Q. We had a long dry spell this spring followed by several days with short but intense rainfall. That seems to cause inconsistency in the concrete we're getting from our producer. Why does a little rain make things so difficult?

A. Aggregate moisture conditions vary tremendously, even within a concrete producer's yard. Outdoor stockpiles can be wet on the outside and dry on the inside if a long dry spell recently was interrupted by rain. When clear, dry weather follows a rainy period, the outside is dry and the inside is wet.

Also, on any given day, the aggregates can be delivered to the batch plant in various moisture conditions and actions, such as washing, cooling, and presoaking aggregates, make a big difference as well. When the loader operator moves aggregates to the conveyors for loading into the bins, each scoop can have a wide variation in moisture condition.

The tricky part is that the mix design typically is based on dry aggregate, so any water in the aggregate has to be subtracted from the water to be used in batching the concrete. The concrete producer may adjust the mix design to account for typical aggregate moisture conditions, such as saturated-surface-dry (SSD), but must either continually confirm these conditions or account for variations.

Concrete producers use a wide range of moisture meters to estimate aggregate conditions—oven-drying techniques usually take too much time. Sometimes the coarse aggregate will be pretty close to SSD condition in the stockpile, especially if the weather has been wet enough to fill most of the pores, but the aggregate has been stored in a well-drained pile so it's nearly dry on the surface. Under this circumstance, moisture compensation for the coarse aggregate is small and can sometimes be ignored. It is less common to have fine aggregate ready to use at SSD, so the sand compensation is almost always needed.

Although the goal is to compute a unique adjustment for each batch based on the specific aggregate in the load, in reality moisture corrections often are based on average moisture conditions over the course of a day or a week of operation.

Q. Can you please explain the terms “saturated-surface-dry” and “absorption?”

A. Saturated-surface-dry (SSD) refers to the condition when the permeable pores of the aggregate are full of water but the surface is dry. SSD aggregates neither contribute nor absorb water from the concrete.

Absorption is the moisture content of SSD aggregates. It is determined by weighing an SSD sample, then oven drying it and weighing it again. The absorption is the difference in weight expressed as a percentage of the aggregate's dry weight.

There are separate ASTM standard test methods—C127 and C128—for measuring the density and absorption of coarse aggregate and fine aggregate. ASTM C566, “Standard Test Method for Total Evaporable Moisture Content of Aggregate by Drying,” describes oven drying of aggregates.

Knowing the absorption for the aggregate you are working with is important because whenever the as-used moisture content is higher than the absorption, there is water on the aggregate surface and the batch water has to be reduced. When the as-used moisture content is less than the absorption, the batch water has to be increased to compensate.

Q. Is it really important to understand all this terminology? I'd like to just let the testing lab take the samples, check the numbers, and run the tests.

A. In short, yes. Despite the standard wording of ASTM C125, C127, and C128, we really do not have a truly standardized terminology for aggregate moisture corrections in the field, and two or three people can look at the same numbers and draw very different conclusions. In one recent project involving mass concrete, the job was delayed for more than a week until all parties realized they each were working from a different set of definitions.

On this project the concrete tickets showed sand moisture content of 3.5% (soaking wet). To the testing company and engineer that meant a total water weight of 3.5% times the oven-dry weight of the sand. The amount of water on the surface of the aggregate would be computed as 3.5% minus the absorption multiplied by the oven-dry weight. But to the concrete producer, it meant that the amount of free water was 3.5% times the as-used wet weight of the sand, and to a consultant it meant that free water was 3.5% times the SSD weight of the sand. Depending on whose interpretation was applied in making the moisture adjustment, the slump on delivery varied by 3 inches and the required dose of superplasticizer more than doubled.

This confusion is not uncommon, and the variable end product often is shrugged off explained as a consequence of an inability to accurately monitor aggregate moisture.

— Kenneth C. Hover, Ph.D., P.E., is a structural/materials engineer and professor of structural engineering at Cornell University, Ithaca, N.Y.