Civil Engineering Reference
In-Depth Information
CONCRETING ON GROUND
Concreting on ground during cold weather involves some
extra effort and expense, but many contractors find that it
more than pays for itself. In winter, the site around the
structure may be frozen rather than a morass of mud. The
concrete will furnish some if not all of the heat needed for
proper curing. Internal concrete temperatures should be
monitored. Insulated blankets or simple enclosures are
easily provided. Embankments are frozen and require less
bracing. With a good start during the winter months,
construction gets above the ground before warmer
weather arrives.
Placing concrete on the ground involves different pro-
cedures from those used at an upper level: (1) the ground
must be thawed before placing concrete; (2) cement
hydration will furnish some of the curing heat; (3) con-
struction of enclosures is much simpler and use of insu-
lating blankets may be sufficient; (4) in the case of a floor
slab, a
vented
heater is required if the area is enclosed; and
(5) hydronic heaters can be used to thaw subgrades using
insulated blankets or to heat enclosures without concern
for carbonation. For more on hydronic heaters, see
“
Heaters
” later in this chapter.
Once cast, footings should be backfilled as soon as
possible with unfrozen fill. Concrete should never be
placed on a frozen subgrade or backfilled with frozen fill;
otherwise once they thaw, uneven settlements may occur
causing cracking.
ACI Committee 306
requires that concrete not be
placed on any surface that would lower the temperature of
the concrete in place below the minimum values shown on
Line 4 in Table 14-1. In addition, concrete placement tem-
peratures should not be higher than these minimum values
by more than 11°C (20°F) to reduce rapid moisture loss and
the potential development of plastic shrinkage cracks.
When the subgrade is frozen to a depth of approxi-
mately 80 mm (3 inches), the surface region can be thawed
by (1) steaming; (2) spreading a layer of hot sand, gravel,
or other granular material where the grade elevations
allow it; (3) removing and replacing with unfrozen fill;
(4) covering the subgrade with insulation for a few days;
or (5) using hydronic heaters under insulated blankets
which can thaw frozen ground at a rate of 0.3 m (1 ft) per
24 hours to a depth up to 3 m (10 ft) (
Grochoski 2000
).
Placing concrete for floor slabs and exposed footings
should be delayed until the ground thaws and warms
sufficiently to ensure that it will not freeze again during
the protection and curing period.
Slabs can be cast on ground at ambient temperatures as
low as 2°C (35°F) as long as the minimum concrete temper-
ature as placed is not less than shown on Line 4 of Table
14-1. Although surface temperatures need not be higher
than a few degrees above freezing, they also should prefer-
ably not be more than 5°C (10°F) higher than the minimum
placement temperature either. The duration of curing
should not be less than that described in Chapter 12 for the
Fig. 14-15. Insulated curing box with thermostat for curing
test cylinders. Heat is supplied by electric rubber heating
mats on the bottom. A wide variety of designs are possible
for curing boxes. (43463)
trolled by a thermostat (Fig. 14-15). When stored in an
insulated curing box outdoors, cylinders are less likely
to be jostled by vibrations than if left on the floor of a
trailer. If kept in a trailer where the heat may be turned
off at night or over a weekend or holiday, the cylinders
would not be at the prescribed curing temperatures
during this critical period.
In addition to laboratory-cured cylinders, it is useful
to field-cure some test cylinders in order to monitor actual
curing conditions on the job in cold weather. It is some-
times difficult to find the right locations for field curing.
Differences in the surface to volume ratios between cylin-
ders and the structure, in conjunction with differences in
mass, make correlating field-cured cylinder strengths to
in-place strengths difficult. A preferred location is in a
boxout in a floor slab or wall with thermal insulation for
cover. When placed on a formwork ledge just below a
heated, suspended floor, possible high temperatures there
will not duplicate the average temperature in the slab, nor
the lowest temperature on top of the slab. Still, field-cured
cylinders are more indicative of actual concrete strength
than laboratory-cured cylinders. Particular care should be
taken to protect compressive strength test cylinders from
freezing; their small mass may not generate enough heat
of hydration to protect them.
Molds stripped from cylinders after the first 24 ± 8
hours must be wrapped tightly in plastic bags or labora-
tory curing started immediately. When cylinders are
picked up for delivery to the laboratory, they must be
maintained at a temperature of 16°C (60°F) to 27°C (80°F)
until they are placed in the laboratory curing room.
Cast-in-place cylinders (ASTM C 873) and non-
destructive testing methods discussed in Chapter 16, as
well as maturity techniques discussed later in this chapter,
are helpful in monitoring in-place concrete strength.
