Rehabilitation of Asphalt Concrete Pavements: Initial Evaluation of The SPS-5 Experiment-Final Report

Chapter 1. Introduction

One objective of the Long-Term Pavement Performance (LTPP) studies is to develop improved methodologies and strategies for rehabilitating flexible pavements. Those factors that can affect the performance of overlaid flexible pavements include, as a minimum, surface preparation, overlay thickness, overlay material, environment, and condition of the original pavement. The LTPP program incorporated all of these factors into a single experiment to study the rehabilitation of flexible pavements–the Specific Pavement Studies (SPS) 5, entitled Rehabilitation of Asphalt Concrete (AC) Pavements.

This controlled field experiment focuses on the study of the specific features noted above for the rehabilitation of hot mix asphalt (HMA) flexible pavements. It is expected that the successful completion of this experiment will lead to improvements in design procedures and standards for overlaying HMA–surfaced pavements. These improvements should contribute to achieving the overall goals of the LTPP program–increased pavement life and better utilization of resources.

Investigating the effects of the specific experimental design features and site conditions (surface preparation, overlay thickness, overlay material, environment, and original pavement condition), as well as their interactions on pavement performance makes it possible to evaluate existing rehabilitation design methods and predict performance of overlaid flexible pavements. It also makes it possible to develop new and improved HMA overlay design equations and to calibrate mechanistic-empirical models.

BACKGROUND

The SPS-5 experimental plans were originally designed to incorporate project sites built in all four LTPP climatic regions on fine-grained subgrade soils. The Strategic Highway Research Program (SHRP), Federal Highway Administration (FHWA), and U.S. State and Canadian Province highway agencies made a major effort to identify appropriate SPS-5 sites and to construct all test sections according to the original experimental design.

A wide range of specific data was collected during and after construction of the HMA overlays. An effort was made to collect field data (profile, cracking, and materials data) before the construction of the HMA overlays in order to quantify the surface condition of the HMA pavement before rehabilitation.

The original expectations for the LTPP program are summarized in the SHRP-P-395 report.⁽¹⁾ Originally, the following objectives were established:

• Evaluate existing design methods.
• Develop improved strategies and design procedures for rehabilitating existing pavements.
• Develop improved design equations for new and reconstructed pavements.
• Determine the effects on pavement distress and performance from traffic loading,environment, materials properties and variability, construction quality, and maintenance levels.
• Determine specific design procedures to improve pavement performance.
• Establish a database to support these objectives and future pavement engineering needs.

The experimental designs for LTPP were developed to achieve these objectives. The following products were identified for the LTPP program:

• General Products: Evaluation of existing design methods and performance equations, new and improved design equations, and calibration of mechanistic models.
• Specific Products: Effects of the specific experimental design features, subgrade soil,traffic and climate, and their interactions (i.e., permeable drainage layers, widened slabs,
• Other Products: Test methods developed specifically for SPS test sections; correlations between materials and technology transfer.

Two objectives of the SPS-5 (rehabilitated flexible pavement) and SPS-6 (rehabilitated rigid pavement) experiments are stated in the same report:

• "The SPS will develop a comprehensive database with information on construction, materials, traffic, environment, performance and other features pertaining to the test sections.
• The primary objective of the experiments on rehabilitation of asphalt concrete and jointed portland cement concrete pavements is to develop conclusions concerning the effectiveness of different rehabilitation techniques and strategies and their contribution to pavement performance and service life."⁽¹⁾

The SPS-5 experiment was designed to evaluate some more common rehabilitation techniques currently used in North America. The experimental factors include the condition of the pavement before overlay (both structurally and functionally), the loading conditions to which the test sections are exposed (both environment and traffic), and the various treatment applications. Five products are expected from the SPS-5 experiment:⁽²⁾

1. Comparisons and development of empirical prediction models for performance of HMA pavements with different intensities of surface preparation, with thin and thick HMA overlays, and with virgin and recycled asphalt pavement (RAP) mixtures.
2. Evaluation and field verification of the American Association of State Highway and Transportation Officials (AASHTO) Guide design procedures for rehabilitation of existing HMA pavements with HMA overlays and other analytical overlay design procedures.⁽³⁾
3. Determination of appropriate timing to rehabilitate HMA pavements in relation to existing surface conditions and types of rehabilitation procedure.
4. Development of procedures to verify and update the pavement management and life cycle cost concepts in the AASHTO Guide using the performance prediction models developed for rehabilitated HMA pavements.
5. Development of a comprehensive database of the performance of rehabilitated HMA pavements for use by State and provincial engineers and other researchers.

The SPS-5 experiment also was designed to identify trends associated with various rehabilitation methodologies on pavement performance and life expectancy. In addition, it is expected to provide data to improve and/or validate current design procedures. With these improved methodologies and procedures, highway agencies should be able to determine more appropriate strategies to rehabilitate flexible pavements. However, the ability of the SPS-5 experiment to meet these expectations has been questioned.^{(4, 5)} Some concerns are:

• Lack of detailed expectations and objectives from the SPS-5 experiment.
• Ability of the SPS-5 experiment to meet expectations in terms of the quality and completeness of the data at present and in the future.
• Deviations between the design and construction features of the in-place project (i.e., layers built to a different thickness or lack of pavement layer compaction).
• Deficiencies in performance data on construction, materials, climate, traffic, and for current and future analysis needs.

