The Superpave Mix Design System: Anatomy of a Research Program (2012)

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20 Chapter 4. Research Phase The research phase began in 1987 after funding for the program was secured. This chapter describes the organizational structure, contracts and other aspects of the research phase. 4.1 ASPHALT PROGRAM STRUCTURE AND STAFFING Although numerous institutional arrangements were considered in the “Blue” and “Brown” books, SHRP was eventually administered as a new operating unit of the National Research Council with a structure as shown in Figure 7. The asphalt program staff included a program manager, technical staff and loaned staff. The A-001 contractor, responsible for leadership and coordination of the asphalt contractors, served as an extension of the program staff. The asphalt program technical staff, working closely with the A-001 contractor, was responsible for preparation of the requests for proposals and evaluation of the proposals. Once the contracts were executed, program staff was responsible for technical, financial and administrative oversight of the contracts; the A-001 contractor provided technical oversight in coordination with the program staff. “Loaned staff,” both national and international, served on the program staff to provide additional technical expertise and/or the perspective of the ultimate end users of the asphalt program results. The Asphalt Advisory Committee, with representatives from government, industry and academia, provided strategic guidance. The Expert Task Groups (ETG), similarly constituted, were a resource for technical review of individual asphalt contracts. An ETG was assigned to each major asphalt contract. 4.2 EVOLUTION AND ORGANIZATION OF THE RESEARCH PROGRAM The SHRP Asphalt Program evolved as it moved from concept to functioning reality. This evolution can be traced from a statement of general objectives in the 1984 recommendations of the Strategic Transportation Research Study (2) through more detailed research plans published by TRB in 1986 (1); to the Contracting Plan for SHRP Asphalt Research approved by the SHRP Executive Committee in 1987 (detailed in Section 4.2.1); and finally to the 1990 Strategic Plan which was presented at the August 1990 “Mid-course Assessment” meeting held in Denver, CO (3). The emphasis on and need for specification development in the SHRP Asphalt Program originated in the 1984 “Blue Book,” which presented the conclusions and recommendations of the STRS project. In that document, the objective of the asphalt research program was stated as follows (2): “To improve pavement performance through a research program that will provide increased understanding of the chemical and physical properties of asphalt cements and asphalt concretes. The research results would be used to develop specifications, tests… needed to achieve and control the pavement performance desired.”

21 This emphasis was reinforced and further defined in the May 1986 “Brown Book.” This document stated that a specific constraint or guideline for the asphalt program was as follows (1): “…the final product will be performance-based specifications for asphalt, with or without modification, and the development of an asphalt-aggregate mixture analysis system (AAMAS).” This document defined the development of the specifications as a task clearly separate from development of the AAMAS. 4.2.1 Contracting Plan In 1987 the SHRP Executive Committee approved A Contracting Plan for SHRP Asphalt Research. The contracting plan combined the multiplicity of tasks identified in the 1986 research plan into a coordinated, manageable structure of six main contracts (which were later expanded to nine, due to the segmentation of the original A-002 and A-003 contracts). This reconfiguration of projects and responsibilities is shown in Table 2. The contracting plan assigned the responsibility for development of the performance-based asphalt binder specification to contract A-001, and the performance-based specification for AAMAS to contract A-006. From the 1987 plan, however, it was clear that the development of the binder specification was the primary objective as is evident from the following (4): “In the asphalt area, the original report, America’s Highways: Accelerating the Search for Innovation, clearly put the dominant focus on asphalt binders. Subsequent discussions by the AASHTO Task Force, the AASHTO Select Committee on Research and the National Research Council’s SHRP Executive Committee have reinforced this initial vision, placing the primary emphasis on research to improve asphalt binders.” Between 1987 and 1990, nine major research contracts and twelve smaller supporting studies were awarded as shown in Tables 3 and 4 (5, 6). In this same time frame, three subtle, but important changes evolved though an ongoing dialogue with those in the highway community who took part in the development, conduct, management and oversight of the program. These included the following: 1) The specification would encompass modified binders as well as unmodified asphalt cements. 2) Accelerated testing was included to validate the binder specification and to improve the AAMAS. 3) The AAMAS would include not only a mix analysis system, but also laboratory mixing and compaction procedures and accelerated performance-related test methods. It was envisioned that the SHRP mix specification development would build upon the NCHRP 9-6(1) work to yield a more robust, well-validated system.

