Development of Shrinkage Crack Control Method due to Combined use of Inert Powder and Chemical Admixture

防止混凝土开裂实际上是混凝土设计的关键问题之一。本研究的目的是研究惰性粉料和化学掺合料复合作用对混凝土收缩裂缝的控制。主要研究结果如下：(1)在砂浆中掺入缓冷高炉矿渣粉、石灰石粉等惰性无机外加剂后，砂浆的自收缩明显降低。然而，添加的有机混合物的种类不同，其效果也不同。另一方面，掺入粉煤灰也显著降低了水泥混合料的自收缩，但促进了其抗拉强度的降低。相反的作用有时会产生较小的拉应力/拉伸强度比，导致抗裂性较低。因此，为了防止由于自生收缩和干燥收缩而产生的裂缝，需要进一步开发复杂的减缩剂。(2)聚乙烯亚胺类减缩剂对水泥有很强的吸附作用，不降低水泥浆体中孔溶液的表面张力。但对自收缩和干缩有一定的抑制作用，且强度发展良好。通过Rietveld分析、选择性溶解和压汞实验数据，可以计算出C-S-H在二元共混体系中的密度。结果表明，随着粉煤灰取代率的增加，C-S-H的密度降低。这表明，C-S-H凝胶的水化体积大于OPC的水化体积，具有自愈作用。因此，利用这种自愈效应来防止裂缝的新方法的开发是值得期待的。

Prevent concrete from cracking is actually one of the key issues for the design of concrete. The purpose of this study is to develop control of shrinkage crack due to combination of inert powder and chemical admixture. Most important results are summarized as follows:(1)When the inert inorganic additives such as slowly cooled blast furnace slag powder and limestone powder were added to the mortar with the organic admixtures which consist of polycarboxylate-based and polyoxyalkylene alcohol based polymer, autogenous shrinkage of mortar decreased markedly. However, the effect varied depending on the kind of organic admixture added. On the other hand, addition of fly ash also reduced significantly the autogenous shrinkage of cement mixes, but promoted to decrease their tensile strength. The opposite effects sometimes brought out a little tensile stress / tensile strength ratio, resulting in a low resistance of cracking. Hence, it was clear that to prevent crack due to autogenous and drying shrinkage, further development of sophisticated shrinkage reducing admixtures were required.(2)The polyethyleneimine based shrinkage reducing admixture absorbed strongly on to cement, providing no reduction of surface tension of pore solution in cement paste. However, it had an effect to reduce autogenous shrinkage and drying shrinkage, and showed good development in strength. Further, the combined use of calcium sulfate enhanced to reduce shrinkage and increase in strength.(3)The density of C-S-H in binary blends system could be calculated from data of Rietveld analysis, selective dissolution and mercury intrusion. As a result, the density of C-S-H became lower as increasing replacement ratio of fly ash. This implied that the volume of C-S-H gel due to fly ash hydration was larger than that of OPC hydration, providing elf-healing effect. Therefore, development of new method to prevent crack by this self-healing effect will be expected in future.