Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Sabrin, Sohana
Master of Applied Science - MASc
University of British Columbia
Alkalinization of problematic soils with silica and alumina rich material to improve their mechanical properties has become a popular practice globally. However, the application of alkalinization process to improve road bases and subbases namely subgrades are limited. Besides, the properties and performance of alkalinization process are dominated by several factors mainly initial curing conditions, concentration of alkaline activators and composition of materials. This study is aimed to investigate the effect of curing condition (temperature and duration) on unconfined compressive strength, elastic modulus (E50), microstructure and freeze thaw durability of silty sand subgrade soil stabilized with bentonite magnesium alkalinization. The selected curing temperatures and durations included ambient temperature, 40°C, 60°C and 80°C for 1 day and up to 3 days. Results revealed that curing at ambient temperature or as low as 40°C is not enough to achieve desirable strength while further increase in temperature favored substantial strength development. However, the properties of samples cured at elevated temperature for longer duration significantly deteriorated at later age (60 days) compared to the properties of samples cured at a slightly lower temperature. The optimal development of unconfined compressive strength and E50 observed when samples cured at 60°C for 3 days and the degree of improvement is 7 times compared to the natural soil. In addition, the scanning electron microscopy (SEM) images revealed presence of cementitious gel resulted from stabilization process. The data from energy-dispersive spectroscopy (EDS) and thermogravimetric analysis (TGA) confirmed the presence of cementitious gel magnesium silicate hydrate (M-S-H) and calcium silicate hydrate (C-S-H) which made the samples more compacted and stable with time. Finally, higher unconfined compressive strength loss was observed during freeze thaw durability test but the samples were capable to withstand all 12 freeze thaw cycles.
HEAT CURING METHOD AND LABORATORY PERFORMANCE EVALUATION OF SUBGRADE SOIL STABILIZED WITH BENTONITE AND MAGNESIUM ALKALINIZATION by Sohana Sabrin B.Sc., Khulna University of Engineering & Technology, 2014 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Civil Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) September 2019 © Sohana Sabrin, 2019 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis/dissertation entitled: Heat curing method and laboratory performance evaluation of subgrade soil stabilized with bentonite and magnesium alkalinization submitted by Sohana Sabrin in partial fulfillment of the requirements of the degree of Master of Applied Science in Civil Engineering . Dr. Sumi Siddiqua, School of Engineering Supervisor Dr. Solomon Tesfamariam, School of Engineering Supervisory Committee Member Dr. Ramy Saadeldin, Adjunct Faculty, School of Engineering Supervisory Committee Member Dr. Jahangir Hossain, School of Engineering University Examiner iii Abstract Alkalinization of problematic soils with silica and alumina rich material to improve their mechanical properties has become a popular practice globally. However, the application of alkalinization process to improve road bases and subbases namely subgrades are limited. Besides, the properties and performance of alkalinization process are dominated by several factors mainly initial curing conditions, concentration of alkaline activators and composition of materials. This study is aimed to investigate the effect of curing condition (temperature and duration) on unconfined compressive strength, elastic modulus (E50), microstructure and freeze thaw durability of silty sand subgrade soil stabilized with bentonite magnesium alkalinization. The selected curing temperatures and durations included ambient temperature, 40°C, 60°C and 80°C for 1 day and up to 3 days. Results revealed that curing at ambient temperature or as low as 40°C is not enough to achieve desirable strength while further increase in temperature favored substantial strength development. However, the properties of samples cured at elevated temperature for longer duration significantly deteriorated at later age (60 days) compared to the properties of samples cured at a slightly lower temperature. The optimal development of unconfined compressive strength and E50 observed when samples cured at 60°C for 3 days and the degree of improvement is 7 times compared to the natural soil. In addition, the scanning electron microscopy (SEM) images revealed presence of cementitious gel resulted from stabilization process. The data from energy-dispersive spectroscopy (EDS) and thermogravimetric analysis (TGA) confirmed the presence of cementitious gel magnesium silicate hydrate (M-S-H) and calcium silicate hydrate (C-S-H) which made the samples more compacted and stable with time. Finally, higher unconfined compressive strength loss was observed during freeze thaw durability test but the samples were capable to withstand all 12 freeze thaw cycles. iv Lay Summary Efficient and well-constructed roadway infrastructure is a key component for growth and economic developments of any region. In addition, durable road pavement with increased service life can save millions of dollars in road maintenance operations. The durability and long-term performance of a pavement structure largely depends on the underlying soil foundation known as subgrade soil. When poor subgrade soils are encountered many highway agencies employ âremove and replaceâ or stabilization techniques to strengthen the subgrade which is not always effective, expensive and environmentally unsafe. This research aims at developing a new environmentally compatible and efficient additive for subgrade soil to comply with the British Columbiaâs Clean Energy technologies. One of the major contributions of this research is to establish a heat curing technique for the newly formed additive. Furthermore, the performance of stabilized samples under adverse environmental condition was evaluated by freeze thaw durability test. v Preface This thesis aimed at establishing a heat curing technique for bentonite magnesium alkalinization which is a newly formed eco friendly additive and determining the performance of stabilized soil under freeze thaw cycles. The research was performed by author under the supervision of Prof. Dr. Sumi Siddiqua. Dr. Siddiqua provided guidance and supervision throughout the entire process. The authorâs responsibilities for this research were completing literature review, experimenting, collection and analysis of data and writing the thesis. The only aspect was not covered by the author was establishing optimum mix design for different test modes, which was done by Nurmunira Muhammad. The laboratory experiments were conducted in the Geomaterial Research Laboratory at the School of Engineering and microstructural analysis was conducted at FiLTER - Fipke Laboratory for Trace Element Research, the University of British Columbia Okanagan (UBCO). Thermogravimetric analysis was carried out at Shared Instrument Facility (SIF), the University of British Columbia Vancouver (UBCV). Ethics approval from the UBC Research Ethics Board was not required for this study. vi Table of Contents Abstract............................................................................................................................... iii Lay Summary ...................................................................................................................... iv Preface ................................................................................................................................. v Table of Contents ...................â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦.â¦â¦â¦â¦â¦â¦â¦.vi List of Tables ...................................................................................................................... xii List of Figures .................................................................................................................. xiiii List of Abbreviations and Symbols ..................................................................................xivv Acknowledgements ......................................................................................................... xvii Dedication ........................................................................................................................ xvii Chapter 1: Introduction ........................................................................................................ 1 1.1 Overview .............................................................................................................. 1 1.2 Motivation of research ........................................................................................... 2 1.3 Novelty of research ............................................................................................... 4 1.4 Objectives ............................................................................................................ 5 1.5 Outline of thesis .................................................................................................... 7 Chapter 2: Literature Review ............................................................................................... 8 2.1 Overview .............................................................................................................. 8 2.2 Road pavement .................................................................................................... 8 2.2.1 Wearing surface ............................................................................................. 9 2.2.2 Base course ................................................................................................... 9 2.2.3 Subbase ........................................................................................................ 9 vii 2.2.4 Subgrade soil ............................................................................................... 10 2.2.5 Special pavement layers............................................................................... 10 2.3 Variation of stress and material in pavement layers .............................................. 10 2.4 Pavement environment ....................................................................................... 12 2.4.1 Pavement moisture regime ........................................................................... 12 2.4.2 Pavement temperature regime ...................................................................... 13 2.5 Pavement behavior in cold regions ...................................................................... 13 2.5.1 Durability of pavement in cold regions ........................................................... 14 2.5.1.1 Frost heave .............................................................................................. 14 2.5.1.2 Thaw weakening ....................................................................................... 15 2.6 Importance of improved subgrade soil.................................................................. 16 2.7 Purpose and ways of soil stabilization .................................................................. 17 2.7.1 Mechanical stabilization ................................................................................ 17 2.7.2 Chemical stabilization ................................................................................... 18 2.7.2.1 Traditional additives .................................................................................. 18 2.7.2.2 Non-traditional additives ............................................................................ 19 2.7.3 Application of salt in soil improvement ........................................................... 20 2.7.4 Geopolymerization ....................................................................................... 21 2.7.4.1 Alkaline activator and heat treatment ......................................................... 21 2.8 Performance criteria of stabilized subgrade soil .................................................... 27 2.8.1 Unconfined Compressive Strength ................................................................ 27 viii 2.8.2 Stiffness....................................................................................................... 27 2.8.3 Durability ..................................................................................................... 28 2.9 Summary of literature review ............................................................................... 28 Chapter 3: Materials and Methods ..................................................................................... 30 3.1 Overview ............................................................................................................ 30 3.2 Materials............................................................................................................. 30 3.3 Mix design .......................................................................................................... 32 3.4 Sample preparation and curing ............................................................................ 32 3.4.1 Unconfined Compressive Strength (UCS) ..................................................... 32 3.4.2 Freeze thaw durability .................................................................................. 35 3.5 Testing Methods .................................................................................................. 35 3.5.1 Particle size distribution of soil ...................................................................... 35 3.5.1.1 Sieve analysis........................................................................................... 36 3.5.1.2 Hydrometer test ........................................................................................ 36 3.5.2 Atterberg limit test ........................................................................................ 37 3.5.2.1 Liquid limit (LL) ......................................................................................... 37 3.5.2.2 Plastic limit (PL) ........................................................................................ 38 3.5.2.3 Plasticity index (PI) ................................................................................... 38 3.5.3 Compaction test ........................................................................................... 38 3.5.4 Specific gravity test ...................................................................................... 39 3.5.5 Swelling index test ....................................................................................... 39 ix 3.5.6 Unconfined Compressive Strength (UCS) ..................................................... 39 3.5.7 Elastic modulus ............................................................................................ 40 3.5.8 Freeze thaw durability test ............................................................................ 41 3.5.9 Field-Emission Scanning Electron Microscopy (FESEM) and Energy-Dispersive Spectroscopy (EDS) .................................................................................................. 42 3.5.10 Thermogravimetric analysis (TGA) ................................................................ 43 Chapter 4: Results and Discussions ................................................................................. 44 4.1 Overview ............................................................................................................ 44 4.2 Material characterization ..................................................................................... 44 4.3 Unconfined Compressive Strength (UCS) ............................................................ 47 4.4 Elastic modulus................................................................................................... 58 4.5 Microstructural analysis ....................................................................................... 60 4.5.1 Field-Emission Scanning Electron Microscopy (FESEM) ................................ 60 4.5.2 Energy-Dispersive Spectroscopy (EDS) ........................................................ 64 4.5.3 Thermogravimetric Analysis .......................................................................... 65 4.6 Effcets of freeze thaw cycles on UCS values ........................................................ 67 Chapter 5: Conclusions and Future works ........................................................................ 69 5.1 Summary ............................................................................................................ 69 5.2 Major findings ..................................................................................................... 69 5.3 Field applications ................................................................................................ 70 5.3.1 Pelletized bentonite magnesium alkalinization ............................................... 71 5.3.2 In-situ dry mixing .......................................................................................... 71 x 5.4 Limitations and future works ................................................................................ 72 Bibliography....................................................................................................................... 73 Appendices ........................................................................................................................ 83 Appendix A: Additional test results ................................................................................. 