Cold Weather Concrete Mix Design

for the Beneficial Use of Coal Fly Ash

as a Supplementary Cementitious Material

This 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 

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 

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.htm. 

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. 

Table 1 Minimum temperatures 

recommended by ACI for fresh 

concrete as placed and maintained

55o F Most slabs, pavements, 

sections less than 12 in. thick. 

50o F Most beams, columns, walls, 

sections 12 to 36 in. thick 

45o F

Large columns, footings, 

pedestals, mats, sections 

36 to 72 in. thick.

40o F Sections over 72 in. thick 

The Slump 

Test
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 lbs. 

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, nonchloride 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.” TurnerFairbank 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.

lmcc.com/news/january2008/january2008-02.asp.

Office of Solid Waste and Emergency Response

EPA-530-F-08-011

June 2008

Recycled/Recyclable

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