Recycled HMA

The National Asphalt Pavement Association (NAPA) makes the claim that HMA is the most recycled product in the U.S. A recent survey by NAPA (Hansen and Newcomb 2011) indicated that in 2010 an estimated 73.5 million tons of "reclaimed asphalt pavement" (RAP) were collected and saved for reuse by the HMA industry with only 4,000 tons nationwide being sent to landfill. This represents a 99.99% recycling rate. Of the tonnage collected, 62.1 million tons were used in new HMA. 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); most commonly created by a milling machine (a.k.a. cold planer). RAP can be used in a variety of ways when properly prepared such as:

• As an addition to new, or virgin, HMA.
• As an aggregate in cold-mix asphalt.
• As a granular base course.
• As a fill or embankment material.

Typically, RAP becomes the property of the contractor doing the work and is transported to a HMA plant or quarry where it is stored in a temporary stockpile for future use. RAP is usually screened and crushed before being included as a component in new HMA. RAP can also be blended with virgin aggregate to be used as a base course, however this application is not the highest and best use because it does not take advantage of the asphalt cement present in the RAP. Therefore, contractors generally prefer to use RAP as a component in new HMA but will also use it as a substitute for unbound aggregate if economics dictate. Using RAP in any capacity allows the contractor to reduce the amount of virgin (new) materials by an equal amount, which can save the contractor money. As a result, RAP is generally viewed as a commodity and is not discarded to landfill without careful thought.

Figure 1: RAP up close.	Figure 2: RAP pile at a HMA plant.

 

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:

1. The RAP aggregate can be used in place of a portion of the virgin aggregate, which lowers cost and reduces waste.
2. The RAP asphalt binder is reheated and used in place of a portion of the virgin asphalt binder, which lowers cost and reduces waste.

In the U.S., 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). 

HAPI Pavement Note on Specifications for RAP in HMA

Eacn major owner agency has a slightly different specifications for how much RAP can be used and in what layers. HDOT. The current specification, dated 2005, and special provisions allow the following amounts of RAP: 20% in “hot mix asphalt (HMA) pavement” – usually used as the surface course (generally the top two inches of a pavement structure. 40% in “hot mix asphalt base (HMAB)” – usually used as the base course. The specification actually allows 30% for batch plants and 40% for drum mix plants, however there are currently no batch plants on O’ahu. Honolulu. The current specification, dated 1986, does not allow the use of RAP in HMA. However, special provisions can, and typically are, added to allow 20% RAP in the surface course and 40% RAP in the base course similar to HDOT. Federal Aviation Administration (FAA). While HDOT administers Hawai’i airports, specifications are largely dictated by the FAA. FAA specifications (found as FAA Advisory Circular AC 150/5370-10), issued in 2009, do not allow RAP in surface mixes (usually the top 2-3 inches) except for shoulders, but allows 30% in other mixtures.  Military. Unified Facilities Guide Specification (UFGS) Section 32 12 15, dated May 2010, does not allow RAP in surface mixes (usually the top 2-3 inches) except for shoulders, but allows 30% in other mixtures. This is the same as FAA specification.

 

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 considered 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.
• Clumps of fine material tend to stick together in RAP giving the appearance of a larger aggregate piece. However, in the HMA plant, these clumps break down and can result in the gradation of the final product being slightly more fine than expected. Properly crushing and sorting RAP before use in new HMA can greatly reduce this problem.

HAPI Pavement Note RAP availibility on O'ahu

A gross estmate for the amount of available RAP on O’ahu is 800,000 tons. For comparison, the total amount of HMA plaed in a given year in Hawai'i ranges from about 750,000 to 1.5 million tons. It is difficult to estimate the total quantity of RAP generated on O’ahu each year or in any specific year since RAP is typically inventoried only as it is used in new HMA and not as it is generated. For 2010, its use in new HMA on O’ahu was on the order of about 100,000 tons (based on estimates from HAPI members). Hansen and Newcomb (2011) report that overall actual RAP use in Hawai'i as a percentage in Hawai’i is about 15% for surface courses and 20% for base courses. It is important to note that there are two main RAP material streams on O’ahu: Pure or “Clean” RAP. Comes directly from milling up old HMA pavement. Contractors usually store this RAP on site at the HMA plant and use it when allowed. Current industry estimates are on the order of 400,000 tons of clean RAP stockpiled on O’ahu. Co-mingled RAP. Comes from demolition and milling jobs where the old HMA pavement is combined in the demolition process with underlying material such as unbound aggregate and soil. This often occurs when a contractor removes a depth of material that extends beyond the existing pavement depth using only one cold-planer pass. Current industry estimates are on the order of 400,000 tons of co-mingled RAP stockpiled on O’ahu. Estimates from HAPI members suggest that clean RAP is used reasonably but with growing inventory suggesting there is room for greater use. Co-mingled RAP is only used infrequently since the non-RAP materials (aggregate and soil) are not allowed to be included (and should not be included) in new HMA. Therefore, stockpiles of co-mingled RAP are growing at a greater rate because it cannot be effectively used. Figure 3: RAP pile at Kalaeloa. Figure 4: Large co-mingled RAP pile(the upper road in this picture is built on the pile) in the Makakilo quarry. Current industry sentiment is that allowable RAP percentages in HMA mixtures in Hawai’i specifications are acceptable. However, RAP inventory is growing. A reasonable solution would be to increase RAP content allowed in surface courses to 25% and to allow RAP to be included in unbound aggregate base courses at 50% so long as it is fully mixed at a central processing facility (see Ooi et al. 2010 and McGarrah and Matsumoto 2010).

 

HAPI Pavement Note Recycled Glass in HMA

HDOT and the Counties allow the inclusion of "construction-grade cullet" (crushed recycled glass) into HMA base course on the order of 10 to 15% by weight of aggregate. This used to be a requirement but has since been relaxed to make cullet allowable and not required. Figure 3: Recycled glass pile at a HMA plant. Figure 4: Recycled glass close-up.

 

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

HAPI Pavement Note on In-Place HMA Recycling

Hawai'i does not perform any large-scale 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.