不同初始缝高比的混凝土断裂试验及边界效应分析

Analysis on fracture experiment and boundary effect of concrete with different initial crack-depth ratios

为了解决直接法(轴向拉伸试验)和间接法(劈裂拉伸试验、弯曲拉伸试验)测试抗拉强度的试验结果相差较大的问题,通过不同初始缝高比早龄期混凝土的楔入劈拉试验,基于已有的Hu-Guan边界效应理论模型,拟合得到两个真实的断裂参数——断裂韧度和抗拉强度,然后基于双K断裂模型,根据试验测得的起裂荷载、峰值荷载及临界裂缝张口位移值,计算初始缝高比为0.05~0.70共8组试件的起裂断裂韧度和失稳断裂韧度。结果表明:Hu-Guan边界效应模型拟合计算得到的抗拉强度值与劈裂抗拉试验值相差4.85%,得到的断裂韧度值介于双K模型计算范围内,验证了已有的边界效应模型适用于确定混凝土材料的真实断裂参数;起裂断裂韧度不受初始缝高比的影响,失稳断裂韧度在0.05≤α≤0.4范围内随初始缝高比的减小而增大,在0.4≤α≤0.6范围内,其值为一常数,在0.6≤α≤0.7时,其值有明显的下降趋势。

In order to solve the larger problems from the results of the experiments for testing the concrete tensile strength with the direct method(axial tensile experiment)and the indirect method(splitting tensile experiment and Bending tensile experiment),two real fracture parameters—fracture toughness and tensile strength are fitted and obtained based on the existing Hu-Guan boundary effect model through the wedge-splitting experiments made on the early-aged concrete specimens with different initial crack-depth ratios,and then both the initial fracture toughness and the unstable fracture toughness of total 8 groups of concrete specimens with the initial crack-depth ratios from 0.05~0.70 are calculated in accordance with the cracking loads,peak loads and the displacements of the critical crack-openings obtained from the relevant experiments on the basis of double—K fracture model.The result shows that the difference between the value of the tensile strength obtained from the fitting calculation of the Hu-Guan boundary effect model and the value from the splitting tensile experiment is 4.85%,while the obtained fracture toughness is within the calculation range of the double-K model.It is verified that the existing boundary effect model is suitable for determining the real fracture parameter of concrete material;from which the fracture toughness is not affected by the initial crack-depth ratio,and the unstable fracture toughness is increased along with the decrease of the initial ratio within the range of 0.05≤α≤0.4,while the value is constant when 0.6≤α≤0.7 with an obvious decreasing trend.