This article originally ran in the March 2011 issue of The Asphalt ProMagazine. This digital reminder, with edits to remove information that has become overly discussed in our industry, is a service for new subscribers and new visitors to theasphaltpro.com. Watch the February 2012 issue for an update on the WMA mix design status.
by Sandy Lender
Perhaps you’ve heard the stories of paving crews working with HMA and not noticing when the foreman surreptitiously signaled for the plant to send warm-mix asphalt (WMA) instead. The crew members kept right on paving and working the mat as if nothing had changed. This anecdotal evidence of WMA’s behavior characteristics proves the mix’s similarities to HMA in the field in a handful of cases.
Paving consultants don’t recommend you trick your crews on a regular basis.
They also don’t recommend you overestimate the ease of compacting a WMA mat. WMA does behave similarly to HMA, according to most experts interviewed for this article, but subtle differences require attention to ensure you get density in the mat. Depending on the mix design, according to Bomag’s Chuck Deahl, some WMA mats have proved easier to achieve density than their HMA counterparts.
First, you want to define WMA by its temperature, not its process. There are two additive processes—chemical and organic—and a mechanical process of foaming to create an asphalt mix in which the temperature can be reduced.
The National Asphalt Pavement Association (NAPA) says conventional HMA is produced at 280 to 320oF (140 to 160oC) while WMA is produced at 212 to 280oF (100 to 140oC). The Texas Department of Transportation (TexDOT) has districts writing the maximum delivery temperature allowed on a warm-mix project as 235oF into their WMA spec. Those are numbers you can take to the bank.
The secret lies in the mix design and lift thickness.
Brad Arnston is the vice president of construction for Knife River Corporation’s north central division in Sauk Rapids, Minn. His division has performed two WMA projects to date and he supports the lift-thickness theory. The first project they performed was an overlay paving 2 inches of Minnesota’s MV3 mix.
“On this job, we were able to get better results on density and lower fuel consumption. On the second job, we paved 1.5 inches of SP440. With this mix, we couldn’t make density no matter how many rollers we put on the road. The conclusion was the material was too coarse for only laying 1.5 inches of mix.”
Bill Rieken, paver application specialist at Terex Roadbuilding, Oklahoma City, agreed that there are minor differences in the way WMA behaves in the field from HMA. Essentially, best practices should prevail.
“There are some minor differences in how warm mix behaves in the field, and much of this will have to do with how it’s compacted,” Rieken said. “Modified rolling patterns to achieve density, the number of compactors in the paving train, and the temperatures that each compactor hits the mat, etc.”
Lay a Test Strip
Don’t forget about the test strip. Richard Kramer of Roadtec, Chattanooga, listed this best practices tip as vital for working with the warm-mix mat.
“The roller men have to learn the proper breakdown and temperature zones for rolling patterns,” he said. “Quality assurance people should be very helpful for this. They should check passes for how much is needed and when.”
That means taking readings with a non-destructive device often. All presenters at the warm-mix seminar at theNAPA annual meeting agreed that testing was vital for a project’s success. Test and re-test throughout the project to make sure you’re achieving density. Those shady spots that affect an HMA mat affect a WMA mat, too. Put your density gauge there and see what kind of reading you get.
The first step of finding the proper breakdown zone is solved for you right here. Everyone agrees: get on the mat directly behind the screed. Don’t wait. In fact, Brodie Hutchins of Vogele, Antioch,Tenn., likes the idea of getting more compaction with the screed so the breakdown roller hits a mat that’s already at 90 percent density.
“When the mix isn’t cooked as hot, it’s not as viscous and the screed floats,” Hutchins explained. He recommended using a tamper bar on the screed to achieve a level of compaction in the WMA mat prior to breakdown rolling. “If compaction’s in the plate first, you get more time. You’re getting on at 90 percent instead of 80 percent.”
While that theory offers the crew more compactive effort at the screed, it slows the paving train. But Hutchins is a proponent for paving speeds of 30 to 40 feet per minute to set up continuous, non-stop paving that’s coordinated with the plant and delivery vehicles.
“That’s an even, steady pace that eliminates starts and stops,” he said. “There’s plenty of time to use a tamper screed that accomplishes up to 90 percent of density before the breakdown roller even approaches the mat.”
No matter what type of density you start with, make sure the roller approaching the mat is approaching at its hottest. Rolling zones are a function of time and temperature. They also require certain equipment and contractors should be wary of removing any of that equipment.
Perhaps you’ve heard stories of crews that were able to take a roller off the train because they were achieving density with only two machines. That’s a great anecdote and a nice fuel and time savings for that particular crew with that particular mix on that particular day. Don’t assume that’s the norm.
