Moisture Content and Density Determination
Moisture content
Moisture content is the mass of water removed by drying, expressed as a percentage of the dry mass of the aggregate. The standard approach: weigh the wet sample, dry it to constant mass (typically in an oven at about 110 °C), cool, and reweigh. Traps include: reporting moisture on a wet basis instead of a dry basis, not drying to constant mass, and failing to cool the sample in a desiccator before weighing (hot samples give low readings on a balance due to convection).
Bulk density and unit weight
Density determination uses a known-volume measure filled with aggregate in a defined manner - either rodded (compacted in layers) or shaken/jigged - then struck off and weighed. The result is bulk density (mass/volume), from which void content is computed using the aggregate's bulk specific gravity: voids (%) = (1 − bulk density / (bulk SG × density of water)) × 100. A common trap is confusing bulk density of the aggregate in the measure with the particle density (specific gravity) of the aggregate itself.
Worked example
A measure of 0.0095 m³ holds 15.2 kg of rodded aggregate. Bulk density = 15.2 / 0.0095 ≈ 1600 kg/m³. If bulk specific gravity is 2.65, then voids = 1 − 1600/(2.65 × 1000) = 1 − 0.604 ≈ 39.6%.
Traps
Watch for: forgetting to tare the measure, using the wrong fill method (rodded vs. loose), and rounding intermediate values too early. Density results are sensitive to fill method, so match the method to the specification.