HMA is one of the most recycled products in the U.S. As much as 100 million tons of HMA are milled off roads during resurfacing and widening projects each year. Of this amount, 80 million tons (80 percent) are recycled as “reclaimed asphalt pavement” (RAP) (APA, 2001). Additionally, in-place recycling techniques can also be used to resurface an existing pavement or pulverize an existing pavement for use as base material.
Reclaimed Asphalt Pavement (RAP)
RAP is essentially old pavement that is reclaimed for use. In its most common form, it is collected in loose granular form as a byproduct of pavement rehabilitation or reconstruction (see Figures 1 and 2). RAP 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.
RAP as a Constituent in HMA
The most common RAP use is as a constituent in HMA. Basically, new HMA is produced at a batch or drum plant to which a predetermined percentage of RAP is added. There is ample evidence that HMA which incorporates RAP performs as well as HMA without RAP. The benefits of RAP use are two-fold:
- The RAP aggregate can be used in place of a portion of the virgin aggregate, which lowers cost and reduces waste.
- The RAP asphalt binder is reheated and used in place of a portion of the virgin asphalt binder, which lowers cost and reduces waste.
Figure 3 shows two dense-graded HMA cores, one with RAP and one without. If used, RAP is most commonly added at 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). For mix design purposes, Superpave, Marshall or Hveem procedures can all be used. Blending charts are typically needed when using high percentages of RAP.
Other general considerations when using RAP are:
- When heated, RAP may give off gaseous hydrocarbons. To minimize these emissions, HMA plants generally heat RAP indirectly (usually it is added after the aggregate is heated and thus it heats up through contact with the already-hot aggregate).
- RAP is typically added cold and thus may require longer HMA plant heating times. This can sometimes reduce plant output by as much as half. This can be overcome by preheating RAP, but the added energy, equipment and emissions concerns often make preheating undesirable.
- RAP usually contains between 3 and 7 percent asphalt by weight or about 10 to 20 percent asphalt by volume (FHWA, 2001). In general, the asphalt binder in RAP will be more viscous than virgin asphalt binder due to aging effects. Therefore, if enough RAP is added, a softer virgin asphalt binder should be used to counteract the more viscous RAP asphalt binder.
- After milling or crushing, RAP gradation is generally finer than pure virgin aggregate because of the degradation that occurs during removal and processing.
- In general, state DOTs allow more RAP in base courses than they do in surface courses.
Table 1. Superpave Asphalt Binder Selection Guidelines for RAP Mixtures (from AASHTO, 2001)
|RAP Percentage||Recommended Virgin Asphalt Binder Grade|
|Less than 15||No change from basic Superpave PG binder requirements.|
|15 - 25||Select virgin binder one grade softer than normal (e.g., select at PG 58-22 if a PG 64-22 would normally be used).|
|Greater than 25||Follow recommendations from blending charts.|
RAP in Cold Plant Mix Recycling
Cold plant mix recycling involves mixing RAP with an asphalt emulsion or foamed asphalt at a central or mobile plant facility. A rejuvenating agent can be added to improve the recycled asphalt binder viscosity and new aggregate can also be added to improve overall performance. The resulting cold mix is typically used as a stabilized base course. Since cold in-place recycling has become more commonplace, cold plant mixing has become less popular.
In-Place HMA Recycling
HMA can also be recycled in-place via hot or cold methods.
- Hot in-place recycling (HIPR). Usually, HIPR can only correct shallow surface distress problems (less than 2 inches). HIPR can be done by heater scarification, repaving or remixing.
- Heater scarification. The pavement surface is heated with radiant heaters, scarified using a bank of nonrotating teeth, rejuvenated using an additive to improve the recycled asphalt binder viscosity, mixed and leveled, then compacted using conventional compaction equipment. Heater scarification is limited in its ability to repair severely rutted pavements, which are more easily rehabilitated with a conventional HMA overlay.
- Repaving. This method is similar to heater scarification, only the top layer is completely removed (rather than left in place) and then placed in either one or two lifts.
- Remixing. This method is used when additional aggregate is required to improve the strength or stability. Remixing is similar to repaving but adds new virgin aggregate or new HMA to the recycled material before it is leveled.
- Cold in-place recycling (CIPR). CIPR essentially pulverizes the existing pavement structure to a predetermined depth, adds a binding agent (such as an emulsion or foamed asphalt), then lays and compacts the resulting product for use as a stabilized base course. This base course is then paved over with an HMA surface course.
- Full depth reclamation (FDR). FDR is a form of CIPR in which the entire pavement structure is pulverized and recycled. Typically some form of BST or HMA overlay is placed on top of the recycled material.
- Asphalt Pavement Alliance (APA). (2001a). Recycling Asphalt Pavement Background. Paper posted on the APA website (http://www.asphaltroads.org) as a .pdf file. Accessed 6 November 2001.↵
- Federal Highway Administration. (2001a). Reclaimed Asphalt Pavement Material Description. Web page on the Turner-Fairbanks Highway Research Center web site. http://www.tfhrc.gov/hnr20/recycle/waste/rap131.htm. Accessed 16 October 2001↵