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Asphalt Treated Base (ATB)
Asphalt
treated base (ATB) is a
dense-graded HMA with a wide
gradation band and lower asphalt content (2.5 - 4.5 percent by weight of
aggregate) 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).
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
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