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.
Editor’s Note: These topics will be discussed at the SCC: Challenges and Barriers to Applications, The Third North American Conference on the Design and Use of Self Consolidating Concrete, to be held at Marriott-O’Hare on Nov. 10–12, 2008. The conference will discuss topics from mix design to field experience. The conference is organized by ACBM and sponsored by a large number of companies and professional organizations including Hanley Wood.
