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Concrete Admixtures and SCC

Source: | Updated: Jan 29, 2016

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.

From the beginning there have been two primary ways to develop mixes for SCC. One method is to create well-graded aggregate concrete mixes. They produce the least amount of segregation and usually don't require the addition of more costly viscosity modifying admixtures (VMAs). The more common approach is to use standard gap-graded mixes with two gradations of large aggregate along with a VMA to control segregation. Gap-graded mixes are the more expensive option but are used regularly because many ready-mix producers don't have bins to hold the additional aggregates required for well-graded mixes. Richard Szecsy, vice president of new product development for Lattimore Materials, McKinney, Texas, says that his company became interested in SCC when it was first announced and their trend now is to produce SCC mixes without VMAs—90% don't include them now.

Designing SCC mixes is much more related to a specific job application now says Szecsy. “In order to keep costs reasonable, we market SCC by the amount of flow (compared to what a mix is capable of) needed for a job. If a contractor only needs to move concrete 20 feet or place concrete in a congested steel reinforcement area, we proportion the mix for that application.” When they proportion a mix, the following things are taken into consideration: the aggregate sizes needed, the amount of reinforcement in a placement, the distance it must flow in the forms, whether the mix will be pumped and at what rate, and the distance to the jobsite from the batch plant.

Sometimes there is the perception that SCC mixes shrink more than other conventional-slump mixes. But Szecsy says this isn't a true statement. Any concrete mix with high loadings of cement shrinks more than those with less cement content, whether or not they are SCC mixes. This partly relates to the top-size aggregates in a mix design, as well as the grading of the aggregates, according to Shah. When SCC is used to achieve good consolidation in placements with congested steel reinforcement, the top-sized aggregate is often 3/8- or ½-inch stone in order to move the concrete between reinforcement. Smaller stone mixes often require higher amounts of cement in the mix, so increased shrinkage can become more of a factor. But Szecsy adds that his company often proportions mixes that include 1-inch, 1½-inch, and even 3-inch top-sized aggregates, with less cement and less shrinkage being the result.

A problem in the past with SCC has been consistency between batches and between different regions of the country. But Szecsy says these issues no longer pose many problems in his area. He credits the ongoing development of polycarboxylate-based superplasticizers.

What it all means

Without doubt the invention of polycarboxylate-based superplasticizers is one of the most important admixture discoveries in recent years. They offered enormous advantages when compared to existing products, but they can come with extra baggage too, as discussed earlier. Fortunately, during the time they've been marketed they have undergone continuous refinement because they are based on engineered molecules. The refinements include extending working time and more consistent results wherever they are used. They are the admixtures that makes SCC possible.

RCAs are just being marketed now so it's hard to tell what their impact will be or where they will find their best use. There are benefits for a very low slump concrete that consolidates well around reinforcement and provides better finishes with minimal labor.

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