Role of Internal Damage Mechanisms in Controlling Bond-Slip Behavior in Pullout Tests in Concrete

The bond-slip relationship is determined from the measured load and the measured relative displacement between the rebar and the concrete in a pullout test. It is considered a characteristic constitutive property of the rebar-concrete, interface and in standards and manuals it is commonly described by a single nondimensional bond stress-slip curve. It is well known that the actual bond-slip relationship depends on the concrete uniaxial compressive strength. Examination of the results of many pullout experiments on normal-strength concrete specimens conducted for the present study indicates that numerous bond-slip curves do not follow the expected single nondimensional curve. Analysis of available data clearly shows two typical normalized bond-slip curves that differ mainly in the slope of the descending branch and the residual bond stress at large values of slip. A series of complementary pullout tests has been conducted for validation and for further analysis of the results. Results of these tests clearly show the two basic bond-slip curve forms, supporting the foregoing observations. Examination of the internal cracking and local damage in the tested specimens at the end of each experiment indicates that the two different curve forms are related to different internal damage mechanisms, which depend on the concrete tensile strength.