“On this project Caltrans is using an asphalt-rubber, gap-graded mix, which in and of itself is very temperature-sensitive,” says Michael Plouff, applications engineer, MWV Asphalt Innovations. “Not only are they using that mix, but they are hauling it 2 1/2 hours to place along the coast, and doing it all at night. This is important for District 1, as they can have very cool temperatures, even in the summer, and they are limited as to what they can use for their pavements by the conditions. They feel that warm mix will give them the opportunity to use rubber mixes and open-graded mixes along the coast.”
They feel that way with good reason: the average nighttime low along the Mendocino coast in June is 48 deg F, and in July, when this work was undertaken, is 49 deg F, according to U.S. Climate Data. The weather often is drizzly and rainy, even in summer.
“The plant is a long ways away, and it’s such a long haul to get here,” said Dave Chang, pavement coordinator for Caltrans District 1. “But when the mix gets here, it still can be rolled and worked at 240 to 250 deg F, while conventional mix has to be at least 280 deg.”
In addition to the 53-mile, 2- to 3-hour-long haul, there are many variables that can pull the mix temperature down, Chang said. “When you dump it in the material transfer vehicle – which is a requirement for our thin blanket overlays – it heats up that big chunk of iron, and that sucks some of the heat out of it. The paver will pull more heat out of it, as will the existing pavement surface. And the coast is cooler, but WMA allows us to put the mix down at cooler temperatures, as last night we got down to 45 degrees F and with conventional asphalt we don’t like to place it below 60 degrees F.”
A similar WMA project in the district in September 2008, California 1 near Point Arena down the coast, placed a polymer-modified, open-graded, WMA friction course containing Evotherm DAT under similar conditions, with long haul and cool ambient temperatures. “Three years into the project, the Point Arena job looks really good, without any signs of raveling,” Plouff said. “Typically three years into an open-graded friction course wheel paths would already have begun raveling away.”
In addition to its functional attributes, Caltrans looks at WMA in terms of ‘green’ technology, as not as much energy is required to make the mix, Chang said, adding rubberized mix also is a green technology. “It utilizes tires that would have gone into a landfill,” Chang said. “But there are functional reasons to use rubber. Rubber ‘gives,’ and as a result should last longer than a conventional open-graded mix. In our district we have severe winters, and an OGFC might last five years along the coast. But we are hoping to get at least seven years out of this thin blanket overlay, which is an inch and a quarter deep.”
The RHMA-G was gap-graded, but not open-graded, and was produced by prime contractor Granite Construction Inc., at its Ukiah North plant. Evotherm DAT warm mix additive and metering equipment was supplied by Telfer Oil Co., Martinez, CA.
At the plant the Evotherm DAT warm mix additive was blended at a rate of 5 percent of total binder into the mix at the plant, which was producing the 1/2-inch-NMAS, open-graded mix at 310 degrees F at about 280 tph using an Astec double-barrel drum plant.
The mix arrived at the jobsite approximately 2 1/2 hours later at 250 to 260 degrees F, where it was dumped into a material transfer vehicle, eliminating the longitudinal windrow often used “out west.” The WMA temperature at the screed was about 240 degrees F as the material was placed and compacted into a 1 1/4-inch lift.
Granite was using three rollers, of 15-, 9-, and 12-ton size. “The first two, including the breakdown roller, are running in vibe mode,” Plouff said. “The breakdown makes a couple of passes in vibratory mode, the intermediate makes one up in vibratory and comes back in static, and way back is the finish roller, which is just taking out marks in static mode.”
When visited, Granite’s crew was placing and compacting about 2,000 tons of rubberized warm mix asphalt on the project, which was two lanes wide plus shoulders, just under eight miles in length, for a total of 13,100 tons.
Because at 1 1/4-inch the lift is so thin, Caltrans does not have a density spec for this mix. “Instead Caltrans has a ‘method’ spec, which specifies how many rollers, and of what weight, are required,” MWV’s Plouff said. “No rubber- or pneumatic-tired rollers are permitted because the rubberized mix would stick to the tires.”