Interfacial Behavior of RC Beams Strengthened with FRP under Fatigue Loading

The paper aims to study interfacial behavior of reinforced concrete (RC) beams strengthened with fiber reinforced polymer (FRP) under fatigue loading. A series of fatigue tests were conducted in the laboratory. The infrared detection technology was used to record the FRP-concrete interfacial fatigue behaviors. The results demonstrated that the fatigue development by RC beams strengthened with FRP was divided into three typical stages: rapid increase, stable-change, and uncontrollable propagation. The period of stable-change accounted for more than 99% of the fatigue lifetime. The average bending stiffness and the average FRP fatigue strain of the stable-change phase obtained under certain fatigue load were close to those obtained under the same monotonic load. The FRP threshold strain of the fatigue failure was about 37% of that of the monotonic load. The infrared thermograms showed that the interfacial fatigue debonding originated from the midspan and expanded to one end of the strengthened beam, which led to the final failure. The FRP fatigue strain and interfacial damage stayed stable in most of the fatigue lifetime, indicating good interfacial anti-fatigue properties. Based on the experimental data obtained in this study, a double logarithm formula was proposed to predict the fatigue life of RC beams strengthened with FRP under cyclic bending loads.