On the one Colton project - K Street, a residential street with some moderate truck traffic, Pavement Recycling crews applied a scrub seal using 5/16-inch recycled aggregate. On the street, crews applied a PASS (Polymerized Asphalt Surface Sealer) scrub seal with RAP chip aggregate covering, and then applied a rubberized emulsion asphalt slurry (REAS) Cape Seal coating.
On the other test project - Key Street, a road serving an industrial area, crews milled three inches of the old road surface, took the material back to the Colton facility for crushing, blended the material with a latex modified emulsion, and then returned the mix to the road for cold placement.
?After the new lift was placed we then sealed one half of the road with Type II RAP slurry and the other half with Type I/Type II (REAS) rubberized emulsion RAP slurry,? Matthews says. ?County road agencies like cold in-place recycling because it?s a very economical way to rehabilitate a road that needs more than just a surface treatment.
?A cold in-place has the structural characteristics of a hot mix, but it also has more air voids that need to be sealed off if you?re not going to put down a wearing course (of hot mix),? he adds. ?That?s why on the Colton (industrial road) project we applied the sealer on the cold mix lift we placed. We also wanted to test the RAP slurry under heavy truck traffic.?
On the residential project the RAP slurry application is usually all that?s needed to maintain the road structure under the traffic load it carries, and most road agencies can?t really afford more than the occasional preservation treatment to maintain residential streets.
According to the International Slurry Surfacing Association (ISSA), the performance characteristics of a slurry mixture make an ideal preservation treatment for hot mix asphalt pavements. Materials used to create a slurry seal include aggregate (1/4-inch or smaller), asphalt emulsion, filler (Portland cement, hydrated lime or aluminum liquid serve as stabilizing agents), and water (for workability) mixed together according to a laboratory?s design-mix formula.
The emulsion serves as the binder to hold the crushed aggregate together and adhere it to the old road surface. Various emulsions and aggregates are used to meet the conditions, specifications and requirements of individual projects.
Slurry equipment, either truck-mounted or self-propelled, is used to transport unmixed materials and then blend them together in a continuous-flow pugmill. The system provides a consistently uniform mixture that can be spread over the road surface with a spreader box linked to the surface slurry-mixing unit. With mixing and spreading accomplished in one continuous operation, treated roads can be reopened to traffic within a few hours.
Emulsion of varying composition and setting times are mixed with one of three grades of aggregates - Type I (fine), Type II (general), and Type III (coarse). Fine aggregate mixtures are used for maximum crack penetration and sealing in low-density/low-wear traffic areas.
Type II aggregates are used on moderate-to-heavy traffic roads to seal, correct moderate-to-severe raveling, rejuvenate oxidized surfaces, and improve skid resistance. Coarse aggregate slurry mixtures are used to correct severe surface conditions and improve skid resistance on roads with heavy traffic loads.
Promoting the RAP advantage
?We?ve done cold placement for years and we?ve also placed thin overlays on projects that we know can last for up to 15 years,? Matthews says. ?Now, we?re trying to introduce another alternative (RAP) into the options we can make available to our customers; and we believe it will deliver the same economical performance that our other preservation methods provide.
?It?s not uncommon for us to save customers 50 percent over conventional rehabilitation methods, and we think RAP products will continue to help our customers maximize their investments,? he adds.
While some of Pavement Recycling road agency customers have experimented with RAP preservation treatments, Matthews expects more to investigate the potential performance and economic advantages of RAP preservation treatments.