Shrinkage of mixes compared at 17 weeks.
Comparison of mixtures with water reducers.

Q. We are planning to bid on some large industrial floor projects this fall. The owners want concrete floors that are low-maintenance, meaning that our contractor-customer wants a mix that minimizes shrinkage to control cracking and curling. The owners claim that excessive concrete shrinkage and curling typically leads to the need for maintenance and repair of joints and cracks after floors have been in service for a while.

How can a producer help minimize shrinkage and curling on floors?

A. Many factors influence shrinkage and curling. Concrete mix design; sub-grade moisture conditions; the location of vapor barriers (directly beneath the concrete or under a layer of compactable fill); weather conditions before, during, and after placement; slab thickness; reinforcement; and joint load-transfer devices are just a few of the variables that the design engineer, producer, and contractor must consider when trying to limit the amount of shrinkage in concrete slabs.

Fortunately, Greg Scurto, president of Scurto Cement Construction of Elgin, Ill., has a strong commitment to quality floors. Since constructing industrial flooring is the firm's primary business, Scurto always looks for better concrete mix designs. “We're very concerned about both shrinkage and curling,” says Scurto. “Curling is a greater problem for us than shrinkage.”

Scurto's concern for quality turned to action when he opted to help fund Walter Flood IV's University of Colorado master's research project. Flood decided to study how changes in concrete mix designs increased or decreased shrinkage.

In his recently published thesis, “Minimizing the Shrinkage of Concrete Mixtures: a Low-Cost Approach,” Flood focused on drying shrinkage. He specifically tried to develop an understanding of how concrete mix ingredients influence shrinkage over time.

Flood developed and tested 14 different mix designs over 17 weeks. To control variables, he chose to keep slump and total cementitious weights constant, with the exception of one mix.

The variables he studied included optimized aggregate mixes (including using the “Fuller” maximum density curve), ¾-inch and 1-½-inch top-sized aggregate mixes, ternary mixes (portland cement, fly-ash, and slag), the affect of mid-range (MRWR) and high-range (HRWR) water reducers, and calcium chloride and non-chloride accelerators.

28-day curing

Flood cast three 4x4x11.5-inch length-change beams, two 4x8-inch cylinders, and one 6x6x36-inch flexural beam for each of the 14 test mixes. Cast at Flood Testing Laboratories in Chicago, the cylinders and flexural beams were placed in a standard curing room 24 hours later and left there for 28 days.

After casting the length-change beams, Flood submerged them in a limewater solution and transported them to the University of Colorado in Boulder. He then removed them from the water to air dry. He measured the air drying samples after four days and at one, two, four, six, seven, nine, 11, and 17 weeks after starting air drying, which was 27 or 28 days after casting.