Influence of steel fiber on compressive properties of ultra-high performance fiber-reinforced concrete

The compressive properties (include compressive strength and modulus of elasticity) of Ultra-high performance fiber-reinforced concrete (UHPFRC) are the most important performance index in structural design. This paper presents experimental results from tests conducted on 36 UHPFRCs with different volume fractions and aspect ratios of steel fiber to investigate the effect of steel fiber on the compressive properties of UHPFRC. The test results indicated that the compressive strength and modulus of elasticity of the hardened UHPFRC increase as the fiber volume fraction or aspect ratio increases. However, the increase trend of compressive strength and modulus of elasticity slowed down when the volume fraction exceeded 2%. It was observed that the steel fiber can restrain the occurrence and development of cracks when UHPFRC specimens are compressed, provided a positive effect for reinforcing UHPFRC, but it also reduced the flowability of fresh UHPFRC, which is negative for reinforcing effect. X-ray CT scanning revealed that the porosity and pore size of hardened UHPFRC increased with the increase of the fiber volume fraction due to its weakened flowability. A prediction model was established based on the analysis of the positive and negative effects of the steel fiber. Semi-empirical prediction formulas for the compressive strength and modulus of elasticity were proposed by regression analysis of the test data in this paper, which were verified and revised by the experimental database of 155 tests from literature. In addition, a relationship formula between modulus of elasticity and compressive strength of UHPFRC was presented, and was verified and revised by the experimental database of 320 tests conducted around the world.