83 Appendix B: Additional images of freeze thaw durability test ........................................... 91 xi List of Tables Table 2.1 Summary of literatures on geopolymerization and heat curingâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦24 Table 3.1 Physical and chemical properties of chemical additives. .......................................... 31 Table 3.2 Summary of mixture and curing design................................................................... 33 Table 3.3 Summary of mixture and curing design for freeze thaw durability test ...................... 35 Table 4.1 Physical properties of the raw silty sand and bentonite ........................................... 46 Table 4.2 Compositional elements of the raw silty sand and calcium bentonite........................ 46 Table 4.3 Unconfined compressive strength test results.......................................................â¦...52 Table 4.4 Concentration of selected chemical elements in optimum sample at age 3, 14 and 60 days. ............................................................................................................... 64 Table 4.5 UCS and moisture content comparison between control and exposed samplesâ¦â¦.â¦68 Table 5.1 Limitations and Recommendationsâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦.â¦â¦â¦â¦â¦..72 Table A.1 Additional unconfined compressive strength test resultsâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦84 Table A.2 Detail mix design for freeze thaw durability testâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦..89 Table A.3 Additional freeze thaw durability test resultsâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦90 xii List of Figures Figure 2.1 Distribution of vehicle induced stresses in pavement system with depth, adapted from FHWA(2017)â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦..11 Figure 2.2 Variation of material quality along with drainage characteristics in a typical pavement system with depth, adapted from FHWA (2017)â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦11 Figure 2.3 Impact of subgrade strength on pavement structural design (source FHWA 2017)â¦.16 Figure 3.1 Image of (a) soil, (b) bentonite and (c) magnesium chloride, MgCl2 â¦â¦â¦â¦â¦â¦.â¦..31 Figure 3.2 Heat curing in oven .............................................................................................. 34 Figure 3.3 Storing of samples (a) Wrapping & (b) Air tightened in zip lock bag and placed in plastic box ...................................................................................................................................... 34 Figure 3.4 Testing of UCS sample ......................................................................................... 40 Figure 3.5 Thawing configuration for freeze-thaw testing (a) Raw sample (b) Heated sample (c) Unheated sample ............................................................................................................ 42 Figure 4.1 Particle size distribution of natural subgrade soil ................................................... 45 Figure 4.2 Compaction curve of natural silty sand subgrade soil â¦â¦â¦â¦â¦â¦â¦.â¦â¦â¦â¦...â¦45 Figure 4.3 Stress strain behavior of raw soil and samples heat cured at 40ºC for 1 dayâ¦â¦â¦..47 Figure 4.4 Stress strain behavior of raw soil and samples heat cured at 40ºC for 2 daysâ¦â¦â¦.48 Figure 4.5 Stress strain behavior of raw soil and samples heat cured at 40ºC for 3 daysâ¦â¦â¦.48 Figure 4.6 Stress strain behavior of raw soil and samples heat cured at 60ºC for 1 dayâ¦â¦â¦â¦49 Figure 4.7 Stress strain behavior of raw soil and samples heat cured at 60ºC for 2 daysâ¦â¦â¦.49 Figure 4.8 Stress strain behavior of raw soil and samples heat cured at 60ºC for 3 daysâ¦â¦â¦50 Figure 4.9 Stress strain behavior of raw soil and samples heat cured at 80ºC for 1 dayâ¦â¦â¦..50 Figure 4.10 Stress strain behavior of raw soil and samples heat cured at 80ºC for 2 daysâ¦â¦...51 Figure 4.11 Stress strain behavior of raw soil and samples heat cured at 80ºC for 3 daysâ¦â¦..51 Figure 4.12 Influence of heat curing temperature and time on compressive strength of bentonite xiii magnesium alkalinization treated soil heat cured at 40ºC for 1, 2, 3 daysâ¦â¦â¦â¦â¦â¦â¦â¦â¦..54 Figure 4.13 Influence of heat curing temperature and time on compressive strength of bentonite magnesium alkalinization treated soil heat cured at 60ºC for 1, 2, 3 daysâ¦â¦â¦â¦â¦â¦â¦â¦â¦..55 Figure 4.14 Influence of heat curing temperature and time on compressive strength of bentonite magnesium alkalinization treated soil heat cured at 80ºC for 1, 2, 3 daysâ¦â¦â¦â¦â¦â¦â¦â¦â¦...56 Figure 4.15 Variation of elastic modulus (E50) of the heat cured samples at 60ºC and 80ºC for 2 and 3 at the age of 3, 14, 60 daysâ¦â¦â¦â¦.â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦..58 Figure 4.16 Micrograph from SEM analysis of raw silty sand subgrade soilâ¦â¦â¦â¦â¦â¦â¦â¦â¦60 Figure 4.17 Micrographs from SEM analysis for samples heat cured at 60ºC for 2 days at curing period of (a) 3 days; (b) 14 days and (c) 60 days. Images are magnified to 10000xâ¦â¦â¦â¦â¦..61 Figure 4.18 Micrographs from SEM analysis of samples heat cured at 60ºC for 3 days at curing period of (a) 3 days; (b) 14 days and (c) 60 days. Images are magnified to 10000xâ¦â¦â¦â¦â¦..61 Figure 4.19 Micrographs from SEM analysis of samples heat cured at 80ºC for 2 days at curing period of (a) 3 days; (b) 14 days and (c) 60 days. Images are magnified to 10000xâ¦â¦â¦â¦â¦..63 Figure 4.20 Micrographs from SEM analysis for samples heat cured at 80ºC for 3 days at curing period of (a) 3 days; (b) 14 days and (c) 60 days. Images are magnified to 10000xâ¦â¦â¦â¦â¦â¦63 Figure 4.21 TGA and DTG analysis of samples heat cured at 60ºC for 3 days after (a) 3 days and (b) 60 days of curingâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦..66 Figure A.1 Typical stress strain curveâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦â¦.83 Figure A.2 Typical calculation of elastic modulus (E50) from stress strain curveâ¦â¦â¦â¦â¦â¦..â¦83 Figure B.1 7 days cured samples after 12 freeze thaw cycles (a) Heated (b) Unheatedâ¦â¦.â¦â¦91 Figure B.2 28 days cured samples after 12 freeze thaw cycles (a) Heated (b) Unheatedâ¦â¦.â¦.91 xiv List of Abbreviations and Symbols USCS Unified soil classification system AASHTO American association of state highway and transportation officials ASTM American Society for Testing and Materials FHWA Federal Highway Administration UCS Unconfined Compressive Strength SEM Scanning electron microscopy EDS Energy-dispersive spectroscopy TGA Thermogravimetric Analysis MgCl2 Magnesium chloride NaOH Sodium hydroxide Na2SiO3 Sodium silicate RO Reverse osmosis AA Alkalinization LL Liquid limit PL Plastic limit PI Plasticity index M Molar concentration L Ratio of sodium silicate (Na2SiO3) to 10M sodium hydroxide (NaOH) BM Bentonite magnesium alkalinization xv CM 3% Magnesium chloride (MgCl2) °C Degree Celsius S Magnesium chloride (MgCl2) g Gram mm Milimeter s Second kPa Kilopascal MPa Megapascal % Percent xvi Acknowledgements First and foremost, my gratefulness to the Almighty Allah for the successful completion of my research. I would like to express my sincere gratitude to my supervisor Dr. Sumi Siddiqua for her immense support and unabating motivation throughout my MASc at UBC. I would never be able to finish this research without her patience, advices, expertise and guidance. I am also grateful to Nurmunira Muhammad, doctoral student at the University of British Columbia Okanagan for her endless help and valuable advices in each step of my research. I am grateful to all my committee members, Dr. Solomon Tesfamariam and Dr. Ramy Saadeldin for their support and insightful comments to improve my research. I would like to thank my colleagues Nurmunira Muhammad, Ahmed ElMouchi, Stephen Renner, Amin Bigdeli, Bigul Pokharel, Priscilla Barretto, Derek Emslie and Biola Salam for their enthusiastic support during my experimental works and creating friendly environment. I would also like to acknowledge the laboratory technicians in the School of Engineering for their assistance particularly to Kim Nordstrom, Michelle Tofteland, David Arkinstall and Bryn Crawford. At last but not the least, I would like to express my greatest gratitude to my parents, husband and brother, for their undying love, encouragement and support throughout my life. I am especially thankful to my husband and best friend Ariful Islam, for his consistent support and ardency throughout my graduate studies. xvii Dedication It is my privilege to dedicateâ¦ My beloved parents Safiuddin Molla &Tahamina Khatun, and my husband Ariful Islam 1 Chapter 1: Introduction 1.1 Overview Roadway infrastructure is a key component for growth and economic developments of any region. Durable road pavement with increased service life can save millions of dollars in road maintenance operations. Road networks are designed and built to provide a safe, comfortable and thriving ride to road users throughout their service life. The two major functional and structural requirements are needed to be fulfilled by road infrastructures to ensure these services. The functional requirement or serviceability is to provide smooth ride experience i.e. skid resistance whereas sufficient level of structural capacity over a reasonable/design time period is the other desired pavement structural requirement (Salour 2015). The author also added that the design of pavement system is complex because its performance is affected by several boundary conditions such as temperature and moisture (water) content; the mechanical behaviour of bound and unbound materials; and the traffic load spectrum and frequency. Among the several boundary conditions, subgrade and unbound granular layers in flexible pavement structure play an important role to the overall pavement performance. These layers support and enhance the performance of road pavement by controlling a safe distribution of vehicle induced stresses and strains (Erlingsson et al. 2017). As such, it is desirable that roadway formation layers should have high strength, bearing capacity and durability. However the performance and stability of subgrade and unbound granular layers are impacted by other boundary conditions such as precipitation, subsurface moisture condition and freeze thaw cycles (Li et al. 2011). The authors also added that moisture related distresses in formation layers leads to cracking, deflection, reduced bearing capacity of pavement and ultimate failure in absence of proper rehabilitation. Understanding of throughout road design process has become essential to establish a sustainable and efficient transportation network by mitigating the impact of adverse environmental changes along with the increased traffic on road pavement. The improvement of subgrade soil quality and investigation of the subgrade material behaviour as well as response to traffic-load induced stresses and 2 environmental factors would be a realistic approach to understand the complex nature of pavement system (Salour 2015). Mechanical compaction, replacement of weak soil and other forms of stabilization are the common ground improvement techniques applied in geotechnical industry. Mechanical compaction increases the soil strength but when this soil come in contact with additional moisture the bearing capacity reduces, and it affects the performance of the pavement (Hopkins et al. 2006). The removal and replacement of poor-quality soil by high quality fill is simplest but expensive method and it is not always a feasible solution. On the other hand, chemical stabilization is practically feasible and cheap to improve large volume of soil. As a result, this technique is extensively used to improve soil quality by most engineers for large construction works (Muhammad et al. 2018). 1.2 Motivation of Research The British Columbia road network is a vital part of the pacific gateway for offering up to 58 fewer travel hours for vessels compared to American ports in Los Angeles and Long Beach (Northern Development Initiative Trust 2014). Furthermore, it connects communities and facilitates the movement of goods across the provinces in Canada. As BC moves into the future, opportunities for new and existing economic developments are booming for example, Liquefied Natural Gas (LNG), oil and gas, forest industries and container movements etc. According to the 2005 British Columbia Ports Strategy, the expected combined increase in port traffic for the Vancouver, Fraser, and Prince Rupert ports from 2003 to 2020 is between 200% to 300% for container cargo, 10% to 55% for resource exports, and 50% to 150% for cruise passengers. This sums of 7 million containers, 96 million tonnes of resource exports, and 2.4 million cruise passengers (BC Ministry of Transportation and Infrastructure 2005). To keep pace with the population growth and new economic developments, BC's road network needs to accommodate the increasing speed, 3 number, and size of vehicles which is a challenge for road designers due to soft compressible soils, limited space and/or alignment opportunities: mountainous terrain and seismic considerations (Ngieng et al. 2014). According to Agriculture and Agri-Food Canada (2018), most of the subgrade soils in British Columbia are either organic soil, or a combination of low-quality sand, silt and clay. These soils are identified as problematic soils and are susceptible to moisture in nature. For example, silty sand exhibits high capillary action during spring (Muhammad et al. 2018) and this behaviour accounts for higher capital investment on road rehabilitation in British Columbia. On the other hand, 92% of BC's total road networks that allow motorized vehicle use are unpaved roads (Environmental Reporting BC 2018). The province has a long history to utilize larger share of its capital investment to maintain this large, unpaved road network. Besides the poor subgrade soil and unpaved road conditions, the road network in BC undergoes severe cold region-related climatic and environmental conditions when it is accompanied with heavy transport traffic loading, can result in seasonal and long term loss of bearing capacity (structural shortage), or loss of surface smoothness due to differential frost action, crack propagation and surface ravelling (serviceability shortage) which increase the needs for maintenance works. According to the British Columbia Ministry of Transportation and Infrastructure (BCMoTI 2015), over the last three years, the government of BC has financed over $380 million to resurface and restore provincial highways and side roads, as well as to improve roadway conditions. Besides there is a larger share of unpaved roads in BC and which contribute the most (92%) to the total length of roads (Environmental Reporting BC 2018). According to Doré et al. (2009), this significantly larger share of unpaved roads are most susceptible to cold climate and responsible for increased maintenance cost. It is therefore essential to rethink and explore efficient options to overcome these problems to adapt, design and construct resilient highways. At the same time, such research would benefit sustaining British Columbiaâs roadway network, especially considering BCâs growing population, 4 increased urban development with new economic developments, continuing evolution of vehicles and the need for adaptation due to climate change. Strengthening and increasing the bearing capacity of the subgrade layer to address the continuously increased vehicle traffic and adverse climatic conditions would be the first step to construct resilient and sustainable road pavement (Erlingsson et al. 2017; Li et al. 2011). Strong subgrade can be constructed by formulating a suitable eco-friendly additive for problematic soils (Muhammad et al. 2018). On the other hand, the province of BC is emphasizing on the opportunities for Clean Energy technologies. There is plentiful of calcium bentonite as industrial by product from mining in British Columbia which is an environmental hazards. The current project targets for developing new material based on mining by-products with reduced environmental and energy impacts. 1.3 Novelty of Research The application of various additives such as cement, lime, fly ash, blast furnace slag, fiber, pozzolanas and polymer well studied by researchers and used for the stabilization of silty sand subgrade. Portland cement and hydrated lime are most commonly used additives to improve the stiffness and reduce stress related to vertical loads of road subgrades (Doré et al. 2009). The authors also stated that this higher stiffness leads to increase in thermal stresses due to frost action related movements and shrinkage in cold regions. So, the application of Portland cement and lime has not gained much popularity to improve subgrade soil in cold regions where more flexible bases are required (Doré et al. 2009; Kolisoja et al. 2005). Besides, the manufacturing process of these calcium based traditional additives produce greenhouse gas (AASHTO Designation: R Draft 2008) and contribute to adverse environmental effects. For example, the production of one ton of Portland cement emits one ton of CO2 into the atmosphere; the production of cement is also energy intensive (Zhang et al. 2013). On the other hand, there are constrains 5 associated with the suitability, availability and cost of non traditional additives (fibers, pozzolanas as well as polymer). So, researchers along with industries are looking to formulate sustainable, environmentally safe and low cost construction materials in order to keep pace with the increasing urban developments and economic growth. Alkali activation of aluminosilicate materials is a new method to form sustainable cementing binder called geopolymer. According to Provis (2017) in addition to its cost effectiveness, the use of an alkali activated geopolymer binder can reduce the emission of CO2 from 40% to 80% and requires less operational energy than the production of Portland cement. The literature on geopolymer binder is limited and there is scope of utilizing industrial waste as the source of aluminosilicate material. In addition, the effects and suitability of heat curing temperature and duration on alkali activated geopolymer still not well studied and organized, especially on silty sand or non-reactive soil. This research investigated the impacts of various heat curing temperature and duration on silty sand subgrade soil stabilized with bentonite magnesium alkalinization, a newly formed green additive by evaluating strength, stiffness and durability and microstructure. 