boards oriented pith-side-up. Analysis of variance (ANOVA) was used to examine the effects of profile type (excluding flat boards) and growth ring orientation on the number of raised latewood slivers in profiled boards. The count data for slivers in profiled boards was transformed (square root) before the final analysis. Statistical computation used the software, Genstat (v. 19). Significant results (p<0.05) are presented graphically and Fisherâs least significant difference (LSD) bar can be used to determine whether differences between individual means are statistically significant (Williams and Abdi 2012). 4.2.2 Preparation and weathering of western larch deck-boards The preparation and weathering of flat and profiled western larch deck-board samples were described in Section 3.2.2. Surfaces of flat-sawn western larch deck-boards were profiled on both sides with one of the six profiles [flat, rib (standard), short rib, tall rib, ribble and ripple] (Figure 4.1). A total of seventy two flat and profiled sample boards were manufactured and oriented either pith-side-up or bark-side-up during a seven month natural weathering trial described in Chapter 3. After the weathering trial, sample deck-boards were removed from subframes and conditioned in a constant climate room at 20 ± 1°C and 65 ± 5 r.h. for two weeks before shelling was assessed using confocal profilometry. 74 Figure 4.1 Custom profile knives used for double-sided profiling of western larch deck-board samples. Geometries for different profiles are listed in Table 3.1 4.2.3 Measurement and imaging of shelling of deck-boards Shelling of flat (unprofiled) weathered western larch deck-board samples was quantified using a non-contact confocal profilometer equipped with a 3 mm optical probe (Altisurf 500®, ALTIMET, France) (Figure 4.2a). The profilometer measured the height variations across the width of each flat deck-board along a 100 mm transverse line at three locations; each line was offset 16 mm from the edges of deck-boards. Height data was extracted from line scans using the software ProfilmOnline (Filmetris, KLA Co., San Diego, CA, USA). Heights of shelled growth rings were measured from the tip of the detached latewood protruding from the wood surface to the adjacent earlywood. A 625 mm2 area on sample boards was scanned using a confocal profilometer and visualized using AltiMap® Premium software to produce 3D images of shelling at weathered surfaces. The scanned surfaces of profiled and flat boards were also photographed using a Canon EOS 80D digital single lens reflex camera equipped with a Canon EF-S 35 mm f/2.8 macro lens (Figure 4.2b). Confocal profilometry and macro-photography were also used to image the shelling of growth rings at the peaks of profiled western larch deck-boards. The number of detached or raised latewood slivers occurring at the peaks of profiled deck-board samples was counted to determine if profile geometry and growth ring orientation influenced the shelling of flat sawn boards. 75 Figure 4.2 Systems used to image the shelling of western larch deck-boards: (a) Altisurf 500 non-contact confocal profilometer; (b) Canon EOS 80D digital single lens reflex camera equipped with Canon EF-S 35 mm f/2.8 macro lens End-grain of flat deck-boards was scanned using a desktop scanner (Hewlett-Packed Office Jet 6700, Palo Alto, C, USA) at 600 dpi resolution. Scanned images were then analyzed with the software ImageJ (https://imagej.nih.gov/ij/) to measure the angle of growth rings to the weathered surface (Figure 4.3). Figure 4.3 Flat western larch deck-board oriented pith-side-up; Examples of shallow (α) or steep (β) angle of growth rings to the deck-board surface exposed to natural weathering 76 4.3 Results 4.3.1 Appearance of shelling in flat western larch boards Shelling of growth rings was evident at the surface of weathered flat western larch deck-boards oriented pith-side-up (Figure 4.4). A 3D confocal profilometry height map and a matching macro-photographic image below show detached latewood protruding above the surface of a board (Figure 4.4a,b). Oblique checking occurs at the interface between latewood and earlywood and the shelled latewood separated from earlywood in the next yearâs growth ring (inter-ring failure rather than intra-ring failure). Figure 4.4 Shelled growth rings at the surface of an unprofiled western larch deck-board (Board 6) oriented pith-side-up and exposed to the weather for 7 months: (a) confocal profilometry height map showing five projecting growth rings at the surface; (b) Macro-photographic image of the matching 3D profilometry map in Figure 4.4a; (c) Confocal profilometry line scan showing the heights of shelled growth rings corresponding with the images in Figure 4.4a,b 77 The extent to which latewood projected above the surface of weathered western larch deck-boards oriented pith-side-up can be also seen in the profilometry height map and line scan in Figure 4.5a,c. The numbered red coloured bands in the height map (Figure 4.5a) and the line scan in Figure 4.5c indicate that one of the latewood bands (number 2) projected approximately 0.3 mm above the adjacent earlywood. Shelling was most pronounced in the central area of a deck-board where growth rings are most to parallel to the wide surface. In this central area, a latewood tip (number 3) detached and protruded approximately 0.15 mm from the surface of the deck-board sample (Figure 4.5b,c). Figure 4.5 Shelling of growth rings at the surface of an unprofiled western larch deck-board (Board 4) oriented pith-side-up and exposed to the weather for 7 months: (a) Confocal profilometry height map showing protruding growth rings labelled 1,2,3,4; (b) Photograph of four shelled growth rings matching the image in Figure 4.5a; (c) Confocal profilometry line scan showing the heights of shelled growth rings corresponding with the images in Figure 4.5a,b 78 Table 4.1 shows the profilometry line scan measurements of the heights of latewood projecting above adjacent earlywood in the six different unprofiled western larch boards with growth rings oriented pith-side-up, and exposed to natural weathering. There is no apparent relationship between wood density or growth rate and the extent to which shelled growth rings projected above the surface of boards. Table 4.1 Average heights of shelled latewood and number of shelled growth rings in flat western larch deck-boards oriented pith-side-up and exposed to the weather for 7 months Board number (Block) Height of shelled growth rings (mm)1 Number of shelled growth rings2 Growth rings/cm Wood density (g/cm3) 1 0.10 (0.16-0.063) 6 (36) 9.0 0.517 2 0.12 (0.31-0.056) 19 (37) 5.0 0.431 3 0.18 (0.48-0.081) 14 (30) 6.0 0.456 4 0.08 (0.21-0.019) 32 (55) 6.0 0.467 5 0.05 (0.07-0.013) 12 (54) 7.5 0.442 6 0.19 (0.52-0.068) 17 (47) 8.5 0.488 1Average Height of shelled growth rings. Max and min in parentheses; 2Total number of growth rings on the exposed surface of boards in parentheses 79 Shelling did not occur at the surface of flat western larch deck-boards oriented bark-side-up, although many checks occurred at the interface of latewood and earlywood (Figure 4.6). However, surface checking was more pronounced in boards with growth rings oriented bark-side-up compared to that in boards oriented pith-side-up (compare Figure 4.6 with Figure 4.5). Figure 4.6a shows that the latewood was slightly raised above the adjacent earlywood in boards