Drying shrinkage can take place over long periods of time. Some shrinkage has even been documented after twenty-eight years. See Figure 4.

Figure 4: Percentage of shrinkage versus time

This long-term shrinkage of concrete may be related to carbonation. The rate of drying shrinkage decreases quickly with time---

• 14-34 percent of the 20-year shrinkage occurs in 2 weeks

• 40-80 percent of the 20-year shrinkage occurs in 3 months

• 66-85 percent of the 20-year shrinkage occurs in 1 year.

The greater the amount of hydrated cement means a smaller volume of unhydrated cement. This leads to a greater drying shrinkage over long curing periods. However, the cement paste becomes stronger with age and can absorb a fraction of its shrinkage without cracking. Drying shrinkage is also affected by the relative humidity of the medium surrounding the concrete. See Figure 5.
 
 

Figure 5: Shrinkage versus time for different relative humidities

The same figure shows that swelling is 6 times smaller than shrinkage in air of relative humidity of 70 percent, or eight times smaller than shrinkage in air at 50 percent (A.M. Neville, 383). Therefore, concrete placed in dry unsaturated air shrinks, but it also swells in water at 100 percent relative humidity.