Q: We are beginning to work on a new set of mix designs for a large commercial project. We are experimenting with some potential changes in the aggregate gradations of our commonly used products.

Since our aggregate supplier is willing to work with us and develop a special concrete aggregate, what parameters should we use? Should we have the supplier create a well-graded aggregate to help increase workability and minimize cement usage? Is there any current research on this subject?

A. The best place to start is the guidelines in the American Concrete Institute's (ACI) document, “Standard Practices for Selecting Proportions for Normal, Heavyweight, and Mass Concrete,” (ACI 211.1). The committee recommends the nominal maximum size of aggregate should be the largest that is economically available and consistent with the structure's dimensions. Large, nominal, maximum sizes of well-graded aggregates have fewer voids than smaller sizes.

One common approach to mix designs is to optimize the packing density of the coarse and fine aggregates. A higher aggregate packing density requires less cementitious paste to fill the voids. Less cement often leads to less shrinkage, improved durability, less heat of hydration, and importantly, lower total mix costs.

Another source is ACI's “Guide to Concrete Floor and Slab Construction,” (ACI 302) which contains several popular approaches producers can use to establish aggregate proportions to attain a maximum packing density for combined aggregates. To help simplify the design, each approach has a chart. These are known as the Coarseness Factor Chart, the 8-18 Chart, and the 0.45 Power Chart.

In recent years, many specifiers have started to look at these charts as absolutes instead of guidelines. In some circumstances, producers who had mixes that had performed very satisfactorily, but whose gradations didn't meet the criteria of a well-graded aggregate as described by one or more of these guides, were forced to adjust their designs. Producers often thought this was an unnecessary expense.

To answer this concern, researchers at the National Ready Mixed Concrete Association (NRMCA) studied the topic. Their research focused on two questions. First, do the empirical approaches represented by the tables in ACI 301 actually result in a maximum aggregate packing density? And secondly, do these lead to improved concrete performance.

Kartha Obla, NRMCA's managing director of research and material engineering, was the primary researcher. Haegin Kim and Colin Lobo, both with the NRMCA, assisted. After considerable effort, the NRMCA has recently released two detailed reports on aggregate grading.

Well-graded combined aggregates

Their work yielded some interesting results. Most importantly, their aggregate packing tests concluded that using well-graded combined aggregates did not necessarily lead to maximum aggregate packing density. And just as important, their concrete performance studies showed that using well-graded combined aggregates did not necessarily lead to lower water demand, lower shrinkage, lower bleeding, or higher strength as is normally anticipated.

The researchers suggest that differences in an aggregate's texture and shape can influence differences in packing density. These same factors also suggest why they observed improvements in finishability and segregation resistance in certain test conditions.

The researchers concluded that Coarseness Factor and 8-18 Individual Percent Retained charts are potential concrete mixture optimization tools. They suggest limiting the ACI tables to evaluation methods by concrete producers.

The researchers strongly suggest that the guidelines found in ACI 301 not be invoked as requirements in project specifications, as they do not assure intended performance.

You can find the complete reports, “Effect of Continuous (Well-Graded) Combined Aggregate Grading on Concrete Performance Phase A: Aggregate Voids Content (Packing Density) and Phase B: Concrete Performance,” at www.nrmca.org/research/eng_articles.asp.