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科技资料原文 Shrinkage of Concrete When concrete loses moisture by evaporation it shrinks. Shrinkage strains are mdependent of the stress conditions in the concrete. If restrained, shrinkage strains can cause cracking of concrete and will generally cause the deflection of structural members to increase with time. The calculation of stress and deformations due to shrinkage is deferred until Chapter 10. A curve showing the increase in shrinkage strain with time appears inFig.2.21. The shrinkage occurs at a decreasing rate with time appeard in shrinkage strains vary greatly, being generally in the range 0.0002 to 0.0006 but sometime as much as 0.0010. Fig.2.21. Typical shrinkage curve for concrete Shrinkage is to a large extent a reversible phenomenon. If the concrete is saturated with water after it has shrunk, it will expand to almost its original volume. Thus alternating dry and wet conditions will cause alternating volume changes of concrete. This phenomenon is partly responsible for the fluctuating deflections of structures (e.g. concrete bridges) exposed to seasonal changes each year. As a rule, concrete that exhibits a high creep also displays high shrinkage. Thus the magnitude of the shrinkage strain depends on the composition of the concrete and on the environment in much the same way as discussed previously for creep. Both the ACI Committee 2092.26 and the CEB-FIP2.27 have proposed empirical methods for the estimation of shrinkage strains. The former approach is described blow. According to ACI Committee 2092.26 for normal weight, sand lightweight concrete (using both moist and steam ouring and types I and III cement), the unrestrained shrinkage strain at any time t is given by Where the coefficients are given below. Ultimate shrinkage strain, The value of can vary widely. In ACI Committee 209 review, was found to be in the range 0.000415 to 0.00107, with mean values of 0.00080 for moist-cured concrete or 0.00073 for steam-cured concrete. These average values s