A number of contractors in Texas are successfully running reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) and warm mix 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," said Mike Brown, vice president of construction, Wheeler/Oldcastle, Round Rock, Texas. "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, 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," said 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 noted 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," said Rand.
"Whereas a less experienced asphalt producer will tend to maximize the specification because it gives him the lowest bid," Rand continued. "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," said Rand. "One is to use more and more recycled material. But the reality 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 acknowledged that some asphalt plants are better at running recycled materials than others. "Some drums have longer dwell times," he said. "An example is an Astec 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, said Malcolm Swanson, president, 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," said Swanson.
"So in many cases you're going to end up introducing the RAP and shingles together," said 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 pointed 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. "Pretty much everybody in the industry, even our competitors, acknowledges that the Double Barrel® is a recycling beast," said Swanson.
Swanson also noted that when you lower the mix discharge temperature from 300 degrees F to 275 degrees F (149 degrees C to 135 degrees C) it sounds substantial. But when you're talking about lowering superheated aggregate temperatures from 600 degrees F to 550 degrees F (316 degrees C to 288 degrees C), it doesn't sound like much. "It really isn't," said 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 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 said: "To do so, you would need to add a complete RAS feeding system—including a weighing RAS Feed Bin, conveyor, and inlet chute."
Another challenge is moisture in the shingles. "Moisture adds another degree of difficulty," said 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, said Chuck Fuller of Ramming Paving Co. Ltd., Austin, Texas. "If you're at 7 to 8 percent moisture in the RAS, it can produce a failure in the field," he said.
"A fundamental problem is that shingles tend to retain water very well," said 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," said 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," said 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 lbs (0.16 tonne) of liquid AC would actually contribute only 288 lbs (0.14 tonne) 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 said producers running RAS need to check moistures often in the stockpile. "And more than that, you really need to keep the shingles dry," he said. "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 preprocessed. 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."
Benifit Of Cover
Shingles need to be covered to keep them dry, Swanson said. A cover over the RAS will also provide shade from the hot sun and help reduce the tendency of the RAS binding together.
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 percent. Running 20 percent RAP at this moisture content as compared to running RAP having 3.5 percent moisture content typically observed in uncovered stockpiles results in a fuel savings of over 6 percent. 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 said 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.
Shingles need to be ground finely, said Rand and producers. The TxDOT specification calls for RAS to be ground to 0.375-in (0.95 cm) minus. "Texas producers typically double-grind the shingles," said 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."
Hot Mix Or Warm
As for whether Brown uses warm-mix or hot-mix asphalt for a given project, he said it depends on the haul distance and a number of other variables. The longer the haul distance, the more Brown is inclined to use hot mix.
"If we discharge a mix at 275 degrees F (135 degrees C) and we're going to have a 20-degree loss during transit, and another 20-degree loss to put it on the ground and through a material transfer vehicle, I get a 40-degree loss," he said. "So that puts the maximum compaction temperature at 235 degrees F (113 degrees C). But that temperature will also diminish rapidly once it has been placed, depending on the thickness of the lift, the wind conditions, and the amount of water on your breakdown roller."
Brown said he likes warm mix, and he says owners like it because you don't oxidize the binder as much. "You're not burning off the light ends of the asphalt and hardening it as much as with hot mix. So, warm mix should give you longer life in the field. I think our owners like it, and I like it, because I want our product to be long-lasting and meet their expectations."BACK TO ISSUE
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