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 [R]-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 white- topping software was nearing the beta testing stage.

"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 effective 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.

A virtual bible for all things whitetopping, 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-shop That 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[TM], 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[TM] 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."

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.

The team established an additional expert advisory group, the Whitetopping 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 investigate the 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.