The time-dependent response of concrete bond to steel under high sustained loads

Design errors, poor construction practices, material deterioration, and other issues can lead to reinforced concrete members being subjected to levels of sustained stresses higher than their expected service loads. These high levels of sustained stress have led to catastrophic RC building collapses around the world. Loading a member greater than 0.75fc' is referred to as high sustained loading and is in the range that could potentially lead to the progressive collapse of a structure. Unfortunately, the behavior of concrete members under this loading condition is not well understood. By gaining understanding of member's behavior when loaded under high sustained stresses, future engineers can better identify elements nearing failure. Preliminary tests performed on simple RC beams tested in flexure showed the beams' tendency to increase in deflection with time--more than the expected creep deflection. This extra deflection occurring at high sustained loading may be accounted for by slippage between the reinforcing bar and the concrete. In order to isolate and study this bond slippage under high sustained loads, beam-end bond tests were loaded to 85-90 [percent] of their short-term capacity and observed for several weeks. The tests show that continued slipping is occurring under high sustained loads. The first sustained load test experienced 22 of its overall slip when under sustained load. The first sustained test also withheld a bond load of 12 [percent] more than the control specimens. The three sustained specimens experienced an increase in free end slip from 0.001" to 0.00254" under constant load which is 90 [percent] to 230 [percent] of the slip at failure in the control specimen. This slip under high levels of sustained load may contribute to the additional deflection under sustained load in reinforced concrete members.