Static and fatigue bond characteristics of interfaces between CFRP sheets and frost damage experienced concrete

Both short and long-term performances of repaired or strengthened concrete structures using external FRP bonding are greatly affected by states of bonding substrates, which are covercrete and may have experienced various damages. One of them is frost damage in cold regions. This paper intends to investigate how the initial frost damages in concrete influence the static and fatigue bond performances of CFRP/concrete interfaces. Concrete specimens were exposed to freeze and thaw cycles before being bonded with CFRP sheets. The initial frost damage of concrete was controlled approximately at three different levels in terms of its relative dynamic modulus of elasticity, which was 100% (non frost damage), 85% and 70%, respectively. Test results showed that failure modes of CFRP/concrete bonded joints with initial frost damage in concrete were the delamination of covercrete. By contrast the joints without initial frost damage failed in a thin concrete layer as usual. Moreover, CFRP/concrete joints with and without initial frost damage showed different manners in their interface bonding strength and stiffness. If the initial frost damage existed in concrete substrate the effective bond length of CFRP/concrete joints was increased due to the decrease of the bonding stiffness and interfacial fracture energy. Fatigue testing results indicated that the linear slopes of S-N curves of CFRP/concrete bonded joints were not influenced by the initial frost damage. The initial frost damage did not shorten the fatigue life of CFRP/concrete joints if a same relative tensile stress level was kept in the FRP sheets, where the relative tensile stress level was defined as a ratio of the applied tensile force in FRP sheets for the fatigue tests to the maximum static pullout one achieved in each test series.