Q.: We have a problem concerning a sea wall which forms a cutoff to a large underground parking garage we are forming in an old dry dock. The contractor favors forming the base of the wall in two pours of about 520 cubic yards each (50 feet long, 14 feet high and 20 feet thick). We are concerned about shrinkage and thermal buildup. Could you please advise on the practicality of pouring this large a volume at a time?
A.: The answer to your question is quite involved because the heat buildup depends on the type of cement, cement content, cement fineness, maximum size of coarse aggregate, the temperature of concrete when placed and the volume-to-surface ratio of the pour. The question can really be answered adequately only by a study of ACI 207.1R-70, "Mass Concrete for Dams and Other Structures," and ACI 207.2R-73, "Effect of Restraint, Volume Changes, and Reinforcement on Cracking of Massive Concrete," with respect to the materials you will be using and the temperatures of the materials and environment.Usually mass concrete is placed in lifts ranging in height from 2 1/2 feet to 10 feet. But lifts as high as 50 feet have been placed. Whether this can be done depends on the amount of temperature rise expected. ACI 207.2R-73, Section 3.4, tells how to estimate heat rise and dissipation based on the volume-to-surface ratio. The "surface" in this case is only the surface exposed to air or forms. Other useful information in the two publications cited should help gauge whether heat buildup would be excessive in a single pour.
Things you can do to reduce the maximum temperature rise are:
- Use larger aggregate to decrease the cement content. (Six-inch-diameter cobbles are commonly used in dam work.)
- Use a pozzolan to replace a part of the cement.
- Use cool water or chipped ice in the mix, counting the ice as part of the batch water.
- Place the concrete in the evening hours when the air temperature is lower.
- Sprinkle the aggregates to cool them by evaporation before batching.
- Use a Type II cement, which has a lower heat of hydration than Type I.