Shrinkage of mixes compared at 17 weeks.
Comparison of mixtures with water reducers.
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 special humidity cabinets maintained at 50% humidity.
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 Colorado'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.
The research yielded the following results:
Applying results to the job site
- The two mixes with the highest shrinkage were the one with the highest dosage of superplastizer and the mixture that used a non-chloride accelerator.
- For several mixes, length-change 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.
- Including 2% calcium chloride had a minimal affect on shrinkage. At 17 weeks it was 0.041% versus 0.037% for the control mix.
- Including a non-chloride accelerator (NCA) resulted in twice the amount of shrinkage. At 17 weeks it was 0.064%.
- A mix with HRWR had the highest shrinkage, but when the amount of cementitious material and water was reduced, 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.
It's important to remember that these specific mixes may or may not relate to those used on a typical job-site. Research studies are designed to hold some variables constant in order to examine the effect of changes in other variables.
For example, 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 effect 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 mi-imal 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.
Scurto Cement Construction should be commended for their willingness to support student research that helps find answers to problems concrete producers and contractors face every day on the job.