Cold Weather Concrete
Residential Concreting:"The COLD Facts"
With the arrival of fall and cooler temperatures, the placement of residential flatwork can continue, provided that the principles of "Cold Weather Concreting" are followed. By definition (ACI 306), cold weather conditions exist when "…for more than 3 consecutive days, the average daily temperature is less than 40°F AND the air temperature is not greater than 50°F for more than one-half of any 24 hr. period."
ACCELERATED EARLY STRENGTH GAIN
To overcome delayed strength and initial set development associated with cold weather conditions, one or a combination of the following mix adjustments are recommended:
* Substituting Type I with Type III cement
* Addition of calcium chloride admixtures
* Addition of non-chloride accelerators
* Increasing the Type I cement content by 100-200 lbs.
PLACEMENT GUIDELINES
* Do not place concrete on a frozen subgrade; upon thawing, uneven settlement and cracking are likely to occur.
* The minimum concrete temperature, as placed and maintained, must exceed 55°F. . . however, caution should be exercised with concrete temperatures above 75°F.
* Appropriate curing and cold weather protection must be incorporated to prevent the concrete from freezing.
CURING AND COLD WEATHER PROTECTION
To develop the strength, durability and permeability potential of the concrete, curing and protection during cold weather conditions are essential. The following guidelines are recommended upon placement:
* The curing period must extend a minimum of 7 days (maintaining the 55°F temperature).
* Do not seal freshly placed concrete. Sealing retains water in the concrete thereby keeping it saturated during freeze/thaw conditions.
* Cold weather protection is best provided through insulating blankets or loose straw (minimum 12 inches deep) sandwiched between a waterproof cover e.g. polyethylene.
MIX DESIGN RECOMMENDATIONS
A minimum specified compressive strength of 4000 psi at 28 days is required for exterior flatwork exposed to cyclic freezing and thawing. The slump, as placed, should not exceed 5 inches with the exception of those mixes incorporating mid or high range plasticizers. The recommended air content is 6.5 ±1.5%.
OWNER CARE - FIRST WINTER
* Deicing salts must not be applied…for traction, sand is recommended.
* Do not allow snow and ice to accumulate…this maintains the concrete in a saturated condition during freeze-thaw conditions.
This information used by permission of the Michigan Concrete Association www.miconcrete.org
Showing posts with label footing. Show all posts
Showing posts with label footing. Show all posts
Monday, February 2, 2009
Pouring Concrete In Hot Weather
SUCCESSFUL HOT WEATHER CONCRETING
Throughout summer, difficulties in mixing, placing and curing concrete often arise. High ambient/concrete temperatures, low relative humidity, and moderately high wind velocities contribute to rapid evaporation of moisture from the surface and accelerated set characteristics (hydration). By reviewing the guidelines presented in this bulletin, you will recognize and be prepared for hot weather conditions.
RECOGNIZING HOT WEATHER
The evaporation chart located below can determine when precautions need to be exercised during hot weather conditions. Any combination of air temperature, relative humidity, concrete temperature, and wind velocity that results in an evaporation rate exceeding 0.10 lbs/ft2/hr will increase the probability of plastic shrinkage crack development and related finishing concerns. Precautions to minimize the rate of surface evaporation are recommended at this point. When the evaporation rate exceeds 0.20 lbs/ft2/hr, the recommended precautions become mandatory.
PLASTIC SHRINKAGE CRACKING
Plastic shrinkage cracks are defined as a network of discontinuous cracks that appear on the surface of a freshly cast concrete slab during or shortly after finishing. The cracks typically form parallel to each other at intervals up to 3 feet apart, penetrating the surface to a depth of 1-2 inches. Plastic shrinkage cracks develop when the rate of evaporation exceeds the rate of bleedwater migration to the surface. During placement, concrete is unable to withstand the tensile force resulting from the rapid evaporation of surface moisture (and associated linear shrinkage) eventually leading to plastic shrinkage crack formation.
CURING
Immediately following placement, cure the concrete with one of the noted techniques: membrane curing compound application, continuous seven day soaking, wet burlap/polyethylene cover, curing paper.
RECOMMENDED CONSTRUCTION PRACTICES
To minimize the potential for plastic shrinkage crack development and related finishing concerns, the following recommendations are listed for planning your next hot-weather related construction project:
1. Prior to placement, dampen forms and subgrade materials.
2. Consult your ready mix supplier for hot weather mix design alternatives.
3. Schedule prompt transport, placement, and finishing of concrete. Whenever possible, schedule early morning placements.
