ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 384

environmental problems. Therefore, the RAP materials are added to the
pugmill (mixer) in the batch plant or added at midlength in the drum at the
drum mix plant. The amount of recycled materials varies from 20% to 70%.
The gradation of new aggregates is selected to correct any deficiency in the
gradation of recycled aggregate. Typically, the grade of new asphalt cement
is soft so that, when it is mixed with the old, hard asphalt, an appropriate
consistency will result. The mix design of asphalt concrete, including re-
cycled materials, is usually performed using either the Marshall or Hveem
procedure.
In addition to hot-mix central plant recycling, cold central plant recy-
cling can use new emulsified or cutback asphalt. However, the cold process
will not have the quality of the hot-mixed material.

■ 9.12.3 In-Place Recycling

In-place recycling is performed by ripping and pulverizing the old pavement
surface and adding new aggregate, water, and asphalt emulsion. The old and
new materials are mixed together in place, graded, and compacted. The sur-
face is left to cure and is then used as a surface layer for low-volume roads.
The recycled layer can also be used as a stabilized base, covered by an as-
phalt concrete surface.

9.13 Additives

Many types of additives (modifiers) are used to improve the properties of
asphalt or to add special properties to the asphalt concrete mixtures (Roberts
et al. 1996). Laboratory tests are usually performed and field performance is
observed in order to evaluate the effect of the additives and to justify their
cost. The effects of using additives should be carefully evaluated; otherwise
premature pavement failure might result. The recyclability of modified as-
phalt mixtures is still being evaluated.

■ 9.13.1 Fillers

Several types of fillers, such as crushed fines, portland cement, lime, fly ash,
and carbon black can be added to asphalt concrete. Fillers are used to satisfy
gradation requirements of materials passing the 0.075 mm (No. 200) sieve,
to increase stability, to improve bond between aggregates and asphalt, or to
fill the voids and thus reduce the required asphalt.

■

9.13.2 Extenders

Extenders such as sulfur and lignin are used to reduce the asphalt require-
ments, thus reducing the cost.