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Development of Mechanistic-Empirical Pavement Design Criteria for Pavement Rehabilitation using Full-Depth Reclamation


In-place full-depth reclamation (FDR) is a proven cost-effective and environmentally sustainable strategy for rehabilitating old, distressed asphalt concrete pavements. It reuses existing materials and has minimal impact on traffic. The recycled materials can be left unstabilized, or stabilized during the recycling process with foamed asphalt, asphalt emulsion, cement, alone or in combination. Although the approach has been used in the United States for more than 15 years, most designs have been empirical and largely based on the experience of a few practitioners. Current mechanistic-empirical design procedures do not have any parameters for FDR layers, nor has any significant testing been done to develop them on a national scale. Foamed asphalt in combination with portland cement is a commonly used stabilizer in FDR. When used individually for stabilization of marginal materials or in full-depth reclamation (FDR) projects, each has both advantages and limitations. Asphalt is generally not appropriate for materials with high fines contents or with plasticity. Portland cement can result in very stiff and brittle layers that are prone to shrinkage, and susceptible to reflected cracking. Observations of projects where foamed asphalt has been used in combination with cement indicate that better performance can be obtained compared to that when the stabilizers are used individually. Laboratory strength tests and field stiffness tests indicate a strong, but relatively flexible material with improved moisture resistance. Electron microscope studies indicate a different stabilization mechanism compared to materials treated with the individual stabilizers. In light of these observed benefits, guidance is required on the long-term behavior of layers stabilized with this combination to ensure that appropriate criteria are used when undertaking a mechanistic-empirical pavement design. Designers currently model these layers as either asphalt or cement, neither of which is appropriate, leading to inaccurate and/or inappropriate pavement designs. Guidance is also required on determining optimal ratios of the two stabilizers to ensure optimum performance.

Objective: The main objective of this research project is to develop mechanistic-empirical design criteria for pavement rehabilitation using full-depth reclamation. The project will also develop guidance on the use of asphalt (foamed or emulsion) in combination with portland cement for stabilization of marginal materials and/or in full-depth reclamation projects. Key issues include the development of performance models/factors that can be incorporated into current mechanistic-empirical pavement design procedures and development of laboratory testing procedures that can be used to select the most appropriate combinations of asphalt and portland cement stabilizers.
Benefits: Meeting the proposed objectives will be of great help to road agencies that have extensive networks requiring upgrading or rehabilitation, and who require the best possible performance from stabilized layers. The development of mechanistic-empirical pavement design functions specific to layers and pavement structures rehabilitated by in-place full-depth reclamation stabilized with asphalt, cement or an appropriate combination of them will allow agencies to design appropriate, cost-effective structures without having to make unsubstantiated assumptions about expected performance. Guidance on the selection of appropriate stabilizer combinations and ratios, mix design testing, performance testing, and specifications will assist practitioners on making informed decisions and potentially reduce the cost of pavement rehabilitation projects.
Related Research: Most stabilization design and construction methods currently specified by highway agencies are based on generic recommendations developed by the Asphalt Recycling and Reclaiming Association (ARRA) and the recommendations of contractors and stabilizer suppliers. They tend to be based more on empirical observations than on mechanistic testing and modeling. Several state agencies have funded state-specific projects on the topic. It would be desirable to accumulate the general knowledge in one source to ensure a uniform process throughout the United States, and to ensure that the guidance is applicable to all types of roads (local, county, state, federal lands) and not only the typically higher volume inter-urban roads managed by state agencies.

Tasks associated with this project should include, but not be limited to:

Task 1. Literature review on research conducted on the topic as well as establishment of a project database of highway sections rehabilitated using full-depth reclamation with and without stabilization. Contact transportation agencies, private contractors and trade organizations as appropriate to augment this assessment.

Task 2. Based on the results of Task 1, develop and implement a laboratory testing framework that compares the performance of materials treated with the individual stabilizers and combinations of them. Testing should focus on mechanistic properties and long-term performance. Electron microscope and other appropriate scanning procedures should be used to understand the micro-mechanical stabilization mechanisms of the different treatments and their relationship to observed performance.

Task 3. Using the results obtained in Task 2, develop a framework for full-scale testing to validate the laboratory test results. Accelerated load testing on instrumented test tracks in combination with testing on in-service pavements should be used to obtain life-cycle performance criteria that can be used to prepare mechanistic design functions that will allow practitioners to confidently design pavements with a combination of stabilizers.

Task 4. Prepare a guideline document on the mechanistic-empirical design of rehabilitation projects using full-depth reclamation. This guideline will include stabilizer selection guidelines based on subgrade, drainage, material, traffic, and climate conditions, mix design testing, performance testing, and suggested specification language.

Task 5. Prepare supporting documentation for incorporating the design approach and design functions into the Darwin and other mechanistic empirical pavement design procedures.

Implementation: It is anticipated that AASHTO and the Federal Highway Administration will take ownership of the planned deliverables

It is anticipated that AASHTO and the Federal Highway Administration will take ownership of the planned deliverables.

Sponsoring Committee:AFS90, Chemical, Mechanical, and Asphalt Stabilization
Research Period:24 - 36 months
Research Priority:High
RNS Developer:David Jones
Date Posted:02/26/2015
Date Modified:02/27/2015
Index Terms:Full-depth reclamation, Cost effectiveness, Rehabilitation (Maintenance), United States, Mechanistic-empirical design, Pavement design, Foamed asphalt, Portland cement, Waterproofing,
Cosponsoring Committees:AFD70, Pavement Rehabilitation; AKC60, Asphalt Pavement Construction and Rehabilitation
Maintenance and Preservation

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