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Weathering Texas Roads
TxDOT Districts
Using More Specialized Asphalt Mixes
With TxDOT districts throughout
the state using more specialized mixes such as PFC and innovative
products such as asphalt-rubber binder, Texas motorists can
look forward to traveling quieter, safer and smoother roads.
By Mary Lou Jay
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| Undersealing and
paving occur in a one-step process as an APAC-Texas Inc.
crew lays the thin-bonded PFC mix in El Paso, where the
district wants to see how pavement might survive the desert
heat. |
Texas thunderstorms can make for unpleasant
traveling for motorists. Visibility can become a problem and
vehicles can hydroplane on the thin skim of water left on
the surface. Pavement markings seem to disappear, making it
harder for motorists to stay in the proper traffic lanes.
Getting the water off the road as quickly as possible can
alleviate some of these problems. That's why TxDOT districts
are specifying more road projects with a type of hot-mix asphalt
noted for its drainage capabilities.
Permeable friction course, or PFC, isn't a new type of asphalt
mix--it has been used with some success in Texas since 1999--but
it is the next generation of the open-graded friction courses
that were used in the 1970s and 1980s. The use of OGFC was
discontinued when state DOTs found that the mixes didn't hold
up over time.
The new PFC mixes seem to last better in all kinds of weather.
"PFC reduces the amount of spray from vehicles during
and after rainstorms," said Marc Shepherd, public information
officer for TxDOT's Beaumont District. "Whereas traditional
asphalt sheds water on top of the road (due to finer materials
in the mix), PFC allows the water to go through. It has more
rock on rock in the mix with less finer materials. In short,
it sort of absorbs the water and pushes it out the sides."
A sealcoat applied to the road below the PFC forces the water
out the sides and protects the underlying layers of asphalt
from water damage.
PFC is also measurably quieter than the majority of hot-mix
asphalt pavements. The reason, experts think, may be because
the PFC is so porous the percussion of the tire hitting the
pavement travels down into the pavement, which disseminates
the noise.
Beaumont District: Reducing
Truck Noise and Spray The first time that
TxDOT's Beaumont District used PFC was in 2005 on the Beaumont
portion of U.S. 59, or the Eastex Freeway, Shepherd said.
The concrete highway, with an average daily traffic count
of 106,000, was in bad shape with many joint repairs.
The road is a major north/south route to Jasper and area
papermills, and it carries a lot of truck traffic. The trucks
were noisy on the concrete road and, when it rained, as it
frequently does in that part of the state, they created a
lot of spray.
After APAC-Texas Inc. performed the placement of an asphalt
PFC overlay, "We noticed a substantial difference,"
Shepherd said. "It reduced noise significantly (a drop
of approximately 10 decibel points). It greatly reduced the
overspray from 18-wheelers after a rainstorm."
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| North of Beaumont,
in another East Texas district, Moore Bros. applied a
sealcoat and PFC to a stretch of U.S. 69 in Tyler County,
where the road was first milled then immediately inlayed
with 4 in. of asphalt. |
This year the district has two PFC projects nearing completion.
One is the resurfacing of a 5.2-mi. section of SH 73 between
SH 82 and FM 366. The four-lane divided highway is a major
route from Port Arthur to Bridge City. It carries about 70,000
vehicles a day, many making their way to the chemical plants
and refineries between Port Arthur and Orange.
The second PFC project is an overlay of a 6-mi. stretch of
U.S. 69 between Port Arthur and Nederland. The four-lane highway
is another major north/south route, with many service-oriented
businesses along the way. APAC-Texas is serving as the general
contractor for both jobs, which cost a total of $13 million.
Motorists will notice once the PFC overlay is in place, Shepherd
said. "It's a far superior road; it's smother and quieter,"
he added.
| Maintaining the
Heat
Keeping asphalt mix at the right temperature is key
to building a smooth and trouble-free pavement, but
it's not easy to know that temperature as the asphalt
is going down.
"One of the continuing problems we have in Texas
is truck-end segregation," said Tom Scullion, program
manager at the College Station-based Texas Transportation
Institute, a state agency and member of the Texas A&M
University System. "At the end of a truckload,
a cold scab sometimes comes through the mix and gets
put on the mat, which prevents getting the right density.
It's difficult to see with the eye when the mix is going
down."
The formation of cold spots can depend on many factors,
including how the material is delivered, how it's getting
into the paver and what the temperature of the air is
during the paving. The cold spots often show up six
months later--when it's too late to fix them--and they
are more susceptible to surface failures such as potholes.
