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
--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
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
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
-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
-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
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.