1、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 wi
2、th 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 rang
3、e 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
4、 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 th
5、e 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 bl
6、ow. 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 tis given by s h s h u t h t h s f e cs s s s s s s Where the coefficients are given below. Ultimate shrin
7、kage strain, shu t o TimeShrinkagestrainto=time at which concrete is subjectde to drying environmentShrinkage strainThe value of shu can vary widely. In ACI Committee 209 review, shuwas 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
8、steam-cured concrete. These average values should be assumed only in the absence of more exact date for the concrete to be used. Time of shrinkage coefficient, St At any time after age 7 days, for moist-cured concrete, 35t tS t (2.17a) Where t = time in days from age 7 days (St=0.46, 0.72, 0.84, 0.9
9、1,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, 55t tS t (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, respe
10、ctively) 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 mo
11、ist-cured concerte hs 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, Sh Sh=1.4-0.01H for 40H80% (2.18a) or, Sh=3.0-0.03H for 80H100% (2.18b) Where H = relative humidity in percen
12、t (Sh = 1.00, 0.80, 0.60, 0,for 40, 60, 80, and 100% relative humidity) Minimum thickness of member coefficient, Sth Sth = 1.00 for 6 in or less and 0.84 for 9 in (1 in = 25.4mm) Slump of concrete coefficient, Ss Ss = 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
13、in =25.4mm) Fines coefficient, Sf Sf =0.86 for 40%, 1.00 for 50%, and 1.04 for 70% fines by weight Air content coefficient, Se Se=0.98 for 4%, 1.00 for 6%, and 1.03 for 10% air Cement content factor, Sc Sc=0.87 for376 1b/yd3, 0.95 for564 1b/ yd3 ,1.00 for705 1b/yd3 and1.09for 940 1b/yd3(1 1b/yd3=0.5
14、93kg/m3) Example 2.2 Estimate 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/yd3 (356kg/m3), an air content of 5%, and is moist cured for 5days after being placed. Solution From Eq.0.16 we have sh = 0 . 0 0 0 8 0 . 9 8 0 . 8 0 0 . 8 4 0 . 8 4 1 . 0 1 0 . 7 8 0 . 9 9 0 . 9 6 = 0 . 0 0 0 3 9 4 原文翻译
