A number of contractors in Texas are successfully running reclaimed asphalt pavement (RAP), recycled asphalt shingles (RAS) and warm mix asphalt (WMA) all at the same time. However, these contractors have faced challenges in running all three at once, and they have climbed the learning curve to do so.
“When you’re running RAP and RAS, you’re doing it for a couple of reasons,” says Mike Brown, vice president of construction, Wheeler/Oldcastle, Round Rock, TX. “One reason is to reduce the amount of virgin asphalt cement, and the other is to reduce or eliminate modifying the virgin asphalt cement with a polymer. So we’re able in a lot of cases to get high-temperature and low-temperature properties – and eliminate the polymers – by using RAP and RAS and warm mix.”
The maximum amount of RAP allowed by the Texas Department of Transportation (TxDOT) in surface mixes is 20 percent. In base courses the limit is 30 percent. And the maximum shingles content is 5 percent. But if a contractor runs 5 percent shingles, the maximum RAP content gets reduced by that 5 percent, to 15 percent.
But hold on, it gets more complicated. “We only allow 20 percent recycled binder in the surface course,” says Dale Rand, Director, Flexible Pavements Branch, TxDOT. “However, if they make a binder grade adjustment, where they go with a softer, what we call our allowable substitute binder, they have to lower the high temperature grade and the low temperature grade of the binder. Then we’ll allow them to go up to 30 percent recycled binder in the surface. And in that case, they could probably use, depending on how the numbers come out, 15 percent RAP and 5 percent shingles. We have a table on how much total recycled binder they can use.”
That said, Rand notes that more experienced asphalt producers do not run the maximum allowable percentages of RAP and RAS. For example, they may use 3 percent shingles and 10 percent RAP together. “They know that if they try to go much above that, that may not get good mixing, and could have other problems,” says Rand.
“Whereas a less experienced asphalt producer will tend to maximize the specification because it gives him the lowest bid,” Rand continues. “In my experience, the more experienced people tend NOT to maximize the specification. They tend to find that spot where they operate the most efficiently.”
One challenge is to transfer enough heat – by conduction through the superheated aggregate in a drum – to the RAP and RAS to melt the liquid asphalt in both, and achieve good mixing.
“We get a lot of mixed messages across the country,” says Rand. “One is, to use more and more recycled material. But the reality of it is that there are limitations on what you can use from a physical standpoint. You just cannot transfer that much heat to a cold material without having some issues. And it can also depend on the type of asphalt plant and whether or not the contractor uses a material transfer vehicle. Some of the contractors will do that. Others won’t.”
Rand acknowledges that some asphalt plants are better at running recycled materials than others. “Some drums have longer dwell times,” he says. “An example is a Double Barrel drum that mixes in the outer chamber and gives you better heat transfer. Some plants handle RAP much better than others. And some of them have a very short mixing zone.”
The ideal way to introduce RAP and RAS to the mixture is to introduce the shingles earlier than the RAP, says Malcolm Swanson, Vice President-Engineering, Astec Inc. That way, the stiffer asphalt in the shingles gets the benefit of the higher temperatures in the superheated aggregate before it before it starts giving up heat to the RAP. “Not a lot of plants are equipped that way, however,” says Swanson.