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Optimized Flexible Pavement Structure – Maximizing Life and Reducing Life Cycle Costs


For more than a decade, the pavement engineering community has recognized the long-life flexible pavement design concept. This concept has been supported by the performance of in-service pavements and laboratory/full-scale testing results. Many in-service pavements constructed 30 or 40 years ago using less sophisticated pavement design procedures, minimally engineered materials, and underestimated traffic projections have provide structural performances much longer than were expected with minimal maintenance and rehabilitation. These pavements have been termed “perpetual pavements”, but the engineering reasons behind these pavements’ performance are not well known or confirmed by rigorous field examination. Many researchers have focused on the limiting strain concept – build a pavement thick enough to reduce the strains at the bottom of the asphalt layers and bottom-up fatigue cracking will not occur. The common value for this strain level is between 70 and 100 microstrain based principally on laboratory test results. While this approach produces design thickness thinner than past empirical based designs and the fatigue damage approach used in the Mechanistic Empirical Pavement Design Guide (MEPDG), it still can result in pavements thicker than currently in-service and thus may unnecessarily increase a project’s construction cost. A research study is required to review existing long-life pavement designs and in particular their performance, to better understand the factors that contributed to their performance that results in an optimal design (i.e., new construction or rehabilitation). The study will look at how existing long-life pavements are performing and how using enhanced engineering materials such as high binder bases, rut resistant high modulus binder layers and high-performing surface layers can best be used in current M-E design procedures to optimize pavement thickness to lower life cycle costs without compromising overall performance.


A study is required to better quantify the factors related to long-life flexible pavements and how these factors can be incorporated into new construction and rehabilitation designs. With the limited federal, state and local funding for new transportation projects and existing facilities, it is critical to identify common factors for long-term structural performance with minimal preservation (i.e., non-structural treatments such as mill and fill). These design and performance factors along with new and better engineered pavement materials should result in lower life cycle costs as well as lower initial construction costs.

The primary structural damage model used in the AASHTO MEPDG is based on bottom up fatigue cracking resulting from repeated horizontal tensile strains at the bottom of the bound layers. The model was developed based on laboratory beam fatigue tests calibrated to a range of pavement thickness from the AASHO Road Test and satellite studies. For the MEPDG additional calibration factors were developed based on a range of pavement thicknesses contained in the LTPP database.

In the original beam fatigue work by Epps and Monismith observed that there was no fatigue damage below 70 microstrains. This endurance limit was used in developing the long life design procedures using limiting strain criteria which is now included in the MEPDG and PerRoad. Subsequent endurance testing by a number of researches has shown a range of endurance limit values resulting in the current range of 70 to 100 microstrains considered for the limiting strain criteria. However there has been very limited work verifying the maximum strain levels found in existing long-life pavements which in many cases have already provided 30 to 40 years of service without any structural deterioration. That limited work suggests that the strain levels for many long-life pavements could be 50% higher than current guidelines for limited strain criteria.

This study will first look at the existing long life pavements that have provided 20 to 40 years of service without experiencing any loss in structural capacity. The range in design features such as pavement thicknesses, material properties, environmental factors and traffic loading, and performance will be documented and assessed. A particular emphasis will be directed toward quantifying the typical strain history predicted for those pavement sections for a range of traffic loading, bearing conditions, and environments. The thinner long life pavements found that are providing long life service will provide clear indications for optimum pavement designs and improved guidance for limiting strain values. Since a large majority of the existing long life pavements will not have been constructed with enhanced engineering materials (rich bottom layers, high modulus intermediate layers etc) procedures will need to be developed to better account for these features in an optimum design processes.


With the current economic situation and future fiscal outlook, funding for new highway construction and rehabilitation projects will continue to be limited. Therefore, agencies must find ways to maximize their investments. The pavement structure represents one of the most costly investments of highway infrastructure and optimizing the pavement structure ensures fiscal responsibility.

Along with the need to optimize the pavement structure, there is also a need to minimize the impact of highway construction on the traveling public. Long-life pavement designs meet both requirements by providing structurally sound pavements that require only non-structural preservation treatments that can be done quickly during off-peak hours. This in turn minimizes user impacts.

Key to long-life pavement designs is determining the appropriate structural thickness required to provide performance and optimizing pavement material selection. Recent advances in pavement materials technology may provide the opportunity to decrease pavement thickness.

The results of this research study will identify the key factors contributing to long-life flexible pavement performance and provide agencies with additional tools to design long-life pavements both as new construction and rehabilitation projects.


This project will be divided into 2 phases.

In Phase 1 the researchers will:

Conduct a literature review to summarize existing long-life pavement design methodologies and research.

Summarize documented long-life flexible pavements within and outside the US. The summary will include identifying the pavement structure, materials used, performance history, traffic data and climatic data.

Perform a series of site visits to long-life flexible pavements. This will include a scanning tour type visit to at least 6 States. The actual locations will be based on the literature survey completed in Step a, and identification of long-life pavements in Step b. The site visits will provide detailed project information and performance history that will be used to better document the range of pavement thicknesses that are providing long life, as well as their engineering properties for a range of traffic loadings and environments.

Prepare an interim report documenting the findings from the literature review and the field visits and a detailed work plan for conducting phase 2 of the project.

In Phase 2 the researchers will use the information collected in Phase 1 to provide adjustments to current M-E based design procedures like the AASHTO MEPDG, PerRoad, or similar programs for optimum pavement designs, and to the extent possible quantify the benefits of improved engineered asphalt materials.

Areas for consideration are:

Relationship of lab limiting strain to field limiting strain with impact on overall pavement thickness using in-service pavements

Properties of newer, engineered asphalt materials in a flexible pavement design procedure (i.e., high-modulus high binder, SMA).

Use of the MEPDG and pavement performance prediction of in-service pavements (back-casting performance)

The procédures should apply to new construction and rehabilitation designs

It is anticipated a maximum pavement structure will be developed to provide long-life. This structure could be used as a cap for new construction designs or rehabilitation designs, independent of the traffic loadings. Long-life pavement structures will be recommended for various categories of road use (i.e., interstate, primary, secondary, urban streets), subgrade support and climate. A design catalog for new construction and rehabilitation projects along with recommended paving materials and specifications will be a deliverable. The framework for a long-life flexible pavement design tool or recommended changes/modifications to the AASHTO MEPDG will be a deliverable.

All the findings from Phases 1 and 2 will be included in the study’s final report. Phase 1 report must be completed and submitted to the project panel for review and approval prior to initiating Phase 2.

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, Flexible pavements, Service life, Life cycle costing, Fatigue cracking, Rutting, Pavement performance,
Cosponsoring Committees: 

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