Tension-stiffening effect in steel-reinforced UHPC composites: Constitutive model and effects of steel fibers, loading patterns, and rebar sizes

The tensile performance of steel-reinforced concrete members is closely associated with the bond interaction between concrete and the embedded rebar. Ultra-high performance concrete (UHPC) is a rapidly emerging concrete material that has an ultra-high compressive strength and bond strength. When it is reinforced with short, discontinuous fibers, it features a tensile strain-hardening behavior and a damage pattern of closely spaced narrow cracks. The present study investigated the tensile behavior of steel-reinforced UHPC members. Sixteen samples were tested, with the experimental variables including embedded rebar sizes, loading patterns, and steel fibers. The tensile responses of the steel-reinforced UHPC samples were evaluated using multiple performance measures, including the damage pattern, stiffness, load-deformation relationship, rebar strain, and the tension-stiffening behavior of UHPC. The test results showed that the enhanced bond strength due to the inclusion of steel fibers transformed the failure pattern of the steel-reinforced UHPC from multiple localized cracks into a single localized crack, which intensified the strain concentration in the embedded rebar. Although the addition of fibers substantially strengthened the tension-stiffening response of the UHPC, it also raised critical concerns about premature failure, especially when UHPC members were reinforced with small steel bars and subjected to monotonic loading. In addition to the experimental study, a tetra-linear constitutive model that was able to reasonably represent the tension-stiffening behavior of fiber-reinforced UHPC up to failure was suggested in this study.