TM 5-822-10/AFM 88-6, Chap. 6
4-6.
Mix 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
exceed 60 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. The selection and evaluation criteria for the new and old aggregate are the same as those for new hot mixes.
a. 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 is determined, as discussed in paragraph 4-6. 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.
b. 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 percent and the amount of reclaimed asphalt concrete used in the recycled mixture is below 50 percent,
the existing asphalt binder can usually be modified with an asphalt cement such as AC-2.5 (ASTM D3381). In this case,
no recycling agent would be needed. When the amount of reclaimed asphalt concrete used in the mixture exceeds 50
percent, or when the penetration of the existing asphalt binder is less than 10 percent, a recycling agent is generally
needed. For many jobs it will be necessary to use an asphalt cement and a recycling agent to properly modify the existing
asphalt at optimum asphalt content.
c. Preparation. The third step consists of preparing recycled mixtures at various asphalt contents with 0, 0.5, and
1.0 percent recycling agent, if a recycling agent is being used. The following data should be plotted for each recycling
agent 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 mixture is to be used. The
amount of recycling agent should be selected so that the recovered asphalt penetration meets the desired limits. It is impor-
tant that the penetration of the recovered asphalt be measured during plant production and that adjustments be made if
necessary to ensure proper asphalt consistency. Paragraph B-2 gives a design example of a hot-mix design for a recycled
asphalt concrete pavement.
4-7.
Recycling hot-mix quality control.
Most recycled asphalt concrete is produced with a drum mixer designed or modified to produce recycled mixtures.
Modified batch plants have also been used successfully to produce recycled hot mix.
a. Drum mixer. when a drum mixer is used for recycling, the new aggregate is added at the high side of the drum
near the flame (fig 4-2). The aggregate absorbs much of the heat from the burner and acts as a shield to protect the
reclaimed asphalt concrete, new asphalt binder, and recycling agent. The reclaimed asphalt concrete is added to the drum
near the midpoint followed by the recycling agent and new asphalt. The flights inside the drum should be in good condition
4-3
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