“For WMA, we’re using the same asphalt plants, just with modifications,” Deahl said. “We’re using the same paving equipment and the same rolling equipment. Here’s where I want to caution AsphaltPro readers. Don’t assume you can remove a roller because you’ve heard of other contractors who’ve done it. We’re using the same equipment for a reason.
“The number and type of rollers you have on a job is mix-dependent,” he continued. “Compaction of warm mix and hot mix asphalt follow basic best practices. Depending on the mix design, warm mix has been easier to compact, and we have reduced the number of rollers, but this is the exception.”
For a project Deahl assisted on, the company added Sasobit at the plant to create a WMA mix design of PG82-22 polymer-modified. The temperatures in this example hover near what most would consider the high end of the WMA spectrum. The crew paved 12-foot wide passes at 25 feet per minute. The lift was 2 ½ inches loose.
* Plant temp: 315oF
* Project temp: 287 to 305oF (off the screed)
* The breakdown zone for the 66-inch double-drum vibratory roller (at 3,000 vpm) was 150 feet in the 287 to 305oF range and achieved 90 to 92% density
* The intermediate zone for the 66-inch double-drum vibratory roller (at 3,000 vpm) was another 150 feet back in the 200oF range and achieved 92 to 94.3% density
* The finish roller was a 66-inh double-drum oscillating drum roller set at vibrate going in and static coming out in a 5-pass pattern; achieved 94.7 to 95.6% density at less than 200oF
Tony Limas of Granite Construction presented his company’s successes with WMA at NAPA’s 56th annual meeting in Orlando in early February. He reported that the company has 60 plants retrofitted with WMA mechanical foaming devices and has placed 400,000 tons of WMA to date. His discussion showed a variety of projects that ranged from normal to challenging.
In one instance, Granite crews performed an emergency repair in the cold and snow after a landslide damaged a pavement. They not only had to de-ice the roadway before paving with WMA, they had to wait on a truck for three hours when it broke down with a load of mix in its bed. Limas reported that the truck arrived with mix at a temperature of 260oF. The mix went down “just fine, with no problems.” Here are the stats from that project.
* Plant temp: 275oF
* Project tamp: 268 to 273oF
* 92 to 93% RICE
* Limas reported that the 3-inch lift helped retain heat in the mat to aid in compaction.
For a project at the Anchorage airport, Limas reported the crew used the Evotherm product to create a PG64-34 polymer-modified mix. It was a ¾-inch dense-graded top lift.
* Plant temp: 235oF
* Project temp: 230oF
* 95% RICE
* For this project, the crew used two DD-130 steel wheel rollers to get compaction and removed the pneumatic-tired roller.
Limas also discussed a project where the crew created a notch wedge joint with no difficulties.
* Plant temp: na
* Project temp: 250oF
* Ambient temp: 55oF
* 92 to 94% RICE
For a three-month detour on SR70, Granite crews worked on a 5 percent grade with a ½-inch PG64-16 with Evotherm additive.
* Plant temp: 240oF
* Project temp: 230oF
* 92 to 94% RICE
The equipment achieving these results depends on well-trained and skilled operators. As Deahl explained, the basics still apply. The paver and roller operators still need training and skill; they still need to know the best practices for achieving best results.
“Best practices still apply,” Deahl said. “Know the specifications of the project. Know the layout. Know the operation of each roller. Balance production. Establish a pattern to achieve coverage, density, smoothness and balanced production. Speed can kill. Double-drum vibratory rollers should be traveling at 2 to 4 miles per hour. Pneumatic-tire rollers at 2 to 3 miles per hour. And static steel wheel rollers at 3 to 5 miles per hour.”
The workers doing hand work have another take on the discussion. Experts agree that WMA makes hand work such as luting and raking more difficult. Be aware of this on urban projects where manhole covers, utility cuts and other structures require detailed touches. Knife River’s Arnston said his crews would rather work with hot mix when it comes to hand work.
“Our crews would rather use HMA when there is a lot of hand work to do; the mix is hotter and easier to rake. With the lower mix temperature of WMA you will have more residue on all of the above [augers, endgates, shovels].”
Clean-up may require a little more of your favorite biodegradable release agent, but crews can breathe easier when working around WMA. Make sure crew members remember to wear safety gloves even though they’re working with a cooler product than they’re used to. Mix cooked to a temperature of 212oF or higher can offer a blister on exposed skin. This is still asphalt and it still requires a healthy respect.
“What contractors have to remember is WMA is basically the same mix design as hot mix,” Terex’s Rieken said. “The only exception is the agent—whether that’s water, as is the case with the Terex system, or an additive—that allows mixing temperatures to be lowered. Therefore, warm mix has the same propensity for material and thermal segregation as if it were produced as hot mix.”
“Listen to the people who have had experience with WMA,” Roadtec’s Kramer advised. “Then apply what you learn. This is true in all paving practices.”