MEPDG – Linking Pavement Design Assumptions and Asphalt Quality Assurance
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.
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.
1. NCHRP 1-47 Results
NCHRP 9-30A Results
NCHRP 9-22, 22A and 22B
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
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
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.
Establish process to accept new mixes
– AC content, gradations, volumetrics, E*, Flow Number, etc.
Establish/Validate process to monitor
mixes during production which may include the examination of production
tolerances and the impacts on mix performance.
Test procedures on 5 projects
Develop Quality Management protocols
for accepting mixes by the SHA and monitoring mixes by the SHA and contractor
Develop draft specifications
|Sponsoring Committee:||AKP30, Design and Rehabilitation of Asphalt Pavements
|Research Period:||24 - 36 months|
|RNS Developer:||Trenton Clark|
|Index Terms:||Pavement design, Mechanistic-Empirical Pavement Design Guide, Pavement layers, American Association of State Highway and Transportation Officials, Asphalt mixtures, Traffic engineering, Guidelines, |