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To increase the percentage of RAP used and keep the asphalt in the mix within tolerance, you really need to screen it into different sizes, like you would with virgin aggregate. "You know the asphalt content and you know what aggregate sizing you are going to have at any given time," Irvine explains. You can then increase RAP content while maintaining mix quality. "If you do increase the RAP, the costs go down."

This makes control of the RAP more critical. "Some asphalt contractors are saying, 'Maybe we ought to buy a mill so we can control ownership of this RAP'," Irvine notes. "Then you can control the scheduling - you can use your milling machine to supplement schedules for peak capacity."

Of course, outsourcing is still a viable alternative. "A lot of times, you can subcontract a large mainline specialty milling contractor," says Irvine. "You can also double haul or haul mix to the jobsite and use your same trucks to collect RAP and bring it back."

Either way, proper operation of the mill becomes more critical as asphalt plants refuse to accept RAP that isn't properly sized. Screening RAP is an energy intensive process that can be minimized but not avoided.

"You can size the material properly with the mills that are out there," says Holland. "But what happens is you will see an operator who will outrun his mill. If he backs off to 90 fpm instead of 110 to 120 fpm, he can get the right sizing out of it. The quality of the millings is becoming more and more of an issue."