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:
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).
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: