Asphalt Treated Base (ATB)

Asphalt treated base (ATB) is a dense-graded (but more permeable) HMA with a wide gradation band and lower asphalt content intended for use as a stabilizing base course and intended to be used in conjunction with a top/ wear course of street/ highway grade HMA (typically 1/2 inch or 3/8 inch HMA as currently defined by Washington state DOT specifications).  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 water resistant 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 rock).  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:

http://www.wsdot.wa.gov/Partners/APWA/

Important Considerations when Substituting ATB for Crushed Aggregate

• The minimum recommended crushed aggregate base thickness is 4 inches.  Placing the crushed rock less thick may be difficult of achieve with many subgrade materials.
• 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.
• If marginal construction conditions are anticipated, ATB can 1.) partially protect the grade from wet weather 2.) serve as a firm, dry staging area for building materials 3.) help identify weak spots in the subgrade prior to final paving and 4.) strengthen the final pavement structure.

Owner’s Note:  ATB is not designed to be a permanent surface (wearing) course.  A dense graded hot mix asphalt pavement needs to be placed over the ATB as the final step in constructing the permanent pavement.  The dense graded HMA will seal the pavement structure, protecting the final paved area from saturated subgrade related moisture damage.

When using ATB as an interim surface for staged construction, it is normal to experience scuffing of the ATB surface and/or localized spalling, especially in high traffic areas or areas with slow speed turning construction traffic (power steering on heavy vehicles).  As ATB is also often placed to protect the subgrade during the construction process (heavy wheel loads, tracked equipment etc.) and during wet/ cold seasonal weather, surface or even localized structural damage to the ATB is not uncommon.  The advantage is that poor subgrade areas that are moisture sensitive may be identified by the ATB damage.  The ATB & subgrade can be then be repaired prior to final paving, ensuring a long-lived final pavement.

Other ATB Information

• Compacted Density ≈ 1.85 tons/yd³

• Layer coefficient (“a”) for use in the 1993 AASHTO Empirical Structural Design ≈ 0.35.  This compares to a crushed rock layer coefficient of 0.13 and a “highway grade” dense grade HMA layer coefficient of 0.50. *

* WSDOT Pavement Policy Guide (Sept. 2018 edition), Table 5.2 – HMA Pavement Assumptions