(Host) Paving Vermont’s highways may seem like it’s strictly a matter of muscles and heavy machinery, but there’s also an exacting science involved.
In the last part of our series on Vermont’s highways, VPR’s Steve Zind takes us behind the scenes for an anatomy lesson in paving.
(Paving and car sounds)
(Hedges) “There’s just one reclaiming progect…”
(Zind) Michael Hedges is the man in charge of Vermont’s highway paving program. Usually he’s behind a desk in Montpelier. But today he’s on his way to the site of one of this year’s sixteen state paving projects – a stretch of Route 7 in the Middlebury-Salisbury area.
The project started with the removal of old pavement. A milling machine with ceramic teeth mounted on drums chips away the asphalt and dumps it into trucks. It’s a process they call mill and fill – or shave and pave. All of the old asphalt is reused.
Workers have begun to put down the first layer of new asphalt – called the leveling course. The asphalt is being produced at the Pike Industries hot mix plant in nearby New Haven.
(Zind) The plant is off the highway, surrounded by land scraped clean by the steady truck traffic and heaped with piles of sand and gravel. A line of swiveling sprinklers splatters water on the road into the facility to keep down the dust kicked up by the trucks.
The heart of the plant is a tower of dark metal that rises like a storm cloud full of noise and heat.
(Buxton) “We’re making 3/8 mix for the Middlebury-Salisbury project “
(Zind) Niall Buxton is a regional coordinator for Pike. He points to a large rotating drum over head that’s being loaded with gravel and sand carried on conveyer belts.
(Buxton) “The stone and the sand go through the dryer and it’s heated up by a 60 million BTU burner and it makes the stone approximately 350 to 400 degrees.”
(Zind) A set of steel stairs leads up into the plant past the burner which rages with the bright intensity of a miniature star.
(Zind) The key ingredient of hot mix is liquid asphalt.
(Buxton) “That’s where the liquid’s being introduced to the stone.”
(Zind) It’s combined with the stone and sand, and dumped into the trucks that haul the hot mix to the paving site. The plant turns out 140 tons of hot mix every hour, ten hours a day.
Underneath the dust, noise and brute power of this place, there lies an exacting science.
A blue cinderblock building next to the hot mix plant houses a laboratory where samples from batches being made are tested. Machines spin and compress the mixture to make sure it’s up to state specifications.
(Buxton) “That’s a Marianne Shaker. And it tells us how much of each size of stone is in the mix and we have a job mix formula which we have to hit.
(Zind) Hitting the right mix specifications within a narrow tolerance is key. Slight variations in the mix can cause the pavement to wear out too soon. The contract the state signs with Pike stipulates bonuses and penalties, depending on whether the job exceeds or falls short of state specifications.
(Buxton) “The potential exists for us to pave all day and either end up having to remove it or receive a considerably reduced payment. It’s a very, very risky business.”
(Zind) A state inspector will test the asphalt mix that’s being trucked to the paving job.
At the site itself, Hedges checks in with Tony Corse, the state engineer oversees the project.
(Hedges) “We just came down through the job and had a peek at it.
(Corse) “Things are running smoothly, I hope.”
(Zind) Corse’s field office is a trailer parked under a line of trees. On a table there’s a thick set of blueprints detailing every aspect of the project.
(Corse) “We inspect. We record. We tally. And we pay for.”
(Zind) Corse will spend the summer here. So will the men and women the contractor has hired to work the project.
(Truck beeping as it backs up.)
(Zind) On this particular afternoon they’re working just south of Middlebury. They cleared out of the downtown area early in the morning to avoid creating traffic tie-ups.
The equipment here is familiar to any sidewalk superintendent. The asphalt trucked from the hot mix plant is laid down by a slow-moving paver.
Rollers trundle back and forth to compact the asphalt. There’s more to them than meets the eye. Their metal drums actually vibrate. How much the soft asphalt is compacted depends on strength and frequency of the vibration. Pike Industries project manager Nick Chen says as the final layer of asphalt is being rolled, a technician will take readings with a density gauge.
(Chen) “And so by that we can tell how effective some of the rolling patterns are, whether or not we need to turn up the amplitude or the frequency on the rollers.”
(Zind) The asphalt needs to be compacted enough to prevent water from seeping in and not too much, so there’s room for the asphalt to expand in hot weather. After the job is finished, the state will do its own testing to measure the density and smoothness of the surface. Once again, there are bonuses or deductions for the contractor, depending on the results.
But despite all the care that goes into paving Vermont’s roads, time, traffic and the elements will eventually take their toll. And this road will again have to torn up and repaved.
For Vermont Public Radio, I’m Steve Zind.
(Paving sounds fade out.)
(Host) For the complete paving series and more information about Vermont’s roads, go to vpr.dot.net.