Changes in mix temperatures and designs, as well as restrictions placed on paving windows, are presenting contractors with a growing list of challenges -- and opportunities -- on asphalt paving projects. Following is a look at three recent paving projects, including the mixes used and the effects on the equipment and techniques applied.
WMA is the cure for hospital lot
Pioneer Paving & Grading, Albuquerque, NM, had an opportunity to bid a large project for Christus St. Vincent Regional Medical Center in Santa Fe, NM. The project included pulverizing and paving of two roads on the hospital grounds, construction of a new parking lot and repair and overlay of nine others.
Don Rooney, Pioneer's owner, approached the project's head engineer with a proposal to use warm-mix asphalt (WMA) for the 3,276-sq.-yd. medical/dental parking lot. "He liked the idea right away, but was concerned about the cost," Rooney says. WMA can cost up to $2 per ton more than traditional hot mix.
Rooney worked out a one-time deal with LaFarge Asphalt, the local hot-mix supplier. "They wanted to get WMA out in the market, and get it down so more people know about it and could come and take a look at it," he says.
WMA is produced at temperatures generally between 50° F and 100° F below that of most hot mix. It requires foaming of the asphalt cement using water or additives during mix production. This foaming reduces the viscosity of the asphalt cement (AC), enabling the aggregate to be coated at lower temperatures.
"Because of the flowability and the way you get density, you can actually pave in cooler weather and extend your season," says Rooney. The mix can also be transported longer distances from the plant.
On the day of the project, Rooney called the plant 60 miles away and ordered 364 tons of WMA for the 2-in. overlay. The WMA was at 275° F at the plant, 250° F coming out of the truck at the jobsite and between 220°F and 230° F as it was laid.
"With the screed heated, we backed in the first truck and unloaded. After the first pull, I asked the crew if they noticed a difference," Rooney says. "They noticed right away that it was easier to shovel and much easier to rake."
The WMA also proved easier on the paver. "Flowability through the paver seemed to be easier and the mix coming through the screed laid a nice mat," Rooney says. "We started at 10 a.m. and by about 1:30 p.m., we had more than 300 tons down. The roller could hardly keep up with the paver."
Because WMA technology reduces mix viscosity, it also reduces the challenges of compaction. "You have more time to get proper density because the temperature isn't as high at the beginning, so it doesn't drop off as quickly," Rooney points out.
Rollers were watered similar to hot-mix asphalt, then a vibratory roller was used for one pass in the breakdown phase. "When it cools enough so you can almost put your hand on it, we hit it with the pneumatic roller, then finished with the steel roller with no vibration. The crew could roll faster than usual and rolling wasn't as much of a chore," says Rooney. "And roller marks seemed to roll out easier. If the pneumatic didn't take them out, the static steel would."
On the third pull, the crew had to wait before hitting it with the roller, but had no trouble compacting the mat. "It was about 195° F when we finally got the roller on it, and it rolled in well and the joints just disappeared," says Rooney. "Then we back-rolled to a smooth finish."
Plate compactors were also used on the job and didn't operate much differently than on a regular mix. "You could wait a little longer to get on it," says Rooney, "and it still smoothed right out."
County uses WMA to attack thermal cracking
Crow Wing County in Minnesota is in the second year of testing WMA to ameliorate thermal cracking and improve binder durability. Following a successful test project in late 2008, a second test was conducted on County Road 2 in 2009. It consisted of placement of two layers of bituminous mix containing a warm mix additive and 30% reclaimed asphalt pavement (RAP).