Later that day Forbes settled on 350 degrees F as the temperature at which the mix should leave the plant, but would revise that temp downward as summer like temperatures returned. "If we return to 90 degree temperatures with bright sun and no wind, the astute paving engineer will look at the exit temperature day by day, hour by hour," Forbes says. "We know from test strips that we were getting the required density behind the screed at about 270 degrees F."
New products seal long joint
New products developed by Heritage Research Group will be used on the track to seal the longitudinal joint from water penetration.
The first is J-Band, a kettle-extruded, modified liquid that will be placed 12 inches wide and 1/8-inches thick, centered beneath the longitudinal joint. "Its purpose is to melt and migrate up into the surface and make that area, left and right of the joint by 6 inches, absolutely impermeable," Pine says.
The second is another extruded product, XJB. "Between passes, we will paint the face of the joint with XJB — for extruded joint bond —which will improve the elasticity and compliance of the joint," Pine says. "So if those joints want to pull away from each other, the joint will stretch and not crack."
Placement and compaction
In September, the first full pass of intermediate course was being placed. This followed over 300 tons of test strips.
Paving was being accomplished with a German-sourced Super 2100 paver from Vogele AG with 12 1/2-foot compacting screed distributed by Wirtgen America Inc. Initial breakdown rolling was being done by two 30,644-pound HD 130 rollers in static mode, from Hamm Compaction Division of Wirtgen. Active compaction was achieved by two Hamm 20,062-pound HD O90V oscillation compactors in oscillation mode exclusively, followed by a single HD 130 finish roller in static mode.
Liquid dish detergent was added to the roller water system to ensure that the SMA would not stick to drums.
"We're pretty confident that we're seeing 89 to 90 percent compaction behind the paver screed, the breakdown rollers are getting us 93 to 93.5, and the HD O90Vs in oscillatory mode are getting us to 95.5, 96 percent compaction," Forbes notes.
The Super 2100 was chosen for its better handling of the SMA. "The three keys to density of SMA in our mind here are temperature, temperature, temperature," Forbes says. "Getting that initial density is a very good thing in case we have issues with mat temperature later on. And getting 89 to 90 percent density at the screed means that the heat we have in the mat will be retained for a much longer time. It means we will have more flexibility in rolling patterns behind the paver."
"I give credit for equipment selection to paving contractor Grady Brothers Inc., especially the paver," Pine says. "Without a doubt we are getting much higher in-place density behind the screed than what we'd get with a normal paver, because of the tamper bar in front of the main screed and the two pressure bars behind.
"We're running in the high-80s in back of the screed, and that does a couple of things," he adds. "We don't have to put as much effort into compaction with the rollers to get up to the same density, and it helps us maintain temperature for a greater length of time, because the more dense the mat is behind the paver when the first rollers get on it, there will be less penetration of drum water into the mat and it won't cool down so rapidly."
On the first day of paving, the Super 2100 moved forward at 25 feet per minute, with rollers at a speed of 100 feet per minute. The two HD 130 breakdown rollers were being used as five pattern pass static, with a five pattern pass using the two HD O90V in oscillatory mode, and a final HD 130 finish roller in static mode. The Hamm roller's oscillation mode was essential in eliminating the risk of crushing the mix aggregate during compaction.
"We wanted to get the initial density with some kind of dynamic action, but were afraid of crushing the aggregate with the vibratory mode. Oscillation works perfectly," Forbes says.
Forbes had not used oscillation mode before, which might have been seen as a risk on such a high-profile, high-stakes project. "We'd never used them before, but our asphalt laboratory research facility, Heritage Research Group, has used them on state highway projects and found the numbers were quite a bit better when using oscillatory mode in conjunction with the static modes on SMA."