oriented bark-side-up possibly due to increased degradation and erosion of earlywood during weathering which is common in softwood species with abruption transitions between growth rings (Williams, Knaebe and Feist 2001). Figure 4.6 Appearance of a flat western larch deck-board oriented bark-side-up and exposed to the weather for 7 months: (a) Confocal profilometry height map showing surface checking and roughening of the surface of a deck-board; (b) Photograph matching the height map in Figure 4.6a; (c) Confocal profilometry line scan showing the variation in height corresponding with the images in Figure 4.6a,b 80 4.3.2 Correlation between the heights of shelled growth rings and growth ring angle There was a significant (p<0.001) negative correlation between the angle growth rings made to the surface of deck-boards oriented pith-side-up and the heights of shelled latewood (Figure 4.7). In other words, latewood tended to project higher above the surface of adjacent earlywood when the angle of growth rings to the surface was shallow. Conversely, the severity of shelling was less when growth rings were inclined at a higher angle to the wide surface. The regression between the heights of shelled latewood and growth ring angle was highly significant (p<0.001). However, the correlation coefficient was low (R2=0.327). This may be caused by my small sample size (six flat boards oriented pith-side-up). Figure 4.7 Linear regression (red line) of heights of shelled growth rings versus growth ring angle to the exposed surface of flat western larch deck-boards oriented pith-side-up and exposed to the weather for 7 months; regression includes 95% confidence intervals (blue-lines) 81 4.3.3 Shelling at the surface of profiled western larch boards Shelling at the interface between latewood and earlywood in profiled deck-boards oriented pith-side-up created lanceolate-shaped latewood slivers (tips) at profile peaks rather than the protruding bands that developed in similarly oriented flat (unprofiled) boards (Figure 4.8). Latewood tips were clearly separated from the adjacent earlywood (Figure 4.8b). The extent to which detached latewood tips projected from the peak of a profiled (rib) deck-board oriented pith-side-up can be seen in Figure 4.8a,c. Figure 4.8 Shelling of a latewood sliver on the peak of a profiled (rib) western larch deck-board oriented pith-side-up and exposed to the weather for 7 months: (a) Confocal profilometry image of a shelled latewood sliver; (b) Photograph of a shelled latewood sliver matching the image in Figure 4.8a; (c) Confocal profilometry scan of a profile peak showing the height of the protruding latewood tip in Figure 4.8a,b 82 Shelling rarely occurred at the interface between latewood and earlywood at the surface of profiled deck-boards oriented bark-side-up (Figure 4.9). However, latewood tips were slightly raised above the peaks of profiled boards (Figure 4.9b). The profilometry height map and line scan of the profile peak in Figure 4.9a,c show that the extent to which latewood tips were raised above the surface was much smaller than that of shelled latewood tips in profiled boards oriented pith-side-up. Figure 4.9 Raised tip of a growth ring on the peak of a profiled (short rib) western larch deck-board oriented bark-side-up and exposed to the weather for 7 months: (a) Confocal profilometry image of a raised latewood tip; (b) Photograph matching the image in Figure 4.9a; (c) Confocal profilometry scan of profile peak showing the height of the same latewood tip in Figure 4.9a,b 83 4.3.4 Number of shelled tips in profiled boards There was a significant (p<0.001) effect of growth ring orientation on the number of shelled tips at the surface of profiled deck-board samples (Figure 4.10). Boards oriented bark-side-up had significantly (p<0.05) fewer raised tips at the peaks of profiles. The interaction of growth ring orientation x profile type on the number of shelled tips was not significant (p>0.05). Figure 4.10 Effect of growth ring orientation on the number of shelled growth rings (tips) at the surface of profiled western larch deck-boards exposed to the weather for 7 months. Results are averaged across boards with different profile types because of the insignificant (p>0.05) interaction of growth ring orientation and profile type on the number of raised latewood tips. Error bars = Least significant difference, p<0.05 There was a small, but statistically significant (p=0.036) effect of profile type on the number of visible detached latewood tips at the peaks of profiled deck-boards (Figure 4.11). Boards with a wavy ribble profile had significantly (p<0.05) fewer raised tips than those of boards with rib profiles (short, standard and tall). 84 Figure 4.11 Effect of profile type on the number of shelled growth rings (tips) at the surface of profiled western larch deck-boards exposed to the weather for 7 months. Results are averaged across boards with different growth ring orientations because of the insignificant (p>0.05) interaction of growth ring orientation and profile type on the number of raised latewood tips Figure 4.12 shows the separation of growth rings at the peaks of different profiled deck-boards oriented pith-side-up. Shelling in boards with rib profiles (Figure 4.12a) appeared to be less severe than that in boards with ripple profiles (Figure 4.12b). Figure 4.12 Appearance of shelled latewood tips at the surface of profiled western larch deck-boards oriented pith-side-up: (a) sample board with a rib profile; (b) sample board profiled with a ribble profile 85 4.4 Discussion My results show that non-contact confocal profilometry was able to characterize shelling in profiled and flat western larch deck-boards exposed to the weather. My profilometry scans and photographs showed that shelling is caused by oblique checking at the interface of latewood and earlywood, in accord with previous findings of Stehr and Ãstlund (2000) and Singh et al. (2010). The severity of shelling varied across the surface of western larch boards due to changes in growth ring orientation (angle) to the deck-board surface. Hence, selective sampling of areas with pronounced shelling was needed to image projecting latewood bands and tips in flat and profiled western larch deck-boards. Macro-photographic images complemented confocal profilometry height maps and line scans and was used to illustrate the morphology and severity of shelling at the surface of boards, contrary to the suggestion of Singh et al. (2010) that photographs captured with a digital camera equipped with a zoom lens cannot accurately represent the âgrain raisingâ of wetted radiata pine panels. Shelling at the interface between latewood and earlywood was more pronounced in western larch boards oriented pith-side-up than in boards oriented bark-side-up. Average heights of shelled latewood bands at the weathered surface of the six different unprofiled sample deck-boards oriented pith-side-up (0.05 to 0.19 mm) are similar to those of protruding latewood in Singh et al.