30 June 2001
higher natural sand contents, there have been repeated problems with rutting under military aircraft.
TM 822-08/AFMAN 32-1131 V8(1)/DM 21.11 provides detailed guidance on aggregate requirements.
b. Mix Design. Mix design of asphalt concrete requires balancing durability, load resistance, and
economics. Relatively lean mixes tend to have high load resistance but suffer environmental aging more
quickly than richer mixes. Rich mixes tend to be unstable but are more resistant to environmental aging.
(1) Military Requirements. Asphalt concrete for military airfields will be designed based on the
75-blow Marshall mix design method. Details are provided in TM 822-08/AFMAN 32-1131 V8(1)/DM
21.11 and the Asphalt Institute MS-2 procedures.
(2) SHRP Mix Design. The SHRP produced an asphalt concrete mix design procedure and
recommended aggregate gradations that are being widely used by state Departments of Transportation.
These gradations and mix design procedures were developed for highway use and have not been
evaluated for airfield use. These SHRP mix design procedures and aggregate gradations are not
approved for military airfields until testing and trials demonstrate their adequacy for airfield loads and
conditions. Approval from HQUSACE (CEMP-ET), appropriate Air Force MAJCOM pavements
engineer, or Naval Facilities Engineering Service Center is needed before these new guidelines are
used on military airfields.
c. Special Asphalt Mixes. Porous friction courses are relatively thin (~ 25 to 38 mm (~1 to 1-1/2
in.)) surface layers of a special open-graded asphalt concrete with clearly visible voids. This mix
provides high skid resistance and combats aircraft hydroplaning, but its open texture allows more rapid
environmental aging of the asphalt binder and makes it very vulnerable to fuel spills. These mixes were
widely used by the Air Force in the 1970s and 1980s, but their use has declined as improved grooving of
conventional asphalt concrete mixes provides similar skid resistance without the disadvantages of the
porous friction courses. Stone mastic asphalt (SMA), sometimes also called stone matrix asphalt, has a
coarse aggregate gradation that provides stone-to-stone contact with the voids between aggregate
particles filled with a relatively rich mastic of asphalt cement, sand, and fibers. The stone-to-stone
contact of the coarse aggregate provides a stiff rut-resistant mineral skeleton, while the rich mastic
provides improved environmental resistance. Two trial applications of SMA by the Air Force for airfield
pavements in the United Kingdom and Italy have performed well to date. Thin applications of fuel
resistant sealers to asphalt concrete pavements provide limited resistance to fuel spills. The fuel-
resistant sealers economically available in the United States are usually coal tar based and are prone to
environmental induced cracking that limits their effectiveness. This cracking often occurs at early ages.
Polymer modification of some of these products has helped but not solved the cracking problem. Slurry
seals are thin applications of emulsified asphalt and sand to oxidized asphalt concrete surfaces to try to
extend the pavement life. They have problems with low skid resistance and are prone to localized
failures that generate FOD. Slurry seals are not allowed on military airfield pavements. Highly
polymerized proprietary systems known as microtexturing that use thin surface applications of a binder
and aggregate to oxidized asphalt concrete surfaces have shown promise but are still in the evaluation
stage. Rejuvenators are composed of lighter-end hydrocarbons that, when sprayed on an oxidized
asphaltic concrete surface, soften the binder and counter some of the aging effect. These materials
have given mixed results in practice and invariably lower the skid resistance of the pavement.
Consequently, they are not allowed to be used on military airfields. The military has used an open-
graded asphalt concrete mix with its voids filled with a proprietary modified hydraulic cement grout to
provide a surface more abrasion and fuel resistant than conventional asphalt concrete. This system is
referred to as resin-modified pavement, and several successful pavements have been built with this
material. HQUSACE (CEMP-ET), appropriate Air Force MAJCOM pavements engineer, or Naval