(by Michael Sheehan)
Hot Weather concrete can be defined as any combination of the following conditions:
- High ambient temperature
- High concrete temperature
- Low humidity
- High wind speed
- Solar radiation
Typically, when the ambient temperature exceeds 80 degrees and the travel time is 35 mins or greater, concrete retarding add-mixtures should be used. Dosage is based on several factors, but a good ready mix company will have a QC Manager consult with the Dispatchers on a daily basis to determine specific needs.
In extreme conditions, refrigerated chilled water or ice are used to lower the concrete temperature to control set time. Using this technique requires special equipment and planning, which is why chemical retarders are the most common approach here in New England.
What happens when we don’t address hot weather conditions?
- Increased water demand equals low PSI strengths
- Maximum chance of shrinkage cracking and cracking in general.
- A temptation to spray the surface of concrete with water, which will later cause pop outs.
Here are just a few guidelines for Hot Weather Concrete:
- Modify concrete mixes with retarders and pozzolanic cements, such as slag which will reduce concrete temps.
- Use fiber mesh to control shrinkage cracking
- Have adequate, skilled manpower to place the concrete.
- Use vapor retarders for slabs.
- Monitor additional onsite water. Follow the 90 minute rule. Once the truck mixes, be prepared to discharge immediately.
- Moisten subgrade. Begin final finishing and curing as soon as possible.
- Protect YOUR cylinders in a curing box!!!
The information used was provided by the NRMCA CIP 12 Document and Mick Albro, QC Technical Manager for Sterling Concrete. Mick has worked on several large construction projects in Massachusetts, including the BIG Dig. Refer to www.nrmca.org and www.sterlingconcrete.net
