Effects of temperature and sustained loading on the mechanical response of CFRP bonded to concrete

Fiber reinforced polymer composites are commonly bonded to concrete structures for strengthening reasons, even though little is known about the evolution of bond strength while the strengthening systems are subject to sustained loads and elevated temperatures. This investigation uses single-lap shear specimens to examine the effects of sustained load and elevated temperature on the time-dependent deformation of a carbon fiber reinforced polymer (CFRP) composite bonded to concrete as well as the pull-off strength of the composite after the sustained loading period. Increased temperature during the sustained loading period led to increased slip of the CFRP, whereas increased curing time of the polymer resin prior to the sustained loading period led to reduced slip. Specimens that underwent sustained loading had increased pull-off strength and interfacial fracture energy following the sustained loading period. This beneficial effect was most significant for roughly 30 days of sustained loading. The analysis of strain distributions and fracture surfaces indicated that stress relaxation of the adhesive occurred in the 30 mm closest to the loaded end of the CFRP during sustained loading, which increased the pull-off strength provided the failure locus remained mostly in the concrete. For longer sustained loading times, the bond failure locus shifted to the epoxy/concrete interface, which diminished part of the strength increase brought on by the stress relaxation of the adhesive.