Better Roads

August 2013

Better Roads Digital Magazine

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RoadScience between the materials and structure of a pavement, and its stresses and strains under load deflection. The concept relates pavement mechanics to "empirical" or experimental performance data obtained in field or lab. The MEPDG uses mathematical models to describe this relationship, and the primary basis for all mechanistic-based pavement performance predictions methods is cumulative axle load applications. The former (1993) edition of the AASHTO Guide for Design of Pavement Structures was based totally on empirical equations derived from the famous, but outdated, AASHO Road Test. That test involved field testing between 1958 and 1960 of a limited number of structural sections at one location, Ottawa, Ill., and with much-reduced traffic levels compared those of the 21st century. Under the new design guide, a designer of any pavement must first consider site conditions such as traffic, climate, subgrade, existing pavement condition for rehabilitation, and construction conditions, in proposing a trial design for a new pavement or rehab. Then, using the software, the trial design will be evaluated through prediction of key distresses and smoothness. If the trial does not meet the demanded performance criteria, the pavement design must be revised until it does. The mechanistic-empirical format of the design guide adapts it to evolution in truckloads, materials, construction techniques, design concepts and even computerization. It's a forward-looking methodology that will take the industry away from the "cookbook" or "recipe" specifications and design methods of the Ottawa tests, and into a future that molds design to anticipated performance. "The benefit of a mechanistic-empirical approach is its ability to accurately characterize in situ material (including subgrade and existing pavement structures)," says the Washington State DOT in its online tutorial. "This is typically done by using a portable device (like a falling weight deflectometer to make actual field deflection measurements on a pavement structure to be overlaid. These measurements can then be input into equations to determine existing pavement structural support (often called 'backcalculation') and the approximate remaining pavement life. This allows for a more realistic design for the given conditions." The MEPDG continues to be fine-tuned. For example, the June GAO report on LCCA affirmed that it "must be calibrated for conditions in individual states because, for example, the same design may perform differently in different climatic conditions and differently based on the available construction materials and stone in the state." And in March 2012, a critique of the MEPDG – AASHTO Mechanistic-Empirical Pavement Design Guide Parametric Study by Ruipeng Li and Steven Cramer, University of Wisconsin-Mad- When a winter storm hits, your goal is to eliminate snow and ice on roads as quickly as possible. The Hi-Way Xzalt makes your streets safer, sooner, by applying a 70:30 mixture of salt/liquid with precision application technology. Improved Deicing: • Less material bounce and improved road adhesion. • Less influence from wind. • Accelerated melting process. Improved Efficiencies: • Broadcast on one, two, or three lanes. • Higher application speeds and wider spread width. • Reduced material usage and longer routes. • One-person installation or removal in under 5 minutes. 800-363-1771 • • © 2013 Highway Equipment Company. All rights reserved. Text INFO to 205-289-3789 or visit 10 August 2013 Better Roads RoadScience_BR0813_old.indd 10 8/1/13 10:54 AM

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