The performance and environmental durability of pultruded glass fibre composite rebars
The work reported is a study of the performance of pultruded unidirectional glass fibre composite rebars, in order to understand and quantify the environmental degradation of the materials in water and alkali solution with, and without, stress. The rebars have been proposed as possible candidates for replacing reinforcing steel in concrete structures. Firstly, the interfacial strength, mechanical properties and dynamic mechanical properties of the rebar composites were investigated using ILSS and a three point bending test, in conjunction with SEM and dynamic mechanical thermal analysis (DMTA). It was concluded that the surface treatments on glass fibres played a major role in the ILSS of the composites. Likewise, the flexural properties and failure behaviour of the rebars were also affected and all were related to the ILSS values. With increasing ILSS, the flexural strength and modulus increased and the failure features varied from compressive, to mixed compressive-tensile, to tensile. This was because higher interphase strengths delayed the occurrence of fibre microbuckling and increased the instability of glass fibres in the matrix. It was found that the glass transition temperature (Tg), as measured by DMTA, can be used to assess interfacial strength. A strong interphase resulted in a relatively higher Tg (and vice versa). This appears to have been caused by the more efficient transfer of the "restraining effect of glass fibres" on polymer chain mobility. The rate and magnitude of flexural property degradation in alkali solution was similar to that in water, except that at 80°C, the modulus was considerably reduced. This was caused by the corrosion of defect inclusions introduced during rebar manufacturing process. The results suggest that the hydroxyl ions in the alkali solution do not diffuse into the polymer matrix. Addition of alkali into water did not lead to a significant difference in water absorption dynamics and DMTA behaviour of pure Atlac 580 vinylester resin. Thirdly, fractured glass rebars, subjected to stress and water or alkali solution at 40°C, were studied. It was observed that low stresses resulted in relatively planar cracks normal to the applied tensile stress but did not pose significant damage to the interphase. The opposite was true for rebars under high-levels of stress. Alkali solutions accelerated the occurrence of stress corrosion in terms of failure times. It was found that crosslinked vinylester resin exhibited a diphasic structure as detected by modulated temperature differential scanning calorimetry (MDSC). Based on these results, it is proposed that microcracks form in the polymer matrix of the composite (under low stresses) due to the different responses of the two phases to the stress field at crack tips. Microcracking then allows transport of aqueous media into the composite by a percolation process.