Concrete Home Building Council - 11/19/2004 (Plain Text Version)

CHBC Chairman
Michael Weber

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In this issue:
Tis the Season, make sure you don't get stuck in the cold!
Concrete Builders Win Top Award at SHOWCASE
The New American Home 2005 Weathered Florida's Hurricanes
Trump This: Project Conversion in Vegas
Paving the Way to Success
ICFs Bloom in the Desert
NAHB to Honor Builders who Help the Disabled
Honoring The Best In Residential Architecture
Concrete Briefs . . . Winner of the Dell Hand Held & More!

Tis the Season, make sure you don't get stuck in the cold!

Cold weather concrete construction can present a new set of challenges to the builder, contractor, and producer. One of the biggest keys to the successful placement of concrete in cold weather is cooperation between all of the parties who play a role in the production of concrete elements in cold weather. There are a number of ongoing research projects that are shedding new light on the methods used to successfully construct concrete in cold weather, and it is becoming apparent that a “one size fits all” approach may not be the best approach (i.e. the practices and mixes used in the construction of foundation walls may not apply to the construction of a slab), but in any case, one thing remains important; in cold weather, concrete cannot simply be placed, finished, and forgotten. In its early stages, it must be continuously protected from damage that can be caused by freezing, and by freeze-thaw cycles.

To understand why it is necessary to protect the concrete from cold weather, it is important to understand the chemistry behind the production of fresh concrete. The setting and strength gain of portland cement concrete results from a chemical reaction known as hydration. It has been generally accepted that for hydration to take place, the temperature of the concrete must be above 40°F. Although some current research projects may find this is not always true, it remains certain that as the temperature of the concrete drops, the hydration process slows, and at some point may stop altogether. In addition, the amount of free water in the mix will have a direct relationship to the damaging effects that freezing has on the concrete.

Below are some general tips to help insure successful cold weather concrete placement:

Make sure the concrete mix has been proportioned for cold weather placement, which can include heating the aggregates, using hot water in the batching process, and using accelerating admixtures.
Remember that accelerators are not antifreeze agents; they simply shorten the set time and can accelerate the strength gain of the concrete.
DO NOT use calcium chloride (or agents containing chlorides) in any amounts as an accelerator in reinforced concrete.
Do not place concrete on frozen ground. Remove all snow, ice, and frost from the areas to receive concrete.
The temperature of embedded items should be above freezing when coming into contact with fresh concrete.
Always cure the concrete after finishing and protect it from freezing. It is advisable to maintain the in-place temperature at 50°F or greater until required strength has been attained. Use insulating blankets, heaters, insulated forms, or other appropriate methods. Edges and corners tend to be more susceptible to freezing so pay extra attention to these areas.
If combustion heaters are used make sure the exhaust is vented out and away from the concrete to reduce the risk of carbonation which occurs when carbon monoxide from the exhaust forms carbonic acid, which can react with the fresh concrete to form a soft dusty surface when it cures.
Consider the use of in place maturity meters to monitor the concrete’s strength gain to determine the length of time that the cold weather protection is needed.
At the end of the protection period it is advisable to cool the concrete gradually to reduce the potential for cracking due to thermal stresses.
Place concrete earlier in the day to gain the advantage of the thermal heat gain produced by the sun in daylight hours.

Remember to check your local codes and project specifications for specific requirements that may be applicable to your project. For additional guidance and recommendations see ACI 306 (Cold Weather Concrete) and the Concrete Foundation Association’s Cold Weather Research Final Report printed in Concrete Construction.