Civil Engineering Reference
In-Depth Information
CHAPTER 14
Cold-Weather Concreting
Concrete can be placed safely without damage from
freezing throughout the winter months in cold climates if
certain precautions are taken. Cold weather is defined by
ACI Committee 306 as a period when for more than 3 suc-
cessive days the average daily air temperature drops
below 5°C (40°F) and stays below 10°C (50°F) for more
than one-half of any 24 hour period. Under these circum-
stances, all materials and equipment needed for adequate
protection and curing must be on hand and ready for use
before concrete placement is started. Normal concreting
practices can be resumed once the ambient temperature is
above 10°C (50°F) for more than half a day.
During cold weather, the concrete mixture and its
temperature should be adapted to the construction proce-
dure and ambient weather conditions. Preparations should
be made to protect the concrete; enclosures, windbreaks,
portable heaters, insulated forms, and blankets should be
ready to maintain the concrete temperature (Fig. 14-1).
Forms, reinforcing
steel, and embedded
fixtures must be
clear of snow and
ice at the time
concrete is placed. Thermometers and proper storage
facilities for test cylinders should be available to verify that
precautions are adequate.
EFFECT OF FREEZING FRESH
CONCRETE
Concrete gains very little strength at low temperatures.
Freshly mixed concrete must be protected against the dis-
ruptive effects of freezing (Fig.14-2) until the degree of sat-
uration of the concrete has been sufficiently reduced by
the process of hydration. The time at which this reduction
is accomplished corresponds roughly to the time required
for the concrete to attain a compressive strength of 3.5
MPa (500 psi) (
Powers 1962
). At normal temperatures and
water-cement ratios less than 0.60, this occurs within the
first 24 hours after placement. Significant ultimate
strength reductions, up to about 50%, can occur if concrete
is frozen within a few hours after placement or before it
attains a compressive strength of 3.5 MPa (500 psi)
(
McNeese 1952
). Concrete to be exposed to deicers should
attain a strength of 28 MPa (4,000 psi) prior to repeated
cycles of freezing and thawing (
Klieger 1957
).
Concrete that has
been frozen just once
at an early age can
be restored to nearly
normal strength by
providing favorable
subsequent curing con-
ditions. Such concrete,
however, will not be as
resistant to weathering
nor as watertight as
concrete that had not
been frozen. The crit-
ical period after which
concrete is not seri-
ously damaged by one
or two freezing cycles
Fig. 14-1. When suitable preparations to build enclosures and insulate equipment have been made,
cold weather is no obstacle to concrete construction. (69876, 43464)
