Control of cracking caused by autogenous shrinkage in mass concrete

研究了水泥种类、配合比、温度和试件尺寸对混凝土自收缩的影响。通过控制试件温度和应力历史的约束试验，研究了自收缩对大体积混凝土裂缝的影响。主要研究结果如下：(1)水泥类型对自收缩的影响可以用矿物组成和水化程度来评价。矿渣细度为6000~8000 cm~2/g时，自收缩增大。(2)提出了混凝土自收缩应变预测模型。它受水泥和外加剂类型、水胶比和有效龄期的影响。(3)在20~60ﾟC温度范围内，混凝土的自收缩速率随温度的升高而增大，可以用成熟度概念来估计。自收缩对约束应力的贡献随着水灰比的减小而增大。对于水灰比为0.20的硅粉混凝土，即使在温度继续升高的情况下也会产生拉应力。这是因为自收缩克服了温度升高引起的膨胀。

Influences of cement type, mix proportion, temperature and specimen size on autogenous shrinkage of concrete were investigated. Influence of autogenous shrinkage on cracking of mass concrete was also investigated with restraining tests where temperature and stress histories of specimens were controlled. Main results of this research are as follows.(1) Influence of cement type on autogenous shrinkage can be setimated with mineral compositon and degree of hydration. Autogenous shrinkage increases by using blast furnace slag with 6000-8000 cm^2/g Blaine Fineness.(2) Predicting model for autogenous shrinkage strain of concrete was proposed. It is function of cement and admixture type, water-binder ratio and effective age. The proposed model is effective for concretes with ordinary Portland cement and with water-binder ratio between 0.2 and 0.56.(3) The rate of autogenous shrinkage increases with temperature of concrete within the range from 20 to 60 ﾟC.It is proved that this effect can be estimated with maturity concept. Autogenous shrinkage during the first 24 hours is lager for a specimen with lager dimensions.(4) Contribution of autogenous shrinkage to restrained stress is increased with decrease in water-cement ratio. For silica fume concrete with water-cement ratio of 0.20, tensile stress generates even when temperature is still increasing. This is because autogenous shrinkage overcomes expansion due to temperature rise.