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UBC Theses and Dissertations

Fiber reinforced concrete : characterization of flexural toughness & some studies on fiber-matrix bond-slip interaction Dube, Āśīsha

Abstract

A chiral HPLC assay for the separation of the four stereoisomers of labetalol in biological fluids was developed. The limit of detection of the individual isomers was 0.15 ng (0.6 ng of injected racemic labetalol). In maternal plasma the concentration of the RR isomer was 25±12% higher than those of the SR, SS and RS isomers throughout the infusion and post infusion period. The RR isomer had the lowest steady-state clearance (50.93±4.64 mL/min/kg) and the longest half-life (5.60±0.86 h). Ofthe 280 mg of labetalol administered, 11.04±2.76 mg was excreted as unconjugated while 65.84±7.19 mg was excreted as glucuronide conjugate. Approximately 32% of the unconjugated labetalol was in the form of the RR isomer compared to 19% for the other isomers. In fetal plasma, the concentration of the RR isomer (24 ± 5 ng/mL at steady-state) was 50% higher than that ofthe SR (11.00±2.38 ng/mL), SS(13.02±3.04 ng/mL) and RS(12.05±2.85 ng/mL) isomers throughout the infusion and post-infusion period, while the concentration of the SR isomer was consistently lower over the same interval. The terminal elimination half-life (t)/2) and the mean residence time (MRT) ofthe RR isomer were 13.07 ± 4.02 h, and 17.19 ± 5.63 h, respectively. Labetalol fetal infusion studies were performed in four chronically instrumented pregnant ewes. In fetal plasma, the concentration of the RR isomer at steady-state was 51.22 ± 4.55 ng/mL, which was significantly higher than the other three isomers ([SR]- 30.33 ± 3.41 ng/mL, [SS]= 33.70 ± 3.22 ng/mL, [RS]=34.93 ± 2.75). The terminal elimination half-life (ti/2) of the RR isomer was 8.62 ± 2.85 li, which was significantly lower than that obtained following maternal infusion of labetalol. Ofthe 9,400 ug of labetalol administered, 73.79 ± 9.64 mg (0.80 + One major problem associated with the testing of fiber reinforced concrete specimens under flexural loading is that the measured post-cracking response is severely affected by the stiffness of the testing machine. As a consequence, misleading results are obtained when such a flexural response is used for the characterization of composite toughness. Unfortunately, many existing standards allow the use of such a flexural response for toughness characterization. As a part of this research program, assessment of a new toughness characterization technique termed the Residual Strength Test Method (RSTM) has been made. In this technique, a stable narrow crack is first created in the specimen by applying a flexural load in parallel with a steel plate under controlled conditions. The plate is then removed, and the specimen is tested in a routine manner in flexure to obtain the post-crack load versus displacement response. Flexural response for a variety of fiber reinforced cementitious composites obtained using the Residual Strength Test Method has been found to correlate very well with those obtained with relatively stiffer test configurations such as closed-loop test machines. A good agreement between the flexural response obtained from the aforementioned methods seems to validate the Residual Strength Test Method. This method is simple, and can be carried out easily in any commercial laboratory equipped with a test machine with low stiffness. The Residual Strength Test Method is found to be effective in differentiating between different fiber types, fiber lengths, fiber configurations, fiber volume fractions, fiber geometries and fiber moduli. In particular, the technique has been found to be extremely useful for testing cement-based composites containing fibers ait very low dosages (< 0.5% by volume). As another major objective of this research program, an analytical model based on shear lag theory is introduced to study the problem of fiber pullout in fiber reinforced composites. The proposed model eliminates limitations of many earlier models and captures essential features of pullout process, including progressive interfacial debonding, Poisson's effect, and variation in interfacial properties during the fiber pullout process. Interfacial debonding is modeled using an interfacial shear strength criterion. Influence of normal contact stress at the fiber-matrix interface is considered using shrink-fit theory, and the interfacial frictional shear stress over the debonded interface is modeled using Coulomb's Law. Stresses required to cause initial, partial and complete debonding of the fiber-matrix interface are analyzed, and closed form solutions are derived to predict the complete fiber pullout response. Analysis shows that the initial debonding stress strongly depends upon fiber length and fiber elastic modulus. The process of interfacial debonding turns catastrophic at the instant the fiber pullout stress begins to drop with an increase in debond length. This condition is satisfied when the difference between the change in the frictional component and the adhesional component of pullout stress occurring due to increase in debond length equals to zero. The magnitude of interfacial frictional shear stress along the embedded fiber length is found to vary as a result of the Poisson's contraction of the fiber. Moreover, Poisson's effect manifests itself in the form of a non-linear relationship between the peak pullout stress versus embedded fiber length plot. Based on energy considerations, an analytical solution is derived to compute the interfacial coefficient of friction. This solution depicts the dependence of the interfacial coefficient of friction on fiber pullout distance. For both steel and polypropylene fibers, interfacial coefficient of friction is found to decrease exponentially with increase in pullout distance. Matrix wear resulting from fiber pullout appears to be responsible for the aforementioned physical phenomena. Parametric studies are carried out to investigate the influence of fiber-matrix interfacial properties (adhesional bond shear strength, normal contact stress and coefficient of friction) and elastic modulus of the fiber. Results suggest that for a given set of interfacial properties, initial debonding stress, maximum pullout stress, stability of debonding process, catastrophic debond length, interfacial shear stress distribution, and overall pullout response significantly depend upon the elastic modulus of the fiber. Given the fiber elastic modulus, recommendations are made as to how efficiency of fiber in pullout may be improved by modifying different interfacial properties..

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