Fracture process zone of high-strength concrete under monotonic and cyclic loading

Abstract

To evaluate the characteristics of the fracture process zone (FPZ) of high-strength concrete (HSC) under monotonic and cyclic loading, three-point bending (TPB) tests were carried out on single-edge notched HSC beams and digital image correlation (DIC) technique was employed to obtain the full-filed deformation of the beams. Based on the displacement and strain fields measured by the DIC technique, a defined strain threshold (εxx) was used to determine the tip of the FPZ and critical crack opening displacement (wc) was used to judge the rear of the FPZ. The length of the FPZ (lFPZ) was defined as the vertical distance between the tip and rear of the FPZ and the width of the FPZ was determined from the strain concentration zone surrounded by the εxx. The results indicates that the envelope of the load versus the lFPZ (P-lFPZ) curve under cyclic loading is consistent with that under monotonic loading and it can be divided into 4 stages, namely linear elastic deformation stage, stable crack propagation stage, unstable stage before fully developed FPZ and unstable stage after fully developed FPZ, respectively. When the FPZ is fully developed, the lFPZ of the beams is about 80.04 mm ∼ 89.72 mm while the maximum width of the FPZ (bm) is about 8.43 mm ∼ 10.31 mm which is equivalent to the maximum size of the coarse aggregates. Both the lFPZ and bm first increases and then decreases in the unloading stage of each cycle and this can be explained by the variation of the stress intensity factor (SIF) which indicates that the driving force of crack growth is greater than the fracture resistance at the initial stage of unloading. Both the lFPZ and bm first decreases and then increases in the early stage of reloading, which may be caused by the lagging recovery of the deformation of the specimen as the rate of reloading is much lower than that of unloading.