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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-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 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 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 ε

should be assumed only in the absence of more exact date for the concrete to be used.

Time of shrinkage coefficient, S t

At 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 h

S h =1.4-0.01H for 40

or,

S h =3.0-0.03H for 80

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 th

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 in =25.4mm)

Fines coefficient, S f

S f =0.86 for 40%, 1.00 for 50%, and 1.04 for 70% fines by weight

Air content coefficient, S e

S e =0.98 for 4%, 1.00 for 6%, and 1.03 for 10% air

Cement content factor, S c

35t t

S t =+55t t

S t =+≤