Asphalt treated base (ATB) is a dense-graded HMA with a wide gradation band and lower asphalt content intended for use as a base course. ATB costs less than typical HMA mixes because it can be produced with less expensive aggregates and lower percentages of asphalt binder. In addition to the site paving benefits, ATB can be advantageous because it can provide:
- A waterproof barrier to prevent fines infiltration into the subgrade and pavement structure. If water accumulates in the subgrade, the repetition of pavement loading can cause subgrade fines to migrate into the base and pavement structure. This can clog the base layer, which impedes drainage and create voids in the subgrade into which the pavement may settle.
- An alternative to untreated base material. Structurally, ATB is about three times as strong as an untreated granular base (such as crushed surface base or top course). Therefore, it is possible to use thinner layers for the same structural support, which can save on excavation costs. In some cases a layer of aggregate base is still needed to provide material to fine grade and to provide a smooth surface on which to pave.
The costs savings of using ATB can add up quickly. On a site that must export material (excess cut), an ATB pavement design can save a considerable amount of excavation, hauling and disposal costs. On a site that must import material (excess fill), ATB can be used to build the pavement over more marginal subgrades (i.e. a structure of gravel borrow and ATB can replace thicker crushed aggregate sections).
The most current ATB mix design specifications are maintained by the American Public Works Association of Washington (APWA-WA) as a Local Agency General Special Provision (GSP). GSPs 4-SA1 and 9-03.6 are used together to define the current ATB specification. These GSPs can be found on the Washington State Department of Transportation Local Agency GSP webpage at:
Important Considerations when Substituting ATB for Crushed Aggregate
- The minimum recommended crushed aggregate base thickness is 4 inches.
- The minimum recommended ATB thickness is about 3 inches. ATB gradation and nominal maximum aggregate size specifications are quite loose, however pavement layers thinner than about 2-3 times the nominal maximum aggregate size may be difficult to compact, tear under the screed, and rollers may crush the larger particles during compaction.
- Consider the original purpose of the crushed aggregate. Sometimes aggregate base is needed to (1) provide material to fine grade and to provide a smooth surface on which to pave or (2) provide frost protection. In these situations, ATB should not be substituted for crushed aggregate base.
- Consider the characteristics of the particular ATB being used. ATB specifications are quite broad and allow for a wide choice of gradation and aggregate quality. For instance, the nominal maximum aggregate size can be anywhere from about 1.5 inches down to 0.375 inches; the gradation can either be fine or coarse; and the aggregate can either be crushed or not crushed. In general, do not assume anything more than what is specified.
Other ATB Information
- Compacted Density ≈ 1.85 tons/yd3
- Layer coefficient (“a”) for use in the 1993 AASHTO Empirical Structural Design ≈ 0.35