Fracture properties of early-age concrete based on digital image correlation technique

In practical engineering, some concrete elements start to bear loads without reaching the standard curing age of 28 days after casting, which results in cracking and crazing, affecting their durability and service life. Therefore, in this study, based on digital image correlation (DIC), threepoint bending beam fracture tests under quasi-static and dynamic loads were conducted for concrete specimens of five curing ages (3, 7, 14, 21, and 28 d) to obtain the crack opening displacements (P-CMODs), fracture energies, crack opening displacements, and crack extension paths, respectively. The changes in horizontal displacement and strain fields in the crack extension region of early-age concrete specimens were further analyzed, and the trends of crack opening displacement and fracture process zone (FPZ) of early-age concrete specimens were investigated to reveal the crack extension law of early-age concrete. The results indicate the fracture energy of concrete under dynamic loading exhibits a significant increase before 21 d of curing, with a maximum growth rate of 20.9 %. At the same curing age, the maximum horizontal strain corresponding to the peak load under dynamic loading was found to be higher than that under quasi-static loading. Moreover, as the curing age increased, the maximum horizontal strain corresponding to the peak load decreased progressively, with the reduction range increasing from 131.8 % at 3 d to 211.7 % at 28 d. The crack mouth opening displacement decreased with the growth of curing age, while the height of the fracture process zone (FPZ) increased steadily, indicating a marked improvement in the crack resistance of the concrete. This study elucidates the crack propagation behavior of early-age concrete, providing valuable insights for optimizing the design and construction of concrete components.