Cold Weather Concrete Mix Design
for the Beneficial Use of Coal Fly Ash
as a Supplementary Cementitious Material
his fact sheet provides
information on cold weather
mix design considerations
for the use of coal fly ash as a
supplementary cementitious
material (SCM). Cold weather
poses special considerations for
the beneficial use of coal fly ash as an
SCM. Cold weather increases the curing
time and decreases the rate of strength
gain of concrete. Concrete containing
coal fly ash is
Table 1 Minimum temperatures
recommended by ACI for fresh
concrete as placed and maintained
55° F
50° F
45° F
40° F
Most slabs, pavements,
sections less than 12 in. thick.
Most beams, columns, walls,
sections 12 to 36 in. thick
generally slower to
cure than concrete
without fly ash,
and this effect is
exacerbated by
cold weather.
The first consideration
for the use of fly ash
as an SCM is the
type of fly ash to
be used. There are
two predominant
types of fly ash used
as SCMs: these are
Class C and Class F
ashes, as defined by ASTM C618. They are both
pozzolanic-that is, they are compounds that
exhibit cementitious properties when combined
with calcium hydroxide in water. Class C ash
Large columns, footings,
pedestals, mats, sections
36 to 72 in. thick.
Sections over 72 in. thick
010cm
30cm
The Slump
Test
slump
020cm
slump flow
United States
Environmental Protection
Agency
also has self-cementing properties and tends to
cure faster than Class F ash, so its use may be
preferable in cold weather. For more information
about the differences between Class C and Class
F ashes, and about the use of fly ash as an SCM,
the Federal Highway Administration web site may
be helpful: http://www.tfhrc.gov/hnr20/recycle/
waste/cfa53.h tm.
Another consideration for cold weather mix
design is that cold weather can actually be
helpful, as long as care is taken to prevent
freezing and other problems. Concrete set in cool
temperatures, while generally slower to cure and
gain strength, will typically have higher ultimate
strength. Fly ash used as a cement replacement
and used in conjunction with liquid admixtures
will further enhance the ultimate strength of
concrete in cooler weather.
The American Concrete Institute (ACI) defines
cold weather as a period of three or more
consecutive days that have an average daily air
temperature of less than 40°F and in which the air
temperature does not exceed 50°F for more than
half of any 24-hour period.
Table 1 shows the benefit of concrete mass on
reducing the effect of lower temperatures on
concrete setting and strength. The larger mass of
concrete is more resistant to damage at somewhat
lower temperatures.
Strategies to Deal with
Fly Ash Concreting in
Cold Weather
There are two general strategies to use to ensure
good performance of fly-ash concrete in cold
weather: one is to adjust the concrete mix to
account for weather's effects, and the other is to
protect the concrete from the cold. Using one or
more of the following methods will allow the user
to achieve the typical benefits of fly ash concrete,
while mitigating the negative effects of the cold.
Reduce water/cement ratio. The slower setting
times and reduced rate of strength gain that result
from decreased hydration can be counteracted
by having a higher cement content, less water, or
a lower slump. Replacing a certain percentage of
cement with fly ash decreases water requirements
so concrete can be placed at a lowered slump.
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Slump is the relative tendency for concrete to
"sag" or "slump" after a special conical form is
removed from a sample of the concrete being
tested. Slump characteristics are important to
proper handling and compaction of concrete.
Require a less-fluid (low-slump) concrete mix.
Lower slump properties are easier to attain
when using fly ash because its spherical shape
results in higher workability even in mixes with
less water. Lower slump will also reduce bleed
water, which is the water that can emerge on
the concrete surface during curing, and that can
result in structural weakness.
Increase portland cement content up to 100 Ibs.
per cubic yard to help develop early strength, or
replace normal cement with rapid-setting Type
III Portland cement.
Add chemical accelerators such as calcium
chloride at a maximum of two percent by weight
to the cement mixture, or add proprietary, non-
chloride accelerators according to specification.
Accelerators increase the rate of hydration,
leading to a shorter set time.
Use air-entrained concrete when exposure to
moisture, freezing, and thawing are expected.
Air entrainment is the deliberate incorporation
of tiny air bubbles into the concrete mix to
improve resistance to freeze-thaw damage.
This is typically accomplished by incorporating
special admixtures into the concrete.
Keep concrete from freezing. If concrete
freezes while still plastic, its potential strength
and durability can be severely affected. Fresh
concrete must be protected from freezing until
the concrete attains a compressive strength of
about 500 pounds per square inch. Concrete
should be protected for a minimum of two days.
Protection methods include:
• Provide insulation blankets and plastics
to help the curing process. Leave for
approximately 7-10 days.
• Provide triple insulation thickness at corners
and edges of walls and slabs.
« Heat the mix by (1) using hot water in the
concrete mix, (2) providing a heated enclosure
for concrete, (3) heating subgrades before
placing concrete, or (4) heating the concrete
framework.
• Do not expose concrete surfaces to a sudden
temperature drop; gradually reduce insulation
or enclosure temperature to control concrete
cooling (no more than a 50°F drop in 24 hours).
• Allow concrete to air dry before exposing
it to freezing temperatures.
• After a sufficient protection period, cool
concrete gradually.
These methods may be used alone or in
combination to reach the setting and strength
gain characteristics required. The appropriate
decision will afford an economically viable
solution with the least impact on the ultimate
concrete properties.
For more information about C2P2 and the use of
coal fly ash, go to: www.epa.gov/c2p2, or contact
the EPA coordinator at RCC-Challenge@epa.gov.
References
ACI (2002) "Cold Weather Concreting."
American Concrete Institute, Committee
306,ACI 306R-88.
ASTM C618-08 Standard Specification for Coal
Fly Ash and Raw or Calcined Natural Pozzolan
for Use in Concrete.
Headwaters Resources (2005) "Fly Ash in Cold
Weather Concrete," Bulletin No. 26. Available
online at: http://www.flyash.com/data/upimages/
press/TB.26%20Fly%20Ash%20in%20Cold%20
Weather%20Concrete.pdf.
FHWA. "Coal Fly Ash User Guideline,
Portland Cement Concrete." Turner-
Fairbank Highway Research Center, Federal
Highway Administration, U.S. Department of
Transportation. Available online at: http://www.
tfhrc.gov/hnr20/recycle/waste/cfa53.htm.
Smith, Philip (2008) "Cold Weather Concreting."
ConcreteNews, L&M Construction Chemcials,
January, 2008. Available online at: http://www.
Imcc.com/news/january2008/january2008-02.asp.
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June 2008
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