Civil Engineering Reference
In-Depth Information
CHAPTER 12
Curing Concrete
Curing is the maintenance of a satisfactory moisture con-
tent and temperature in concrete for a period of time imme-
diately following placing and finishing so that the desired
properties may develop (Fig. 12-1). The need for adequate
curing of concrete cannot be overemphasized. Curing has a
strong influence on the properties of hardened concrete;
proper curing will increase durability, strength, watertight-
ness, abrasion resistance, volume stability, and resistance to
freezing and thawing and deicers. Exposed slab surfaces
are especially sensitive to curing as strength development
and freeze-thaw resistance of the top surface of a slab can
be reduced significantly when curing is defective.
When portland cement is mixed with water, a chemical
reaction called hydration takes place. The extent to which
this reaction is completed influences the strength and dura-
bility of the concrete. Freshly mixed concrete normally con-
tains more water than is required for hydration of the ce-
ment; however, excessive loss of water by evaporation can
delay or prevent adequate hydration. The surface is partic-
ularly susceptible to insufficient hydration because it dries
first. If temperatures are favorable, hydration is relatively
rapid the first few days after concrete is placed; however, it
is important for water to be retained in the concrete during
this period, that is, for evaporation to be prevented or sub-
stantially reduced.
With proper curing, concrete becomes stronger, more
impermeable, and more resistant to stress, abrasion, and
freezing and thawing. The improvement is rapid at early
ages but continues more slowly thereafter for an indefinite
period. Fig. 12-2 shows the strength gain of concrete with
age for different moist curing periods and Fig. 12-3 shows
the relative strength gain of concrete cured at different
temperatures.
60
8
Moist-cured entire time
50
In air after 28 days moist curing
6
40
In air after 7 days moist curing
In laboratory air entire time
30
4
20
2
10
0
0
0 7
28
90
365
Fig. 12-1. Curing should begin as soon as the concrete
stiffens enough to prevent marring or erosion of the sur-
face. Burlap sprayed with water is an effective method for
moist curing. (69973)
Age at test, days
Fig. 12-2. Effect of moist curing time on strength gain of
concrete (
Gonnerman and Shuman 1928
).
