Asphalt pavements often are being cold planed to correct surface irregularities, maintain curb lines and remove poor quality pavement prior to asphalt overlay. This has created large quantities of stockpiled reclaimed asphalt pavement (RAP).

Although RAP is used for a variety of pavement maintenance applications, a very effective use is for the production of maintenance mixes. The RAP is cold central plant recycled (CCPR) with an asphalt emulsion. These mixes can be produced for immediate use or for stockpiling and later placement. Besides being very high quality and salvaging pavement materials, RAP mixes result in good cost savings as can be from 25 to 50 percent less expensive than new mixes.

Collecting and Stockpiling RAP

Cold planing is recommended for pavement removal as individual layers of different types of mixes can be obtained and placed in separate stockpiles, if desired. Also, the size of the RAP will be smaller than if the pavement is obtained by ripping with sizing by screening and crushing much easier. A very important part of the removal process is preventing contamination of the RAP.

The location chosen for stockpiling the RAP, if possible, should be where the mix can be both produced and stored for maintenance uses to keep material handling to a minimum. This is important to keeping the cost of the final product well below that of new maintenance mixes.

Proper stockpile management is essential to having a successful cold central plant recycling operation. RAP of varying quality and uniformity can be separated to create more uniform stockpiles. Dumping of concrete, soil, wood and trash into RAP stockpiles should not be permitted. RAP tends to compact and so the lowest stockpile height that the space will permit should be used with a maximum height of about 10 feet (3 meters) preferred. A large bucket loader is recommended with the bucket having teeth to loosen the RAP. Also, if the RAP has a high asphalt content or the asphalt is not old/hardened so tacky, a bulldozer may be necessary to loosen and mix the material.

Asphalt Emulsions
Asphalt emulsions are used as the asphalt binder for RAP maintenance mixes with the type dependent on how the mix is to be used. When the mix is to be stockpiled and extended workability is needed, an emulsion containing a petroleum distillate is used, such as a high float emulsion. For immediate placement after mixing, slow setting emulsions also may be used. RAP can be sampled from stockpiles and a mix design completed to determine an emulsion percentage.

Mixture Production

A cold central recycling plant is preferred for producing RAP maintenance mixes. This equipment results in good control of both material size and asphalt emulsion content. The plant consists of a feeder hopper, screening and crushing unit, and a twin-shaft mixer for adding the asphalt emulsion. The mixer has a belt scale and computer controlled liquid additive system for accurately adding the emulsion. The mix is removed from the mixing unit by a belt conveyor and stacked for immediate use or for stockpiling at the mixing or other site for later use.

Maximum material size is controlled by the screening and crushing unit with no large pieces as can occur if RAP is used directly from the stockpile. Also, the coarseness of the mixture can be adjusted by changing the screen size with a coarser gradation for deep patching or made finer if to place in a thin layer as for skin patching. Typical mix sizes are 100% passing a 7/8 inch or 5/8 inch screen. Eliminated is tearing of the mat by the asphalt paver or motor grader due to large over-sized material.

Mixture Stockpiling

Stockpile height must be maintained as low as possible, preferably not greater than about 8 to 10 feet (2.5 to 3 meters), to prevent the asphalt coated particles from sticking together and producing chunks. This is of particular concern as mix moisture content decreases and air temperatures rise. Also, bucket loader front wheels should be kept off of stockpiles as large chunks may result due to compaction by the wheels. Stockpiles should be checked periodically with a bucket loader to determine if the mix is maintaining workability and a reduction in asphalt content possibly is needed. A thin crust may form on the stockpile surface but this is typical for these types of mixes.

Paving Procedures

Paving procedures are similar to those used for new maintenance mixes. The existing pavement is swept to clean the surface and a tack coat of diluted emulsion applied for good bonding. RAP maintenance mixes can be placed with a conventional asphalt paver or a motor grader. For motor grader placement, the blade should be straight and sharp and the tires worn or of the turf type to prevent leaving tread marks in the finished mat.

Initial compaction normally is with pneumatic-tired rollers and the final rolling sometimes with a double-drum steel roller. Vibratory compaction can increase density and smooth the surface for better ride quality and usually is completed at low amplitude. Vibration should not be used if damaging the mat with severe checking or slippage or when mix thickness is not at least 1 inch (25 mm). If the mat is tender at placement, it may be possible to re-roll later after losing moisture and/or warmer weather. Roller drums and tires normally must be kept wetted to prevent picking up of the cold recycled mix.

Surface Treatment

Some type of surface treatment is required for cold central plant recycled maintenance mixes. A fog seal of diluted asphalt emulsion can be applied as a temporary treatment, primarily to prevent raveling. However, a seal coat with a cover aggregate (sand, sand-gravel or chips) is recommended if only to be seal coated. Also, the surface can be an overlay of another type of cold mix or hot mix asphalt (HMA). A fog seal can be applied immediately after placement but other surface treatments typically are not placed until some curing period. This is to allow for moisture loss and mix strength increase.