âs (2010) coated and wetted flat-sawn radiata pine panels (~0.15 mm). The severity of shelled latewood at the surface of flat western larch boards oriented pith-side-up diminished as growth ring angle became greater to the surface. This observation accords with the predictions of a finite element model of samples with growth rings inclined at different angles to the surface and exposed to cyclical wetting and drying (Stehr and Ãstlund 2000). My observations of shelling at the surface of profiled western larch deck-boards oriented pith-side-up and exposed to the weather accord with a previous observation by Evans cited in Heshmati et al. (2020). In addition, I described the morphological features of shelled latewood tips that appeared at the peaks of profiles in profiled deck-boards. These shelled tips were less visible than the detached bands and complete tips in flat boards. Profiled boards oriented pith-side-up were more susceptible to shelling than boards oriented bark-side-up, in accord with my observations of the shelling of unprofiled boards. 86 Clearly, flat and profiled western larch deck-boards should be installed bark-side-up rather than pith-side-up to avoid shelling at the surface of boards, even though the latter orientation was more effective at reducing the checking and cupping of western larch deck-boards exposed to the weather, as observed in Chapter 3. Adverse checking and cupping of boards with growth rings oriented bark-side-up could be reduced by treating the boards with preservatives containing wax/oil or using a pigmented finish (Feist 1990; Akhtari and Nicholas 2014b). In addition, my results showed that boards with a wavy profile (ribble and ripple) developed fewer shelled latewood slivers than boards with rib profiles. However, the latter profile type was better at reducing the checking and cupping of profiled western larch boards (as observed in Chapter 3). However, growth ring orientation and profile type should be optimized to restrict shelling at board surfaces because of the potential for shelled latewood tips to injure people walking on decks (Williams and Knaebe 1995; Heshmati et al. 2020). Williams and Knaebe (1995) commented that builders do not always orient boards bark-side-up when decks are being built. As a result, localized areas of shelling and associated paint failure have been observed at the surface of decks and also siding (Williams and Knaebe 1995). An intuitive and visual indicator is needed to help builders orient deck-boards bark-side-up rather than pith-side-up. One promising solution for profiled deck-boards would be to machine grooves on the pith-side of deck-boards rather than ribs. Sub-surface grooves are employed for indoor flooring and deck-boards to improve their dimensional stability during service (Hislop 2006; Wengert 2007). Commercially available profiled deck-boards that have trapezoidal grooves on the underside of boards are currently sold across North America and Europe (Cheng 2015), but no attention is paid to whether such grooves are machined into the pith- or bark- side of boards. If deck-board manufacturers machined ribs on the bark-side and grooves on the pith-side, it would be easy to correctly orient deck-boards bark-side-up. However, there is little information in the literature on the effect of groove geometry on the checking and cupping of profiled deck-boards (Englund 2010). One potential concern is that an imbalance of stress between the profiled surface and grooved underside of boards might result in excessive cupping of boards (Mallet et al. 2018). Accordingly, in the next chapter, I examine the effect of sub-surface groove geometry 87 on the checking and cupping of profiled pacific silver fir and western hemlock deck-boards oriented bark-side-up and exposed to the weather. 4.5 Conclusions Non-contact confocal profilometry was able to quantify shelling of western larch deck-boards and reveal a correlation between the height of shelled latewood and corresponding angle of growth rings to the exposed surface. My findings indicate that flat and profiled western larch deck-boards should be oriented bark-side-up to avoid shelling, even though boards oriented pith-side-up are less susceptible to checking and cupping. Profiled boards with a wavy (ribble) profile had fewer shelled latewood tips at their surfaces than boards with rib profiles. These findings and conclusions may also be relevant to other commercially important species with distinct growth rings and could help reducing shelling at the surface of boards made from them. 88 Chapter 5: Reducing the Checking and Cupping of Pacific Silver Fir and Hemlock Deck-boards with Sub-surface Profiles 5.1 Introduction Pacific silver fir and western hemlock are commonly harvested together and sold as a species mix called hem-fir (Crawford and Oliver 1990). Hem-fir wood is not durable and hence preservative treatments are invariably employed to improve its durability. Pacific silver fir is the more treatable of the two species. Therefore, it has been suggested that hem-fir should be sorted by wood species so that pacific silver fir boards can be preferentially used for products as deck-boards that require treatment (Morris 1995). However, this suggestion has not gained traction. Nevertheless, research has continued with the aim of improving the performance of treated pacific silver fir deck-boards and their resistance to the adverse effects of weathering, particularly checking and cupping (Morris and McFarling 2008; Evans et al. 2010; Cheng and Evans 2018). As mentioned in Chapter 2, Cheng and Evans (2018) carried out research to determine the profile that was most effective at reducing the checking of pacific silver fir deck-boards. They confirmed earlier findings that single-sided profiling reduced the checking of pacific silver fir deck-boards (Morris and McFarling 2008; Evans et al. 2010) and rib profiles were more effective than wavy profiles (ribble and ripple) at restricting checking of boards. However, single-sided profiling increased the boardsâ tendency to cup during weathering (Cheng and Evans 2018). Subsequently, Heshmati et al. (2020) found that double-sided profiling significantly reduced the problem of the cupping of treated western hemlock, Douglas fir and white spruce deck-boards exposed to the weather. In addition, they found that deck-boards oriented pith-side-up checked and cupped less than boards oriented bark-side-up. However, flat and profiled boards oriented pith-side-up were more susceptible to shelling which is a severe defect (Koehler 1929; Williams and Knaebe 1995). Hence, they concluded that profiled boards needed to be oriented bark-side-up (Heshmati et al. 2020). This conclusion makes it problematic to recommend double-sided profiling as a way of reducing the checking and cupping of deck-boards because of the difficulty in practice of orienting boards bark-side-up when both faces of boards are similar in appearance. However, a different profile on the pith-side of deck-boards, which can relieve drying stresses could help orient boards correctly. The aim of this chapter is to select such a profile to replace rib profiles 89 on the pith-side of profiled pacific silver fir and western hemlock deck-boards exposed to the weather. 