Bond between 0.7-inch-diameter steel strands and UHPC: Pullout test, finite element simulation, and estimation of transfer length

In prestressed structures, the combination of Ultra-High Performance Concrete (UHPC) and large-diameter steel strands can have a synergistic effect, but whether this synergy can be achieved is dependent on the bond between them. This study aimed to investigate the bond property of 0.7-inch-diameter steel strands embedded in UHPC by conducting nine sets of orthogonal pullout tests. The experimental design focused on examining the influence of steel fiber content, embedment length, and concrete cover depth. Variance analysis was utilized to determine the relative significance of these factors, the results of which revealed that the steel fiber content had the most significant impact compared to embedment length and concrete cover depth. Moreover, the steel fiber content was found to affect the failure mode that occurred during the pullout tests. To accurately replicate the two observed failure modes, i.e., pullout failure and splitting failure, two types of finite element models were developed to simulate the test. Furthermore, based on the thick-walled cylinder theory, an empirical formula for the transfer length of steel strands in UHPC was proposed. A comparative study of the proposed formula, existing formulas, ACI code specification, and experimental data was conducted to demonstrate the accuracy of the proposed formula.