Effects of elevated temperatures and of loading procedures on the bond performance of reinforcement in concrete

The decay in the local bond performance of reinforcement bars in concrete due to exposure to high temperature is often investigated performing pull-out tests on steel rods embedded in the center of concrete specimens. The tests are generally performed in residual state i.e. upon cooling down to room temperature. However, this approach neglects not only the detrimental effects of the temperature on splitting strength of bond, but also the effects of the application of a sustained load during the heating phase, as in real applications. This paper investigates the bond performance between steel and concrete after exposure to high temperatures as a function of the rebar position, maximum exposure temperature and heating and loading sequences. Three different rebar positions are investigated, namely, rebar located in the center of the specimen (clear cover to both edges equal to 72mm), rebar at the edge (clear cover to closer edge 24mm and to the other edge 72mm), rebar at the corner (clear cover to both edges 24mm). Two different heating and loading sequences are considered. In a first series, the load is applied after cooling down of the specimen from the desired target temperature, while in a second series a constant load is applied prior to temperature increase. Results obtained through the two procedures are compared and discussed, also with respect to existing code provisions.