What is the FDOT standard for testing unit weight of fresh concrete?

• A. Use slump test to infer unit weight.
• B. Use ASTM C138 procedure to determine unit weight to verify density and mix consistency.
• C. Use a measured volume test with no standard.
• D. Use ASTM C745.

Answer: (B)

Determining the unit weight of fresh concrete means measuring its density directly, not guessing from how the concrete flows. Slump tests assess workability, not density, so they can’t verify unit weight or mix consistency. The FDOT standard uses ASTM C138, which defines a repeatable method: take a container with a known volume, fill it with freshly mixed concrete following a specified process, level and tamp as required, then weigh the filled container. Subtract the container’s tare weight to get the concrete’s mass, and divide by the container volume to obtain the unit weight (density). Performing this test (often multiple determinations) provides a direct check of density and helps confirm the mix proportions and moisture content are behaving as specified. Using a nonstandard measured-volume test or relying on another standard not intended for unit weight would not give the consistency and reliability required for FDOT QC.