Long-term moisture diffusion in vinylester resin and CFRP rebars: A 20-year case study

Moisture absorption in polymer composites is a slow process that can take several decades, so it is often modelled without experimental verification. This unique study reports the moisture diffusion in epoxy vinylester (VE) resin and carbon fibre reinforced pultruded (CFRP) rebars over a period of 20 years at ambient temperature. Non-Fickian diffusion in VE is modelled by the unified two-phase model, which combines the bound water or hin-dered diffusion and the polymer relaxation concepts. Calculations based on the short-term projection with an unknown true saturation moisture content result in highly underestimated predictions. The true secondary equilibrium is 30-60% higher than the Fickian "pseudo-equilibrium". Moisture absorption of CFRP rebars at 98% RH was studied on short and long samples to identify the axial and radial diffusivities. The concept of apparent diffusivity, combining edge and anisotropy effects, is extended to the case of cylindrical rebars. Rebars absorb up to 0.55% of moisture following the Fickian behaviour, and the predictions are consistent with the calculations using the microstructural considerations. Calculations based on short-term projections result in highly over-estimated axial diffusivity and the ratio of diffusion anisotropy. Long-term ageing of VE resulted in up to a 40% reduction in tensile strength and a 10 degrees C reduction in glass transition temperature. The interlaminar shear strength of CFRP rebars decreased by 30%, and its partial retention after moisture desorption indicated inter-phase degradation. The results will contribute to an understanding of water diffusion and environmental ageing of VE-based composites, bridging the gap between real-life degradation performance and accelerated durability predictions.