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Road Science
Perpetual Pavement, Two Years Later
The asphalt industry’s concept for long-life pavements is getting a
good look by state and local agencies and highway engineers.
by Tom
Kuennen, Contributing Editor
After two years of promotion to road agencies and contractors, the
asphalt industry’s perpetual pavement campaign is shifting to the field
as technical investigations begin and in-field construction proliferates.
The pace of development has been rapid.
Many states have launched hot-mix asphalt construction projects that
meet perpetual pavement design criteria, and more are on the way for 2004.
The asphalt industry has released a new software program that will
help road agencies and contractors design perpetual pavements. PerRoad 2.4
was introduced in January and its first version may be downloaded now
(details below). Training sessions will be held throughout the United
States in 2004.
A new National Cooperative Highway Research Program project (NCHRP
9-38) will study the resistance of asphalt pavements to fatigue cracking.
The project was awarded to a research contractor in January. When it
concludes in two years, the project likely will provide additional support
for propagation of perpetual pavement designs.
Field validation of an HMA mix that uses perpetual pavement design
began in October in the second phase of testing of the National Center for
Asphalt Technology’s Pavement Test Track near Auburn, Alabama.
The asphalt industry has found that the cracking and seating of
deteriorated portland cement concrete pavements, followed by thick asphalt
overlays, perfectly fits perpetual pavement criteria. The industry has
created demonstrations out of such projects around the country. The
crack-and-seat projects help make up for the slow pace of true
remove-and-replace reconstruction projects involving perpetual pavements,
and the inherent strength of this base permits the use of thinner asphalt
sections.
 As the concept matures, perpetual pavement promotional attention is
shifting from decades-old existing projects that meet perpetual pavement
criteria, to new projects in the field.
In addition, the marketing campaign has moved to local venues, with
state asphalt pavement associations becoming very active in promoting
perpetual pavements among their state departments of transportation and
local road agencies through publications, personal outreach, project open
houses, and seminars.
“From my view of the world, perpetual pavements have taken off
rapidly,” said Jim Huddleston, P.E., executive director, Asphalt
Pavement Association of Oregon. “The concept is really exciting, not
only for DOTs, but for local agencies as well.”
In 2004, state associations will be critical in getting the word out to
interested road agencies, Huddleston told Better Roads editors. “A lot
of local agencies are asking,” he said. “And it’s not just DOTs. I
was asked by our local American Society of Civil Engineers chapter to make
a presentation. And I spoke in January to the metro Portland counties and
cities. The idea of being able to manage a pavement at-grade, forever,
rather than reconstruction or overlays, is very appealing.”
Durable pavements
First introduced in the trade press in these pages two years ago (The
Quest for Long-Life Asphalt Pavement, February 2002, pp 30-37), perpetual
pavement is a consolidation of the asphalt industry’s best concepts for
durability to compete with long-lived concrete pavements.
The perpetual pavement concept was launched by the Asphalt Pavement
Alliance in a joint promotional effort with the Asphalt Institute, the
National Asphalt Pavement Association, and the State Asphalt Pavement
Associations, representing local contractor associations in 36 states.
A perpetual pavement is defined by APA as an HMA pavement designed and
built to last longer than 50 years without requiring major structural
rehabilitation or reconstruction, and needing only periodic surface
renewal in response to distresses confined to the top of the pavement.
 Perpetual pavements are designed with thick layers of asphalt of
different mix formats, with a sacrificial driving course on top. This
driving or friction course is intended to be periodically cold-milled and
overlaid with more HMA to restore condition.
Ideally, scheduled preventive maintenance and periodic renewal of that
sacrificial driving course would be the only work required on a perpetual
pavement after initial construction. Such work could be done in a classic
mill-and-fill operation at night, with reopening of the roadway in the
morning, minimizing impact on the driving public.
