Research On Early Age Shrinkage And Cracking Of HPC And Measures To Suppression

The early age shrinkage of HPC is serious which cause severe cracking risk. The method of non-contact shrinkage test and edge cracking test were adopted to study the early shrinkage and cracking property of high performance concrete. At first, the difference property of shrinkage and cracking between high performance concrete and ordinary concrete were compared, then different shrinkage model were discussed in this paper. Secondly, the effect of fly ash, silica fume and fine slag powder on the shrinkage of HPC were discussed. At last, saturation ceramsite and super absorbent polymer were admixed in the HPC. The striking conclusions are subjected as follow:(1) The shrinkage of HPC is mainly during 13h which is longer than ordinary concrete. And the higher concrete strength, the value of concrete shrinkage at early age will be bigger. The course of early shrinkage of HPC could be divided into rapid shrinkage and slow shrinkage. HPC shrinkage modle was presented according to k1(x) and k2(x) which described to two phase of shrinkage. The two-line model could incarnate the truly shrinkage of HPC. It could be summarized by cracking test that high shrinkage will advance the crack risk. The maximum relatively cracking area of HPC was 627 mm2·m-2, but the value of ordinary concrete was less than 100 mm2·m-2.(2) The effect on early shrinkage property of HPC was difference accoding to the mineral kind. More fly ash in concrete can reduce the shrinkage value more pronounced. When the content of fly ash is 25%, the decline is 46.5% compared to without fly ash. The fine slag powder has the same function with FA at very early age, but it will boost shrinkage after about 14 hours. If the content of fine slag powder was more than 7%, it is not good for eliminate the shrinkage. Silica fume has the opposite effect to fine slag powder.(3) Both saturation ceramsite and super absorbent polymer could effectively decrease the shrinkage of HPC. The best volume substitution rate of ceramsite to coarse aggregate is 41%.the value of 72h shrinkage could decline to 43%. More super absorbent polymer admixed into HPC, shrinkage will be eliminate more effective. But they have different negative effects on compress strength and slump of HPC. Super absorbent polymer could reduce the HPC shrinkage and at same time decrease cracking risk effectively. However, for saturation ceramsite, it slight increases cracking risk. It could be reasoned by ascent of ceramsite. So, there will be more interfacial transition zone on formed surface of concrete. It predisposes to more thin cracks than HPC without ceramsite.