Civil Engineering Reference
In-Depth Information
and steam temperature in the enclosure (
ACI Committee
517 1992
).
80
80
°
C (175
°
F) max steam temperature
Insulating Blankets or Covers
60
65
°
C (150
°
F)
Layers of dry, porous material such as straw or hay can be
used to provide insulation against freezing of concrete
when temperatures fall below 0°C (32°F).
Formwork can be economically insulated with com-
mercial blanket or batt insulation that has a tough mois-
tureproof covering. Suitable insulating blankets are
manufactured of fiberglass, sponge rubber, cellulose
fibers, mineral wool, vinyl foam, and open-cell poly-
urethane foam. When insulated formwork is used, care
should be taken to ensure that concrete temperatures do
not become excessive.
Framed enclosures of canvas tarpaulins, reinforced
polyethylene film, or other materials can be placed around
the structure and heated by space heaters or steam.
Portable hydronic heaters are used to thaw subgrades as
well as heat concrete without the use of an enclosure
Curing concrete in cold weather should follow the
recommendations in
Chapter 14
and
ACI 306 (1997)
,
Cold-
Weather Concreting.
Recommendations for curing concrete
in hot weather can be found in
Chapter 13
and
ACI 305
,
Hot-Weather Concreting.
52
°
C (125
°
F)
40
20
Note: Steam temperature increased
22
F)/hr up to maximum
Type I cement
°
C (40
°
0
1
3
5
7
9
11
13
15
Delay period prior to steaming, hours
17
15
13
11
9
7
5
3
Steaming period, hours
Fig. 12-10. Relationship between strength at 18 hours and
delay period prior to steaming. In each case, the delay
period plus the steaming period totaled 18 hours (
Hanson
1963
).
Electrical, Oil, Microwave, and
Infrared Curing
may result in damage. It is recommended that the internal
temperature of concrete not exceed 70°C (160°F) to avoid
heat induced delayed expansion and undue reduction in
ultimate strength. Use of concrete temperatures above
70°C (160°F) should be demonstrated to be safe by test or
historic field data.
Concrete temperatures are commonly monitored at
the exposed ends of the concrete element. Monitoring air
temperatures alone is not sufficient because the heat of
hydration may cause the internal temperature of the con-
crete to exceed 70°C (160°F). Besides early strength gain,
there are other advantages of curing concrete at tempera-
tures of around 60°C (140°F); for example, there is reduced
drying shrinkage and creep as compared to concrete cured
at 23°C (73°F) for 28 days (
Klieger 1960
and
Tepponen and
Eriksson 1987
).
Excessive rates of heating and cooling should be
avoided to prevent damaging volume changes. Tempera-
tures in the enclosure surrounding the concrete should not
be increased or decreased more than 22°C to 33°C (40°F to
60°F) per hour depending on the size and shape of the
concrete element.
The curing temperature in the enclosure should be
held until the concrete has reached the desired strength.
The time required will depend on the concrete mixture
Electrical, hot oil, microwave and infrared curing methods
have been available for accelerated and normal curing of
concrete for many years. Electrical curing methods
include a variety of techniques: (1) use of the concrete
itself as the electrical conductor, (2) use of reinforcing steel
as the heating element, (3) use of a special wire as the
heating element, (4) electric blankets, and (5) the use of
electrically heated steel forms (presently the most popular
method). Electrical heating is especially useful in cold-
weather concreting. Hot oil may be circulated through
steel forms to heat the concrete. Infrared rays and
microwave have had limited use in accelerated curing of
concrete. Concrete that is cured by infrared methods is
usually under a covering or enclosed in steel forms.
Electrical, oil, and infrared curing methods are used pri-
marily in the precast concrete industry.
CURING PERIOD AND TEMPERATURE
The period of time that concrete should be protected from
freezing, abnormally high temperatures, and against loss
of moisture depends upon a number of factors: the type of
cementing materials used; mixture proportions; required
strength, size and shape of the concrete member; ambient
weather; and future exposure conditions. The curing