Understanding that fatigue cracking is the Achilles Heel of asphalt
pavement longevity — and that fatigue cracking begins at the bottom of a
pavement structure and works its way to the top — perpetual pavements
are planned from the bottom up: The lowest layer is designed to resist
bottom-up fatigue cracking; the middle layer uses an asphalt mix designed
to support anticipated traffic loads; and the top layer can be of any HMA
mix design.
Perpetual pavement’s increased durability is a product of mechanistic
design, instead of long-standing empirical design. In a mechanistically
based pavement, designers analyze how traffic stresses induce strain that
will affect the pavement’s performance, taking into account material
qualities and thickness.
By contrast, an empirical design takes the results of existing
experience — in the industry’s case, the AASHO Road Tests — and
makes the design conform to a test design that successfully withstood
loadings that mirror those anticipated for the new construction.
“By designing the pavement to keep strain below the critical level,
fatigue failure is avoided and perpetual performance can be assured,”
said Fred F. Frecker, P.E., president and executive director, Flexible
Pavements of Ohio, the state contractor association. “Structural and
aeronautical engineers have used mechanistic design principles for years.”
New software aids design
New pavement design software introduced in January will help road
owners, engineers, and builders design to perpetual pavement criteria. The
first public version of this software, PerRoad 2.4, is available now.
Developed by NCAT’s Dr. David H. Timm, assistant professor of civil
engineering at Auburn University, the software is a perpetual or
long-life, flexible-pavement design tool, to be used in conjunction with
applicable design standards.
This design tool is a mechanistic, not empirical, procedure for design
of long-life asphalt pavements. “PerRoad is a new tool for the industry,”
said Mark Buncher, P.E., director, field engineering support, The Asphalt
Institute. “This is huge, because now we have a tool that will help
agencies start designing perpetual pavements.”
The big challenge to design always has been how to get started, Buncher
told Better Roads editors. “A mechanistic analysis is needed to
determine the stresses and strains in the asphalt layers, and to make sure
you get below the 70 microstrain figure in the bottom layer,” he said.
“You can’t do that without mechanistic analysis, which a lot of
agencies did not have the ability to do. Now they do.”
PerRoad 2.4 performs two levels of analysis. The first, deterministic,
is based purely on nominal design values — for example, average
stiffness or tire load. The second level of design incorporates
reliability analysis and predicts the amount of risk associated with a
particular design. A design is not considered complete without the
reliability analysis, according to Timm.
With this software, a design engineer will define the trial pavement
structure, including the number of pavement layers, material types,
material properties, variability and perpetual pavement thresholds.
Climate is a part of the equation; the engineer must specify the duration
of the seasons and material properties in each season. After a trial
structure has been entered, the analyst must enter the levels of loading
anticipated for the pavement.
Worst-case pavement responses are calculated, as well as the
probability that user-defined thresholds will be exceeded during the
design life. A trial design is judged to be non-perpetual if any of the
performance thresholds have been exceeded.
If that’s the case, changes in the design thicknesses can be made
until the pavement responses are below the threshold. The program’s
deterministic mode may be used to estimate appropriate design thicknesses,
and probabilistic mode may be used as the basis for final design.
Finally, a cost analysis can be performed to estimate the cost per
lane-kilometer or lane-mile, based on the design thicknesses and materials
in the pavement structure.
Ohio tests the water
In the meantime, state DOTs have been busy with their own perpetual
pavement projects, many involving rubblizing or crack-and-seating of
existing concrete pavements.
Over the last two years, major perpetual pavement projects have been
launched in California, Wisconsin, Michigan, Texas, Illinois, Ohio,
Oregon, Pennsylvania, Maryland, and Kentucky.
Ohio began its first perpetual pavement project on I-77 near Canton in
2002. The 2-mile project involved removing a four-lane concrete highway
and replacing it with a new six-lane asphalt perpetual pavement.
“Perpetual Pavement is going to change the way we build roads in
Ohio,” said Fred F. Frecker, P.E., president and executive director of
Flexible Pavements of Ohio, the state contractor association. “The type
of congestion you see on I-77 as they replace the worn out pavement will
not be repeated. Transportation departments won’t have to dig up and
replace a perpetual pavement because it won’t wear out. This means a
curtailment of construction-site traffic jams and eventually big savings
in the long run.”