Now an invention from the Texas Transportation Institute
may help solve that problem. Funded by a grant from
TxDOT, Scullion has developed a device that can make
the critical laydown temperature measurement.
The device is a bar that hangs like a footplate off
the back of a paver. It has 10 infrared, built-in sensors,
and the information they collect is fed into a miniature
computer. The display shows what the mix temperature
is, and the computer also stores data for future analysis.
"This is a quality assurance matter," Scullion
said. "Contractors say they can fix the problem
if we can tell them that there is one. This bar will
tell them when there's a problem."
The infrared bar provides a more uniform, more accurate
measurement than individual measuring devices do, added
Tony Yrigoyen, transportation supervisor for TxDOT's
Houston District. "With the individual devices,
the angle where you shoot the surface is different every
time, so you get different readings," he added.
"But infrared gives you a homogeneous reading."
The infrared bar was initially tested in 2005 as an
independent stand-alone device. During the summer, Yrigoyen
used it on a road project in its new, mounted format.
Having a record of the readings can alert contractors
to problem areas and allow them to make adjustments
to their procedures, Yrigoyen said. "Our goal is
to place asphalt according to specifications, and this
is another aid that contractors will have to make sure
that what they're doing is in compliance," he said.
He added that the device allows evaluating in "real-time
conditions and provides a well-documented record of
operations every day."
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Tyler County: Changing
Specifications for Smoother Travel About 200 mi. north, Moore
Bros. of Sulpher Springs, is working on another section of
U.S. 69 in Tyler County that will end up with a PFC surface.
The two-year, $8.5 million project was to be completed by
the first of this month.
Most of the road is a two-lane highway with 12-ft. driving
lanes and 9-ft. shoulders, with some center turn lanes. The
work starts at the Woodville city limits and continues 8 mi.
to the south.
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| Moore Bros. recently
finished work on U.S. 69 near Woodville and was preparing
to apply a sealcoat, followed by a 1.5-in. layer of PFC. |
The pavement on the section required some work before it
could receive its top layer of PFC, said David Moore, vice
president of Moore Bros. "We had a total of 12,000 yds.
of deep-base repair, where we milled out the bad spots 10
in. deep and put in 10 in. of asphalt-stabilized base,"
Moore added.
At that point, plans called for the milling of the road 4
in. before laying additional asphalt base. "We knew the
road wouldn't hold up to that--to have traffic come through
after it had been milled--so we suggested milling and then
immediately inlaying the road with 4 in. of asphalt right
behind it," Moore said. He said another 3-in. layer of
asphalt stabilized base was laid on top.
Next came a 1-in. layer of type D asphalt, which is made
with a smaller aggregate than the asphalt base. "It really
smoothes the road out," Moore added. The company finished
the work during the summer and was preparing to apply a sealcoat,
which will be followed by a 1.5-in. layer of PFC.
"During construction of this road last year, we had
Hurricane Katrina come through," Moore said. "The
refineries in Port Arthur were down. Our liquid asphalt was
coming from Atofina in that area, so we had to wait a month
to start back to work."
Because of new TxDOT specifications, which call for a surface
temperature of 60°F during paving, work was suspended
from December though January.
Dallas: Asphalt Section Quiets Concrete Highway PFC is being
used to quiet a stretch of IH-30 through Dallas. APAC-Texas
Inc. of Dallas working under general contractor Kiewit Construction
of Fort Worth, was already doing the asphalt underlay for
5 mi. of the road when TxDOT asked it to place a PFC overlay
on a .25-mi. section of the road.
"On the east end of the project, IH-30 goes through
neighborhoods of some nice, old Dallas homes," said Kirk
Morris, area manager for the Texas Bitulithic Division of
APAC-Texas. "They had built screening walls for noise
control, but they couldn't get them high enough."
Bill Hale, TxDOT's Dallas District engineer, had the idea
of using PFC in the section.
APAC-Texas is conducting a test of two paving methods for
the PFC on the road. On the eastbound side, on top of the
concrete, it laid a PFC and a thin, polymer-modified emulsion
membrane using a specialized laydown machine called a Nova-Paver.
With the paver, crews could seal the underlay and place the
top course of PFC in one step, saving time and labor.
On the westbound side, the company laid one course of chip
seal then placed the PFC on top of that. The two sections
should eventually show if the new polymer-modified emulsion
membrane works as well and lasts as long under the PFC as
the traditional chip-seal coat.
In both cases, the PFC has reduced noise levels, Morris said.
Although no sound tests have been run yet, he said there's
no doubt the PFC is doing what it was designed to do. "As
you're riding along the concrete road and you hit this section,
the noise just goes away," Morris added.
