Question: I'm working on creating a mix design submittal based upon a specification that requires closed-gap aggregate gradation. To meet the specification, I need to find, or create, an evenly graded 1 +-inch top size aggregate. This means that the commonly used middle testing sieves, such as the No. 8, No. 4, and No. 16, will retain 8% - 18% of the total material.

Lately I have come across more concrete mix specifications that include requirements for a closed-gap gradation. My problem is that we could be dealing with a number of aggregate sources. So I have to recalculate a mix design using several different material gradations. This is a time-consuming process.

Do you know of a computer program that allows the user to calculate the materials' gradation needs automatically? It would be even better if the program has a database that includes the gradation of all available aggregate sources. The user then could enter a specification's target gradation limits, and the program would select the aggregate or combination of aggregates that best fit the specification - and it would select from the fewest possible aggregate sources.

Answer: Just after this question was posted, a number of forum members quickly responded. Jay Shilstone, a concrete consultant at the Shilstone Companies, Dallas, offers a few words of warning to those trying to attain the 8%-18% retained material spec. Just meeting the requirements of a uniform combined aggregate grading doesn't always result in the best aggregate blend, he notes.

To avoid placing problems, Shilstone recommends that the mix design technician also calculate the starting blendÆs coarseness factor and workability factor. He advises clients to target a coarseness factor of 50-65 and a workability factor about five to seven points above the optimum blend.

Shilstone has a program that was displayed at World of Concrete last month. With Shilstone's, see MAT-A program, the user can enter a gradation spec for up to 12 sieves and a gradation of up to 12 aggregates. The program calculates the best blend of those aggregates to meet the spec in less than one minute. The user then can lock out aggregates that only need a few percent and recalculate the blend using the reduced set of aggregates.

For more information about the software, contact Shilstone at 214-361-9681, or you can download a demo disk that is available at www.shilstone.com.

Russell E. Neudeck, from the Haskell Company, a leading design-build firm in Jacksonville, Fla., responded by saying that he offers a similar program. It's designed to allow the user to enter the available aggregates and modify the proportions to obtain the desired results. The program allows for up to five aggregates: two fine and three coarse. For more information, call Neudeck at 904-791-4563 or by e-mail: reneudec@thehaskelco.com.