Ruhlin Construction was the general contractor and Northstar Asphalt
was the paving subcontractor. Construction on northbound lanes continued
through the end of the 2002 season. In spring 2003, traffic was diverted
to the newly paved northbound lanes while the southbound lanes were
rebuilt.
Costing just over $16 million, the project used more than 164,000 tons
of asphalt. The 4-inch fatigue-resistant asphalt base layer was bid at
$4.45 per square yard. The rest of the pavement, 13.25 inches of perpetual
pavement layers, cost $19.76 per square yard, according to FPO. “This
cost is approximately the same as it would have been using traditional HMA
paving methods,” they said.
The project evolved through the subcontractor’s initiative. During
the bidding process, Northstar Asphalt suggested Ohio DOT give perpetual
pavement design a try as a demonstration project.
“Using a perpetual pavement [on I-77] was a means of insulating us
against warranty remedial work,” said Jeff Wenger, Northstar Asphalt, at
the 57th annual Ohio Transportation Engineering Conference last November.
“That is, perpetual pavement is a premium design and as such would not
likely require maintenance during the [seven-year] warranty period. Using
a fatigue-resistant layer provided a measure of confidence that the
pavement would not suffer damage from excessive bending occurring during
and after construction.”
To get maximum exposure, FPO partnered with Ohio DOT, the Federal
Highway Administration, the APA, and contractors Ruhlin and Northstar in
sponsoring an open house. Nearly 200 engineers from all over the state and
even from other parts of the country attended the event.
In 2004, work will begin on another section of I-77 in the Akron-Canton
region. This $18 million, Ohio DOT project will be constructed as a
perpetual pavement as the result of a change order between Ohio DOT
District 4 and contractors Ruhlin and Northstar, and will complement the
2002-03 project.
“The first direction of this project was completed last year, and the
second direction will be built this year,” Frecker told Better Roads
editors. “This whole job will have been built to perpetual pavement
standards. Instrumentation placed in last year’s project indicates that
‘microstrain’ levels at the bottom of the pavement were in the 30s,
and it’s generally accepted that 70 is what we’re shooting for. The
pavement design obviously is doing what it’s supposed to be doing.”
Also this year, U.S. 30 in Wayne County, Ohio, will be reconstructed as
an entirely new four-lane pavement on new alignment, Frecker said. But
there’s a twist: one set of lanes will be full-depth asphalt, and the
other set of lanes will be completely reconstructed top to bottom as a
concrete pavement, thus providing the DOT with a long-term
apples-to-apples comparison of performance.
“One direction is going to go asphalt, one direction concrete,”
Frecker told Better Roads editors. “Concrete will [also] have an
opportunity to show what they can do for a long-life pavement.”
Perpetual Pavements in Illinois, Texas
In Illinois, state legislation required the Illinois DOT to demonstrate
construction of pavements designed for a 30-year life cycle. IDOT chose a
section of I-70 in Clark County near Casey, Illinois, to be reconstructed
with an asphalt pavement designed for 30 years of traffic. This section of
I-70 is heavily traveled, with trucks accounting for 49% of the traffic.
In this instance, in-place rubblization and seating of the existing
concrete pavement was included in the project. The 11-mile project used
550,000 tons of asphalt.
At the project’s peak last July, IDOT, the FHWA, and the Illinois
Asphalt Pavement Association cosponsored a perpetual pavement open house,
bringing together road agency and private sector observers from Illinois,
neighboring states, and Canada. A report on this project appeared in our
January 2004 issue (Full-Depth Asphalt Gets Tested in Illinois, pp 64-67).
The Texas DOT is applying the perpetual pavement philosophy to its
interstate renewal projects, with three such projects let in late 2003.
