Fracture toughness of UHPC mixtures: Effects of w/b ratio, cement and silica fume contents

Ultra -high performance concrete (UHPC) is an innovative concrete known for its exceptional strength, durability, and toughness, making it highly desirable for various applications. However, a comprehensive understanding of the factors influencing UHPC's fracture toughness, such as water/binder (w/b) ratio and cement and silica fume (SF) contents, is still needed. This study aims to address this knowledge gap. Twelve different UHPC mixtures were prepared, considering different w/b ratios, cement and SF contents, while employing an optimal steel fiber content across all mixtures. The UHPC specimens were fabricated and subsequently tested to ascertain their compressive strength (f'c), elastic modulus (E), and fracture toughness. The stress intensity factor, Kic, and fracture -energy release rate, Gf, were determined as indicators of the fracture toughness. The statistical analysis of the experimental data revealed that the cement content had a major effect on the fracture toughness whereas the w/b ratio and SF content had a relatively minor effect. The empirical models for Kic and Gf (in terms of w/b ratio, cement, and SF contents), were developed through regression analysis. These models showed that fracture toughness decreases with an increase in the w/b ratio but increases with higher cement and SF contents. Notably, Kic and Gf correlated well with the key mixture parameters but showed no significant correlation with the measured f'c of the UHPC mixtures. The empirical models developed for Kic and Gf with a high degree of accuracy can be effectively utilized to optimize the design of UHPC mixtures. These models enable the achievement of desired fracture toughness within the range of the key mixture parameters investigated in the current work. The findings of this study enhance the understanding of UHPC fracture toughness and provide valuable insights for the practical application and optimization of UHPC mixtures.