Asphalt
Stone Mastic Asphalt (SMA) vs Dense-Graded Asphalt: Bitumen Requirements Compared
SMA costs more per tonne and needs more bitumen — but on the roads it’s designed for, it earns that cost back in service life.
Stone Mastic Asphalt (also called Stone Matrix Asphalt, both abbreviated SMA) and conventional dense-graded asphalt look similar once laid, but their internal structure and behavior under load are meaningfully different — and that difference shows up directly in bitumen content.
Gradation is the fundamental difference
Dense-graded asphalt uses a continuous, well-graded blend of aggregate sizes, from coarse down to fine, that interlocks to form the load-bearing skeleton, with bitumen filling the remaining voids. SMA uses a gap-graded blend — mostly coarse aggregate that forms a stone-on-stone contact skeleton, with a rich "mastic" of bitumen, filler, and fine sand filling the gaps between the coarse stones, plus a stabilizing additive (typically cellulose or mineral fiber) to prevent that rich mastic from draining out of the mix during transport and laying.
| Property | Dense-Graded Asphalt | Stone Mastic Asphalt (SMA) |
|---|---|---|
| Aggregate gradation | Continuous/well-graded | Gap-graded, coarse-dominant |
| Typical bitumen content | 5.0–5.5% | 6.5–7.5% |
| Load-bearing mechanism | Aggregate interlock + binder | Stone-on-stone contact + rich mastic in voids |
| Rutting resistance | Good | Excellent — stone skeleton resists deformation |
| Durability / cracking resistance | Good | Excellent — rich mastic resists fatigue and low-temp cracking |
| Relative cost per tonne | Baseline | 15–30% higher, driven by bitumen content and fiber additive |
| Typical use | General roads, most traffic levels | High-traffic highways, intersections, bus lanes, airports |
Why SMA needs a stabilizing additive and dense-graded doesn’t
Because SMA’s bitumen-rich mastic sits in a gap-graded structure with relatively large void spaces between coarse stones before mastic fills them, that mastic is prone to draining downward during hot storage, transport, and laying — a defect called "binder drain-down." Fiber (usually cellulose) is added specifically to increase the mastic’s viscosity enough to prevent this, which is why SMA mix designs include a fiber percentage that dense-graded mixes typically don’t need.
When the extra cost is worth it
SMA earns its higher material and bitumen cost on high-stress applications: intersections with heavy braking/turning loads, bus and truck lanes, and airport pavements, where its superior rutting and cracking resistance extends service life enough to offset the upfront premium. On a low-traffic residential road, that premium usually isn’t justified.
What this means for your bitumen calculation
If you’re comparing quotes or planning a mix change from dense-graded to SMA, don’t just adjust the bitumen content percentage in isolation — SMA mix designs also typically use a slightly different target density and often a polymer-modified binder (which may have a different density than standard penetration-grade bitumen), so update both figures together for an accurate quantity estimate.
Model both mix types side by side using the calculator