| Summary: | Although glass fibre reinforced polymer (GFRP) bars do not corrode in the same way as conventional steel reinforcement, their physical and mechanical properties are prone to degradation following exposure to a variety of aggressive environments. Despite the relatively large amount of research on durability of FRP reinforcement in concrete environments, the available design models are still not able to account for all of the most typical in-service conditions, especially the effect of sustained stress on long-term properties. Thus, there is a need to develop a reliable prediction model to estimate the rate of degradation of GFRP bars and long-term performance in real structural applications. A comprehensive test programme was carried out on 348 GFRP specimens subjected to different environments (concrete, alkali solution and tap water), different temperature levels (20, 40, 60°C) and two different levels of sustained stress. The mechanical performance of the GFRP specimens, as well as their physical and chemical characteristics, were evaluated through the implementation of a complementary set of techniques, including direct tension test, flexural tests, inter-laminar shear tests, moisture absorption, SEM-EDX, FTIR, and DMA. The material tests were complemented by accelerated tests on GFRP RC tension ties and small-scale beams to examine the effect of the studied environments on the long-term bond and flexural behaviour of GFRP RC members under service conditions. While no significant reduction was observed in the elastic modulus of the tested GFRP bars, tensile strength, flexural strength and transfer properties (ILSS) were found to be affected by the conditioning environment. The most significant cause of GFRP bar degradation in concrete was found to be driven by chemical reactions, which tend to be accelerated by a range of physical processes, with elevated temperatures playing a key role in triggering and accelerating the development of critical degradation mechanisms. The test results on the long-term performance of GFRP bars in concrete showed that stressed specimens conditioned in a wet environment underwent a reduction in tension stiffening response as a result of bond degradation and a reduced stress transfer from the bar to the surrounding concrete. The results also indicated that the accelerated aging conditions affected overall flexural behaviour and led to overall higher deflections and larger crack widths. A new framework, based on the implementation of the Arrhenius principle and a TSF concept, was developed to account for the effects of exposure temperature, moisture, sustained stress and service life on residual long-term properties of GFRP bars in concrete. A modified design equation was developed and proposed, along with a revised set of environmental reduction factors.
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