The full extent of the deviations and deficiencies, and the potential impact of those deficiencies, are not yet quantified for the SPS-5 experiment. Issues of experimental design, construction quality, data quality, and data completeness (with respect to both current data-collection guidelines and anticipated pavement engineering needs) also need to be addressed.

The SPS-5 projects were constructed between 1989 and 1997. Therefore, at the time this review was performed in 1999-2000, the data were sparse in many of the above-listed areas. However, several of the SPS-5 sections had begun to exhibit distress; thus, it was possible to make preliminary evaluations.

As of 2000, the only in-depth assessment of the SPS-5 experiment was Performance of Rehabilitated Asphalt Concrete Pavements in the LTPP Experiments–Data Collected Through February 1997 (using the LTPP data public release of February 1997). That study summarized early performance trends and observations of the 17 SPS–5 projects built as of 1996.⁽⁶⁾ The study neither focused on nor addressed the completeness of the experimental data, nor did it evaluate the adequacy of the experiment to provide data necessary to ensure that the broader expectations of this experiment could be attained. Therefore, the effort described in this report was initiated to conduct a comprehensive review of all SPS-5 experimental sites to determine the adequacy and potential of data from this experiment to satisfy future pavement engineering needs.

This review compared the experiment sites, as they existed in 1999-2000, with the original expectations and measured the projects against new expectations for the 21^st century. For example, there was a greater emphasis on mechanistic-based design in 2000 than existed a decade previously. This review provides a sound basis for:

• Planning remedial actions that may be warranted given various deficiencies in construction or data collection.
• Determining future monitoring and data collection activities.
• Planning future analysis of the collected or monitored data.

This evaluation of the SPS-5 experiment was conducted at the same time as and in cooperation with the evaluation of the SPS-1 (new flexible pavement), SPS-2 (new rigid pavement), and SPS-6 (rehabilitated rigid pavement).

STUDY OBJECTIVES

The primary objective of the SPS-5 experiment on rehabilitation of flexible pavements is to determine the relative influence and long-term effectiveness of factors that influence the performance of overlaid flexible pavements. The study described in this report was to conduct a detailed review and to determine the extent to which this experiment would provide the necessary data to ensure that the objectives and expectations are attained. This review concentrated on the core experimental test sections and on the supplementary test sections that were built by the individual agencies for each project. Five specific activities were completed for this review:

1. Evaluation of the set of core and supplemental test sections constructed within the SPS-5 experiment in relation to their ability to support the objective and characterize the overall "health" and analytical potential of the SPS-5 experiment. This included: (a) identifying areas of strength and weakness and developing a plan of recommended corrective measures as appropriate to strengthen the SPS-5 experiment to accomplish its objectives; and (b) developing analysis plans for both short-term and long-term goals. This objective was subdivided into two areas:
   • Evaluate the quality and completeness of the SPS-5 construction data (in relation to current data-collection requirements) and provide recommendations for the resolution and correction of anomalous or poor quality data.
   • Evaluate the adequacy of existing data and current data-collection requirements in relation to anticipated analytical needs; identify areas where current requirements were excessive or deficient; and provide recommendations where adjustments (in quantity, quality, frequency, or data type) were warranted.
2. Identification of any confounding factors introduced into the SPS-5 experiment by construction deviations or other factors not accounted for in the original experimental design.
3. Consideration of both short-term and long-term horizons in the evaluation and preparation of recommendations for data analysis.
4. Evaluation of the opportunities for local, regional, or national analysis of the core and supplemental test sections.
5. Identification of specific objectives and expectations that should be pursued for the SPS-5 experiment, considering the original expectations and future needs. As appropriate,expectations at the local agency, the regional, and the national level were considered.

Specifically, this report focused on four areas of the SPS-5 experimental data:

1. Review of data quality.
2. Detailed discussions on the quantity and percentage of data that were at Level E (the highest quality data, which has passed specific checks) in the LTPP Information Management System (IMS) database.
3. Comparison of designed versus as-constructed section parameters, especially those used to design the experiment (i.e. experimental deviations and construction problems).
4. Preliminary evaluation of performance and identification of future analyses that can be performed on the data.

It should be understood that the LTPP database is dynamic– data are continually checked and entered. This review and detailed assessment of the experiment represents a "snapshot" of the database and the Level E data at a particular point in time.

SCOPE OF REPORT

The report is subdivided into six chapters including the introduction. The second chapter is an overview of the status, as of 2000, of the SPS-5 experiment in comparison to the original experiment designs. The third chapter looks at the project requirements for each SPS-5 project. The fourth chapter is an overall summary of each project detailing the construction difficulties, experimental deviations, and data completeness; in other words, it summarizes each SPS-5 project that had been built (as of 2000), notes the data that are available for each project, and identifies construction difficulties and any data deficiencies. The fifth chapter presents an analysis of the initial observations of the key distress and performance indicators completed on a project-by-project basis and across the entire experiment. Chapter six summarizes effects that data deficiencies may have on the results that can be obtained from this experiment.

More detailed information and data are provided in the appendices. Appendix A presents a summary of the construction and deviation reports, as well as other data elements that were available for each project. Appendix B presents a summary of the available construction data for each project.