July/August 2002
New
Software Promises to Put Whitetopping on the Map
by Robert Otto Rasmussen, George K.
Chang, J. Mauricio Ruiz, W. James Wilde, Patricia Kim Nelson, Jason
Dick, and Dan K. Rozycki
Mending
deteriorated asphalt pavements with portland cement concrete is a
familiar technology. Highway engineers used whitetopping—concrete
overlays placed on top of asphalt—as early as 1918. Offering
benefits that include long life and superior bonding to underlying
material, whitetopping overlays grew in popularity through the mid-1970s,
and ultra-thin whitetopping burst onto the scene in the early 1990s.
Until recently, however, pavement engineers had no one clear resource
or set of guidelines that they could refer to when determining where,
when, or how to use whitetopping as a pavement option.
In
2001, to fill this knowledge void and help validate whitetopping as
a viable alternative, the Austin, TX-based transportation engineering
firm, The Transtec Group, developed design, construction, and rehabilitation
guidelines for whitetopping. Capitalizing on state-of-the-art computer
modeling technologies, the firm is developing a Windows®-based
software that pavement practitioners can use to analyze and compare
different whitetopping strategies. Balancing cutting-edge research,
field-tested best practices, and construction and traffic restraints
with economics, the project teams goal is to help make whitetopping
a more competitive alternative for roadway construction and rehabilitation
projects. By June 2002, the whitetopping software was nearing the
beta testing stage.
Through
Thick and Thin
Highway
engineers have met great success using conventional whitetopping
overlays—20 centimeters (8 inches) or more—for more than
60 years. And for the last 10 years, ultra-thin whitetopping—5
to 10 centimeters (2 to 4 inches)—has satisfied the need for
an effective low-cost overlay for intersections and low-traffic and
low-speed applications. What was missing, according to Jim Mack, executive
director of the American Concrete Pavement Association Northeast
Chapter, was a unified program to design thin overlays from 5 to 20
centimeters (2 to 8 inches).
Conventional
and ultra-thin whitetopping overlays are based on two different technologies
and bonding interactions, Mack says. The computer program
will bridge the gap between ultra-thin and conventional whitetopping,
enabling pavement engineers to design whitetopping overlays effectively
for any road application from residential streets to high-volume interstates.
With the whitetopping software, pavement practitioners will be able
to analyze all three whitetopping applications—ultra-thin, thin,
and conventional. The software will help construction and materials
engineers, construction supervisors, and contractors produce more
effection concrete mixtures, pavements, specifications,
and repairs using whitetopping overlays. The product will help engineers
choose the proper overlay thickness, joint spacing, and the optimum
surface preparation.
We
want States to think about whitetopping as another tool that they
can use to rehabilitate roadways, but they need to know how to use
it properly, says Ken Fults, director of the materials and pavement
section in the Construction Division of the Texas Department of Transportation
(TxDOT). The software will enable States to make better and
more rational decisions about whitetopping.
A
Systems Approach to Pavement Engineering
A
virtual bible for all things white-topping, the software demonstrates
the inherent value of approaching the world of whitetopping through
a systems approach. Rather than view the processes of white-topping
design, construction, and rehabilitation as independent sets of procedures—which
easily could have led to authoring three different sets of guidelines—the
software developers elected to employ a systems approach to the project.
By
integrating all three sets of procedures into one unified software
program, the project team created a practical and reliable one-stop-shopthat will
enable State highway agencies, contractors, and pavement designers
to design and build white-topping overlays efficiently, based on the
best data on materials, cost and safety available in the industry.
Highway
engineers used a similar approach when developing the original philosophy
behind the asphalt industry's Superpave which combines three
distinct components—binder specification, mix design, and performance
prediction testing—into one comprehensive system.
"The
whitetopping software will be for the concrete industry what Superpave
is for the asphalt industry." Bob Risser, executive director
of the Michigan Concrete Paving Association,
says. But more than just a set of design principles, the white-topping
software will provide a usable tool that highway agencies can use
on a daily basis to explore pavement overlay alternatives.
A
New Overlay Option
Highway
engineers traditionally perceive portland cement concrete pavements
as an option for new construction only, primarily for heavy-duty pavements.
But for pavement rehabilitation, agencies generally view hot-mix asphalt
(HMA) overlays as the first option, regardless of the existing pavement
structure. HMA overlay designs, however, are not usually as robust
as concrete. Economics and construction restraints often drive the
design of HMA overlays, resulting in typical thicknesses of 10 to
15 centimeters (4 to 6 inches), independent of the design procedure.