5.2 Material and methods 5.2.1 Experimental design and statistical analysis My experiment was a randomized block design which examined the effects of different wood species (pacific silver fir and western hemlock) and sub-surface profiles on the checking and cupping of deck-board samples exposed to the weather. Five replicate blocks of ten samples (5 replicates x 2 species x 10 sub-surface profiles) were prepared for the experiment. One pacific silver fir board and one western hemlock board were randomly selected and allocated to each of the five replicate blocks. Ten deck-board samples were produced from each board and assigned at random to one of the ten sub-surface profile types [wide rib (control), short 5, short 6, short 7, med 5, med 6, med 7, tall 5, tall 6 and tall 7]. A wide rib profile was used for the top profile because a previous study showed that it was highly effective at reducing the checking of Pacific silver fir deck-boards exposed to the weather (Cheng 2015). Analysis of variance (ANOVA) for a randomized block design was used to examine the effects of wood species and sub-surface profile type on the checking and cupping of western hemlock and pacific silver fir deck-board samples exposed to the weather. Statistical analyses used the software, Genstat (v.19). In addition, I sought a contrast between the cupping and checking of the double-sided profiled control and the profiled and grooved specimens, averaged across all groove types. A sub-routine (convstrt) within Genstat was used to provide a contrast between the cupping and checking of the control versus grooved samples. Diagnostic checks were carried out on data to determine if they conformed to the assumptions of ANOVA (normality, equal variance and independence). Data on the average area of the ten largest checks in each deck-board were transformed (natural logarithm) before the final analysis. Significant results (p<0.05) are presented graphically and Fisherâs least significant difference (LSD) bar can be used to compare the means of the double-sided profiled control and sub-surface grooved samples and to determine whether differences are statistically significant (Williams and Abdi 2012). 90 5.2.2 Sub-surface profiles 5.2.2.1 Survey of sub-surface profiles used commercially Before manufacturing deck-board samples with sub-surface grooves, I obtained ten commercial deck-boards which contained sub-surface grooves (Table 5.1). The geometry of sub-surface grooves in the commercial boards (width of board, thickness of board, depth of grooves, width of grooves, angle and pitch) were measured. One end of each grooved deck-board sample was sanded with 220 grit abrasive paper until the cross-section was smooth. The cross-sections were scanned with a desktop scanner, as described in Chapter 4. Scanned images were analyzed with the software ImageJ (https://imagej.nih.gov/ij/) to characterize the ten commercial sub-surface grooved boards. Table 5.1 Description of ten commercially available sub-surface grooved deck-boards Cross sectional image of boards Description 1 Species: Scots pine Origin: Finland Width of board: 148 mm Thickness of board: 33 mm Number of grooves: 11 Depth of grooves: 2.91 mm Width of grooves: 2.08 mm Angle: 0° Pitch: 12.02 mm 2 Species: Scots pine Origin: Finland Width of board: 144 mm Thickness of board: 26 mm Number of grooves: 8 Depth of grooves: 3.91 mm Width of grooves: 6.96 mm Angle: 59.66° Pitch: 15.97 mm 3 Species: Scots pine Origin: Finland Width of board: 122 mm Thickness of board: 32 mm Number of grooves: 2 Depth of grooves: 4.36 mm Width of grooves: 3.11 mm Angle: 0° Pitch: 39.03 mm 91 4 Species: Scots pine Origin: Finland Width of board: 122 mm Thickness of board: 28 mm Number of grooves: 8 Depth of grooves: 2.95 mm Width of grooves: 2.97 mm Angle: 0° Pitch: 13.51 mm 5 Species: Scots pine Origin: Finland Width of board: 94 mm Thickness of board: 26 mm Number of grooves: 5 Depth of grooves: 1.32 mm Width of grooves: 4.95 mm Angle: 49.60° Pitch: 16.05 mm 6 Species: Douglas fir Origin: Canada Width of board: 147 mm Thickness of board: 28 mm Number of grooves: 9 Depth of grooves: 3.94 mm Width of grooves: 7.13 mm Angle: 49.66° Pitch: 15.01 mm 7 Species: Southern pine Origin: United States Width of board: 145 mm Thickness of board: 28 mm Number of grooves: 7 Depth of grooves: 2.96 mm Width of grooves: 5.95 mm Angle: 33.66° Pitch: 14.09 mm 8 Species: Southern pine Origin: United States Width of board: 145 mm Thickness of board: 22 mm Number of grooves: 3 Depth of grooves: 4.90 mm Width of grooves: 3.33 mm Angle: 0° Pitch: 31.01 mm 92 9 Species: Spruce Origin: United Kingdom Width of board: 142 mm Thickness of board: 26 mm Number of grooves: 7 Depth of grooves: 2.95 mm Width of grooves: 5.13 mm Angle: 49.67° Pitch: 17.52 mm 10 Species: Cumaru Origin: South America Width of board: 128 mm Thickness of board: 24 mm Number of grooves: 6 Depth of grooves: 4.95 mm Width of grooves: 5.49 mm Angle: 19.07° Pitch: 19.07 mm The design of the sub-surface grooves in commercial profiled deck-boards varied. For, example, there were five different configurations of profiling and grooving machined onto surfaces of deck-boards made from Scots pine (commercial sample 1,2,3,4,5). Furthermore, six of the surveyed commercial deck-boards (profiles 1,5,6,7,8,9) contained an odd number of grooves whereas the other four sub-surface grooved deck-boards had an even number of grooves (profiles 2,3,4,10). Six of the grooved deck-boards contained trapezoidal shaped grooves. Profile 7 left ungrooved areas on the underside of the board to accommodate deck fasteners. Three of the boards contained sub-surface grooves but no top profile (like flooring). 93 5.2.2.2 Design of sub-surface groove profiles The design of new groove profile knives was informed by my survey of commercial sub-surface profiles and was based on three parameters: angle, pitch and height (Figure 5.1). The angle of tapered trapezoid grooves (θ) was set at 15° to reduce the total number of groove types. Furthermore, grooves were offset approximately 28 mm from the edges of boards so they didnât interfere with the fastening of boards to the deck sub-frame. Figure 5.1 Parameters used to design sub-surface grooved profiles The height and pitch of grooves were varied to produce nine different sub-surface groove types. The geometries of the different designed grooved profiles are shown in Table 5.2. Table 5.2 Dimensions of groove types applied to the underside of pacific silver fir and western hemlock deck-boards Sub-surface groove type Number of grooves/15cm Groove pitch (mm) Groove depth (mm) Wide rib1 (dbl) - - - Low 5 5 18.0 2.0 Low 6 6 15.0 4.0 Low 7 7 12.0 6.0 Med 5 5 18.0 2.0 Med 6 6 15.0 4.0 Med 7 7 12.0 6.0 Tall 5 5 18.0 2.0 Tall 6 6 15.0 4.0 Tall 7 7 12.0 6.0 1Wide rib profile acted as the control: Groove depth = 2 mm; Groove radius = 0.11 mm; Peak radius = 1.70 mm. Number of peaks per 150 mm was 24 for wide rib 94 Accurate engineering drawings of each groove type were produced using the software, AutoCAD 2019. Engineering drawings for each groove type can be found in Appendix 1. Two 150 mm long custom grooved knives (Figure 5.2) and associated backing plates were fabricated by the industrial tooling company, Great Lakes Custom Tool Mfg. Inc., Pestigo, WI, USA. Figure 5.2 Custom-designed knives for different sub-surface groove types and top profile used to manufacture deck-boards from pacific silver fir and western hemlock 5.2.3 Manufacture of surface profiled and sub-surface grooved deck-boards Flat-sawn hem-fir boards measuring ~36 x 140 x 3670 mm3 were donated by FPInnovations. A thin (~30 µm) section was cut from the radial face of each board using a sharp single edged stainless steel blade (GEM® Super Stainless Steel Blades, Personna, Verona, VA, USA). Each section was mounted on a glass microscope slide and the section was covered with a droplet of water followed by a glass cover slip. The section was examined at different magnifications using a binocular light microscope (Model CH30RF100, Olympus, Tokyo, Japan). Western hemlock was distinguished from pacific silver fir by the presence of ray tracheids in western hemlock and their absence from pacific silver fir (Kukachka 1960). Subsequently, samples measuring ~ 15 x 15 x 30 mm3 were cut from each of the hem-fir boards. The basic density of the hem-fir boards was calculated using their oven dry weight (obtained by oven drying samples at 105°C to a constant weight) divided by their water-saturated volume (obtained by Archimedean displacement). As expected, western hemlock samples were denser than pacific silver fir samples (Table 5.3). 