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MEPDG – Linking Pavement Design Assumptions and Asphalt Quality Assurance

Description:

With the development of the mechanistic-empirical pavement design guide (MEPDG) under the National Cooperative Highway Research Program (NCHRP) 1-37A and 1-40D and the subsequent adoption by the American Association of State Highway and Transportation Officials (AASHTO), many transportation agencies have initiated the process of implementing this new pavement design procedure. The MEPDG is a paradigm shift for most transportation agencies; no longer can agencies segment the interconnected functions of pavement design, traffic engineering, materials, and construction. Each discipline in an agency must work together to effectively use the MEPDG for new or rehabilitation pavement designs. One major difference between the MEPDG and previous AASHTO pavement design guides is the asphalt materials characterization. AASHTO design guides prior to the MEPDG use an asphalt layer coefficient (ai) as the primary means of quantifying material strength and potential performance. The empirical based value, developed from the AASHO Road Test (1957-61), is roughly related to the asphalt modulus and the asphalt use – surface or base. This value, typically between 0.40 and 0.44, was not calibrated or verified by most agencies nor was any check performed during production to compare the modulus of the asphalt to a layer coefficient based on the tables in the AASHTO Pavement Design Guides. Today, the MEPDG has abandoned the layer coefficient approach and has instead incorporated mechanistic properties of asphalt mixes and binders to calculate a dynamic modulus. The recently completed NCHRP 1-47 project found that the pavement predicted performance was highly sensitive to the master curve of the asphalt material, which relates dynamic modulus to material temperature and traffic loading. In addition to the dynamic modulus, NCHRP 9-30A recommended the permanent deformation characteristics (i.e., flow number?) of the mix be incorporated into the ME pavement design guide.
While dynamic modulus and flow number (?) may be the best property for estimating the future performance of an asphalt material, it is not routinely measured by transportation agencies or asphalt producers. Therefore, a gap exists between the pavement design process and the materials produced for a project. Specifications and quality assurance procedures are needed to ensure the material properties used in the MEPDG process can and are produced during construction. This may result in new procedures for accepting mix designs; proper cataloging of existing mixes used by an agency including master curves, gradations, asphalt binder contents and volumetrics; new material specifications to match production variability with design assumptions; and other significant changes to current workflow processes used by agencies and contractors.

Objective:

Most state highway agencies (SHAs) have a formal process to approve asphalt concrete (AC) mixtures and to monitor these mixtures as part of a Quality Assurance program. AC mixes must be designed using a state approved process such as SUPERPAVE™. Additionally, some SHAs require additional performance testing (i.e., rut testing and crack testing) before a mixture is approved. Once approved, the SHA’s QA program is performed to ensure the mixture approved is actually produced.

Although the AASHTO Mechanistic-Empirical Pavement Design Guide allows a pavement designer to choose the level of detail desired when providing asphalt material property inputs (which is typically based on the information available), the end result is the ‘master curve’—the relationship between material stiffness, temperature, and loading frequency . As shown in the sensitivity analysis performed in NCHRP 1-47, the master curves associated with the asphalt materials play a major role in the predicted performance of a pavement structure. Therefore, it is critical the AC material assumptions used by the engineer match the materials specified in the contract documents and are produced by the contractor in order to achieve the pavement structure designed.

Related Research:

1. NCHRP 1-47 Results

  1. NCHRP 9-30A Results

  2. NCHRP 9-22, 22A and 22B

Tasks:

The objective of this project will be to tie together the pavement design assumptions, construction documents, and quality assurance programs into an approach for SHAs to implement in conjunction with the MEPDG. Proposed tasks for this project would include:

  1. Literature Survey – This survey will review the results of the 1-47 project for flexible pavements. It will identify the key performance parameters. Using this information, transportation agency specification books will be reviewed to see what agencies have processes that may link the 1-47 parameters with quality assurance procedures.

  2. Transportation Agency Survey – Many surveys have been recently completed to assess where agencies are in the ME implementation process; however, no surveys have addressed how an agency will link the design assumptions with production quality assurance. This survey will focus on the current QA processes for materials acceptance and monitoring, and what efforts agencies are taking to link the new design process with mix production.

  3. Establish process to accept new mixes – AC content, gradations, volumetrics, E*, Flow Number, etc.

  4. Establish/Validate process to monitor mixes during production which may include the examination of production tolerances and the impacts on mix performance.

  5. Test procedures on 5 projects

  6. Develop Quality Management protocols for accepting mixes by the SHA and monitoring mixes by the SHA and contractor

  7. Develop draft specifications

  8. Final Report

Sponsoring Committee:AKP30, Design and Rehabilitation of Asphalt Pavements
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Trenton Clark
Date Posted:01/27/2014
Date Modified:01/28/2014
Index Terms:Pavement design, Mechanistic-Empirical Pavement Design Guide, Pavement layers, American Association of State Highway and Transportation Officials, Asphalt mixtures, Traffic engineering, Guidelines,
Cosponsoring Committees: 
Subjects    
Highways
Materials
Pavements

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