UFC 3-250-03
15 May 2001
content of the existing pavement mixture is generally near the optimum asphalt content; hence, the
addition of more asphalt cement or recycling agent may result in an excessive asphalt content. If the
existing asphalt binder is not modified with a low viscosity asphalt or recycling agent, a brittle mixture
with a short service life will be produced.
f. Mixture design. The mix design is conducted to determine the percentages of reclaimed asphalt
mixture, each new aggregate, recycling agent, and asphalt cement to be used in the mixture. The
amount of reclaimed mixture used in a recycled mixture is usually based on the amount of reclaimed
materials available, the desired physical properties of the recycled mix, requirements of the aggregate
gradation, economical considerations, and the type of asphalt plant. A drum mixer can prepare recycled
asphalt mixtures using up to a maximum of 70 percent reclaimed mixture. However, in order to ensure
that the quality of the mix is controlled, the amount of reclaimed asphalt concrete used in the production
of recycled hot mix should not normally exceed approximately 40 percent. When a modified batch plant
is used to produce the recycled mixture, the maximum amount of reclaimed materials that can be added
to the mixture generally varies between 50 and 60 percent because at least 40 to 50 percent new
superheated aggregate is needed to obtain sufficient heat transfer to the reclaimed asphalt pavement
material. In general practice the amount of reclaimed asphalt pavement used ranges from 10 to
40 percent of the total weight of the recycled mixture. The selection and evaluation criteria for the new
and old aggregate are the same as those for new hot mixes.
(1) Percentage of aggregate. The first step in the mixture design is to determine the percentage
of each new aggregate and reclaimed asphalt concrete that should be used. The amount of reclaimed
asphalt concrete that can be practically recycled was discussed in the preceding paragraph. The
gradation of the aggregate extracted from the reclaimed asphalt and the gradations of the new
aggregates are then determined. The percentage of each aggregate to be used in the recycled mixture
is then selected so that the blended gradation of all aggregates used, including the aggregate in the
reclaimed asphalt concrete, meets the specification requirements.
(2) Type of binder. The second step is to determine the type of binder or recycling agent to be
used in the mixture. A recycling agent is usually required to modify the oxidized asphalt binder. When
the penetration of the old asphalt binder is more than 10 and the amount of reclaimed asphalt concrete
used in the recycled mixture is below 40 percent, the existing asphalt binder can usually be modified
with an asphalt cement such as an AC-2.5 (ASTM D 3381). In this case, no recycling agent would be
needed. When the amount of reclaimed asphalt concrete used in the mixture exceeds 40 percent, or
when the penetration of the existing asphalt binder is less than 10, a recycling agent is generally
needed. The use of a recycling agent is not normally recommended for hot-mix recycling.
(3) Preparation. The third mix design step consists of preparing recycling mixtures at various
asphalt contents with 0, 0.5, and 1.0 percent or more of new asphalt cement. The following data should
be plotted for each asphalt content being evaluated: (1) density versus additional asphalt content,
(2) stability versus additional asphalt content, (3) flow versus additional asphalt content, (4) voids in the
total mix versus additional asphalt content, and (5) voids filled with asphalt versus additional asphalt
content. These graphs, with the exception of stability, take the same shape as those developed when
conducting a mix design for conventional hot-mix asphalt concrete. The plot of stability versus additional
asphalt content generally indicates the highest stability at 0 percent additional asphalt and a reduction in
stability as the asphalt content is increased. The optimum asphalt content should be determined by
averaging the asphalt contents at the peak of the density curve, middle of the voids in the total mixture
requirements, and middle of the voids filled with asphalt requirements. The requirements for voids in the
total mix, voids filled with asphalt, stability, and flow are the same as those for conventional hot-mix
asphalt concrete. Mixtures at optimum asphalt content for each recycling agent content should be
prepared and the asphalt recovered from these mixtures. The penetration of the recovered asphalt
should be a minimum of 60 percent of the desired original asphalt penetration for the area in which the
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