El Paso: Riding on Rubber
Although PFC can be a good choice in areas that receive a
lot of rain, that's not a big concern for TxDOT's El Paso
District. It is more interested in seeing how a pavement can
survive the blistering desert heat.
For the past two years, the district has been testing an
asphalt mixture that incorporates a higher oil content and
added some recycled tires into the mix.
"We had some of our hot-mix pavements out here falling
apart on us, showing some premature stress and cracking just
three to five years after we've put them down," said
Tomas Saenz, El Paso District materials engineer.
| Warm Mix: The Next Hot Trend?
To make a hot-mix asphalt pavement, producers have
to heat the aggregates and the liquid asphalt binder
to extremely hot temperatures (between 300°F and
350°).
The process produces a strong and durable paving material
but requires a great deal of fuel and involves the release
of emissions during the manufacturing process.
Now TxDOT and other state DOTs are studying a new paving
mix called warm-mix asphalt, or WMA, that could reduce
emissions and cut fuel requirements.
"Basically, WMA includes an additive to facilitate
mixing and compacting," said Dale Rand, director
of TxDOT's Construction Division, Flexible Pavements
Branch. "It allows the plants to run at a lower
temperature, which lowers the emissions."
Plant temperatures for WMA are generally 250° and
at times even lower.
WMA has several other advantages. Because its production
releases fewer emissions, it is better for workers and
can be used in areas where air quality is a concern.
WMA doesn't require new technologies, since it is laid
like traditional hot-mix asphalt; in fact, it's easier
to work with because the mix is less stiff at lower
temperatures. And because outdoor temperature isn't
as big a concern, the paving season could be extended.
One drawback is the additional cost, which may run
as much as $5 a ton for the additives.
There are four WMA technologies currently under study.
European countries have been testing WMA since the late
1990s, and their initial success has led to several
U.S. trials.
TxDOT's San Antonio District plans to place a test
section on Loop 368 (Austin Highway), from Eisenhower
to Ritteman roads in the next few months.
The Texas Transportation Institute has also received
funding for a study that will evaluate the various WMA
paving mixtures and study the results of at least two
test sections in Texas that have used WMA mixes.
These and other studies should help determine how well
WMA pavements endure over time and if they are a viable
choice for Texas roads.
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After some research, the El Paso District theorized that
the hot, dry climate was prematurely oxidizing the pavements,
which had a low oil content. The small amount of binder used
in previous asphalt mixes--plus greatly increased traffic
on the roads in the district--were probably causing the problems.
TxDOT investigated Arizona's successful results with asphalt-rubber
binder and decided to attempt its own tests on several road
maintenance projects in the city of El Paso and El Paso County.
In 2005, TxDOT applied the mix as an overlay (usually 2 in.)
to approximately 50 lane mi. This year, the mix will be used
on another 70 lane mi.
The mix starts with regular asphalt oil, then introduces
15 to 20 percent rubber chips into the blend.
"These are actually ground up tires," Saenz said.
"We put the rubber chips into the digesting tank with
the oil for about an hour to let the oil absorb into the rubber.
Once that's done, the binder gets run into the hot-mix plant
like any regular binder."
The mixes that TxDOT is using on these projects now have
a 7 to 8 percent oil content as opposed to the 3 to 4 percent
used previously.
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| Night paving was
done in El Paso to avoid traffic congestion. APAC - Texas
Inc. used a Nova-Paver on the thin-bonded PFC mix made
with a tire-rubber-modified oil from Wright Asphalt. |
Although night paving is not the norm in the El Paso District,
much of the work on the asphalt-rubber binder projects has
been done at night to avoid traffic congestion.
The mix goes down much like any other, although the crews
use steel-wheeled rollers, not pneumatic, because the pneumatic
roller tends to pick up the fine aggregates. "The mix
itself retains heat, which allows the contractor to work with
it a bit longer," Saenz said.
The resulting road is a more attractive, darker color than
those with less oil and no rubber. "And the asphalt-rubber
binder gives us a more elastic and weather-resistant material,"
Saenz added.
There are other benefits as well. "We've been overlaying
some sections of IH-10 and it's a lot smoother and quieter
than what we've put down before," Saenz said.
Ride quality has improved, too, with the profilers used to
measure the road giving better numbers than in the past. Recycling
old tires into the roadway instead of stockpiling them in
landfills is also a plus.
"This is a new direction we've taken in the El Paso
District," Saenz added. "The Texas Transportation
Commission has told TxDOT it wants smoother, more durable
pavements, and our district is making progress toward that
goal."
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