Which term best describes the ratio of attained dry density to maximum dry density expressed as a percentage?

• A. Percent compaction
• B. Relative density
• C. Density ratio
• D. Compaction factor

Answer: (A)

Percent compaction is the ratio of attained dry density to maximum dry density expressed as a percentage. This term comes from Proctor tests, where the maximum dry density is established as a benchmark, and field or lab compaction is reported as a percent of that benchmark. It directly communicates how close the achieved density is to the densest possible state, which is exactly what you need to know for meeting specifications. For example, if the maximum dry density is 1.90 g/cm3 and you achieve 1.80 g/cm3, the percent compaction is about 94.7%.

Other terms describe different ideas. Relative density relates to how dense a granular soil is compared to its loose and dense states, not to a maximum dry density. Density ratio and compaction factor refer to other concepts about mass and density that don’t express the achieved density as a percentage of the maximum. In FDOT practice, specifying a percentage of maximum dry density is the standard way to express compaction requirements.