Many project documents specify a water-cementitious materials ratio (w/cm) for concrete, but it's often difficult to confirm that the specified value has been attained. Verification usually is based on batch records for the amounts of cement and water used. However, the water-content values on these records are sometimes suspect because water content is difficult to estimate precisely due to changes in aggregate moisture content, water left in the truck, and water added at the jobsite.
AASHTO has a provisional standard for an onsite testing method, "Water Content of Freshly Mixed Concrete Using Microwave Oven Drying." The procedure has been used on several projects for verifying w/cm while there is still time to adjust it. But how precise is the test, and what are reasonable tolerances when evaluating the test results? Casimir J. Bognacki, manager, materials engineering division, the Port Authority of New York and New Jersey, Jersey City, N.J., and Colin Lobo, vice president, engineering, National Ready Mixed Concrete Association, Silver Spring, Md., offer some answers these questions.