Covering stockpiles is not just important for shingles. RAP must also be kept as dry as possible since it can gain significant environmental moisture resulting in higher production costs . Though logistics often prevent it, just-in- time milling and processing can reduce RAP moisture to near 0.5%. Running 20% RAP at this moisture content as compared to running RAP having 3.5% moisture content typically observed in uncovered stockpiles, results in a fuel savings of over 6%.
Additionally, with less moisture in the RAP from the beginning, less heat transfer is required. Since less heat transfer is required of the device that mixes the superheated virgin aggregate and RAP together, the transfer of heat occurs more quickly. Paradoxically, laboratory heat transfer tests show that at least some moisture in the RAP/RAS enhances heat transfer – the steam that flashes off the RAP/RAS as it contacts the superheated virgin aggregate actually serves to help transfer heat.
Brown says he and Chuck Fuller generally use tear-off shingles in central Texas. But in cities where asphalt shingles are produced, contractors often use manufactured shingle waste. Those shingles are not oxidized as much as tear-off shingles are.
“The good news with manufactured waste shingles is that the asphalt is softer and it’s more workable,” says Brown. “The bad news is that, when a stockpile of RAS is subjected to the sun, some of that RAS will glue itself back together and make clumps. That’s the other reason you might cover it.”
Shingles need to be ground finely, say Rand and producers. The TxDOT specification calls for RAS to be ground to 3/8-inch minus. “Texas producers typically double-grind the shingles,” says Rand. “Whether it’s twice, or three times, or however many times, the finer the grinding on the shingles, the better they work. They blend in better.”
Fuller of Ramming Paving agrees. “The finer you get it the better,” he said in a presentation at World of Asphalt. “Some of the companies who grind shingles say they can get it in one pass. But we double grind so that we get 100 percent to pass the 3/8-inch specification.”
Brown of Wheeler/APAC said his company successfully ran 20 percent RAP and 3 percent RAS for a base course mixture used on Parmer Lane in Austin.
“And we foamed it,” he said. “We used an Astec Double Barrel Green system. We discharged the mix at 275 degrees; that was our target. It was a short haul to the project. We also ran the IR bar on our paver, and we had no thermal segregation.”
(An IR bar refers to a paver attachment made by MOBA Corp. to measure temperatures across an asphalt mat in real time. It is used to detect thermal segregation.)
Brown said Wheeler/APAC had no problems with moisture on the Parmer Lane project. “We used an Astec Pro-Sizer to crush and fractionate the RAP. We would sample the stockpiles and determine moisture content, and we really didn’t have any problems with it. We did use a Shuttle Buggy to transfer mix from the trucks to the paver.
“We’re not running real high percentages of RAP and RAS,” says Brown. “We could have run higher recycle contents, but we did not. And the reason we did not is that we look at two things – Hamburg rut resistance, which is a high-temperature rut resistance measure – and we looked at indirect tensile strength, which helps us determine low-temperature cracking resistance. And what we have found is that when we kept our recycle below the maximums, we were able to get both the high-temperature rut resistance and the low-temperature cracking resistance.”
For the Parmer Lane project, Brown said his company hired a company, Sustainable Pavement Technologies, to clean, test, and grind the shingles to assure that the particle size met the 3/8-inch spec. “It turned out well,” Brown said. “We achieved our bonuses on density, and we monitored the thermal segregation for our information only.”
Brown said Wheeler/APAC uses separate cold feeds for the RAP and RAS, but the company weighs the combined RAP and RAS over a single weigh belt. “And it works well for us. We did calibrate the RAS feeder with the weigh belt, and then we calibrated the RAP feeder with the weigh belt. We made sure throughout, that our calibrations were staying true.”