Investigation of bonding behavior of steel ribbed rebar embedded in ultra-high performance concrete reinforced with steel microfiber using multi-scale finite element method

In this study, by developing the finite element model, the pull-out behavior of ribbed steel rebar from ultra-high performance fiber reinforced concrete was investigated. For this purpose, the multi-scale finite element model was simulated in three separate phases, including ultra-high performance concrete in the form of isotropic material, steel microfibers and ribbed rebar. In order to consider more realistic assumptions, Rib pattern of rebar were simulated for the first time and the interaction between concrete with rebar and fibers was modeled by using the adhesive contact zone model. The constant parameters of the adhesive contact zone model were determined by calibrating the results of the finite element model and those of the experimental pull-out test. Finally, after validating the multi-scale finite element results, the maximum pull-out force, the corresponding displacement and the work required for the rebar pull-out were determined using the force-slip curves. Effective parameters such as bond length and rebar diameter, and the volume percentage of steel microfibers, on the bond strength were evaluated. The results show that the bond length has a significant effect on the bonding behavior of the rebar and with increasing the bond length the pull-out force also increases. According to the results, for rebar with a diameter of 20 mm, by increasing the amount of fibers volume from 0 to 2%, the pull-out force and the work increase by about 30% and 22%, respectively.