Which density measurement is used as the reference for QC testing?

• A. AASHTO maximum density
• B. Standard Proctor maximum density
• C. Modified Proctor density
• D. Laboratory density index

Answer: (B)

The main idea is that a fixed lab reference density sets the target for field compaction. The Standard Proctor maximum dry density provides the highest dry density achievable under a defined compaction effort and moisture condition, so it serves as the benchmark for QC testing. In practice, you test a lab sample with the Standard Proctor to get the maximum dry density (and its optimum moisture content). In the field, you measure the in-situ density and express it as a percent of that lab maximum to determine whether compaction meets specs. If a project specifies a heavier compaction energy, the Modified Proctor maximum density would be the reference instead, but by default the Standard Proctor is used. AASHTO maximum density isn’t the typical reference for this QC context, and a laboratory density index is a derived ratio, not the baseline density used for acceptance.