Investigating the effect of steel micro- and macro-fibers on the bond behavior of steel rebar embedded in ultra-high performance concrete

In recent years, the use of ultra-high performance concrete (UHPC) has been widely considered in special structures and retrofitting of existing structures. Since most of the relations proposed by the regulations are based on the behavior of concrete with normal strength concrete, it is necessary to conduct more experimental studies to predict the behavior of UHPC. One of these notable behaviors is the bond and cohesion between rebar and UHPC, which plays a critical role in the design and seismic behavior of structures. In the present study, a series of experimental tests of uniaxial compressive strength, splitting (Brazilian) tensile strength, elastic modulus, and pull-out of ribbed steel bars from ultra-high performance fiber-reinforced concrete (UHPFRC) were performed. The tests were done considering the four-volume fractions of hooked-end steel macro-fibers, straight steel micro-fibers, a hybrid combination of these two fibers, three different diameters of ribbed steel bars, and several embedded lengths. Moreover, direct tensile and compressive stress-strain curves, Young's modulus, bonding properties of rebar with UHPFRC, and fracture mechanisms were considered. The results indicated that applying micro-fibers may enhance the bond stress between rebar and UHPC better than macro-fibers. Also applying hybrid fibers resulted in the concurrent enhancement of bond stress and the optimum point slip of UHPC in the stress-slip diagram. This finding maybe attributed to the satisfactory performance of micro-fibers in reducing the growth speed of micro-level cracks and the suitable effect of macro-fibers in decreasing the progression speed of macro-level cracks. Furthermore, it was revealed that the increase in rebar diameter and embedding length leads to the reduction of maximum bond stress in UHPFRC.