15 May 2001
reclaimed materials, the mix design should be performed on the reclaimed materials as recovered. The
maximum-size particles of the reclaimed asphalt concrete should not exceed the requirement which is
usually 3.5 centimeters (1-1/2 inches). When new aggregates are to be added, the design should be
performed on the desired mix of reclaimed aggregate and new aggregate.
d. Mix design. Development of the mix design accomplished two objectives: it determines the
amount of new binder and rejuvenator, modifier, or recycling agent required to obtain a durable and
stable mixture, and it determines the amount of moisture needed to provide maximum density.
(1) Additives. A rejuvenator, modifier, or recycling agent can be used in place of new asphalt
cement in some instances to improve the old asphalt cement properties. A thorough blending of these
materials and oxidized asphalt cement does not immediately occur when mixing the recycled cold mix.
In fact, the additive initially coats the old asphalt mixture pieces and with time, probably months, will
penetrate the old asphalt binder and produce an improved binder. During the first few months, the
recycled cold mix may be unstable because of the film of additive around the oxidized asphalt and
aggregate. After the additive penetrates the old asphalt and the binder material becomes more
homogeneous, the recycled cold mix should perform satisfactorily. Because of the initial instability and
increased costs created by additives, asphalt emulsions are usually used by themselves in recycled cold
mixes when the desired asphalt properties can be achieved.
(2) Asphalt content. A guide for the selection of the type of asphalt emulsion to be used is given
in ASTM D 3628. The amount of new asphalt binder needed in the recycled cold mix should be
determined by conducting a conventional hot-mix design on the recovered aggregate. The laboratory
density obtained in the hot-mix design is approximately equal to the maximum density that will be
obtained in the field under traffic. The amount of asphalt added should be varied by 0.5 percent
increments from 0 percent to the high side of optimum asphalt content. The samples should be
compacted by the required effort, either 50 blows for low-pressure tires or 75 blows for high-pressure
tires, and determinations should be made for density, stability, flow, voids total mixture, and voids filled
with asphalt. These determinations should be plotted and curves drawn to select the optimum asphalt
content. The optimum additional asphalt will often be between 0 and 1 percent. When the optimum
additional asphalt to be selected is 0 percent, no additional asphalt should be added since it may cause
the mixture to become unstable. When no asphalt is needed, only water should be added to lubricate
the mixture so that the needed density can be obtained in the field. There are several other design
methods in use by other state agencies and private organizations. The mix design method of the
Asphalt Institute is similar to others in that it is based mainly on aggregate properties and gradation. In
comparison to these methods, the Corps of Engineers procedure should normally require the highest
percentage of asphalt cement or the most conservative design.
(3) Compacted samples. After the optimum asphalt content has been determined, samples
should be made at the optimum asphalt content with varied water contents. These samples should then
be compacted at room temperature using the same compaction effort as that used to determine
optimum asphalt content. Next, the dry density for each of the compacted samples should be
determined, a moisture/ density curve should be plotted, and the moisture content that provides
maximum dry density should be selected as the optimum moisture content. A design example is given
in Appendix B.
e. Removal of in-place material. The material to be used in the recycled cold mix can be removed
or loosened from the in-place pavement by a number of methods. The reclaimed material for central
plant cold-mix recycling can be removed by cold milling or ripping and crushing the existing asphalt
concrete pavement. For In-place recycling the same methods can be used or the pavement can be
pulverized and mixed in place. The three more common methods are identified below.