Successful use of perpetual pavement design in two TxDOT districts led to
two more of these HMA projects there, I-35 in Hill County and I-35 in Webb
County, according to the APA.
In Texas, APA reported, SH 114 in the Fort Worth District includes
segments where an HMA perpetual pavement and a thick, continuously
reinforced concrete pavement were being constructed in segments with
heavily loaded aggregate truck traffic. This will provide a direct
comparison between HMA and PCC performance.
The Hawaii DOT now uses a design period of 50 years for high-volume
urban highways and tunnels, 30 years for medium volume urban and high
volume rural roads, and 20 years for other pavements. “The High Volume
Urban Highways and Tunnels design period of 50 years essentially makes
these pavements perpetual pavements,” notes the Hawaii Asphalt Paving
Industry, the state association.
West Coast perpetual pavements
In January 2004, the Oregon DOT let the contract for reconstructing I-5
near Albany with a rubblization of the existing concrete, followed by
placement of 12 inches of asphalt. This design was developed by Dr. Jim
Lundy, Oregon State University, and OSU student Sarah Bultena, using a
mechanistic analysis.
Lundy’s analysis incorporated predictions of material properties and
performance using Superpave materials, and the fatigue limit concept
coming from recent research on perpetual pavements.
The analysis indicated that the proposed pavement will develop no
fatigue damage in the base layers after more than 40 years of traffic.
This will allow the Oregon DOT to manage this pavement with thin surface
course treatments for an indefinite length of time — but more than 40
years — following the perpetual pavement playbook.
This project will encompass 140,000 tons of asphalt. It will have a
10-inch base of 19 mm Superpave mix using a PG 70-22 binder, and 2-inch
surface course of 19 mm open-graded friction course with PG 70-28 binder.
“The DOT has chosen to mill the old asphalt off, rubblize the
concrete beneath, and then overlay, finishing with an open-graded friction
course,” APOA’s Huddleston told Better Roads editors. “The
open-graded is the Oregon F-Mix, which is our version of an OGFC.” The
project is about 4 miles in length and located about 15 miles south of
Salem, the state capital.
In California, the first “new” pavement to be built to perpetual
pavement standards was the reconstruction of the Long Beach Freeway,
I-710, in 2001-02. This design placed asphalt over cracked-and-seated
concrete, except under overpasses where overhead clearance did not allow
additional buildup of pavement. Under overpasses, the existing concrete
was completely removed and replaced with full-depth asphalt.
The full-depth sections under bridges consisted of a total thickness of
13 inches of HMA, including a top layer of open-graded friction course
over 3 inches of a rut-resistant mixture containing an engineered binder.
The next-down HMA layer was a 6-inch-thick dense-graded mix made with a
relatively stiff binder. The bottom, fatigue-resistant layer, was a 3-inch
binder-rich mixture.
Elsewhere, the cracked concrete was overlaid with 5 inches of
dense-graded mix, followed by a 3-inch rut-resistant layer. The overlaid
PCC sections had an open-graded surface mixture.
“The existing PCC pavement along the 2.3-mile stretch of I-710 in
Long Beach was rehabilitated successfully with long-life asphalt concrete
in eight 55-hour weekend closures, two weekends earlier than initially
planned by Caltrans,” said concept designer Carl L. Monismith, P.E.,
University of California-Berkeley, in a paper at this year’s
Transportation Research Board meeting in January in Washington, D.C. “The
project proved that asphalt concrete can be used as the material in
long-life fast-track pavement rehabilitation projects, even on the most
heavily truck-loaded route in the state.”
Alternate bidding in Kentucky, Maryland
In June 2003, construction was completed on a long-life asphalt
pavement on I-65 in Kentucky. The pavement was designed for a 40-year life
and carried a 10-year warranty, reported the Plantmix Asphalt Industry of
Kentucky, the state contractor association.
The project was bid in December 2001 with alternate long-life asphalt
and concrete pavement designs, each meeting the 40-year ESAL criteria. The
project was advertised with a minimum warranty of five years, extendable
(with a credit) up to 10 years.
