Time-dependent crack propagation in concrete under constant loading

Abstract

This paper presents an experimental study on the fracture characteristics of concrete under constant loading, focusing on the crack propagation and the unstable fracture criterion. The test involves three-point bending (TPB) notched beams and compact tension (CT) specimens with different heights, and four constant loads are considered. The results show that the specimens fail within 0.37–4566 h and a higher constant load results in a shorter lifetime of concrete. Furthermore, the time-dependent crack mouth opening displacement and crack length, CMOD(t) and a(t), show three stages with the loading time, t. In the first stage, CMOD(t) and a(t) increase rapidly whereas their growth rates decrease. In the second stage, CMOD(t) and a(t) increase at constant growth rates, indicating cracks propagate stably. In the third stage, CMOD(t) and a(t) increase at greater growth rates than those in the second stage and the growth rates are increasing, indicating the unstable fracture occurs. Accordingly, the time of the boundary point between the second and third stages is defined as the critical time of the unstable fracture of concrete under constant loading, tc. CMOD(tc) and a(tc) are found to be greater than the critical CMOD and effective crack length under static loading, CMODc and ac, and decrease with increasing the constant load. Moreover, it is found that the unstable fracture toughness of concrete under static loading, KunIC, can also be used to determine whether the unstable fracture occurs under constant loading, i.e., when the stress intensity factor at tc reaches KunIC, the unstable fracture occurs.