1.4 Objectives The primary goal of the current research is to understand the effects of heat treatment on strength, microstructure and durability of problematic subgrade soil stabilized with bentonite magnesium alkalinization, a newly formed green additive. In order to achieve these goals the following objectives were identified: 1. Develop heat curing technique to stabilize problematic road subgrade using mining product 6 2. Understand the efficacy of curing temperature and duration on strength, elastic modulus and microstructure of stabilized subgrade soil 3. Performance evaluation of treated subgrade soil by laboratory durability analysis at cold climate These objectives are accomplished by implementing the following primary tasks: 1. Study the strength and stiffness of the optimally prepared samples, heat cured at three different curing temperatures and for each temperature three separate curing periods. A series of unconfined compressive strength tests were carried out under this task. 2. Investigate the aggregation of soil particles by image analysis and presence of cementing bond (magnesium silicate hydrate and calcium silicate hydrate) within the treated samples in objective 1. These tasks were completed by performing Scanning Electron Microscopy and Thermogravimetric analysis. 3. Compare the strength of samples went under 12 freeze thaw cycles with the strength of treated samples in objective 1. A series of unconfined compressive strength tests were carried out to accomplish it. 7 1.5 Outline of Thesis In order to fulfill the above mentioned objectives, this thesis consists of five chapters as follows: Chapter 1: presents a brief background on subgrade soil and its influence on roadway pavement performance. Further, it highlights the importance of this research along with its novelty. Finally emphasizes the objectives of this research and tasks performed to accomplish the listed objectives. Chapter 2: provides a comprehensive literature review on roadway pavement systems and subgrade soil, soil stabilization and explains the necessity of heat treatment in detail. Finally, the results obtained from earlier studies are also incorporated in this chapter. Chapter 3: discusses soil characterization along with the properties of bentonite, magnesium chloride and alkaline activator. In addition, provides a description of the sample preparation, test protocols adopted to conduct experimental analysis and equipment used in this research. Chapter 4: demonstrates the effects of heat curing on silty sand subgrade soil stabilized with bentonite magnesium alkalinization through an in-depth analysis of test results and microstructural investigations. Chapter 5: provides a brief conclusion of the test results and research findings obtained throughout the study. Also, outlines the field applications, limitations encountered at this research effort along with recommendations for future works. 8 Chapter 2: Literature Review 2.1 Overview Strong subgrade with higher strength, bearing capacity and durability is the prerequisite for enhanced pavement performance and increased service life with minimal maintenance works (Erlingsson et al. 2017; Li et al. 2011).This present study aims at finding a sustainable and eco-friendly way to stabilize problematic siltysand subgrade soil to enhance subgrade performance under adverse environmental conditions. The following sections include the literature reviews conducted for this research. At first a description of road pavement systems, necessity of high quality subgrade soil and influences of different factors on it are presented. The ways of soil stabilization and necessity of heat treatment in geopolymerization also reviewed, followed by the incorporation of results obtained from earlier studies. 2.2 Road Pavement Road pavement structures are linearly spreading out across various topography and climatic conditions (Doré et al. 2009). According to the authors this pavement structure acts as a means of interactions between the superimposed traffic loads and the underlying soil. The pavement structure performs this role by distributing the top down loads and lessen numerous geotechnical effects at bottom. The aspects of first role include sustaining repeated moving axle loads of vehicles and proper distribution of stress to bottom layers. On the other hand, the second role responsible to mitigate environmental impacts: natural stresses, displacements, temperature, moisture, negative pore pressure and differential frost heave for structural and functional performance (Doré et al. 2009). The design and construction of pavement structure are divided into different components in order to successfully fulfill these aspects. The typical key components of pavement system namely wearing surface; bound structural layer; unbound base and subbase; subgrade and drainage systems are constructed with processed materials above the natural soil. According to Federal Highway Administration (2019) the proper design and functioning of all the 9 key components of the pavement system dominates the satisfactory pavement performance which includes optimal structural capacity as well as durability, ride quality and safety. 2.2.1 Wearing Surface The wearing surface or surface layer is the stiffest layer in pavement structure and typically consist of hot or cold mix asphalt and bituminous surface treatments. The structural and functional role of this layer is to provide sufficient smoothness, adherence, and sealing or drainage of surface water whereas bound structural layers (i.e., asphalt or Portland cement concrete) perform to yield sufficient load-carrying capacity as well as prevent water intrusion into the underlying unbound materials. The appearance of road embankment and pavement markings are also dependent on this layer (Doré et al. 2009; FHWA 2019). 2.2.2 Base Course The base course is immediately underneath the surface layer and contribute to the structural performance of the overall pavement structure. The key functions of this layer include additional distribution of load and rapid infiltration of infiltrated water (drainage). The ability to fulfill these functions depends on characteristics and density of granular base material (gradation, roughness and shape) along with moisture content (Doré et al. 2009; Pavement interactive 2019). 2.2.3 Subbase Subbase is the layer in between base course and subgrade layer (Pavement interactive 2019). The importance of this layer more evident in cold and humid environment. The three key roles of subbase layer is to act as drainage layer: minimize and regulate the moisture content of the overall pavement structure more especially base layer, separation layer: prevent the intrusion of fine particles to base layer from subgrade and finally frost protection layer (Doré et al. 2009). 10 2.2.4 Subgrade Soil Subgrade soil is a key component of the pavement structure thought it is not considered as a part of the whole system (Doré et al. 2009). According to FHWA (2019) subgrade soil ensures a uniform and sufficiently stiff, strong, and stable foundation for the overlying layers. This complex layer mostly constructed with local materials: barely modified natural soils or engineered filled material. This practice always increases the compressibility, higher susceptibility to moisture and frost of this layer and deteriorate the desired pavement performance. Adaptation of subgrade improvement techniques: homogenization, stabilization, reinforcement and replacement of subgrade soil are required in order minimize these problems initiated by poor subgrade soil (Doré et al. 2009). 2.2.5 Special Pavement Layers Drainage, separation, reinforcement and insulation layer are the special layers introduced in pavement structure to fulfill a particular role (Doré et al. 2009). 2.3 Variation of Stress and Material in Pavement Layers According to FHWA (2019) upper layer in a pavement system experience the highest vehicle induced stress whereas it decreases with depth. The typical attenuation of load-induced stresses with depth are as follows in Figure 2.1: 11 Figure 2.1 Distribution of vehicle induced stresses in pavement system with depth, adapted from FHWA (2017) Accordingly the upper wearing surface layer made with expensive, well-graded, durable and frictional material, mixed most commonly with bitumen in modern road pavements. Besides less expensive materials are used in lower layers to minimize the cost of construction as presented in Figure 2.2. However this practice increases the rehabilitation and reconstruction cost associated with pavement failure (FHWA 2017; Dhir et al. 2017). Figure 2.2 Variation of material quality along with drainage characteristics in a typical pavement system with depth, adapted from FHWA (2017) 12 2.4 Pavement Environment Pavement structures are always in constant interaction with atmospheric climatic factors in every region. The physical processes similar to the atmospheric climatic factors act on the pavement structure consist the pavement environment. However, the pavement environment is only governed by the physical laws of the pavement porous mineral system instead of atmospheric gaseous environment. Temperature and moisture are the main environmental factors acting on the pavement systems. The continuous evaluation of surrounding atmospheric environment leads to the daily and seasonal variation along with the spatial distribution of these parameters in pavement system (Doré et al. 2009; Salour 2015). Similar statement was provided by FHWA (2019) that moisture and temperature are the prominent environmentally driven variables which impact pavement performance and subgrade properties. So, temperature and moisture conditions in pavement systems significantly impact the pavement design and performance, most importantly strength, durability and load bearing capacity (Li et al. 2011). 2.4.1 Pavement Moisture Regime Moisture condition in pavement structure significantly changes throughout the year (Erlingsson et al. 2017; Doré et al. 2009; Salour 2015). The unbound layer compacted to the optimum moisture content during the construction phase eventually reach towards natural equilibrium with time (Zapata et al. 2009). This moisture in pavement systems can be accumulated from different sources such as precipitated water enters through surface irregularities (cracks, shoulder edges, joints), seepage from high ground and capillary action due to high ground water table. The percentage of natural equilibrium moisture depends on the material properties and distance to the groundwater table level and its variation. In pavements systems subsurface layers are most susceptible to excess moisture due to use of poor quality materials. The maximum dry density of this weak soil momentarily reduces due to excessive moisture and subsequent reduction in 13 strength and bearing capacity. These lead to local distresses like potholes, excessive roughness, deflection, cracking etc. and increase the cost of road maintenance (FHWA 2017; Li et al. 2011; Salour 2015). 2.4.2 Pavement Temperature Regime Solar radiation, geothermal heat are the main factors contributing to heat intake whereas emitted convection & turbulence responsible for heat extraction in pavement system. Pavement temperature regime varies between stable bottom temperatures towards continually changing surface temperature. The variation of temperature in pavement structure depend on the surrounding environmental conditions, material quality and depth (Salour 2015). Temperature variation due to environmental conditions significantly observed during summer high to winter low temperatures and extent of variation with depth can be obtained from âtrumpet curveâ. The excess moisture in pavement structures turns into ice lenses at winter months due to emerging frost and permafrost conditions (Doré et al. 2009). Likewise, thawing occurs at spring due to elevated temperature. This phenomenon results in reduction of bearing capacity and detrimentally impact the durability of pavement (FHWA 2017; Li et al. 2011). To counter these deformation stresses engineers are looking for sustainable ways in design stages 2.5 Pavement Behavior in Cold Regions Freezing of pavement results from extensively low temperature in cold region and the extent of freezing vary depending on the geography as well as climate of the region. In North America, most of Canadian provinces and half of northern America experience enough cold temperature that induces frost penetration underneath pavement structure i.e. in subgrade soil (Doré et al. 2009). Whereas moistures are present in pavement from general sources as described in previous section of this literature. These two conditions (low temperature and excess moisture) 14 satisfy the needs to occur frost heave action in pavement system. This frost heave action causes expansion in interstitial water and expands soil volume. These changes in volume lead to differential settlement and change in soil density combined with spring thawing responsible for reduced bearing capacity and undesirable pavement performance (Doré et al. 2009; FHWA 2017; Hermansson et al. 2005; Salour 2015). 2.5.1 Durability of Pavement in Cold Regions Durability is a serious pavement structural design concern in the cold regions of the world (large parts of Asia, northern Europe, Alaska, Canada and about a third of the USA) (Simonsen et al. 1999). Frost heave and freeze thaw cycles are the main concerns associated with pavement durability (Parker 2008; Roper 2007). Frost heave occurs due to differential volume changes in base or subgrade layer resulted from the formation of ice lenses at winter months and thawing at spring. This mechanism of consecutive freeze thaw significantly impact the structural capacity of pavement structure. According to Parker (2008) the integrity of pavement structure is deteriorated by the formation of ice lenses in soil pore structure during winter months and thawing at spring. Mechanical properties of the pavement greatly affected by this seasonal changes. However, the extent and depth of ice accumulation varies by the duration and intensity of negative temperature (Hermansson et al. 2005; Simonsen et al. 1999; Teltayev et al. 2019). According to the authors in mild winter ice accumulation occurs near the surface and thawing starts by increase in temperature. In contrast, extreme cold weather such as in Canada prevails the formation of ice in subgrade layer of the pavement. In this case detrimental impacts start to occur at spring. The overall frost action encounters in following two steps: frost heave and thaw weakening. 2.5.1.1 Frost Heave Frost heaving of subgrade soil resulted from the expansion granular soil moisture when it turns into ice. The ice in soil voids crystallizes due to changes in temperature and subsequently try to 15 extend in soil mass by forming continuous ice lenses, layers, veins, or other ice masses. It propagates through capillary rise or cracks and thickens in the direction of heat transfer. This process continues until the complete depletion of granular moisture and the freezing condition is unfavourable for crystallization. As the ice lenses grow and expand in volume the surrounding soil heaves up (Hermansson et al. 2005; Li et al. 2011). This uneven ice formation favours reduction in pavement serviceability by facilitating cracks and surface roughness (Al-Mala Yousif 2015; FHWA 2017; Li et al. 2011; Simonsen et al. 1999). The authors also pointed that following three conditions triggers the formation of ice lenses and thus frost heave: ï· Frost-susceptible soil (significant amount of fines) ï· Sub-freezing temperatures ï· Availability of water 2.5.1.2 Thaw Weakening In contrast during spring thawing releases water as ice melts. The top down thawing progresses almost entirely from the pavement surface to downward. This phenomenon expedites reduced permeability of frozen sub surface layers and increases thaw weakening period. As a consequence water get trapped in between surface layer and still frozen layer below. This increased moisture content elevates the risk of water saturation and high pore water pressure in unbound layers (subgrade) than the layers close to surface. Consequently, this results a weakened subgrade condition with significantly reduced strength, bearing capacity, stiffness and damage the overall structural integrity (Al-Mala Yousif 2015; FHWA 2017; Li et al. 2011; Simonsen et al. 1999). According to FHWA (2019) excessive moisture content also restricts the recovery of soil structure by destructing the arrangements and cementitious bonding of soil particles. 