Mechanism of Developing Cold Joint in Concrete, Its Pore Structure and Repair Technique

发现两层混凝土之间的浇筑时间间隔和浇筑时的环境条件是混凝土构件产生冷缝的主要原因。这是由于第一层混凝土在浇筑第二层混凝土前较长时间才开始水化，且温度较高。为了定量讨论这两个参数的影响，对不同放置间隔和不同温度下的砂浆试样进行了液体渗透性测试。根据实验结果，提出了由间隔时间和温度组成的渗透率计算公式。对于室外环境对透气性的影响，采用夏、冬两个季节间隔时间制作冷缝试件，讨论了季节对冷缝形成的影响差异。此外，通过计算样品的温度来估计每个样品的渗透率。该方法对冷接头渗透率估算的有效性也得到了实际应用的验证。本文提出了一种研究冷接头结构的新方法，即在高压下将镓注入试样的孔隙中，然后通过EPMA测量来绘制镓的分布。结果表明，沿混凝土冷缝形成不连续平面，间隔时间越长，温度越高，不连续平面越明显。为使冷缝水密，在低压下注入以氟化硅镁和硅酸钙为主的3种修补剂，并测定其渗透性。结果表明，它们在冷接头中产生了一些结晶体，降低了接头的渗透性。对冷节理孔结构的观察也证实了冷节理孔结构变得致密。

A time gap in placing between two concrete layers and environmental condition at concrete placing is found out to be the main cause for producing a cold joint in a concrete member. It is because that the first concrete layer started hydration at long interval time before placing of the second concrete layer and at higher temperature. To discuss quantitatively effect of the two parameters, liquid permeability of mortar specimens that were produced at several placing intervals and at various temperatures were measured. The equation for calculating permeability composed of interval time and temperature was proposed on the basis of the experimental results.As for the effect of outdoor environment on permeability, specimens with cold joints were made at several interval times of placing in summer and winter and the difference in influence of the seasons on formation of a cold joint was discussed. Furthermore permeability of each specimen was estimated by calculation using temperatures of the specimens. The usefulness of the above method to estimate to permeability of a cold joint is also confirmed in an actual field.A new technique, in which gallium is intruded m pores of a specimen at high pressure and then the distribution of it is mapped by an EPMA measurement, was developed for studying the structure of a cold joint. It was made clear that a discontinuous plane was formed along the cold joint in concrete and it became more noticeable at longer interval time and at higher temperature.To make a cold joint watertight three kinds of repair agents, mainly composed of magnesium silicofluoride and calcium silicate, were injected in cold joints at low pressure, and their permeability were measured. It was found out that they produced some crystals in cold joints and decreased their permeability. It was also confirmed by the observation of pore structures at the cold joints that the structures became dense.