STUDY ON PREVENTION METHOD FOR THERMAL CRACK OF CONCRETE PLACED IN HOT WEATHER AMBIENCE

通过对高温环境下混凝土楼板的温度和水化特性的试验和数值分析，探讨了高温环境下混凝土楼板面层早期裂缝的预防方法。首先，作为温度裂缝的基础试验，测量了砂浆试件从浇筑后即刻到24小时的温度分布和水分蒸发随时间的变化，并定量讨论了外部条件(环境温度、环境湿度、风、太阳辐射)和内部条件(材料、配合比)的影响。其次，给出了防止温度裂缝的流程图。给出了流程图中用到的热应变分析方法，分析中用到的水蒸气蒸发速率的估算方法，测量了由温度分布计算表层热应变的幼龄期试件的热膨胀系数，以及作为裂纹发生的边界线的拉伸能力。利用这些结果分析了板坯表面温度分布引起的拉应变，并阐明了当拉伸能力最小时，拉应变同时达到最大值。由此可见，最外层与中心层温差引起的裂缝在上述时期最易发生。并定量地阐明了外部条件和内部条件的影响，最后讨论了养护作为预防热裂的效果。刚开始固化时，表层的拉伸应变较小。也就是说，养护对防止热裂是非常有效的。并阐明了硫化开始时间是重要的，必须在拉伸性能达到最小之前开始。

In this study, prevention method for early age crack at surface layr of concrete slab placed in hot weather ambience was discussed by experiment and numerical analysis on properties of temperature and water vaporization.At first, as basic experiment of thermal crack, time-dependent change of temperature distribution and water vaporization at mortar specimens were measured from immediately after placing toward twenty four hours, and effect of outer conditions (ambient temperature, ambient humidity, wind, solar radiation) and inner conditons (materials, mix proportion) were quantitatively discussed.Next, flow chart for prevent thermal crack was shown. Further, analysis method of thermal strain used in flow chart, estimation method of water vaporizing rate used in analysis were shown, coefficient of thermal expansion of specimens at young age to calculate thermal strain at surface layr from temperature distribution, and tensile capacity as border line of crack occurence were measurd.Using these results, tensile strain at surface layr of slab specimens due to temperature distribution was analyzed, and it was clarified that tensile strain bacame maximum at the same time when tensile capasity became minimum. So it was obvious that crack due to temperature difference between outermost and central layrs was most liable to occur at above mentioned period. And effect of outer conditons and inner conditions were quantitatively clarified.At the end, effect of curing as prevention of thermal crack was discussed. Immediately after starting of curing, tensile strain at surface layr bacame smaller. In the other words, it was cirtificated that curing was very effective to prevent thermal crack. And it was clarified that starting time of curing was important, and it must be started before the period when tensile capasity became minimum.