Continuous paving is a key factor to achieving consistent density and is paramount when it comes to smoothness with the P-401 WMA mixture,” states Kevin Riley, quality control manager for Aggregate Industries Northeast Division, when talking about the company’s recently completed rehabilitation of Runway 15R-33L at Boston Logan International Airport.
Aggregate Industries produced and laid down 61,000 tons of polymer-modified warm mix asphalt (WMA) during five phases that started mid-July and ended mid-September 2012. The Massachusetts Port Authority (MassPort), the owner and operator of Boston Logan International Airport, has been specifing WMA for runway and taxiway rehabilitation since 2008. An additional 18,000 tons of P-403 hot mix asphalt was produced for runway shoulders.
The P-401 WMA mixture is produced using 5.4% asphalt cement modified with latex and Sasobit wax additive. The mixture contains 20% RAP and 1% lime for anti-strip. Nominal maximum aggregate size is 3/4” (19 mm). Plant output temperature is controlled at 270° F. The asphalt plant, located in Saugus about 15 miles north of Logan, produces around 275 tons per hour.
“We know quite a bit about production of warm mix in general and the P-401 mix in particular,” says Riley. “And we’ve learned the workmanship it takes to get acceptable mat and longitudinal joint density. The trick is managing the laydown and compaction processes so certain key elements — paving speed, mat thickness, rolling patterns and mat temperature — have no significant variation.”
Planning for consistent paving
Ed Bisnette is the paving superintendent with primary responsibility for production planning and the paving process. Bisnette says planning was excellent because of the cooperation between the three management arms of the project. “We have a strong relationship with the owner and the prime contractor,” Bisnette explains. “MassPort and (prime contractor) Barletta Heavy Division helped so much in their execution of phasing and closures.”
Bisnette is also adamant that before discussing the paving process, he has to acknowledge the role of the milling division, which milled 200,000 square yards during the project.
“On a runway project, when your fills (paving depths) are based on the grid, the milling has got be done right. We had three crews from our milling division working ahead of us. They made as many corrections as they could. The grid they cut for us was excellent. We had some deep fills in the first lift but most of the corrections we made were in the quarter inch to half inch range.”
That meant that paving depths were consistent as the paving crew hit the grades. “By the time we were paving the top lift, we made few grade corrections and those were in eighth inch to quarter inch range,” says Bisnette.
The crew set up the grade control system to create a fast screed response in order to make grade corrections, if needed, within the 25-foot grid stations. Grade sensors on both sides were positioned just ahead of the auger shaft. Right side grade control made grade corrections. Left side grade control matched the longitudinal joint height.
Consistent mat thickness, which the crew handled well, is one of the key elements to high quality results mentioned by Riley. Another factor is consistent paving speed. That factor gets a lot of attention.
“There are many challenges when it comes to getting a steady flow of mix to the paver due to delays caused by security inspections, escorted travel within the restricted area and the normal rush hour traffic in and around Boston from the production facility to Logan,” says Riley. “As a section of the airfield was paved to full width each day that meant backing up and taking off 12 times per day — material delivery was calculated around that process.”
Bisnette goes on to say, “When we start a pull, we pave at one speed and never stop.” Using a material transfer device (MTD), in this case a Blaw-Knox Mobile Conveyor, permits non-stop paving – as long as there are enough trucks to allow completion of a pull. How does Bisnette plan for enough trucks?