Effects of fibers on the mechanical properties of UHPC: A review

Ultra-high performance concrete (UHPC) developed rapidly in research and commercial use during the recent decade. Significant progress has been achieved in its material science and technology, including why and how to add discontinuous fiber reinforcement in it. This paper reviews the researches on understanding the effects of various fibers on the mechanical properties of UHPC, focus on the straight steel fibers but involving also deformed steel fibers, non-steel fibers as well as hybrid fibers. It also discusses the research methodology, prediction of mechanical properties by fiber factors, and the classification of UHPC mechanical properties related to this topic. It shows that (1) the experimental research is the main methodology for investigating the effect of the fibers on the mechanical properties of UHPC; the tensile performance of UHPC should be studied by uniaxial tensile tests and its representative indicators should include tensile strength, initial cracking strength, and peak tensile strain; (2) fiber plays an essential role in the reinforcement of the tensile strength, compressive strength, modulus of elasticity, and other material properties of UHPC, but in weakening the flowability of fresh UHPC. The positive and negative effects of fibers on the mechanical properties of UHPC should be considered, and the technology should be developed to maintain the flowability when high volume fraction of fibers is added in the UHPC; (3) the parameters of steel fibers affecting the mechanical properties of UHPC include volume fraction, size, shape, orientation and distribution, average bonding strength and minimum tensile strength, etc., which are mainly studied independently in the existing research. The studies on the combined effect of these parameters are limited but worthy of further investigation; (4) hybrid fibers could efficiently produce reinforcement effects for UHPC. It has great practical and research significance to conduct in-depth studies though the theoretical analysis and quantitative prediction are complex.