Owners want low maintenance concrete floors; to deliver that contractors want less shrinkage in order to control cracking and curling. Excessive 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.
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 need to be considered when trying to limit the amount of shrinkage in concrete slabs.
Constructing industrial flooring is the primary business of Scurto Cement Construction, Elgin, Ill. “We're very concerned about both shrinkage and curling,” says Greg Scurto, the company's president. “Curling is a greater problem for us than shrinkage. When a slab rocks under the weight of material handling equipment, it becomes a problem for us.” Scurto is always looking for better concrete mix designs for floor construction. So, when he had the opportunity to help fund Walter Flood IV's University of Colorado master's research project on changes in concrete mix designs that increase or decrease shrinkage, he accepted—provided that aggregates and materials from the Chicago area be used for the study.
Flood's research model
In his thesis “Minimizing the Shrinkage of Concrete Mixtures: a Low-Cost Approach,” Flood defines four types of shrinkage in concrete:
- Autogenous—when water-cement ratios (w/c) are less than 0.40
- Plastic shrinkage—the rapid loss of moisture from fresh concrete surfaces
- Carbonation shrinkage—the reaction between concrete and atmospheric carbon dioxide over time
- Drying shrinkage—when the relative humidity of the hardened concrete is less than 100%
Flood's study focused on drying shrinkage, specifically how concrete mix ingredients influence shrinkage over time. This includes aggregates (sizes and gradations), admixtures, cementitious materials, and water content. In all, 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 chose to look at included optimized aggregate mixes (including use of the “Fuller” maximum density curve), 3/4-inch and 1-1/2-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.
The testing procedure
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. These specimens were cast at Flood Testing Laboratories in Chicago and the cylinders and flexural beams were placed in a standard curing room 24 hours later and left there for 28 days. The length-change specimens were placed in a lime-saturated water bath.
After casting the length-change beams in Chicago, Flood submerged them in water and transported them to the University of Colorado in Boulder. He then removed them from the water to air dry. He then measured the air drying samples after 4 days and at 1, 2, 4, 6, 7, 9, 11, and 17 weeks after starting air drying, which was 27 or 28 days after casting. In the interval between testing, the samples were stored on a shelf, separated by pieces of 1x2-inch pine. Flood used a “length-comparator” instrument to measure the beams, as specified in ASTM C 157-04.
Flood followed ASTM C 157-04 “Standard Test Method for the Length Change of Hardened Hydraulic-Cement Mortar and Concrete” in the casting, curing, storing, and length-change measurement of the beams, with the exception of the humidity requirements. C 157 requires that after the initial curing period, samples be stored in humidity cabinets, maintained at 50% humidity to ensure that the moisture content of each sample is the same. Since shrinkage is related to the amount of moisture in concrete, maintaining uniform humidity keeps this variable constant, allowing tests performed at different times and locations to be compared. Although Flood knew that Col-orado's humidity, which averaged 23.9% during the measurement period, was lower than the ASTM requirement, he reasoned that the humidity of each beam would be the same, so the results for the different mixes could be compared.
Flood reached the following conclusions from this research:
- Generally speaking, the concrete mixes with the lowest total water content achieved the highest strengths.
- The mix with fly ash and the highest amount of superplasticizer achieved the highest strength. (sample 7)
- Concrete with stone aggregates performed much better than mixes with gravel aggregates.
- Ternary mixes displayed higher concrete strengths.
- Fuller-optimized aggregate concrete mixes fell in the center of the rankings for compressive strength.
- From the highest flexural strength to the lowest was only a difference of 354.5 psi.
- The concrete mix with the highest flexural strength was a Fuller-optimized ternary mix with 3/4-inch top-sized aggregate.
- The mixes with the highest shrinkage were the one with the highest dosage of superplastizer and the one that used a non-chloride accelerator (samples 7 and 8).
- For several concrete mixes, measurements taken between the 28th and 122nd day were significantly different. This suggests that specifications for final shrinkage measurements should be conducted beyond 28 days or that the specifier should be aware that different mixtures shrink at different rates.
- Mixes using Fuller's curve to increase aggregate density didn't reduce shrinkage (samples 4 vs. 12; 14 and 1 vs. 11 and 13).
- Inclusion of 2% calcium chloride had a minimal affect on shrinkage. At 17 weeks it was 0.041% versus 0.037% for the control mix (sample 4 vs. 9).
- Inclusion of a non-chloride accelerator (NCA) resulted in twice the amount of shrinkage. At 17 weeks it was 0.064% (sample 4 vs. 8).
- A mix with HRWR had the highest shrinkage, but when the amount of cementitious material and water was reduced (sample 2), the amount of shrinkage decreased significantly. This may indicate that lower cementitious mixtures may be used with HRWR to lower cost without sacrificing performance.
- Increases in aggregate sizes decreased shrinkage (sample 1 vs. 4).
Using the results
Flood's research gathered information about the concrete mix designs he developed and tested. These mixes may or may not relate to those used on a typical jobsite. Research studies are designed to hold some variables constant in order to examine the effect of changes in other variables. In Flood's study the weight of cementitious materials and the slump of the mixes were held fairly constant in order to focus on aggregate sizes and gradations, the affect of pozzolans, and the use of admixtures. But again, these mix proportions aren't necessarily those that would be used in the field. For instance, designing a well-graded or “optimized” aggregate mix with minimal space between aggregates also involves calculating the amount of portland cement needed to coat the aggregates, to bond them together. The result would be less portland cement (perhaps as much as 50 pounds per cubic yard) and therefore less water than the mixes used in Flood's research. Reduced shrinkage would likely be the result since lower cementitious paste content leads to less shrinkage.
The concept of shrinkage in concrete is well understood, but a simple and effective method for testing has not been found. This is especially the case for concrete mixes with large aggregate. Ring beam molds with steel rings cast into the concrete being tested (to provide restraint) were developed to measure both the time it takes the concrete to crack and to measure the total shrinkage represented by the cracks. But there are problems with using ring beams to measure concrete containing 1 1/2-inch aggregate. The development of electronic strain gauges offer opportunities for measuring internal restraints in concrete resulting from shrinkage forces; however, these methods are not yet accepted by ASTM C 157.
Scurto Cement Construction should be commended for their willingness to support student research that helps find answers to problems contractors face every day. Their support confirms that contractors strive to learn more about their product and make it better.