Air Voids and Asphalt Pavement Durability

Air voids are the small spaces remaining between coated aggregate particles after asphalt is mixed, placed and compacted. Their percentage is a basic volumetric property, yet it has a strong influence on pavement life. The right void content allows the binder to hold the aggregate structure together while leaving enough room for controlled thermal expansion and traffic densification.

For Australian road authorities, contractors and testing laboratories, air-void control is particularly important because pavements face large temperature ranges, intense rainfall, heavy freight, urban congestion and repeated maintenance activities. A well-designed mix can still fail early if the delivered material is too open, too dense, poorly compacted or exposed to water through connected voids.

What Air Voids Measure

Air voids, commonly written as Va, express the volume of air within a compacted asphalt specimen as a percentage of its total volume. The calculation compares the bulk density of the compacted specimen with its theoretical maximum specific gravity, which assumes that no air remains in the mix:

Va = 100 × (1 − Gmb/Gmm)

Here, Gmb is the bulk specific gravity of the compacted asphalt and Gmm is the maximum theoretical specific gravity. Laboratory technicians may determine these values using compacted briquettes, gyratory specimens or pavement cores, depending on the project specification and investigation purpose.

Air voids are different from voids in the mineral aggregate, or VMA. VMA includes the spaces between aggregate particles and contains both binder and air. The voids filled with bitumen, often called VFB or VFA, show how much of that available space is occupied by binder. These related measurements must be interpreted together because a target air-void percentage can produce different performance outcomes in different aggregate structures.

A visual reference such as this street-level example may help explain how pavement sits within a wider streetscape, but appearance cannot confirm density or durability. Pavement performance depends on measured properties, construction records, drainage and loading history rather than surface appearance alone.

Why Void Content Changes Pavement Performance

Excessive air voids create a more permeable asphalt layer. Water can move through connected pores and reach the binder–aggregate interface, where it may weaken adhesion and promote stripping. In climates with intense rainfall, such as parts of Queensland and northern New South Wales, this can accelerate ravelling, pothole development and loss of skid-resistant aggregate.

Higher void content also exposes more binder to oxygen. Oxidation gradually makes the binder stiffer and more brittle, increasing the risk of fatigue cracking and thermal cracking. A pavement with apparently adequate early strength may therefore lose flexibility as it ages. The effect is more pronounced when the layer is thin, the binder is already hard or the surface receives substantial ultraviolet exposure.

Very low air voids create a different set of risks. If the mix is over-compacted or contains too much binder, aggregate particles may lose the stone skeleton needed to resist movement. Under heavy wheel loads, binder can migrate towards the surface, causing flushing or bleeding. Rutting becomes more likely when the mix has insufficient internal structure, particularly at intersections, bus stops, climbing lanes and freight routes.

For that reason, durability is not achieved by simply pursuing the lowest possible void percentage. The aim is a stable, low-permeability mix with adequate binder film thickness and sufficient resistance to permanent deformation. The correct range depends on traffic, aggregate grading, binder grade, layer position and the governing specification.

How Mix Design Controls Durability

During mix design, engineers select aggregate gradation, particle shape, binder content and compaction effort to establish a balanced volumetric structure. Dense-graded asphalt commonly targets approximately 3 to 5 per cent air voids at the specified design compaction level, although the applicable range varies by mix type and Australian project requirements. Open-graded and porous asphalt intentionally use higher void contents to provide drainage and noise benefits.

Aggregate shape has a major influence on void structure. Angular, rough-textured particles interlock and create a strong skeleton, while rounded or excessively smooth particles can make the mixture easier to compact but less resistant to rutting. The proportion of manufactured fines, natural sand and mineral filler also changes VMA, binder demand and the risk of tenderness during rolling.

Binder content must be sufficient to coat the aggregate and provide durability without filling too much of the available space. If the selected gradation produces low VMA, increasing binder may reduce air voids below the preferred level and create a mix that is vulnerable to flushing. If VMA is too high, the mix may require more binder and may remain permeable or unstable if the aggregate framework is poorly controlled.

Australian designers commonly refer to Austroads guidance and project-specific road authority requirements when setting volumetric criteria. A mix suitable for a cool Melbourne arterial may require different binder and compaction considerations from one used on a heavily trafficked Perth road or a wet tropical route near Darwin. Laboratory performance tests should support, rather than replace, sound volumetric design.

Compaction Determines the In-Situ Result

A mix can meet its laboratory air-void target and still perform poorly if field compaction is inadequate. Temperature at delivery, paver speed, layer thickness, roller pattern, haul distance and weather all affect the final density. Asphalt that cools below its effective compaction range before rolling may retain excessive voids that cannot be removed later.

Construction control should compare field density with the reference density required by the contract. Cores provide direct evidence of bulk density and in-situ voids, while calibrated non-destructive gauges can help identify uniformity and guide coring locations. Results should be assessed across the lane, including wheel paths, joints, edges and areas around utility covers or drainage structures.

Longitudinal joints deserve particular attention. They often receive less confinement and may have higher voids than the centre of the mat. Water entering an open joint can cause localised stripping and edge breakdown, eventually allowing defects to spread into the trafficked lane. Joint construction, echelon paving and appropriate rolling sequences are therefore essential parts of air-void management.

