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Suggested Citation:"Objective and Scope." National Academies of Sciences, Engineering, and Medicine. 2021. Investigating the Relationship of As-Constructed Asphalt Pavement Air Voids to Pavement Performance. Washington, DC: The National Academies Press. doi: 10.17226/26219.
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Page 19
Suggested Citation:"Objective and Scope." National Academies of Sciences, Engineering, and Medicine. 2021. Investigating the Relationship of As-Constructed Asphalt Pavement Air Voids to Pavement Performance. Washington, DC: The National Academies Press. doi: 10.17226/26219.
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Page 19

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18 construction will potentially be less than that of the maintenance, preservation, and rehabilitation that may be needed to address a reduced pavement life. A previous study conducted in 2001 under NCHRP Project 20-50(14) "Significance of As-Constructed AC Air Voids to Pavement Performance" reviewed available LTPP data but concluded that there was limited LTPP data for the study. That study was unable to develop findings due to an insufficient dataset to analyze. (Seeds, 2002) For this 2018 study, the number of LTPP test sections used was primarily constrained by the type of pavement test section, as-constructed data availability, length of distress measurement history over the period from initial construction until a new pavement treatment was applied, and the distresses that emerge during the construction cycle of interest. Figure 1-1 shows the difference between the number field performance data collection events for each LTPP section available for the 2001 study (2 to 6 events) and the current 2018 study (4 to 18 events). The main differences include: • For LTPP test sections selected for fatigue analysis, the minimum amount of data available for each section increased from 2 to 4 years. • For LTPP test sections selected for rutting analysis, the minimum amount of data available for each section increased from 8 to 14 times that rutting was measured. • Another advancement is that LTPP distress data now distinguishes between types of longitudinal cracking. Longitudinal cracking in the wheel path, which is more logically associated with fatigue, is separated from cracking outside the wheel path often associated with longitudinal construction joints. Figure 1-1. Availability of LTPP data. In light of these comparisons of data available to the past NCHRP 20-50 (14) study and what is currently available, it is clear that there is a need to re-examine the study objective using currently available LTPP data.

19 C H A P T E R 2 Research Approach Objective and Scope The objective of this research was to determine the effect of in-place AV on the performance of asphalt pavements using data from the LTPP database and other appropriate sources. For clarity, the term in-place AV was defined as the as-constructed AV measured during construction. The research focused on four primary distress types related to asphalt pavement performance: rutting, fatigue cracking, transverse cracking, and ride. Three distress types used measures as described in the Distress Identification Manual (Miller, 2003) for the LTPP and ride was measured as international roughness index (IRI). Methodology The research team applied three analysis methods to the LTPP data and conducted validation efforts with data from other sources. The complexity of the influence of as-constructed AV on performance was examined with multiple methods to capture varying levels of available data and the impact of other variables, such as climate and traffic. Method 1 isolated the influence of as-constructed AV by dividing the projects into common subsets, Method 2 used a regression modeling technique to account for incomplete data, and Method 3 used a computer-based information processing technique commonly called artificial neural network (ANN). Overview of the LTPP Dataset A mining approach was used to extract data for this study. A query was created and the key parameter that eliminated the use of most of the LTPP database was the availability of as-constructed AV. The query included the following key boundaries: asphalt pavements, as-constructed AV, more than five years of LTPP monitoring, and more than three distress surveys. A total of 426 LTPP test sections met the criteria. Table 2-1 lists the number of sections from each LTPP group. As shown in this table, there is a reasonable split between new construction (226 sections) and rehabilitation projects (200 sections). Table 2-1. LTPP sections used in this study. LTPP Study Study Description No. of Sections GPS 1 Asphalt concrete on unbound granular base 7 GPS 2 Asphalt concrete on bound base 4 GPS 6B AC overlay with conventional asphalt cement on AC pavement, no milling 40 GPS 6C AC overlay with modified asphalt cement on AC pavement, no milling 16 GPS 6D Multiple AC Overlays with Conventional Asphalt Cement on AC Pavement, No Milling 6 GPS 6S AC Overlay on AC Pavement with Milling and/or Fabric Pretreatment 31

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Several controlled laboratory studies have shown that air voids (AV) can have a large effect on the performance of asphalt pavements. AVs that are either too high or too low can cause a reduction in pavement life.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 299: Investigating the Relationship of As-Constructed Asphalt Pavement Air Voids to Pavement Performance determines the effect of in-place AVs on the performance of asphalt concrete (AC) pavements.

The document also has supplemental appendices that are available by request to Ed Harrigan. They include data sets for LTPP, Pavement ME Design Validation, MnROAD Validation, and NCAT Validation.

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