Aggregate Base Testing: Compaction, Strength, and Acceptance
What Base Testing Requires
Aggregate base testing focuses on whether the placed layer meets density, gradation, and strength requirements. Level 2 candidates should connect laboratory results to field acceptance decisions.
Compaction control. The laboratory reference is a moisture-density curve giving maximum dry density and optimum moisture content. Field density is measured and expressed as a percentage of that maximum. Know that relative compaction = (field dry density / lab maximum dry density) × 100. Trap: comparing wet field density directly to lab maximum dry density without correcting for field moisture - always convert to dry density first.
Moisture correction. Field moisture content must be determined so wet density can be converted: dry density = wet density / (1 + moisture content as a decimal). A common exam trap gives wet density and moisture and expects you to notice the correction is required.
Gradation of base course. Base gradations are typically full-grading bands; check percent passing at each control sieve and confirm the material is neither too coarse (stability loss, segregation) nor too fine (excess fines, moisture sensitivity).
Strength and acceptance. Understand that bearing strength tests (such as a California Bearing Ratio–type index) relate to support capacity, and that acceptance combines density, gradation, and thickness. Thickness shortfalls reduce structural number even when density passes.
Worked Example
Lab maximum dry density = 2.10 Mg/m³. Field wet density = 2.20 Mg/m³ at 6% moisture. Dry density = 2.20 / 1.06 = 2.075 Mg/m³. Relative compaction = 2.075 / 2.10 × 100 = 98.8%. If the specification requires 95% minimum, the layer passes.
Traps to Expect
- Forgetting the moisture correction before computing relative compaction.
- Using optimum moisture as the acceptance criterion instead of density.
- Ignoring thickness when density passes.
- Confusing a strength index with a density requirement.