Gap Graded Asphalt
Last updated: 2026-10-08
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Gap Graded Asphalt
Gap graded asphalt is a hot mix asphalt (HMA) whose aggregate gradation curve deliberately omits one or more intermediate sieve sizes, typically the 4.75 mm to 2.36 mm range, creating a "gap" between coarse and fine fractions. A typical gap graded mix contains 60–75% coarse aggregate, 20–30% fine aggregate, and 5–7% binder by mass, with air voids of 4–6%. It is used mainly for wearing courses and stone mastic asphalt-type surfaces where high macrotexture and rutting resistance are required.
What Is Gap Graded Asphalt?
Gap graded asphalt is defined by what it leaves out of the gradation, not what it contains. In a standard dense graded mix, aggregate particles are distributed continuously across every sieve size. In a gap graded mix, the intermediate sizes are removed, so the coarse skeleton and the fine filler do not interlock through a full range of particles.
The result is a mix with a coarse aggregate skeleton carrying the load and a fine mortar (fines plus binder) filling part of the void space. The large voids left between coarse particles improve surface texture, drainage, and skid resistance.
Typical gap graded asphalt composition:
| Component | Percentage by mass | Notes |
|---|---|---|
| Coarse aggregate (>4.75 mm) | 60–75% | Crushed stone or slag, often 10–14 mm nominal |
| Fine aggregate (0.075–4.75 mm) | 20–30% | Manufactured sand |
| Mineral filler (<0.075 mm) | 2–6% | Limestone dust, hydrated lime |
| Bitumen binder | 5–7% | Pen 50/70, PG 64-22, or polymer modified |
| Air voids (compacted) | 4–6% | Design target |
| Voids in mineral aggregate (VMA) | 15–19% | Higher than dense graded |
The gap is usually created by removing the 2.36 mm to 4.75 mm fraction, though some specifications remove the 1.18 mm to 4.75 mm range. The exact gap depends on the target surface texture and the nominal maximum aggregate size (NMAS).
Gap Graded vs. Dense Graded vs. Open Graded Asphalt
The three gradation families behave differently in service. The table below compares standard values for a 10 mm NMAS wearing course.
| Property | Gap Graded | Dense Graded | Open Graded |
|---|---|---|---|
| Coarse aggregate content | 60–75% | 40–55% | 75–85% |
| Binder content | 5–7% | 5.0–6.0% | 4.0–5.5% |
| Air voids (design) | 4–6% | 3–5% | 18–25% |
| VMA | 15–19% | 14–16% | 22–30% |
| Interlocking mechanism | Coarse-to-coarse contact | Full particle interlock | Coarse-to-coarse contact |
| Rutting resistance | High | Medium to high | High |
| Skid resistance | High | Medium | Very high |
| Noise reduction | Moderate (2–4 dB) | Low (0–2 dB) | High (4–6 dB) |
| Drainage | Semi-permeable | Impermeable | Fully permeable |
| Typical layer thickness | 25–50 mm | 30–60 mm | 30–50 mm |
| Service life (wearing course) | 8–12 years | 10–15 years | 6–10 years |
Gap graded asphalt sits between dense graded and open graded. It keeps an impermeable or semi-permeable structure like dense graded, but borrows the coarse skeleton of open graded. That combination delivers texture and rutting resistance without the drainage and durability penalties of fully open mixes.
Key differences in practice:
- Dense graded relies on continuous gradation for strength; it is the default choice for most pavements and offers the widest tolerances.
- Gap graded sacrifices some workability and density for texture and stability; it demands tighter plant control.
- Open graded maximizes permeability and noise reduction but has lower structural contribution and shorter life.
Gap Graded Asphalt Mix Design and Properties
Mix design for gap graded asphalt follows AASHTO or EN procedures, but the gap changes the design targets. The Bailey method and the "dominant aggregate size" concept are commonly used to balance the coarse skeleton and the mortar.
