Consideration of Preservation in Pavement Design and Analysis Procedures (2015)

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1 Introduction Background and Problem Statement The methodology commonly used to design pavements in the United States was developed from pavement perfor- mance data collected during the American Association of State Highway Officials (AASHTO) road test conducted in Ottawa, Illinois, from 1958 to 1960. [The pavement design procedure is presented in reports that are alternately referred to as the AASHTO Guide for Design of Pavement Structures, the AASHTO Design Guide, and the Guide. Here the term “guide” is used generally to refer to the AASHTO pavement design procedure and associated versions of its documentation prior to the release of the new Mechanistic-Empirical Pavement Design Guide (MEPDG) developed under NCHRP Project 1-37A (ARA 2004; AASHTO 2008).] This methodology uses empirical performance models that were based on the limited range of site conditions at the road test, including the types of support materials, the types of applied loads, the environ- mental conditions, and the short duration of the data collec- tion effort. The experiment at the road test was not set up to observe the long-term effects of maintenance actions, although some routine/corrective maintenance was performed on the test pavements (e.g., crack sealing and patching). Also, because the original design models were based on the observed performance of roads that were exposed to nearly continual loadings over a relatively short time, these models did not account for the effects of maintenance or environment on pavement performance. The draft design procedure was first published in 1962, and several enhancements were introduced in subsequent revi- sions; all are incorporated in the AASHTO Guide for Design of Pavement Structures (AASHTO 1993). In particular, the guide added content on the rehabilitation of pavements with and without overlays and encouraged user agencies “to build a continuous and accurate performance database to increase the overall accuracy and confidence level of performance predictions” (AASHTO 1993). The Mechanistic-Empirical Pavement Design Guide: A Manual of Practice (MEPDG; AASHTO 2008) was introduced in 2008. It notes that “preservation programs and strategies are policy decisions which are not considered directly in the distress predictions” and that “preservation treatments applied to the surface of hot-mix asphalt (HMA) layers early in their life may have an impact on the performance of flex- ible pavements and HMA overlays.” It further notes that “the pavement designer needs to consider the impact of these pro- grams in establishing the local calibration coefficients or to develop agency-specific values.” These remarks suggest that the effects of pavement preservation are not fully considered in the MEPDG performance prediction models. Preventive maintenance—the practice of keeping good roads in good condition—is a key component of pavement preservation. Preventive maintenance encompasses a variety of treatments whose application could have a positive effect on pavement performance, such as the following (from Transpor- tation Research Circular E-C078 2005): • Preventing or Slowing Down Infiltration of Moisture and Incompressibles. Crack and joint sealant materials, mem- brane seals applied over a pavement’s entire surface, and cer- tain patches will reduce the amount of water that infiltrates the pavement system. Sealing cracks and joints also keeps incompressibles from entering the pavement structure and impeding the expansion/contraction of the pavement. • Providing Protection Against Aging and Oxidation of Bitu- minous Surfaces. The application of a new thin surfac- ing seals a bituminous surface and protects the underlying structural layer from some environmental effects. The pro- cess can be repeated several times after the surfacing ages and wears out, as long as the overall pavement remains structurally sound and the environmental effects are not too severe. • Restoring Surface Integrity. Preventive maintenance treat- ments, such as slurry seals, chip seals, and partial-depth C H A P T E R 1

