Freezing Temperature Protection Admixture
for Portland Cement Concrete
CHARLES J. KORHONEN AND JOHN W. BROOK
serve its intended purpose. Finishing operations
INTRODUCTION
take longer as temperatures dip to 5C (40F) and
below, and forms cannot be stripped as fast as
Background
Development of an admixture capable of al-
they can during the summer. The rate of concrete
lowing fresh concrete to gain strength at below-
strength gain is slowed. At a few degrees below
freezing temperatures without causing detrimen-
zero, the hydration rate of cement continues to
tal effects to the final product has long been a goal
slow and the mix water begins to turn into ice; at
3C (27F), 90% of the water will freeze (Korhonen
of the concreting industry. Work on the problem
began several decades ago, with contributions
1990). If freezing occurs, upon thawing the con-
made by researchers from the former Soviet Union,
crete may lose half its strength.
Scandinavia, and elsewhere (Korhonen 1990) who
There are procedures today to protect newly
showed that certain chemicals can significantly
placed concrete from freezing and to ensure ad-
depress the freezing point of the concrete mix
equate strength to produce concrete that meets
water, and that other chemicals can accelerate the
construction needs for strength and durability.
hydration rate of cement at very low tempera-
However, this protection is costly. It has been
tures. To date, however, there has been no com-
estimated that the U.S. construction industry
parable advancement of these or other chemicals
spends 0 million (Civil Engineering 1991) ev-
in the United States. Concerns over their poten-
ery year on measures to protect fresh concrete
tial adverse effects, such as increased risk of cor-
from freezing. An admixture that would alleviate
rosion or chemical reaction with aggregate, have
this expense would be of great economic benefit.
discouraged serious consideration.
Master Builders (MB) established renewed in-
As a result, current U.S. winter concreting prac-
terest in this topic in the late 1980s by marketing
tices have remained unchanged for the past sev-
this country's first nonchloride, low-temperature
eral decades. Concrete ingredients such as stone,
admixture: Pozzutec 20. Though Pozzutec 20 de-
sand, and water must still be heated to melt all
presses the freezing point of water a few degrees,
ice, but not heated so highly as to cause rapid set
its major cold weather advantage is that it has
within the concrete mixing and handling equip-
been specially formulated to accelerate setting time
ment, and to create a mix temperature that is well
and strength gain in concrete. When used at rec-
above freezing. The substrate on which fresh con-
ommended dosages, Pozzutec 20 greatly increases
crete is placed must be thawed, and the concrete
the rate of cement hydration, generating more
must be kept warm and moist long enough to
heat earlier than would be generated by normal
ensure adequate strength to allow early removal
concrete, even those containing conventional ac-
of forms for their reuse.
celerators. This extra heat usually provides enough
The American Concrete Institute (ACI) sets the
protection to prevent concrete from freezing until
standards for winter concreting. It recommends
it has developed sufficient strength to resist ice
that freshly placed concrete must be protected
damage. After the concrete has reached this level
from freezing by maintaining its temperature at
of self-protection, it continues to gain strength
or above 5C (40F), preferably at or above 10C
even if its internal temperature should fall below
(50F) (ACI 1988) until it has sufficiently cured to
freezing. Pozzutec 20 is recommended for use at
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