各种级配大坝混凝土双K断裂参数试验研究——兼对《水工混凝土断裂试验规程》制定的建议

An experimental study on double-K fracture parameters of concrete for dam construction with various grading aggregates

为配合水工混凝土断裂试验规程在我国的制定,提供更充足的基础数据,受规程编制组委托,进行了最大尺寸为S×D×B=2200 mm×550 mm×240 mm的三点弯曲梁试件和最大尺寸为2H×D×B=1200 mm×1200 mm×250mm的楔入劈拉试件共140个试件的断裂试验。通过电阻应变片,测得不同布置测点荷载应变变化曲线,由此确定了不同级配及湿筛法大坝混凝土实测的起裂荷载和失稳荷载,计算了双K断裂韧度参数,讨论分析了双K断裂参数的尺寸效应和形状效应。结果表明:电阻应变片法测定起裂荷载的方法可行,Pini/Pmax=0.6￣0.85;三点弯曲梁法所得混凝土双K断裂韧度值无明显的尺寸效应;楔入劈拉试件由于竖向加载荷载PV引起附加力矩MV对裂缝尖端应力场的影响,导致所得混凝土断裂韧度值表现出一定程度的尺寸效应,但当采取措施消除此影响或此影响较小时,楔入劈拉试件的断裂韧度值也将无明显的尺寸效应;在相同配合比、相同高度条件下,不同试件形式的双K断裂韧度值无明显的差异。因此,可认为混凝土双K断裂参数是描述混凝土结构裂缝起裂及失稳断裂过程的材料常数,与试样类型和尺寸无关。

To further provide sufficient test data for formulating the norm fracture of hydro-engineering concrete in China, as suggested by the committee on norm concrete fracture test, a series of fracture tests were performed on a total of 140 specimens via three-point bending and wedge-splitting, with the maximum dimensions being S×D×B=2200 min×550 min×240 mm and 2H×D×B=1200 mm×1200 mm×250 mm respectively. With the arrangement of electric resistance strain gauges, the load vs. strain curves are obtained, and the initial cracking load and maximum load are specified. Double-K fracture parameters are calculated, and discussions on the size effect and geometry effect are also made. The results indicate that it is feasible to measure the initial cracking load by using electric resistance strain gauge and the ratio of the initial cracking load to the unstable fracture load that approximately fails into the range from 0.6 to 0.85. Based on the test results of three-point bending of beams, the double-K fracture parameters are rather size-independent. While for the wedge-splitting specimens, the influence of additional vertical load induced moment on the stress field around the crack tip enables the double-K fracture parameters to exhibit size effect to some extent. However, when eliminating or reducing such influence, wedge-splitting geometry has no obvious size effect on the double-K fracture parameters. The double-K fracture parameters, determined from using the same concrete and depth but different geometries are comparable, indicating that the double-K fracture parameters are independent of size and geometry and can be treated as the fracture parameters to describe fracture initiation and instability for crack propagation in concrete structures.