There were two bidders, one asphalt and one concrete. Both bidders
chose 10-year warranties, but asphalt was the winner, bidding $18.7
million versus $21.4 million for concrete. The contractor earned the
maximum bonus for early completion.
And last summer in Maryland, the Maryland Asphalt Association reported
that the State Highway Administration took alternate bids for pavement
type on a project incorporating a perpetual pavement design.
The large project, an extension of Maryland 43 (Whitemarsh Boulevard)
from U.S. 40 to Maryland 150 northeast of Baltimore, was open to both
asphalt and concrete bids; the asphalt specification called for 135,000
tons of HMA, while the concrete specification called for 117,000 square
yards of 12-inch, non-reinforced PCC.
There were eight bidders for the project, but only one bid a concrete
pavement. The successful asphalt bidder was $3 million below the only
concrete bid.
The perpetual pavement design for the Maryland 43 project consisted of
a 2-inch stone matrix asphalt surface course, 12 inches of Superpave, 6
inches of graded aggregate base, and 12 inches of select borrow for
capping.
NCAT tackles perpetual pavements
In October, field validation of an HMA mix, which incorporates elements
of perpetual pavement philosophy, started at NCAT’s Pavement Test Track
near Auburn, Alabama. This work is part of the second phase of testing at
the track (see NCAT Pavement Test Track Puts Superpave to
Test, February
2003, pp 42-51).
During the track’s downtime last summer, a test section was
constructed with a rich HMA base course with a binder content of optimum
plus 0.5%. This rich base has the potential to minimize propagation of
fatigue cracks from the bottom upwards.
“This work will substantiate some of the perpetual pavement issues,”
said Dr. Marvin Traylor, P.E., director of engineering & research,
Illinois Asphalt Pavement Association. “The [track’s] second phase is
taking a look at the total pavement structure to find out how stresses and
strains develop in thick, full-depth HMA pavement, and add to the body of
knowledge that suggests properly constructed full-depth pavements don’t
fail structurally, but merely wear out at the top.”
NCHRP study nears launch
Another important study of perpetual pavement will launch this year
under the sponsorship of NCHRP. NCHRP 9-38, Endurance Limit of Hot Mix
Asphalt Mixtures to Prevent Fatigue Cracking in Flexible Pavements, is a
$750,000, 24-month project that should define an endurance limit for
strain in the lower layers of HMA mixtures. This, in turn, should result
in more efficient structural design of pavements for mixtures of different
characteristics. Staff contact is Ed Harrigan.
Study objectives are to test the hypothesis that there is an endurance
limit in the fatigue behavior of HMA mixtures, measure its value for a
representative range of mixtures, and recommend a procedure to incorporate
the effects of the endurance limit into mechanistic pavement design
methods.
Fatigue cracking originating at the bottom of an asphalt structure has
long been acknowledged as the most costly form of distress to correct
through rehabilitation, NCHRP observes. “Bottom-up fatigue cracking
occurs when repeated wheel loads impose tensile strains of sufficient
magnitude to initiate cracking that eventually propagates up to the
surface. Factors contributing to this form of distress include inadequate
pavement structure, weak underlying materials, and HMA mixtures with
inadequate material properties.”
HMA pavements that exhibit good long-term performance have
characteristics that prevent bottom-up fatigue cracking, NCHRP says. They
have a sufficient thickness of asphalt to limit the tensile strain at the
bottom of the HMA structure so that bottom-up fatigue cracking is not
initiated, they have a sound foundation to support the structure, and the
HMA mix exhibits sufficient flexibility to counter the initiation of
bottom-up cracking at low levels of tensile strain — all elements of
perpetual pavement design.
“Field experience suggests that an endurance limit, that is, a level
of strain below which fatigue damage does not occur for any number of load
repetitions, is a valid concept for HMA mixtures; its quantification could
aid in the efficient design of long-life flexible pavements with a
significantly reduced life cycle cost,” NCHRP said. This could result in
deep HMA pavements that are not over designed for their application, thus
not more expensive than they need be.
