Evaluation of Microsilica and Nanosilica on Bond Properties between Moderate–High Strength Concrete and Plain–Ribbed Steel Rebar

Individual and synergistic effects of microsilica (MS) and nanosilica (NS) on the bond properties between moderate–high strength concrete and plain–ribbed steel rebar were investigated. Two different concrete series with water–cementitious material ratio (W/Cm) of 0.55 and 0.36 were studied. Pull-out tests were performed to analyze the bond strength and bond-slip behavior. Compressive strength, splitting tensile strength, and elastic modulus of the samples were also reported. Results have revealed that although the compressive strength showed a considerable increase after adding MS and/or NS, the modulus of elasticity and splitting tensile strength were not affected considerably. With the addition of MS and/or NS, while an increment was seen in the bond strength for the plain rebar, no significant effect was observed for the ribbed one. In addition, despite residual bond strengths showing an increase in the plain rebar, a reduction was seen for the ribbed one. In terms of slip energy, the use of MS and/or NS for plain rebar made a positive contribution, i.e., the slip energies for both concrete strength groups increased. On the other hand, for the ribbed rebar case, slip energy was negatively affected for the concrete with W/Cm of 0.55, while no significant trend was observed for the 0.36 W/Cm group. Based on the bond-slip data obtained, new prediction models were proposed to estimate bond strength and bond-slip behavior for both rebar types. These are compared with other models available in the literature. For the ribbed rebar, the slip amount, where the plateau region on the bond stress versus slip curves begins, was significantly lower than the assumptions considered in the other models. Considering the descending parts of bond stress versus slip relations, the residual bond stress can be maintained more efficiently in plain compared to ribbed rebar.