Enhancing the High-Temperature Fracture Toughness of Ultrahigh-Performance Concrete through Optimization of Ternary Cement Matrix and Plastic Fiber Geometric Properties

Abstract

This study aims to enhance the high-temperature resistance of ultrahigh-performance concrete (UHPC) by optimizing the cement-fly ash-silica fume ternary cement matrix composition and the geometric characteristics of the plastic fibers. The UHPC was designed and prepared with the cement-fly ash-silica fume ternary cement matrix through simplex centroid method and 2% volume fraction steel fiber. Polypropylene fibers with different geometric characteristics were incorporated to enhance the high-temperature resistance. The fire resistance is examined through the high-temperature exposure test. The initial bursting temperature is recorded, and the deterioration of mechanical performance is characterized. It was found that the decrepitation temperature of UHPC first increases and then decreases with the fly ash (FA) content. Meanwhile, the influence of the silica fume (SF) and cement content on the decrepitation temperature is not obvious. The optimum mix ratio of 50%–65% cement, 20%–30% FA, and 10%–20% SF is recommended to prepared the cementitious matrix with 450°C or higher initial burst temperature. The crack mouth opening displacement (CMOD) test assisted with digital image correlation (DIC) examination is conducted to characterize the fracture performance before and after high-temperature exposure. It was found that the fracture resistance of UHPC first increases and then decreases with the silica fume content, which reaches the maximum value of 16.64 N/mm with 27% SF content. Meanwhile the high-temperature resistance of UHPC increases and then decreases with the increase of the length of the doped polypropylene (PP) fibers. The added 1.2% volume fraction PP fiber can resolve the spalling issues of the UHPC materials, and the residual compressive strengths after exposure to 1,000°C of UHPC samples containing 15 mm–18 μm and 15 mm–33 μm fibers can exceed 50 MPa. This study can serve as a solid base for the fire resistance design of UHPC materials in field construction.