Every asphalt pavement is built in layers, and each layer has a job. The surface course takes tires, water and sun. The layers under it spread the load down to the base. The mix in each lift is picked for that job, and the number on the plant ticket tells you most of what you need to know.
State DOTs name their mixes differently, and your plant's names may not match anyone else's. Ask the plant for the mix design sheet on anything you price regularly. It lists the aggregate size, the binder grade and the design density.
What the number means
The 9.5, 12.5, 19 or 25 mm on a mix is its nominal maximum aggregate size. Roughly, it's one sieve size larger than the first sieve that holds back more than 10% of the stone. You don't need the definition on a lot. What matters is:
- Smaller stone gives a tighter, smoother surface that sheds water and looks finished.
- Bigger stone gives a stiffer layer that carries load and costs less per ton, with a coarser texture.
The size also sets how thin you can lay it. A common rule of thumb is a compacted lift at least three times the nominal maximum aggregate size, and some specs ask for four times on coarse mixes. Lay a 19 mm mix at an inch and the stone drags and won't compact.
| Mix | Typical use | Minimum lift, 3× rule |
|---|---|---|
| 9.5 mm | Thin surface lifts, driveways, patches, overlays | About 1.1 in |
| 12.5 mm | Surface course on lots and driveways | About 1.5 in |
| 19 mm | Binder or intermediate course | About 2.25 in |
| 25 mm | Base course | About 3 in |
Surface mixes: 9.5 and 12.5 mm
The 12.5 mm surface mix is the everyday top course on parking lots and driveways in much of the country, usually laid at 1.5 to 2 inches. Where the plant runs a 9.5 mm mix, it's handy for thin overlays and hand work, and it gives a tighter finish that some residential customers prefer.
The binder is listed as a performance grade, like PG 64-22. The first number is the high pavement temperature it's graded for and the second is the low, in Celsius. Your plant and state spec pick the grade for your climate. A polymer-modified grade with a higher first number resists rutting where trucks stop and turn, like a loading dock or a drive-through lane.
Binder and base mixes: 19 and 25 mm
A 19 mm binder course goes under the surface on heavier work: commercial lots with truck traffic, entrances and drive aisles. It's stiffer than the surface mix and cheaper per ton.
A 25 mm base mix (or larger, in some states) is the structural bottom lift of a thick asphalt section. It's coarse, and it isn't meant to be the surface. Left open to traffic, it ravels and lets water in.
Plants often put more reclaimed asphalt (RAP) in binder and base mixes than in surface mixes. That's normal, and it's one reason those mixes cost less.
Picking the section for the job
The section is a design question, and on commercial work it's usually in the engineer's plans. When it isn't, these are common starting points, not a design:
- Residential driveway: a compacted aggregate base, then 2 to 3 inches of surface mix in one or two lifts.
- Light commercial lot, cars only: aggregate base, then a binder course and a surface course, often around 3 to 4 inches of asphalt total.
- Truck routes, dumpster pads and entrances: a thicker section than the rest of the lot. Heavy loads in small areas are where lots fail first.
What sits under the asphalt matters as much as the mix. Soft subgrade under a good section still fails.
Before you price the job, call the plant and confirm they're running the mix you're specifying, and when. Not every plant runs every mix every day, and a substitution changes the density you used to figure your tons.
Related articles
- Asphalt tonnage calculator: tons from square feet and depth
- How to price an asphalt overlay: tons, crew days and the margin you need
- Asphalt vs. concrete driveways: walking a homeowner through the cost
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