Effect of concrete cover and embedment length on bond strength of steel reinforcement in UHPC: An experimental and numerical approach

This study investigates the bond behavior between steel bars and ultra-high performance concrete (UHPC) through experimental testing and numerical modeling. A total of 80 pull-out tests were conducted on UHPC specimens reinforced with 13 mm and 16 mm steel bars, examining the effects of varying concrete cover thicknesses and embedment lengths. Bond–slip curves, failure and cracking patterns, and bond strength were compared and discussed. Results showed that increasing concrete cover thickness led to a notable enhancement in the average peak bond stress, with improvements of up to 1.5 and 3.0 times observed and contrasting longer embedded lengths results. Numerical simulations using the LS-DYNA program validated the experimental findings, accurately predicting bond behavior and failure modes. Based on these findings, a modified peak bond stress predictive equation was proposed, demonstrating high accuracy for UHPC with compressive strengths between 150 and 200 MPa. This comprehensive analysis provides an experimental and numerical basis for calibrating bond–slip models and for developing future design recommendations for UHPC-reinforced structures.