16 2.6 Importance of Improved Subgrade Soil The overall structural capacity and integrity of pavement substantially depends on subgrade layer (FHWA 2017; Papagiannakis et al. 2008; Yoder et al. 1975). According to Dhir et al. (2017) stat the California Bearing Ratio (CBR) of subgrade soil controls the thickness of base layer. Similar statement was made by FHWA (2019) and established the following relationship between subgrade CBR to base thickness and structural capacity contribution. Figure 2.3 Impact of subgrade strength on pavement structural design, adapted from FHWA (2017) From Figure 2.3 it is evident that the increase of subgrade CBR 2 to 50 can reduce the base thickness from 24 inch to 0 inch. This reduction in base thickness can significantly reduce the pavement construction and material cost. Likewise improved subgrade material protects the pavement structure from adverse environmental impact as well as reduce the cost of road maintenance and rehabilitation works (Li et al. 2011; Simonsen et al. 1999). But the natural subgrade soil is often unsuitable to deliver the desirable performance in terms of strength, 17 stiffness, durability and drainage etc. for enhancing pavement structural capacity. So, it is inevitable to improve subgrade soil. 2.7 Purpose and Ways of Soil Stabilization Soil stabilization is the process of altering or improving the physical and engineering properties of natural soil to meet an engineering purpose. Improvements include higher bearing capacity, strength, durability, stiffness, dry density and resistance to softening by water (Afrin 2017; Firoozi et al. 2017; Latifi et al. 2015a & 2015b; Sherwood 1993). This technique of improving natural soil being used for millennium in different ways (Firoozi et al. 2017; Kowalski et al. 2007). In modern era the soil stabilization technique can be classified in two categories: mechanical stabilization and chemical stabilization (Afrin 2017; Makusa et al. 2013; Patel 2019). The success of stabilization depends on the proper selection of technique and additive. It is also important to ensure that the properties of stabilized geomaterials and mixtures are suitable for specific design purposes such as foundations, embankments, shoulders, subgrades, bases, and surface courses (Das 2000). 2.7.1 Mechanical Stabilization Mechanical stabilization is the process of improving soil physical properties by altering gradation. Typically it is done by induced vibration or compaction, mixing two or more soil together (Afrin 2017; Das 2000; Makusa et al. 2013; Patel 2019). However geosynthetics are also used as mechanical stabilizer where the soil strength and other engineering properties are further enhanced (due to particles interlocking, confinement, frictional resistance, tensile strength) (Das 2000; Patel 2019). 18 2.7.2 Chemical Stabilization Chemical stabilization is the process of stabilizing soil by adding single or multiple additives to achieve desired properties to meet the specific project needs (Kowalski et al. 2007; Makusa et al. 2013). According to Makusa et al. (2013) the overall process is governed by chemical reactions between additives (cementitious material) and soil minerals. Wherever the interactions of the soil and additives depend on the composition, micro and macro structure of different soils, and their physical and chemical behaviour with the air and water (AASHTO Designation: R Draft 2008). Jabar (2018) and Firoozi et al. (2017) reported that the type and amount of additives for stabilization varies with the physio-chemical characteristics of the soil. The stabilization of different soils are accomplished by using traditional calcium based additives and non-traditional additives. 2.7.2.1 Traditional Additives The application of conventional additives (cement, lime, fly ash) on clayey as well as sandy soil has been studied and proved efficient by several researchers (Abu-Farsakh et al. 2015; Hashemi et al. 2015; Saadeldin and Siddiqua 2013; Sukmak et al. 2013; Tariq & Maki 2014). Among these additives cement is the oldest and considered only primary stabilizing agent due to its proactive nature (Makusa et al. 2013). Whereas lime is well known as an economical additive to improve strength, optimum water content, shrinkage limit and workability (Firoozi et al. 2017; Makusa et al. 2013). On the other hand fly ash is considered as a secondary additive as it is not able to bring desired effects by its own (Makusa et al. 2013). According to Little (1999) the mechanisms of stabilization by these additives take place through cation exchange, flocculation & agglomeration, pozzolanic reaction and carbonate cementation. Firoozi et al. (2017) and Makusa et al. (2013) reported that calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) are the ultimate cementitious reaction products from the stabilization. The authors also mentioned that these cementitious reaction products responsible to enhance the overall properties and performance of soil. Although the inarguable effectiveness of these additives it has adverse 19 environmental effects. Nowadays global warming is the key environmental concern and carbon dioxide (CO2) is one of the main factors responsible for this (Ali et al. 2011; Taha et al. 2013; Zhang et al. 2012). According to Ali et al. (2011) and Zhang et al. (2013) manufacturing of a ton of Portland cement emits a ton of CO2 to the atmosphere and the production process is energy intensive. Similarly, Liska et al. (2008) reported that cement industry alone produces about 10% of total CO2 emission. Besides studies found that cement or lime is not suitable to stabilize soil containing sulfates and organic carbon. When sulfate enriched soils treated with lime or cement undergo excessive swelling which leads to loss in strength and ultimate pavement failures (Nair et al. 2011; Puppala et al. 2014; Verástegui-Flores et al. 2014). On the other hand presence of organic carbon in soil hinders pozzolanic reaction needed to gain strength (Hampton et al. 1998; Ling et al. 2013). 2.7.2.2 Non-traditional Additives Non-traditional soil stabilization additives consist of a variety of commercially available chemical and liquid agents with diverse characteristics (Tingle et al. 2007 & 2004). Blast furnace slag, fiber, polymer, resins and pozzolanas are the frequently used non-traditional additives to enhance the geotechnical properties of soil. Blast furnace slag is not a cementitious compound by itself. But it possesses latent hydraulic properties to be used as an additive when comes in contact with lime or alkaline material (Sherwood, 1993). Stabilization of soil using fiber is considered one of the best methods in the prevention of cracks (Firoozi et al. 2017). The authors also concluded that the application of fiber increases the strength but not effective for all type of soils especially clayey soils. Pozzolanas (siliceous and aluminous materials) are not cementitious in nature. But fine grained pozzolanas in presence of moisture chemically react with calcium hydroxide to form weak cementitious compound (ASTM C595). For example rice husk ash, an agricultural waste and make an excellent pozzolana as rich in silica (Sherwood, 1993). It applied for soil improvement and proved not suitable to increase the strength at greater extent (Behak et al. 2016; Rahgozar 20 et al. 2018). Likewise the application of polymer also been studied by some researchers but it is not cost effective and feasible at all conditions (Al-Saad 2005; Latifi et al. 2016b; Srinath et al. 2013). 2.7.3 Application of Salt in Soil Improvement The effects of chloride salts of sodium (Na), calcium (Ca), magnesium (Mg) and potassium (K) on engineering and microstructural properties of natural soil have been investigated by researchers and proved effective (Helle et al. 2014; Latifi et al. 2016a; Latifi et al. 2015a & 2015b; Muhammad et al. 2018; Siddiqua et al. 2014; Turkoz et al. 2014). According to Helle et al. (2014) the improvement in soil properties depends on the adsorbed ions in mineral surfaces. The authors also incorporated that the addition of salt changes ion concentration and composition in pore water which determines position of adsorbed ions in mineral surfaces. This chemical phenomenon positively alters the soil microstructure and expedite soil improvement. But it is required to identify the proper concentration and suitability of a salt for a particular soil due to differences in diffusion characteristics. The potential of sodium chloride (NaCl) to improve the dry density, optimum moisture content, unconfined compressive strength and shear strength characteristics of silty clay soil and with mixture of soils has been proved (Singh et al. 1999; Jafer 2013). The implication of calcium chloride (CaCl2) has notable influence on compressibility, suction and shear strength characteristics of bentonite-sand mixture and silty clay (Siddiqua et al. 2014; Jafer 2013). Helle et al. (2014) similarly documented the effectiveness of calcium chloride (CaCl2) on quick clay to improve geotechnical strength properties. Another salt magnesium chloride (MgCl2) which belongs to halide group applied to clay, residual as well as peat soil and studies found that it is an effective stabilizer to improve unconfined compressive strength, reduce swell pressure along with dispersive characteristics of subgrade soil (Latifi et al. 2016a; Latifi et al. 2015a & 2015b; Muhammad et al. 2018; Turkoz et al. 2014). Besides the effectiveness of MgCl2 to improve soil properties, it has the ability to absorb moisture from atmosphere and dust control. In addition 21 William et al. (2009) documented that the concentration of magnesium in soil is below the standards set by Environment Canada and Health Canada (2001) to be harmful to aquatic life. For this reason the application of magnesium will not create any environmental consequences. These characteristics prove the potential of MgCl2 to be used as a good source of green soil stabilizer for roadway construction. 2.7.4 Geopolymerization Alkali activation of silica and alumina rich materials to form sustainable cementing binder also known as geopolymer has recently received increased interests (Provis 2017; Sukmak et al. 2013; Zhang et al. 2013). During the geopolymerization, the amorphous aluminosilicate materials dissolved in alkaline activator to form reactive silica and alumina, then harden into synthetic aluminosilicate material with strong binding property through nucleation process (Ilkentapar et al. 2017; Provis 2017; Palomo et al. 1999; Van Jaarsveld et al. 1997; Zhang et al. 2013). Rios et al. (2016) mentioned that the high concentration of hydroxyl ions in highly alkaline solution destruct the covalent bonds in silica and alumina. The authors also added that alkaline cations act as building blocks for the structure, by compensating the excess negative charges, and by the ongoing nucleation process which turns into a stable C-S-H / M-S-H gel in the presence of calcium or magnesium respectively. However, according to the authors, thermal treatment is required to ensure the optimal precipitation of aluminosilicate substances during the nucleation process. 2.7.4.1 Alkaline Activator and Heat Treatment For alkali activation potassium hydroxide (KOH), sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) are the most commonly used solution and potential of these solution as an alkaline activator has already shown success (Atis et al. 2015; Heah et al. 2011; Kamarudin et al. 2011; Vargas et al. 2011). Palomo et al. (1999) concluded that besides the concentration of alkaline activator, the curing condition, especially the curing temperature and time, significantly impact the 22 mechanical properties of a geopolymer. Further, recent studies evaluating the effects of heat curing for conditioning alkali activation, have determined that the compressive strength significantly increases due to heat curing (Atis et al. 2015; Heah et al. 2011; Rovnaník 2010; Vargas et al. 2011). Heah et al. (2011) investigated the effects of heat curing on kaolin based geopolymer at four temperatures (40, 60, 80, 100ËC) with curing periods of 1, 2, 3 days. The authors concluded that the reaction rate of noncalcinated material is too slow to start the initial setting at ambient temperature, but increasing temperature triggers the dissolution of reactive species facilitating higher strength development. Similar conclusion was made by Atis et al. (2015), when they studied the influence of heat curing on geopolymer mortar consisting of standard sand and fly ash (class F) at different temperatures as low as 45ËC and as high as 115ËC for heat curing of 1,2, and 3 days. Vargas et al. (2011) investigated the effects of heat curing temperature (50, 65 and 80ËC) for a period of 24 hours on fly ash based geopolymer. According to the authors, elevated temperatures are responsible for higher strength development within a shorter period by effective alkaline attack to fly ash. Improved microstructure of heat treated geopolymer is desirable for maintaining structural integrity for longer age along with high strength; however, few researchers have focused on that. Sukmak et al. (2013) studied the relation of specimen size and fly ash/clay ratio on the heat curing duration to gain highest strength and defined the relation with optimum heat energy. The authors also advised that rapid drying should be avoided during curing, and that a lower relative humidity is preferable. Atis et al. (2015) defined the relation of curing temperature with sodium (Na) content in the alkaline activator and concluded that Na concentration in the alkaline medium controls the curing temperature. Kamarudin et al. (2011) focused on curing regime humidity and found that curing at higher temperature results in lower compressive strength and cracks due to a lack of moisture. Similarly, Heah et al. (2011) also stated that curing for longer periods at higher temperature reduces the compressive strength due to gel contraction, leading to a loss of water molecules, shrinkage, and damage of granular structure, and failure of the sample at later age. The authors also added that some amount of 23 water is needed in the sample to maintain structural integrity and to reduce cracking. When heat curing was performed for longer durations, the increase in compressive strength at later age was significantly smaller due to a reduction in consumable reactants for the geopolymeric reaction (Ilkentapar et al. 2017). These findings conclude that the extent of heat curing temperature and duration of curing on strength development of geopolymer depends on the type of materials, concentration of alkaline activator, size of sample, as well as solid to liquid ratio. Therefore, it is critical to formulate a separate heat curing technique for different materials. 24 Table 2.1 Summary of literatures on geopolymerization and heat curing Reference Type of soil and additives used Curing temperature Curing duration Optimum result Remarks Atis et al. (2015). Standard sand, Fly ash (class F), Alkali activator (NaOH 97% pure) 45,55,65,75,85, 95,105,115ËC 1,2,3 days 14% NaOH and cured at 115ËC for 24 hours ï· Geopolymeric reaction rate influenced by curing temperature and duration. ï· At higher temperature all the water evaporates before dissolution of reactive species (silica) so no compressive strength develops. Bonder et al. (2012). Natural pozzolans size less tham 75 micrometer, 7.5 M KOH Solution 20, 40, 60, 80ËC 27 days 60 ËC ï· Compressive strength increases with curing temperature and pozzolanic activity. Gunasekara et al. (2015). River sand, Fly ash (class F), NaOH solution (15M), Sodium silicate solution (Na2SiO3): Ms ratio 2 80ËC 1 day 80ËC ï· A smaller negative zeta potential of the geopolymer is an indication of more gel formation and high compressive strength. 25 Reference Type of soil and additives used Curing temperature Curing duration Optimum result Remarks Heah et al. (2011). Kaolin powder, NaOH powder,Sodium silicate solution (Na2SiO3) 40,60,80,100ËC 1,2,3 days 60ËC at 3 days ï· For geopolymerization non calcined materials require longer reaction time compared to calcined materials and ambient temperature is not feasible. ï· Curing for a longer period of time at higher temperature reduces the compressive strength due to gel contraction that leads to loss of water molecules, shrinkage and damage of granular structure and failure of sample at later age. Ilkentapar et al. (2017). Standard sand, NaOH (purity greater than 97%), fly ash 75ËC 4h, 1, 2, 3, 7 days Compressive strength increases with curing temperature but the increase in strength gain decelerates due to heat curing duration ï· When heat curing was done for longer duration, the increase in compressive strength at later age is significantly small due to reduction in consumable components of fly ash into geopolymeric reaction. ï· Abrasion resistance decreases and water absorption increases with increase in heat curing duration. Kamarudin et al. (2011). Fly ash (class F), NaOH solution (12M), Sodium silicate solution (Na2SiO3): Ms ratio 3.2 50, 60, 70, 80ËC 24 h 60ËC ï· Curing regime humidity has significant influence the long term performance of geopolymer. 26 Reference Type of soil and additives used Curing temperature Curing duration Optimum result Remarks Lemaire et al. (2013). Plastic silty soil, 1% lime + 5% cement 20ºC, 50ºC 1,3,7,28,90, 300 days Unconfined compressive strength at 20ËC for 300 days is similar to 50ËC for 28 days ï· At higher temperature leads to gaining faster strength Rovnaník (2010). Metakaolin, Alkaline silicate solution with silicate modulus 1.39, quartz sand 10, 20, 40, 60 & 80ËC 4h, 1, 2, 3 days 80ËC for 24 hours ï· Higher curing temperature makes the hardened sample less dense. Sukmak et al. (2013). Silty clay, Fly ash (class F), NaOH solution (10M), Sodium silicate solution (Na2SiO3): Ms ratio 0.3 55 to 140ËC for 24 to 168 h 75ËC for 48 h ï·Heat energy (product of curing temperature and time) controls the strength gain and requires more heat energy for larger specimen with higher weight. ï· The optimum heat energy depends on FA/Clay ratio. 