22 National Research Council Concrete & Structures Highway Operations & Maintenance LTPP Asphalt Contractors A-001 Contractor Technical Staff Loaned Staff Expert Task Groups Executive Committee SHRP Executive DirectorInformation Transfer Asphalt Program Manager Asphalt Advisory Committee Figure 7 Asphalt Program Structure (dashed lines indicate advisory status)

23 Table 2 Major Asphalt Research Contracts: Proposed vs. Actual (based on 1 and 4) Brown Book – Proposed Research Projects (1986) Actual Contracts Awarded (1987 – 1993) 00 1 00 2A 00 2B 00 2C 00 3A 00 3B 00 4 00 5 00 6 Project 1 – Asphalt Properties 1.1 Asphalt Chemical Composition  1.2 Physical Properties of Asphalt  1.3 Relationships Between Asphalt Chemical and Physical Properties  1.4 Relationships of Asphalt Chemical and Physical Properties to Pavement Performance   1.5 Fundamental Properties of Asphalt-Aggregate Interaction Including Adhesion and Absorption  1.5a Physiochemical Properties of Asphalt at the Asphalt-Aggregate Interface  1.5b Physiochemical Properties of Asphalt Used with Absorptive Aggregates  1.6 Survey of Current Manufacturing Practices  1.7 Asphalt Modification  Project 2 – Performance-Based Testing and Measuring Systems 2.1 Testing and Measuring Systems for Asphalt (with and without asphalt modification)   2.2 Testing and Measuring for Asphalt-Aggregate Systems (with and without asphalt modification)   2.2a Fatigue Cracking of Asphalt-Aggregate Systems  2.2b Permanent Deformation of Asphalt-Aggregate Systems  2.2c Low-Temperature Cracking of Asphalt- Aggregate Systems  2.2d Aging of Asphalt-Aggregate Systems  2.2e Water Sensitivity of Asphalt-Aggregate Systems  2.3 Relationship of Asphalt Chemical and Physical Properties to Asphalt-Aggregate Mix Properties  Project 3 – Pavement Performance Studies 3.1 Model Development   3.2 Asphalt Performance Studies   3.3 Evaluation Procedures for Prediction Models  Project 4 – Performance-Based Specifications for Asphalt and Asphalt-Aggregate Systems 4.1 Performance-Based Specifications for Asphalt   4.2 Performance-Based Specifications for Asphalt- Aggregate Systems (AAMAS)    Project 5 – Coordination 5.1 Research Project Coordination  5.2 Operate Materials Reference Library  5.3 Experiment Design  5.4 Economic Considerations  5.5 Implementation Packages  For tasks with more than one “,” bold denotes primary responsibility.

24 Ja n- M ar Ap r- Ju n Ju l-S ep O ct -D ec Ja n- M ar Ap r- Ju n Ju l-S ep O ct -D ec Ja n- M ar Ap r- Ju n Ju l-S ep O ct -D ec Ja n- M ar Ap r- Ju n Ju l-S ep O ct -D ec Ja n- M ar Ap r- Ju n Ju l-S ep O ct -D ec Ja n- M ar Ap r- Ju n Ju l-S ep O ct -D ec Ja n- M ar Ap r- Ju n Ju l-S ep O ct -D ec Contract Number and Name Contractor/Location Amount ($1,000) A-001: Asphalt Experimental Design, Coordination, and Control of Materials University of Texas at Austin, Austin, TX Tom Kennedy, PI $6,188 A-002A: Binder Characterization and Evaluation Western Research Institute, Laramie, WY Claine Peterson, PI Ray Roberston and Dave Anderson, Co-PI $9,033 A-002B: Novel Approaches for Investigating Asphalt Binders University of Southern California Costas Synolakis and Victor Chang, Co-PI $893 A-002C: Nuclear Magnetic Resonance (NMR) Investigation of Asphalt Montanta State University, Bozeman, MT Wyn Jennings, PI $601 A-003A: Performance-Related Testing and Measuring of Asphalt-Aggregate Interactions and Mixtures University of California-Berkeley Carl Monismith, PI Fred Finn and Gary Hicks, Co-PI $9,500 A-003B: Fundamental Properties of Asphalt-Aggregate Interaction Auburn University, Auburn, AL Christine Curtis, PI $3,000 A-004: Asphalt Modification Southwestern Labs, Houston, TX David Rowlett, PI $3,363 A-005: Performance Models and Validation of Test Results Texas A&M Research Foundation, College Station, TX Robert Lytton, PI Rey Roque, Co-PI $3,249 A-006: Performance-Based Specifications for Asphalt- Aggregate Mixtures ** University of Nevada at Reno, Reno, NV PI, Chuck Hughes $895 $36,722 1992 1993 ** A-006 subsequently folded into A-001 Contract following 1990 "Midcourse Assessment" 1987 1988 1989 1990 1991 Table 3 Major Asphalt Contracts (4)

