Q: We need to guarantee that the temperature of the concrete we deliver to a job next month will be less than 75° F. We have urged the contractor to consider early morning pours to help avoid high summer temperatures.
Even so, we need to plan for those pours that must occur in the middle of the day. We are considering using ice to control the temperature. Can you suggest any guidelines?
A: Your best source of information is ACI 305, Hot Weather Concreting. Included in the committee's report covering all aspects of pouring, placing, and protecting fresh concrete in hot and dry conditions, is a section on using ice. The committee suggests substituting ice for batch water can be an effective way to reduce a mix's temperature. But the committee warns that even under the best conditions, the maximum temperature reduction from ice substitution for most concrete mixes is about 20° F.
The committee recommends that the ice be weighed to ensure accurate proportioning. Most batching systems allow for the plant operator to set up a recipe using both ice and water.
The decision to switch from cooled water or chillers to ice can be expensive. In the past, many producers would buy block ice and then rent equipment to either chip or crush the blocks. Today, most producers opt to secure dedicated ice plants. Plant operators can plumb these self-contained units into existing waterlines, hook up operating power, and then make ice.
In its document, the ACI 305 committee offers the following tips on using ice:
- Shave, crush, or chip the ice, and place it into the mixer as part of the batch water.
- Do not allow ice to melt before batching. But it should be melted completely by the time the mixing process is completed.
- Only 75% of the batch water requirements should be comprised of ice.
- Aggregates should be well-drained of free moisture.
To learn the latest on concrete heat monitoring, you'd be interested in the information presented at the technical session on Heat Development: Monitoring, Prediction, and Management at the ACI meeting in Atlanta in April.
Editors Kejin Wang and Anton Schindler have gathered the session's nine papers on a CD entitled Concrete Heat Development: Monitoring, Prediction & Management. Topics include innovative technology for concrete heat monitoring, using heat measurements to characterize concrete mixtures, evaluating mixture performance, detecting potential incompatibilities of concrete materials, heat management in precast concrete, and modeling and predicting in-place concrete temperatures. You can order the CD from ACI for $57.50 at www.concrete.org.