TM 5-852-4/AFM 88-19, Chap. 4
157
adequately insulated concrete surfaces after the onset of
heat and from strong C02 atmosphere . The harmful
effects of C02 can be avoided by the use of heaters
extreme low temperatures are seldom successful due to
which do not create combustion gases in the heated
the press of other work during such critical periods. It
area or by the removal of gases from oil burning heaters
should be remembered that, to be effective, the
through proper venting to the atmosphere. Membrane
insulating material must be kept in close contact with the
curing compound applied to exposed surfaces of
concrete or formed surface and air should not be
concrete is helpful for preventing detrimental effects of
permitted to circulate between it and the concrete.
2
C0 but should not be used as a substitute for venting oil
Particular attention should be given to assure that edges
burning heaters. Heaters also create severe safety
and corners are provided with extra protection.
problems due to the hazards of fire and asphyxiation.
Protection of the insulating material with heavy moisture
Passage of electric current through the concrete or
proof canvas or plastic sheeting from wind, rain, snow, or
reinforcement for heating purposes is a Soviet technique
other wetting is essential. The insulation should have a
2
conductance of not more than 0.25 Btu/hr-ft -F and
that has serious complications and drawbacks in danger
should be maintained in such a condition that this value
to personnel and possible local overheating of concrete;
is not exceeded during the period of protection. As a
it is not recommended. Any mechanical devices used to
general rule, concrete includes in forms and/or adequate
maintain heat after placing should be backed up by
202
will not lose enough moisture to
protective insulation
standby equipment.
h. Protection after the curing period. After the
require additional curing water. Table 6-2 from the
203
is a useful
Bureau of Reclamation Concrete Manual
specified curing and protection period, the insulation and
guide to insulated forms using bat or blanket insulation.
forms or the enclosure should be removed only after
Polystyrene or polyurethane foam equivalents require
careful review of the temperature observations made and
less thickness than bat or blanket insulation and are
recorded throughout the period and the results of the test
more suitable for re-use. On larger projects special
specimens. When it is assured that the concrete has
commercial insulated forms of plywood and foamed
attained the desired strength, the protective and curing
plastic board may be considered. Various substitute
measures may be discontinued. Rapid chilling of the
materials as well as foamed-in-place techniques may be
concrete surface must be avoided to preclude shrinkage
used.
Insulation can usually suffice without
cracking and serious damage to the structure. Care
supplementary heat down to an ambient air temperature
should be used to avoid the rapid exposure of large
of, say, 25F; this value, and also rule-of-thumb
areas of fresh concrete to low ambient temperatures.
temperature estimates, involve considerable uncertainty
This can be accomplished by the gradual removal of the
because convection losses depend strongly on wind
forms in the presence of a source of heat, the dissipation
speed and because such variables as type of cement
of which is prevented by tarpaulins or similar sheltering
and the thickness of concrete section are also involved.
devices, by removal of forms and immediate application
(2) Heating of forms. Below about 25 F
of insulation, by slow reduction of heat supplied to
ambient temperature, heat is advisable, together with the
enclosures, and preferably by delaying removal of forms
insulation. Forms may be heated by steam jacketing or
or other protection until suitable temperatures prevail.
electrically. A recent product for efficient electrical
The time, trouble, and expense of heating the materials,
heating consists of panels of woven glass, coated with
and using protective measures during the mixing,
graphite and impregnated with epoxy resin. The panels
transporting, placing and curing of the concrete will be
are readily incorporated into the insulation of the
wasted if the stability, durability and soundness of the
formwork or may be sandwiched with plastic protection.
structure are jeopardized by a lack of concern at this last
(3) Heated enclosures. A number of types
stage. The temperature of the concrete should not be
of heated enclosures may be used to protect concrete
allowed to decrease at a rate of more than 2 F/hr.
i.
Temperature records. A comprehensive
from loss of heat. Live steam is considered the best
heating agent since it also insures that the exposed
temperature record should be kept during and after the
concrete surfaces are not subjected to moisture
period of protection and included in the permanent
132
evaporation . The enclosures must be reasonably tight
records of the job. The temperature of the heated and
and securely braced against wind and snow loading with
unheated materials and the freshly mixed concrete
particular care given to tightness of openings in the
should be a part of this record. It is desirable to obtain
enclosure. The use of hot air blowers is the second best
the temperature history of the interior concrete for each
method of heating the air in an enclosure. Rapid drying
part of the structure during and after the necessary
of the concrete surface will result from hot air, however,
period of protection and until the concrete temperature
and the curing surfaces must be kept damp. The use of
stabilizes.
This can best be accomplished with
open fires and salamanders, while still in somewhat
thermocouples; however, the embedment of this-walled
general use, should be avoided wherever possible due to
possible damage to concrete from direct exposure to
6-8
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