减缩剂与聚丙烯纤维对混凝土早期收缩开裂的影响

目的研究减缩剂、聚丙烯纤维及其混合掺入对混凝土早期收缩开裂的影响规律及作用机理．方法采用板状试件法测试混凝土在两种干燥条件下的早期开裂状况，利用非接触式混凝土收缩仪测定混凝土早期自收缩与干燥条件下的总收缩，并通过压汞仪和扫描电子显微镜分析了各混凝土孔隙结构和微观形貌．结果加入减缩剂明显降低了混凝土早期自收缩和干燥收缩，因而显著地减轻了混凝土早期开裂．聚丙烯纤维能在一定程度上降低早期自收缩，但由于早龄期部分纤维-砂浆界面处结合不良且在混凝土中引入一些大孔，使干燥条件下混凝土的总收缩值增大；它使混凝土早期收缩裂缝分散、宽度减小，但却增加了小裂缝数量和开裂面积．结论减缩剂、聚丙烯纤维的掺入均能改善高性能混凝土早期开裂，其作用效果优劣次序为：减缩剂〉减缩剂＋聚丙烯纤维〉聚丙烯纤维．

Effects of Shrinkage Reducing Admixture and Polypropylene Fiber on Early Age Shrinkage Cracking of Concrete

Effects of shrinkage reducing admixture(SRA) and polypropylene fiber(PPF)on early age cracking tendency of concrete were studied.Non contact concrete shrinkage measuring apparatus and plate type concrete cracking frame were adopted in this experiment.The pore characteristic and the microstructure of concrete were determined by mercury intrusion porosimetry and scanning electron microscopy.The results show that the adding of SRA reduces markedly early age shrinkage cracking of concrete because it reduces greatly both autogenous shrinkage and dry shrinkage of concrete at early ages.The microstructure of SRA concrete is uniform and denser than the control concrete.The adding of PPF restricts early age autogenous shrinkage of concrete some extent.There are some weak interface zones between PPF and mortar matrix and some coarser pores induced by PPF in the concrete,so the PPF concrete has a higher shrinkage than the control concrete when exposed to the dry condition.The adding of PPF disperses the early age shrinkage cracks and diminishes the crack width,but it leads to smaller cracks occurring and a higher cracking area.According to the efficiency of mitigating early age shrinkage cracking of concrete,the following order from high to low is obtained:shrinkage reducing admixture,shrinkage reducing admixture plus polypropylene fiber and polypropylene fibre.