27 2.8 Performance Criteria of Stabilized Subgrade Soil The performance testing provides essential inputs of subgrade material properties for mechanistic-empirical pavement design and further assess the influence of subgrade material in premature pavement failure. In addition, the properties and qualities of treated subgrade soil determine the suitability along with effectiveness of an additive and stabilization process at different climatic conditions for successful road construction project. Besides the appropriate performance tests assure the exact proportion of admixtures in a volumetric mix design and curing condition (Doré et al. 2009). The following testing procedures are adopted to assess properties as well as seasonal and spatial variation of stabilized subgrade soil. 2.8.1 Unconfined Compressive Strength Strong subgrade with higher strength are required to resist vehicle induced cyclic loading and improve structural integrity of the upper layers in a pavement system. Unconfined compressive strength test is one of the most common laboratory testing procedure to determine the strength of treated subgrade soil. . According to ASTM D4609-08 (ASTM 2008), an increase of 0.345 MPa or more in UCS proves the effectiveness of an additive for stabilizing the soil. 2.8.2 Stiffness The stiffness of subgrade soil is very important in mechanistic pavement design and analysis. The proper distribution of superimposed load and vehicle induced stresses on subgrade soil depends on its stiffness. Besides it also determines the stress, strain and displacement of overall pavement system. Stiffness of a material can be determined from the stress strain curve of unconfined compressive strength test, cyclic triaxial test or CBR test. 28 2.8.3 Durability The durability of subgrade soil plays an important role on the satisfactory pavement performance throughout their design service life. The durability of subgrade material is a major concern in cold regions to reduce the freeze thaw effects and material segregation. Different agencies developed several techniques to assess the impacts of frost heave action and freeze thaw cycling on the durability of stabilized pavement material. The freeze thaw durability test, tube suction and vacuum saturation are common techniques to determine the durability of stabilized subgrade soil. First two tests are done according to ASTM D560 and ASTM C593 respectively and vacuum saturation durability test is performed based on Texas Department of Transportation Test Method Tex-144-E. 2.9 Summary of Literature Review High quality subgrade material is essential for the overall pavement structural capacity and performance. Most of the natural subgrade soils are not suitable to provide the desired properties needed for satisfactory pavement performance. This poor pavement performance leads to lower ride quality and higher pavement maintenance cost. So, it is required to ensure desirable qualities of subgrade soil by developing proper stabilization technique. Portland cement and lime are not suitable for cold regions due to stresses related to thermal changes among the frequently used additives. On the other hand, production of traditional additives (cement, lime, fly ash, blast furnace slag) has significant environmental consequences. However, alkalinization of aluminosilicate material has potentiality to develop suitable binding property and the development of this technique for soil stabilization is still in its early stage. From the available literature it is evident that this alkalinization technique dominated by several factors including curing condition, type of materials, concentration of alkaline activator, as well as solid to liquid ratio. For this reason it is required to establish alkalinization technique separately for different soils and additives. There are only a few studies which investigated the application alkalinization technique for silty sand 29 subgrade soil. Likewise, the performance of this treated subgrade soil is not well studied against environmental effects and under repeated traffic loads. This study intended to evaluate the efficacy of curing condition (temperature and duration) on silty sand subgrade soil stabilized with bentonite magnesium alkalinization. Furthermore, freeze thaw durability test was conducted to assess the laboratory performance of the treated subgrade soil at cold climatic condition. 30 Chapter 3: Materials and Methods 3.1 Overview This research is a comprehensive approach to establish a new chemical stabilization technique along with heat curing for weak subgrade soil and the applicability of the additive validated through experimental investigation of the untreated and treated soils. To evaluate the effectiveness of newly formed additive and laboratory performance of treated subgrade soil two separate tests were conducted requiring preparation and testing of 216 specimens. This chapter presents the procedures and protocols used during the research project, including material characterization, specimen preparation and testing. 3.2 Materials The soil used in this research was collected from the subgrade layer exposed during the widening of Trans-Canada Highway 1, near Thompson River in Kamloops, British Columbia. After collecting the soil sample it was oven dried at 105°C until constant mass is reached. Then sieve analysis, standard compaction test, Atterberg limit and specific gravity tests were carried out. The soil is then classified in accordance to the AASHTO M145-91 (AASHTO 2017) and ASTM D2487-11 (ASTM 2011) soil classification system. Furthermore, an EDS analysis was performed to obtain elemental composition of the soil. Bentonite was supplied in dry powder form by Pacific Bentonite Ltd., a local miner from Kamloops, BC. The characterization of bentonite revealed from physical properties and EDS analysis. MgCl2 was chosen for this research due to its popularity as a sustainable additive to improve compressive strength, reduce swell pressure, dispersive characteristics of subgrade soil (Latifi et al. 2016; Latifi et al. 2015a & 2015b; Muhammad et al. 2018; Turkoz et al. 2014). Additionally, it can facilitate the formation of magnesium silicate hydrate (M-S-H) with silica rich soil in highly alkaline medium (Muhammad et al. 2018). Anhydrous magnesium chloride (MgCl2) 31 was used in this research to mix with soil in solid-state. Soil, bentonite and magnesium chloride (MgCl2) used in this research presented in figure 3.1. A 10M sodium hydroxide (NaOH) solution with a purity of 98%, and sodium silicate (Na2SiO3) in liquid form, were used as alkaline activators. The properties of MgCl2 and alkaline activators obtained from manufacturer are summarized in Table 3.1. (a) (b) (c) Figure 3.1 Image of (a) soil, (b) bentonite and (c) magnesium chloride, MgCl2 Table 3.1 Physical and chemical properties of chemical additives. Properties Magnesium Chloride (MgCl2) Sodium Hydroxide (NaOH) Sodium Silicate (Na2SiO3) Physical state Solid Liquid Liquid Appearance White to yellow crystal Colorless Colorless pH 5-6.5, 5% aq. solution 14 11.2 Relative density (g/cm3) 2.32 2.13 1.4 Solubility (g/L) 540 Soluble in water Soluble in water Molecular weight (g/mol) 95.205 39.997 122.062 Specific gravity - - 1.38-142 32 3.3 Mix Design The magnesium alkalinization additive consists of a mixture of MgCl2 and alkaline activator solution. For the magnesium alkalinization the percentage of MgCl2 was 3% of the total dry weight of mixture and the ratio of sodium silicate (Na2SiO3) to 10M sodium hydroxide (NaOH) was chosen as 0.5, whereas the ratio of magnesium chloride to alkaline activator was kept constant (0.7). The optimal mix design for magnesium alkalinization was selected based on Muhammad et al. (2018). In addition to magnesium alkalinization, raw calcium bentonite is used in this study as a source of calcium (Ca) and Si to improve the properties of the weak subgrade soil. Four different percentages of calcium bentonite (10%, 20%, 30% and 40%) were added to the dry weight of soil and magnesium alkalinization considering the total dry weight of mixture 100%. Increased percentages of calcium bentonite in the mixture indicated higher compressive strength. Finally, sample prepared with 57%, 40%, and 3% percentage of raw soil, calcium bentonite, and magnesium chloride respectively to the total of 100% dry weight of solids (raw soil + calcium bentonite +MgCl2) were chosen for heat curing. The optimum moisture content (alkaline activator + water) was calculated from a standard proctor test. The amount of water was calculated by subtracting the quantity of alkaline activator (0.7*dry weight of MgCl2) from the optimum moisture content. Deionized water was used throughout the experimental program to avoid any undesirable error from water quality. For the durability analysis samples prepared with 97% soil and 3% MgCl2 with alkaline activator and without alkaline activator were also tested. Finally the samples ere categorized as heated, unheated, 97% soil+3% MgCl2 (CM), 97% soil+3% MgCl2 +AA (CM Alkalinization). 3.4 Sample Preparation and Curing 3.4.1 Unconfined Compressive Strength (UCS) Homogeneity in soil mass was ensured for all samples by oven drying the collected soils at 105°C for 24 hours, and then sieving through a No. 10 (2mm) standard sieve. Soils retained on No. 10 33 sieve discarded considering the size of UCS samples because presence of larger particles can create inconsistency in the structure of different samples. Samples were prepared for UCS tests according to ASTM D4219-08 (ASTM 2008) by mechanically mixing a predefined quantity of soil with measured amounts of calcium bentonite, magnesium chloride, alkaline activator, and RO water for 5 minutes coherent to the mix design presented in Table 3.2. Table 3.2 Summary of mixture and curing design Mix mode Alkaline activator ratio (L) L/S ratio Soil Bentonite MgCl2 (Mass ratio of materials for 1kg mixture) Curing Temperature Duration Sample 1 0.5 0.7 0.57 0.4 0.03 40ËC 1,2,3 days Sample 2 0.5 0.7 0.57 0.4 0.03 60ËC 1,2,3 days Sample 3 0.5 0.7 0.57 0.4 0.03 80ËC 1,2,3 days The sample was compacted in three layers of equal height in a 76 mm height and 38mm diameter mold, using an instron machine. Minimum compaction energy was applied during the time of static compaction of each layer of the sample to achieve the maximum dry density without damaging the structure of soil particles. Next, the sample was extruded from the mould using a hydraulic jack. The samples were heat cured in three different temperatures (40°C, 60°C, and 80°C) for 1, 2, and 3 in an electrical oven to evaluate the effect of the heat curing temperature and duration days as depicted in Figure 3.1. The samples were placed in oven immediately after casting to avoid the loss of moisture. After the end of the curing period, the samples were removed from the oven, wrapped with a thin polyethylene sheet and stored in an air tight box to avoid escape of moisture from the sample as evident in Figure 3.2. The boxes with samples were stored in a temperature-controlled room at 22°C until the date of testing. The UCS of the samples was tested at 3, 7, 14, 28, and 60 days of age. 34 Figure 3.2 Heat curing in oven (a) (b) Figure 3.3 Storing of samples (a) Wrapping & (b) Air tightened in zip lock bag and placed in plastic box 35 3.4.2 Freeze Thaw Durability Freeze thaw durability analysis conducted on the raw subgrade soil, optimal heat cured, unheated sample, CM and CM Alkalinization. The mix design, curing condition and freeze thaw cycles are noted in table 3.3. The sample was prepared by following the same procedure used in UCS sample preparation. After preparing the sample and initial heat curing, it was stored in the temperature-controlled room at 22°C following the same procedure used to store the UCS samples. The freeze thaw cycles of the raw soil started right away after preparing the samples. On the other hand the freeze thaw cycles of the samples: heated, unheated, treated with magnesium chloride, Magnesium Chloride + Alkaline Activator started at 7 and 28 days of age. Table 3.3 Summary of mixture and curing design for freeze thaw durability test Mix mode Alka-line ac-tivator ratio (L) L/S ratio (S) Soil Bentonite MgCl2 (Mass ratio of materials for 1kg mixture) Heat curing temper-ature Heat curing dura-tion Start of freeze thaw cy-cles at age Raw soil 0 0 0 0 0 0 ËC 0 days 0 days Heated BM 0.5 0.7 0.57 0.4 0.03 60ËC 3 days 7, 28 days BM 0.5 0.7 0.57 0.4 0.03 0 ËC 0 days 7, 28 days CM 0 0 0.97 0 0.03 0 ËC 0 days 7, 28 days CM Alka-linization 0.5 0.7 0.7 0 0.03 0 ËC 0 days 7, 28 days 3.5 Testing Methods 3.5.1 Particle Size Distribution of Soil Particle size distribution of the soil was determined by sieve analysis followed by hydrometer test. The sieve analysis performed to determine the distribution of the coarser or larger-sized particles, and the hydrometer test was used to determine the distribution of the finer particles (i.e., material passing No. 200 sieve=0.075 mm) in soil. The results from these tests were used to classify the soil in accordance with the Unified Soil Classification System (USCS). 36 3.5.1.1 Sieve Analysis Sieve analysis of the soil was performed according to ASTM D6913 (ASTM 2017). Around 500g of oven dried raw silty sand was taken and passed through a stack of sieves (3/4", 3/8", No.4, No.10, No.20, No.40, No.40, No.60, No.140 and No.200). The coarse sieve was placed at the top of the stack, followed by increasingly finer sieves below. A pan was placed below the bottom sieve to collect the soil that passed the finest sieve. By weighing the fraction retained by each sieve, points on the gradation curve was calculated. 3.5.1.2 Hydrometer Test Hydrometer test was completed following ASTM D7928 - Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis (ASTM 2017). Around 50g of fine soil that passed the No. 200 sieve during the mechanical sieve analysis was used for this test and which was soaked in 125 ml sodium hexametaphosphate (40 g/L) solution for 16 hours. After that the slurry was transferred to an ASTM D7928 specified dispersion cup and blended in the stirring apparatus at a rate of 10,000 rpm for 1 minute. The slurry was then transferred to the sedimentation cylinder, RO water was added to the cylinder until the 1000 ml mark and the cylinder was repeatedly turned upside down and back for one minute. The mechanism behind hydrometer analysis is the soil particles initially are suspended in the liquid mixture, but settle over time. Larger particles settle faster in accordance with Stokesâ Law, which states the diameter of a spherical particle is proportional to the square root of its settling velocity. As smaller and smaller particles settle past the center of mass of the hydrometer with the passage of time, the density of the slurry affecting the buoyancy of the hydrometer decreases, and the hydrometer floats lower and lower in the slurry. Measurement of the hydrometer position in the slurry was recorded as a function of time at 2, 4, 8, 16, 30, 60, 120, 240 and 1440 minutes, and was used to calculate grain size versus percent passing for the 37 gradation curve. Readings for correction factor from control cylinder and water temperature were also recorded at each time interval. The results obtained from sieve and hydrometer analysis produced a tabulation of particle size versus percent passing that was graphically presented as the particle size distribution curve. 3.5.2 Atterberg Limit Test In the laboratory the Atterberg limits were determined as per ASTM D4318 - Standard Test Method for Liquid Limit, Plastic Limit, and Plasticity Index of Soils (ASTM 2017). Each test was repeated three times to ensure the validity of data and the average was reported as final result. 3.5.2.1 Liquid Limit (LL) The liquid limit is the moisture content that defines where the soil changes from a plastic to a viscous fluid state. Soil sample for liquid limit test was prepared by air drying the soil and then sieving through a standard #40 (0.475 mm) sieve. Then 100g of sieved soil was taken and thoroughly mixed with small amount of RO water until it turns into a smooth uniform paste. The test was performed using the Casagrande liquid limit apparatus. Then the soil paste was placed in the cup and using grooving tool a 1/2â (13 mm) groove was made down the center of the cup. The handle of liquid limit test device was revolved at the rate of 120 rpm. The number of blows was recorded to make the two halves of the soil pat come into contact at the bottom of the groove along a distance of 13 mm (1/2â).This process was repeated 5 times with different water contents. A graph was plotted between water content vs. no of blows in a semi-log paper. From this curve the water content was determined against 25 blows which the liquid limit of soil. 38 3.5.2.2 Plastic Limit (PL) The plastic limit is the moisture content that defines where the soil changes from a semi-solid to a plastic (flexible) state. The soil was prepared similarly as the liquid limit test. RO water was added and the moist soil paste was rolled on a ground-glass plate into a thread of 3.2 mm diameter by hand. The process was repeated by squeezing and re-rolling the sample until starts to crumble for a thread diameter larger than 3.2 mm. The process was repeated for five times and five moisture content was obtained. The average moisture content was noted as the plastic limit of the soil sample. 