Glass fibre reinforced polymer (GFRP) materials have emerged as a practical alternative material for producing reinforcing bars for non prestressed concrete structures. This is due to their relative low cost-to-performance, and non-corrosive nature. However, their durability in alkaline environment is still concern. The internal concrete environment has high alkalinity with pH between 10.5 - 13 depending on the design mixture of the concrete and type of cement used, this alkaline environment damage glass fibre through loss in toughness, strength and embrittlement. The embrittlement of fibres is due to the nucleation of calcium hydroxide on the fibre surface. In addition, calcium, sodium and potassium ions found in concrete pore solution are highly aggressive toward glass fibres. Although resin matrices provide certain level of protection to fibres from this degradation, migration of high pH solutions and alkali salts through resin (or void, crack, interface between fibre/matrix) to the fibre surface is possible. The extent of composites degradation depends on the severity and duration of exposure, glass fiber types, chemistry of the polymer matrix, and the interface, curing degree, manufacturing processes. Yet the mechanisms responsible for degradation of glass-fiber-reinforced polymer composites in concrete environment are not clearly understood. In addition to the effect of alkaline environment on the durability of GFRP reinforcing bars, there is also level of concern relate to the moisture associated degradation, especially as related to the vinylester systems, which are commonly used as resin matrix in GFRP reinforcing bars.
Due to ease processing, vinylester resin systems are commonly used to produce GFRP reinforcing bars. Vinylester resin systems are combinations of methacrylated epoxy compounds and styrene, wherein styrene is used as a reactive diluent and the vinylester serves as the crosslinking agent. Cure is achieved by free radical bulk polymerization with rates and degrees of cure being highly dependent on actual system formulation and cure regime used. The network formation process can be described as a combination of three ongoing processes, homopolymerization, of vinylester and styrene separately, and the copolymerization of both with progression taking place at different rates in the same time frame. Although the rate of fractional conversion of styrene double bonds is initially less than that of the vinylesters, the styrene monomer may continue reacting after the vinylester double bond conversion has stopped. This difference in rates can result in the formation of microgel structures with domains of high cross-link density in a pool of unreacted monomers leading to distinct heterogeneities. This incomplete polymerization, or undercure, can not only lead to changes in properties with time due to slow residual cure progression, but can also induce lower heat stability, lower resistance to hydrolysis, and increased susceptibility to moisture associated degradation and a greater degree of susceptibility of swelling in solvents Hence, there is a critical need to further the understanding of mechanisms leading to moisture-associated degradation, especially as related to the ambient cured vinylester systems, for which there is still a significant lack of data.
The principle objectives of this work are to determine the effect of moisture and alkaline environments on the long-term durability of glass-fiber composites and to elucidate mechanisms of property degradation. More specifically
Results, obtained from these accelerated ageing tests, provide state of art experimental data related to long-term durability of E-glass/vinyl ester reinforcing bars. Measurement of physical-chemical properties can yield critical information on the degradation mechanisms of E-glass/vinylester bars, which can be used not only to optimize material performance, but also to provide information in the design of accelerated ageing tests.
For all samples aged at room temperature and 40°C for 150 and 300 days in distilled water and simulated alkaline solution, mechanisms of E-glass/vinyl ester bars is mainly post-cure dominated, micro-cracks in resin matrix are observed on samples aged in alkaline solution, the micro-cracks become big with increased exposure time. For samples aged in distilled water and alkaline solution at 60oC for 300 days, fibre degradation are observed on parts which are near to surface coatings, the mechanisms are mainly physical-chemical dominated. From the combined analysis of FTIR, DSC, and SEM, it is not clear whether alkali ions could penetrate through vinyl ester resin.
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