Super-smooth rides result from Texas urban repaving
Houston one-pass, hot in-place recycling wins performance bonus, while eliminating tack coat and keeping driveways clean.
Experience in Texas is proving that one-pass, hot in-place recycling of existing urban asphalt roads can be done swiftly and with bonus-winning smoothness.
In summer 2000, a hot in-place recycling contractor repaving a seven-lane, highly congested urban arterial highway in suburban Houston with such attention to detail that it qualified for 85% of a smoothness bonus, as subsequently determined by the customer, the Texas Department of Transportation, Houston District.
"State Highway 6, from south of I-10 to the Harris County line is a 45,000-vehicle per day project in a congested business/commercial area of Houston," says Jim King, Cutler Repaving Inc., Lawrence, Kansas.
"The traffic made it a hard job," King says. "The road is flanked by shopping malls and apartment complexes, a total business environment through the whole project. But we only disturbed traffic one time in each lane. We cut the traffic disruption by at least half. And while that may not seem like a lot, when you consider how many lane closures would be required, the saving are substantial."
Under these circumstances, Cutler earned a smoothness bonus of $46,000 out of a possible $50,000 on the 281,000-square yard, two month, $1 million-plus project.
In Texas, ride bonuses can be awarded on projects which involve pavement removal and leveling course, or overlays. The result can be smoother pavements.
"Based on the current criteria, the ride quality was excellent, and they got the bonus in accordance with the provisions of the specifications," says J.A. Yrigoyen, P.E., design manager, West Harris Area Engineer's Office, Texas DOT Houston District.
"The bonus was nice, but what we were trying to achieve was customer satisfaction," says John Rathbun, Cutler vice president-sales. "There were monetary benefits that we would realize, but we were more concerned with the state feeling our process had a lot of benefit and value for the money they spent, especially when they had not had any experience with the process up until then."
The state used a unique, on-pass hot in-place recycling process that reuses the existing deteriorated asphalt as a leveling course, and on it places a fresh layer of virgin hot-mix asphalt.
"The pavement is heated to about 350 degrees F," Rathbun says, "and once it's in a softened, pliant condition, it's scarified to a depth of 1 inch. A recycling agent that restores the viscosity of the aged asphalt is applied and mixed in., This material is then reapplied and distributed with a screed as a 1-inch leveling course."
While that material removes at 225 degrees F, a 1-inch virgin hot-mix asphalt overlay is places over the recycled leveling course. "What's unique is that the machine that does the scarification, applies the recycling agent, and places the leveling course, also applies the new overlay simultaneously," Rathbun says.
That benefits road users because there is no delay between the time the pavement is recycled and the time a riding or friction course is placed, resulting in a safer road for users.
But there is an engineering benefit as well "We achieve a thermal bond between the recycled layer and the new layer," Rathbun says. "From an engineering point of view, there is no declamation between the recycled layer and the new overlay. That's very important in predicting life-cycle performance. The same heat that's used to take the road apart is used to put it back together, and the two layers are effectively compacted into one lift."
The virgin surface is applied by a separate, vibrating screed no more than 3.0 feet behind the leveling course screed. It's feed from a hopper at the front of the repaver via a drag/slat conveyor chain which brings the hot-mix asphalt through a tunnel along the length of the machine, to the paving screed.
The result is a monolithic, 2-inch finished pavement that is equivalent in ride to a 2-inch mill and overlay. Because the machine travels 18 to 20 feet per minute, traffic barricades can come down with great speed, with all reclaimed material used on the spot without hauling. Traffic can drive on the new pavement as quickly as with conventional paving, while driveways and intersections are blocked no longer than 15 minutes from start to finish.
The process also reheats the edge of adjacent repaved lanes, resulting in a more durable, higher-density seam between the driving lanes, Rathbun says.
The lack of a tack coast was one of the considerations of the Texas DOT in awarding the contract. For the client, the West Harris Area office of TxDOT's Houston District, customer satisfaction drives its choice of paving processes.
In this congested commercial area, elimination of the tack coat and its being tracked all over customer parking lots during paving was important as it improves site relations with the traveling public, Yrigoyen says.
"The reason we selected this process was the location is highly commercial, with a lot of driveway traffic," Yrigoyen says. "We wanted a process that was customer-friendly, allowing users to drive from the highway without tracking the tack coast that normally is required when we have an asphalt overlay. The tack coat is the most disturbing element for the traveling public because when vehicles cross the lanes to be paved, they pick up the tack coat, and it actually can get onto the cars - and into businesses - creating a nuisance.
"This process does not require any tack coat," he says. "In advance of the paving operation there is no tack in the roadway, and as we required ingress and egress to driveways from the open lanes, the process avoids spreading of the tack coat of passenger vehicles that would be traveling the roadway near the rehab operation, or into the parking lots of the businesses.
"State Highway 6 is a six-lane urban arterial with a continuous turning lane," Yrigoyen says. "They treated the existing asphalt surface down to 1-inch deep, and added a rejuvenating agent. Immediately behind they placed 1 inch of new asphalt, and compacted the two layers. A good bonding was created between the new and recycled surfaces." A specification permitting hot in-place recycling with new asphalt overlay has been in place in Houston for about a year, Yrigoyen says. So the year of 2000 represented the first year large-scale hot in-place recycling has taken place there.
"So far we have not had any problems, although the process is new in our area," he says. "We expect that this will be a good way to overlay our roadways, but time will tell."
Yrigoyen's office undertook two other repaving contracts by Cutler in summer 2000, on completely different types of roadways. They were the I-10 frontage roads, a two-month project of approximately 220,000 square yards of full-service highways paralleling an interstate expressway; and FM159, a two-lane highway in Waller County, Texas. There, 92,000 square yards were repaved in and around Hempstead, Texas, including sections with curb-and-gutter.