3.5.2.3 Plasticity Index (PI) Plasticity index is defined as the difference between liquid limit and plastic limit. The value was calculated from the difference of liquid limit and plastic limit of the soil sample. PI=LL-PLâ¦â¦â¦â¦â¦â¦â¦â¦â¦â¦ (1) 3.5.3 Compaction Test The standard proctor test was performed according to ASTM D698 (ASTM 2015) to determine the maximum dry density and optimum moisture content of the soil. At first, 3 kg of soil was taken which was sieved through a #4 sieve (4.75 mm). The target moisture content was started from 8% and continued in an increment of 2% until the soil mass started to decrease. RO water was used throughout the entire test. The mold along with the clamp was assembled to the solid base on floor. The soil was compacted in three layers using 25 blows in each layer. The compaction collar was removed and extended soil at top of mold was scraped using a knife. Then the mass of soil and mold was measured. The soil was collected from each layer of soil column in the mold and placed in oven for dry mass. Finally necessary calculations were made to calculate maximum dry density and optimum moisture content. 39 3.5.4 Specific Gravity Test Specific gravity is the ratio of the mass of unit volume of soil solid to the mass of the same volume of gas-free deionized water at a stated temperature (20ºC). The specific gravity of the soil was determined according to ASTM D854 (ASTM 2014). The soil was air dried and sieved on sieve #10 (2 mm). The pycnometer was calibrated in standard manner. Then 50 gm of soil sample was taken and dispersed in 100 ml of RO water by a mixer. Then the soil water slurry was transferred in a 500 ml pycnometer and filled with equilibrium temperature de-aired RO water to half of original volume of pycnometer. Any soil adhering to the pycnometer rinsed into the slurry. After that, vaccum was applied for removal of all air bubbles and it took around 2 hours. After vacuuming, more de-aired RO water was added until the 500 mL mark was reached, then the pycnometer was placed in an insulated container over night for temperature equilibrium. The mass and temperature of pycnometer, soil, and water was measured together next day. 3.5.5 Swelling Index Test The swell index test procedure according to ASTM D5890 (ASTM 2006) was used to determine the general swelling characteristics of bentonite clay. 90 ml RO water was taken into a 100 ml graduated cylinder. 2g sample of dried and finely ground bentonite clay was dispersed into the cylinder in 0.1g increments to perform the test with a minimum interval of 10 minutes passed between additions to allow for full hydration and settlement of the clay to the bottom of the cylinder. These steps were followed until the entire 2g sample was added to the cylinder. The sample was then covered and protected from disturbances for a period of 16 - 24 hours, at which time the level of the settled and swollen clay was recorded to the nearest 0.5 ml. 3.5.6 Unconfined Compressive Strength (UCS) The UCS test is a widely used testing method to assess the improvement in strength of treated soil with different admixtures. According to ASTM D4609-08 (ASTM 2008), an increase of 0.345 40 MPa or more in UCS proves the effectiveness of an additive for stabilizing the soil. Compressive strength of the samples was measured using a GDS automatic oedometer system for fine grained soil as depicted in Figure 3.3 at the end of the curing periods of 3, 7, 14, 28, and 60 days. The tests were conducted according to ASTM D4219-08. The vertical load was kept constant at 1 mm/min until the failure occurred. For consistency, at least four replicates of each sample were tested to report the result for a single condition. Figure 3.4 Testing of UCS sample 3.5.7 Elastic Modulus Soil Young's modulus (E) commonly represents soil elastic modulus, and a measure of soil stiffness. It is defined as the ratio of the stress along an axis over the strain along that axis in the range of elastic soil behaviour. Elastic modulus of the treated soil was measured from the stress strain behaviour obtained from UCS data. E50 was calculated by selecting axial strain at 50 % of maximum deviator stress and calculating the secant modulus (Siddiqua et al. 2014). The elastic modulus is often used as an indicator of soil settlement and elastic deformation properties. 41 3.5.8 Freeze Thaw Durability Test Durability refers to the ability to remain functional longer design life without requiring excessive maintenance. In cold region the freeze and thaw cycles lead to the deterioration of subgrade and a matter of major concern for satisfactory pavement performance. In this study the freeze thaw durability of the raw and treated soil at two different conditions evaluated. Three replicates sample for each conditions were prepared and freeze thaw test was performed according to ASTM D 560. For each additive concentration and curing conditions every sample was prepared, compacted, extruded, and cured as described in section 3.4. The raw sample after preparing immediately placed a chest freezer at -21°C for 24 hours. On the other hand the heated sample and unheated sample placed in the chest freezer at age 7 and 28 days. Following the freezing period, specimens were removed from the chest freezer and thawed at room temperature for 24 hours as displayed in Figure 3.4. This process of consecutive freezing and thawing comprised one freeze-thaw cycle. As prescribed in ASTM D 560, specimens were subjected to 12 freeze thaw-cycles in total. After 12 cycles were completed, the circumference of each specimen was measured and then the UCS testing conducted as described previously using a GDS automatic oedometer system for fine grained soil. During each thawing period, care was taken to place specimens treated with the same stabilizer together in order to prevent cross contamination of stabilizers in the event that leaching occurred. 42 (a) (b) (c) Figure 3.5 Thawing configuration for freeze-thaw testing (a) Raw sample (b) Heated sample (c) Unheated sample 3.5.9 Field-Emission Scanning Electron Microscopy (FESEM) and Energy-Dispersive Spectroscopy (EDS) The morphology of the raw and treated silty sand subgrade soil structures (i.e. size, shape, orientation, aggregation of soil particles) was examined by FESEM. A TESCAN MIRA3 XMU was used for SEM. EDS analysis was performed to characterize the major components of the treated soil using an Oxford Aztec X-max EDS system. Fine powder was made for both the SEM and 43 EDS analysis by grinding the soil collected from the mid layer of sample treated at different curing conditions and temperatures in a trace free environment and dried at a glued surface at 105°C for 24 hours. After drying, the powdered samples were cooled to reach ambient temperature and finally coated using palladium and platinum at a ratio of 20/80 in a high vacuum argon atmosphere until the coating thickness reached 15 µm at 0.002 kPa. 3.5.10 Thermogravimetric Analysis (TGA) The instrument used for thermogravimetric analysis was a NETZSCH TG 209 F1 Libra. The analysis was carried out on approximately 20 milligrams of fine samples in an aluminum crucible in a nitrogen environment. Temperatures ranged from room temperature to 1000ºC with an increase of 10ºC/min. Results of the TGA analysis were presented as mass loss versus temperature (TGA) curve, and first derivative of mass loss as a function of time versus temperature (DTG) curve. 44 Chapter 4: Results and Discussions 4.1 Overview This chapter outlines and analyzes the results obtained from the experimental approach described in chapter 3. The objective of this chapter is to characterize the raw subgrade soil, evaluate the effectiveness of newly formed additive to stabilize problematic silty sand subgrade soil by UCS test and microstructural analysis, finally performance evaluation of treated sample at adverse environmental condition by freeze-thaw durability test at laboratory conditions. 4.2 Material Characterization Particle size distribution curve of the silty sand subgrade soil used in this research is obtained from the results of sieve and hydrometer analyses. The particle size distribution curve in Figure 4.1 revealed that the soil has approximately 11% gravel, 64% sand, and 25% fines (i.e. silt and clay). Whereas Figure 4.2 presents the compaction curve obtained from standard proctor test of the raw silty sand. The physical properties of the soil are summarized in Table 4.1. The soil is classified as A4 in accordance to the AASHTO M145-91 (AASHTO 2017) soil classification system. Correspondingly, ASTM D2487-11 (ASTM 2011) characterizes the soil as well graded non-plastic silty sand (SM). Both classification systems describe the soil as fair to poor subgrade material. The result of EDS analysis of the raw soil presented in Table 4.2 and the dominant quantity of compositional silica as well as alumina proves the potentiality of the soil for alkali activation. Physical properties and compositional analysis of the bentonite were performed and enlisted in Table 4.1 & 4.2 respectively. The bentonite is classified as low swelling calcium bentonite, containing 41.1% montmorillonite. Odom (1984) stated that calcium bentonite falls in the Ca-Mg smectite category of smectite clay minerals. Odom also added that smectite clay minerals consist of two tetrahedral and one octahedral sheet, and these sheets contain tetravalent 45 (Si), trivalent (Al3+, Fe3+), and divalent (Mg, Fe) which are exchangeable. These characteristics satisfy the suitability of calcium bentonite for alkali activation as reported above. Figure 4.1 Particle size distribution of natural subgrade soil Figure 4.2 Compaction curve of natural silty sand subgrade soil 0204060801000.001 0.01 0.1 1 10 100% Finer by WeightParticle Diameter (mm)1.61.661.721.781.841.90 5 10 15 20 25Dry density (g/cm3)Water content (%)46 Table 4.1 Physical properties of the raw silty sand and bentonite Properties Soil Bentonite Liquid limit, LL (%) Plastic limit, PL (%) Plasticity index, PI (%) Specific gravity Fines fraction (passing size 75µm, %) Maximum dry density (kN/m3) Optimum moisture content (%) Swelling Index (mL/2g) Cation Exchange Capacity (meq/100 g) 20.63 19.80 0.83 2.7 25.5 17.85 12.8 - - 209211882.49-2.7297.2- - 12 51 Table 4.2 Compositional elements of the raw silty sand and calcium bentonite Elements Soil (wt. %) Bentonite (wt. %) O - 56.52 Si 49.46 24.24 Al 15.2 9.83 Fe 18.93 3.61 Na 3.09 1.94 Ca 9.85 1.41 Mg 3.46 1.34 Ti - 0.56 K - 0.55 47 4.3 Unconfined Compressive Strength (UCS) Figure 4.3 â 4.11 represents the stress strain curves obtained from unconfined compressive strength test for untreated raw soil and heat cured specimens. The results show that the heat curing significantly increased UCS of bentonite magnesium alkalinized samples in comparison to untreated raw samples. It is evident from analyzing the stress strain curves at all curing conditions that samples heat cured at 60ºC for 2 and 3 days as well as 80ºC for 1 to 3 days (Figure 4.7- 4.11) showed similar stress strain behavior through out the curing time. In addition, curves showed peak UCS which indicates dense structure and ductility also reduced. Figure 4.3 Stress strain behavior of raw soil and samples heat cured at 40ºC for 1 day 02004006008001000120014000 1 2 3 4 5Axial Stress (kPa)Axial Strain (%)Raw soil3 days7 days14 days28 days60 daysTemperature: 40ºCDuration: 1 day48 Figure 4.4 Stress strain behavior of raw soil and samples heat cured at 40ºC for 2 days Figure 4.5 Stress strain behavior of raw soil and samples heat cured at 40ºC for 3 days 02004006008001000120014000 1 2 3 4 5Axial Stress (kPa)Axial Strain (%)Raw soil3 days7 days14 days28 days60 daysTemperature: 40ºCDuration: 2 days02004006008001000120014000 1 2 3 4 5Axial Stress (kPa)Axial Strain (%)Raw soil3 days7 days14 days28 days60 daysTemperature: 40ºCDuration: 3 days49 Figure 4.6 Stress strain behavior of raw soil and samples heat cured at 60ºC for 1 day Figure 4.7 Stress strain behavior of raw soil and samples heat cured at 60ºC for 2 days 02004006008001000120014000 1 2 3 4 5 6 7Axial Stress (kPa)Axial Strain (%)Raw soil3 days7 days14 days28 days60 daysTemperature: 60ºCDuration: 1 day02004006008001000120014000 1 2 3 4 5Axial Stress (kPa)Axial Strain (%)Raw soil3 days7 days14 days28 days60 daysTemperature: 60ºCDuration: 2 days50 Figure 4.8 Stress strain behavior of raw soil and samples heat cured at 60ºC for 3 days Figure 4.9 Stress strain behavior of raw soil and samples heat cured at 80ºC for 1 day 02004006008001000120014000 1 2 3 4 5Axial Stress (kPa)Axial Strain (%)Raw soil3 days7 days14 days28 days60 daysTemperature: 60ºCDuration: 3 days02004006008001000120014000 1 2 3 4 5Axial Stress (kPa)Axial Strain (%)Raw soil3 days7 days14 days28 days60 daysTemperature: 80ºCDuration: 1 day51 Figure 4.10 Stress strain behavior of raw soil and samples heat cured at 80ºC for 2 days Figure 4.11 Stress strain behavior of raw soil and samples heat cured at 80ºC for 3 days 02004006008001000120014000 1 2 3 4 5Axial Stress (kPa)Axial Strain (%)Raw soil3 days7 days14 days28 days60 daysTemperature: 80ºCDuration: 2 days02004006008001000120014000 1 2 3 4 5Axial Stress (kPa)Axial Strain (%)Raw soil3 days7 days14 days28 days60 daysTemperature: 80ºCDuration: 3 days52 Table 4.3 shows peak UCS of treated samples obtained from stress strain curves. As discussed in Chapter 3, UCS test was performed on four replicate specimens for each curing condition to show specimen variation. Table 4.3 presents the average UCS from four replicate specimens at each curing condition. Table 4.3 Unconfined compressive strength test results Curing time Curing temperature Heat curing duration (days) Axial stress (kPa) 3 days 40â¦C 1 647.55 2 916.504 3 832.96 60â¦C 1 770.001 2 979.637 3 881.992 80â¦C 1 1145.59 2 1065.38 3 1307.21 7 days 40â¦C 1 674.146 2 930.599 3 791.207 60â¦C 1 540.3 2 926.014 3 932.899 80â¦C 1 1145.13 2 1055.6 3 1242.2 14 days 40â¦C 1 597.83 2 890.43 3 823.15 60â¦C 1 719.7 2 833.37 3 943.13 80â¦C 1 947.19 2 993.31 3 1016.1 53 Curing time Curing temperature Heat curing duration (days) Axial stress (kPa) 28 days 40â¦C 1 508.52 2 683.11 3 725.93 60â¦C 1 806.1 2 850.7 3 902 80â¦C 1 1012.6 2 927.37 3 1048.3 60 days 40â¦C 1 494.58 2 606.67 3 561.34 60â¦C 1 829.2 2 834.16 3 894.07 80â¦C 1 935.04 2 923.30 3 1002.6 Figure 4.12 â 4.14 graphically represents the average of the maximum UCS and standard deviation obtained from the four replicates of sample cured at different heat curing conditions at the end of curing time (3, 7, 14, 28, and 60 days). As shown in Figure 4.12 â 4.14, a noticeable increase in UCS was observed for all heat curing conditions compared to the UCS of raw silty sand 141 kPa. The accelerated temperature lead to gaining high initial strength at 3 days of curing, but the strength decreased at later age due to rapid setting of the sample (Rovnaník 2010). The reduction in strength at the end of 60 days is higher for samples cured at 40ºC and 80ºC. 54 Figure 4.12 Influence of heat curing temperature and time on compressive strength of bentonite magnesium alkalinization treated soil heat cured at 40ºC for 1, 2, 3 days It can be observed from Figure 4.12 that samples heat cured at 40ºC began to experience loss in strength after 7 days of curing. At the end of 60 days, the difference between initial strength at 3 days and strength at 60 days was around 150 kPa, 310 kPa, and 260 kPa for 1, 2, and 3 days heat curing duration respectively. 55 Figure 4.13 Influence of heat curing temperature and time on compressive strength of bentonite magnesium alkalinization treated soil heat cured at 60ºC for 1, 2, 3 days Variation in strength was observed for samples cured at 60ºC throughout their age, as shown in Figure 4.13. Samples heat cured at 60ºC for 1 day showed a greater variability compared to the other heat curing durations (2 and 3 days). Initially high strength gain was noticed for samples heat cured for 2 days at 60ºC; however, a reduction in strength continued until 14 days of age, then increased slightly at 28 days age, and then remained almost constant until 60 days age. The most prominent strength gain observed for samples heat cured at 60ºC for 3 days. In this case, the initial strength continued to increase and reached 943 kPa at 14 days. A very unnoticeable reduction in strength was observed at 28 days of curing. The samples did not experience any further strength loss and the compressive strength was around 900 kPa at the end of 60 days curing age. 56 Figure 4.14 Influence of heat curing temperature and time on compressive strength of bentonite magnesium alkalinization treated soil heat cured at 80ºC for 1, 2, 3 days On the other hand, in Figure 4.14, samples heat cured at elevated temperature 80ºC exhibited significant increase in strength during all heat curing periods as the elevated curing temperature increased the alkali activation of aluminosilicate materials present in the sample. Samples which cured at 80ºC temperature for 1 and 2 days exhibited same strength until 7 days of age, but at 14 days, the strength decreased by 200 kPa for 1-day heat cured samples and ended at 935 kPa at 60 days. The reduction of strength for 2 days curing duration was lower than the 1 day heat cured sample, with a final strength of 923 kPa at the end of 60 days. For heat curing duration of 3 days the strength dropped by 300 kPa at the end of 60 days, compared to the strength at 3 days age. 57 The trends observed in UCS results of this study comply with the findings of Heah et al. (2011); Kamarudin et al. (2011); Khale and Chaudhary (2007). However, the setting and hardening mechanism of the geopolymer is complex. The variations in strength throughout the curing time suggest that curing temperature and duration of geopolymer greatly influence both its initial setting and hardening for final strength development. In addition, only a sufficient amount of heat is needed for reactivity of aluminosilicate material because the unreactive aluminosilicates act as an aggregate in the geopolymeric binder to improve strength (Provis et al. 2005). For example, heat curing at lower elevated temperature (40ºC for 1, 2, 3 days and 60ºC for 1, 2 days) initiate early age setting but not sufficient for the increased reactivity of aluminosilicates and unable to form a hard, compacted structure (Phair et al. 2000). This results in non consistent strength development. Whereas, desirable strength development observed due to heat curing at 60ºC for 3 days because the favorable presence of unreactive aggregate particles formed stronger geopolymeric structure. On the other hand, high elevated curing temperature for longer duration increases dissolution of aluminosilicates at higher rate and leads to form larger crystals. According to Cioffi at al. (2003) larger crystals loosely pack at the binder phase and thus form porous structure. Besides, heat curing at higher temperature for prolonged duration leads to quick setting of the structure and rapid solidification to gain high early strength. But it retards the rate of the pozzolanic reaction due to gel contraction and lack of unreactive aluminosilicate aggregate materials produce weak structure with increased possibility of shrinkage (Heah et al. 2011; Kamarudin et al. 2011; Khale and Chaudhary 2007). According to the authors, the formation of porous and weak structures lead to higher strength loss at later age with increased curing time which is identical to the findings of this study. So, temperature beyond 80ºC would not be ideal for the proposed mix design of soil and additive used in this study. The recommended temperature for heat curing of alkali activated bentonite and magnesium additive is 60ºC for 3 days when a prolonged curing duration is preferable because these samples indicated most consistent gain in strength after 28 days of curing. Consequently, a curing period of 1 day is sufficient for higher 58 temperature (80ºC) when quick early strength gain is required. The anticipation of these findings is based on curing up to 60 days only, to obtain more insight on strength behaviour study at longer curing time is recommended. 