Experimental study on local crack propagation of concrete under three-point bending

To investigate the propagation process of local cracks in concrete, three-point bending tests were performed on the beams with a local rectangular pre-crack. Pre-cracks with different sizes were set in the specimens with the widths ranging from 30 mm to 100 mm and the heights ranging from 30 mm to 60 mm. The three-point bending tests were carried out with a controlled displacement rate of 0.024 mm/min. The compressive strength, elastic
modulus and tensile strength of the concrete were 40.89 MPa, 32.57 GPa and 3.40 MPa, respectively. By monitoring the initiation and propagation of local cracks on the specimen surfaces, the fracture toughness of the concrete specimen with a local crack was determined and the crack propagation process along the thickness and height directions was investigated. The experimental results indicated that, under three-point bending loading, the local cracks always propagated firstly on the bottom surface of the specimen and formed a through crack in the thickness direction. After that, the crack in the specimen began to propagate in the height direction until full failure. In addition, the initial fracture toughness obtained from the specimens with bottom-through cracks was almost the same as the one from the specimens with local cracks. However, the unstable fracture toughness obtained from the specimens with bottom-through cracks was not consistent with the one from the specimens
with local cracks, where the average unstable fracture toughness calculated using the critical crack lengths at different thicknesses was taken as the unstable fracture toughness of the local crack specimens. This indicated that the initial fracture toughness could be considered as a material characteristic, while the unstable fracture toughness was dependent on the crack geometry.