How to Resolve ASTM D6938 vs ASTM D698 Density Testing Disputes

How to Resolve ASTM D6938 vs ASTM D698 Density Testing Disputes

A contractor reports that the gauge shows 95% compaction, while the laboratory report appears to show something different. Before rejecting the work, separate the two jobs being performed: ASTM D6938 measures in-place density and moisture, while ASTM D698 establishes a laboratory moisture–density relationship. This ASTM D6938 vs ASTM D698 density testing issue is usually resolved by checking the reference curve, material, test location and equipment records in the right order.

1. Define what each result actually represents

Start by writing the acceptance calculation in full. A typical percentage compaction result is the in-place dry density from ASTM D6938 divided by the maximum dry density from the approved ASTM D698 curve, multiplied by 100. The two numbers do not come from the same test and should not be expected to match.

ASTM D698 is a laboratory compaction test. Its Section 1 scope covers the relationship between water content and dry unit weight for soil compacted with a standard effort. Section 4 identifies the significance and use of the test, while Section 5 addresses the apparatus and Section 7 the compaction procedure. The standard compaction effort is 12,400 ft-lbf/ft³ (600 kN·m/m³).

ASTM D6938 is an in-place test using electromagnetic and nuclear measurements to determine wet density, dry density and moisture content. Review the current edition’s Sections 5 through 9 for significance, equipment, calibration, standardisation and the field procedure. It is a field measurement method, not another way of producing the Proctor curve.

2. Freeze the approved reference before testing

Obtain the approved D698 report before the crew starts acceptance testing. Confirm the soil description, source, sample date, method designation, maximum dry density, optimum moisture content, mould size and any oversize-particle correction. The reference must represent the material placed in the test area, not simply the nearest previous laboratory result.

The ASTM D698 proctor test differences matter when the material gradation changes. Method selection depends on the soil’s particle-size distribution and the mould and rammer specified by the selected D698 method. A curve made with one method or one blend should not be used automatically for a materially different blend.

Record whether the project specification calls for standard Proctor, modified Proctor or another project-defined reference. ASTM D698 supplies standard effort; ASTM D1557 supplies modified effort. Substituting one curve for the other can create an apparent field-testing failure even when the gauge is operating correctly.

3. Prepare and verify the field gauge

Use the ASTM D6938 nuclear gauge method only with trained, authorised personnel and in accordance with the applicable radiation licence or state, provincial, territorial or national requirements. In the US, this commonly involves the NRC Agreement State or NRC requirements; in other jurisdictions, follow the relevant radiation regulator and licence conditions.

Before taking production readings, complete the checks required by the gauge manufacturer and the current D6938 procedure. In practical terms, this normally includes:

1. Inspect the gauge, source rod, detector housing, scraper ring and reference block.
2. Confirm the daily standardisation or stability check and retain the printed record.
3. Verify the correct test mode, depth, project identification and soil calibration.
4. Check that the gauge calibration and leak-test records are current where required by the licence or manufacturer.
5. Prepare a firm, smooth test location with no loose particles, standing water or visible voids under the gauge.

Do not treat a standardisation check as a substitute for calibration. D6938 Section 8 deals with calibration concepts, and the manufacturer’s calibration model and correction data remain essential to reliable readings.

4. Run the tests in a controlled sequence

First identify the lift, station, offset, elevation and material source. Then record the layer thickness and the compaction equipment used. Place the gauge in the specified position, take the required reading period, and record wet density, moisture and calculated dry density together with the gauge serial number and operator.

Use the same test area, or a documented nearby area with the same material, when collecting a validation sample for laboratory testing. Avoid placing the gauge over an edge, a utility, a large stone, a wet pocket or a repair patch. Surface geometry and non-uniform material can affect the reading even when the instrument passes its checks.

If the project requires a minimum density, compare the measured dry density with the approved D698 maximum dry density. Also compare field moisture with the D698 optimum moisture content. A density shortfall at a substantially different moisture content may point to placement or moisture conditioning rather than a simple gauge error.

This is the practical point of an ASTM D6938 vs D698 comparison: D6938 supplies the numerator for the acceptance calculation, while D698 supplies the laboratory denominator and moisture relationship.

5. Investigate a discrepancy before rejecting the lift

When results disagree, repeat the measurement at a second location and document the spacing. If the reading changes materially, investigate local variability before changing the acceptance basis. Check the gauge’s surface contact, test depth, count time, operator entries and whether the reading was taken on the correct lift.

Next, compare the field material with the D698 sample. Look for a changed borrow source, additional crushed rock, segregation, excessive oversize particles, recycled material or a different moisture condition. If the project specification permits, obtain a representative sample and ask the laboratory to confirm the applicable D698 method and curve.

Common recovery actions are:

• Re-standardise and repeat the gauge reading when the daily record or instrument setup is doubtful.
• Take a sand-cone or other approved independent field test when the specification allows it; ASTM D1556 may be relevant for sand-cone testing.
• Request a laboratory review when the reported maximum dry density, moisture basis or oversize correction appears inconsistent.
• Quarantine the affected lift and map the test locations instead of averaging unrelated stations.
• Have the engineer of record approve any new reference curve before using it for acceptance.

Do not “correct” a failed result by selecting the higher of two laboratory maximum densities. The acceptance basis must come from the contract documents and an appropriately representative test.

6. Close out the record and prevent repeat disputes

The final report should link every field result to the D698 report used for comparison. Include the standard and edition, test method, gauge identification, calibration or correction status, standardisation result, location, test depth, wet and dry density, moisture content, maximum dry density, optimum moisture content and calculated compaction percentage.

Retain the raw gauge printout or electronic file, laboratory worksheets and any independent verification results. If the discrepancy resulted from a changed material, update the inspection and test plan so that a new D698 curve is triggered by a defined source or gradation change.

Closing takeaway

The safest answer to “ASTM D6938 vs ASTM D698 density testing” is not to choose one method over the other. Use D698 to establish the approved laboratory reference and D6938 to measure the placed material. When the numbers conflict, verify the reference, instrument, location and material in that order; then use an approved independent test or laboratory review to close the evidence gap.