U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: July/August 2002|
Issue No: Vol. 66 No. 1
Date: July/August 2002
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 team's 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 Iowa's 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 whitetopping's 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 team's 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 FHWA's 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 pavement's 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.
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:
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 software's 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 strategy's estimated cost as well as a projected cash flow over the life of the pavement.
The software's 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 that's 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 FHWA's Accelerated Loading Facility, located at the Turner-Fairbank Highway Research Center in McLean, VA.
Researchers at FHWA's 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:
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.
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.
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 engineer's 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.
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.