NRI Bridge Report

October 10, 2009 by admin
Filed under In the News

Cold Weather Concrete

October 5, 2009 by admin
Filed under Cold Weather Concrete

10 Tips for Pouring Concrete in Cold Weather

1. Always plan in advance. Have your equipment and materials ready before cold weather hits. Be ready with heaters, insulated curing blankets, and/or enclosures.

2. Use air-entrained concrete.

3. Don’t place concrete on a frozen subbase. Be sure that all ice, snow and frost are removed from surfaces the concrete will touch. Thawing frozen ground is highly recommended.

4. For max durability & strength, fresh concrete should be kept at 55ºF or higher for thin sections. Consider using high-early strength concrete.

5. Cure concrete to prevent loss of moisture. When heated enclosures are used, provide extra moisture by sprinkling or use steam for heating. Vent salamanders and other fuel-burning heaters. Concrete should be allowed to cool slowly to prevent thermal cracking. Best solution we’ve observed is a wet cure concrete curing blanket with insulated backing.

6. Do not use “antifreeze” compounds in an attempt to lower the freezing point of concrete.

7. Leave the forms in place as long as the job schedules permit. Reshoring is necessary until concrete reaches required design strength.

8. Keep job condition records. Record, at least twice daily: weather conditions, temperatures of the air and the concrete surface.

9. If the concrete is to cure below 60ºF, water reducers or retarders may prolong the set.

10. The use of calcium chloride or admixtures containing soluble chlorides is not recommended under certain conditions. Concrete curing blankets are always prefered.

Ideally, concrete should cure for 7 days at temperatures between 65-85°F, with humidity levels under 100%. Due to seasonal weather changes, ideal conditions rarely occur naturally.

Powerblanket Concrete Curing Electric Blankets

Use Powerblanket™ Concrete Curing Blankets

Pouring concrete slabs can be a challenge even in ideal weather conditions. Cold temperatures can increase the risk that your concrete will not cure properly. Proper concrete curing is a key to a strong, long-lasting concrete slab.

Never pour concrete on frozen ground. Use powerblanket™ or other ground thawing electric blankets to bring the temperature up to at least 65°F. Why? Not only can the ground freeze the concrete from the bottom up, cracking can occur when the ground thaws. Hollow spots under the concrete may also be created – and driving over the hollow spots will create tension which in turn creates cracks.

Likewise, never pour concrete over snow – you’ll simply add unnecessary water to the concrete.

If you’re pouring a foundation slab, temperature control is even more critical. A foundation wall is simply a vertical slab – but it’s exposed on three sides once the forms are pulled. Foundation forms should be protected with powerblanket™ or other concrete curing blankets for at least three days, depending on weather conditions.

If you remove forms too quickly, your foundation walls will have less protection, especially if you also remove the concrete curing blankets. Temperatures below 14°F completely stop the hydration (crystal growing) process, demanding the use of curing blankets or other meothods of applying heat.

If the walls are backfilled before proper curing has taken place, the concrete may not be strong enough to withstand the pressure. Once temperatures rise above that point, the chemical reaction resumes, but very slowly. It could take a long time for your foundation walls to gain enough strength to resist the pressure of backfill dirt.

Use Low Slump Concrete

Low slump concrete is hard to work with, but in cold weather it can mean the difference between success and failure. While labor costs can be higher, you’ll have less water bleed problems, and you can place the powerblanket™ concrete curing blankets on the slab more quickly, reducing the effects of cold weather.

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