Fatigue Behavior of Large-Scale Reinforced Concrete Beams Strengthened in Flexure with Fiber-Reinforced Polymer Laminates

Abstract

The aim of the present research is the experimental investigation of the response of carbon-fiber-reinforced polymer (CFRP) strengthened reinforced concrete (RC) beams of large scale under fatigue loading. Reinforced concrete beams were cast for this purpose, both with rectangular and T cross-sections, with web dimensions of 200×500 mm and 3,050 mm in length. Beams were strengthened in flexure with externally bonded CFRP laminate [externally bonded reinforcement (EBR)] or with CFRP laminates bonded into slits on the concrete cover surface [near-surface-mounted (NSM)]. Transverse CFRP sheets were placed on the shear spans of the beams to avoid fragile shear failure. All specimens were tested under fatigue loading, with load-unload cycles of 2 Hz frequency. Two different amplitudes of cycles were investigated. The different fatigue life and modes of failures are discussed. Beams subjected to the low loading range, simulating the service loads of a bridge beam, could sustain more than 2 million cycles. Beams subjected to the high loading range, according to the maximum limit of industry standards, presented fatigue failure of the reinforcing steel in cycles less than 1.5×106. The ratio of the axial rigidity of the tensile steel bar ks to the axial rigidity of fiber-reinforced polymer (FRP) strengthening kf plays a significant role in extending the fatigue life of steel and thus of RC FRP strengthened beams. The lower the ks/kf ratio, the higher the fatigue life of the retrofitted member. NSM strengthening may maintain a full bond with the concrete substrate even after the fatigue fracture of steel bars. Therefore, it may provide residual capacity to the retrofitted member to survive detrimental collapse.