Fracture Toughness of Concrete after Sustained Loading

The fracture toughness of concrete after sustained loading is a parameter for evaluating residual bearing capacity of cracked concrete structures in service. To investigate the fracture toughness of concrete after sustained loading, creep tests were conducted on 125 three-point bending beams under different sustained load levels, and then static fracture tests were performed at different creep time. The fracture toughness after sustained loading was calculated based on the peak load and net critical crack length, i.e., the difference between the critical crack length in static tests and crack propagation length during creep. The results indicate that when the sustained load is less than the initial cracking load, the crack does not propagate during creep, and the fracture toughness is greater than unstable fracture toughness under static loading. When the sustained load exceeds the initial cracking load, the crack propagates during creep, and the fracture toughness is lower than the unstable fracture toughness. Moreover, the fracture toughness decreases with increasing loading time and crack tip opening displacement. The specimens subjected to lower sustained load levels experienced longer loading time for the same crack tip opening displacement, resulting in a smaller fracture toughness. Particularly, for the creep failure condition, the fracture toughness at the critical failure state increases with load levels. A calculation model was established to calculate the fracture toughness after sustained loading using loading time and crack tip opening displacement. This model can be used to evaluate the stability of crack propagation and residual bearing capacity of cracked concrete structures.