Multi-physics Modelling of Moisture Diffusion in the FRP-Concrete Adhesive Joints

FRP-to-concrete adhesive joints are being used increasingly often across a wide range of structural problems, as they provide the possibilities to overcome the uneven stress transfer and stress concentration often found in conventional mechanical anchor systems. For hybrid FRP/concrete substrates, however, the interfacial bond strength is mainly affected by the behavior of adhesive joints, which are known to be highly sensitive to environmental conditions such as moisture uptake and temperature variations. More specifically, polymers are perceptible to moisture ingress, which diffuses out of concrete. The related water uptake leads to a series of changes in mechanical properties, summarized as hydrolytic degradation. A nonlinear FEM-based model was used to simulate the moisture diffusion into the adhesive, which was then coupled with a mechanical degradation model for thermoset polymers. The multi-physics computational framework is able to capture the moisture transport between concrete and adhesive, and adhesive and air. Also, the associated local changes in mechanical response can be obtained. An experimental test campaign in different humidity conditions was selected to calibrate and validate the numerical model, which shows good agreement. The load-slip curves of the single-lap shear test were predicted. The results provide valuable insights regarding the underlying moisture diffusion and interface degradation mechanisms.