95 Table 5.3 Wood properties of the five replicate blocks (boards) of pacific silver fir and western hemlock Board number Pacific silver fir Western hemlock Basic density (g/cm3) Growth ring per cm Basic density (g/cm3) Growth ring per cm 1 0.431 10 0.418 7 2 0.368 3 0.434 7 3 0.362 7 0.458 18 4 0.320 8 0.479 6 5 0.370 6 0.408 9 Mean 0.370 7 0.439 9 The bark-side surface of flat sawn parent boards was first profiled with a wide rib profile. The wide rib profile was selected because it has been shown to be one of the most effective profiles at reducing the surface checking of profiled pacific silver fir deck-boards exposed to the weather (Cheng 2015). The underside (pith-side) of boards were grooved with one of the nine custom designed grooved knives. The underside of the control was profiled using the same wide rib profile used for the upper, bark-side, of the boards. Ribs and grooves were machined into the surfaces of pacific silver fir and western hemlock boards using a moulding machine (Weinig Powermat 700, Michael Weinig Inc., Mooresville, NC, USA). Two 150 mm wide knives for the wide rib profile were inserted into a 125 mm, two-wing, cylindrical rotary cutter head as described in Chapter 3. The procedures used to secure knives in the cutter head and position the cutter head in the moulder are described in Chapter 3 (Section 3.2.2). The ten parent boards were oriented bark-side-up and fed into the moulder at a feed speed of 8 m/min and operating at a spindle speed of 7000 rpm. The width of sample boards was reduced to the final size during the profiling of the top surface. Each parent board was then cross-cut using a pendulum saw (Stromab ps 50/f, Campagnola Emilia, Italy) to produce ten 300 mm long sample boards. Samples within each parent board were then randomly assigned to one of nine sub-surface groove types or the wide rib control (Table 5.1 and Figure 5.3). Two relevant knives for the sub-surface profiles and associated backing plates were secured into the cutter head, as above. The pith-side of profiled sample boards were then grooved using a moulder (as above) with only the top cutter head active. A one metre long spacer board was used to ensure 96 that sample boards passed smoothly through the space between infeed and outfeed rollers. A total of one hundred flat-sawn surface profiled and sub-surface grooved western hemlock and pacific silver fir deck-boards each with a final dimension of 300 (length) x 135 (width) x 32 mm (thickness) were manufactured (Figure 5.3). Figure 5.3 Cross-sectional views of surface profiled (wide rib) and sub-surface grooved pacific silver fir deck-boards. Dashline indicates the centre of boards End-grain of samples was sealed with two coats of epoxy resin (Intergard® 740, International Paint Singapore Pty Ltd., Singapore) to reduce preservative uptake via end grain and to reduce end-checking of boards. Samples were stored in a conditioning room at 20 ± 1°C and 65 ± 5% r.h. for one month before preservative treatments. All sample boards were treated with a 2.0% alkaline copper quaternary preservative solution in a pilot-scale pressure treatment cylinder at FPInnovations (Figure 5.4). The preservative treatment cycle consisted of an initial vacuum (30 minutes at -74.5 kPa), a pressure period (1 hour at 1034.2 kPa) and a final vacuum (15 minutes at -74.5 kPa). Preservative retention of profiled pacific silver fir and western hemlock deck-boards samples were 3.7 kg/m3 (min = 0.6; max = 8.6; SD = 2.1) and 2.5 kg/m3 (min = 0.5; max = 8.0; SD = 1.7), respectively. The difference in preservative retentions between pacific silver fir and hemlock is significant (p=0.002) in accord with the findings of Morris (1995). Conversely, there was no significant (p=0.781) effect of profiling on the preservative retention of treated 97 hem-fir boards. After treatment, deck-board samples were dried in a constant climate room at 20 ± 1°C and 65 ± 5% r.h. for 3 months before the outdoor weathering trial. Figure 5.4 Pilot-scale preservative treatment cylinder used to pressure treat pacific silver fir and western hemlock deck-board samples 98 5.2.4 Natural weathering of deck-board samples Profiled and treated pacific silver fir and western hemlock deck-board samples from each of the five different parent blocks were fastened onto separate supporting racks made from preservative-treated 2 x 4 lumber (Figure 5.5). Within each rack, a separate sub-frame was prepared for each species. Figure 5.5 Photograph of surface profiled and sub-surface grooved western hemlock and pacific silver fir deck-board samples on the five weathering racks on the first day of the weathering trial Each deck-board sample was secured at its four corners onto the sub-frame using galvanized decking screws (CAMO® fastening system, National Nail Corp. Grand Rapids, MI 49519, USA. http://www.camofasteners.com/). In addition, boards cut from preservative-treated lumber measuring ~ 600 x 90 x 40 mm3 were installed at the ends of each rack to prevent the edges of samples at the end of each sub-frame from being exposed to the weather. A gap of 10 mm was left between each of the ten sample deck-boards and the sacrificial end boards. Five mini-decks each containing ten pacific silver fir and ten hemlock decking samples were constructed. Each assembled mini-deck was 60 cm long, 155 cm wide and 61 cm high. The five test decks were exposed to the weather at FPInnovationsâ Vancouver test site for 6 months from February to August 2019. Table 5.4 shows the weather conditions at the test site during the outdoor exposure trial. 99 Table 5.4 Weather conditions in Vancouver, British Columbia, Canada during the 6-month outdoor weathering trial (February to August 2019) Month Temperature (°C) Total Precipitation (mm) Mean Maximum1 Minimum1 February 0.4 3.8 (8.5) -3.0 (-8.8) 75.0 March 6.0 10.6 (17.3) 1.3 (-4.7) 31.2 April 9.5 13.1 (15.9) 5.9 (1.1) 110.8 May 14.2 18.4 (23.9) 9.9 (5.3) 30.4 June 16.5 21.1 (29.9) 11.9 (7.9) 26.2 July 18.5 22.8 (29.2) 14.1 (10.4) 30.8 August 18.7 23.1 (26.0) 14.2 (10.0) 18.7 1Mean minimum and maximum temperatures (with extremes in parentheses). Data are for Vancouver International Airport (49.196, -123.182), obtained from the Government of Canada website http://climate.weather.gc.ca/historical_data/search_historic_data_e.html. 5.2.5 Measurement of surface checking and cupping The methods and equipment used to measure the checking and cupping of sample boards were described in detail in Chapter 3. Sample deck-boards were stored in an open covered area and remained fastened onto the sub-frames until just prior to checking and cupping measurements. The purpose-built system, described in Section 3.2.4, was used to measure the cupping of deck-board samples. Three cupping measurements were carried out per face for each sample. The three measurements of each face were then averaged. Cupping and crowning of boards were expressed as positive and negative displacements (in mm) from the flat reference line to the highest point of distortion, respectively. The length and width of surface checks were manually measured using a transparent ruler and an optical eyepiece containing a calibrated graticule, as described in Section 3.2.5. The average areas of the ten largest checks in deck-board samples were analyzed because only large checks are visible to the naked eye and detrimental to the overall surface appearance of deck-boards. 