2repairs, can correct non-severe, non-structural deterioration that is limited to the surface of a pavement (e.g., weathering and raveling, bleeding, loss of friction, roughness, and some HMA rutting). • Improving Surface Texture. Preventive maintenance treat- ments, such as chip seals, thin overlays, and diamond grind- ing, improve the surface characteristics of the pavement by restoring the macrotexture of the pavement surface and influencing pavement surface friction and noise. These effects contribute to improved overall performance (in comparison to the pavement without treatment) and a delayed need for rehabilitation (i.e., the pavement with pres- ervation will reach a rehabilitation threshold much later); these effects should be reflected in the pavement performance prediction models. Figure 1 illustrates the effect of successive application of preventive maintenance treatments on pave- ment performance. While the effects of preservation are easy to illustrate, their benefits are not easily quantified, for the following reasons: • Preservation has not been widely practiced for a long time, and there remain many questions about its effect on com- monly used measures of pavement performance. • In general, preservation has not been practiced as part of a documented program (in contrast with capital projects, which more easily enter into an agency’s formal records), making it difficult to distinguish between pavements that have and have not received preservation treatments. • In some agencies, the practice of preservation varies among districts and is often influenced by fluctuations in funding and nontechnical factors. As a result, sustained effects are not adequately measurable. • The effects of preservation are highly variable and depend on the existing pavement condition, treatment type, materials, treatment timing, construction quality, environment, traffic volume, and other factors. Therefore, a substantial amount of data is needed to adequately analyze the effect of preserva- tion on pavement performance. • The metrics used for monitoring pavement performance may not appropriately reflect the short- or long-term effects of preservation. The MEPDG performance models were calibrated using data from in-service pavement sections included in the Long- Term Pavement Performance (LTPP) program. It is highly likely that these pavements were maintained over their lives, but the percentage of the sections that included the application of preservation treatments, as well as the type and time of applica- tion, are not known. Most likely, a preservation treatment was applied to some sections but not to others. Also, it is more likely that the MEPDG models incorporate the routine maintenance component of preservation but not necessarily the preventive maintenance component. Ideally, pavement design and perfor- mance models should consider the effects of preservation on performance. A procedure for calibrating the MEPDG models to account for the effects of preservation on pavement perfor- mance and design is needed. Research Objective The research was initially intended to develop procedures for incorporating pavement preservation treatments into the MEPDG design analysis process that would become part of the MEPDG Manual of Practice. However, it was determined in the early stages of the research that sufficient data were not available to support the development of such procedures. The research objective was then modified to focus on identify- ing and developing processes that would serve as a basis for developing these procedures. Source: Peshkin et al. 2004. Preventive Maintenance Treatments Applied Good Pa ve m en t C on di tio n Initial Performance Period Poor Time No Maintenance Treatment Applied Figure 1. Illustration of the effect of preventive maintenance treatments on pavement performance.

3 Research Scope and Approach To accomplish the research objective, the project docu- mented the effects of preservation on performance by (1) con- ducting a literature review and telephone interviews with state highway agency (SHA) personnel and industry representatives and (2) identifying procedures that consider such effects in the design and analysis process. The literature review covered recent or ongoing studies dealing with (1) pavement preservation practices for HMA and Portland cement concrete (PCC) pavements (or PCCPs), in terms of treatment usage and performance and the effect on pavement life and performance trends; (2) MEPDG evaluation and implementation activities (e.g., sensitivity testing, verifica- tion testing, local calibration, and other performance model refinements) and MEPDG use; and (3) pavement design appli- cations that consider preservation. Telephone interviews were held with representatives of 14 SHAs, selected on the basis of experience with pavement preservation and the MEPDG and on the possible availability of data on the effects of preservation on pavement performance. Also, telephone interviews were held with representatives of five industry organizations. To better understand the extent to which the effects of pres- ervation treatments were considered in the MEPDG per- formance prediction models, the test sections used in the development and calibration of these models (LTPP and non- LTPP sections) were identified and their maintenance and rehabilitation (M&R) history was examined. The results of the literature review and interviews were used to further evaluate and define three possible approaches for considering the effects of preservation in the MEPDG procedures. These approaches consider developing pave- ment preservation response models, calibrating the models for preservation, or modifying material properties to account for the effects of preservation. The data required to fully develop these approaches were then identified, and their availability within SHAs was evaluated. It was concluded that sufficient data were not readily available to support the development of these approaches. The research then focused on describing and illustrating possible uses of the approaches. Organization of Report This report is presented in seven chapters, including this introductory chapter. Chapter 2 briefly describes the state of the practice with regard to pavement preservation. Chapter 3 describes the MEPDG process, its implementation and use, the extent of its consideration of preservation, and the avail- ability of data to support developing models for incorporation into the MEPDG analysis procedures. Chapter 4 describes an approach for developing response models for considering the effects of preservation in the MEPDG procedures. Chapter 5 describes an approach for calibrating MEPDG performance models to account for the effects of pavement preservation. Chapter 6 describes an approach that considers the changes in material and pavement structural properties caused by preservation and addressing those changes in MEPDG mod- els to reflect the effects of preservation. Chapter 7 summa- rizes the research findings and presents recommendations for further research. Nine appendices for this report are available on the TRB website. Appendix A is a bibliography that describes the doc- uments that were reviewed. Appendices B and C describe preservation strategies for HMA-surfaced and PCC-surfaced pavements, respectively, their use in the MEPDG, and their expected effect on distress. Appendices D and E contain brief syntheses on the topics of pavement preservation and the MEPDG, respectively. Appendices F and G summarize the responses of SHA and industry group representatives, respec- tively. Appendix H provides a listing of the LTPP test sec- tions used in developing and calibrating the MEPDG models and identifies those sections whose performance data were influenced by applied preservation treatments. Appendix I examines the available SHA data and their suitability to sup- port the development of approaches. These appendices can be found on the report summary web page by searching for NCHRP Report 810 at www.TRB.org.