HOT WEATHER CONCRETING

“One or a combination of the following conditions that tends to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration, or otherwise causing detrimental results: high ambient temperature; high concrete temperature; low relative humidity; and high wind speed.”

REFERENCE MATERIALS

PORTLAND CEMENT

• Design and Control of Concrete Mixtures (15th Edition) by Steven H. Kosmatka and Michelle L. Wilson Chapter 16 – Hot Weather Concreting (pg. 315-326)

AMERICAN CONCRETE INSTITUTE

• ACI 305R-10 Guide to Hot Weather Concreting Reported by ACI Committee 305  
• ACI 305.1-14 Specification for Hot Weather Concreting Reported by ACI Committee 305

AMERICAN CONCRETE INSTITUTE

• Manual of Concrete Practice – by American Concrete Institute This manual can be purchased in hard copy from the ACI or can be accessed on line through the ACI website. (www.concrete.org)

THE CONTRACTOR'S GUIDE TO QUALITY CONCRETE CONSTRUCTION

• American Concrete Institute and American Society of Concrete Contractors by Ward Malisch, Lindsay Kennedy, Rich Heitzmann, Becky Hartford with many contributions from other members of the ASCC. Chapter 3: The Concrete Specification Pg 31 Chapter 10: Concrete Placement and Finishing pg. 127 Chapter 11: Common Field Problems – Cause and Prevention pg133-146

HOLCIM WEATHER APP

• Holcim WeatherAPP – Apple App. Store, Android App. Store

HOT WEATHER CONCRETING

THE PROBLEMS WE FACE

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• Increased Rate of Cement Hydration
• Increased Rate of Moisture Evaporation

FACTORS THAT AFFECT THE RATE OF CEMENT HYDRATION

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• Ambient air temperature
• Concrete Material Temperature
• Fineness and quantity of cement
• Supplementary cementitious materials
• Admixtures

FACTORS THAT INCREASE THE RATE OF MOISTURE EVAPORATION

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• Wind
• Solar Radiation
• Concrete Temperature
• Air Temperature
• Relative Humidity

Increased rate of cement hydration and increased rate of moisture evaporation result in:

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• More water required to make the concrete workable (decreased strength) Our slump diminishes more quickly (requires more water, yields decreased strength) Concrete is faster setting, harder to finish well. Consistent broom finish is most difficult to achieve

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• More likely to have plastic shrinkage cracking and differential thermal cracking Potential for decreased durability due to cracking
• Variations in color and finished appearance due to changing water/cement ratios between trucks and cold joints in slabs, walls, etc.

WHAT CAN WE DO?

You’re going into battle…plan well and have a strategy to overcome the difficulties you are about to face

Strategies

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• Adjust the size of your pour. Don’t bite off more than your crew can chew.
• Adjust the size of your crew. If you have less time to do the job, it will take more people to get the job done quickly.

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• Increase or decrease your delivery rate. The concrete needs to be placed and struck off while the slump is consistent from truck to truck. You will have more success consolidating the concrete and finishing the surface consistently
• Make sure your method of conveyance will satisfy your need to place the material quickly
• Space for two trucks to back up to the pump is critical

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• Place wind breaks to prevent air movement across the surface of your slab. Otherwise the surface may lose moisture and set before the mid-depth of your slab will support finishing equipment and foot traffic.

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• Pour at night when possible to reduce the solar radiation effect of direct sunlight. Difficulties result from differences in sunlight exposure. Partially shaded areas wreak havoc on your ability to produce a consistent finish.

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• Design your slab placement sequence to accommodate exposure to direct sunlight. We prefer to finish in the shade versus starting the pour in the shade

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• Work with your ready-mixed supplier to purchase a mix design that has a proven track record of success in similar conditions. It should be easy to place and should not have a tendency to flash set. High early strength mixes that are often used in post-tensioned slab pours are particularly difficult.

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• Plan to use an evaporation retarder to slow the loss of moisture at the surface.
• You will need to have a plan to reach all areas of your freshly placed and screeded concrete with a sprayer to be effective with an evaporation retarder.

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• Spraying only convenient, easy to reach areas will not hep you. Today's typical large slab pour makes it difficult to reach all areas.
• Spraying a leading edge is effective when your concrete placement is delayed

REDUCING THE TEMPERATURE OF THE DELIVERED CONCRETE WILL HELP

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• Cooling the aggregates
• Keep the concrete drums out of direct sunlight
• Using chilled water or ice

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• Using a hydration stabilizer or retarding admixture can be helpful
• However, this can cause problems in slab pour by retarding the center of the slab cross section while the surface hardens prematurely.

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• The surface is exposed to wind and sunlight which can cause crusting at the surface while the middle of your slab is still in the plastic state

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• If possible increase the humidity above your slab surface. Misting water above your slab surface will increase the humidity that your slab is exposed to thus decreasing the rate of evaporation at the surface.

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• Saw cut your slab as soon as you can without damaging the slab with the saw.
• Saw your slab in strategic fashion. The need for relief in your slab varies. Some areas are more urgent than others. For example: inverted corners create trouble spots.

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• Apply curing compound as soon as you can without damaging the concrete surface with foot traffic, equipment, etc.

HOW DO I KNOW WHEN WE HAVE A PROBLEM AHEAD?

Conditions

• When the rate of evaporation exceeds the rate at which bleed water reaches the surface of your slab, the likelihood of plastic shrinkage cracking is very high.

WHAT IS THE RATE?

Bleed water varies for every mix design. A rule of thumb is 0.2 lb/sf/hr

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• Check your probability for plastic shrinkage cracking.
• The Holcim Weather App
• Check your forecast and calculate the anticipated rate of evaporation

Figure 4.2 in ACI 305R-10

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To use Figure 4.2 you will need:

• Wind Meter (anemometer)- measure at 20" above slab surface
• Concrete Thermometer
• Humidity Monitor - Measure 4'- 6' above concrete surface
• Thermometer for ambient air temperature- Measure 4'-6' above slab
• Check temperature of concrete being produced in the hours leading up to the pour to help know what to expect

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Summary

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• Increased Rate of Cement Hydration
• Increased Rate of Moisture Evaporation

AMERICAN CONCRETE INSTITUTE

• ACI 305R-10 Guide to Hot Weather Concreting Reported by ACI Committee 305  
• ACI 305.1-14 Specification for Hot Weather Concreting Reported by ACI Committee 305