“So in many cases you’re going to end up introducing the RAP and shingles together,” says Swanson. In a counterflow plant, that will occur at the RAP collar or, with an Astec Double Barrel, at the RAP chute to the mixing chamber. As Swanson points out, the Astec Double Barrel plant has a mixing chamber that permits the RAP and RAS to mix with the superheated aggregate in an inert, non-oxidizing environment.
Swanson also notes that when you lower the mix discharge temperature from 300° F. to 275°F., it sounds substantial. But when you’re talking about lowering superheated aggregate temperatures from 600° F. to 550° F., it doesn’t sound like much.
“It really isn’t,” says Swanson. “So you can get adequate melting of the shingle asphalt within the superheat temperature range that you’re going to use, even when you’re making it as warm mix.”
In new asphalt plants, Astec can provide the capability to add RAS first, then the RAP. Or, Astec can retrofit a Double Barrel plant, by adding another inlet to the mixing chamber. Many Double Barrel plants in the field can add lime upstream of the RAP, and RAS would be no different. Swanson says to add a complete RAS feeding system – including a weighing RAS Feed Bin, conveyor, and inlet chute – would cost in the range of $300,000.
Moisture a challenge
Another challenge is moisture in the shingles. “Moisture adds another degree of difficulty,” says Brown. A problem arises when moisture in the shingles is inconsistent throughout a stockpile. For moisture testing, samples are small, but the plant must run tons of RAS.
Moisture tests in shingles can produce satisfactory results in the laboratory, but the stockpile can still present problems, says Chuck Fuller of Ramming Paving Co. Ltd., Austin, TX. “If you’re at 7 to 8 percent moisture in the RAS, it can produce a failure in the field,” he says.
“A fundamental problem is that shingles tend to retain water very well,” says Swanson. “And the water content can be more than 20 percent and quite varied within a given quantity of shingles, so it’s hard to know how much water is going into the plant with the shingles at every instant. As with all materials, it is important to keep them dry. But with shingles, it’s even more important because they can hold so much moisture.
“If you don’t do a pretty good job of keeping your shingles dry and moisture fairly consistent, you can suddenly be putting proportionately a larger amount of water into the system,” says Swanson. “And if you don’t know what that percentage of water is with a fair degree of accuracy, because it all weighs as shingles, you can introduce an error into your binder content.”
For example, if the shingles contain 12 percent water, a producer will subtract that from the weight of shingles that is used to calculate liquid AC content.
“If it’s 12 percent water, then it’s 88 percent shingles,” says Swanson. “So that’s the mass that you calculate your asphalt on. But, if the shingles contain 20 percent water, then the material going into the mixer as shingles is actually on 80 percent shingles. So, what is weighed as a ton of shingles and, if bone dry, would contribute say 360 pounds (0.18 ton) of liquid AC would actually contribute only 288 pounds (0.14 ton) of AC. That being the case, the importance of knowing how much water is in the shingles is important and minimizing it is even more important.”
Swanson says producers running RAS need to check moistures often in the stockpile. “And more than that, you really need to keep the shingles dry,” he says. “Another problem is that shingles tend to be delivered pretty wet. Most of the contractors are not processing the shingles themselves. They’re buying them pre-processed. And processing the shingles is typically a wet process. The water doesn’t drain out too well. So I don’t know to what degree the shingle processor has control over the amount of water that is residual in the shingles, but that is a factor in the water content of the shingles going into the plant.”
Shingles need to be covered to keep them dry, Swanson says. A cover over the RAS will also provide shade from the hot sun and help reduce the tendency of the RAS binding together.