RAP! What is it good for?

By Paul Brown

Asphalt pavement is one of the few products that is 100% recyclable. Highway agencies generate recycled asphalt products or RAP, from cold planing or milling pavements and from utility cuts. Many have found RAP to be a valuable resource for hot asphalt mixes, cold asphalt mixes, full-depth recycling and aggregate replacement. This article summarizes these uses of RAP.

RAP in Hot Mix

RAP is useful for hot asphalt overlays if the agency develops a mix design that meets criteria established by State and Federal specifications. The limiting factor is the percentage of RAP which asphalt companies are able to use to produce high performance asphalt mixes. The New Hampshire DOT allows from 15% to 35% RAP. Many engineering firms and some asphalt suppliers can analyze an agency’s RAP and develop a mix design.

RAP in Cold Mix

Other common uses of RAP require less engineering and are lower in cost. Cold mix made with 100% RAP can perform as well as hot mix if road managers consider gradation, residual asphalt content, curing time, and compaction are considered. Cold mixes perform well when gradation is controlled. The RAP is crushed and sized to meet the typical hot mix gradation requirements.

The residual asphalt content determines the amount of emulsion added to the material to create a stable mix. The content can vary based on the origin of the RAP. If the road was old and oxidized, the residual asphalt content could be quite low. Because of the variable asphalt content, the resulting mix could require from 3-10 gallons of asphalt emulsions to create a stable cold mix. It is therefore recommended that cold mix be made only from tested materials.

To have a long-lasting asphalt surface, the mix should have sufficient time to cure. Emulsions, used with RAP, are made from soap and water. Adequate curing time is vital to allow the moisture to evaporate from the mix.

The millings should be of uniform size. Cold mix is typically laid in 2"-3" lifts. Larger lifts are difficult to compact. Good compaction is critical for cold mix performance. Heavy rollers are a must for proper cold mix compaction. A properly compacted mix will have 10-15% voids content. All cold mixes must be sealed before the winter to prevent water penetration into the voids.

On low volume roads cold mixes that are done correctly have the same performance and structural capacity as hot mix asphalt. Placing cold mix on roads that have excessive heavy and/or turning traffic is generally not recommended the key to success is to do it right.

Full-Depth Recycling

Full-depth recycling generates RAP. It utilizes 100% of the existing asphalt by grinding up and mixing the whole asphalt surface with the underlying (base) material. The resulting mix should have a gradation similar to crushed gravel. It then serves as the base material for a new pavement surface.

The RAP gravel base material will produce a mixture that has 30% to 50% more load-bearing capacities than virgin aggregate. This means that 6

inches of asphalt reclaimed base can have the equivalent strength of 9 inches of virgin gravel. These results will vary depending on the materials are mixed with the RAP.

This can provide a valuable resource to regrade the subbase, correct drainage problems, and add more aggregate to the road base. These advantages provide municipalities with less expensive methods of reconstruction.

Full depth recycling is a relatively inexpensive method of reconstruction that can provide 10-15 years of service life for the road.

To achieve long service life agencies must pay close attention to material gradation, subbase material, compaction and moisture content.

Material gradation depends on the speed of the reclaimer. When it moves too fast, the resulting aggregate mixture will contain chunks and large pieces. An uneven mixture is difficult to grade and usually leads to long-term compaction problems. A well-graded aggregate and evenly sized mixture is easy to compact to achieve maximum density. Proper graduation is vital to achieve a long life cycle.

The agency must pay close attention to pre-existing problems in the subbase. RAP mixed with unsuitable material might provide short-term results, but the problems will reappear and have to be addressed later only if the subbase contains suitable materials will the RAP mixture provide an adequate road base.

Local road managers must ensure adequate compaction and optimum moisture content. Soil additives can improve compaction because they attract moisture that will help attain optimum moisture content. Additives allow contractors to meet compaction requirements with less job site effort and cost.

Conclusion

RAP is viable replacement for virgin aggregate. With depleting natural resources municipalities should consider RAP as an alternative product. Aggregates in older pavements contain some of the best materials available today. Future municipal budgets may dictate that highway departments look to lower-cost alternatives such as RAP for overlays, shoulder material road base, and surface gravel for unpaved roads.

I would be pleased to provide you with more detailed information as would the UNH T² Center.

Paul Brown is the President of Roadtech, Inc.

Return to UNH T2 Center, 33 College Road, Durham NH 03824 603-862-2826 Fax 603-862-2364