What are Reclaimed Asphalt Pavement (RAP) Materials?

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The Federal Highway Administration (FHA) of the US Department of Transportation (DOT) defines reclaimed asphalt pavement (RAP) materials as removed and/or reprocessed pavement materials that contain asphalt and aggregates. 

RAP materials are either milled or crushed and used for various highway construction activities like subgrade preparation, recycled hot and cold mix asphalt pavements, backfills, etc.

This article explains the production, properties, and applications of reclaimed asphalt materials as per FHA.

Production of RAP Materials

RAP materials are generated when asphalt pavements are removed for resurfacing, reconstruction, or accessing buried utilities. After properly crushing and screening, RAP forms a mixture of well-graded and high-quality aggregates coated by asphalt cement.

FHA mentions the following ways of producing RAP materials for pavements:

1. The asphalt pavement is removed either by milling or full-depth removal at the site. The milling is conducted using a milling machine that can remove the pavement surface up to 50 mm thickness in a single pass. To perform full-depth removal, a rhino horn on a bulldozer or pneumatic pavement breakers are used to rip and break the pavement. These RAP materials are taken to a central processing plant, where they undergo further crushing, screening, conveying, and stacking.
2. Asphalt pavement can also be pulverized at the site using a self-propelled pulverizing machine. These pulverized materials are used with a stabilized base course at the same time.
3. In addition to the above methods, there are hot-in-place and cold-in-place recycling of asphalt pavements, consisting of a continuous train of operations:
   1. Partial removal of pavement surface
   2. Mixing the reclaimed material with additives like a binder, virgin aggregates, or softening agents
   3. Placing
   4. Compacting

As per FHA, around 41 million metric tons of RAP are produced each year in the United States. RAP produced is recycled but not used completely the same year it is produced. Only 20 percent of the annual RAP generated is disposed of safely in landfills or used for construction activities.

Properties of RAP Materials

The properties of RAP are dependent on:

1. Properties of the constituent materials
2. Type of asphalt concrete mix
3. Quality, size, and consistency of aggregates

Table-1 below shows the particle size distribution of RAP generally obtained either by milling or crushing:

The physical and mechanical properties of RAP as per FHA are given in Table-2:

Applications of RAP in Highways

Milled or crushed RAP can be used for various highway applications. The applications of RAP in highways, along with their processing requirements, are mentioned below:

1. RAP is used as an aggregate substitute for new or recycled asphalt mixes.
2. RAP is used to produce granular base or subbase aggregate. It is crushed, screened, blended with conventional granular aggregate or any reclaimed concrete material.
3. RAP is used for hot mix and cold mix asphalt pavement. When produced in a  central RAP processing facility, finished RAP material that is processed to the desired gradation is incorporated as an aggregate substitute to hot mix asphalt paving mixture. Similarly, cold mix asphalt is produced by combining finished RAP material as an aggregate substitute to cold mix asphalt paving mixture.  
4. The in-place recycling process can also produce cold mix asphalt and hot mix asphalt pavement. Here, RAP is prepared and processed, mixed with asphalt emulsion, and placed and compacted either as a single or multiple pass operation. 
5. Stockpiled RAP material is used for granular fill or base for embankment or backfill construction. 

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