Bond behaviour between reinforcing steel bars and FRC under monotonic loading

Bond phenomenon allows the variation of the bar tensile action to be transferred to the surrounding concrete by means of shear stresses developing along the lateral surface of the bar. This mechanism entails the formation of radial transversal pressure as a result of the wedge action of the crashed concrete between the ribs. When the tensile strength is reached in the concrete cover, the wedge action may cause the onset of longitudinally splitting cracks which impair bond behaviour, unless a proper confinement is present. The latter may be provided by an external pressure when the bar is near to a support, by transverse reinforcement and by the post-cracking resistance of the cracked cover. This resistance mainly depends on the post-peak behaviour of concrete, which is usually negligible in normal concrete but can be significantly enhanced with the addition of fibers. Based on the available results from the literature, in the first part of the paper an analysis on the bond behaviour between steel rebars and FRC is presented. The influence of several parameters, such as concrete strength, fibers content, aspect ratio and concrete cover is evaluated. Eventually, two sets of experimental results of pull-out tests on short anchorages embedded in plain concrete and fiber reinforced concrete, will be also presented. The first one aims to simulate the influence zone of one stirrup. A volume content of steel hooked fibers equal to 0.38% has been investigated. The second one aims to study the influence of fiber orientation by adopting a volume content of steel hooked fibers equal to 0.5%. The test results show that the addition of hooked steel fibers increases bond strength and reduces splitting crack opening but it significantly depends on fiber orientation.