Recycling Background

HMA is one of the most recycled products in the U.S.  It is estimated that as much as 100 million tons of HMA are milled off roads during resurfacing and widening projects each year (Video 1).  Of this amount, 80 million tons are recycled as "reclaimed asphalt pavement" (RAP) (APA, 2001).  RAP (Figure 1 and Figure 2) is typically generated by rehabilitation or reconstruction projects and can be used in a variety of ways such as:

• As an addition to regular HMA.
• As an aggregate in cold-mix asphalt.
• As a granular base course when pulverized.
• As a fill or embankment material.

This section will discuss RAP as a constituent in regular HMA, which is its most common use.

RAP Used in HMA

Basically, new HMA is produced in a plant and a predetermined percentage of RAP is added.  RAP addition, when done, is typically 10 to 30 percent by weight although additions as high as 80 percent by weight have been done and additions as high as 90 to 100 percent by weight are feasible (FHWA, 2001).  There is ample evidence that HMA which incorporates RAP performs as well as HMA without RAP.  Figure 3 shows two dense-graded HMA cores, one with RAP and one without. Some typical precautions when using RAP are:

• When heated, RAP may produce emissions in the form of gaseous hydrocarbons.  To minimize these, HMA plants generally heat RAP indirectly (usually it is added after the aggregate is heated and thus heats up through contact with the already-hot aggregate).
• RAP addition may require longer HMA plant heating times.  This can sometimes reduce plant output by as much as half.  
• RAP generally contains between 3 and 7 percent asphalt binder by weight or about 10 to 20 percent asphalt binder by volume (FHWA, 2001).  In general, RAP will be more viscous (stiffer) than new HMA because of asphalt binder aging.  Therefore, if enough RAP is added, a less viscous (softer) asphalt binder should be used. 
• In general, state DOTs allow more RAP in base and binder HMA courses than they do in surface courses.
• After milling or crushing, RAP gradation is generally finer than pure virgin aggregate because of the degradation that occurs during removal and processing.