Influence of restraint thermal effects on the bond strength of externally bonded CFRP-concrete joints

Elevated temperatures lead to a reduction of the bond strength of FRP (fibre reinforced polymer) reinforcement glued on concrete by ambient cured epoxy adhesive. This degrading behaviour is mainly observed for exposure temperatures beyond the glass transition temperature of the epoxy (T-g similar or equal to 60-80 degrees C) which is usually 2-3 times lower than the T-g of the matrix of pultruded FRP reinforcement. Contrary, lap shear tests on FRP-concrete systems made with carbon fibre reinforced polymer (CFRP) revealed an increase of the bond strength at temperatures close to the T-g of the epoxy adhesive. Several explanations have been expressed for this unexpected trend. The most accredited hypothesis describes the increase of bond strength as a consequence of the induced restraint thermal effect between the CFRP and the concrete substrate, because of the different thermal elongation of the materials. In this work, the influence of the restraint thermal effect is investigated further in terms of influence on the ultimate load recorded by lap shear tests at elevated temperature and to understand better the underlying mechanism. The obtained outcomes have revealed that the evaluation of the restrained thermal action has to consider the influence of the bond length of the joint and the thermal degradation of the adhesive.Elevated temperatures lead to a reduction of the bond strength of FRP (fibre reinforced polymer) reinforcement glued on concrete by ambient cured epoxy adhesive. This degrading behaviour is mainly observed for exposure temperatures beyond the glass transition temperature of the epoxy (T-g similar or equal to 60-80 degrees C) which is usually 2-3 times lower than the T-g of the matrix of pultruded FRP reinforcement. Contrary, lap shear tests on FRP-concrete systems made with carbon fibre reinforced polymer (CFRP) revealed an increase of the bond strength at temperatures close to the T-g of the epoxy adhesive. Several explanations have been expressed for this unexpected trend. The most accredited hypothesis describes the increase of bond strength as a consequence of the induced restraint thermal effect between the CFRP and the concrete substrate, because of the different thermal elongation of the materials. In this work, the influence of the restraint thermal effect is investigated further in terms of influence on the ultimate load recorded by lap shear tests at elevated temperature and to understand better the underlying mechanism. The obtained outcomes have revealed that the evaluation of the restrained thermal action has to consider the influence of the bond length of the joint and the thermal degradation of the adhesive.A