Effect of Crystalline Waterproofing Admixture for Enhancing the Mechanical and Fracture Properties of Ultrahigh-Performance Fiber-Reinforced Concrete

This research investigates the enhancement of mechanical and fracture properties of ultrahigh-performance fiber-reinforced concrete (UHPFRC) through the incorporation of a crystalline waterproofing admixture at dosages of 0.5, 0.75, and 1.0% relative to the cement content. The goal is to improve the interfacial bond between the cementitious matrix and steel fibers, thereby enhancing the overall mechanical and fracture performance of UHPFRC. The chemical modifications induced by the admixture within the matrix were evaluated using thermogravimetric analysis and porosity measurements, while physical alterations were assessed through workability and setting time tests. In addition, porosity and stereo microscope analyses were conducted to elucidate the interaction between the crystalline waterproofing admixture and the cementitious matrix, as well as its influence on steel fiber bonding. Mechanical and fracture properties were examined using standardized flexural and compressive strength tests specifically designed for steel fiber–reinforced concrete. The evaluation of the fresh-state properties indicated a decrease in workability with increasing crystalline waterproofing admixture content. Moreover, the addition of crystalline waterproofing admixture led to a linear reduction in both initial and final setting times. Stereo microscopy images revealed enhanced interaction and bonding between fibers and the matrix, attributed to the filling of macropores by the crystalline waterproofing admixture. Notably, the addition of the admixture–cement ratio of 0.75% resulted in a 17.6% increase in compressive strength and a 74.6% enhancement in flexural strength. Furthermore, the results revealed a strong linear correlation between residual strengths, with coefficients of 0.96 for fR1, indicating efficient fiber distribution. However, a decline in reinforcement efficiency was noted as crack openings exceeded 2.5 mm.