30 June 2001
Facilities Engineering Service Center should be consulted for up-to-date guidance on these and other
specialty asphalt mixes.
(1) Aging and Oxidation. Asphalt oxidizes and stiffens over time which leads to a loss of
cohesion and flexibility. This eventually leads to cracking and raveling. Asphalt cements from different
sources oxidize and age differently. Research suggests that additives to the asphalt cement may slow
oxidation, but firm conclusions and guidance are not available yet.
(2) Cold Weather Cracking. As the temperature drops, asphalt cement becomes stiffer and
more brittle. With repeated exposure to cold temperatures and in conjunction with other stiffening and
aging mechanisms, the asphalt concrete will develop cracking. The SHRP PG grading system of rating
asphalt binders that has been adopted by the military specifically tries to select binder characteristics to
resist this cracking based on the exposure at the project location.
(3) Fuel Spillage. Fuels, oils, hydraulic fluids, and similar liquids are solvents for the asphalt
binder. Hence asphalt concrete should not be used where it will be exposed to such materials. Resin-
modified pavement may be used as a surfacing over conventional asphalt concrete to obtain fuel
resistance. Coal-tar based fuel resistant sealers have only a temporary life expectancy before cracking
reduces their effectiveness.
(4) Stripping. Several mechanisms contribute to moisture damage to asphalt concrete and are
generally referred to as stripping. These mechanisms include displacement of the asphalt film coating
the aggregate by water, emulsion of the asphalt cement, and pore pressure development. Stripping
seems to require water, stripping susceptible aggregates (e.g., siliceous aggregates), and repeated
loads. Lime and proprietary liquid antistrip agents can combat the problem. Also, proper aggregate
selection, and drainage to reduce the asphalt concrete's exposure to water can help mitigate the
dangers of stripping. Fortunately, stripping seems to be relatively uncommon in military airfield
pavements. Stripping potential and the need for countermeasures should be addressed in the mix
e. Construction. Production and placement of high-quality asphaltic concrete suitable for military
airfields is a demanding and skillful operation. Proper mixing and delivery of the asphaltic concrete,
proper placement procedures that prevent segregation, skillful construction of the longitudinal joints, and
compaction with equipment of adequate size and at appropriate temperatures are all required to achieve
a suitable final product.
5. RECYCLED MATERIALS. Today, portland-cement concrete and asphaltic concrete are routinely
recycled as aggregate for subbase and base course material, drainage layers, fill, and as aggregate in
new asphaltic and portland-cement concrete. In all recycling operations, maintaining consistency in the
recycled product is a challenge. If the recycled product all comes from a single project with consistent
properties and constituents, the recycled product will probably have consistent properties and can be
incorporated into construction without difficulty. If recycled materials from different projects are
intermingled, the recycled product properties are likely to be highly variable, and meeting stringent
airfield pavement material requirements with such mixed-source materials is highly problematic.
Including debris from building demolition in the recycled product to be used in the airfield pavement
structure is not allowed as contamination with undesirable material such as brick or gypsum board is
likely and the recycled material from such sources tends to be highly variable. Recently, major problems
developed on a project that used recycled portland-cement concrete as fill and as base course in an