1 Mar 98
well and are expected to provide a service life equal to conventional hot-mix asphalt concrete
8. Construction Considerations.
a. Contaminated soils considered for use in ASB may contain cutback or emulsified asphalt,
which may require additional considerations in the construction process. Cutback asphalts are no
longer widely used in pavement construction due mainly to environmental concerns. Some
contaminated soils, depending on the type and level of contaminant, can contain hydrocarbon
levels similar to those found when using a cutback asphalt cement (Dineen 1991).
b. If ASB mixtures contain emulsified or cutback asphalt cements, they will need time to
cure. This cure takes place as the water or solvent that was combined with the asphalt cement
evaporates and only the asphalt cement binder remains. Generally, a 100 mm (4 in.) layer of ASB
should be allowed to cure for at least 5 to 7 days depending upon weather conditions. The
mixtures will cure faster in warmer, drier climatic conditions. The manual TM 5-822-8 provides
guidance on cold-mix and stabilized materials.
c. The use of a test section prior to starting full production is recommended to verify the
acceptability of the mix design and the construction procedures. A test section can be used to
judge the ability of the construction procedures to satisfactorily mix and place the material.
Usually, a percentage (normally 86 percent) of the theoretical maximum specific gravity is the
amount of compaction required. The test section allows for verification of the compaction
equipment and roller patterns. Obtaining samples for density verification can be difficult
depending on the characteristics of the ASB material. Normally, asphalt concrete pavements are
cored to obtain samples for density determination. However, the ASB may break apart under
coring or subsequent handling. Other options that can be used are to saw cut a square or
rectangular section, or a nuclear density gage calibrated with sand cone or water balloon densities
may be utilized. When placing the mixture with a paver, the longitudinal joints should be
constructed as a hot joint would be in hot-mix asphalt placement. This should increase joint
density and increase the durability of the ASB mixture. Where this type of construction can not
be accomplished, cutting back the compacted edge to a width at least equal to the depth of the
layer, prior to placing the adjoining material should increase the density and durability of the joint.
d. Placement of a contaminated soil within an ASB will require a high degree of mixture
control and uniformity. To achieve this the ASB should be mixed in a stationary pugmill mixer.
These mixers allow various materials to be mixed, usually by weight, and allow for close
gradation control and for accurate metering of asphalt and water into the mixture.