Effect of Aggregate Grading and Steel Fiber on the Properties of Ultra-High Performance Fiber Reinforced Concrete

Coarse aggregate is one of the factors affecting the workability and the properties of Ultra-High Performance Concrete (UHPC). In addition, adding steel fiber into UHPC constituents enhances the toughness of the material under tensile stress. In the present study, the aggregate consisted of quartz sand and crushed stone, while the steel fiber content was varied at 0.0, 1.0, 1.5, 2.0 and 2.5% by volume, respectively. The findings show that the compressive strength reaches the highest value at the age of 28 days, measuring 151.2 MPa, which corresponds to 2.0% steel fiber content. The compressive strength at 3 days achieves approximately 65–75% of the 28 day strength, and at 7 days, it reaches 75–85% in comparison. The steel fiber content has a minor impact on the compressive strength but significantly affects the flexural strength of concrete. The flexural strength increases by 72% with increasing fiber content from 1% to 2%. Moreover, the samples with 2.5% steel fiber content result in a notable properties decline of both fresh and hardened concrete.