Many agencies regard an HMA overlay as an intermediate fix before
major rehabilitation or reconstruction is required. In many cases,
the length of service is expressed as a minimum requirement but not
geared to any type of service-related distress.
If
agencies used the same operational and economic criteria to evaluate
whitetopping options, the project team believes, portland cement concrete
overlays could become a more competitive alternativeto HMA,
especially in the category of 10- to 20-centimeter (4- to 8-inch)
overlays. According to Ken Fults with TxDOT, when most people think
of whitetopping, they think of ultra-thin whitetopping "because
it's the new kid on the block," he says.
Fults
adds, "State looking at flexible pavement deformations might
think that if they put down 4 inches of whitetopping, they've cured
the problem, but they may be creating a more severe problem if they
don't do it right. We want to make sure that any whitetopping job
gives the State the best product for the money."
Laying
the Groundwork
Four
primary objectives for the whitetopping project were identified. The
first of which was to document the performance of the three classes
of whitetopping overlays—ultra-thin, thin, and conventional—when
subjected to heavy loads. Researchers in several States previously
had constructed whitetopping sections and installed instruments to
monitor their performance.
The
software team evaluated several whitetopping projects in Colorado,
Georgia, Iowa, Minnesota, Missouri, and Tennessee for factors such
as bond strength and overall performance. The data gathered from the
study sites proved invaluable in the calibration and validation of
the design procedures selected for the project.
The
second objective was to develop a whitetopping design procedure for
each class of overlay, taking into account critical parameters and
site conditions such as surface preparation and the condition of the
interface between the concrete and the asphalt. Recognizing that a
systems approach to design would include mechanistic models and consider
both agency costs and user impacts, the project team determined that
the best way to meet this objective would be to develop a unified
framework for designing whitetopping overlays that optimizes not only
thickness but also the many other aspects of design, construction,
and rehabilitation alternatives.
The
third objective called for developing best practices and quality control
guidelines for each class of whitetopping to ensure the construction
of quality pavements. Toward this end, an expert review panel of experienced
industry and State engineers was assembled to share their experiences
and knowledge of whitetopping concepts, construction, and quality
control guidelines.
|
On
this resurfacing project at an intersection in Tennessee, the
State constructed a whitetopping overlay to mitigate a repeated
shoving problem with the underlying hot-mix asphalt. |
|
The
project team used a wealth of data collected from a whitetopping
project on Iowas Route 21, shown here, to validate the
numerous models in the analysis system used to develop the whitetopping
software. |
The
final objective aimed to complete the whitetopping life cycle by identifying
potential rehabilitation alternatives for each class of existing whitetopping.
To rehabilitate a degraded whitetopping successfully, highway engineers
would need to be familiar with design parameters and quality control
specifications, as well as whitetoppings sensitivity to environmental
conditions. The project team focused on predicting and isolating causes
of distress and relating those to suggested repair techniques. The
team also considered the remaining lives of underlying asphalt pavements
and their relationship to distresses in white-topping overlays.
The
team established an additional expert advisory group, the White-topping
Internal Technical Advisory Panel. Composed of representatives from
the Colorado, Michigan, and Texas highway departments and the concrete
paving industry, the panel provided invaluable consultation in shaping
the beta version of the software into a user-friendly format that
could be implemented readily in their home States and beyond.
Bob
Risser, with the Michigan Concrete Paving Association, was a member
of this panel. The goal that [the software developers] had all
along was that the tool would be usable by engineers on an everyday
basis, Risser says. We were the reality check for the
Ph.D.s.
Anticipating
value in using both synthetic and steel fibers in whitetopping concrete,
the project teams second objective involved partnering with
Synthetic Industries, Inc., and Master Builders, Inc., to investigatethe
effects of using fiber reinforcement in whitetopping concrete.
Developing
the Design Procedure
Using
the best available technologies, Dr. George Chang led the team of
software developers in creating a product that integrates environmental,
material, traffic, pavement response, pavement distress, and economic
(life-cycle cost) modeling. Carefully coded and thoroughly tested,
the end result is an accurate and practical software application that
makes performance predictions possible.
Environmental
modeling in the whitetopping software uses pavement profile temperature
models based on technology similar to that used in FHWAs HIPERPAV
system. (See Paving the Way on page 20.) By employing
finite-difference methods—mathematical procedures that determine
the stress deformation in a system such as whitetopping—the team
could correct some of the mistakes common to pavement temperature
methods used in the past. The developers tested and validated the
environmental model extensively, using field data.
