Towards a better understanding of the mechanisms controlling the durability of FRP composites in concrete
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Wide adoption by the construction industry of Fibre Reinforced Polymer (FRP) rebars - a relatively recent construction material that offers numerous advantages of corrosion resistance, higher strength, lighter weight, etc. over conventional reinforcing materials for concrete, such as steel - is at least partially impeded due to a lack of an effective long term in-service performance prediction model and relatively high initial costs. A reliable service life prediction model for FRP composites in concrete depends on a clear understanding of the transport mechanisms of potentially harmful chemical species into the FRP composites and their subsequent contribution to any potentially active degradation mechanism(s). To identify which mechanisms control the degradation of Glass Fibre Reinforced Polymers (GFRP) in alkaline environments, GFRP rebars were immersed into simulated concrete pore solutions and subjected to accelerated ageing tests (Phase 1). The conditioned samples were analyzed by various electron microscopy (SEM, EDS) and spectroscopic methods (FTIR). Analyses of these tests revealed that fibre-matrix debonding took place in few samples exposed to 75 °C (the highest temperature considered in this study), and tested after one year, despite the fact that the glass fibres and polymer matrix remained essentially intact and that no penetration of alkalis into the GFRP rebars was observed. Hence, this study shows that the Vinyl Ester (VE) polymer matrix used acts as an effective semi-permeable membrane by allowing the penetration of water while blocking alkali ions. The findings showing that most of the damage seems to be confined to the fibre-matrix interphase (or interface), under the considered test conditions, stimulated an investigation on the effects of sizing on the strength retention and water up-take of GFRP rebars in Phase 2 of the testing program. In order to study the effects of sizing on the properties of GFRP rebars, GFRP custom plane sheets with sized and desized glass fibers were produced and exposed to deionized water at 4 °C, 23 °C, and 50 °C. Irrespective of sample types, the tensile strength decreased with temperature while the mass gain and moisture diffusivity increased with temperature. However, the sized samples showed a similar mass gain behavior as the desized ones, at the same exposure environment. This study confirms that sizing in GFRP custom plane sheets contributes not only to the initial strength of the composite by enhancing the adhesion between the glass fibre and a matrix, but also to the strength retention (i.e., durability) when exposed to harsh environments. The experiments of Phase 2 were carried out at 100% relative humidity (RH). However, field service conditions vary with respect to RH and temperature for GFRP composites in concrete. Therefore, a further study was conducted to investigate the effects of RH and temperature on the properties of GFRP rebars in Phase 3. The effects of RH were investigated by exposing GFRP rebars to nine RH environments (9%-100%) while monitoring mass changes during drying and wetting. Moreover, the thermal effects of GFRP rebars on water uptake in deionized water at 4 °C, 23 °C, and 50 °C were studied and compared with those for GFRP custom plane sheets. The effects of RH on drying and wetting for GFRP rebars exhibited a hysteretic behavior. The percent of mass gain at 100% RH showed a significant difference from that in other RH environments. Mass gain and moisture diffusivity were found to increase for both rebars and custom sheets with increasing temperature. A typical Fickian behaviour of water absorption was observed for both types of samples at all exposure conditions, except the GFRP rebars at higher temperatures (starting at 50 °C) which showed non-Fickian behaviour for water absorption. The dependence of the diffusion coefficient on temperature was found to follow the Arrhenius equation. Literature reports severe matrix cracking and fibre dissolution of GFRP in accelerated ageing tests. The results of this investigation confirmed that no matrix or fibre degradation was found in any sample up to 75 °C. However, the interface of samples exposed to 75 °C started to show signs of debonding at the fibre-matrix interface. Hence, any likely candidate mechanism must be related to some degradation at the interface/interphase of the GFRP composite. It was proposed here to assess whether preexisting microdefect locations could serve as sites for potential formation and growth of a microblister (local osmotic cells). Based on the thermodynamics of microblister formation and growth, a rational model has been proposed to address the mechanisms of microblister formation at the interphase of FRP composites. Results of the analyses, show that the critical pressure that needs to be overcome by the osmotic pressure before a microblister can grow is much higher than this latter one for both GFRP rebar and GFRP sheet samples until 75 °C for the temperatures considered in this study. Therefore, one can conclude that no microblister is formed at the given experimental conditions of samples immersed at a pH 7 with the temperatures of 4 °C, 23 °C, 50 °C, and 75 °C. The same model also predicted that if temperature is spiked to very high values (around 95 °C), a preexisting microblister would be able to grow, which could eventually lead to matrix volume changes and/or cracking, even if the temperature remains below the glass transition temperature (Tg) of the matrix.
DegreeDoctor of Philosophy (Ph.D.)
CommitteeEvitts, Richard; Wegner, Leon; Oguocha, Ike; Lin, Yen-Han; Nemati, Mehdi
Copyright DateSeptember 2011