What are Air Voids in Asphalt Pavement Mixtures?
Air voids (Va) are the air pockets in compacted asphalt mix, given as a percentage of the total volume of the mix.
What air voids are
Hot mix asphalt has three parts: air voids, mineral aggregate and binder. The percent air voids is the volume of the voids divided by the total volume of the mix, times 100. A lab finds it from two specific gravities: Gmb for the compacted mix, and Gmm for the loose mix with no voids. The formula is Va = 100 × (Gmm − Gmb) / Gmm. A value has meaning only with its compactive effort, for example a 75-blow Marshall specimen or a Superpave mix at Ndesign.
Design values
A mix design sets a target for air voids, and the design must also meet a minimum for voids in the mineral aggregate (VMA). The remaining volume holds the binder. FHWA gives an example: 14% VMA and 4% air voids leave 10% of the volume for binder.
| Specification | Air voids at mix design | Acceptance limits |
|---|---|---|
| Superpave for roads (FHWA) | 4% at Ndesign | not stated in the source |
| FAA Item P-401 | 3.5% | 2.0% to 5.0% |
| FAA Item P-403 | 3.5% | lot average 2% to 5% |
| FAA Item P-404, fuel-resistant | 2.5 ±0.2% | 1.5% to 3.5% |
The FAA values come from AC 150/5370-10H. The P-401 and P-404 limits apply to the percentage-within-limits acceptance method. P-403 accepts or rejects a lot on its average.
Low and high air voids
Low air void content can cause bleeding. FAA AC 150/5380-6C says binder fills the voids of the mix in hot weather and then expands onto the surface. Extensive bleeding can reduce skid resistance severely.
High air void content has the opposite risk. Interconnected voids let water enter the mix, and raveling becomes a significant problem above about 8% air voids. Raveling can show that the binder aged and hardened, and it can be a source of foreign object debris (FOD). A rule of thumb from Linden and others (1989) says that each 1% of air voids above 7% costs about 10% of pavement life.
Air voids and inspection
The specification tests above use samples from the plant and cores from the finished pavement. A visual survey does not measure air voids. It records distresses such as bleeding and raveling, and the maintenance guidance links both to the condition of the mix.
Frequently Asked Questions
- What is the design air void content of airfield asphalt?
- In the FAA specification for hot mix asphalt (Item P-401), the mix design criterion is 3.5% air voids. Item P-403 also uses 3.5%. The fuel-resistant mix in Item P-404 uses 2.5 ±0.2%. A Superpave mix design for roads uses 4% at the design number of gyrations (Ndesign).
- How do labs measure air voids?
- A lab finds the bulk specific gravity (Gmb) of a compacted specimen or core. It also finds the maximum specific gravity (Gmm) of the loose mix. It then calculates Va = 100 × (Gmm − Gmb) / Gmm. The FAA lists ASTM D3203 as the test method for percent air voids. It lists ASTM D2726 for bulk specific gravity and ASTM D2041 for maximum specific gravity.
- What does a low air void content cause?
- It can cause bleeding. FAA AC 150/5380-6C says that too much binder in the mix, low air-void content, or both cause bleeding. The binder fills the voids in hot weather and then expands onto the surface. Extensive bleeding can severely reduce skid resistance.
References
- coree-ctre-volumetrics
- HMA Volumetrics Revisited: A New Paradigm (B. J. Coree) Center for Transportation Research and Education, Iowa State University
- fhwa-hif-11-031
- Superpave Mix Design and Gyratory Compaction Levels (TechBrief FHWA-HIF-11-031) FHWA
- faa-ac-150-5370-10h
- AC 150/5370-10H, Standard Specifications for Construction of Airports (21 Dec 2018) FAA
- faa-ac-150-5380-6c
- AC 150/5380-6C, Guidelines and Procedures for Maintenance of Airport Pavements (10 Oct 2014) FAA
- faa-paver-asphalt
- PAVER Distress Identification Manual for Asphalt Surfaced Airfields (US Army ERDC-CERL, June 2009) US Army ERDC-CERL
- pavement-interactive-compaction
- Compaction Importance Pavement Interactive
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