Effect of polypropylene fibers on the bond-slip performance of HSS bars in HPC and UHPC

To control the structural performance of reinforced concrete (RC) members, enough bonding between rebars and concrete should be provided. Different parameters affect the bond interaction between rebars and concrete. This investigation tends to assess the bonding resistance behaviour of high-strength steel (HSS) bars in concrete considering the effect of two types of concrete: high-performance concrete (HPC) and ultra-high-performance concrete (UHPC). In addition to the type of concrete, the effect of fibers incorporation is measured. For this aim, a total of thirty-six specimens were cast and evaluated. Two diameters (12 mm and 16 mm) and three embedded lengths (1, 2, and 3 times the diameter of rebars) were also used, and the impact of the rebar's diameter and embedded length on the load-bearing capacity, stress and slip of rebars were examined. To boost the bonding characteristics of reinforcements, three various polypropylene fibres (PF) contents were added: 0%, 0.5% and 1%. A pull-out test was carried out on samples. In addition, the obtained results and previous models proposed by literature have been employed to generate new models to predict the bond-slip characteristics of HSS bars in HPC and UHPC when different PF contents are incorporated. The results showed that the maximum peak of slip between the HSS bars and concrete deteriorated with the utilisation of PF, and this peak declined more for UHPC. Additionally, the load capability of specimens was significantly enhanced when PF were added. Finally, the model suggested in this paper may be used to forecast the ultimate stress and bond-slip characteristics of HSS bars in conventional and PF-reinforced HPC and UHPC, with a good level of correctness with the experimental results.