Design parameters and typical values:
| Parameter | Typical Range | Test Method |
|---|---|---|
| NMAS | 10 mm or 14 mm | EN 13043 / ASTM D448 |
| Binder content | 5.0–7.0% | EN 12697-1 |
| Air voids | 4–6% | EN 12697-8 |
| VMA | 15–19% | Calculated |
| Voids filled with bitumen (VFB) | 65–78% | Calculated |
| Marshall stability | >8 kN | ASTM D6927 |
| Flow | 2–4 mm | ASTM D6927 |
| Wheel tracking rate (60°C) | <0.05 mm/min | EN 12697-22 |
| Indirect tensile strength (dry) | 0.6–1.0 MPa | AASHTO T283 |
| TSR (moisture resistance) | >80% | AASHTO T283 |
| Binder draindown (SMA-type) | <0.3% | EN 12697-18 |
| Permeability | 10⁻⁵–10⁻⁶ m/s | Falling head test |
Performance characteristics:
- Rutting resistance: the coarse skeleton carries load through stone-to-stone contact, so gap graded mixes typically show wheel tracking rates below 0.05 mm/min at 60°C, compared with 0.08–0.15 mm/min for dense graded mixes.
- Skid resistance: macrotexture depth of 0.6–1.2 mm is achievable, versus 0.3–0.6 mm for dense graded surfaces.
- Noise: gap graded surfaces reduce tire-pavement noise by 2–4 dB(A) compared with dense graded asphalt.
- Durability: lower binder content than SMA and higher void content than dense graded make gap graded mixes more sensitive to aging and moisture damage.
- Stiffness: dynamic modulus at 20°C is typically 8,000–12,000 MPa, compared with 5,000–9,000 MPa for dense graded mixes.
Applications and Global Usage
Gap graded asphalt is specified where texture, stability, and durability must be balanced. It is common on high-speed roads, urban arterials, airfield pavements, and industrial hardstands.
Where it is used:
- High-speed trunk roads and motorways — wearing courses on curves and gradients where rutting risk is high.
- Airfield pavements — taxiways and runways where fuel resistance and texture are required.
- Urban roads — noise-sensitive corridors and intersections with heavy braking.
- Industrial areas — container terminals, ports, and hardstands subject to slow, heavy traffic.
- Bridge decks — thin wearing courses where weight is a constraint.
Regional specifications and practice:
| Region | Specification | Typical Binder | Notes |
|---|---|---|---|
| UK | BS EN 13108-1, Clause 4.5 | Pen 50/70, PMB | Gap graded wearing course widely used |
| Germany | TL Asphalt-StB 07 | PmB 25/55-55 | SMA variants with gap gradation |
| USA | AASHTO M 325, state DOT specs | PG 64-22, PG 70-22 | Gap graded used in SMA and OGFC families |
| Australia | AS 2150, VicRoads 407 | C320, C450 | Gap graded for heavy duty |
| India | MoRTH Specifications | VG-30, VG-40 | Gap graded for high rainfall zones |
| Middle East | Qatar, UAE MOT specs | PG 76-10 (PMB) | High temperature performance |
Global consumption of gap graded and related coarse-skeleton mixes is estimated at 150–200 million tonnes per year, with Europe and North America accounting for roughly 60%. In the UK alone, gap graded wearing courses represent around 15–20% of all surface course tonnage.
Prices vary by region and binder type. As of 2024–2025, ex-works prices for gap graded asphalt typically fall in these ranges:
- Europe: €85–€120 per tonne
- North America: $95–$140 per tonne
- Middle East: $70–$100 per tonne
- South Asia: $60–$90 per tonne
Polymer-modified gap graded mixes add 15–30% to the base price. Bitumen binder itself accounts for 35–45% of the total mix cost, which is why binder grade selection drives project economics.
Construction, Placement, and Quality Control
Gap graded mixes are less forgiving than dense graded mixes. The coarse skeleton can segregate during transport and placement, and the low fines content reduces workability.
