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Concrete Pavement Technology Update
MEPDG Development Continues
Pavement design has come a long way and is preparing to take a new turn—a turn to the future with the development and eventual implementation of a new pavement design procedure. Referred to as the Mechanistic–Empirical Pavement Design Guide (MEPDG), the procedure analyzes basic pavement system responses (stresses, strains, and deflections) and relates them to pavement performance in terms of common distress types (e.g., cracking, faulting). This process allows engineers to design for allowable levels of distress over the selected design period.
Development of the MEPDG was sponsored by the National Cooperative Highway Research Program (NCHRP) under Project 1-37A. Initiated in 1996, the project developed a comprehensive approach to the design of pavement structures, with formal documentation produced in 2004 and ongoing refinements to the accompanying software program culminating in the release of version 1.0 in 2007.
Primary benefits of the new design guide are the improved reliability of the resultant designs and the ability to handle any and all combinations of materials, traffic, and climatic conditions. This capability is a major improvement over the widely used 1986/1993 AASHTO Design Guide for Pavement Structures, which was based on the limited materials, short test period, single climatic zone, and low traffic levels associated with the conduct of the AASHO Road Test from 1958 to 1960.
The MEDPG allows engineers to predict how a pavement system with given material properties might respond to climatic conditions and traffic loadings over time.
Strictly speaking, the MEPDG is not a design tool, but rather a pavement analysis tool. In other words, an initial design is first selected, along with definitions of material properties, climatic conditions, and traffic loadings. The response of that initial design to the climatic and traffic loading conditions is determined, and related to the development of pavement damage and distress. If the projected distress is within tolerable limits, the design is considered adequate; if not, the pavement design is modified and another iteration is performed to determine its suitability. This process is illustrated below, left.
Since the release of the research version in 2004, a number of state highway agencies have been conducting investigatory and feasibility studies on the MEPDG, with an eye towards implementing the procedure. A separate report providing recommended calibration procedures for the MEPDG will be available from NCHRP soon, which should help agencies in their implementation efforts.
During the summer of 2007, the American Association of State Highway and Transportation Officials (AASHTO) Highway Subcommittee on Design and Highway Subcommittee on Materials approved the MEPDG, and in late 2007 the AASHTO Standing Committee on Highways voted to adopt the MEPDG as an interim specification. Much of the work now is focusing on enhancement and refinement of the current version of the MEPDG software and turning it into an AASHTOWare® software program, DARWin ME.
The overall timeline for DARWin ME calls for the development of the program to begin in late 2008 or early 2009 and to be completed in 15 to 18 months. Following the release of DARWin ME, the current version of DARWin will be retired within 12 months.
Article prepared by Kurt Smith, CPTP Implementation Team (firstname.lastname@example.org).
Ongoing Surface Characteristics Research Activities
On March 31, 2008, the American Concrete Pavement Association (ACPA) Noise and Surface Characteristics Task Force was updated on ongoing research activities by Iowa State University (ISU), the Federal Highway Administration (FHWA), Purdue University, and ACPA.
Paul Wiegand, ISU, reported the results of on-board sound intensity (OBSI) testing of 500 unique surface textures and 1,200 unique pavement test sections. Noise emissions ranged from 98 to 109 dBA. Results indicated that sub-100-dBA projects can be obtained using any of the conventional tining or diamond grinding treatments. The causes of excessive noise levels were identified as a too-smooth or too-bumpy surface texture, presence of an ill-designed repeating pattern, and impulse loading effects from joints. ISU will produce a report on texturing guidelines.
Two of the unique surface textures tested for on-board sound intensity at Iowa State University. Above, diamond grinding; below, longitudinal tining.
Mark Swanlund, FHWA, highlighted some activities of the Pavement Surface Characteristics Program, which covers smoothness, friction, tire–pavement noise, and vehicle splash/spray:
Robert Bernhard, Ph.D., provided an overview of the three-phase research effort on diamond grinding, sponsored by ACPA, at Purdue. One of the more interesting findings is that the results tend to be tire dependent. The grooving experiment involved including an acoustic medium in the groove, and this appeared promising. Innovative textures of various geometric patterns (circles, waffles, or variations) are being evaluated.
Larry Scofield, ACPA, briefed the Task Force on several topics. Current noise and friction testing results on the Next Generation Concrete Surface were presented; three test sections have been placed, and four more potential locations are being pursued. ACPA is collecting friction data to compare longitudinal- and transverse-tined concrete surfaces. In addition, the use of longitudinal grooving to affect roadway surface drainage is being evaluated. ACPA is developing an interactive sound demonstration system that will provide audio comparisons of roadways with different surfaces and textures, to be available soon.
It is only recently that significant research has been devoted to functional performance of concrete pavements, which is heavily influenced by pavement surface characteristics. Perhaps the most challenging task will be to relate those characteristics to the prediction of highway crashes. The results of this research will help bring safe, quiet, durable, and economical pavements to the traveling public.
Article prepared by Roger Larson (email@example.com), CPTP Implementation Team.
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