Cold Weather Concrete

COLD WEATHER CONCRETING

When the average daily temperature falls below 40 F degrees for more than three days, special precautions need to be taken when placing, finishing, curing, and protecting concrete.

Let's take a look at the properties of concrete which cold weather effects!

--In its "plastic" state, concrete will freeze if its temperature falls below 25 degrees. If concrete freezes at this state, the potential strength can be reduced by more than 50% and durability will be adversely affected.

YOU SHOULD PROTECT CONCRETE UNTIL...
--It attains a minimum of 500 psi........
--This is about 2 days after placement if maintained temp. is 50 degrees.

--Lower temperatures have a major effect on the rate of cement hydration, which results in slower set time and strength gain.

"Rule of Thumb"
A drop in temperature of 20 degrees will double the set time.
(Keep this in mind when scheduling operations...such as form removal!)

Anytime concrete is in contact with water and freeze/thaw cycles air entrainment should be used.
Newly placed concrete should be protected from freeze/thaw cycles until it is has reached a compressive strength of 3500 psi.

The process of cement hydration generates heat, and fresh concrete should be insulated to retain this heat and maintain favorable curing temperatures.
Differences between the surface and interior temperatures should be prevented.....cracking may result when difference exceeds 35 degrees.
Use protective insulating measures to avoid thermal shock!!!

How do I place concrete in COLD WEATHER?

Placing concrete in cold weather provides the opportunity for better quality...cooler initial temperature will ultimately result in higher strength.

Accelerators are chemical admixtures which can "accelerate" the rate of setting and strength gain. Calcium chloride may be added at the plant, but should not exceed a maximum of 2% by weight of cement.
Non-chloride accelerators should be used when corrosion of steel reinforcement for contact with metal is a concern.

Accelerators DO NOT prevent concrete from freezing and protective measures should still be taken with curing and freeze protection.

Concrete should be placed at the lowest possible slump to reduce bleeding and set time. Adding 1-2 gallons of water per cubic yard will delay set time anywhere from 1/2 hour to 2 hours depending on the conditions.

Do not start finishing while concrete continues to bleed...this will result in a weak surface!

Preparations for Placing Concrete in Cold Weather

-Snow, ice, and frost should be removed.
-Temperatures of surfaces and metals in contact with concrete should be above
freezing...may require insulating or heating prior to placement.
-Have materials and equipment in place to protect concrete during and after
placement.
-Insulated blankets, tarps, straw covered with plastic sheets
-Heaters (make sure to vent to avoid carbonation..thus dusting)
-Enclosures and possible insulated forms

What to watch out for!!!
-Pay particular attention to corners and edges...suseptible to heat loss.
-Do not let surface dry out while in "plastic" state...causes shrinkage cracks.
-Make sure to adequately cure!
-Water curing is not recommended at freezing temperatures.
-Use curing compounds or impervious paper & plastic sheets for slabs.
-Extremely cold temperatures may require insulated blankets or forms.
-Forms should not be stripped for 1-7 days, depending on the setting characteristics and existing conditions.

BASIC COLD WEATHER CONCRETING GUIDELINES

1) Use air-entained concrete when exposed to freeze/thaw conditions.

2) Keep surfaces in contact with concrete free of ice & snow, and at a temperature above freezing.

3) Place and maintain concrete at recommended temperature.

4) Place at lowest possible slump.

5) Protect "plastic" concrete from freezing or drying.

6) Protect from early-age freeze/thaw cycles until adequate strength is achieved.

7) Limit rapid temperature changes when protective measures are removed.