PREDICTION OF DRYING SHRINKAGE CRACKING OCCURRED IN CONCRETE

本研究的目的是探讨混凝土干缩裂缝的预测方法。(1)对普通轻骨料混凝土和人工轻骨料混凝土按JIS(草案)进行了干缩开裂试验，未考虑龄期对开裂时间的影响。(2)利用约束钢筋的应变分布计算了约束混凝土试件产生的拉应力，无论起始龄期如何，产生裂缝时的平均应力都是混凝土抗拉强度的70%~80%。(3)人工轻骨料混凝土比普通混凝土更容易开裂。而这是由于干燥收缩应变的大小造成的；人工轻骨料混凝土的干缩应变约为普通混凝土的两倍。(4)有限元应力分析系统。该系统考虑了干燥条件下水泥水化过程的影响。(5)利用该应力分析系统得到了混凝土试件的应力分布，并与混凝土抗拉强度进行了比较。将应力与抗拉强度之比称为抗拉强度比，求出抗拉强度比大于1.0的截面面积。当该面积占整个面积的比例达到40%~50%时，可以观察到裂缝的发生。今后应考虑裂纹发生的适当标准。

The purpose of this research project is to discuss the method for predicting the cracking due to drying shrinkage of concrete. The results of this study are summarized as follows :(1) The drying shrinkage cracking tests for normal and artificial lightweight aggregate concrete were performed according to JIS(draft), and the effects of the age at the start of drying on the cracking time were not recognized.(2) The tensile stress produced in the restraint concrete specimen was calculated by using the distribution of strain of the restraint steel, and the mean stress at the occurrence of cracking was 70 to 80 % of the tensile strength of concrete regardless of the age at the start of drying.(3) The artificial lightweight aggregate concrete was easy to crack compared with the normal concrete, and this is due to the amount of drying shrinkage strain ; the drying shrinkage strain of the artificial lightweight aggregate concrete was about twice that of the normal concrete.(4) The stress analysis system using the finite element method(F.E.M.) for the concrete laid in the dry air from early ages was developed, and the influence of the progress of cement hydration in the drying condition was considered in this system.(5) The stress distribution of concrete specimen was obtained by use of this stress analysis system, and the calculated stresses were compared with the tensile strength of concrete. The ratio of the stress to the tensile strength was called tensile strength ratio, and the area of the section where the tensile strength ratio was greater than 1.0 was obtained. The occurrence of the cracking was observed when the ratio of this area to the whole area reached 40 to 50 %. The proper criterion for the occurrence of cracking should be considered in the future.