Impact of UHPC Tensile Behavior on Steel Reinforced UHPC Flexural Behavior

Ultrahigh-performance concrete (UHPC) typically contains short steel fibers with a fiber volume of 2% or larger. This relatively high fiber volume leads to high UHPC tensile strength, typically larger than 8 MPa, and constitutes 30%–40% of the UHPC material cost. Recent studies show that if the reinforcing ratio is low, the high tensile strength of UHPC results in a low structural drift capacity (e.g., less than 2.5%). To lower the cost and improve the ductility of steel-reinforced UHPC (R/UHPC), this study experimentally and numerically explores the relationship between R/UHPC flexural behavior and UHPC material tensile behavior, which is affected by the fiber volume. Experimental variables include reinforcing ratios (0.96% and 2.10%), fiber volumes (0.5%, 1%, and 2%), and two proprietary UHPC materials. Seven simply supported R/UHPC beams are tested. Experimental results show that (1) reducing the fiber volume has little impact on the maximum crack width at the service limit state and the crushing resistance at the ultimate limit state; and (2) with a low reinforcing ratio, reducing fiber volume increases the structural member ductility and provides more failure warnings at the peak load. To understand the relationship between R/UHPC flexural behavior and more possible variations of material properties, a two-level, five-factor factorial experiment was conducted numerically with different combinations of reinforcing ratios, reinforcing steel properties, and UHPC tensile properties. Numerical results show that higher reinforcing ratios, higher steel postyield hardening strength, and lower UHPC tensile strength lead to a more ductile flexural failure. Finally, a minimum reinforcing ratio is validated to mitigate the possibility of low structural drift capacity (e.g., less than 2.5%).