Q: Is it necessary to use a curing compound when a concrete slab is placed during the spring or fall and temperatures approach 60ºF during the day but are sometimes below freezing at night? Would the curing compound possibly retain moisture that might make the concrete more susceptible to damage caused by freezing?

A:Cold Weather Concreting (ACI 306R-88) advises that concrete exposed to cold weather (temperatures below 50ºF) isn't likely to dry at an undesirable rate.

However, since new concrete may be vulnerable to freezing when it's in a critically saturated condition, protection such as a heated enclosure may be needed. Floors are especially prone to rapid drying in a heated enclosure.For such enclosures, ACI 306 suggests using steam to heat the concrete and prevent excessive evaporation. When dry heat is used, the report advises covering the concrete with an impervious material or a curing compound. Water curing isn't recommended since it increases the likelihood of concrete freezing in a nearly saturated condition when protection is removed.

For the conditions you mention, in which a hard freeze is unlikely, covering the concrete with insulated blankets might be the best strategy. The blankets will help to retain moisture and heat needed for curing but, when removed, will allow the concrete to dry before it's exposed to freezing temperatures.

Reader's Response:

In your response question regarding the best cold-weather curing methods, you should have gone one step further. After the sentence saying that concrete exposed to cold weather "isn't likely to dry at an undesirable rate," you should have stressed that appropriate curing procedures should be provided to minimize moisture loss, as stated in ACI 302.1R-96, Guide for Concrete Floor and Slab Construction. We should always stress curing no matter what the temperature, whether ponding, membrane-forming cures or sheet materials are used. Too many people feel curing is not necessary at lower temperatures.

- John C. Hukey Dayton Superior Corp. Oregon, Ill.