4.4 Elastic Modulus Soil strength and deformation properties determine the stability of the subgrade (Thompson 1979). Higher values of elastic modulus indicate structured materials (Hicher 1993) and resistance to elastic deformation. Consequently, the elastic modulus is an important parameter for the subgrade soil treated with a new form of additive, as used in this study. Youngâs modulus of elasticity was calculated from the stress strain curve obtained from the UCS test. E50 represents the secant modulus at 50% of the maximum stress obtained from the UCS test (Siddiqua et al. 2014). The E50 values of heat cured samples at 60ºC and 80ºC for 2 and 3 days were measured at curing ages of 3, 14, and 60 days, and are represented in Figure 4.15. The E50 of raw silty sand was calculated as 4.55 MPa. Samples heat cured at 60 ºC for 3 days and 80ºC for 2 days attained 7 times more elastic modulus at 60 days of curing compared to the raw soil due to the addition of newly formed additive and heat curing. 59 Figure 4.15 Variation of elastic modulus (E50) of the heat cured samples at 60ºC and 80ºC for 2 and 3 at the age of 3, 14, 60 days Based on Obrzud & Truty (2010) compiled from Kezdi (1974), the elastic modulus value of 30 MPa indicates a dense sample structure. It is evident from Figure 4.15 that the heat curing time and temperature has significant influence on the elastic modulus. Initially, the elastic modulus was lower for samples heat cured at 60ºC for 2 days, compared to other heat cured samples which possess higher elastic modulus. The modulus of elasticity decreased for all heat cured samples at 14 days from their highest values observed at 3 days of age. Further increase in the elastic modulus was again observed at 60 days for all samples except the samples heat cured at 80ºC for 3 days. This observation is identical to the conclusion made by Lydon et al. (1995), strength and stiffness are related to porosity. The formation of geopolymeric cementitious gel reduced the porosity and increased the strength as well as stiffness of the subgrade material. 60 4.5 Microstructural Analysis 4.5.1 Field-Emission Scanning Electron Microscopy (FESEM) Microstructural morphology of raw soil and heat cured samples were investigated by FESEM analysis. The SEM micrograph for raw silty sand subgrade soil is presented in Figure 4.16. From the image, it appears that soil particles are dispersed, discontinuous and flocculated without the presence of any visible bonding among them. These orientation and lack of aggregation of soil particles indicate higher porosity and voids due to absenteeism of any hydration products (Latifi et al. 2015b; Muhammad et al. 2018). Figure 4.16 Micrograph from SEM analysis of raw silty sand subgrade soil Similarly, FESEM analysis was conducted on stabilized samples heat cured at two desired curing temperatures 60ºC, 80ºC for 2 and 3 days at the curing time of 3, 14, and 60 days to investigate their morphological changes throughout the curing process. The treated samples for FESEM analysis were chosen based on the UCS results. Figure 4.17(a-c) presents the SEM micrographs of samples heat cured at 60ºC for 2 days at 3, 14, and 60 days of curing respectively. From these images it is evident that the existence of gel like products are visible at 3 days of curing and the development of this gel like substance was not significantly with time (14 days and 60 days). Although this gel like substance reduced porosity in soil structure and increased 61 aggregation of soil particles compared to the raw soil but still there is notable indication of void spaces in soil structure. (a) (b) (c) Figure 4.17 Micrographs from SEM analysis for samples heat cured at 60ºC for 2 days at curing period of (a) 3 days; (b) 14 days and (c) 60 days. Images are magnified to 10000x (a) (b) (c) Figure 4.18 Micrographs from SEM analysis of samples heat cured at 60ºC for 3 days at curing period of (a) 3 days; (b) 14 days and (c) 60 days. Images are magnified to 10000x Whereas, in Figure 4.18(a-c), the presence of similar substance also identified in the SEM micrographs of samples heat cured at 60ºC for 3 days. From Figure 4.18(a) it is evident that the gel like substance started to grow at 3 days of curing and increased in amount with curing time 62 (14 days). Finally, at 60 days of curing the gel like substance apparently filled all the pores and strongly bonded the soil particles together. The soil structure appeared as a continuous connected material with noticeably compacted structure. The comparison of micrographs in Figure 4.17 and 4.18 for all curing days indicates that samples heat cured at 60ºC for 3 days indicated higher development of gel like substance and highly compacted continuous soil structure compared to samples heat cured at 60ºC for 2 days. Furthermore, Figure 4.19(a-c) and Figure 4.20(a-c) illustrate the SEM micrographs of samples heat cured at 80ºC for 2 and 3 days respectively. Noticeable changes in microstructure was observed from both of the figures. It is evident from the micrographs that the formation of gel like substance started at early age and evolved with time for samples heat cured at 80ºC for 2 days , this behavior is similarly recognized for samples heat cured at 60ºC. This gel like products which made the samples compacted and aggregated over time, further recognized as M-S-H/ M-A-H and C-S-H from EDS as well as thermogravimetric analysis. The M-S-H/ M-A-H and C-S-H gel are cementitious in nature and the main hydration products of cement stabilization (Ilkentapar et al. 2017; Latifi et al. 2015 a&b). The development of these gel-like materials was triggered by the dissolution of silica and alumina in the soil in highly alkaline medium created by NaOH (10M) and Na2SiO3, and by the high concentration of Mg2+ ions resulted from destabilization of magnesium chloride in a heated environment (Bernard et al. 2017; Muhammad et al. 2018). The production of this cementitious gel increased the UCS value as observed in section 4.3 of this research and confirmed the similarly findings made by Latifi et al. (2015a) and Muhammad et al. (2018). However, samples heat cured at 60ºC for 2 days and 80ºC for 3 days initially showed a more compacted structure at 3 days of curing and with time the gel like substance contracted and soil particles became segregated. These results comply with the statements made in earlier studies by Bernard et al. (2017) and Kamarudin et al. (2011). According to the authors the ratio of solid to water should be sufficient for initiating reaction kinetics for optimal cementitious product 63 development at different heat curing conditions as too low temperatures would not be conducive for reaction kinetics due to high moisture content, and too high temperatures would dry out the sample. (a) (b) (c) Figure 4.19 Micrographs from SEM analysis of samples heat cured at 80ºC for 2 days at curing period of (a) 3 days; (b) 14 days and (c) 60 days. Images are magnified to 10000x (a) (b) (c) Figure 4.20 Micrographs from SEM analysis for samples heat cured at 80ºC for 3 days at curing period of (a) 3 days; (b) 14 days and (c) 60 days. Images are magnified to 10000x 64 4.5.2 Energy-Dispersive Spectroscopy (EDS) The elemental composition of the selected optimum sample from the EDS analysis is presented in Table 4.4. The major components observed in the samples were: Si, Mg, Al, Na, Ca, and O. The concentration of Si, Al, and Mg increased with curing time as the heated environment favoured the development of these products due to the dissolution of alkali activated materials. According to Vargas et al. (2011), the ratio of Si/Al should be in the range of 2.5-3.8 to develop the optimum strength in alkali activated geopolymer. The ratio of Si/Al used in this study is consistent with the desirable ratio for all curing durations. A noticeable increase in the Mg concentration observed after 14 days and at 60 days is almost around 3 times that of the initial concentration. Over time, these increased concentrations of Si, Al, and Mg led to the formation of a more cementitious gel, developing a compact structure without further strength loss (Muhammad et al. 2018; Favier et al. 2013). This result is also supported by the obtained SEM image. Table 4.4 Concentration of selected chemical elements in optimum sample at age 3, 14 and 60 days. Element 3 days 14 days 60 days Heat cured at 60°C for 3 days Heat cured at 80°C for 2 days Heat cured at 60°C for 3 days Heat cured at 80°C for 2 days Heat cured at 60°C for 3 days Heat cured at 80°C for 2 days Si 28.76 34.56 31.07 37.49 46.29 46.61 Mg 3.62 3.58 4.98 4.27 13.23 12.22 Al 12.12 10.63 13.21 13.65 18.39 19.74 Na 2.25 2.95 0.95 1.25 0.9 0.77 Ca 1.3 0.89 0.88 - 0.7 0.76 O 51.95 47.39 48.91 43.34 - - 65 4.5.3 Thermogravimetric Analysis TGA analysis was conducted to further investigate the cementitious bond observed from the SEM analysis. Although the SEM analysis indicated the presence of gel like substances for both samples heat cured at 60ºC for 3 days and at 80ºC for 3 days; but only the samples heat cured at 60ºC for 3 days were chosen for thermogravimetric analysis due to consistent strength pattern throughout the curing time. The results obtained from TGA analysis are presented in Figure 4.21 as mass loss versus temperature (TGA curve), and the first derivative of mass loss versus temperature (DTG curve). (a) 66 (b) Figure 4.21 TGA and DTG analysis of samples heat cured at 60ºC for 3 days after (a) 3 days and (b) 60 days of curing Notable mass loss or endothermic peak has been observed from both curves after 3 days and 60 days of curing at around 50ºC to 200ºC, and 200ºC to 300ºC, due to the loss of pore water or water present in the pozzolanic reaction products (C-S-H, M-S-H, C-A-H, C-A-S-H) (Haha et al. 2011; Zhang et al. 2014). Few exothermic and endothermic peaks were observed around 850ºC due to the crystallization of M-S-H; increased intensity of these peaks with curing time indicates increased M-S-H crystallization with time as a result of ongoing hydration (Zhang et al. 2014). The C-S-H does not possess a strong structure, but its ability of forming continuous bonds makes the particle more aggregated, thereby increasing the strength. On the other hand, 67 the development of M-S-H gel is quite low at lower curing periods, but the extent of formation increases with time (Zhang et al. 2018). The authors also added that M-S-H gel forms a shell-like structure on the surface of the particles and does not fill the pore spaces. The combination of C-S-H and M-S-H for aggregation and surface bonding of the particles resulted in a more stable development of compressive strength with increased curing time. 4.6 Effcets of Freeze Thaw Cycles on UCS Values The physical damage of the samples after 12 freeze thaw cycles examined by UCS test. Table 4.4 represents the comparison of UCS, moisture content of control samples and samples exposed to freeze thaw conditions. Hyphens in the table indicates data that were not determined due to specimen deterioration. All the specimens tested showed a reduction in unconfined compressive strength and increase in moisture content as depicted in Table 4.5. The phenomenon of losing strength was higher for the samples introduced at freeze thaw cycles after 28 days of curing. Samples treated with magnesium chloride and magnesium chloride alkalinization were deteriorated after single freeze thaw cycle and completely unable to handle. On the other hand, the optimally prepared heat cured sample (60°C for 3 days) retained very negligible amount of its initial strength after the 12 cycles of freeze thaw. The retained strength was almost same for both 7 and 28 days curing period. Whereas the raw soil retained 25.5% of UCS value after 12 cycles of freeze thaw. The highest percentages of UCS i.e.74% and 32.07% at 7 days and 28 days curing condition respectively was conserved by optimally prepared unheated sample. Solanki et al. (2013) explained that the reduction in UCS triggered by the combined effects of pore structure and increase in moisture content during the thawing of freeze thaw cycles. An increase in moisture content during the thawing phase results in the formation of ice lenses within the void space of the specimens in the freezing phase, distorts the soil structure. 68 Table 4.5 UCS and moisture content comparison between control and exposed samples Sample type Control moisture content (%) Exposed moisture con-tent (%) Control UCS (kPa) Exposed UCS (kPa) Retained UCS (%) 0 days curing Natural soil 12.8 14.9 141 36 25.5 7 days curing BM 22.2 24.5 133.8 99.12 74 Heated BM 20.5 25.1 932.9 34.77 3.7 CM 16.8 - 145 - 0 CM alkalinization 15.2 - 143.25 - 0 28 days curing BM 20.4 25.5 168.98 54.2 32.07 Heated BM 19.1 26.9 902 30.53 3.8 CM 17.9 - 165.9 - 0 CM alkalinization 18.6 - 204 - 0 Similar phenomenon observed in the results of this study, Magnesium chloride has the natural tendency of absorbing higher moisture which resulted in more ice lens formation in the samples treated with only magnesium chloride and magnesium chloride alkalinization. However optimally prepared heated samples absorbed 2.3% and 2.7% more moisture at 7 days and 28 days curing conditions respectively compared to the unheated samples. This absorption of higher moisture content by heated samples resulted from the reduction of granular moisture content from heat curing. The alkalinization process of heat cured samples consumed some structural moisture during heat curing. Likewise after 28 days of curing samples lost some of their inter-granular moisture. This reduction in moisture content increased the affinity to absorb more moisture during thawing period and prompted to more ice lenses formation during freezing in both cases. Alternatively, raw soil absorbed only 2.1% of moisture as sandy soil has less affinity to absorb water but the poor microstructure of the soil prompted to more loss of strength after the freeze thaw cycles. 69 Chapter 5: Conclusions and Future works 5.1 Summary Identifying new eco friendly soil stabilizers for stabilizing weak subgrade soils to obtain sustainable roadway infrastructure has become essential. This research focuses to establish a new green additive for silty sand subgrade soil and also to evaluate the consequences of heat treatment on geopolymerization process of the newly formed additive. The effectiveness of a newly formed additive and the durability of a stabilized subgrade soil at adverse environmental conditions were also investigated. This study is the first step towards developing a new sustainable road pavement material for significant cost savings in road maintenance works. 5.2 Major Findings In this research, the effect of heat curing temperature and its duration on silty sand subgrade, treated with bentonite magnesium alkalinization, was studied by evaluating the UCS with increasing age. The results obtained were analyzed in terms of elastic modulus, morphology, and composition, and the following conclusions were made: 1. Heat curing temperature and duration has significant impact on bentonite magnesium alkalinization. Curing at ambient or low temperature is not enough to attain desirable strength development, and poor structural integrity of the samples resulted from too high temperatures due to a lack of hydration water, leading to failure at later age. The extent of heat curing duration varies with temperature; when the temperature is low, a longer curing duration is required. Further, a shorter heat curing duration is suitable for higher temperatures. 