Determining Initial Cracking Load Of Concrete Using Stain Gauge Method And Study Of The Toughening Effect Of UHTCC-concrete

Fracture process of concrete includes three different stages:crack initiation, stable crack propagation and unstable fracture. Double-K criterion can describe this process appropriately with initial cracking toughness and unstable fracture toughness as two parameters. In2012, RILEM TC-TDK committee was set up to formulate an international standard testing method of double-K criterion. As a part of the round-robin testing plan, this paper determines double-K fracture parameters of four kinds of concretes using three-point bending notched beams, the sizes of which are500mm*100mm*60mm and1000mm*200mm*120mm. Some problems in the experiments, especially the stabilition of strain gauge method, are discussed. Then, this paper uses strain gauges with different lengths to achieve initial cracking load of low strength concrete. And equivalent crack lengths are calculated to make a comparison with actual ones. Besides, we attempt to mix concrete and UHTCC together to see the toughening effect.Results show that usual strain gauge method does well in measuring initial cracking load of high strength concrete. And5mm long strain gauges have more chance to work well than10mm long ones for C25concrete with aggregates no bigger than25mm in this paper. The difference between actual crack length and equivalent crack length calculated with data determined by strain gauge method is almost no more than10%.Compression strength of concrete in this paper is37%higher than UHTCC mixed in. When volume content of UHTCC is10%～20%, compression strength of UHTCC-concrete is about6%lower than that of plain concrete. Initial cracking toughness of UHTCC-concrete becomes smaller while unstable fracture toughness seems to be slightly larger as volume content of UHTCC is bigger. If we calculate fracture energy by simulating the tail segment of load-displacement curve with exponential function, it can be concluded that:when volume content of UHTCC is10%, fracture energy of UHTCC-concrete has a118%increase; when volume content is20%, fracture energy has a315%increase. The toughening effect of UHTCC-concrete is quite obvious, but discreteness of the experimental results also cannot be ignored. Some improvements in mixing process may be needed.