New Perpetual Pavement Award Winners
The Asphalt Pavement Alliance announced the winners of its Perpetual
Pavement Awards for 2003 in January.
The award recognizes asphalt pavements that are a minimum of 35 years
old, have never had a structural failure, have not been overlaid more
frequently than an average of 12 years, and demonstrate the qualities
expected from long-life asphalt pavements.
In making the awards, APA demonstrates that perpetual pavement design
concepts are not new.
The awards will be presented to the owners of the pavements at a
breakfast ceremony March 16 during World of Asphalt 2004 in Nashville,
March 15-18. For more information, visit www.asphaltalliance.com.
The winners, as determined by the National Center for Asphalt
Technology, are:
- Arizona DOT, for a section of I-17.
- Minnesota DOT, for a section of Trunk Highway 71.
- Missouri DOT, for a section of U.S. 63.
- Nebraska DOT, for a section of U.S. 20.
- New Jersey Turnpike Authority, for the Garden State Parkway.
- Ohio DOT, for State Route 73.
- Oklahoma DOT, for a section of I-35 60 miles north of Oklahoma City.
- City of Toronto, for the Don Valley Parkway.
Each winner will receive an engraved crystal obelisk and will have his
or her name and project added to a permanent plaque, which is kept at NCAT.
Long-life Asphalt Symposium
The National Center for Asphalt Technology at Auburn University in
Alabama will hold an international symposium on Design and Construction of
Long-Lasting Asphalt June 7-9 this year.
The event is sponsored by the International Society for Asphalt
Pavements and co-sponsored by the National Asphalt Pavement Association
and the Federal Highway Administration.
Emphasis will be on materials, mix design, and construction, and
procedures used for pavement design for long-lasting pavements. Topics
will include construction issues, quality control/quality assurance,
contracting methods, structural design, and more.
For more information, contact NCAT’s Carol Tapley at 334-844-6228, or
at ctapley@eng.auburn.edu, or visit NCAT at its new Web address of
www.ncat.us.
For More Information
More information is available about hot-mix asphalt perpetual pavements
from these sources.
Design Software. The new PerRoad 2.4 perpetual pavement design
software can be downloaded at www.eng.auburn.edu/users/timmdav/PerRoad.msi.
This initial release is free, to be used as-is with no warranties as to
accuracy or correctness at this time.
Rubblization. Rubblization of existing concrete pavements is a prime
component of today’s perpetual pavement applications. Last summer
Asphalt Pavement Alliance released an interactive CD-ROM entitled
Rubblization: The Quick, Cost Effective, Environmentally Friendly Fix for
Failed Concrete Pavement. The CD incorporates video segments and
PowerPoint presentations with voice-over narration, and allows users to
navigate menus for more information. It’s available at www.asphaltalliance.com.
Asphalt Pavement Alliance. The Asphalt Pavement Alliance has a
variety of information on perpetual pavements, including an interactive
CD. Documents for download at no charge include the essential, 26-page
Perpetual Pavements: A Synthesis; the 118-page TRB Circular 503 on
Perpetual Bituminous Pavements, the groundbreaking Perpetual Pavement
Concept Paper by Huddleston, Buncher, and David Newcomb of NAPA; and a
Perpetual Pavements PowerPoint presentation. Access all of them by
visiting www.asphaltalliance.com, and selecting Perpetual Pavements.
NCHRP 9-38. More information about NCHRP 9-38, Endurance Limit of Hot
Mix Asphalt Mixtures to Prevent Fatigue Cracking in Flexible Pavements,
can be accessed at www4.trb.org/trb/crp.nsf/
All+projects/NCHRP+9-38.
Rubblization Guide. The Minnesota Asphalt Pavement Association’s
Web site has a construction guide on rubblization. Locate it and other
related documents at www.asphaltisbest.com/resources.asp.
Reprinted from Better Roads Magazine
March 2004 |