Temperature and logistics are especially relevant on large Australian sites. A long haul from a regional asphalt plant to a metropolitan project can reduce workability before the rollers reach the mat. In Sydney or Brisbane, traffic management restrictions may compress the paving window, while in regional areas a small crew may need to coordinate delivery, laying and testing with limited plant availability.

Testing, Sampling and Field Investigation

Reliable air-void results begin with representative sampling. Asphalt should be sampled from the delivery stream or pavement according to the relevant method, with records of location, lot, mix designation, production time, temperature and compaction conditions. A single core may identify a problem, but a pattern of results provides a stronger basis for acceptance, maintenance planning or forensic assessment.

Laboratory testing may include bulk density, maximum theoretical density, binder content, aggregate gradation, voids in mineral aggregate and moisture sensitivity. Core orientation, trimming, surface damage and specimen dimensions need to be controlled because small errors in volume or mass can materially affect the calculated percentage. Testing facilities working on public infrastructure should use documented procedures, calibrated equipment and competent personnel.

When density or structural performance is in doubt, field investigation can extend beyond air voids. Falling-weight methods can assess pavement response and help identify weak areas beneath the asphalt; a guide to falling weight testing provides useful background on this type of investigation. Deflection results do not directly replace air-void measurements, but they can show whether a local density issue is associated with subgrade weakness, moisture or inadequate layer support.

Surface friction also needs separate attention. A pavement may have acceptable voids yet offer inadequate wet-weather grip because of aggregate polishing, flushing or contamination. Skid assessment methods, explained in this skid testing guide, complement volumetric and structural testing when investigating crashes, intersections, steep grades or high-risk wet-weather locations.

A laboratory and field unit operating under a quality system such as SNI ISO/IEC 17025 can provide traceable test results, while ISO 45001 supports safer investigation and sampling practices. For Australian clients, the equivalent expectation is clear documentation of methods, equipment verification, sampling control and reporting limitations so that test data can be used confidently in project decisions.

Australian Conditions and Service Life

Australian pavements experience highly variable environments. Perth’s hot summers can soften binder and intensify rutting in slow-moving traffic, while Melbourne’s cooler conditions place greater emphasis on flexibility and resistance to cracking. Brisbane and coastal New South Wales receive heavy rainfall that can expose permeability and drainage weaknesses, and roads around Darwin face tropical storms, heat and moisture-related deterioration.

Freight loading is another decisive factor. Routes serving ports, mining operations, distribution centres and regional agricultural areas may experience repeated heavy axle loads that rapidly densify asphalt in wheel paths. A mix with initially acceptable air voids can become too dense under traffic if its aggregate skeleton is weak. Designers must consider expected post-construction densification rather than relying only on the laboratory endpoint.

Surface treatments are common across the Australian market and should not be confused with asphalt air-void control. Spray seals and chip seals use a different structure, with aggregate chips and sprayed binder forming the wearing course. Dense-graded asphalt overlays, open-graded friction courses and stone mastic asphalt each require their own acceptance criteria. Comparing their void percentages without considering function can lead to incorrect quality judgements.

Drainage, shoulder condition and maintenance history also influence durability. Water trapped at the pavement edge, blocked table drains or poorly sealed utility reinstatements can create failures that appear to be caused by asphalt density alone. A practical assessment should combine core results with site levels, visible distress, rainfall exposure, traffic data and construction records.

Practical Comparison of Air-Void Conditions

Air-void condition	Typical pavement behaviour	Main durability concern	Useful investigation response
Excessively high	Open, permeable and prone to further oxidation	Water ingress, stripping, ravelling and fatigue cracking	Core density, permeability indicators, joint review and moisture assessment
Within design range	Balanced coating, stability and controlled permeability	Performance still depends on binder, aggregate, drainage and construction	Routine density, volumetric and surface-condition monitoring
Excessively low	Dense and potentially binder-rich or over-compacted	Flushing, bleeding, rutting and loss of texture	Review binder content, VMA, wheel-path rutting and surface friction
Highly variable across a lot	Uneven performance and localised weak points	Early defects at joints, edges, cold areas or isolated patches	Additional cores, paving records, temperature mapping and targeted deflection testing
Deliberately high in porous asphalt	Designed drainage and noise characteristics	Clogging, ravelling and moisture-related loss of cohesion	Check connected voids, clogging, permeability and aggregate retention

Air voids should therefore be treated as a performance indicator rather than an isolated pass-or-fail number. Sound decisions connect the result to mix design, layer function, environmental exposure, traffic demand and observed distress. This approach is especially valuable when determining whether a pavement needs routine maintenance, local repair, resurfacing or a more comprehensive structural investigation.

Use a competent testing provider to verify asphalt density, volumetric properties, pavement response and surface safety before defects become widespread. UP. PPP supports laboratory and field investigations for roads, bridges, drainage assets and pedestrian infrastructure, giving project owners evidence for construction acceptance, maintenance planning and durable infrastructure decisions. Contact the unit to arrange testing suited to the pavement, site conditions and applicable quality requirements.