15 May 2001
truck. The grader mixer by working the materials back and forth and the action of the blade blends the
materials together. Depending on conditions the water may be mixed prior to the addition of the asphalt.
After the required mixing and aeration have occurred the grader can be used to grade and level the
mixture prior to compaction. Rotary mixers are usually self propelled and have a chamber that
contains 1 or 2 transverse shafts with cutting blades.
(2) Placement. The recycled mixtures produced at central plants or by travel plant mixers are
usually placed by a paver to achieve a more uniform appearance, structure, and density. Mixtures
produced by graders and rotary mixers are normally windrowed and placed with graders. Widowing
prior to placement allows for aeration, when it is required. Moisture from emulsified asphalts and water
used for compaction must be allowed to escape. Therefore the recycled mixtures should be placed in
uncompacted layers of at least 50 millimeters (2 inches) and not more than 100 millimeters (4 inches).
The upper limit may be increased to 150 millimeters (6 inches) if an open graded mixture is used.
(3) Curing. Cold-mix recycled mixtures gain strength and stability with time, as they cure.
Factors that effect the cure time are; type of emulsified asphalt, mix water content, gradation, ambient
and mixture temperatures, wind velocity, and humidity. When multiple lifts are required, the cure time
between them should be from 2 to 5 days depending on the amount of moisture and porosity of the
(4) Compaction. Compaction should begin as soon as the emulsified asphalt has broken or
when the mixture has been properly aerated. Compaction is usually accomplished by either or a
combination of both rubber-tire, steel-drum and vibratory rollers. The type and size used will depend on
the type of mixture and the thickness that it has been placed. Since it is difficult to establish a laboratory
density for comparison with field density, the theoretical maximum density (TMD) should be used to
establish field density requirements. The theoretical maximum density is that density at which there
would be zero air voids in the mixture. At least 86 percent of the theoretical maximum density should be
obtained in the field to ensure that the voids in the field mixture are not excessive.
(5) Field density measurements. The field density should be determined from cores removed
from the in-place pavement. With some mixes it will be difficult to obtain undamaged cores prior to
curing the cold mix for a few days. When cores cannot be obtained within 24 hours of paving, other
methods of obtaining samples should be considered. For example, ice placed on the sample locations
for 1 to 2 hours prior to coring samples will cool the material and reduce damage caused by heat
developed during the coring operation. Another approach is to use a concrete saw to cut small cubes
from the pavement. Nuclear density gages can be used to obtain an indication of density, but actual
samples should be taken to determine acceptability for the density of the in-place mixture.
4. RECYCLED HOT-MIX ASPHALT.
a. General. Recycled hot-mix asphalt should be considered as an alternative anytime pavement
reconstruction is planned. Recycling will create a greater savings in material cost and total job cost than
the use of new hot-mix that for an asphalt for reconstruction projects. The performance of recycled hot
mix should be considered near equivalent to that expected with conventional hot mix. Recycled hot mix
can be used to construct asphalt base, intermediate, and surface courses (Surface courses require prior
approval.). Figure 7-3 provides an outline of the hot-recycling process.
b. Equipment. The equipment required for pavement removal and crushing will include either
conventional equipment for ripping and crushing or a cold-milling machine. A batch or drum plant, either
designed or modified to mix recycled materials is also required. Placement is with a conventional paver,
and trucks, front-end loader, and asphalt distributor are also required.