Study On Cause Of Crack Formation And Crack Mechanism Of High-Flowability Concrete

Since pump high-flowability concrete commenced in late 70's, it has been put to extensive application due to its advantages of fast, high-efficiency and stable quality. But its high flowing property and pumpability also cause some negative effects, such as serious shrinkage cracks at early age, which arouses wide social concern. This paper first discusses the cause of crack formation and crack mechanism of high performance concrete. Then carries out crack tests of three groups of concrete slabs using aggregate type, sand rate and dosage of fly ash as design parameter. At the same time, the non-contacting electrical measurement device is used to monitor the electrical property of fresh concrete. Based on above experimental results, an optimum proportion of high-flowability concrete with high crack resistance is brought forward. Then according to temperature gradient and humidity gradient, which are the essential factors inducing concrete cracks, distribution of thermal and moisture within young concrete and numerical methods to calculate temperature stress and shrinkage stress are systematically analyzed. Based on these theoretical studies, the temperature stress and shrinkage stress of plain concrete slab and reinforced concrete slab are calculated using finite element analysis software ADINA, and the differences between them are also made.The experimental results show that as far as crack resistance of concrete is concerned, the surface feature of coarse aggregate is more sensitive than aggregate shape. If the strength and workability are approximate identical, concrete tends more easily to crack with the increasing sand rate. And compared with ordinary concrete, high performance concrete adding fly ash shows better workability. But there is an optimum mixing dosage of fly ash for crack resistance of concrete. The electrical resistivity curves of fresh concrete have the same developing tendency. They can all be divided into three stages: dissolving period, competition period and setting and hardening period. And in the same experiment batch, those that have higher crackresistance have a little lower electrical resistivity at their early age. The finite element analysis results show that the rising temperature stage of concrete sheet caused by hydration heat is short, the occurrence of peak temperature value is earlier and the value is lower. And the slabs soon have decreasing temperature trend. The above phenomenon is due to the features of thinner thickness, larger area and easily heat-diffusion of slabs. Shrinkage stress rates high in the total amount of stress caused by temperature and humidity gradient. So in the analysis of such concrete members as thin wall and sheet, the influence of drying shrinkage can't be ignored. Embedding some steel bars in concrete slabs doesn't have much influence on temperature stress. But it can reduce shrinkage stress of concrete effectively, and delays the occurrence of cracks.