Key takeaway
An asphalt air-void percentage depends on its specific-gravity pair. A plausible result can be wrong because the reference mixture or specimen basis was mismatched.
Find the reference behind percent density
An asphalt quality archive retains a bulk-specific-gravity result and a percentage called density. A later calculation supplies a maximum-specific-gravity value from another row. The resulting air-void number looks reasonable, but its meaning depends on whether the two inputs actually belong together. The owner needs the measured specimen and the applicable mixture reference, not just a formula.
WSDOT’s January 2026 Materials Manual describes air voids through maximum specific gravity Gmm and compacted-mixture bulk specific gravity Gmb. Its WAQTC procedure gives Va = 100 × (Gmm − Gmb)/Gmm, expressed as a percentage of total volume. Gmm and Gmb identify different measurement bases; their familiar decimal values should not be treated as interchangeable fields.
A wrong mixture reference creates a believable percentage
Hypothetical example: an invented compacted specimen S8 has Gmb = 2.375. Its explicitly assumed applicable mixture reference M8 has Gmm = 2.500. Another mixture M9 has Gmm = 2.450 and is stipulated to be inapplicable to S8. These are dimensionless specific gravities, not density values in kg/m³. No physical specimen was tested.
The applicable pair gives 100 × (2.500 − 2.375)/2.500 = 5.0% air voids. Pairing S8 with M9 instead gives 100 × (2.450 − 2.375)/2.450 = approximately 3.0612%, or 3.1% at the procedure’s stated reporting precision. The error is about 1.9 percentage points after rounding, even though both results look like ordinary percentages.
| Illustrative calculation | Gmb | Gmm and basis | Air voids | Review decision |
|---|---|---|---|---|
| S8 with applicable M8 | 2.375 | 2.500; assumed applicable | 5.0% | Reproducible under the stated pairing |
| S8 with inapplicable M9 | 2.375 | 2.450; different mixture | 3.1% rounded | Reject this pairing |
| S8 with missing reference | 2.375 | Unknown | Unresolved | Do not fill from the nearest row |
Make applicability inspectable before automating the join
Retain specimen identity, mixture or production reference, sample event, relevant binder/content basis, method and issued result. Link the technical basis that makes a particular Gmm applicable. Some approved procedures may use a representative or calculated reference rather than a separate maximum-specific-gravity measurement for every specimen. The records reviewer should preserve that actual rule, not invent a one-to-one requirement.
A timestamp-nearest join is not evidence of applicability. In the example, the correct decision is already stipulated, so the arithmetic exposes the effect of choosing the wrong reference. In a real archive, the quality owner must establish the pairing from project records and the applicable method. If that evidence is absent, leave Va unresolved rather than selecting the value that makes an acceptance percentage look favorable.
Keep laboratory and field compaction distinguishable
FHWA’s LTPP inventory guide directs calculations to use data applicable to individual samples and separates laboratory-compacted and field-compacted sample sheets. Those distinctions matter when historical files are merged. A specimen prepared in a laboratory and a core from placed pavement can describe different conditions even when the mixture designation matches.
Preserve sample type and compaction basis with the result instead of averaging unlike values into one unlabeled material history. Retain whether a reported maximum value was measured or derived under a documented procedure. This article reads the agency explanations; it does not claim to have inspected the full AASHTO or ASTM standards they reference or selected a current project’s testing method.
A reproduced percentage still needs its actual decision rule
The completed S8 review supports the stated 5.0% calculation and rejects the stipulated wrong-reference calculation. It supplies no acceptance range, pay factor or prediction of pavement performance. Those conclusions need the project’s actual requirements, sampling basis and authorized technical disposition. Correct arithmetic cannot create missing acceptance authority.
For archive preparation, describe which raw measurements, mixture references, specimen types and calculation versions survive. Preserve a superseded result beside the corrected calculation and its reason. Readiness can assign missing reference evidence; Inventory can describe the retained relationships. A prospective discussion can use this fictional pair without exposing a contractor’s mix design or implying a real quality outcome.