Civil Engineering Reference
In-Depth Information
A salamander is an inexpensive combustion heater
without a fan that discharges its combustion products into
the surrounding air; heating is accomplished by radiation
from its metal casing. Salamanders are fueled by coke, oil,
wood, or liquid propane. They are but one form of a
direct-fired heater. A primary disadvantage of salaman-
ders is the high temperature of their metal casing, a defi-
nite fire hazard. Salamanders should be placed so that
they will not overheat formwork or enclosure materials.
When placed on floor slabs, they should be elevated to
avoid scorching the concrete.
Some heaters burn more than one type of fuel. The
approximate heat values of fuels are as follows:
No. 1 fuel oil 37,700 kJ/L (135,000 Btu/gal)
Kerosene 37,400 kJ/L (134,000 Btu/gal)
Gasoline 35,725 kJ/L (128,000 Btu/gal)
Liquid-propane gas 25,500 kJ/L (91,500 Btu/gal)
Natural gas 37,200 kJ/m
3
(1,000 Btu/ft
3
)
The output rating of a portable heater is usually the heat
content of the fuel consumed per hour. A rule of thumb is
that about 134,000 kJ are required for each 100 m
3
(36,000
Btu for 10,000 ft
3
) of air to develop a 10°C (20°F) tempera-
ture rise.
Electricity can also be used to cure concrete in winter.
The use of large electric blankets equipped with thermo-
stats is one method. The blankets can also be used to thaw
subgrades or concrete foundations.
Use of electrical resistance wires that are cast into the
concrete is another method. The power supplied is under
50 volts, and from 7.0 to 23.5 MJ (1.5 to 5 kilowatt-hours)
of electricity per cubic meter (cubic yard) of concrete is
required, depending on the circumstances. The method
has been used in the Montreal, Quebec, area for many
years. Where electrical resistance wires are used, insula-
tion should be included during the initial setting period. If
insulation is removed before the recommended time, the
concrete should be covered with an impervious sheet and
the power continued for the required time.
Steam is another source of heat for winter concreting.
Live steam can be piped into an enclosure or supplied
through radiant heating units. In choosing a heat source, it
must be remembered that the concrete itself supplies heat
through hydration of cement; this is often enough for
curing needs if the heat can be retained within the
concrete with insulation.
cement type and amount, whether accelerating admix-
tures were used, and the loads that must be carried.
Recommended minimum periods of protection are given
in Table 14-3. The duration of heating structural concrete
that requires the attainment of full service loading before
forms and shores are removed should be based on the
adequacy of in-place compressive strengths rather than an
arbitrary time period. If no data are available, a conserva-
tive estimate of the length of time for heating and protec-
tion can be made using Table 14-3.
Moist Curing
Strength gain stops when moisture required for curing is
no longer available. Concrete retained in forms or covered
with insulation seldom loses enough moisture at 5°C to
15°C (40°F to 55°F) to impair curing. However, a positive
means of providing moist curing is needed to offset
drying from low wintertime humidities and heaters used
in enclosures during cold weather.
Live steam exhausted into an enclosure around the
concrete is an excellent method of curing because it
provides both heat and moisture. Steam is especially prac-
tical in extremely cold weather because the moisture
provided offsets the rapid drying that occurs when very
cold air is heated.
Liquid membrane-forming compounds can be used
for early curing of concrete surfaces within heated
enclosures.
Terminating the Heating Period
Rapid cooling of concrete at the end of the heating period
should be avoided. Sudden cooling of the concrete surface
while the interior is still warm may cause thermal crack-
ing, especially in massive sections such as bridge piers,
abutments, dams, and large structural members; thus
cooling should be gradual. A safe temperature differential
between a concrete wall and the ambient air temperature
can be obtained from
ACI 306R-88
. The maximum uni-
form drop in temperature throughout the first 24 hours
after the end of protection should not be more than the
amounts given in Table 14-2. Gradual cooling can be
accomplished by lowering the heat or by simply shutting
off the heat and allowing the enclosure to cool to outside
ambient air temperature.
DURATION OF HEATING
FORM REMOVAL AND RESHORING
It is good practice in cold weather to leave forms in place
as long as possible. Even within heated enclosures, forms
serve to distribute heat more evenly and help prevent
drying and local overheating.
If the curing temperatures listed on Line 4 of Table
14-1 are maintained, Table 14-3A can be used to determine
the minimum time in days that vertical support for forms
After concrete is in place, it should be protected and kept
at the recommended temperatures listed on Line 4 of
Table 14-1. These curing temperatures should be main-
tained until sufficient strength is gained to withstand
exposure to low temperatures, anticipated environment,
and construction and service loads. The length of protec-
tion required to accomplish this will depend on the
