DETERMINATION OF WATER-CONTENT OF FRESH CONCRETE USING A MICROWAVE-OVEN

As part of the Strategic Highway Research Program (SHRP) contract, Optimization of Highway Concrete Technology, a microwave-oven method for measuring water content of as-delivered fresh concrete was evaluated. Water contents of a series of concrete mixed prepared in the laboratory were gravimetrically determined by drying the concrete in a microwave oven and comparing these measurements with water contents used for making these mixes. A relatively high-power microwave (900 W) equipped with a turntable to provide uniform drying was used. Mixes used in the study included conventional concrete mixes with three types of aggregates with low, moderate, and high water absorptions and mixes with silica fume, fly ash and latex. Field ruggedness of the procedure was verified by testing under actual field conditions at placements of bridge deck overlays and full-depth pavement repair sections. Test results showed that this technique is sufficiently reproducible for purposes of field control and is independent of absorption of aggregates or consistency of concrete. The test is quite simple and does not require a skilled operator. The total test time is 16 min, including time for sample manipulation and check-weighting twice during the drying process. The method can be applied to latex-modified and silica-fume concretes, as well as more conventional mixes.