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板表层早期裂缝的试验和温度、蒸发特性的数值分析，探讨了高温环境下混凝土板表层早期裂缝的防治方法。首先，作为热裂缝的基础实验，从放置24小时开始，测量砂浆试件温度分布和水分蒸发随时间的变化，定量讨论外部条件（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，给出了防止热裂的流程图。进一步给出了流程图中采用的热应变分析方法、分析中采用的水蒸发速率估算方法、试样在幼龄期通过温度分布计算表层热应变的热膨胀系数以及作为裂纹发生边界线的拉伸能力。利用这些结果，分析了温度分布对板坯试样表层拉伸应变的影响，明确了拉伸应变在拉伸能力最小的同时达到最大。可见，在上述阶段最易发生由最外层和中间层温差引起的裂缝。定量地阐明了外部条件和内部条件的影响。最后讨论了固化对热裂纹的预防作用。固化开始后，表层拉伸应变立即变小。也就是说，证明了固化对防止热裂是非常有效的。阐明了固化起始时间的重要性，必须在拉伸能力最小的时期之前开始。

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.