How is the unit weight of fresh concrete typically determined?

• A. By estimating from material densities.
• B. By sampling a known volume and measuring its mass according to ASTM C138.
• C. By weighing the truck before loading.
• D. By calculating from batch water content.

Answer: (B)

Measuring the unit weight of fresh concrete is done by directly weighing a known volume of the material. Using a calibrated container with a precise, known volume, fill it with fresh concrete, consolidate as required, and strike off to a true level. Weigh the container with the concrete, subtract the container’s tare weight, and divide the resulting mass by the container’s volume. This gives the unit weight (mass per unit volume) in units like pounds per cubic foot or kilograms per cubic meter. This direct approach reflects the actual density of the fresh mix, including how much air, paste, and aggregate are present at the time of measurement. It aligns with ASTM C138, ensuring a standardized method and reliable comparison across batches.