大初始缝高比混凝土试件双K断裂参数的试验研究

双K 断裂准则能够定量描述混凝土裂缝的起裂、稳定扩展和失稳断裂。该文采用混凝土三点弯曲梁试件,通过在试验中测得的起裂荷载P_ini、最大荷载P_max及临界裂缝口张开位移CMOD_C计算了初始缝高比为0.3~0.9共7组试件的起裂断裂韧度K_ICini 和失稳断裂韧度K_ICun 。结果表明,当初始缝高比为0.3~0.7时,混凝土裂缝扩展经历起裂、稳定扩展和失稳破坏3 个阶段,双K 参数均是与初始缝高比无关的材料参数；当初始缝高比大于或等于0.8 时,混凝土裂缝起裂后便进入失稳扩展阶段,起裂荷载即为最大荷载,且计算得到的K_ICini 仍与初始缝高比无关。因此,在确定K_ICini 时,仅需测得初始缝高比大于或等于0.8试件的P_max,将P_max作为P_ini直接计算得到K_ICini。同以往的试验方法相比,其结果更为准确且试验方法简单。

EXPERIMENTAL INVESTIGATION ON DOUBLE-K FRACTURE PARAMETERS FOR LARGE INITIAL CRACK-DEPTH RATIO IN CONCRETE

The fracture process including crack initiation,stable crack propagation and unstable fracture in concrete can be described quantitatively by the double-K fracture criterion.In this paper,the three-point bending concrete beam specimens are classified in 7 groups,with the initial crack-depth ratios set from 0.3 to 0.9,respectively.The initial fracture toughness and unstable fracture toughness are calculated from the initial load Pini,the maximum load Pmax and critical crack mouth open displacement CMODC.The results indicate that if the initial crack-depth ratios are between 0.3 and 0.7,the concrete crack propagation process will go through crack initiation,crack propagation and unstable fracture,and the double-K fracture parameters are independent of an initial crack-depth ratio.If the initial crack-depth ratio is greater than 0.8,the concrete crack enters the stage of unstable fracture when initial cracking happens,and the initial load is equal to the maximum load,and is still independent of an initial crack-depth ratio.Therefore,can be calculated from Pini,which equals to Pmax when the initial crack-depth ratio equals to or greater than 0.8.Compared to the previous method,the results are more accurate,and the test method is simpler.