Major advances in three software systems are improving the state of the art in concrete pavement design. 

The long-awaited Mechanistic-Empirical Pavement Design Guide was expected to gain final approval this fall by AASHTO, the state transportation officials group. By May, the M-E PDG had been unanimously approved by AASHTO’s Joint Technical Committee on Pavements and had moved up to the next level of the association’s approval process.

A second is a new version of StreetPave, a mechanistic design system developed for streets, local roads, and even highways by the American Concrete Pavement Association.

The third is ACPA’s AirPave software system for airport design. The association hopes to update it by the end of this year.

“We want to improve the user interface and make it more user friendly,” said Gerald F. Voigt, P.E., ACPA President and CEO. “And, within this software, we want to include a design method for pavements that will accommodate planes as large as the Airbus 380, the largest passenger aircraft in history.”

Simpler with StreetPave

“We feel that our StreetPave program fills a need for a design method that is less complex than the Mechanistic-Empirical Pavement Design Guide,” says Voigt. “We don’t feel that county or city agencies will be very comfortable with the M-E PDG.”

StreetPave is mainly designed for use on streets and county roads, but it can design highways as well. It includes an asphalt pavement design system that is based on the Asphalt Institute’s method. Plus, the program permits lifecycle cost analysis of both asphalt and concrete pavements for comparative purposes.

Voigt said StreetPave represents a significant advance over PCAPave, its predecessor program. The newer program has improved concrete-fatigue modeling and gives the designer more flexibility regarding when and how concrete pavement joints should be doweled. Plus, PCAPave did not include any asphalt pavement design capability.

ACPA staff developed most of the StreetPave program, except the concrete-fatigue models, which Applied Research Associates, Champaign, Illinois, developed. The program provides joint spacing and load transfer recommendations, as well as thickness, rounding, and reliability considerations. “For example, it will recommend a maximum joint spacing and load transfer dowels of a certain diameter depending on the required thickness. This is the first publicly available design

procedure to provide these types of jointing recommendations.” Voigt said. “And if you don’t need dowel bars, it won’t recommend them.” StreetPave costs $100 and ACPA will provide technical support.

Progress with M-E PDG

The M-E PDG integrates pavement design, materials selection, construction, and pavement management systems, says the Federal Highway Administration. A new design system is needed to replace the current system, which is based on an updated version of observed pavement performance from the AASHO Road Tests of the 1950s near Ottawa, Illinois. (AASHO was the forerunner organization for AASHTO.)

M-E PDG permits direct consideration of specific traffic levels, climate changes, material properties, and road base support, says the FHWA. For concrete pavements, designers can predict levels of distress over time.

Designers using M-E PDG can incorporate new pavement features and analyze them on a performance basis, says Michael Ayers, PhD., ACPA’s director of pavement technology services. Such features include widened lanes, tied concrete shoulders, stabilized vs. unstabilized bases, and dowel bar sizes and spacings.

The mechanistic portion of the M-E PDG calculates stresses, strains, and deflections of the pavement in response to loading levels. The empirical portion of the design guide relates the calculated values of stresses and deflections to observed performances. “And that correlation of mechanistic to observed values is done with transfer functions,” says Ayers.

Following a large volume of calculations, the new software produces expected values for distresses — transverse cracking, faulting, and ride deterioration — over the design life of the pavement. “If you specify that you will accept 15 cracked slabs per 100 slabs after year 30, it will show you graphically whether you meet or exceed that value,” says Ayers. “Then, if you exceed your design threshold, you go back and tweak something in the design — pavement thickness, base construction, or some other consideration — to make your design acceptable.”

“This is the first time that pavement design has been based on scientific design procedures,” says Ayers. “Up to now, design has been based on observed performance, which is really a kind of seat-of-the-pants method. You’d build a test section, observe the performance, and correlate the performance of a certain thickness with the loadings received by the pavement.”

Ayers says the M-E PDG’s empirical data base uses Long-Term Pavement Performance data that stems from actual results taken nationwide under the Strategic Highway Research Program begun in 1987.

As of June 2007, Missouri was the only state using M-E PDG, but a fair number of states had begun calibration efforts, Ayers said. Calibration involves changing the transfer functions so that they are specific to your given region of the country.

As it has with other federal pavement programs, the FHWA is using the Lead States Concept for M-E PDG. Besides Missouri, Lead States include Washington, Montana, Minnesota, Wisconsin, Utah, Arizona, New Mexico, Virginia, North Carolina, Pennsylvania, Florida, and New Jersey. The Lead States concept calls for the first-adapter states to share experiences and learn from each other as they calibrate and use the M-E PDG.  

ACPA Develops Training Program

The American Concrete Pavement Association has developed a set of 10 training modules for field-level concrete paving managers. Eight of the 10 modules were piloted last January at the North Dakota Department of Transportation, and the program received excellent feedback, says Michael Ayers, PhD, ACPA’s director of pavement technology services.

Developed and taught by Ayers, the training program is called the Contractor-Agency Training Program. The first eight modules cover: Design and Construction Basics; Fundamentals of Concrete Pavement Materials; Mixture Design Basics and Mixture Optimization; Quality Control/Quality Assurance Basics; Slipform Paving Operations; Curing and Texturing; Pavement Jointing and Load Transfer; and Trouble-Shooting and Problem Solving. The two new modules cover Concrete Overlays and Fast-track Concrete Paving.

“We would like to raise the bar on construction quality,” says Ayers. “We would like to see methods that we know work become standard practice. During the training, we make creative suggestions to the effect that, ‘We know this procedure has produced excellent results in the past.’”

Each module takes about an hour to teach, and more time is spent with questions, Ayers says. All eight modules can take from one to three days to teach.

Ayers also teaches Concrete Pavements 101 for upper level engineers, consultants, and DOT designers. The course covers the design, construction, and rehabilitation of concrete pavements. Concrete Pavements 201 covers the Mechanistic-Empirical Pavement Design Guide. “We talk about development of the inputs, how to run the program and how to interpret the results,” says Ayers.