Placement parameters:
| Parameter | Recommended Value |
|---|---|
| Production temperature | 160–175°C |
| Delivery temperature at paver | 145–165°C |
| Compaction temperature | 130–150°C |
| Roller passes | 4–8 (static + vibratory) |
| Layer thickness | 25–50 mm |
| Compaction target | 93–97% of Marshall density |
| Joint type | Hot joint, echelon paving |
Quality control checks:
- Gradation: at least one extraction test per 500 tonnes of mix.
- Binder content: at least one test per 500 tonnes.
- Air voids: minimum 3 cores per 1,000 m².
- Texture depth: sand patch test every 500 m of lane.
- Permeability: spot checks on semi-permeable variants.
Common failure modes and prevention:
- Segregation — caused by dropping mix from height or over-handling. Prevent by using paver-mounted transfer devices and limiting stockpile drops.
- Binder draindown — binder drains off coarse aggregate during transport. Prevent with cellulose or mineral fibers at 0.3–0.5% by mass.
- Moisture damage — high void content allows water ingress. Prevent with hydrated lime at 1–2% or liquid antistripping agents.
- Raveling — loss of aggregate from the surface. Prevent with adequate compaction and polymer-modified binders.
- Fat spots — localized binder-rich areas. Prevent with uniform mixing and consistent feeder rates.
Contractors should expect 10–15% higher plant control costs for gap graded mixes compared with dense graded, largely due to the tighter gradation tolerances.
Cost, Supply, and the Role of Bitumen Binder
Bitumen is the single largest variable cost in any asphalt mix, and gap graded mixes use 5–7% binder by mass — roughly 10–20% more than a comparable dense graded mix. This makes binder procurement strategy critical.
Cost breakdown for a typical gap graded mix (per tonne, Europe, 2025):
| Cost Component | Share of Total |
|---|---|
| Bitumen binder | 35–45% |
| Coarse aggregate | 20–25% |
| Fine aggregate and filler | 8–12% |
| Plant and mixing | 10–15% |
| Transport and placement | 8–12% |
Binder grades commonly used:
- Pen 50/70 — standard for temperate climates.
- PG 64-22 — North American equivalent for moderate climates.
- PG 76-10 / PMB 45/80-65 — for high-temperature, heavy-traffic applications.
- Crumb rubber modified — increasingly specified for sustainable gap graded surfaces.
Gap graded asphalt is supplied by regional asphalt plants and by international bitumen traders who bundle binder supply with mix design support. For exporters and importers, the key variable is consistent binder quality across shipments, since a 0.5% swing in binder content can shift mix performance noticeably.
Suppliers typically quote in 20–30 tonne truckloads for domestic delivery and 1,000–5,000 tonne parcels for export. Lead times run 3–10 days for domestic supply and 20–45 days for international binder shipments, depending on port and vessel availability.
FAQ: Gap Graded Asphalt
What is gap graded asphalt used for?
Gap graded asphalt is used for wearing courses on high-speed roads, airfield pavements, urban arterials, and industrial hardstands. It is chosen where high macrotexture, rutting resistance, and moderate noise reduction are needed. Typical layer thickness is 25–50 mm.
How does gap graded asphalt differ from dense graded asphalt?
Gap graded mixes contain 60–75% coarse aggregate and omit intermediate sieve sizes, while dense graded mixes have a continuous gradation with 40–55% coarse aggregate. Gap graded mixes offer better texture and rutting resistance but lower workability and tighter production tolerances.
What is the binder content of gap graded asphalt?
Binder content typically ranges from 5.0% to 7.0% by mass of total mix. Polymer-modified binders are common in heavy-traffic or high-temperature applications. Binder accounts for 35–45% of the total mix cost.
What are the disadvantages of gap graded asphalt?
The main disadvantages are segregation risk, lower workability, and higher sensitivity to moisture damage due to void content of 4–6%. Production costs are 10–15% higher than dense graded mixes because of tighter quality