Comparing the Volumetric and Mechanical Properties of Laboratory and Field Specimens of Asphalt Concrete (2016)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

3 Introduction Maintenance and growth of U.S. infrastructure is vital to the economic and social prosperity of the country. For this reason, significant resources must be allocated to ensure that adequate paving mixtures are designed, produced, and constructed. In 2005, of the approximately 4 million miles of roads in the United States, 2.6 million were paved with either Portland cement or asphalt cement concrete. Approx- imately 94% of the paved roads were surfaced with asphalt concrete mixtures. This scale of infrastructure has enabled the American public and business to travel 3 trillion vehicle miles annually (Brown et al. 2009). 1.1 Asphalt Mixture Design Asphalt mixture design is most commonly defined as the process by which an aggregate gradation and optimum asphalt binder content are determined to meet prescribed criteria associated with pavement performance (Brown et al. 2009). From the 1940s to the 1990s, most asphalt concrete mixtures were designed using the Marshall or Hveem methods. During the 1990s, states began implementing the Superpave mixture design method as a result of the Strategic Highway Research Program (SHRP). The purpose of this program was to develop mixture design methods that could be used to predict pave- ment performance. In the Superpave procedure, volumetric properties in association with expected traffic levels are used to determine the optimum asphalt binder content. As of 2012, most state DOTs have implemented the Superpave mixture design. Regardless of the mix design method selected, the pri- mary reason for conducting mixture design procedures is to determine a suitable combination of aggregates and asphalt binder for optimum pavement performance. The resulting “recipe” is termed the job mix formula (JMF). During produc- tion, the design JMF should be verified and revised through the plant to accommodate production and field conditions (Brown et al. 2009). 1.2 Asphalt Mixture Production The basic purpose of an asphalt mix plant is to proportion, heat, and combine the components of the mixture design as per the design. The aggregate structure in the JMF is typically a blend of three or four different aggregates, while the asphalt binder is normally a performance grade (PG) asphalt binder with or without additives (e.g., antistrips or polymers). Large- scale production of the mixture in the plant is difficult to dupli- cate during laboratory design protocols (Brown et al. 2009). For this reason, quality control (QC) and quality acceptance or quality verification (QV) testing is conducted to ensure that the mixture produced is appropriate for what is designed. In this project, the combination of QC and quality acceptance activi- ties will be defined by the AASHTO definition of quality assur- ance (QA). QA testing is used as a basis of pay for the contractor. 1.3 QA Testing Adequate QA practices, which include testing conducted by the contractor and acceptance testing conducted by the state, are the keys to obtaining a satisfactory product and ensuring that a constructed hot mix asphalt (HMA) pave- ment is what the designer specified (AASHTO R 10). Years of experience indicate that deviation from either material or construction specifications often leads to premature pave- ment distress or even failure (Hughes 2005). 1.4 Problem Statement There is a need to identify and quantify causes, sources, and levels of variability in volumetric and mechanical properties of mixtures from the design, production, and construction of the mixture. This requires evaluation of three possible scenarios for production of asphalt mixture specimens: (1) laboratory-mixed, laboratory-compacted specimens (LL), produced during the design process; (2) plant-mixed, C H A P T E R 1

4laboratory-compacted specimens (PL), involving volu- metric acceptance testing of plant-produced mix; and (3) plant-mixed, field-compacted specimens (PF), used dur- ing density acceptance testing of in situ pavement and forensic evaluation of as-built pavement. Although research studies have evaluated some aspects of this problem, a comprehensive national study is needed to provide a complete evaluation of all volumetric and mechanical properties of interest includ- ing, but not limited to, the recently introduced dynamic com- plex modulus. Additionally, with the increased emphasis on mechanical-empirical pavement design, an evaluation of variability among specimen types and its effect on pavement performance prediction is needed. 1.5 Objectives and Scope The objectives of this project, as stated in the request for proposals, were to (1) determine causes of variability and the precision and bias for volumetric and mechanical proper- ties of dense-graded asphalt mixtures measured within and between laboratory-mixed and -compacted [design (LL)] specimens, plant-mixed and laboratory-compacted [produc- tion (PL)] specimens, and plant-mixed and field-compacted [construction (PF)] specimens; and (2) prepare a recom- mended practice in AASHTO standard format for state DOTs to incorporate these results in their specifications and criteria. These objectives were accomplished by evaluating and com- paring common volumetric and mechanical properties of the three specimen types through (1) a meta-analysis of existing data and (2) a laboratory experiment using 11 mixtures from various states across the United States. Variation in key pro- duction process factors—specifically the return of baghouse fines, delay in specimen fabrication, aggregate absorption, aggregate hardness, and stockpile moisture content—was evaluated in the laboratory experiment. For each mixture, the following volumetric and mechanical properties were mea- sured for the three specimen types: • Volumetric properties: air voids, voids in mineral aggre- gate, voids filled with asphalt, bulk specific gravity of the aggregate blend, mixture maximum specific gravity, asphalt binder content, and gradation. • Mechanical properties: loaded-wheel tracking (LWT) rut depth, axial dynamic modulus (E*), and indirect tension dynamic modulus (IDT E*). 1.6 Research Method To achieve the aforementioned objectives, the project was conducted in two phases (I and II) as follows: Phase I • Task 1: Conduct literature review • Task 2: Survey, collect, and perform a meta-analysis on data from past research studies that relate to the following issues: – Levels of variability in asphalt mixtures – Factors causing variability between specimen types Phase II • Task 3: Develop the laboratory experimental plan • Task 4: Execute the approved laboratory experiment • Task 5: Conduct data analysis – Individual mix analysis to quantify magnitude of varia- tion within each mix – Combined mix analysis to evaluate causes of variation • Task 6: Develop specification recommendations based on results of the analysis – Evaluate effects on predicted performance using the Mechanistic Empirical Pavement Design Guide (MEPDG) • Task 7: Prepare final report Figure 1-1 summarizes the research method applied in Phases I and II of this study. In Phase I, the researchers col- lected and analyzed data from previously completed research projects that could be used to determine a solution to the problem statement. At the conclusion of Phase I, the research team and NCHRP agreed that the data collected were not sufficient to adequately answer the problem statement. There- fore, an experimental factorial was developed and conducted, completed as Phase II of the project. As shown in Figure 1-1, identifying and acquiring asphalt mixtures meeting the research criteria was an iterative process because some mixtures iden- tified in the experimental factorial were not practical for field production. Once a mixture was identified, samples were col- lected during production and sent to the Louisiana Trans- portation Research Center (LTRC) where specimens were prepared and the laboratory evaluation of the mixture was conducted. Along with the production samples, contractor QC data were collected for analysis. The process was repeated until all the mixtures were collected to complete the experi- mental program. After all the mixtures were collected and analyzed, the individual data sets were combined into a meta- data set and analyzed to answer the project objectives. 1.7 Report Outline This report has eight chapters, including this introduc- tory chapter (Chapter 1). Chapter 2 describes the preliminary research and analysis conducted in Phase I to support the development of the experimental program. Chapter 3 presents the development of the experimental program, and Chap- ter 4 describes the methods used. Chapter 5 summarizes the individual mixture analyses and results. Chapter 6 presents the combined data analyses of the 11 asphalt mixtures. Chap- ter 7 presents the proposed tolerances and conversion factors developed from the statistical analyses of the individual and combined results. Finally, Chapter 8 summarizes findings and conclusions of the research.

5 Phase I Phase II Figure 1-1. Research method flowchart.