Batch Ticket Reveals Mix Design

Accounting for moisture

The second complicating factor is that since aggregates can be wet, dry, or damp on the surface, and their pores can be anywhere from full to empty, the batch weights of the aggregates must be adjusted to account for the influence of moisture content on the relationship between aggregate batch weight and aggregate volume. Further, water on the surface adds to the batch water, and water absorbed into unsaturated pores subtracts from batch water. To control workability, setting time, shrinkage, strength, and durability, the concrete producer must monitor aggregate moisture condition and correct the amount of batch water accordingly. (For a more in-depth discussion on correcting for aggregate moisture, read the Problem Clinic from August 2007.)

To accomplish this, the concrete producer uses any of a number of techniques to estimate the weight of water present as a percentage of the weight of aggregate; this percentage is called the “Moisture Content.” While standard industry documents typically define this water weight as a percentage of the Oven-Dry weight of the aggregate, other definitions are used locally, such as the common variant of defining moisture content as a percentage of SSD weight. So, for example, a testing lab using the standard definition might find a sand sample to have moisture content of 3%, with just enough water to fill pores but leave the surface dry. The concrete producer, defining moisture content based on SSD weight, would report a moisture content of 0% for exactly the same sand sample!

Either value can eventually lead to a reasonable batch-water correction, but only if the differences in definitions are recognized. The key takeaway here is that the batch ticket will show aggregate moisture corrections based on the concrete producer’s aggregate monitoring system and on his definition of moisture content. You have to chat with the producer to understand and then verify its method.

In this example, the concrete producer has assumed (from local experience) that moisture corrections can be ignored for these coarse aggregates and for this particular storage condition (see the column headed %MOISTURE). For the sand, however, the plant’s sensors are reporting a moisture content of 6% (above SSD based on local definition and sensor calibration). That is one wet sand! Given that the design batch weight calls for 1255 lb of SSD sand/CY, at 6% moisture (above SSD), the required batch weight of sand has to be increased by 6% to 1330.5 lb/CY. For 2 CY of concrete this requires 2661 lb of wet sand, which when released into the weighing container already holding 3570 lb of coarse aggregates, will bring the target total weight up to 6231 lb, just like the batch ticket shows.

Water weight

Now that the sand weight has been corrected for moisture, we must correct the batch water or else the concrete will end up too wet because of the water on the sand. Of the total sand weight of 2661 lb, about 151 lb are surface-water and 2510 lb are SSD sand. Water weighs 8.34 lb per gallon, so the batch water must be decreased by 151 lb/(8.34 lb water/gal) = 18.1 gallons. This corrects the design batch water from 2 x 31 = 62 gallons to 62 – 18 = 44 gallons.

But the “Required” column only shows 24.0 gallons—is this a mistake? Before we panic about the missing 20 gallons of water, and start mentally composing back-charges to the producer for cold joints, rapid setting, and all manner of catastrophe, look through the words and numbers below the mix design columns to find all of the information concerning water. There you will see the assumption “WATER IN TRUCK: 20.0 gl.” Mystery solved; the mixture has been corrected for aggregate moisture and has included an operational assumption for the volume of water already in the truck drum before addition of the batch water.

The batch ticket also shows the actual weights batched (BATCHED) and the small variation between Required (target) values and actual batched values. Given the actual weights batched, the producer has computed that the water volume is less than required by a total of 0.4 gallons, and has shown this as “TO ADD: 0.4 gl” meaning that this amount can be added without exceeding the w/c for the approved mixture. The producer and contractor could have agreed to hold more water back to allow for metered water addition onsite as described in ASTM C94, and in such case the ticket would have shown just exactly how much water could be added.

So now when you look at the batch ticket—and you should look at every batch ticket before you sign it—you will be able to decipher the fine print, have some idea of how long since the load was batched and when it’s likely to set, understand what you are paying and why, and be able to check the batch ingredients against the submittals. With all this information on one little sheet of paper, you should give these tickets the respect they deserve!

Kenneth C. Hover is a professor of civil engineering and the Stephen Weiss Presidential Fellow at Cornell University, Ithaca, N.Y. He is a former ACI president.