科技资料原文Shrinkage of ConcreteWhen 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 concreteShrinkage 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-FIP 2.27 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 byWhere 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 valuesS h r i n k a g e s t r a i n sh shu t h th s f e c s s s s s s s εε=shu εshu εshu εshould be assumed only in the absence of more exact date for the concrete to be used.Time of shrinkage coefficient, S tAt any time after age 7 days, for moist-cured concrete,(2.17a)Where t = time in days from age 7 days(St=0.46, 0.72, 0.84, 0.91,and 0.98 for t = 1 month, 3 months, 6months, 1 year, and 5 years, respectively)or, at any time after age 1 to 3 days for steam-cured concrete,(2.17b)Where t = time in days from age 3 days(St=0.35, 0.62, 0.77, 0.87,and 0.97 for t = 1 month, 3 months, 6months, 1 year, and 5 years, respectively)For shrinkage considered from greater ages than given above, the difference may be determined use Eq.2.17a or 2.17b for any period after than time. That is shrinkage for moist-cured concrete between, say, 1 month and 1 year would be equal to the 7-day to 1-year procedure assumes that the moist-cured concertehs been cured the shrinkage needs to be multiplied by 1.2; a linear interpolation between 1.2 at 1day and 1.0 abd 1.0 at 7 days may be used.Relative humidity coefficient, S hS h =1.4-0.01H for 40<H<80% (2.18a)or,S h =3.0-0.03H for 80<H<100% (2.18b)Where H = relative humidity in percent(S h = 1.00, 0.80, 0.60, 0,for 40, 60, 80, and 100% relative humidity)Minimum thickness of member coefficient, S thSth = 1.00 for 6 in or less and 0.84 for 9 in (1 in = 25.4mm)Slump of concrete coefficient, SsSs = 0.97 for 2 in ,1.00 for 2.7 in, 1.01 for 3in, 1.05 for 4 in, and 1.09 for 5 in (1 in =25.4mm)Fines coefficient, S fS f =0.86 for 40%, 1.00 for 50%, and 1.04 for 70% fines by weightAir content coefficient, S eS e =0.98 for 4%, 1.00 for 6%, and 1.03 for 10% airCement content factor, S c35t tS t =+55t tS t =+≤S c =0.87 for376 1b/yd 3, 0.95 for564 1b/ yd 3 ,1.00 for705 1b/yd 3 and1.09for 940 1b/yd 3(1 1b/yd 3=0.593kg/m 3)Example 2.2Estimate the free shrinkage strain that can be expected to occur in a 9 in (230mm) thick concrete wall from the age of 7 days during a 5-year period at a relative humidity of 60%. The concrete has a slump of 3 in (76mm), a fines content of 34% by weight, a cement content of 600 1b/yd 3 (356kg/m 3), an air content of 5%, and is moist cured for 5days after being placed.SolutionFrom Eq.0.16 we have原文翻译 混凝土的收缩当混凝土由蒸发丢失水分时便产生了收缩现象。

混凝土的收缩是一种自发的变形，并不依靠作用在混凝土上的外力。

因此当收缩变形不能自由进行时，有可能导致混凝土产生裂缝，并且裂缝的数量随着时间的增加而增多。

在后面的第10章具体介绍收缩产生的变形和裂缝的计算。

图2.2.1反应了混凝土的收缩随时间发展的规律，早期混凝土的收缩变形速率随时间的增加变化较快，但是到了后期混凝土的收缩变形和初期相比有很大的不同，收缩的总量一般为（2～5）×10-4，有时达到1×10-3。