早龄期混凝土表面开裂问题的试验研究

研究太阳辐射和风速影响下早龄期混凝土表面开裂问题.用400mm×400mm×160mm混凝土试件,构成一个半无限的平面模型,进行了两组试件的比较.第一组试件直接暴露在阳光下,第二组试件暴露在空气中但有遮阳设施,这两组试件的大气温度与相对湿度等基本一致,但经风条件与遮阳条件不同.试验发现:第一组试件的表面产生了毛细裂纹,第二组试件则没有.温湿度观测值显示:第一组试件温度变幅很大,湿度梯度更大,第二组则相对较小.综合分析认为:过大的湿度梯度和过高的温度变幅是混凝土表面产生收缩和约束裂缝的决定性因素.

Experimental Study on Surface Cracking of Early-age Concrete

This paper focuses on relative humidity (RH) and temperature (T) difference between the internal part and the surface of concrete caused by solar radiation and wind. An experimental model was set up with early-age concrete specimens under sun shinning, windy condition. The concrete specimens were shaped in the 400mm×400mm×160mm mold. One side of the specimen was exposed to air, while the other five sides were covered with adiabatic and impermeable material, so as to make up a semi-infinite plate model. Two groups of specimens were compared. Specimen in Group1 was exposed to sunlight directly, and specimen in Group2 was exposed to air but covered with a sunshield, so that the air temperature, wind velocity, as well as the relative humidity of air were nearly the same for the two groups. Hairline cracks were observed on the surface of the specimen in Group 1, but no cracks was found on the specimen of Group 2. Experimental results also indicated that both large relative humidity gradient and temperature variation did exist simultaneously in the drying zone. Evaporation rates and total weight loss during test for two groups were calculated, and the critical evaporation rate of concrete was discussed as well.