Influence of fibers on tensile behavior of ultra-high performance concrete: a review

Ultra-high performance concrete (UHPC) or fiber-reinforced concrete (FRC), one of the representatives of cementitious composites, has ultra-high compressive strength and high tensile strength, as well as excellent durability. Fibers are crucial for the tensile strength, post-cracking toughness, and control of cracks in UHPC. However, there are still many questions about the reinforcing effects and mechanisms of fibers on the tensile behavior of UHPC. The parameters, such as fiber type, shape, length-to-diameter ratio, etc., affect the tensile behavior of UHPC to different degrees, and the determination of the optimal mixing ratio of hybrid fibers is a difficult task. Therefore, it is necessary to review, summarize, and compare the studies on the influence of fibers to the tensile behavior of UHPC. The physical and mechanical properties of various fibers used in UHPC are thoroughly reviewed, and the effects of fiber type, content, shape, length, orientation, and ratio of hybrid fibers on the tensile behavior of UHPC are analyzed. Furthermore, three main enhancement mechanisms of fibers, including crack-resistant toughening, fiber shape effect, and chemical bonding, are discussed to improve the tensile behavior of UHPC. The constitutive models of UHPC in tension is also summarized and discussed. Finally, some recommendations and suggestions for future research and development of UHPC in tension are provided.