100 5.3 Results 5.3.1 Cupping of grooved western hemlock and pacific silver fir deck-boards There was a significant (p<0.001) effect of sub-surface groove type on the cupping of deck-board samples, and a significant (p=0.031) interaction of sub-surface groove type x wood species on cupping. There was no significant (p>0.05) difference in cupping of the control versus grooved boards (averaged across all groove types). The interaction occurred because seven of the fir boards with different groove geometries cupped less than those of hemlock boards, but fir boards machined with Med 7 or Tall 5 grooves cupped significantly (p>0.05) more than similarly profiled western hemlock boards (Figure 5.6). Figure 5.6 Interaction of sub-surface groove type and wood species on the cupping of sample deck-boards exposed to the weather for 6 months. Note that boards with 6 grooves per 15 cm do not contain a groove running along their centre-line 101 Cupping of sub-surface grooved pacific silver fir boards, except for Med 7 and Tall 5 boards, were not significantly (p>0.05) different from that of the wide rib control. Figure 5.7 shows the pronounced cupping of a grooved (Tall 5) sample after it was exposed to the weather for 6 months. Cupping of western hemlock deck-board samples machined with the nine different sub-surface groove types were not significantly (p>0.05) different from that of the control. In Figure 5.6, groove types are ordered by depth from left to right and by number of grooves (5, 6 and 7) within each depth class (low, medium and tall). There does not appear to be a discernable trend in cupping with groove depth for western hemlock, although cupping of fir boards with the deepest grooves appears to be greater than those of boards with shallower grooves. However, western hemlock sample boards with an odd number of grooves (5 and 7 grooves) cupped less than that of boards with similar groove depth but profiled with an even number of grooves (6 grooves). The cupping difference between odd and even number of grooves was also evident in pacific silver fir boards with low (2 mm) grooves at different pitches. Boards with an odd number grooves (5 or 7) contained a sub-surface groove running along the centre-line of boards whereas boards with six grooves did not. Figure 5.7 Cross-sections of pacific silver fir deck-boards after weathering for six months: (a) double-sided profiled wide rib control; (b) Sub-surface groove profile (Tall 5). Note the difference in cupping between the two boards 102 5.3.2 Surface checking of grooved western hemlock and pacific silver fir deck-boards There was no significant (p>0.05) difference in the checking of the profiled control versus grooved samples (averaged across all groove types) However, sub-surface groove type, wood species and the interaction of sub-surface groove type x wood species had significant (p<0.001) effects on the area of the ten largest checks in sample boards. The interaction occurred because hemlock boards with shallow grooves (Low) had smaller visible checks than boards profiled on both sides, but the opposite effect was observed in similarly grooved pacific silver fir deck-boards (Figure 5.8). In comparison to the control, all groove types, except for the Tall 5 profile, reduced the size of large checks at the surface of hemlock boards. However, only two groove types (Med 5 and Tall 7) significantly (p<0.05) restricted checking in pacific silver fir boards. Figure 5.8 Interaction of sub-surface groove profile and wood species on the average area of ten largest checks in deck-board samples exposed to the weather for 6 months. Note that boards with 6 grooves per 15 cm do not contain a groove running along their centre-line 103 5.3.3 Cupping and checking of western hemlock and pacific silver fir deck-boards The cupping and checking of western hemlock boards plotted in Figures 5.6 and 5.8 shows peaks and troughs across the different groove types. Cupping peaks are centred on groove types with six grooves, whereas the opposite is the case for checking. This effect can be more clearly seen Figure 5.9 which plots groove type against cupping and checking of sub-surface grooved western hemlock deck-boards. Figure 5.9 Surface check area and cupping of grooved western hemlock deck-boards exposed outdoors for 6 months. Labels on the x-axis refer to the sub-surface groove types listed in Table 5.2. The y1-axis (left) refers to the average area of ten largest checks at the surface of grooved western hemlock deck-boards. The y-2 axis (right) refers to the cupping of western hemlock deck-board samples 104 This inverse relationship between groove type and checking versus cupping is less pronounced for pacific silver fir boards. Nevertheless, there are few cases where a groove type is highly effective at reducing both cupping and checking (Figure 5.10). Figure 5.10 Surface check area and cupping of grooved pacific silver fir deck-boards exposed to the weather for 6 months. Labels on the x-axis refer the sub-surface groove types machined into the underside of pacific silver fir deck-boards. The y1-axis (left) refers to the average area of ten largest checks at the surface of grooved pacific silver fir deck-boards. The y-2 axis (right) refers to the cupping of pacific silver fir deck-board samples 105 5.4 Discussion The effect of sub-surface grooving on the cupping and checking of profiled deck-boards varied depended on groove geometry, wood species and unexpectedly groove number. Ideally, sub-surface grooves should reduce both cupping and checking of boards in comparison to double-sided profiled boards. However, in western hemlock, there tended to be an inverse relationship between the ability of groove types to restrict cupping and reduce checking. In other words, the profiles that were most effective at restricting cupping were the least effective at reducing checking. Nevertheless, there was an exception, machining five 2 mm grooves into the sub-surface of profiled western hemlock boards was able to restrict both checking and cupping, and was equally effective as double-sided profiling. The inverse relationship between groove geometry and cupping and checking was less evident in pacific silver fir, but boards with five 4 mm grooves did not increase cupping or checking in comparison to the double-sided profiled control. There was an unexpected effect of groove number (pitch) on cupping and checking, as mentioned above. Boards with six grooves appeared to behave differently to those with five or seven grooves. The latter two groove configurations have a groove located in the centre of boards whereas grooves in boards with 6 grooves are offset from centre line. For example, cupping of western hemlock boards with six grooves were more pronounced than