The
material models include ones for concrete, HMA, subbase, and subgrade
materials. The team developed a number of concrete property conversion
modules to maximize the practical side of the software, allowing the
user to correlate various types of concrete strengths and moduli.
The HMA model selected for the software includes an innovative damage-adjusted
modulus model in addition to a sophisticated model to
consider
traffic speed, asphalt binder type, and aging. The soils model includes
a modulus estimation tool that enables users to enter a value back
calculated from falling weight deflectometer data—which provides
data on a pavements response to dynamic wheel loads—or
even just the soil classification.
The
traffic model includes a convenient tool to convert equivalent single-axle
loads to axle load spectra, which corresponds with the upcoming American
Association of State Highway and Transportation Officials 2002
Design Guide. The response and distress models also include state-of-the-art
methods such as finite element modeling.
To
meet the varying demands of the users, the whitetopping software provides
a range of analysis levels that enable users to run the program at
one of three different speeds. As a result, the software can serve
as a planning tool, a day-to-day analysis tool, and as a final design
tool.
Software
Development
Jason
Dick and the software development team handled the task of organizing
the data to generate optimized designs for whitetopping pavement.
The team used a strategy consisting of three components: a set of
inputs, an analysis of the inputs, and a life-cycle cost analysis.
The whitetopping software organizes similar inputs under one of the
following categories:
- General—Basic
strategy and analysis information, including names, locations,
distress thresholds, times, etc.
- Design—Items
such as the dimensions of the pavement and the geometry of the
items contained in the whitetopping layer.
- Materials,
Whitetopping Layer—Aspects
of the materials that make up the whitetopping layer and the characteristics
of the interface between the whitetopping and the underlying HMA
surface.
- Materials,
Support Layers—Inputs for
the existing HMA, base, and subgrade layers. The software can
estimate many of these inputs, so it is unnecessary for the user
to have knowledge of all the properties of these layers.
- Environment—Season
length and geographic location. Environmental data—based
on an average of 30 years of weather data—loads automatically.
- Traffic
Loading—Current level of
actual or estimated traffic loading, along with expected growth
rate over the analysis period.
The software enables the user to
change the units of inputs easily, and each input is checked automatically
before the analysis to ensure that it is within the valid range.
|
This
screenshot shows how the whitetopping software organizes similar
inputs together into one of several basic categories. By clicking
on the Whitetopping Design folder, a user can view
and adjust details about the whitetopping design, including
overlay thickness, dowel layout, and joint spacing. |
Depending
on the level of detail a user requires, the whitetopping software
offers three levels for analyzing a strategy with increasing accuracy:
preliminary, intermediate, and final. The final level is the most
accurate but takes more time to run than a preliminary analysis. Using
a new PC, run times vary from 15 seconds to upwards of 30 minutes,
depending on the level of sophistication the user wishes to achieve.
Having
keyed in the relevant project details, a user then can use the softwares
comparison module to select from a variety of pavement distresses,
including joint faulting, joint spalling (cracking), corner cracking,
transverse cracking, longitudinal cracking, International Roughness
Index (IRI), and serviceability (PSI). For each distress factor, the
user can compare alternative pavement thicknesses to determine the
overlay thickness that will provide the desired performance. The software
also displays user-defined threshold values to show how close the
distresses come to reaching these terminal values. Finally,
entering user-defined economic inputs, the user can calculate a strategys
estimated cost as well as a projected cash flow over the life of the
pavement.
The
softwares ability to help users ensure that the whitetopping
product performs as intended and provides the best value per dollar spent
is a key feature. Do you need fibers in the whitetopping? If so, what
kind—polypropylene, steel, etc.? How much saw cutting is needed?
|
By
selecting the Comparison tab in the whitetopping
software, a user can plot two different strategies on a graph
side by side. This screen capture demonstrates how the software
can generate a graph showing how a distress feature (Ride Index)
can vary by altering an input (in this case, white-topping thickness).
|
Anything
you do to the pavement structure, adding fibers or cutting, costs
time and money, Ken Fults at TxDOT says. If you can select
a less expensive fiber or reduce the amount of sawing and get as good
a product or better in the long run, then thats the kind of
information you want to know upfront. It all goes back to how cost-effective
whitetopping will be compared to other rehabilitation strategies.
