Influence of sustained loading on fracture properties of concrete

Abstract

To investigate the effects of sustained loading on the fracture properties of concrete, basic creep and three-point bending (TPB) tests were conducted on the pre-notched beams. The specimens were first subjected to two sustained loading levels, i.e. 30% peak load and the initial cracking load over 115 days. Then, they were moved out from the loading frames and tested under TPB loading until failure. The critical crack propagation length (Δac), the peak load (Pmax) and the fracture energy (Gf) were measured in the tests, and the unstable fracture toughness (KICun) was calculated accordingly. Furthermore, based on the load-displacement curves obtained in the TPB tests, the energy dissipation was derived using the modified J-integral method. By enforcing balance between the energy dissipated and the energy caused by the fictitious cohesive force acting on the fracture process zone, the tension-softening constitutive laws under the two sustained loading levels were established and also simplified as bilinear forms for practical applications. Finally, the effects of sustained loading on the fracture properties were examined by comparing with the tested results from the aging specimens in the static TPB tests. The test results indicate that low sustained loading had no effects on all fracture properties of concrete investigated in this study, while under high sustained loading, Δac and KICun increased and Gf and Pmax almost remained unchanged. Meanwhile, a smaller free-stress crack opening displacement was obtained under the high sustained loading level, which indicates a shorter FPZ length formed, resulting in the increase in brittleness of concrete.