15 May 2001
(3) Multiple-pass method.
(a) General. The multiple-pass recycling technique applies the recycled material and the
wearing surface in separate operations. The recycled material is used only as an intermediate or
leveling course whereas a new asphalt mixture is used as the wearing course. The multiple-pass
method usually involves less complicated equipment. The original heater-planer equipment fits into this
type of method. The multiple-pass method involves the use of equipment that heats the pavement,
scarifies or mills the heated asphalt material, adds rejuvenator as required, and places the recycled
asphalt material. This normally involves several separate pieces of equipment including heaters,
sacrificers, hot milling machines, and pavers. The recycled material is placed and compacted before
cooling and is used as an intermediate or leveling course. A hot-mix asphalt wearing surface is later
applied to the recycled surface within a predetermined period of time. The asphalt surfacing is required
to protect the recycled surface from the effects of traffic.
(b) Heating. The heating methods and procedures used are the same as those used for
single-pass recycling method.
(c) Material displacement. The material can be removed with either scarifying teeth or a
mill. The majority of the multiple pass methods use scarifying teeth to loosen or displace the pavement
surface. The scarifying teeth are usually located at the end of the heating vehicles. In most methods,
the second vehicle would not only heat the pavement but also scarify, level, and screed the pavement.
The scarifying teeth are normally spring-loaded and "give" when a solid obstacle is encountered. These
springs also help prevent any fracturing of the aggregate during scarification. When a preheating unit is
used, the scarifying teeth can be set to penetrate up to 30 millimeters (1-1/4 inch) to obtain an effective
overall depth of 25 millimeters (1 inch). The depth of scarification required will control the speed of the
(d) Recycled mixture. As with single-pass methods, the rejuvenator is added somewhere
during the scarifying, mixing, or leveling (screening) processes. The mixing in this method is usually
accomplished by auguring/mixing drums located behind the scarifying teeth.
(e) Placement. The recycled mix is usually leveled and placed by auger drums and a
screed. The recycled mix is then compacted with rollers to the required density. This recycling method
cannot normally change grades; however, it can smooth some bumps, depressions, and minor ruts.
d. Rejuvenating. The application of a chemical rejuvenator provides the penetration of chemicals
into the asphalt pavement which act to soften the binder. Rejuvenators can be based on either asphalt
or coal-tar binder materials. Rejuvenators can be sprayed directly on the surface of an asphalt
pavement or they can be used in conjunction with many types of recycling processes.
(1) Properties of rejuvenators. Some of the properties of rejuvenators are described below.
(a) The application of a rejuvenator to an asphalt pavement partially restores its original
asphalt properties. However, for a rejuvenator to be successful, it must penetrate the pavement surface
and soften the asphalt binder. Most rejuvenators penetrate 5 millimeter (1/4 inch) or more into the
surface of a pavement, depending on the type of rejuvenator, the weather conditions, and the
permeability of the pavement surface.
(b) When a rejuvenator is applied to a pavement in which the asphalt binder is oxidized, it
will retard the loss of surface fines and reduce the formation of additional cracks. Application of a
rejuvenator will also tend to reduce the skid resistance of the pavement, in some cases, for up to 1 year.
While this reduction in skid resistance should not be a significant factor for parking aprons and taxiways,