To improve the functionality of the software, the developers also
added a library feature that enables agencies, companies, and other
users to develop a library of information that can be saved, reused,
or shared easily. Using the library feature, a State can save and
later recall commonly used strategies, mix designs, or aggregates.
Just like opening a spreadsheet file, a click of the mouse enables
the user to access a previously created strategy and automatically
import or export customized data for reuse or sharing with other users.
Another
key feature is the ability to compare strategies. The white-topping
software will not only plot the analysis results of two strategies
at the same time, but also will show the input differences between
these strategies. Users can save, e-mail, and share entire strategies
as easily as transmitting a word-processed document or spreadsheet,
facilitating comparisons of strategies created by different users.
Finally, the white-topping software contains a full-featured, nonproprietary
print engine with the capability to print all inputs, outputs, and
analysis charts.
Field
Calibration and Validation
To
build a sturdy house you need a solid foundation. Developing a software
tool that would analyze whitetopping design alternatives accurately
required extensive model calibration and validation. J. Mauricio Ruiz
and his team collected information for this purpose from existing
whitetopping sites across the country and coupled that data with experimental
ultra-thin whitetopping data from tests at FHWAs Accelerated
Loading Facility, located at the Turner-Fairbank Highway Research
Center in McLean, VA.
|
Researchers
at FHWAs Accelerated Loading Facility (ALF) in McLean,
VA, constructed eight examples of different types of ultra-thin
whitetopping. Each consisted of a different thickness, joint
pattern, use of fibers, and type of underlying hot-mix asphalt.
As expected, the research identified differences in the performance
of the various pavements under load. The researchers used data
gathered during this study to further validate the whitetopping
predictive models. |
The
team divided the model calibration and validation data into two categories:
pavement response models and pavement distress prediction models.
The pavement response models measure pavement stress and the deflection
inflicted by environmental and traffic loading. The models for predicting
pavement distress included those that measure structural distresses
in the field, such as cracking, faulting, and spalling as well as
functional distresses like the loss of ride quality.
The
developers evaluated a total of four existing whitetopping projects
in the ultra-thin, thin, and conventional categories. These sites
include projects in Colorado, Iowa, Mexico, and Tennessee. The team
collected the following information for each field site:
- Pavement
design information
- Construction
procedures
- Pavement
responses
- Evaluations
of the bond characteristics at the asphalt-concrete interface
- Structural
and functional evaluations, in terms of pavement distress and
ride quality From FHWAs Accelerated Loading
Facility,
the team collected experimental
data regarding white-topping design, construction, lab tests, responses,
and distress history. Research at FHWA included pavement deflection
testing, ground penetrating radar, core testing for material characterization,
bond strength testing, and joint and crack movement analysis.
Laboratory
Evaluation
A
robust laboratory evaluation was key to developing accurate models
to be used in the systems analysis tool. Dr. Patricia Nelson led a
team of researchers, with assistance from Synthetic Industries and
Master Builders, in conducting conventional testing methods such as
strength and modulus testing. To characterize the impact that fibers
may have on whitetopping pavements, the team also conducted residual
strength tests (ASTM C1399), fracture toughness tests, and fiber pullout
tests.
The
team tested a factorial of concrete designs with and without fibers
to determine what types of fibers work best for whitetopping. The
researchers also tested specimens of concrete from the various field
sites to supplement the data needed for calibrating and validating
the models. The laboratory testing enabled the team to evaluate thetrue
nature of the fiber reinforcement, ultimately leading to a more accurate
and reliable analysis tool.
Construction
and Rehabilitation Guidelines
Although
the use of whitetopping to rehabilitate existing asphalt pavement
is not a new practice, many in the pavement industry have had little
experience in building, maintaining, or rehabilitating white-topping
pavements. Dr. W. James Wilde led a team in developing two sets of
guidelines aimed at providing the best practical industry knowledge
of whitetopping pavements.
Topics
covered in the construction guidelines range from surface preparation
and traffic control to achieving adequate bond between the portland
cement concrete and HMA layers, and from curing and temperature management
to saw cut timing and the effects of tining.
Tapping
into the knowledge of experienced practitioners, the team also developed
whitetopping rehabilitation guidelines to provide agencies and contractors
with direction in planning maintenance and rehabilitation. Since the
rehabilitation guidelines cover ultra-thin, thin, and conventional
thicknesses, the team assigned limits that identify the most appropriate
rehabilitation options available for each thickness classification.