4. Erect temporary windbreaks to reduce wind velocity over the concrete surface.
5. Erect temporary sunshades to reduce concrete surface temperatures.
6. To minimize surface evaporation, incorporate fog misting or evaporation retarders. Continue fog misting until curing is initiated. Evaporation retarders (usually polymers) can be spray applied following screeding with little or no affect on finishing or adhesion of membrane curing compounds.
7. Incorporate synthetic fibers (polypropylene) to resist shrinkage cracking.
Figure. 1 - Effect of concrete and air temperatures, relative humidity, and wind velocity on the rate of evaporation of surface moisture from concrete. This chart provides a graphic method of estimating the loss of surface moisture for various weather conditions. To use the chart follow the four steps outlined above. If the rate of evaporation approaches 0.2 lb/ft2/hr (1kg/m2/hr), precautions against plastic shrinkage cracking are necessary (Lerch 1957).
This information used by permission of the Michigan Concrete Association www.miconcrete.org
Throughout summer, difficulties in mixing, placing and curing concrete often arise. High ambient/concrete temperatures, low relative humidity, and moderately high wind velocities contribute to rapid evaporation of moisture from the surface and accelerated set characteristics (hydration). By reviewing the guidelines presented in this bulletin, you will recognize and be prepared for hot weather conditions.
RECOGNIZING HOT WEATHER
The evaporation chart located below can determine when precautions need to be exercised during hot weather conditions. Any combination of air temperature, relative humidity, concrete temperature, and wind velocity that results in an evaporation rate exceeding 0.10 lbs/ft2/hr will increase the probability of plastic shrinkage crack development and related finishing concerns. Precautions to minimize the rate of surface evaporation are recommended at this point. When the evaporation rate exceeds 0.20 lbs/ft2/hr, the recommended precautions become mandatory.
PLASTIC SHRINKAGE CRACKING
Plastic shrinkage cracks are defined as a network of discontinuous cracks that appear on the surface of a freshly cast concrete slab during or shortly after finishing. The cracks typically form parallel to each other at intervals up to 3 feet apart, penetrating the surface to a depth of 1-2 inches. Plastic shrinkage cracks develop when the rate of evaporation exceeds the rate of bleedwater migration to the surface. During placement, concrete is unable to withstand the tensile force resulting from the rapid evaporation of surface moisture (and associated linear shrinkage) eventually leading to plastic shrinkage crack formation.
CURING
Immediately following placement, cure the concrete with one of the noted techniques: membrane curing compound application, continuous seven day soaking, wet burlap/polyethylene cover, curing paper.
RECOMMENDED CONSTRUCTION PRACTICES
To minimize the potential for plastic shrinkage crack development and related finishing concerns, the following recommendations are listed for planning your next hot-weather related construction project:
1. Prior to placement, dampen forms and subgrade materials.
2. Consult your ready mix supplier for hot weather mix design alternatives.
3. Schedule prompt transport, placement, and finishing of concrete. Whenever possible, schedule early morning placements.
4. Erect temporary windbreaks to reduce wind velocity over the concrete surface.
5. Erect temporary sunshades to reduce concrete surface temperatures.
6. To minimize surface evaporation, incorporate fog misting or evaporation retarders. Continue fog misting until curing is initiated. Evaporation retarders (usually polymers) can be spray applied following screeding with little or no affect on finishing or adhesion of membrane curing compounds.
7. Incorporate synthetic fibers (polypropylene) to resist shrinkage cracking.
Figure. 1 - Effect of concrete and air temperatures, relative humidity, and wind velocity on the rate of evaporation of surface moisture from concrete. This chart provides a graphic method of estimating the loss of surface moisture for various weather conditions. To use the chart follow the four steps outlined above. If the rate of evaporation approaches 0.2 lb/ft2/hr (1kg/m2/hr), precautions against plastic shrinkage cracking are necessary (Lerch 1957).
This information used by permission of the Michigan Concrete Association www.miconcrete.org
Concrete Tools For Contractors
Bogen Concrete is the place to purchase quality concrete tools, blades for the professional contractor and home owner as well.
From edgers to bullfloats we carry a variety of tools and supplies to fit our customers needs.We are a supplier of tools for Kraft Tools and also Marshalltown. If there is a tool you need you can find it in there catalog. Give tool time dennis a call and he will place the order for you if we don't have it in stock.
From bullfloats to mag floats
chances are we have what you need in stock.
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