Investigations on the FPZ evolution of concrete after sustained loading by means of the DIC technique

The three-point bending (TPB) specimens were first subjected to two levels of sustained loading, 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 digital image correlation (DIC) technique was utilized to investigate the fracture process in concrete after sustained loading. According to the variations of the crack opening displacement from the experimental observations, the crack propagation lengths and the crack widths with respect to different loading points were derived, which provides the information on the evolution of the fracture process zone (FPZ) in concrete after sustained loading. The results indicated that, compared with the tested results from the aging specimens in the static TPB tests, the low sustained loading did not show obvious effects on the FPZ evolution. However, under the high sustained loading, the crack initiated during the loading duration, resulting in a longer critical crack propagation length. In addition, the decrease in the FPZ length for the specimens under the high sustained loading is more significant at the post-peak load stage, resulting in the increase in brittleness of concrete. Finally, a test method for determining the crack propagation length using clip gauges was proposed, which is more convenient in lab. The crack propagation lengths tested using clip gauges fairly well agreed with those by means of the DIC and verified the proposed measure method in this study.