Common Sense, Communication and Care

There are always brief stretches of weather during any season when it is not feasible or advisable to place concrete. Fortunately in Wisconsin, these usually don't last too long. A little bit of common sense, communication between all the parties involved, talent, and care will produce a quality concrete product.

Adverse conditions do not have to mean the end of a building project. However, it does force the builders, contractors, and architects to make additional decisions based upon the working environment. One area frequently overlooked, or misunderstood, is the relationship between temperature and concrete.

Difficult weather does not have to mean the end of placing concrete. Wisconsin, with its warm and gentle summers and mild autumns, often provide the perfect weather for placing concrete. Temperatures frequently range between 50º F. and 70ºF. and do not create problems for the concrete.

However, high heat, driving winds, or marginal temperatures provide trying conditions for the placing of concrete. Depending on the weather, preparation, and finishing, temperature may effect the strength, appearance, and price of concrete -- factors important to the decision making process. Yet, with a basic understanding of sound concrete practices, concrete may be place successfully in less than ideal situations.

Placing Concrete During Cold Weather

If the temperature of fresh concrete is 55ºF., or greater -- and if the concrete is maintained at a temperature of 55ºF. or higher -- then winter concreting should be trouble-free. Approximate set time for concrete at 70ºF. is six hours. Set time jumps to just over 14 hours if the concrete temperature drops to 40ºF. If it drops below this point and the concrete actually freezes early in the process, loss of strength, up to 50 percent, increased permeability and a lower resistance to weather may result.

The key is to start with warm concrete and keep it warm. The internal heat of the concrete mix may be raised by heating the materials, using extra or special cements, or by the addition of accelerators. The environment may be also altered by using enclosures and moist heat, applying insulating blankets, polystyrene sheets, or hay, and leaving the forms in place.

Basic Guides for Winter Concreting

1. Plan in advance. Have equipment and materials ready before cold weather hits. Be set with heaters, insulating materials and enclosures.
2. Use air-entrained concrete.
3. Don't place concrete on a frozen subbase. Be sure that all ice, snow and frost are removed from surfaces the concrete will touch.
4. For durability, the fresh concrete should be kept at 55ºF. or higher for thin sections. Consider using high-early strength concrete.
5. Cure concrete to prevent loss of moisture. When heated enclosures are used, provide extra moisture by sprinkling or use steam for heating. Vent salamanders and other fuel-burning heaters. Concrete should be allowed to cool slowly to prevent thermal cracking.
6. Do not use "antifreeze" compounds in an attempt to lower the freezing point of concrete.
7. Leave the forms in place as long as the job schedules permit. Reshoring is necessary until concrete reaches required design strength.
8. Keep job condition records. Record, at least twice daily: weather conditions, temperatures of the air and the concrete surface.
9. If the concrete is to cure below 60ºF., water reducers or retarders may prolong the set.
10. The use of calcium chloride or admixtures containing soluble chlorides is not recommended under certain conditions:

• In concrete containing aluminum or prestressing strand because of corrosion.
• Where discoloration of troweled surfaces cannot be tolerated.
• Where galvanized steel will remain in permanent contact with the concrete.
• In concrete subjected to alkali-aggregate reaction or exposed to soils or water containing sulfates.

11. Be especially careful in protecting cylinders for strength tests.

12. Concrete placed in late fall or winter should not be exposed to salts applied as deicers or salts which drip from parked vehicles.

For further information reference PCA's Design and Control of Concrete Mixtures, Chapter 12.

Placing Concrete in Hot Weather

Caution needs to be applied when placing concrete in hot weather. Without the proper care, concrete may have reduced strength and will be very prone to cracking due to rapid drying. It also may stiffen quickly making finishing quite difficult.

At some point, usually between 75ºF. and 100ºF., hot weather problems for concrete may begin. The combination usually causing the most problems is low relative humidity and high wind velocity. These conditions, when added to sun and high temperature, create a very high potential for problems.

There are several methods of cooling concrete. The most efficient way is to cool the aggregates, which may be done as simply as sprinkling them with water and allowing the evaporation process to cool them. Other methods of cooling the concrete include using ice or injecting liquid nitrogen into the mixer. However, both methods add cost to the concrete. The contractor should also be prepared with sunshades, windbreaks and other means to prevent rapid drying.

Basic Guides for Hot Weather Concreting

1. Plan in advance. Have equipment and materials ready before the hot weather arrives.
2. Keep the subgrade and forms moist so they will not absorb water from the mix.
3. Keep sunshades and windbreaks available and use them whenever possible.
4. Have everything prepared before the ready mix truck arrives. Don't make the truck wait for you.
5. Keep in constant communication with the ready mixed concrete provider. Coordination between contractor and producer is key.
6. Concrete should be placed, struck off and darbyed immediately.
7. Use evaporation retardants, fogging or misting with water, or cover with a vapor-proof sheet after screeding. This will help prevent rapid drying, crusting, plastic shrinkage and rubber sets.
8. Temporary covers, like continuously moistened burlap, may be placed over the fresh concrete and removed in small sections immediately ahead of the finishers.
9. Substituting a burlap drag or broom finish will eliminate other high risk finishing practices, such as smooth trowel finish.
10. Curing should take place when the surfaces are hard enough to resist marring.
11. Seal with a good, quality sealer a minimum of 30 days after placing and curing.
12. Remember to protect the crew in high temperatures. Drink plenty of fluids and be careful with long exposure to the sun.

For further information reference PCA's Design and Control of Concrete Mixtures, Chapter 11.

Common Sense, Communication, and Care

There are always brief stretches of weather during any season when it is not feasible or advisable to place concrete. Fortunately in Wisconsin, these usually don't last too long. A little bit of common sense, communication between all the parties involved, talent and care will produce a quality concrete product .