Engineering and Construction

Concrete expands slightly with a gain in moisture and contracts with a loss in moisture. This contraction of the concrete with a loss of moisture has been termed drying shrinkage. The United States Department of the Interior (1947) performed drying shrinkage tests on small concrete beams (4 × 4 × 40 in.) and they measured the drying shrinkage for a period of 38 months. The small concrete beams were initially moist-cured for 14 days at 70°F and then stored for 38 months in air at the same temperature and 50 percent relative humidity. The results of the study showed that 34 percent of the drying shrinkage occurred within the first month and 90 percent of the drying shrinkage occurred within 11 months. The remaining 10 percent of the shrinkage occurred from 11 months to the end of the test (38 months). Hence the results of this study on small concrete specimens showed that most drying shrinkage occurs within the first year after placement, with the amount of shrinkage decreasing with time.

It has also been observed that concrete near the surface dries and shrinks faster than the inner concrete core (Hanson, 1968). Other studies have shown that the volume-to-surface ratio is very important in the amount of drying shrinkage (Hansen and Mattock, 1966). Concrete having a low volume-to-surface ratio will shrink much more than the same concrete having a high volume-to-surface ratio. In addition, it has been observed that drying shrinkage continues longer for larger masses of concrete.

For a constant volume-to-surface ratio of concrete, the factors governing the amount of drying
shrinkage are as follows (PCA, 1994):

1. Water per unit volume of concrete. The amount of water per unit volume of concrete is by far the most important factor affecting the amount of drying shrinkage. The more water in the concrete mix, the greater the amount of drying shrinkage. One study at M.I.T. showed that for each 1 percent increase in mixing water, concrete drying shrinkage was increased by about 2 percent (Carlson, 1938). Another study showed that doubling the water in the concrete mix from 200 to 400 lb/yd3 caused a five times increase in the amount of drying shrinkage.

2. Increased water requirement. Any alteration of the mix design that increases the water requirement of the cement paste will lead to an increase in drying shrinkage. For example, additional mix water may be needed to increase the slump or to compensate for high temperatures of the freshly mixed concrete. Likewise, more water per unit volume will be required for those concrete mixes that have a high proportion of fine aggregates.

3. Coarse aggregates. The coarse aggregates physically restrain the drying shrinkage of the cement paste. Hence dense and relatively incompressible coarse aggregates will be more difficult to compress and will reduce the amount of drying shrinkage as compared to soft and deformable aggregates. Examples of dense and relatively incompressible aggregates that reduce the amount of drying shrinkage include aggregates composed of quartz, granite, and feldspar (ACI, 1980).

4. Curing. Curing of concrete is accomplished by maintaining a satisfactory moisture content and temperature in the concrete for a specified period of time immediately following the placement and finishing of the concrete. In essence, the objective of curing is to prevent the loss of moisture from the concrete. Curing can improve many desirable concrete properties, such as strength, dura- bility, watertightness, abrasion resistance, and resistance to freezing and thawing. In addition, because the loss of water from concrete causes it to shrink, the process of curing will reduce the amount of drying shrinkage.

5. Admixtures. Some admixtures require an increase in the water content of the concrete mix and hence they will cause an increase in drying shrinkage. In addition, studies have shown that accelerators, such as calcium chloride, can substantially increase the drying shrinkage of concrete.

6. Cement. The type of cement, cement fineness and composition, and the cement contents may have some effect on the amount of drying shrinkage, but the effect is usually small.

The basic reason for the development of shrinkage cracks in concrete slab-on-grade foundations is that drying shrinkage produces tensile stresses that are in excess of the concrete’s tensile strength.

Development of Shrinkage Cracks in Concrete Slab-on-grade Foundations.   There can be many different reasons for the development of excessive concrete shrinkage. For example, excessive shrinkage cracking can develop in concrete if the mix was inadequately prepared with a water-cementitious material...