Question: Is there an accurate, quick and inexpensive way to determine the percent of water in aggregate?

Answer: I batch up to 100 cubic yards of concrete a week for our in-house projects, using all applicable admixtures. Because the water in the aggregate varies, I have to guess at the amount of water for each mix. This takes unnecessary time and extra trial batches.

Here are three possible approaches:

Rule-of-thumb.

Assuming your sand and gravel or stone aren't unusual in nature, you can estimate the free moisture that the aggregates may contribute to the mixing water. Sand can be expected to contribute about 2 to 6 percent free moisture by weight, and gravel or stone about 1 or 2 percent.

Under average conditions that means that 104 pounds of sand would give you the equivalent of 100 pounds of sand in what is called a saturated surface dry (SSD) condition and 4 pounds (1/2 gallon) of free moisture. This 4 pounds of moisture or water becomes part of the mix water. Newly delivered sand, in the early part of the day, will have more free moisture. After a rain the sand may briefly have even more than 6 percent moisture.The coarse aggregate contains less free moisture and varies less. You may have as little as 1 pound of free moisture per 100 pounds of gravel or stone, and seldom more than 2 pounds (1/4 gallon).

Oven drying.

For quick quality control some operators have found a microwave oven to be useful. You place several pounds of the sand, gravel or stone in a Pyrex dish of convenient size, such as 9 x 13 inches. Weigh the filled dish, then deduct the weight of the dish. Place the dish in the oven for about 30 to 40 minutes at full power setting. Using oven mitts, remove the dish, cool it to room temperature (about 45 minutes) and weigh. The difference in weight represents the total moisture content of the aggregate.

For making batch calculations it is handiest if moisture has been calculated as percent of the SSD weight of the aggregate. The total moisture measured in the test represents both the free moisture and the moisture that the aggregate absorbs. Only the free moisture becomes part of the mix water. The absorbed water, usually about 1 percent, remains within the aggregate. If the coarse aggregate is unusually porous, the absorption should be determined by a separate test.

Example for test on sand:

  • Weight of wet sand sample............4.16 pounds
  • Oven dried weight ........................3.96 pounds
  • Saturated surface dry (SSD) weight, (1 % absorption)3.96 x 1.01 ..................................4.00 pounds
  • Free moisture, 4.16 - 4.00 ...........0.16 pound
  • Free moisture, percent of SSD weight(0.16 / 4.00) x 100 .....................4.0 percent. In the example, 104.0 pounds of aggregate would represent 100 pounds of SSD aggregate and 4.0 pounds of mix water. A hot plate may also be used for this kind of testing but without the same fringe benefit of lunch preparation that the microwave offers.

    The Speedy Moisture Tester.

  • One of the simplest methods of testing, using equipment specifically designed for moisture tests, is the Speedy Moisture Tester, made by a British manufacturer and marketed for about $1000 by several concrete testing equipment suppliers in this country, such as those listed below. The equipment is packaged so that it can be used in either the field or the laboratory. The test uses calcium carbide as a reagent and requires about 3 minutes for testing a 200-gram sample. Several tests are recommended to verify the results. The equipment is calibrated to indicate percent moisture based on the wet weight of the test sample within a claimed accuracy of 0.5 percent. You could use a conversion factor to change the percent moisture from the wet basis to the SSD basis.You may be interested in Chapter 3, "Properties of Freshly Mixed Concrete as Influenced by Aggregate," in ACI 221R-84, Guide for Normal Weight Aggregates in Concrete, published by the American Concrete Institute, Box 19150, Detroit, Michigan 48219. Addresses known to us of U.S. sources of the Speedy Moisture Tester are: Forney Inc., Route 18 South, RD 2, Wampum, Pennsylvania 16157; Humboldt Manufacturing Company, 7300 West Agatite, Norridge, Illinois 60656; Soiltest Inc., 2205 Lee Street, Evanston, Illinois 60202.