Hot Weather Concreting
Any period of high temperature
in which special precautions need to be taken to ensure proper handling, placing, finishing, and curing of concrete. High
winds, low relative humidity, and solar radiation all have major effects on concrete.
Hot weather conditions
can produce a rapid rate of evaporation of moisture from the surface of the concrete and accelerated setting time, amoung
other problems. Generally, high relative humidity tends to reduce the effects of high temperatue.
Considerations for HOT
High temperatures cause:
-Increased water demand, thus increasing the water to cement
ratio. This will result in a lower potential strength.
-Accelerated slump loss and loss of entrained air.
-Decrease in set time, resulting in more rapid
Concrete cured in at high temperatures at an early age will not be
as strong at 28 days as
the same concrete cured at temperatures in the range of 70 degrees (F).
High Temperature - High Wind
Velocity - Low Relative Humidity
These conditions will result in a high rate of evaporation and may result in early plastic
shrinkage cracking (drying shrinkage cracking), and the evaporation rate can remove the surface water that is necessary
for hydration...unless proper curing methods are used. (See "Curing" under "Concrete Practices" section).
If the concrete is placed
on a hot day, followed by a cool night, thermal cracking may occur due to the rapid drop in temperature. In massive
structures, high temperature also accelerates cement hydration resulting in thermal cracking.
So How Do I "Concrete
in HOT WEATHER?"
1. Recognize the factors that affect concrete.
2. Plan to minimize their effects.
Use recommended adjustments of batch proportions, possibly using water reducing and retarding admixtures.
ADVANCE TIMING AND SCHEDULING SHOULD BE COMMUNICATED IN ORDER TO AVOID DELAYS IN DELIVERY, PLACING, AND FINISHING!!!
When possible, deliveries should be scheduled to avoid the hottest part of the day. In case of extreme temperatures,
and the project must be completed in the prescence of hot weather, ice may be added to the mix at the plant.
If high winds and low
humidity are predicted use windbreaks, sunscreens, mist fogging, or evaporation retardants to avoid plastic shrinkage cracking
(see "Plastic Shrinkage Cracking" in "Problem Solving")
SUMMARY FOR "HOT WEATHER CONCRETING"
1. MODIFY MIX USING PROPER
ADMIXTURES. REDUCE THE CEMENT CONTENT OF THE MIX AS MUCH AS POSSIBLE WITHOUT INTERFERING WITH THE STRENGTH.
2. HAVE ADEQUATE MANPOWER TO QUICKLY PLACE, FINISH, AND CURE THE CONCRETE.
3. LIMIT THE ADDITION
OF WATER AT THE JOBSITE---ADD ONLY TO SLIGHTLY ADJUST THE SLUMP. WATER ADDITION SHOULD NOT EXCEED ABOUT 2.5 GALLONS PER YARD.
4. SLABS ON GRADE SHOULD NOT BE PLACED DIRECTLY ON POLYETHYLENE SHEETING OR OTHER VAPOR RETARDERS. VAPOR RETARDER SHOULD
BE COVERED WITH A MINIMUM OF
4" LAYER OF COMPACTIBLE, GRANULAR FILL MATERIAL.
5. BEGIN FINAL FINISHING
AS SOON AS THE WATER HAS LEFT THE SURFACE. START CURING AS SOON AS FINISHING IS COMPLETED. CONTINUE CURING FOR AT LEAST 3
DAYS...COVER CONCRETE WITH WET BURLAP AND PLASTIC SHEETING TO PREVENT EVAPORATION OR USE A CURING COMPOUND, OR CURE SLABS
7. PROTECT TEST CYLINDERS AT THE JOBSITE BY SHADING AND PREVENTING EVAPORATION.
8. DO NOT USE ACCELERATORS UNLESS IT IS COMMON PRACTICE TO AVOID PLASTIC SHRINKAGE CRACKING AND EXPEDITE FINISHING
FELL FREE TO CALL COLE COUNTY INDUSTRIES IF YOU HAVE ANY FURTHER QUESTIONS!!!!