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55 C H A P T E R 7 Summary This research was initially intended to develop procedures for incorporating pavement preservation treatments into the MEPDG design analysis process. However, in reviewing the data available from several SHAs, it was determined that suf- ficient data were not available to achieve this objective. The project objective was modified to focus on identifying and describing processes for developing such procedures. The research included a review of information relevant to pavement preservation and pavement design (primarily as related to the MEPDG) and interviews with representatives of selected SHAs and pavement industry groups to assess pave- ment preservation and pavement design practices and avail- ability of data to support the development of procedures for incorporating preservation into the MEPDG. Also, the LTPP and other test sections used in the development and calibration of the MEPDG performance prediction models were identified and examined to determine if any preservation treatments were applied to those sections and thus already accounted for in these models. Based on this work, three approaches that would allow the consideration of preservation in the MEPDG design and analysis procedures were identified and illustrated by examples. Pavement Preservation State of the Practice Although there is growing use of pavement preservation by state highway agencies, there is a lesser tendency to apply these treatments to high-volume roads and in severe climates. The most commonly used preservation treatment types for HMA-surfaced pavements are crack sealing/filling, micro- surfacing, thin HMA overlay, and drainage maintenance. For PCC pavements, diamond grinding, partial- and full-depth repair, joint resealing, load transfer retrofit, and drainage maintenance are commonly used. These treatments, and variants thereof, are appropriate candidates for incorporat- ing into the MEPDG design. Many studies on pavement preservation performance have been conducted over the past 20 years. Early studies generally focused on subjective, experience-based estimates of per- formance or on historical records (treatment application frequency) as a basis for estimating performance. More recent studies have focused on objective measures of performance involving the collection and analysis of time-series performance data from in-service pavement sections or experimental test sections. Treatment performance can be assessed in terms of treat- ment service life, pavement life extension, and performance benefit area. Historical data on how the structural and perfor- mance indicators are influenced by preservation are needed for considering preservation in the design process. SHAs have evaluated preservation treatment performance, although there were some issues with tracking the locations of preservation treatment projects in the databases and the reliability of the collected data. However, some agencies estab- lished performance models that often focused on just one performance parameter (e.g., IRI). Specific types of data in adequate amounts and format are needed to support consid- eration in the design process. MEPDG Evaluation, Implementation, and Use Many SHAs have been or are currently engaged in the evaluation, implementation, and use of the MEPDG process. At least three studies have addressed the design of pavements considering the effects of preservation. In one of these studies, the developed ME-based flexible pavement design program (CalME) allows a user to schedule one or more predefined M&R or preservation treatments as part of the design, and accounts for their effects on material and pavement structure mechanical properties. The investigation of the LTPP pavement sections used in developing and calibrating the MEPDG models indicated Summary and Recommendations for Research
56 some degree of influence on the models by preservation treatments. However, incorporating preservation directly into the MEPDG will remain a difficult task because of the lack of specific information on the effects of preservation. Approaches for Incorporating Preservation into the MEPDG Three approaches for considering preservation in the design and analysis procedures were identified. Each approach has distinct advantages and disadvantages influencing its poten- tial for implementation and use. One approach accounts for all aspects of structural and functional performance. Another approach builds off the calibration/validation process outlined in the AASHTO Local Calibration Guide (AASHTO 2010) but requires a substantive set of preservation-treated test sections and the collection of time-series performance data to support development of calibrated models. A third approach considers the changes in pavement materials and structure properties resulting from treatment application but involves a high level of complexity to accurately define the immediate and long- term changes resulting from a treatment application. Several SHAs indicated a lack of the data needed to fully develop and validate the alternative approaches. A few states have several years of network-level preservation treatment per- formance data, but there are various issues with the data (e.g., inaccurate; hard-to-access location, cross-section, and history information; incompatibilities with MEPDG param eters) that would make their use questionable. A few states have good but limited project-level data available in terms of the quantity of pavement sections or the time-series performance. Preservation treatments have not typically been consid- ered in the pavement design process because of the insignifi- cant contributions to pavement structural capacity and the inability to quantify their effects. The inability to accurately quantify both initial and long-term effects of preservation treatments on performance makes their inclusion in pave- ment design and analysis procedures difficult. Recommendations for Future Research The information and findings from this study advance the goal of considering the effects of preservation in the pave- ment design process. However, further research is needed to fully develop and validate one or more of the approaches pre- sented in this report, including the following: ⢠MEPDG Performance Model Calibrations for Preservation. There is a need to perform calibrations of the MEPDG models using data from the SPS-3 and SPS-4 test sections and other test sections and to develop calibration factors for the various flexible and rigid pavement preservation treatments. ⢠LTPP Pavement Materials and Structure Properties. There is a need to evaluate in situ and laboratory testing data for the various SPS-3 and SPS-4 test sections to determine effects of preservation treatments on surface permeability, asphalt aging with depth, and structural response. ⢠Evaluate the Pavement ME Design Software Ability to Incor- porate Preservation in the Design. Research is needed to develop procedures for incorporating the effects of preser- vation in the models contained in the Pavement ME Design software. ⢠Evaluate the Effects of Preservation on Material Properties. Actual data on the effects of preservation treatments on HMA surface layer material properties and the moisture and thermal profile of the existing pavement are limited. Further research is needed to provide direct inputs for use into the MEPDG procedures.
