PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 274

7.4 Properties of Hardened Concrete

It is important for the engineer to understand the basic properties of hard-
ened portland cement concrete and to be able to evaluate these properties.
The main properties of hardened concrete that are of interest to civil and
construction engineers include the early volume change, creep, permeability,
and stress-strain relation.

■ 7.4.1 Early Volume Change

When the cement paste is still plastic it undergoes a slight decrease in volume
of about 1%. This shrinkage is known as plastic shrinkage and is due to the
loss of water from the cement paste, either from evaporation or from suction
by dry concrete below the fresh concrete. Plastic shrinkage may cause crack-
ing (Figure 7.27); it can be prevented or reduced by controlling water loss.
In addition to the possible decrease in volume when the concrete is still
plastic, another form of volume change may occur after setting, especially at
early ages. If concrete is not properly cured and is allowed to dry, it will
shrink. This shrinkage is referred to as drying shrinkage , and it also causes
cracks. Shrinkage takes place over a long period of time, although the rate of
shrinkage is high early, then decreases rapidly with time. In fact, about 15%
to 30% of the shrinkage occurs in the first two weeks, while 65% to 85% oc-
curs in the first year. Shrinkage and shrinkage-induced cracking are in-
creased by several factors, including lack of curing, high water-cementitious

FIGURE 7.27 Plastic shrink-

age cracking.