Q: I'm fairly new in the concrete business and I know that a mix's water-cement ratio affects how well the concrete turns out, but as a contractor why should I care about something called “aggregate moisture?”

A: This is an important question because admixture dosage, workability, strength, air, and durability all depend on the mix having the right water content, which also requires adjusting batch water for aggregate moisture. Moisture adjustments are becoming important with the growing use of workability-controlling admixtures and the increased use of specialty concrete such as self-consolidating and pervious concrete. But first, some background.

When aggregates are “wet,” it means that water is on the surface of the particles, and this “aggregate-free water” becomes part of the total batch water as soon as the aggregate goes into the mixer. For a “dry” aggregate, some of the batch water will be absorbed into the pores of the aggregate during mixing and transport. The concrete producer will add extra batch water to account for dry aggregates, and hold back on batch water to compensate for wet aggregates. The amount of the adjustment depends on aggregate porosity, moisture condition, and batch weights, but it is not unusual for the amount of adjustment to be in the range of 1 to 3 gallons of water per yard, or 10 to 30 gallons in a 10-yard load.

That adjustment can increase or decrease slump by as much as 1 to 3 inches. If batch water is not held back for wet aggregates, the mix can arrive onsite at a slump 3 inches higher than expected, and the reverse can happen if water is not added to compensate for dry aggregates. Even though a mix that is too dry can be made workable with normal, midrange, or high-range water reducers, the required dosage is highly sensitive to water content. When superplasticizers are used, it is common to batch the mix for a predetermined slump based on water alone (“water-slump”). Corrections for aggregate moisture are necessary or the preplasticized slump can be way off target, leading to a mix that is far too fluid or so dry that a very high dose of superplasticizer is needed.

In addition to influencing concrete's workability, moisture adjustments also affect concrete strength. For a six-sack mix (6 sacks x 94 pounds of cement per sack, per cubic yard) an additional gallon of water can reduce the 28-day cylinder strength by about 130 psi. It does not matter if the water was added intentionally or as a result of an uncompensated increase in aggregate moisture content. If a typical water adjustment is in the range of 1 to 3 gallons per yard, that alone can account for a 100 to 400 psi strength variation.

So, who makes this correction? Fortunately, the aggregate moisture correction is routinely applied by the concrete producer at the time of batching, estimating the moisture condition of the aggregates and changing to the batch weights as required. The calculations usually are done automatically in the batching computer based on input values for aggregate moisture and absorption. Aggregate moisture data and the number of pounds or gallons of adjusted water usually are printed on the batch ticket.

As a contractor, part of your responsibility is to make your requirements for concrete properties and uniformity clear to your concrete producer. Next, find out how sensitive your mix is to changes in water content. If the producer's submittal packet includes a “three-point-curve,” this can be an ideal tool for showing how strength varies with water content in your specific mix. You might also conduct some field trials with carefully monitored water addition to find out how slump or admixture dose vary with water.

Early in the job spend some time with the concrete producer and walk through a typical batch ticket, item by item, to find out exactly what the moisture numbers mean, how they are measured, and how the corrections are made. Find out how the aggregates are being processed. Are they presoaked? Are they stored undercover? Are they replenished from the quarry about as fast as the concrete is shipped? Increasing your familiarity with the whole process will make you and your producer better prepared, and will mark you as a discerning customer.

— Kenneth C. Hover, Ph.D., P.E., is a structural/materials engineer and professor of structural engineering at Cornell University, Ithaca, N.Y., and a popular speaker at Hanley Wood's World of Concrete.