Bond Behavior between Near-Surface-Mounted CFRP Strips and Concrete at High Temperatures

This paper presents an experimental study concerning the bond behavior at high temperatures between concrete and carbon fiber–reinforced polymer (CFRP) strips installed according to the near-surface-mounted (NSM) technique. Double-lap shear tests were performed on concrete blocks strengthened with CFRP strips installed into slits and bonded with either an epoxy adhesive (series EP) or a mixed grout composed of epoxy and cement binders (series MG). Specimens were first heated up to predefined temperatures (from 20 to 150°C, measured in the adhesive) and then were loaded up to failure. The obtained results allow drawing the following conclusions: (1) the axial strains along the bonded length became closer to linear (at high temperature) because of the softening (glass transition) of the bonding materials; (2) the effective bond length increased for elevated temperature; (3) the bond-slip curves exhibited considerable and consistent stiffness and maximum shear stress reduction with temperature; (4) the failure modes in series EP changed from cohesive (in the concrete) at ambient temperature to adhesive (at the CFRP-adhesive interface) at elevated temperatures; and (5) the bond strength was considerably reduced with temperature. The mixed grout, although providing similar bond strength at ambient temperature and in spite of containing cement binders, suffered much higher performance reduction with elevated temperature compared to epoxy. With this latter adhesive, very high residual strength was measured for temperatures well above the Tg of the epoxy. The results of series EP show also that for all temperatures tested the bond strength of CFRP-concrete interfaces installed according to the NSM technique is much higher than that of similar interfaces installed according to the more conventional EBR technique; furthermore, the NSM technique also leads to lower reduction of residual strength with temperature.