Cold weather concreting

Concrete Construction, Sept, 2006

October 1956

Q. How soon after concrete has been poured can it be frozen without sustaining damage?

A. In "Recommended Practice for Winter Concreting," ACI Committee 604 [now ACI 306] noted that the principal factor is the rate at which the concrete gains strength. Experience indicates that concrete may not be seriously damaged by one or two cycles of freezing if it has attained a strength of more than 500 psi. Limited tests indicate that concrete placed at a temperature of 75[degrees] must be subjected to severe freezing within about 6 hours for any serious damage to be sustained. If it is placed at 40[degrees] F, even a mild freezing at 25[degrees] may result in a 50% strength loss.

May 1977

Q. Is it true that concrete will no longer freeze after it reaches 500 psi and, if so, what is the best method for field-testing the concrete to determine when it reaches 500 psi?

A. It is not true that concrete will not freeze after it reaches a strength of 500 psi. However, it is a rule of thumb that concrete will not be destroyed by a few cycles of freezing and thawing after it has attained about 500-psi strength. In other words, 500 psi is the minimum strength required to resist the first exposure to freezing. In most cases it is not really practical to determine that concrete has attained this minimum "safe" strength. Most engineers would be more inclined to estimate the rate of strength gain on the basis of the known low-temperature behavior of the concrete mix being used. Then if there is any likelihood whatever that the concrete might not attain 500 psi before it freezes, they would make provision for heating or insulating it enough to prevent freezing. Actually, the recommended method is to not rely at all on either tests or estimates but to follow the practices for protecting concrete against freezing in accordance with ACI 306 "Recommended Practice for Cold Weather Concreting." That standard provides tables which show the amount of protection required for various concrete sections, types of cement and degree of load as well as the length of time the protection should be supplied.

October 2004

Q. How long must concrete be protected from freezing temperatures, with and without accelerators or admixtures?

A. Fresh concrete should be protected from freezing for the first 24 hours after placement, by which time it should have a compressive strength of 500 psi. After that, the amount and duration of protection will depend on the desired rate of strength development.

Generally, the greater the cement content and the section thickness, the greater the heat of hydration and thus more rapid strength gain of the concrete. You can use the maturity method from ASTM C 1074 if you want to correlate the temperature-time factor of the in-place concrete to its strength.

Other references that may be of interest are ACI 306 "Cold Weather Concreting," and the first section in ACI 201 "Guide to Durable Concrete." Beyond that, the 14th edition of Design and Control of Concrete Mixtures gives a maximum saturation of capillary pores at 91.7% filled with water as the point at which concrete with or without air will not be affected by freezing and thawing cycles. The 9% expansion of water as it freezes has the room for expansion it needs to avoid disruptive internal pressures.

Another good resource on this subject is T.C. Powers' Prevention of Frost Damage to Green Concrete, which is available as a free PDF file at www.cement.org/bookstore (search for "frost damage"). That is the reference for the 500 psi minimum for a single frost cycle in ACI 306. Remember that the resaturation of pores requires very little water from an external source to render a non-air entrained concrete vulnerable to damage.