Bond stresses along plain steel reinforcing bars in pullout specimens

Mechanics-based relationships are established between bond stress, bar force, slip at the unloaded end of the bar and slip along the length of plain steel reinforcing bars in pullout specimens. Two 200 mm (7.9 in.) diameter by 800 mm (31.5 in.) long pullout specimens reinforced with instrumented built-up hollow reinforcing bars were tested. Maximum pullout resistance was observed just before slip initiated at the unloaded end of the bar and the bond resistance subsequently reduced as slip increased. It is shown both analytically and experimentally that bond stress magnitudes vary along the length of the bar at all applied loads. The location of the peak bond stress shifts from the loaded end toward the unloaded end of the specimen with increasing applied load. A theoretical two-step bond stress-slip model is shown to capture the essential features of bond behavior observed experimentally in pullout specimens.