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17 C H A P T E R 1 Background Over the years, several controlled laboratory studies have shown that air voids (AV) can have a large effect on the performance of asphalt pavements. The National Center for Asphalt Technology (NCAT) Report 16-02R (Tran, 2016) is a literature review on asphalt mixture compaction and provides numerous references on laboratory and field studies examining the effect of AV on performance. Air voids that are either too high or too low can cause a reduction in pavement life. Air voids that are too low can cause bleeding, rutting, and shoving. High AV, on the other hand, leads to an increased potential for water infiltration, accelerated oxidation, raveling, and cracking. In addition, high AV pavements will continue to consolidate under traffic, causing rutting in the wheel path. The terms âas-constructedâ and âin-placeâ air voids are often used interchangeably. As-constructed AV is defined as the measured AV after the pavement is constructed but before the pavement is open to traffic. It is a measure that an agency can control by construction specification and is routinely measured as part of pavement construction acceptance. In-place AV can be defined as the asphalt mixture AV as they change over time, which agencies do not typically measure and cannot be controlled. In this study, the term âas- constructedâ AV is used or implied throughout this report. As-constructed AV is a complicated variable to quantify. An asphalt pavement structure is generally paved in multiple lifts and the lifts may not have the same asphalt concrete (AC) mixtures. Most agencies do not change their specified minimum as-constructed AV for depth of the AC lift, but construction quality assurance measurements indicate that AV measurements vary. A further complication is the influence of AV on performance. Surface rutting is dominated by AV in the upper 4 inches, fatigue cracking is influenced by AV over the entire AC thickness, and thermal transverse cracking is dominated by AV in the surface lift. While ride is a common performance measure, it is generally believed to be influenced mostly by quality of construction and other performance conditions, so the influence of AV on ride is expected to be mixed. Since the early 1900âs, there have been numerous methods established to compute AV in asphalt mixtures. The correct method to compute AV is based on the relationship between the measured mixture maximum specific gravity (Gmm) and measured mixture bulk specific gravity (Gmb). The relative density of an asphalt mixture is computed as Gmb divided by Gmm expressed as a percentage, and AV is the computed difference (100% minus the relative density percent). Hence, the unit of measure for AV is also percent of Gmm (%Gmm). In this report, the abbreviation %Gmm is used in a number of equations to refer to the value of AV, not the relative density of the mixture. There is a need to determine the effect of as-constructed AV on the performance of asphalt pavement. The challenge is to be able to isolate other factors that can affect pavement performance such as climate, traffic, and pavement structure. The Long-Term Pavement Performance (LTPP) program constitutes the most comprehensive source of information currently available to quantify this effect. If lower AV proves to have a positive effect on pavement performance, any cost associated with higher density during
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.
