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Introduction
Results of a recently completed project in
the San Antonio district show that an overlay with Permeable
Friction Course (PFC) can dramatically improve the performance
of a Continuously Reinforced Concrete Pavement (CRCP). This project
is the first of it's kind in Texas. However, this practice has
gained wide-spread acceptance in Arizona. As a result of their
previous successes, the Arizona Department of Transportation
(ADOT) recently launched a campaign to use PFC to overlay most,
if not all, existing CRCP in the greater Phoenix area.
Results from the Texas Department of Transportation's
(TxDOT)'s project in San Antonio show that an overlay of only
1.5 inches of PFC:
- improved the ride quality of the existing CRCP by approximately
61%
- improved the skid resistance by over 200%
- reduced the noise levels by an average of 14 decibels (dB).
A noise reduction of 3 to 6 dB is normally
considered to be very good. The noise reduction of 14 dB experienced
on the San Antonio project is not only considered outstanding
but is very possibly the largest noise reduction ever recorded
on a TxDOT project.
PFC overlays on CRCP appear to merit serious
consideration, given that Texas has a large amount of existing
CRCP that currently ranks poorly in terms of ride quality and
noise.
Background
The project is located on IH 35 between mile marker
166 (near Walzem Road) and 168 (near Weidner Road). The New Braunfels
Area Engineer (Greg Malatek, P.E.) and his assistant (Michelle
Kopp, P.E.) were the engineers in charge of the project. The
project was let in May of 2002, awarded to Dean Word Company,
Ltd, and completed in the fall of 2002.
The existing CRCP was constructed in the early
1980s. The existing CRCP was generally sound, with only minor
distresses.
Safety concerns were the primary reasons for placing a hot
mix overlay on the CRCP because skid resistance of the existing
CRCP was low and the roadway had a history of numerous wet weather
accidents. In addition to the safety concerns, the existing CRCP
was also extremely rough and, therefore, extremely loud. Complaints
were common. In some ways it represented a "worst- case-scenario"
of pavement per-formance. It was not comfortable, but it was
durable. In other words, it was "a problem that wouldn't
go away."
The Area Engineer chose an asphalt rubber (AR)
version of PFC for the overlay. AR (which contains approximately
18% crumb rubber) was chosen as the binder due to its excellent
adhesive characteristics and history of successful performance
on CRCP overlays in Arizona. PaveTex Engineering designed the
PFC mixture and Cox Paving provided the AR binder. The mix was
designed to have 8.3% binder and a total of more than 18% air
voids.
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To date, TxDOT has approximately 20 overlays
with PFC. PFC mixtures
are used extensively throughout the southern region of the United
States.
The most common mixture of PFC utilizes fibers and polymer modified
asphalt. PFC with AR is the second most common. PFC is the surface
mix
of choice in Florida, Georgia, Alabama, South Carolina, and Arizona.
It is
gaining popularity in Texas, New Mexico and other states due
to its benefits:
- reduced hydroplaning
- improved skid resistance
- reduced splash and spray
- improved visibility of pavement markings
- reduced traffic noise
- improved ride quality
Results
Before and after ride quality results are presented in Table
1. Ride quali-ty
was measured using the International Roughness Index (IRI) with
a
high-speed inertial profiler. On average, the roughness was reduced
by
128 inches per mile with the PFC overlay. This represents approximately
a
61% improvement in ride quality. Research has shown that improving
the
ride quality of CRCP pavements
can significantly
extend its performance
life by reducing the
dynamic loading associat-ed
with roughness.
Table 1. Average Ride Quality Measurements
The before and after sound pressure (noise)
measurements are presented in Table 2. The measurements were
taken along the edge of the pavement using handheld noise meters.
The results in Table 2 are the
average readings taken
at several locations along the project.
The locations were documented so that
the before and after readings could be
taken at the exact same locations. On
average, the noise was reduced from 85 to 71 dB. The reduction
of 14 dB is significant because sound pressure is measured on
a logarithmic scale. In laymen's
terms, the noise was reduced by more than half. Noise reduction
of this
magnitude can be considered even more significant when compared
to
what is normally achieved by constructing noise walls along the
highway. Such construction is relatively time consuming and expensive.
Costs typically run over 1 million dollars per linear mile.
After the PFC overlay on this project, numerous
compliments were received related to noise reduction from local
business owners and residents. Numerous positive comments were
even received on a radio "call-in" talk show.
Conclusions
Early results from TxDOT's first PFC overlay
on CRCP are extremely positive. If this project has the long-term
success experienced in Arizona, this paving strategy should dramatically
improve the performance of CRCP
pavements in Texas. This combination strategy has the potential
to optimize the durable properties normally associated with CRCP
and add the safety and comfort properties associated with hot
mix asphalt pavements. PFC and CRCP may be a winning combination
to address TxDOT's vision to provide pavements that are comfortable,
safe, and
durable.
If you would like more information, contact
Dale A. Rand
(512.506.5836) or Amitis Meshkani (512.506.5847) at the Flexible
Pavements Branch of the Construction Division.
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