Asphalt producers/contractors continue to leverage the value of reclaimed asphalt pavement (RAP) by investing in equipment to process that resource to be incorporated in new mixes for commercial and state agency customers. Rising material costs -- both virgin aggregate and liquid asphalt cement binder -- have encouraged both producers and customers to consider the cost-savings benefit of RAP mix designs.
Many state department of transportation agencies have gained an increased acceptance of RAP designs, realizing they not only save money and expand budgets, but also that they perform as well as virgin mix designs.
For most asphalt production operations, a horizontal shaft impactor and screen to size material is all that's needed to process old asphalt. Milled RAP material can quickly be processed with a screen, with 80% of the millings passing through the ½-inch minus openings.
Larger material can then be moved through a closed-circuit conveyor system to a crusher for further processing before being returned to the screen. A horizontal crusher will sufficiently break down larger particles without crushing the aggregate content.
For asphalt producer/contractors who want better control over their ability to add maximum RAP content to their mix designs, a crushing/screening system that fractionates (separates) the material components of the RAP is considered the best way to go.
Without the capability to fractionate, most producers/contractors typically add 15% to 20% RAP to their mix designs. If DOT designs allow for more, especially in base course mixes, a fractionating system is the only way to control the actual material components of the old RAP you want to add back into the new mix.
Whenever you begin to add more than 20% RAP into a hot mix or warm mix design, fractionating old material into two sizes (generally ½-inch to ¼-inch and ¼-inch minus) not only controls the actual quantity of material size, it also allows you to more precisely determine the amount of liquid asphalt cement that old material contains.
Having that control minimizes segregation between old and new material and allows more control over the air void content of the mix you're producing. A high-frequency fractionating system works best to efficiently separate fine material (1/4-inch and smaller) from the RAP you're processing.
Working with both
Troy Kutz, vice president of the materials division for Williams Charles Construction of Loves Park, IL, says he's had experience with both types of asphalt recycling equipment and both have worked well for the purpose intended.
Located 60 miles northwest of Chicago, Williams Charles Construction operates four asphalt production facilities and portable UltraMax 1200-25CC Eagle crusher to process RAP for the four asphalt plants. The Eagle is a closed-circuit crushing and screening plant with a vibrating grizzly feeder and a double-deck screen designed to produce two cubical spec products in a single pass.
"The Eagle crusher works fine to process asphalt down to the ½-inch minus IDOT (Illinois Department of Transportation) requirements for acceptable RAP, and we'll move it from plant to plant to process 6,000-7,000-8,000 tons of RAP at a time," Kutz says.
"We generally don't like to process more than that simply because piles of RAP tend to bind together into larger chunks if left unused for any length of time," he continues. "In an average year we'll process 75,000 to 125,000 tons of RAP depending on the jobs and the spec requirements for allowable RAP content. Also, state projects only allow us to recycle millings from state highways back into new mix designs."
Williams Charles first experience with fractionating RAP came last year while working on the Illinois Tollway I-90 project near Rockford.
"We hired a contractor to provide the fractionating service in order to produce mix designs that allowed 40-45% RAP," Kutz says. "Typically, IDOT allows up to 30% RAP (1/2-inch minus) in binder mixes for state project, but the Tollway allowed 40-45% FRAP."