15 May 2001
the compacted mixture to become overfilled and thus create an unstable mixture. To determine when a
mixture is unstable, samples of the mixture should be obtained and compacted at 121 degrees C
(250 degrees F) using the standard compaction effort for the job. When the voids of the compacted
samples are less than 3 percent, the mixture should be considered unstable, and the amount of
rejuvenator should be reduced. Pollution caused by smoke from the heating of the asphalt surface may
be a problem. But the amount of smoke can usually be controlled within an acceptable range on most
asphalt mixtures. Heating of pavements that have numerous sealed cracks may present a problem
since the sealer material usually causes an increase in the amount of smoke during the heating
(2) Single-pass method.
(a) General. Single-pass recycling methods were first developed in Europe and have
recently started to gain wide usage in the United States. The single-pass method will involve one of two
procedures outlined in table 7-3. The first procedure, called remix, involves the combining of a new
asphalt mixture with the reclaimed (scarified or hot-milled) asphalt mixture prior to placement on the
pavement surface. The second procedure, called repave, involves overlaying the recycling material with
new asphalt mix prior to rolling or cooling of the recycled material. The single-pass method usually
involves several pieces of equipment, including heaters, sacrificers, hot milling heads, spray bars, or
pavers in various combinations or as separate pieces of equipment. The single-pass recycling method
usually involves larger and more complex pieces of equipment than are used in the multiple-pass
(b) Heating. Prior to displacing or loosening the pavement, the pavement surface is heated.
The heating units are generally a refractory type and typically use liquid propane as the fuel to heat the
inside of the heater unit. In this way there is never a direct flame placed on the pavement surface.
Despite this lack of contact between flame and asphalt, it is not unusual for the surface asphalt to be
burned. The severity of this problem is increased by excess asphalt or oils on the pavement surface.
The amount of heat applied to the pavement is controlled by the amount of propane burned, the height
or distance of the heater from the pavement, and the speed of the heater traveling over the pavement
surface. Many methods employ two heating units for increased productivity. The first heating unit
preheats the pavement; the second machine follows closely to provide additional heat to bring the
pavement to the desired temperature. The heating vehicles can be either towed or self propelled.
(c) Material displacement. The single-pass methods all use hot-milling to remove
approximately 25 to 40 millimeters (1 to 1-1/2 inches) of the existing asphalt concrete pavement with one
pass. For greater depth of recycling two separate machines each with hot-milling heads are used.
These machines usually contain pavement heating elements, tanks and spray apparatus for adding
rejuvenator, or possibly new asphalt cement, some sort of mixing screws or a pugmill for mixing
conveyors, and storage apparatus for new asphalt concrete, and a paving screed at the end in addition
to the hot milling drum.
(d) Recycled mixture. The rejuvenator or new asphalt cement is usually either added during
milling or during the succeeding mixing operation. The mixing can be accomplished by a series of
augers perpendicular to the direction of travel or by a traveling pugmill. The new asphalt concrete, if it is
required, is usually added to the recycled mix during this mixing. Some equipment can apply new
asphalt concrete on top of the recycled mixture.
(e) Placement. The recycled mixtures are usually placed with a screed, often attached at
the end of a recycling train. After placement, the mix is compacted with rollers to achieve the desired
density. Single-pass methods can provide a relatively small amount of corrections from the existing