Concrete is both marvelously simple and marvelously complex. Mix appropriate amounts of portland cement, aggregates, and water, and you have one of the world's most versatile and durable construction materials. On the other hand, if you vary the proportions or the character of any of the components, you may find it to be a far different material, either before it has begun to harden or as it matures. Add any of a number of admixtures or supplementary cementitious materials, and again you alter the nature of the concrete.
There has always been a need for specialists in concrete design, production, and placing who understand concrete and can therefore anticipate the effects of changing ingredients. With the complexities of modern concrete, it has become even more important to know how even subtle changes in the concrete mixture can have significant effects.
Stuck in the middle
One early explanation of modern concrete's fundamental behavior was put together in 1918 by Duff A. Abrams, the “professor in charge of laboratory” at the Structural Materials Research Laboratory in Chicago. In a presentation to the annual meeting of the Portland Cement Association in December of that year, Abrams summarized the results of nearly 50,000 tests carried out over the previous 3 years to investigate concrete proportioning.
One of his primary findings, which has come to be known as the “water-cement ratio law,” was that “for given concrete materials the strength depends on only one factor—the ratio of water to cement.” Abrams said this relationship would hold “so long as the concrete is not too dry for maximum strength and the aggregate not too coarse for a given quantity of cement; in other words so long as we have a workable mix.”
Variations that naturally occur in the aggregate being used to make concrete, such as gradation and moisture content changes, can and should be monitored. Based on that information, adjustments can be made to the other components to maintain a consistent mixture.
A second equally important conclusion was based on the gradation of the aggregate expressed as the fineness modulus, which is the cumulative area under the sieve analysis curve. Abrams observed that regardless of the aggregate size used, the lower the fineness modulus of the aggregate, the more water was required to achieve a given plastic condition. Hence, the aggregate gradation affects concrete strength by affecting the amount of water required to obtain a workable mix.
Despite the passage of time, these two principals remain unchanged. Today the concrete contractor often is stuck in the middle between an engineer who requires a resubmittal whenever the mix design is altered and a concrete producer who has to make periodic adjustments to the mix to maintain consistency.
In reality, the nature of the aggregate varies over time. Changing weather conditions introduce one variable the producer must account for when batching the concrete. Another predictable change is the variation in gradation as the batch materials are taken first from the top of the stockpile, then from the middle, and finally from the bottom. ASTM guidelines say the concrete mixture should be redesigned when the fineness modulus changes by more than 0.2. But even if the change is detected and a new mixture developed, it can take several weeks for approval of a new mix design. The contractor may then end up working with an inconsistent product because the producer's flexibility to make the necessary adjustments to the mix design is limited by a rigid submittal process.
The more concrete contractors understand the dynamics of mixture design, the better equipped they are to work with concrete producers when these variations arise. While experience is a great teacher, the learning process can be accelerated by using some of the sophisticated analytical tools now available. One example is seeMIXII, a concrete mixture design and analysis computer program offered by The Shilstone Companies.
Interpreting the data presented in reports such as these generated by seeMIXII can allow concrete contractors to be proactive customers and to recognize when specifications are suspect. The gap graded individual percent retained chart (left) shows a high percentage retained on the ½-inch sieve and a low percentage retained on the #4 and #8 sieves. On the well graded chart (right), these numbers come closer to the others in a profile sometimes called the “leaning haystack.”
The analysis feature, in particular, should help anyone trying to better understand concrete technology. Once you enter the mixture design information (the quantities of each component per cubic yard of concrete), the program generates a set of quantitative analyses. In addition to looking at fineness modulus, as Abrams did, this program calculates numerous other indicators, such as a coarseness factor—a rough indicator of whether the mixture is rocky or sandy—and a workability factor.
The results of the program's high-powered calculations are only numbers, though. It's the program user's job to learn how to evaluate that quantitative data to make qualitative evaluations. Throughout the documentation, users are also reminded to test mixes before they are used on the job. Numerical analysis can be a great tool, but it doesn't replace the need to run trial batches.