The guidelines explain, for example, that speed and cost are two major
considerations that drive the selection of rehabilitation options.
For a whitetopping pavement to be a feasible construction alternative,
it must last a long time and be repaired quickly while disrupting
traffic as little as possible.
Implementation
Pavement
practitioners constantly seek new and improved methods to meet the
ever-increasing demands for safe, durable, and cost-effective pavements.
Under the guidance of industry experts Ted Ferragut of TDC Partners
and Dan Rozycki, the whitetopping project team met this demand by
providing a software tool that will help pavement specifiers provide
better performing overlays that improve safety and reduce the impact
on road users during construction.
|
In
Mexico, the cement industry has worked closely with the government
to use whitetopping as a means to extend pavement life. One
of several whitetopping projects constructed in the last decade,
this major highway runs through Queretaro. |
Predicting
pavement performance and designing the most appropriate whitetopping
overlays demands thorough research, careful analysis, and well-informed
recommendations. Pavement practitioners demand simplicity, customizability,
and user-friendliness in a software tool. The project team successfully
marries these to create a whitetopping software tool that is both
comprehensive and easy to use.
Whitetopping is another option that cities, counties, and States
can use to maintain their roadways, says Steve Waalkes, director
of engineering and rehabilitation for the American Concrete Pavement
Association. This software is an excellent tool for the pavement
engineers toolbox. According to Waalkes, beta testing
and rollout are the next steps for the whitetopping software. For
more information about whitetopping or the software development project,
see www.whitetopping.com.
Acknowledgement
The
authors would like to thank all the entities and individuals who contributed
in providing guidanceand
expertise during this project, including the Task 3/5 Project Panel,
the Internal Technical Advisory Panel, Synthetic Industries, Master
Builders, and the Colorado, Iowa, Tennessee, and Texas departments
of transportation. The project team also recognizes Jim Mack of the
American Concrete Pavement Association for his leadership in this
project and support of the project panel. Cooperation from numerous
individuals across the Nation helped the project team assess the whitetopping
state of practice and develop a framework for the proper execution
of this effort.
Dr.
Robert Otto Rasmussen is vice president
and chief engineer of The Transtec Group in Austin, TX. He serves
as principal investigator of the Task 3/5 Project entitled, Performance
and Design of Whitetopping Overlays for Heavily-Loaded Pavements.
See also Paving the Way.
Dr.
George K. Chang is a project manager
with The Transtec Group. He serves as chief modeler and technical
code manager for the FHWA HIPERPAV II project and the whitetopping
project. Chang received his B.S. in agricultural engineering from
National Taiwan University and his M.S.E in environmental engineering
and Ph.D. in civil engineering from the New Jersey Institute of Technology.
Chang is active in the research fields of material modeling, nondestructive
testing and analysis, pavement structure modeling, and pavement system
analysis integration. He is a registered professional engineer in
New Jersey.
J.
Mauricio Ruiz serves as the field
validation team leader for the whitetopping project. See also Paving
the Way .
Dr.
W. James Wilde is a project manager
for The Transtec Group. He serves as the team leader in developing
the construction and rehabilitation guidelines for Task 3/5 on whitetopping
overlays. Wilde serves on the Transportation Research Board committee
A2A07 and the ASCE Airfield Pavements Committee. He received his BS
in civil engineering at Brigham Young University and M.S.E and Ph.D.
at The University of Texas at Austin. He is a registered professional
engineer in Texas.
Dr.
Patricia Kim Nelson serves as the
leader of the laboratory testing team on the whitetopping project.
See also Paving the Way.
Jason
Dick is a software developer at The
Transtec Group. He serves as a lead software developer on the Task
3/5 whitetopping project. Dick received his BS in electrical engineering
from the University of Texas at Austin. He is a member of the Institute
of Electrical and Electronics Engineers and an engineer-in-training
in Texas.
Dan
K. Rozycki is president of The Transtec
Group. He serves as implementation team leader of the white-topping
project. Rozycki received his BS in civil engineering from the University
of Texas at Austin. He is active in several organizations, including
the Transportation Research Board, Pyrogenesis, and the American Concrete
Pavement Association.
Other
Articles in this issue:
Taking Concrete to the Next Level
Getting It Together
Fine-Tuning Innovative Technologies
On the Road Testing Roads
Paving the Way
Making Roads Better and Better
Texas Tests Precast for Speed and Usability
The Biggest Bang for Your Buck
New Software
Promises to Put Whitetopping on the Map
Road
Map to the Future