The Double-K Fracture Toughness of Concrete with Different Coarse Aggregate Volume Fractions

This study examines how coarse aggregate volume fraction (<i>V</i>_a) affects the double-<i>K</i> fracture toughness and fundamental mechanical properties of concrete. Wedge-splitting tests were conducted on specimens with six different <i>V</i>_a values: 19%, 25%, 31%, 37%, 43%, and 50%. The results indicate that compressive strength (<i>f</i>_c) and elastic modulus (<i>E</i>) consistently increase with <i>V</i>_a, reaching 59.8 MPa and 37.9 GPa at <i>V</i>_a = 50%, respectively. Conversely, tensile strength (<i>f</i>_t), double-<i>K</i> fracture toughness (including initiation toughness KICini and unstable toughness KICun), and fracture energy (<i>G</i>_IF) initially increase before decreasing, peaking at an optimal <i>V</i>_a of 37%. Specifically, KICini, KICun, and <i>G</i>_IF reached their maximum values of 0.54 MPa·m^1/2, 1.20 MPa·m^1/2, and 225.0 N/m at <i>V</i>_a = 37%. The tortuosity of crack paths follows a similar trend, becoming more pronounced up to <i>V</i>_a = 37% before diminishing. Furthermore, quantitative exponential relationships were established between <i>f</i>_t and KICini, KICun, and <i>G</i>_IF. A safety warning parameter (<i>δ</i>), derived from the double-<i>K</i> fracture toughness, was proposed to quantitatively assess the pre-peak ductility, with values ranging from 0.88 to 0.72 in this study. The findings offer valuable guidance for optimizing concrete mix design, suggesting that a <i>V</i>_a range of 25% to 31% provides an optimal balance between high crack initiation resistance and adequate safety warning capacity for critical engineering structures.