A Study On Critical Crack Length Of Concrete

The critical crack length of concrete is a key parameter for calculating the concrete fracture toughness,and it plays an important role in the study of concrete crack propagation behavior and the safety assessment of cracked concrete structures.In the early research,the Linear Elastic Fracture Mechanics（LEFM）analytical method is often used to calculate the critical equivalent crack length ac IEFM of concrete,but its essence is a calculation method based on elastic equivalence,and there is still a difference between ac LEFM and the actual critical crack length of concrete.Therefore,in this paper the difference between the critical equivalent crack length ac LEFM calculated by the LEFM analytical method and the real critical crack length of concrete is studied quantitatively,as well as the influence of the concrete specimen size and the initial crack length-depth ratio on this difference.Mode I fracture tests were carried out on concrete three-point bending beams with four different specimen sizes and three different initial crack length-depth ratios,and three methods were used to determine or calculate the critical crack length of concrete.The first method is pasting strain gauges on both sides of the crack propagation path,the second is arranging the clip gauges in the height direction,the third is a numerical simulation by using the propagation criterion based on crack initiation toughness.The results show that the critical crack lengths of concrete measured by the three methods have good consistency,which proves the accuracy of the critical crack lengths measured in this study.a_c^LEFM calculated by the LEFM analytical method was compared with the measured critical crack length of concrete and the difference between the two was calculated.The results show that the difference between ac LEFM and the critical crack length of concrete decreases with the increase of the initial crack length-depth ratio of the specimen.When the initial crack length-depth ratio of the specimen is 0.5,the difference between ac LEFM and the critical crack length is within 10%,and the ac LEFM calculated by the LEFM analytical method is approximately equal to the critical crack length of concrete;when the initial crack length of concrete is less than 0.5,the difference between the two is obvious.Similarly,the difference between aC LEFM and the critical crack length of concrete decreases with the increasement of specimen size.When the height of the specimen reaches 400mm,the difference between the two is less than 10%,and the ac LEFM calculated by the LEFM analytical method is approximately equal to the critical crack length of the concrete;when the height of the specimen is less than 400mm,the difference between the two is obvious.