CONCLUSIONS

This chapter has discussed the durability of GFRP bars in concrete environments and their long- term performance in concrete structures. The purpose was to review and summarise the current knowledge and research to identify the areas where further research is still required. A number of aggressive agents that affect the durability of GFRP bars used in civil engineering applications were identified and presented. Although it is widely known that GFRP rebars have different advantages in terms of mechanical characteristics compared to conventional steel reinforcement, the influence of potential synergies on the mechanical properties of loading and environmental exposure in real environments needs further investigation. Exposure to high alkalinity of concrete, moisture and under loading conditions at elevated temperature was identified as the most harmful

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form of synergy affecting the lifetime of GFRP bars in concrete structures. Current results indicate that GFRP bars are prone to degradation in alkaline environments due to the presence of hydroxide ions and water molecules [147]. The diffusion of these hydroxide ions and water molecules into the rich resin layer of the bars can have adverse influences on long-term durability of GFRP bars in concrete elements; hence it is necessary to understand fully the long-term durability and performance of GFRP bars in order to have confidence in using these bars in concrete. Although the resin matrix in GFRP bars around individual filaments is expected to protect the glass fibres from attack by harmful agents, the alkaline pore solution in the surrounding concrete can expedite the degradation process of resins, resulting in significant reductions in tensile strength and degradation in the concrete/GFRP reinforcement bond. The factors that were identified as affecting this degradation are matrix type, fibre type, manufacturing processes, installation procedures and the short- and long-term loading and exposure condition (physical and chemical). The resins matrix on GFRP bars, in general, plays an important role in protecting glass fibres from moisture ingress and transferring the load to glass fibres. However, moisture diffusion into the resins matrix of GFRP bars subjected to stress at different temperature levels with respect to solution concentrations has not been comprehensively assessed and its deterioration effects on the bond behaviour with surrounding concrete have not been evaluated so far. In this context, the bond degradation of GFRP bars under sustained stress and subjected to different environmental conditions needs to be examined to provide a safe design. However, the long-term performances of GFRP in concrete structures under loading conditions generally have not been extensively investigated and are not fully understood. Potential synergies of sustained stress, high alkalinity and elevated temperature associated with moisture can have significant effects on degradation mechanisms of GFRP materials. It is therefore important to evaluate their influence on the long- term properties of materials in order to prevent undesirable outcomes in the structure’s lifetime. No study has been carried out on degradation mechanisms of GFRP bars subject to various combinations of different sustained stress, different alkaline environments and different temperatures associated with wet environments for gaining comprehensive understanding of degradation processes. The rates of degradation of mechanical properties (tensile properties, flexural properties, transfer properties) are also not available to designer engineers, either in the form of tables or design charts, with respect to the expected environmental exposure and loading conditions. Gaining this knowledge requires in-depth investigation of the changes that occur in chemical, physical and microstructural properties before and after aging.

After reviewing the literature on durability testing approaches, it was observed that there is no unified standard method to use as guidance. Each investigation or research study has used its own

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experimental method to produce experimental data for modelling potential GFRP bars degradation. Correlations between accelerated aging (laboratory aging) and natural exposure have been derived by using different models. Most of these models were generated based on Fick’s law or the Arrhenius principle or by simple regression analysis. Through reviewing previous work for measuring the durability of GFRP, it was decided to design an experimental programme, based on accelerated test methods (see Chapters 3-5). The selected accelerated test methods included high alkalinity environments, wet environments, elevated temperature associated with sustained stress, since the potential synergies of these parameters were identified as the most aggressive agents affecting the durability of GFRP bars in concrete structures. Different combinations and scenarios of these parameters were therefore proposed to study the change over time of the bars’ mechanical properties (i.e. tensile strength, flexural strength, bond strength and modulus of elasticity). In an attempt to achieve a more fundamental understanding of their long-term behaviour, several techniques were proposed to investigate the changes in chemical, physical and microstructural properties using moisture absorption tests, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX) and dynamic mechanical analysis (DMA). As criteria for assessing the long-term performances of GFRP in concrete structures, their bond performances with concrete and the flexural behaviour of GFRP RC beams were selected. It is also worth mentioning that the available information in the literature addressing the long-term performance of GFRP bars in concrete elements is very limited in terms of the conditions considered in this research.

It appears that by understanding the degradation mechanisms of GFRP bars in concrete environments under loading conditions and by accumulating and analysing the experimental results, the model originally proposed in fib Bulletin 40 [5] can be modified and improved to predict long-term mechanical performances of GFRP bars in multiple environments under loading conditions which represent the exposure conditions of GFRP RC structures in service. This model could enable structural engineers to predict long-term degradation of GFRP bars, in the form of tensile strength reduction and or bond strength reduction, without resorting to running a long- term and costly testing program.

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