Fluctuation of bond stress-slip behaviour of deformed bar under displacement control

The fluctuation of the bond stress as a function of the slip of deformed bar was determined experimentally, under displacement control, using 200 mm concrete cubes with centrally embedded 16 mm deformed bar designed to be in a confined condition. The observed global fluctuation in the bond stress, measured by linear variable differential transformers, was verified by independent measurement of the local strains along the length of the embedded bar using bonded electrical resistance (ER) strain gauges. In the pull-out tests, the failure mode was by shear. The ER strain gauges were installed inside grooves carefully machined in the steel bar to prevent direct friction from the concrete damaging the strain gauges. This approach also prevented damage to the wires and wire/gauge connection during the process of bar slip. The global fluctuation of the bond stress-slip behaviour was determined graphically and analysed using a number of statistical methods and independently verified from the local strain variation along the bar obtained from the strain gauge readings. Based on these observations, the fluctuation in bond stress with slip under displacement control has been shown to be mainly dependent on the transverse rib pattern of the bar