Moisture Content and Density on a Coarse Granular Base
A field technician is compacting a 200 mm lift of crushed aggregate base. The specified density is 98% of the maximum dry density from a modified Proctor (AASHTO T 180) on the minus 19 mm fraction. The field sand cone test gives a wet mass of 5,420 g, a dry mass after oven drying of 5,180 g, and a volume of 2,610 cm³. The lab Proctor maximum dry density is 2,180 kg/m³. The technician computes field dry density and reports 97.1% compaction, which fails. The contractor argues the material is too coarse for the sand cone and that the test is invalid. The lift is 200 mm thick and the base course contains about 30% gravel retained on the 19 mm sieve.
1. Recompute the field dry density and percent compaction from the given data. Does the arithmetic support the reported 97.1%?
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Wet mass 5,420 g, dry mass 5,180 g, so moisture content = (5,420 − 5,180)/5,180 = 240/5,180 = 4.63%. Field dry density = dry mass / volume = 5,180 g / 2,610 cm³ = 1.985 g/cm³ = 1,985 kg/m³. Percent compaction = 1,985 / 2,180 = 91.1%, not 97.1%. The reported value is inconsistent with the raw data, so the technician's calculation is in error regardless of the method dispute. The correct field dry density is about 1,985 kg/m³ and compaction is about 91%, which is a clear failure. The arithmetic must be corrected and the result re-reported before any methodological argument is considered.
2. Is the sand cone method appropriate here given 30% gravel retained on 19 mm? What correction or alternative should be applied, and how does that affect the acceptance decision?
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The sand cone (AASHTO T 191) is generally limited to material passing the 19 mm sieve or with only minor oversize; with about 30% retained on 19 mm, the in-place volume is affected by large particles and the test is not directly comparable to a Proctor run on minus 19 mm. The standard approach is to apply an oversize correction to the laboratory maximum dry density (and optimum moisture) using the bulk specific gravity of the oversize fraction, producing a corrected target density for the full material. The field dry density must then be compared to the corrected maximum, not the minus-19 mm value. If the oversize correction raises the target, the percent compaction changes; however, a 91% result is far enough below 98% that a correction of typical magnitude (a few percent) would not close the gap. The lift should be reworked and retested, and the density target should be recomputed with the oversize correction for future tests.
3. What does the moisture content tell you about likely compaction problems on this lift?
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The measured moisture is 4.63%, which for a crushed base is on the dry side. Without the lab optimum moisture it cannot be conclusively judged, but a dry field moisture combined with low density is a classic sign of insufficient water for lubrication during compaction. The technician should compare 4.63% to the Proctor optimum (corrected for oversize) and, if the field is more than about 1–2 percentage points dry of optimum, recommend moisture addition before re-rolling. This is a judgement point: low density alone does not prove inadequate effort; moisture state must be evaluated because dry material can resist densification even with adequate roller passes. The correct action is to add water to near optimum, recompact, and retest density and moisture together.