2. Curing at 60°C for 3 days, or at 80°C for 2 days, was identified as an optimum combination of heat curing temperature and duration for the additive-mix used in this research. Due to elevated curing temperature the higher strength development was observed at 3 days age, 70 but the strength continued to decrease until 14 days. No loss in strength was noticed at age 28 days and further, as the development of the cementitious gel compacted and hardened the soil structure over time. 3. Stiffness is one of the most important properties for desirable performance of subgrade soil. A noticeable improvement was observed in the elastic modulus (E50) in the treated soil, which is seven (7) times higher compared to raw silty sand. 4. The presence of cementitious gel was identified from the FESEM and EDS analysis for all curing ages. Over time, the gel appeared stronger with the increase of Si, Al, and Mg, and resulted in a stiffer and more durable material. 5. The presence of C-S-H and M-S-H was identified from the thermogravimetric analysis, and the formation of these bonds made the samples more stable and compact with time. 6. Optimally prepared unheated samples illustrated better resistance to freeze thaw cycles compared to the heated samples. It is evident from the analysis of the results that optimal heat-treated bentonite magnesium alkalinization can be a cost-effective and green additive for increasing the strength and stiffness of silty sand for a sustainable subgrade. Although some exception observed at freeze thaw durability test. Whether microstructure and modulus of elasticity are equally important along with strength at evaluating the performance of materials during freeze thaw durability test provided that the harshness of freeze thaw cycles and materials were capable of withstanding all 12 cycles. 5.3 Field Applications Developing proper field application techniques and quality control measures are essential to ensure that the final product will be adequate for its intended use. Following initial field application techniques are identified for bentonite magnesium alkalinization: 71 5.3.1 Pelletized Bentonite Magnesium Alkalinization Pelletized bentonite magnesium alkalinization additive can be produced by mixing predefined amount of raw silty sand subgrade soil, bentonite, magnesium alkalization and water through ordinary truck distributors. Then the pellets can be placed in subgrade layer with reasonable compactive efforts. This method seems feasible as already being used in road construction and very efficient to reduce dust during construction. 5.3.2 In-situ Dry Mixing The concept involved in this technique is mixing dry soil with designed amounts of additives using an auger at required depths in the actual site of construction. The predefined amount of solid additives can be mixed with soil using an auger to required depths of subgrade layer. Solution of alkaline activator and water can be used as mixing fluid. Finally hot air can be sprayed to apply the proper heat along with predefined compactive effort. Although this technique looks easy and less expensive but pulverization, additive content, moisture content, uniformity of mixing, time sequence of operations, compaction and curing would be the most important factors during this operation. 72 5.4 Limitations and Future Works A few limitations and their recommendations for future studies have been listed below in Table 5.1: Table 5.1 Limitations and Recommendations Limitations Recommendations The utilization of additives and heat curing technique was only investigated on the silty sand which is most common subgrade soil in BC. The mineral composition and properties differ from one subgrade soil to another. So, it is im-portant to investigate the efficacy of the newly developed additive and heat curing technique on other subgrade soils. Strength analysis was performed under static loading in this study. Road pavements experience repeated stress from the movement of vehicles and goods. It is recommended to perform strength analysis using cyclic triaxial test. Durability tests were performed using freeze thaw cycles. This method was selected as it is applicable to a cold region like Canada. Other type of durability tests such as vacuum saturation and tube suction tests can be done to compare results from freeze thaw tests. Af-ter completing all these tests a better under-standing of the proposed additive will be es-tablished. The scope of this research was limited to la-boratory tests. Small scale field test can be used to check la-boratory results in the field condition. 73 Bibliography AASHTO M 145-91 (2017). Standard specification for classification of soils and soil-aggregate mixtures for highway construction purposes. Washington, D. C. 20001. United States. AASHTO Designation: R Draft (2008). Standard recommendation practice for stabilization of subgrade soil and base materials, Washington, DC 20001. United States. 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Journal of Cleaner Production, 27, 118â125. 83 Appendices Appendix A: Additional Test Results Figure A.1 Typical stress strain curve Figure A.2 Typical calculation of elastic modulus (E50) from stress strain curve 010020030040050060070080090010000.00% 0.50% 1.00% 1.50% 2.00% 2.50% 3.00%Axial Stress (kPa)Axial Strain (%)0200400600800100012000.00% 0.50% 1.00% 1.50% 2.00% 2.50% 3.00% 3.50%Axial Stress (kPa)Axial Strain (%)E50= 495/0.0167 = 29.64 MPa 84 Table A.1 Additional unconfined compressive strength test results Curing time (days) Curing tempera-ture Heat curing dura-tion (days) After curing weight (g) After cur-ing height (mm) After curing diame-ter (mm) Target speed (mm/s) Target posi-tion (mm) After curing den-sity Axial stress (kPa) Aver-age ax-ial stress (kPa) 3 days 40â¦C 1 124.0 75.2 37.2 0.0125 11.274 1.4552 659.66 647.55 125.4 75.0 37.3 0.0125 11.253 1.474 598.43 125.8 75.2 37.5 0.0125 11.286 1.4745 620.7 125.2 75.0 37.4 0.0125 11.255 1.4711 662.29 2 122.4 75.4 37.6 0.0126 11.315 1.4312 904.1 916.504 122.5 75.3 37.5 0.0126 11.3 1.4333 850.31 122.7 75.2 37.6 0.0125 11.276 1.4393 926.72 121.7 75.3 37.5 0.0125 11.289 1.4253 918.69 3 122.8 75.2 37.4 0.0125 11.279 1.4402 834.42 832.96 122.2 75.1 37.4 0.0125 11.267 1.4349 827.7 122.5 75.1 37.3 0.0125 11.265 1.4378 850.91 122.5 75.3 37.3 0.0125 11.292 1.4353 818.8 60â¦C 1 121.6 75.7 37.6 0.0126 11.355 1.4163 788.38 770.001 121.5 75.8 37.8 0.0126 11.363 1.4138 614.51 121.6 75.7 37.8 0.0126 11.36 1.4158 788.85 122.4 76.1 37.7 0.0127 11.414 1.4183 732.77 2 121.6 75.5 37.6 0.0126 11.325 1.4196 997.59 979.637 120.9 75.5 37.7 0.0126 11.321 1.4126 961.68 121.2 75.8 37.7 0.0126 11.376 1.4091 859.4 121.4 75.7 37.9 0.0126 11.357 1.4139 868.05 3 121.8 75.9 37.7 0.0126 11.378 1.4153 958.9 881.992 121.9 75.8 37.7 0.0126 11.375 1.4171 813.17 120.4 75.7 37.7 0.0126 11.351 1.4034 721.71 121.3 75.8 37.6 0.0126 11.376 1.4098 873.91 80â¦C 1 121.0 75.6 37.7 0.0126 11.336 1.4117 1181.6 1145.59 121.1 75.5 37.6 0.0126 11.322 1.4141 1083.3 120.3 75.8 37.5 0.0126 11.366 1.3999 1171.9 2 120.5 75.8 37.9 0.0126 11.369 1.402 988.06 1065.38 120.1 75.7 37.7 0.0126 11.352 1.3994 1031.8 120.1 75.5 37.4 0.0126 11.322 1.4033 1084.3 119.9 75.5 37.4 0.0126 11.319 1.4015 1080.1 3 122.7 75.7 37.0 0.0126 11.354 1.4295 1210.5 1307.21 120.1 75.6 37.6 0.0126 11.333 1.4018 1396.2 120.2 75.4 37.8 0.0126 11.304 1.4063 1255.3 120.7 75.5 0.0126 11.328 1.4096 1366.9 85 Curing time (days) Curing tempera-ture Heat curing dura-tion (days) After curing weight (g) After cur-ing height (mm) After curing diame-ter (mm) Target speed (mm/s) Target posi-tion (mm) After curing den-sity Axial stress (kPa) Aver-age ax-ial stress (kPa) 7 days 40â¦C 1 124.1 75.0 37.3 0.0125 11.253 1.4584 688.71 674.146 124.0 75.2 37.4 0.0125 11.283 1.4537 694.28 123.5 75.2 37.4 0.0125 11.276 1.4491 603.47 123.4 75.3 37.4 0.0125 11.288 1.4457 639.44 2 122.5 74.6 37.4 0.0124 11.19 1.4473 983.43 930.599 122.4 75.2 37.4 0.0125 11.273 1.4358 936.43 121.8 75.1 37.5 0.0125 11.261 1.43 871.93 122.2 75.0 37.6 0.0125 11.244 1.4375 857.88 3 121.8 75.0 37.4 0.0125 11.25 1.4324 963.04 791.207 122.1 75.4 37.5 0.0126 11.312 1.4273 841 122.5 75.1 37.4 0.0125 11.267 1.4383 758.69 122.7 75.2 37.3 0.0125 11.285 1.4385 773.93 60â¦C 1 128.8 72.5 38 0.0121 10.875 1.5662 - 540.3 126.3 72.4 38.1 0.0121 10.8525 1.5393 - 128.5 75.9 37.9 0.0126 11.385 1.4931 - 127.5 72.3 37.5 0.0121 10.845 1.5549 540.3 2 121.3 75.4 37.4 0.0126 11.316 1.4172 793.47 926.014 121.5 75.8 37.4 0.0126 11.363 1.4141 811.64 121.8 75.7 37.6 0.0126 11.355 1.4191 1032.2 121.9 75.7 37.8 0.0126 11.352 1.4207 934.18 3 121.7 75.7 37.5 0.0126 11.361 1.4163 706.98 932.899 121.5 75.7 37.5 0.0126 11.352 1.4156 894.03 121.8 75.9 37.6 0.0126 11.379 1.4153 953.96 121.7 75.7 37.5 0.0126 11.351 1.4186 950.71 80â¦C 1 120.6 75.5 37.7 0.0126 11.325 1.4089 983.69 1145.13 120.8 75.7 37.7 0.0126 11.349 1.4076 1080.6 120.5 75.7 37.7 0.0126 11.358 1.4032 1231.9 121.0 75.8 38.0 0.0126 11.363 1.4085 1122.9 2 120.9 75.9 37.6 0.0126 11.381 1.4047 1063.7 1055.65 121.1 75.6 37.7 0.0126 11.343 1.4116 1065.1 120.7 75.7 37.8 0.0126 11.352 1.4059 1038.2 120.7 76.0 37.7 0.0127 11.4 1.4008 964.47 3 120.4 75.3 37.6 0.0126 11.3 1.4092 1246.5 1242.22 120.4 75.7 37.7 0.0126 11.354 1.4029 1248.3 120.7 75.6 37.8 0.0126 11.334 1.4086 1231.9 121.0 75.9 37.8 0.0127 11.385 1.4061 1132.9 86 Curing time (days) Curing tempera-ture Heat curing dura-tion (days) After curing weight (g) After cur-ing height (mm) After curing diame-ter (mm) Target speed (mm/s) Target posi-tion (mm) After curing den-sity Axial stress (kPa) Aver-age ax-ial stress (kPa) 14 days 40â¦C 1 124.8 74.7 37.4 0.0125 11.211 1.4724 600.07 597.834 124.8 75.1 37.5 0.0125 11.267 1.4654 627.93 124.4 75.3 37.5 0.0126 11.297 1.4563 583.58 124.7 75.1 37.5 0.0125 11.268 1.4638 579.76 2 122.7 74.6 37.3 0.0124 11.192 1.4502 876.29 890.43 123.1 75.4 37.4 0.0126 11.315 1.4388 904.57 123.2 75.2 37.4 0.0125 11.283 1.4438 767.53 122.8 75.3 37.3 0.0126 11.297 1.4381 728.71 3 122.4 74.9 37.3 0.0125 11.237 1.4411 840.94 823.158 122.7 75.3 37.4 0.0125 11.294 1.4374 835.99 122.4 75.2 37.3 0.0125 11.276 1.4362 816.15 124.6 75.1 37.5 0.0125 11.259 1.4638 799.55 60â¦C 1 127.1 74.7 37.5 0.0121 10.935 1.5369 705.16 705.023 126.9 74.1 37.7 0.0119 10.74 1.5630 734.37 127.7 75.7 37.9 0.0121 10.89 1.5507 624.16 126.7 74.6 37.8 0.0117 10.53 1.5915 620.64 2 121.6 75.7 37.5 0.0126 11.348 1.4172 843.34 833.375 121.9 75.6 37.7 0.0126 11.336 1.4222 695.27 121.9 75.8 37.4 0.0126 11.369 1.4187 823.41 121.6 75.5 37.8 0.0126 11.328 1.42 647.5 3 121.7 75.2 37.5 0.0125 11.285 1.4268 815.55 943.131 121.7 75.6 37.8 0.0126 11.334 1.4202 880.27 121.9 75.3 37.6 0.0126 11.295 1.4277 968.21 121.9 75.4 37.5 0.0126 11.316 1.4243 980.92 80â¦C 1 121.1 75.6 37.6 0.0126 11.334 1.4127 1057.8 947.199 121.0 75.8 37.5 0.0126 11.373 1.4068 945.79 121.4 75.9 37.6 0.0126 11.382 1.4104 956.58 121.9 76.0 37.9 0.0127 11.394 1.4153 939.23 2 120.6 75.5 37.8 0.0126 11.325 1.4085 978.27 998.003 120.8 75.4 37.6 0.0126 11.313 1.4128 1004.2 120.7 75.6 37.6 0.0126 11.337 1.4085 983.97 120.5 75.4 37.3 0.0126 11.31 1.4096 991.77 3 120.5 76.0 37.6 0.0127 11.394 1.3989 911.71 1016.07 123.6 75.6 37.7 0.0126 11.339 1.4419 420.87 120.8 75.5 37.7 0.0126 11.324 1.411 1148.2 121.3 75.7 37.5 0.0126 11.348 1.4142 988.25 87 Curing time (days) Curing tempera-ture Heat curing dura-tion (days) After curing weight (g) After cur-ing height (mm) After curing diame-ter (mm) Target speed (mm/s) Target posi-tion (mm) After curing den-sity Axial stress (kPa) Aver-age ax-ial stress (kPa) 28 days 40â¦C 1 126.5 75.6 37.6 0.0126 11.342 1.4751 517.07 508.525 124.0 75.6 37.3 0.0126 11.337 1.446 506.52 123.7 75.9 37.5 0.0126 11.378 1.4383 452.35 124.1 75.6 37.6 0.0126 11.337 1.4481 501.98 2 122.1 75.7 37.7 0.0126 11.36 1.4212 671.91 683.115 122.5 75.5 37.5 0.0126 11.324 1.4303 734.55 122.6 75.6 37.4 0.0126 11.34 1.43 518.55 122.6 75.3 37.5 0.0126 11.3 1.4352 642.89 3 122.7 75.4 37.4 0.0126 11.309 1.4347 648.95 725.937 122.6 75.6 37.4 0.0126 11.339 1.4298 757.95 122.6 75.6 37.6 0.0126 11.336 1.4304 715.39 122.4 75.6 37.5 0.0126 11.345 1.4274 704.47 60â¦C 1 119.96 68.0 37.9 0.0113 10.2 - 618.82 743.702 121.5 69.7 37.7 0.0116 10.455 - 320.87 121.5 69.8 37.8 0.0116 10.47 - 750.21 120.8 68.3 38.1 0.0113 10.245 - 862.08 2 121.8 75.5 37.7 0.0126 11.33 1.422 816.62 850.704 122.2 75.7 37.5 0.0126 11.348 1.4237 872.54 122.2 75.3 37.5 0.0126 11.3 1.4304 879.75 122.2 75.5 37.6 0.0126 11.321 1.4275 833.9 3 121.6 75.5 37.6 0.0126 11.325 1.4204 681.86 902.006 122.9 75.9 37.5 0.0127 11.385 1.4276 872.86 122.2 75.5 37.6 0.0126 11.322 1.4278 910.58 121.8 75.3 37.4 0.0126 11.297 1.4264 922.58 80â¦C 1 121.0 75.7 37.8 0.0126 11.358 1.4087 1035.4 1012.64 120.8 75.6 37.8 0.0126 11.343 1.409 988.09 121.0 75.3 37.9 0.0125 11.289 1.4178 1033.2 121.2 75.9 37.9 0.0126 11.379 1.4085 993.93 2 120.6 75.7 37.6 0.0126 11.352 1.4055 931.36 927.37 120.9 75.5 37.7 0.0126 11.318 1.4134 813.63 121.6 75.7 37.8 0.0126 11.358 1.416 923.38 121.5 75.9 37.8 0.0126 11.378 1.412 887.85 3 120.9 75.8 37.7 0.0126 11.372 1.4057 1012.1 1048.37 120.7 75.8 37.7 0.0126 11.37 1.4038 1054.9 120.6 75.7 37.8 0.0126 11.355 1.4043 1078.1 120.2 74.8 37.7 0.0125 11.22 1.4166 980.58 88 Curing time (days) Curing tempera-ture Heat curing dura-tion (days) After curing weight (g) After cur-ing height (mm) After curing diame-ter (mm) Target speed (mm/s) Target posi-tion (mm) After curing den-sity Axial stress (kPa) Aver-age ax-ial stress (kPa) 60 days 40â¦C 1 125.0 75.5 37.2 0.0126 11.327 1.4593 492.16 494.589 120.8 75.6 37.8 0.0126 11.346 1.4077 496.12 124.3 75.5 37.2 0.0126 11.328 1.4508 488.76 123.8 75.3 37.2 0.0126 11.301 1.4487 501.31 2 122.8 75.0 37.6 0.0125 11.25 1.4442 619.31 606.678 122.8 74.9 37.2 0.0125 11.234 1.4455 633.5 122.8 75.2 37.5 0.0125 11.28 1.4402 567.21 123.2 74.6 37.5 0.0124 11.184 1.4571 470.16 3 119.0 75.1 37.2 0.0125 11.265 1.3967 507.26 561.344 121.8 75.3 37.1 0.0126 11.301 1.4249 519.76 122.6 75.3 37.6 0.0125 11.288 1.437 657.02 121.0 75.5 37.9 0.0126 11.325 1.4131 346.28 60â¦C 1 124.81 73.5 37.7 0.0122 11.019 1.4981 736.06 875.764 131.1 72.9 37.8 0.0121 10.941 1.5847 596.76 125.7 73.4 37.8 0.0122 11.01 1.5100 935.25 129.7 73.5 37.4 0.0122 11.0175 1.5565 816.28 2 121.9 75.4 37.4 0.0126 11.306 1.4257 787.75 822.556 121.8 75.0 37.6 0.0125 11.25 1.4314 848.88 122.9 75.8 37.6 0.0126 11.366 1.4301 862.98 122.7 75.6 37.6 0.0126 11.346 1.4302 790.61 3 0 0 #DIV/0! 894.075 0 0 #DIV/0! 121.7 75.3 37.6 0.0126 11.3 1.4246 878.7 122.1 75.4 37.6 0.0126 11.306 1.428 909.45 80â¦C 1 120.1 73.5 37.6 0.0123 11.031 1.4394 894.51 935.048 120.9 76.0 37.6 0.0127 11.394 1.4032 1024.7 121.1 75.9 37.7 0.0126 11.384 1.4074 963.95 121.4 76.2 37.8 0.0127 11.43 1.4052 946.69 2 120.5 75.6 37.8 0.0126 11.343 1.4048 840.17 923.306 121.1 75.7 37.4 0.0126 11.349 1.4117 885.11 120.7 75.7 37.6 0.0126 11.349 1.4066 793.07 122.0 75.9 37.7 0.0126 11.382 1.4174 961.5 3 120.5 75.6 37.7 0.0126 11.333 1.4062 1032.2 1002.6 120.5 75.6 37.7 0.0126 11.333 1.4062 971.5 121.0 75.8 37.6 0.0126 11.376 1.4069 987.98 121.0 75.5 37.6 0.0126 11.33 1.41 1018.8 89 Table A.2 Detail mix design for freeze thaw durability test Type Mix-ture No. SS/SH ratio (L) L/MgCl2 (weight ratio) Soil (%) MgCl2 (%) Ben-tonite (%) Weight of soil (g) Weight of MgCl2 (g) Weight of Ben-tonite (g) Weight of L (g) Weight of wa-ter (g) Total mix-ture (g) MDD (KN /m3) OMC (%) L (%) Total mass per sam-ple (g) Raw soil Raw soil 0 0 100 0 0 850 0 0 0 108.80 850 17.9 12.8 0 156.83 Soil (0.97) +MgCl2 (0.03) CM 0 0 97 3 0 892.4 27.6 0 0 131.56 920 19.29 14.3 0 169.49 Soil (0.97) +MgCl2 (0.03) +L (0.5) CM Al-kalini-zation 0.5 0.7 97 3 0 742.05 22.95 0 16.06 83.39 765 16.2 13 2.1 142.34 Soil (0.57)+ Bentonite (0.4) +MgCl2 (0.03)+ L (0.5) BM 0.5 0.7 57 3 40 421.8 22.2 296 15.54 150.96 740 16.64 22.5 2.1 146.20 Heated Soil (0.57)+ Bentonite (0.4)+MgCl2 (0.03)+ L (0.5) Heated BM 0.5 0.7 57 3 40 421.8 22.2 296 15.54 150.96 740 16.64 22.5 2.1 146.20 90 Table A.3 Additional freeze thaw durability test results Cur-ing days Sample types Weight (g) After curing weight (g) After cur-ing height (mm) After curing diame-ter Target speed (mm/s) Target posi-tion (mm) After curing den-sity Axial stress (kPa) Aver-age Ax-ial stress (kPa) 0 days Raw soil 156.41 140.1 77.57 38.78 0.013 11.636 1.59 34.25 35.99 152.9 136.9 78.53 38.82 0.013 11.78 1.54 37.76 155.2 140.1 78.11 38.83 0.013 11.717 1.58 35.95 7 days CM 170.0 Ë Ë Ë Ë Ë Ë 0.00 168.5 Ë Ë Ë Ë Ë Ë Ë 168.5 Ë Ë Ë Ë Ë Ë Ë CM Al-kaliniza-tion 142.82 Ë Ë Ë Ë Ë Ë Ë 0.00 142.4 Ë Ë Ë Ë Ë Ë Ë 142.2 Ë Ë Ë Ë Ë Ë Ë BM 146.6 139.4 76.82 38.15 0.013 11.523 1.60 98.24 99.12 146.4 139.6 77.00 38.46 0.013 11.55 1.60 92.62 146.3 139.6 76.58 38.32 0.013 11.487 1.61 106.5 Heated BM 120.4 136.9 76.62 38.33 0.013 11.493 1.58 33.73 34.77 121.0 136.5 76.90 38.20 0.013 11.535 1.57 34.2 121.0 137.7 77.97 38.57 0.013 11.696 1.56 35.81 28 days CM 170.1 Ë Ë Ë Ë Ë Ë Ë 0.00 168.4 Ë Ë Ë Ë Ë Ë Ë 168.7 Ë Ë Ë Ë Ë Ë Ë CM Al-kaliniza-tion 143 Ë Ë Ë Ë Ë Ë Ë 0.00 141.6 Ë Ë Ë Ë Ë Ë Ë 142.1 Ë Ë Ë Ë Ë Ë Ë BM 146.5 144.3 77.07 38.25 0.013 11.561 1.65 57.12 54.20 146.3 143.7 76.82 38.32 0.013 11.523 1.65 58.52 146.4 145.2 77.41 38.06 0.013 11.612 1.65 46.96 Heated BM 120.5 141.2 77.90 38.14 0.013 11.685 1.60 29.8 30.53 120.9 141.9 77.69 38.34 0.013 11.654 1.61 30.5 120.6 141.3 76.90 38.10 0.013 11.535 1.62 31.3 91 Appendix B: Additional Images of Freeze Thaw Durability Test (a) (b) Figure B.1 7 days cured samples after 12 freeze thaw cycles (a) Heated (b) Unheated (a) (b) Figure B.2 28 days cured samples after 12 freeze thaw cycles (a) Heated (b) Unheated
Thesis/Dissertation
eng
Civil Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Heat curing method and laboratory performance evaluation of subgrade soil stabilized with bentonite and magnesium alkalinization
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