It's always interesting to know what the changes are in the admixture industry because they have so much impact on the properties of concrete as well as how it places and finishes. Admixtures also have a profound impact on how contractors do their work and how their bottom line is affected.
In the past few years, many new admixtures have been introduced into the marketplace. The most important new admixture has been the polycarboxylate-based high-range water reducer or superplasticizer—a large improvement over naphthalene and melamine-based superplasticizers. Also recently introduced is a new class of admixture referred to as “rheology controlling admixtures” (RCA).
The introduction of SCC in the United States is approximately a decade old. As a type of concrete, it has shown both promise and problems in terms of producing a reliable, consistent product. But recent developments have reduced the frustrating problems, making SCC more contractor friendly.
Here are the ways that recent developments have improved the product that contractors install.
User-friendly polycarboxylates
The nice thing about polycarboxylates and the change that they introduced to the way we develop admixtures is that scientists first decided what properties they would like a superplasticizer to have and then engineered molecules to achieve that result. Previous to this moment in time, scientists experimented with materials to see how they might perform. But over time, it's been learned that many things influence the way polycarboxylates affect concrete behavior and admixture companies have been working to solve the problems. There have been many announced changes as a result.
What the changes are really all about is the recognition that conditions regarding concrete and cement differ by region of the country, so polycarboxylate-based admixtures have to be tailored to provide consistent performance for each region. Developments include the following:
- Longer pot life—concrete that stays fluid or retains slump longer
- Earlier strength development
- Product that doesn't cause additional air entrainment
- Admixture that performs the same between regions of the country
All manufacturers of polycarboxylate-based superplasticizers face the same problem. These admixtures by nature can produce undesired amounts of air entrainment—air that doesn't serve the need for freeze/thaw resistance. The trick is to limit their ability to produce air while allowing air-entraining admixtures to develop the proper air-void structure. Products on the market now will do that.
Rheology controlling admixtures
RCAs are new admixtures, designed for low-slump concrete mixtures in the range of 1 to 3 inches, such as concrete for porous paving or concrete that is slip formed. This concrete typically won't move down the chute of a ready-mix truck easily on its own. By adding RCAs, concrete will move down a chute and make placement much easier—behaving like 4-inch slump concrete. But it also prevents concrete from slumping over after placement, making it ideal for slip forming and extruded concrete operations.
Adding RCA to a concrete mix with no special requirements adds only about 2% to the cost of the concrete. They have no effect on the structural properties of the concrete. The benefit includes better consolidation around steel reinforcing, a better finished appearance, and no handwork.
The changing world of SCC
When SCC was first introduced, it was thought that it could move on its own as much as 120 feet from the point of placement. Practical experience, however, points more in the direction of 60 to 80 feet from the point of placement. This flowability is measured by filling an inverted (or upright) slump cone with SCC and lifting it so that the concrete flows onto a flat surface. The diameter of the puddle is measured as the “spread” of a mix. The range that defines SCC mixes is considered now to begin at a spread of 18 inches, extending to a spread of 32 inches, beyond which segregation tends to become significant.
The challenge for designing SCC mixtures is to make them very flow-able without causing aggregate segregation, says Surendra Shah, director of the Center for Advanced Cement-Based Materials at Northwestern University, Evanston, Ill. “But developing a test to measure segregation for field application isn't easy. Tests must be science based yet be easy to measure in the field,” he says. Tests for segregation that can meet these criteria are still under development.
