ASTM D6938 vs D698: Field Density Testing Requirements

When you're running compaction control on a job site, the two standards you'll pull out most often are ASTM D6938 and ASTM D698. They serve different but linked roles: D698 defines the laboratory compaction reference (the Proctor curve), while D6938 covers the nuclear gauge used in the field to measure in-place density and moisture. Neither standard is optional if you're doing earthwork under a spec that cites them—both carry mandatory clauses that bind the contractor and the testing agency.

This article walks through the normative requirements of each standard, focusing on what you must do to stay compliant. We'll look at the clauses that impose obligations, the exact limits and tolerances, and the documentation you need to prove you followed the methods.

ASTM D698: The Laboratory Proctor Reference

ASTM D698, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort, defines the maximum dry density and optimum moisture content for a given soil. It's the baseline your field results get compared against.

What the standard requires:

• Effort level: Section 4.1.1 specifies a standard Proctor effort of 12,400 ft-lbf/ft³ (600 kN·m/m³). This is non-negotiable—using modified effort (ASTM D1557) changes the reference and invalidates your comparison.
• Mold size: Section 9.1 requires a 4-inch (101.6-mm) diameter mold for soils passing the 4.75-mm (No. 4) sieve. If your soil has larger particles, you must switch to the 6-inch mold or use a different method.
• Moisture content: Section 10.3 requires that you test at a minimum of five different moisture contents to establish the curve. The optimum moisture is the peak of that curve, and you must report it to the nearest 0.1%.

Proof of compliance: You need a completed Proctor curve report that shows the test method (D698), the soil classification, the mold size, and the moisture-density relationship. This document is the reference for your field density ratio.

ASTM D6938: The Nuclear Gauge Field Method

ASTM D6938, Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth), is the field test you'll run on compacted lifts. It's a rapid, non-destructive method, but it's only as good as your calibration and operational discipline.

What the standard requires:

• Calibration: Section 6.1 mandates that the gauge be calibrated at least annually, or after any repair that could affect measurement. Calibration must be performed by an accredited lab and traceable to a national standard (like NIST in the US or NATA in Australia).
• Standard count: Section 8.2 requires you to take a standard count at the start of each day and whenever you suspect drift. The count must be within the manufacturer's specified limits—typically ±1% of the original calibration count.
• Measurement depth: Section 7.1 specifies that the gauge measures to a depth of about 12 inches (300 mm) for density and about 4 inches (100 mm) for moisture. If your lift thickness exceeds that, you need to test in 6-inch (150-mm) increments.
• Test location: Section 8.4 requires a minimum of one test per 1,000 ft² (93 m²) per lift, but your project spec may impose a higher frequency. Check your contract.

Proof of compliance: You need a field density report that includes the gauge serial number, calibration certificate date, standard count values, test location, lift number, and the measured wet density and moisture. The report must also show the calculated dry density and the percent compaction relative to the D698 reference.

Comparing the Two: What Each Standard Actually Controls

The key difference is scope: D698 sets the laboratory benchmark; D6938 measures the field result. Neither standard tells you the acceptable percent compaction—that's set by the project specification (often 95% of standard Proctor, but it varies).

Where they interact:

• Moisture control: D6938 gives you field moisture content, which you compare to the D698 optimum. Many specs require field moisture within ±2% of optimum.
• Density ratio: The percent compaction is calculated as (field dry density / lab max dry density) × 100. Both standards must be cited in the report for that ratio to be valid.
• Calibration traceability: D6938 calibration requirements are separate from D698, but the lab reference curve must be generated with a calibrated oven and balance—so both standards rely on traceable equipment.

Calibration Requirements for ASTM D6938

Calibration is where most compliance failures happen. ASTM D6938 calibration requirements are explicit:

1. Annual calibration (Section 6.1) by an accredited lab.
2. Daily standard count (Section 8.2) before any tests.
3. After repair (Section 6.1.1), the gauge must be recalibrated before further use.
4. Documentation: Keep the calibration certificate on site and available for inspection. It must list the reference block density, the date, and the technician's signature.

If you're working in Australia, your gauge must also comply with ARPANSA regulations for radiation safety—that's separate from ASTM, but it affects your operating license.

Common Compliance Pitfalls

• Using D6938 with a D1557 (modified) reference: If your spec says D698, you can't substitute D1557. The density values are different.
• Skipping the daily standard count: If the count drifts beyond ±1%, your field data is invalid. Stop work and recalibrate.
• Testing too shallow: If your lift is 12 inches thickaren't you're only measuring the top 12 inches—but if it's thicker, you must test in increments to cover the full lift.
• Not adjusting for oversize particles: Both D698 and D6938 require correction for particles retained on the ¾-inch (19-mm) sieve. Section 9.2 of D698 and Section 8.5 of D6938 spell this out. If you ignore it, your percent compaction is wrong.

Takeaway: What You Must Have to Be Compliant

To run a defensible compaction control program, you need:

• A valid D698 Proctor curve for the exact soil being placed.
• A D6938 gauge with a current calibration certificate and daily standard counts on record.
• Field reports that show the calculation from wet density to dry density to percent compaction.
• A clear line of traceability: gauge calibration → field measurement → lab reference.

Both standards impose mandatory clauses that bind you. Follow them, document the evidence, and you'll pass any audit or inspection.