boards with five or seven grooves. This finding accords with a previous study by Ratu et al. (2007) on the effects of the number of kerfs on the cupping of flat (unprofiled) southern pine deck-boards exposed to the weather. They found that southern pine boards with two underside kerfs offset from the centre of boards cupped more than those boards with one or three kerfs (both of which had a centre-kerf). Furthermore, Mallet et al. (2018) mapped strain developments in radiata pine deck-boards during cyclical wetting and drying, using 3D digital image correlation. Resulting strain maps showed that the middle of the transverse section of a deck-board experienced the most tensile strain. Hence, an opening in the middle of boards might relieve strains at the surface of the board via cupping (Ratu et al. 2007). However, Ratu et al. (2007) also found that a centered kerf that reduced cupping did not appear to have the same positive effect on checking. The same effect was noted here with western hemlock, suggesting that stress relief in deck-boards exposed to 106 cyclic drying occurs via checking and cupping. Therefore, it is unlikely that stress relief profiles and grooves will completely eliminate checking and cupping, but the combinations that restrict both forms of degedration to levels that do not adversely affect the appearance of boards may be possible. 5.5 Conclusions In conclusion, certain sub-surface groove types could be a viable replacement for ribs in double-sided profiled boards because some groove types can restrict cupping without adversely affecting the ability of a rib profile to restrict checking. An odd number of shallow or medium sized sub-surface grooves showed equal or better performance compared to sub-surface ribs in restricting cupping and checking in deck-boards exposed to natural weathering. Thus, such grooves could replace rib profiling on the pith-side of boards and provide a visual indicator to help builders to correctly install boards bark-side-up as desired. Further research should be performed to examine the suitability of sub-surface grooving for profiled boards made from other commercially important softwood species that are susceptible to checking and shelling such as southern pine and western larch. 107 Chapter 6: General Discussion, Conclusions and Suggestions for Further Research 6.1 General discussion In Chapter 1, I hypothesized that the effectiveness of profiling at reducing checking and cupping of flat-sawn softwood deck-boards exposed to the weather could be improved by optimizing surface and sub-surface profile types. My findings partially support this hypothesis. Results in Chapter 3 showed that standard and tall rib profiles were significantly more effective than other profile types at restricting the checking of western larch deck-boards that were profiled on both sides. However, there was no significant effect of profile type on the cupping of western larch boards, which runs counter to my general hypothesis. The best profiles at reducing the checking of western larch deck-boards were particularly effective with boards exposed pith-side-up. Checking of such boards was much less pronounced than that of boards oriented bark-side-up, even though the boards had the same profiles applied to their upper surface. Hence, based on the results in Chapter 3, the best profile and board type combination to reduce the adverse effects of checking and cupping of boards was standard and tall rib profiles with the pith-side of boards facing up, However, results in Chapter 4 showed that both profiled and unprofiled western larch deck-boards oriented pith-side-up developed pronounced separation of latewood at the surface of boards during weathering. Such separation is known as shelling (Koehler 1929; Williams and Knaebe 1995). Shelling of profiled boards resulted in sharp tips of latewood projecting above the surface of profiled boards, which have potential to create stick-injuries (Johnson and Bradner 1932; Miniutti 1963). Shelling was only pronounced in profiled and flat boards oriented pith-side-up. For this reason, I suggest that profiled western larch boards should only be oriented bark-side-up. When this restriction is applied to western larch deck-boards, the tall rib profile was the best profile at reducing checking of double-sided profiled western larch deck-boards oriented bark-side-up. Western larch is a wood species with distinct growth rings and a high contrast in density between earlywood and latewood (Johnson and Bradner 1932). Southern pine is a species with similar characteristics, however, a previous study suggested that a ripple profile was more effective than a rib profile at restricting the checking of southern pine during artificial accelerated 108 weathering (Evans et al. 2010). My findings run counter to this previous research on profiled southern pine, but accord with more recent studies of the effectiveness of different profiles at reducing the checking of pacific silver fir, Douglas fir, western hemlock and white spruce (Cheng and Evans 2018; Heshmati et al. 2018; 2020). Previous research showed that profiled boards should be profiled on both sides to produce a balanced board which is less susceptible to cupping (Heshmati et al. 2020). However, in practice such a board could be oriented either bark- or pith-side-up. The latter orientation is undesirable as my results in Chapter 4 indicated. Hence, ideally, the pith-side surface of double-sided profiled boards should contain a different type of profile to the bark-side to make it easier to orient boards correctly and avoid shelling of latewood tips during outdoor exposure. My research in Chapter 5 examined whether sub-surface grooves could replace profiles on the underside of boards. I chose pacific silver fir and western hemlock as deck-board species because previous research has optimized profile geometry for pacific silver fir and western hemlock (Cheng and Evans 2018; Heshmati et al. 2018) and furthermore western hemlock is susceptible to shelling (Heshmati et al. 2020). Both species are traded commercially as hem-fir. My results showed that the difference in checking between the double-sided profiled control and sub-surface grooved samples was insignificant, in accord with the findings of Ratu et al. (2007) that different configurations of underside kerfing had no significant effects on surface checking. Cupping of grooved hem-fir boards (averaged across all groove types) was significantly less than that of double-sided profiled boards. My results suggested that five 2 mm deep grooves and five 4 mm grooves are the optimal configurations for restricting both checking and cupping of profiled western hemlock and pacific silver fir boards, respectively. Interestingly, the depth of the recommended grooves for western hemlock is the same as the depth of profiles used in previous studies that have optimized profiles in softwoods (Cheng and Evans 2018; Heshmati et al. 2020). Hence, the use of sub-surface grooving potentially provides a solution to the orientation and shelling issue that became apparent in Chapter 4. In addition, there was a noteworthy finding on the effect of the number of grooves (pitch) on the cupping of western hemlock boards. Boards with a groove in the middle of the underside (5 or 7 grooves) cupped less than boards without a groove in the middle (6 grooves). This finding could be important for companies