CONCRETE knowledge from a CONCRETE source!

The National Ready Mixed Concrete Association (NRMCA) defines hot weather concreting as anytime during high temperatures in which special precautions need to be taken to ensure proper handling, placing, finishing and curing concrete. During the summer months you will most generally experience problems that require precautions being taken for hot weather concreting, but the associated climatic factors of high winds and dry air can occur at any time. These conditions often produce a rapid rate of evaporation of moisture from the surface of the concrete and an accelerated set time among other problems.

Hot weather is a major factor to consider when planning a concrete project. Too often it is assumed that not much can be done to combat hot and dry conditions. Many times the addition of large amounts of water is used to fight off rapid set times or to ease the placing and finishing process. These assumptions and practices often times prove to be detrimental to the concrete.

Problems that can occur due to high temperature, high wind and low humidity are:

• Increase slump loss, making concrete more difficult to place and finish.

• Accelerated set time, decreasing the amount of finish time.

• Increasing the potential for plastic and drying shrinkage cracking.

• Decreased air content.

• Reduced strength if water is added to offset slump loss.

• Thermal cracking may result from a rapid drop in concrete temperature when placed on a hot day followed by a cool night.

Here are some useful tips for successful Hot Weather Concreting:

1. Use proven recommendations for adjusting concrete proportions, such as water reducing agents. Set retarding admixtures, and Flyash.

2. Advanced timing and scheduling, to avoid delays between trucks. Trucks should be able to discharge immediately and adequate personnel should be available to place and finiish the concrete.

3. When possible, deliveries should be avoided during the hottest part of the day.

4. Limit the addition of water at the job site. You should add no more than 2 to 3 gallons of water per cubic yard.

5. Slabs on grade should not be placed on polyethylene sheeting. If a vapor barrier is a must then a damp bed of compactible granular fill material should be placed over the grade.

6. Wind breaks and shade should be used if at all possible.

7. Finish as soon as the sheen has left the surface of the concrete.

Some Tips for Curing Hot Weather Concrete:

The curing process should start as soon as finishing is complete. A temporary moisture retaining mononuclear film can be placed over the surface. Cover the slab surface with white plastic sheeting, damp burlap or possibly straw; be sure to keep the straw and/or burlap damp. NRMCA suggests a curing period of 4 to 9 days depending on the ambient temperature.