Admixtures can work to alter the rate of heat generation but at the same time can result in lower concrete strengths. In mixes where low water/cement ratios are required for durability (such as bridges, parking structures or marine facilities), concrete is often substantially over-designed for strength. In cases like this, heat reducing admixtures can reduce the likelihood of cracking.
What Causes Plastic Shrinkage Cracking?
Plastic shrinkage occurs as fresh concrete loses its moisture after placement but before any strength development has occurred.
This type of shrinkage is affected by environmental effects of temperature (concrete and ambient), wind and relative humidity. It is a particular problem in hot weather concreting.
What is Autogenous Shrinkage?
This type of shrinkage is difficult to measure or separate from drying shrinkage which is a much greater concern.
Autogenous shrinkage takes place as the portland cement hydrates and without loss of water from the mix as opposed to the drying shrinkage process. Admixtures may reduce this type of shrinkage but are not specifically designed to do so.
What to Ask When Considering Using Shrinkage Reducing Agents?
Shrinkage reducers control drying shrinkage and minimize cracking. Terry Holland in his recent article in Concrete Construction, “Using shrinkage-reducing admixtures” suggests asking manufacturers the following:
- What are the effects on fresh concrete?
- What are the effects on hardened concrete?
- What is the effect on durability?
- Is there any information on how have similar structures behaved in service?
- Is the same cement going to be used as in the test specimens or comparison project?
- What is the impact on cost?
Before using a shrinkage reducing agent, ask the manufacturer how it will affect your project and materials.
Are There Cost Tradeoffs When Using Shrinkage Reducers?
The cost of adding shrinkage reducers to concrete can be compared to the advantages of not having to either repair or retest.
This can be particularly important if the project is construction of a concrete water tank or reservoir. Passing the required hydrostatic test the first time more than offsets the cost of using the admixture.
Other examples of cost savings could relate to expansion joints, number of prestressing strands required, or reduction in curling of floor slabs.
In addition, in projects where a cheaper high shrinkage aggregate cost must be compared to a more expensive better aggregate, the cost of improving the concrete by using shrinkage reducing admixtures should be included in the cost comparison.
Do Admixture Effects Integral Color?
While all other admixtures have been tested and found to have no long-term detrimental effects on colored concrete, they may shift the color lighter or darker. This makes it that much more important to maintain batch-to-batch consistency.
Never switch admixtures or stop using them in the middle of a colored concrete project! The accompanying chart shows the effects of common admixtures on colored heated concrete floors