that employ sub-109 surface grooves in deck-boards. I noted that some of the profiled and grooved deck-boards that are manufactured commercially do not contain a center groove. Hence, I would recommend an odd number of grooves to produce one that is in the middle of the underside of boards. Mallet et al. (2018) mapped compressive and tensile strains during cyclical drying and wetting of profiled boards, and found that strains were concentrated in the middle of boards. Hence, a sub-surface groove in the middle would allow boards to flex freely and relieve drying stresses (Ratu et al. 2007). Overall my findings on profiling and sub-surface grooving may help to improve the performance of solid wood deck-boards and overcome some of the objections to such boards, for example cupping and checking. Additional preventive measures can be used to further improve deck-board performance. For example, deck-boards could be treated with preservatives containing wax/oil additives to improve their dimensional stability and reduce checking (Akhtari and Nicholas 2014b). Secondly, semi-pigmented penetrating finishes could be used to reduce the adverse effects of weathering such as discolouration and checking (Nejad and Cooper 2011a). Freedonia Group (2016a) conducted a 10-year lifecycle cost analysis showing that the average maintenance cost for solid wood decking is $450 compared to that of wood plastic composites decking, $150. Even factoring the higher cost of reapplying preservatives and exterior finishes annually, the 10-year cost including initial installation and annual maintenance for a deck made from preservative-treated wood ($12,540) is still cheaper than the cost of a deck made from wood plastic composites ($14,350) (Freedonia Group 2016a). Hence, the imperative to develop better approaches to maintaining the aesthetics of wooden deck-boards remains, and my findings indicate that there is further scope for improving the performance of many deck-board species that are traded commercially. 6.2 Conclusions The findings in this thesis demonstrate that the combination of surface and sub-surface profiling influences the checking and cupping of deck-boards. My results for western larch confirm the effectiveness of the standard and tall rib profile at restricting the checking of softwood deck-boards. Profiled deck-boards oriented pith-side-up were less susceptible to checking and cupping, but they developed shelling. Therefore, I conclude that profiled western 110 larch deck-boards and potentially other deck-board species that are susceptible to shelling should be oriented bark-side-up when they are exposed to the weather. I conclude that confocal profilometry is a suitable technique to quantify shelling at the surface of flat and profiled western larch deck-boards. This conclusion may also be relevant to other softwood species that are prone to shelling, in particular, those with distinct growth rings and conspicuous high density latewood, for example Douglas fir and southern pine. Finally, I conclude that sub-surface grooving shows promises as a viable replacement for ribs on the underside (pith-side) of double-sided profiled hem-fir deck-boards. Two groove profiles performed equally well as the sub-surface wide rib profile. Furthermore, I discovered a noteworthy design criterion for sub-surface grooving. I conclude that deck-boards that are 140 mm wide should be machined with an odd number of sub-surface grooves which creates a groove down the center of the underside of boards because such boards show less tendency to cup than boards with an even number of grooves (no central opening). This finding provides useful information for manufacturers of commercial deck-boards which employ sub-surface stress relief grooves. 6.3 Suggestions for further research Surface profiling in combination with sub-surface profiling and grooving improved the performance of wooden deck-boards exposed to the weather. Further exploration of such physical treatments is needed as I only applied them to three wood species. Additional trials of a similar scope are needed on a variety of different softwoods. Such trials should occur in a range of different climates as geographical location has a significant effect on the weathering of deck-boards (Morris et al. 2016). Confocal profilometry height maps and macro-photographic pictures of unprofiled western larch deck-boards in Chapter 4 showed the inter-band separations of latewood and earlywood when growth rings in boards oriented pith-side-up shelled. It would be worthwhile to image the growth ring interface using scanning electron microscopy to observe whether growth ring separation is due to interfacial or cell wall failure. 111 My results in Chapter 5 clearly revealed that sub-surface groove geometry influences the cupping of deck-boards exposed to the weather. Boards that contained a central groove tended to be less susceptible to cupping compared to boards without an opening in the center. Hence, a controlled experiment should be carried out to investigate in more detail the effect of this central opening on strains that develop in surface profiled and sub-surface grooved deck-boards exposed to drying cycles. There have been suggestions in the literature that the grooves of profiles could trap water and dust/nutrients/fungal inoculum during weathering, favouring the biological degradation of profiled boards (Morris and McFarling 2008; Heshmati et al. 2020). If this suggestion is correct, it might influence the choice of profile or wood preservative in different climates. Furthermore, fungal degradation in the grooves of profiles might influence the checking of boards. Further research to examine the effect of groove geometry and preservative treatment on microbial colonization of profiled boards is needed. 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Corrosion Science. 52(12):3943â3948. doi:10.1016/j.corsci.2010.08.014. 127 Appendices Appendix 1: AutoCAD images of grooving knives designed for the experiment described in Chapter 5 Appendix figure 1 Groove profile (Tall 5): Height of grooves = 6 mm; Pitch = 18 mm Appendix figure 2 Groove profile (Tall 6): Height of grooves = 6 mm; Pitch = 15 mm Appendix figure 3 Groove profile (Tall 7): Height of grooves = 6 mm; Pitch = 12 mm Appendix figure 4 Groove profile (Med 5): Height of grooves = 4 mm; Pitch = 18 mm 128 Appendix figure 5 Groove profile (Med 6): Height of grooves = 4 mm; Pitch = 15 mm Appendix figure 6 Groove profile (Med 7): Height of grooves = 4 mm; Pitch = 12 mm Appendix figure 7 Groove profile (Low 5): Height of grooves = 2 mm; Pitch = 18 mm Appendix figure 8 Groove profile (Low 6): Height of grooves = 2 mm; Pitch = 15 mm Appendix figure 9 Groove profile (Low 7): Height of grooves = 2 mm; Pitch = 12 mm 129 Appendix 2: Statistical analyses for Chapter 3 Please contact Professor Philip D. Evans (phil.evans@ubc.ca) Appendix 3: Statistical analyses for Chapter 4 Please contact Professor Philip D. Evans (phil.evans@ubc.ca) Appendix 4: Statistical analyses for Chapter 5 Please contact Professor Philip D. Evans (phil.evans@ubc.ca) Appendix 5: Paper published in open access journal Coatings https://doi.org/10.3390/coatings10090862
Thesis/Dissertation
eng
Forestry
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Optimizing profile geometry to reduce the checking, shelling and cupping of profiled softwood deck-boards exposed to natural weathering
Text