15 May 2001
(3) Hot-mix recycling. Hot-mix recycling is a process which involves removing the existing
asphalt mixture, crushing it if necessary, and mixing it in a hot-mix plant with new aggregate, asphalt,
and recycling agent, when required. The hot-mix recycled asphalt mixtures can be designed for use in
all types of pavements. Crushed portland cement concrete has also been used as aggregate for hot
c. Potential problems. Recycling asphalt concrete pavements generally requires more time and
effort for a proper material design. Also, asphalt mixtures with highly variable material properties
become very difficult to recycle. Pavement areas containing one or more overlays with varying mix
designs make it difficult to establish baseline properties for a subsequent recycling design. When
recycling such a pavement, separate material designs are required for each pavement area containing
unique pavement materials.
2. SURFACE RECYCLING.
a. General. When a pavement is structurally sound and rehabilitation is needed to correct a surface
problem, surface recycling should be considered. The three surface recycling processes discussed in
the following paragraphs are hot in-place recycling, rejuvenating, and cold milling. Hot in-place recycling
can be divided into either single or multiple pass methods. The basic procedures for surface recycling
are shown on the flow chart in figure 7-1.
b. Equipment. The major pieces of equipment required for the various types of surface recycling
techniques include: brooms, trucks, front-end loaders, graders, asphalt distributors, self-contained
heating units, heater-scarifiers, hot-milling machines, pavers, trucks, and/or cold-milling machines.
c. Hot in-place recycling.
(1) General. Hot in-place recycling of the pavement surface prior to overlay can minimize some
existing pavement surface problems such as bonding of overlays if the existing pavement surface has
become polished over a period of years from the action of traffic. In this case, the surface can be
scarified or hot milled to promote a good bond between this existing polished surface and the overlay.
Hot in-place recycling also breaks up the existing crack pattern and reduces the amount of reflective
cracks that will appear in the overlay. The hot in-place recycling procedure is often used to maintain
secondary roads to provide a tight waterproof surface. In most instances, this process is followed by an
asphalt concrete overlay. The quality of mixes obtained by hot in-place recycling is difficult to control.
The quality of the mixture depends on the depth of recycling, time of heating, amount of rejuvenator
added, and amounts of compaction. When no overlay is applied in conjunction with the recycling
material, it is necessary that the heated material be rolled immediately to ensure that a satisfactory
density is obtained. Multiple-pass methods which treat to a total depth of about 25 millimeters (1 inch)
can cause an excess of coarse aggregate on the surface. This coarse aggregate makes the material
difficult to compact and results in a surface that tends to ravel. The multiple-pass method without an
overlay should only be used to improve the surface of mixtures on secondary roads. An overlay in
conjunction with multiple-pass method is recommended. The overlay may be added prior to compaction
of the recycled material and both layers compacted simultaneously, or the recycled material may be
compacted prior to addition of the overlay. To ensure better bond and better overall density, it is
recommended that the overlay be placed immediately after recycling the surface and the entire depth of
material compacted. Although this recycling and overlay approach has been used on airfields, generally
other alternatives should be selected. The asphalt sampled from the recycled and rejuvenated material
should show an improvement in properties over the existing asphalt properties. The desired properties
of the recovered asphalt binder will depend upon the local climatic conditions and the existing properties
of the asphalt binder. Generally, the penetration and viscosity values should range from 30 to 70 and
1,000 to 3,000 poises, respectively. The amount of binder material added should not cause the voids in