California project combines RAP and tire rubber in a cape seal treatment
By Edgard Hitti
The R-factor. We’re not talking about a band, statistics or a home project – we mean, “recycle.”
Over the years, many forms of recycled materials have been used in paving applications, including tire rubber (TR) derived from used tires and recycled asphalt pavement (RAP).
Many agencies in California actually own RAP piles. With decreasing construction and maintenance funds, these piles can become a pivotal way to stretch the maintenance budget. The key is how to incorporate as much RAP and TR in multiple preventive and maintenance applications.
In this application, workers combined RAP and TR in a two-layer, cape seal system utilizing hot-applied binder and slurry emulsion.
The challenge of a hot-applied chip seal is obtaining the hot and precoated chip, which requires high quality aggregates, one-size gradation, relatively small quantities and a hot plant to pre-coat the aggregate chips. Pavement Recycling Systems, Inc. (PRSI) is a company that has a method to process the RAP into a 3/8-inch chip while meeting existing local agency specifications for chip gradation.
Hot-applied chip seals require the use of pre-coated rock, usually 0.5 to 1 percent binder by weight of the chip. When using RAP, the extra cost of having the chips pre-coated is eliminated via the retained binder adhering to the RAP.
Several samples were evaluated in the lab prior to the project in order to determine the best application rate and to answer several concerns regarding the RAP chip retention – mainly dust. A comparison was made utilizing RAP and virgin aggregate with emulsion chip seal and hot applied chip seal binders. After many tests of pre-heated and non-preheated aggregates, the conclusion was that the non-preheated RAP chip performed as well as virgin aggregates.
In April 2013, highly distressed streets in the city of Bakersfield, California were selected for the TR and RAP cape seal treatment. The application process started with 0.45 gallons per square yard of PG 76-22TR (with 15 percent tire rubber) produced by Paramount Petroleum Corporation and heated to 330 to 350˚F. Application of 3/8-inch chips made of 100 percent RAP followed. Rolling with a rubber-tire roller and a steel-wheel roller followed by sweeping completed the construction process.
Even more RAP can be used by capping the rubberized, hot-applied RAP chip seal with emulsion slurry seal (type II) using 100 percent RAP. The combination of these two applications provides an interlocking system of the TR/RAP chip and the slurry that offers resistance to crack propagation from the bottom and traffic stresses from the top.
“Evaluating this new, promising use of recycled material is an innovative way to save scarce agency dollars and provide another tool for serving the driving public with better roads,” said Bakersfield City Road Superintendent Mike Conner.
Many of these roads were constructed with 1 to 1.5 inches of what was previously called oil sand. With innovative tools like these, many streets can be brought back to life at a good service level. Utilizing products with tire rubber not only improves the binder properties, but will also divert used tires from going into landfills.
Over a year later, the residents of the Bakersfield neighborhoods, where the streets have been treated with the RAP and TR cape seal, are still enjoying the smooth ride with no sign of distress propagating to the surface or any aggregate loss.
Hitti is the Director of Asphalt Technical Services for Alon Asphalt Company.