Effect of bond loss of tension reinforcement on the flexural behaviour of reinforced concrete beams

An experimental programme has been conducted in order to investigate the flexural behaviour of concrete beams with variable un-bonded length of tension reinforcement. A test series of six simple beams containing different nominal length without bond close to the support had been conducted in this investigation. The tested beams are of 2250 mm total span loaded at the middle third with two equal concentrated loads. The bond loss had been introduced with plastic tubes surrounding the longitudinal tension reinforcement leaving short bonded lengths over supports and at positions of stirrups crossing the longitudinal reinforcement. The effect of reinforcement bond loss on the response, cracking load, crack propagation, deflection, ultimate capacity, reinforcement strain at mid span and mode of failure of beams is examined in this paper. The cracking load, number of cracks in the flexural zone, and the crack width are affected significantly with increasing the area of bond loss. On the other hand, the reduction in the ultimate load capacity is surprisingly low even with 73% loss of bond. This refers to the creation of high bond forces at the small bonding areas at the crossing stirrups which compensates the high bonding loss in the longitudinal bars between stirrups.