PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 267

7.3 Curing Concrete

Curing is the process of maintaining satisfactory moisture content and tem-
perature in the concrete for a definite period of time. Hydration of cement is
a long-term process and requires water and proper temperature. Therefore,
curing allows continued hydration and, consequently, continued gains in
concrete strength. In fact, once curing stops, the concrete dries out, and the
strength gain stops, as indicated in Figure 7.19. If the concrete is not cured
and is allowed to dry in air, it will gain only about 50% of the strength of
continuously cured concrete. If concrete is cured for only three days, it will
reach about 60% of the strength of continuously cured concrete; if it is cured
for seven days, it will reach 80% of the strength of continuously cured con-
crete. If curing stops for some time and then resumes again, the strength gain
will also stop and reactivate.
Increasing temperature increases the rate of hydration and, consequently,
the rate of strength development. Temperatures below 10

F) are unfa-
vorable for hydration and should be avoided, if possible, especially at early
ages.
Although concrete of high strength may not be needed for a particular
structure, strength is usually emphasized and controlled since it is an indica-
tion of the concrete quality. Thus, proper curing not only increases strength,
but also provides other desirable properties such as durability, water tight-
ness, abrasion resistance, volume stability, resistance to freeze and thaw, and
resistance to deicing chemicals.
Curing should start after the final set of the cement. If concrete is not
cured after setting, concrete will shrink, causing cracks. Drying shrinkage
can be prevented if ample water is provided for a long period of time. An ex-
ample of improper curing would be a concrete floor built directly over the

° C (50 °

150

Moist-cured entire time

125

In air after 7
In air after 3

100

75

In air entire time

50

25

0

FIGURE 7.19 Compressive strength
of concrete at different ages and curing
levels.

7 28 90 180

3

Age, days