The production and placement of asphalt is an evolving business. With fuel costs continuing to rise, contractors are looking for ways to cut costs to remain competitive. Stricter environmental regulations require the reduction of emissions at the plant and jobsite. Contractors are being asked to adapt to these difficult circumstances while still meeting density and smoothness requirements.
How can contractors meet this challenge? As is often the case, new technologies hold the answer. While warm-mix asphalt has been in use for some time in Europe, it is still relatively new to the U.S. That is starting to change as more contractors and federal and state transportation departments are experimenting with warm-mix asphalt to determine the pros and cons of the technology.
One contractor interested in exploring warm-mix asphalt is the Knife River Corp. in Sioux City, Iowa. One of the reasons that Knife River looked into warm-mix asphalt is because the company was awarded a paving project in Tabor, S.D., early in the paving season. This project was 60 miles from the asphalt plant.
“Because the job in Tabor was a smaller job, it was not going to be cost effective to move the asphalt plant closer to the jobsite,” said Mike Collins, Midwest quality control manager for Knife River. “Sixty miles is a long haul for asphalt. If it’s too costly to move the plant, how can we still haul asphalt that distance without the asphalt cooling down to where we can’t get compaction?”
Keeping it warm
Collins believed that the answer to that question might be warm-mix asphalt. When Knife River was awarded a mill-and-fill project on Highway 12 in northeast Nebraska, also 60 miles from the asphalt plant, Collins decided to experiment with warm-mix asphalt and pitched the technology for the job. The Nebraska Department of Roads was open to the idea.
“Sixty miles can become a little bit of a juggling act, as far as how the mix is going to react,” said Collins. “Some of the conventional mixes set up fast and can create problems with the equipment. If you can make a mix pliable longer, it allows us to haul 60 miles.”
The project covered almost 7 miles on Highway 12 between Verdel and Monowi, Neb. Throughout the project, 1.5 in. of pavement was milled off and a single 2-in. lift was placed. The time frame for the project was four days. While there are a lot of paving factors that Knife River could investigate with warm-mix asphalt, the short time of the project meant that the company could only really experiment with the temperature of the asphalt. What Knife River wanted to determine was the lowest temperature the asphalt could be mixed at the plant before it would be too cold after a 60-mile haul to achieve the required compaction densities on the jobsite.
To create the warm mix, Knife River decided to include a Sasobit additive in the PG 64-28 liquid binder of the asphalt. Since this was the first time Knife River or the Nebraska Department of Roads had used this product, it was decided that the project would be split into three different sections. Of the 11,570 tons of asphalt to be placed on the job, 6,000 tons would be conventional asphalt and the remaining 5,570 tons would be split between 1.5% Sasobit and 2.5% Sasobit in the liquid binder.
“We needed to use conventional asphalt on part of the job to get a baseline on temperatures,” said Collins. “The conventional asphalt temperatures were charted at the plant, after the haul, behind the screed and during the compaction process.”
Using 2.5% Sasobit in the liquid binder had never been tried before in the U.S., even though a 2.5% mixture is common in Europe. Binders in Europe are harder than those over here, requiring more Sasobit. Most warm-mix projects completed in the U.S. have used 1.5% Sasobit. Knife River decided to try 2.5% to see if there were any additional benefits in increasing the additive in the liquid binder.
Trucks using flowboy trailers transported asphalt to the jobsite, and a windrow fed material to the paver throughout the project. “We like to use the flowboy trailers because they give us a longer window to run the paving laydown consistently,” said Collins. “Consistent paving produces better results in smoothness.”
Knife River used Ingersoll Rand DD118 compactors for vibration during breakdown and intermediate compaction. An Ingersoll Rand DD130 compactor was used for finish compaction. “We like the DD118 compactors, because they allow us to be quicker, more consistent and use less equipment on the jobsite, which in turn allows us to be competitive in this market,” said Collins.
An Ingersoll Rand Blaw-Knox PF3200 paver was used to place the asphalt. “We’ve done research and we get the best smoothness numbers with the Blaw-Knox pavers, so we stay with them,” said Collins. In April 2007, Volvo Construction Equipment purchased the Ingersoll Rand road machinery line.
Using a conventional asphalt mix design, the asphalt was mixed at the plant at 320°F. “The mix lost 10 to 15° on the haul from the plant to the windrow,” said Collins. “An additional 20 to 25° was lost from the windrow to the screed.”
Average temperatures recorded with the asphalt were 285°F at breakdown compaction, 245° at intermediate compaction and 175° at finish compaction.
The specification for compaction on the job is 93.5% or higher. With the conventional asphalt, Knife River achieved an average of 94% compaction.
