Civil Engineering Reference
In-Depth Information
125
this, membrane-forming curing compounds may not
retain enough water in the concrete. Therefore, fogging
and wet curing become necessary to maximize hydration
(
Copeland and Bragg 1955
). Fogging during and after
placing and finishing also helps minimize plastic cracking
in concretes with very low water-cement ratios (especially
around 0.30 or less).
When moist curing is interrupted, the development of
strength continues for a short period and then stops after
the concrete's internal relative humidity drops to about
80%. However, if moist curing is resumed, strength devel-
opment will be reactivated, but the original potential
strength may not be achieved. Although it can be done in
a laboratory, it is difficult to resaturate concrete in the
field. Thus, it is best to moist-cure the concrete continu-
ously from the time it is placed and finished until it has
gained sufficient strength, impermeability, and durability.
Loss of water will also cause the concrete to shrink,
thus creating tensile stresses within the concrete. If these
stresses develop before the concrete has attained adequate
tensile strength, surface cracking can result. All exposed
surfaces, including exposed edges and joints, must be pro-
tected against moisture evaporation.
Hydration proceeds at a much slower rate when the
concrete temperature is low. Temperatures below 10°C
(50°F) are unfavorable for the development of early
strength; below 4°C (40°F) the development of early
strength is greatly retarded; and at or below freezing
temperatures, down to -10°C (14°F), little or no strength
develops.
In recent years, a maturity concept has been introduced
to evaluate the development of strength when there is vari-
ation in the curing temperature of the concrete. Maturity is
the product of the age of the concrete and its average curing
temperature above a certain base temperature. Refer to
Chapter 14
for more information on the maturity concept. It
follows that concrete should be protected so that its tem-
perature remains favorable for hydration and moisture is
not lost during the early hardening period.
105
85
65
Casting/curing temperature,
°
C (
°
F)
23/23 (73/73)
32/32 (90/90)
10/10 (50/50)
23/10 (73/50)
45
25
0
20
40
60
Age, days
120
100
Casting/curing temperature,
C (
F)
°
°
80
23/23 (73/73)
32/32 (90/90)
10/10 (50/50)
23/10 (73/50)
60
0
20
40
60
Age, days
Fig. 12-3. Effect of curing temperature on strength gain
(top) relative to 28-day strength and (bottom) relative to the
strength of concrete at 23°C (73°F) (
Burg 1996
).
The most effective method for curing concrete
depends on the materials used, method of construction,
and the intended use of the hardened concrete. For most
jobs, curing generally involves applying curing com-
pounds, or covering the freshly placed and finished con-
crete with impermeable sheets or wet burlap. In some
cases, such as in hot and cold weather, special care using
other precautions is needed.
Concrete mixtures with high cement contents and low
water-cement ratios (less than 0.40) may require special
curing needs. As cement hydrates (chemically combining
with water) the internal relative humidity decreases caus-
ing the paste to self-desiccate (dry out) if no external water
is provided. The paste can self-desiccate to a level where
hydration stops. This may influence desired concrete
properties, especially if the internal relative humidity
drops below 80% within the first seven days. In view of
CURING METHODS AND MATERIALS
Concrete can be kept moist (and in some cases at a favor-
able temperature) by three curing methods:
1. Methods that maintain the presence of mixing water
in the concrete during the early hardening period.
These include ponding or immersion, spraying or
fogging, and saturated wet coverings. These methods
afford some cooling through evaporation, which is
beneficial in hot weather.
2. Methods that reduce the loss of mixing water from
the surface of the concrete. This can be done by cov-
ering the concrete with impervious paper or plastic
sheets, or by applying membrane-forming curing
compounds.
