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3 âThe importance of compaction in highway construction has long been recognized.â (Pauls and Goode, 1939) In-place density of asphalt mixtures is the single most important property of asphalt mixtures in pavement. The relationship between in-place density and performance of the mixture has been studied for decades. The process of achieving in-place density involves creating the proper mix design, under- standing aggregate and asphalt binder properties, understanding paver and roller operations, and adjusting the process to account for environmental conditions. Many details are involved in the process. Literature Review NCHRP published two previous documents on the subject of achieving compaction of asphalt concreteâSpecial Report 131: State of the Art: Compaction of Asphalt Pavements (Highway Research Board, 1972) and NCHRP Synthesis 152: Compaction of Asphalt Pavement (Hughes, 1989). Deacon established the relationship between density and laboratory fatigue fracture life in 1965. Work by Harper and Gallaway in 1967 laid some of the early groundwork for justification for density requirements for asphalt mixtures comparing laboratory and field testing. Epps and Moni- smith (1969) continued that work relating mixture density to fatigue performance. Epps et al. (1969) further established that density variations impact the fatigue life of asphalt mixtures, noting that the mechanical properties of the mixture are largely determined by the mixture density. More recently, Mogawer et al. (2011) reported similar results from multiple laboratory tests. The importance of density in asphalt concrete mixtures has been established through multiple studies including Bell et al. (1984), Linden et al. (1989), and Seeds et al. (2002). These reports show a general trend of a 10 percent decrease in pavement life for each 1 percent increase in air voids in the mixture above some maximum level. The increase in air voids relates to a decrease in density. Significant changes in mix design procedures have occurred since the Hughes report due to the implementation of the results from the Strategic Highway Research Program. Superpave was the principal family of products relating to asphalt concrete. The changes made during the Superpave implementation process have not changed the fundamental relationships between density and performance of asphalt pavements, only the design process was affected. However, Superpave did not recommend a standard on how agencies should measure and set criteria for pavement density. As a result, there is widespread variability on how density is specified. Brown (1998) provides an overview of the issues relating to the implementation of Superpave. Key changes from previous asphalt technologies are (1) a new asphalt binder grading system, C h a p t e r 1 Introduction
4 Specifying and Measuring asphalt pavement Density to ensure pavement performance (2) revised aggregate consensus properties, (3) new aggregate size definitions, (4) revised lift thickness to maximum size aggregate ratio requirements, and (5) a new laboratory compaction protocol. The change in the aggregate size definition had a significant impact on the achievement of density in the field. Prior to Superpave, the typical ratio between lift thickness and maximum aggregate particle size was 2:1. With the Superpave change in definition of nominal aggregate size to include aggregate particles larger than the maximum size, the ratio is now recommended to be 3:1 in order to adequately achieve density in the asphalt concrete. Some agencies have increased the ratio for coarse aggregate mixtures. There is inherent variability of asphalt mixture production and placement. Variability can, for example, be the result of aggregate gradation changes that might result in the segregation of the mix. Such changes can have a significant impact on the ability to achieve and/or properly measure density of the asphalt concrete. Variations in aggregate absorption can also affect the measurement of the maximum theoretical specific gravity (Gmm) and, thereby, influence the mea- surement of the in-place density. Large aggregate mixes may create variability issues due to the small sample sizes usually associated with sampling and testing methods. These are just a few examples of the impact variability can have on the measurement of density. Agencies must strive to understand the inherent variations in the process when writing specifications. This project was focused on understanding the state-of-the-practice for measuring and speci- fying density in asphalt pavements. This report, therefore, will be concentrated on issues impact- ing the ability to achieve roadway density. A two-pronged approach was used for the project. The first was a literature review and the second a survey of the asphalt industry. As directed in the statement of work, the literature review was limited to resources after 1990. Prior work by Hughes (1989) had provided an excellent coverage of previous research activities. Survey It has long been recognized by asphalt technologists that the level of density of an in-place asphalt pavement plays a critical role with respect to the long-term performance of the pave- ment. However, a standard on how agencies measure and establish specification criteria for pavement density does not exist. The objective of this survey is to collect specific information on the current state-of-the-knowledge of in-place density of asphalt pavements relating to the pavement performance as well as the current practices of agencies regarding how in-place den- sity is measured and specified. To be consistent with current terminology in the industry, the following definitions will be used throughout this report: â¢ Density: the weight per unit volume of the asphalt mixture â¢ Compaction: the process of increasing the density of the asphalt mixture through paving and rolling operations (Therefore, compaction refers to the process of achieving a desired density.) â¢ Reference density: the value to which the in-place density is compared to determine percent density (This value is often the theoretical maximum density but can also be a lab or field determined value.) â¢ Percent density: the percentage of the reference density achieved during the compaction pro- cess (Percent density is the desired engineering material property around which specifications are written.) This survey was distributed to state departments of transportation (DOTs) through the AASHTO Subcommittees on Construction and Materials members as well as through the
Introduction 5 membership of the National Asphalt Pavement Association (NAPA) and the state asphalt pave- ment associations. In addition, provincial Ministries of Transportation in Canada, and inter- national associations were solicited for contributions. In total, 100 responses were received. The survey was not distributed to local agencies that often rely on state DOT specifications for their jurisdiction. Responses were received from all 50 state DOTs and the District of Columbia DOT. A total of 60 agency responses were received, including a few multiples from the same state (Subcommittees on Construction and Materials members) and from five Canadian provinces (Newfoundland, Ontario, Saskatchewan, Nova Scotia, and Alberta). A total of 38 responses were received from private industry personnel: 15 from contractors, 16 from state asphalt pavement associations, 2 from material suppliers, and 5 from others. There was no private industry response from 25 states. Responses were rounded to the nearest whole number in the text for ease of reading but shown to the nearest tenth in the tables. In many cases, the values do not add up to 100 percent due to multiple responses being given or rounding of values. Six broad categories were addressed in the survey: â¢ General issues in the density of asphalt pavements, â¢ Specification types for control of density, â¢ Density measurement techniques, â¢ Construction parameters affecting density, â¢ Longitudinal joint construction, and â¢ Emerging technologies for achieving density. This report is divided into four chapters. Chapter 1 is the introduction, Chapter 2 provides the background and survey analysis, Chapter 3 presents the findings, and Chapter 4 provides the conclusions. Please note that the survey, âSummary of Agency Survey Responses,â is available for down- load from the project webpage at http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp? ProjectID=3997.