Next, Knife River produced and placed the mix with 1.5% Sasobit in the liquid binder. Typical warm-mix asphalts with 1.5% Sasobit are produced at the plant with a temperature between 50 and 60° cooler than conventional mixes. Knife River produced this mix at the plant at 265°F, 55° cooler than the conventional mix. At the windrow, temperatures were recorded at 250°F and 225° behind the screed, 220° at breakdown compaction, 175° at intermediate compaction and 150° at finish compaction.
“One of the first things we noticed when we switched over to the warm-mix asphalt was that the screed’s angle of attack flattened out slightly,” said Collins. “We had to make a correction in order to make sure it didn’t affect smoothness.”
Even at lower temperatures than the conventional asphalt, the 1.5% Sasobit mix still achieved 100% pay for reaching mat density requirements.
With the 1.5% Sasobit mix successfully placed, it was time to try the 2.5% mix. The 2.5% mix was produced at the plant at a temperature of 255°F, 65° cooler than the production temperature of conventional asphalt. In front of the windrow, the 2.5% Sasobit asphalt was 240°. Behind the screed the asphalt was 215°, at breakdown compaction the asphalt was 210°, at intermediate compaction 175° and 150° at finish rolling.
“The operator of the intermediate compactor told me he could tell the difference in the asphalt because he was getting feedback into the compactor,” said Collins. “The operator felt like he was trying to compact the previous day’s pavement. The pavement sets up almost instantly in the intermediate phase.”
In order to correct this problem, Collins said he had the intermediate compactor operator work closer to the breakdown compactor. Another solution considered was to remove the intermediate compactor from the job, but removing the intermediate compactor did not make sense because of the short time that Knife River was working with the 2.5% Sasobit. Had there been more time to work with the warm-mix asphalt, Collins would have removed the compactor to see what kind of results would have been achieved.
Initial core densities for the first try of the 2.5% Sasobit mix showed 94 to 95% compaction. Since the required compaction densities for the job were still being achieved, Collins decided to lower the mix temperature of the 2.5% at the plant even further to a temperature of 225°F. Beginning at this temperature point meant that the asphalt was 210° at the windrow, 200° behind the screed, 195° at breakdown compaction, 160° at intermediate compaction and 130° at finish compaction.
Even when mixing the asphalt at 225°F, Knife River was recording between 93 and 95% densities on the finished mat with a nuclear gauge.
“By the time we got done lowering temperatures, we cut 95° off the production temperature of the mix,” said Collins. “Most of the literature on warm mix says that 50° to 70° can be cut off the production temperature. We achieved that with the 1.5% Sasobit. Once we went to the 2.5% Sasobit, the asphalt was even more pliable.”
In fact, Knife River never hit the lowest temperature of production of the 2.5% Sasobit asphalt. At 225° at production, the asphalt was still achieving required production at compaction.
“What we came up against was the capabilities of the plant, especially in keeping the baghouse warm enough,” said Collins. “When the temperature of asphalt production is dropping, the baghouse temperature is dropping too. You can’t have temperatures below 212° in the baghouse for very long because water condensation occurs on the side of the baghouse. We knew we could produce the mix cooler and that we could compact it, but we couldn’t go down any farther because the plant couldn’t do it.”
Future No. 1 pick?
One of the things that Collins learned about warm mix is training the plant operators how to produce the asphalt. “The training of the people running the plant to produce this mix at the lower temperatures is the biggest obstacle to overcome because they’re not used to working at these temperatures,” said Collins. Collins is interested in trying to change the flight of the asphalt plant or looking into other types of plants to see if he can avoid the water condensation on the baghouse in the future.
Producing warm-mix asphalt at cooler temperatures than conventional asphalt means that less fuel is used for the burner. Less burner fuel means lower production costs. “We saw a 15 to 20% decrease in fuel and that’s significant considering all of the starting and stopping we did on this project,” said Collins. “A contractor could save 30 to 40% on fuel depending on how low the temperature is and if it’s produced at a consistent rate.”
The next time Collins works with warm-mix asphalt, he wants to monitor the difference in emissions. “There were no blue smoke emissions, including at the plant and during laydown,” said Collins.
With emission limits getting stricter, Collins believes that the elimination of blue smoke will be one factor that pushes the use of warm-mix asphalt to the forefront. However, the costs involved in using warm-mix asphalt also will play a role. While less fuel is used to produce the asphalt, the Sasobit additive to the liquid binder to the asphalt is more costly than the fuel savings.
“I don’t think the transportation departments look at warm-mix asphalt as an option yet, but they will,” said Collins. “What we found is the fuel savings alone is not going to cover the cost of the warm-mix product. You have to find a combination of things you can do to reduce cost. For instance, we could have removed the intermediate compactor from our job, but we didn’t because we worked over a short time frame. As contractors do more warm-mix asphalt projects, they will be able to figure out how to cut things they don’t need for the job to make it more cost effective.”
The benefits of warm-mix asphalt in terms of environmental impact and time allowed for compaction over long-haul distances has convinced Collins that contractors and transportation departments will experiment and find the best way to use this innovative pavement.
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