Uniaxial Compression Behavior of Ultra-High Performance Concrete with Hybrid Steel Fiber

This study investigated the effects of hybrid steel fiber of different sizes (6 and 13 mm) on the uniaxial compression performance of ultra-high performance concrete (UHPC). The failure pattern, stress–strain relationship, and toughness under the uniaxial compression of a reference UHPC mixture with no fiber and five UHPC mixtures with mono or hybrid fiber content of 2%, by volume of concrete, were studied. The results indicated that the incorporation of hybrid fiber significantly improved compression properties, with increased strength and toughness. An UHPC mixture with 1.5% long and 0.5% short fiber showed the best compressive behavior, whereas those with 2% short fiber showed the worst properties. Based on the obtained peak stress, strain at peak stress, elastic modulus, and compression toughness index, an analytical model was proposed to generate the complete stress–strain relationship. The proposed model agreed well with the experimental results.