25 Contract Number and Name Contractor/Location Amount ($1,000) A-001: Asphalt Experimental Design, Coordination, and Control of Materials University of Texas at Austin, Austin, TX Tom Kennedy, PI $6,188 A-002A: Binder Characterization and Evaluation Western Research Institute, Laramie, WY Claine Peterson, PI Ray Roberston and Dave Anderson, Co-PI $9,033 A-002B: Novel Approaches for Investigating Asphalt Binders University of Southern California Costas Synolakis and Victor Chang, Co-PI $893 A-002C: Nuclear Magnetic Resonance (NMR) Investigation of Asphalt Montanta State University, Bozeman, MT Wyn Jennings, PI $601 A-003A: Performance-Related Testing and Measuring of Asphalt-Aggregate Interactions and Mixtures University of California-Berkeley Carl Monismith, PI Fred Finn and Gary Hicks, Co-PI $9,500 AIIR-01: Asphalt Characterization by Supercritical Fluid Chromatography/Fourier Transorm Infrared Micospectometry University of Connecticut, Storrs, CT J Stevens, PI $230 AIIR-02: Air Permeability of Asphaltic Materials, and Gas Permeability and Thermal Oxidative Stability Studies on Asphalt Materials using Electrodynamic Balance Research Triangle Institute Research Triangle Park, NC ??? PI $250 AIIR-04: Fluorometric Characterization of Asphalts Pennsylvania State University, University Park, PA A Davis and G Mitchell, Co-PI $143 AIIR-05: Rheological Studies of Asphalts Correlations with Structural Parameters Pennsylvania State University, University Park, PA I Harrison, PI $135 AIIR-06: Asphalt Binder Characterization and Evaluation: Thermal Chromatography - Mass Spectrometry David Sarnoff Research, Princeton, NJ B Benz, PI $88 AIIR-07: Surface Analysis by Laser Ionization of the Asphalt- Aggregate Bond SRI International, Menlo Park, CA T Mills, PI $219 AIIR-09: Significance of Intermediate Principal Stress, Principal Plane Rotation, and Evaluation of Loading Spectra on Fracture and Permanent Deformation of Asphalt Concrete Texas A&M Research Foundation, College Station, TX W Crockford, PI $180 AIIR-10: Evaluation of Donor-Acceptor Properties of Asphalt and Aggregate Materials and Relationship to Asphalt Performance David Sarnoff Research, Princeton, NJ M Labib, PI $152 AIIR-11: Fundamentals of the Asphalt-Aggregate Bond SRI International, Menlo Park, CA D Ross, PI $246 AIIR-12: Innovative Techniques to Distinguish Performance of Asphalt-Aggregate Mixtures CTL International, Inc. Columbus, OH O Abdulshafi, PI $177 AIIR-13: Microscopial Analysis of Asphalt-Aggregate Mixtures Related to Pavement Performance The Danish National Road Laboratory, Roskilde, Denmark K Erickson, PI $184 AIIR-14: Advanced High Performance with Permeation Chromatography Methodology Montanta State University, Bozeman, MT W Jennings, PI $179 $2,183 1992 19931987 1988 1989 1990 1991 Table 4 AIIR (Asphalt, Independent, Innovative Research) Supporting Asphalt Contracts (4)

26 To achieve the research goals, the program was envisioned to progress in four phases as follows and as shown in Figure 8: 1. Conceptualization - The physicochemical properties of asphalt binders and mechanical/structural properties of asphalt-aggregate mixes that affect pavement performance were to be identified. 2. Definition - The effects of binder properties were to be validated in asphalt-aggregate mixes through laboratory testing and, to a lesser degree, through accelerated pavement testing. Concurrently, standardized test methods to support the binder specification and AAMAS were to be developed. 3. Validation – Field performance data were to be used to validate binder and mix properties that affect pavement performance. 4. Adoption – Final recommendations for the binder and mix specifications would be made and implementation would begin. As articulated in the 1990 strategic plan, the scope and objectives of the major contracts were as follows (4): Contract A-002A (Binder Characterization and Evaluation): Identify the chemical and physical properties of asphalt binder believed to influence the performance of asphalt-aggregate pavement systems. Refine into test methods those chemical and physical characterization processes that appear to offer the most practical basis for specification testing in terms of the following: correlation between binder properties, mixture performance and pavement performance established by contracts A-003A and A-005; reliability; cost; ease of use; and other features of the tests themselves. Contract A-003A (Performance-Related Testing and Measuring of Asphalt-Aggregate Interactions and Mixtures): Validate in asphalt-aggregate mixtures the candidate relationships identified in contract A-002A (and to a lesser extent, A-003B and A-004) between the physical and chemical properties of asphalt binder and asphalt pavement performance (first-stage validation). Develop standardized, accelerated test methods for asphalt-aggregate mixtures that may be employed in a mixture analysis system to support a performance-based specification for mixtures. Contract A-003B (Fundamental Properties of Asphalt-Aggregate Interaction): Develop a fundamental understanding of the chemistry of the asphalt-aggregate bond and how it affects adhesion and water sensitivity. Develop a fundamental understanding of the mechanical and chemical basis of asphalt absorption into highly porous aggregates. Prepare reliable, practical test methods that measure asphalt-aggregate adhesion, water sensitivity and absorption and estimate their effects on pavement performance. Contract A-004 (Asphalt Modification): Adapt as necessary performance-related test methods for binders and mixtures to permit their use with the full range of modified systems. Explore innovative refinery processes to enhance the performance of modified asphalt binders. Develop a modifier evaluation protocol to permit evaluation and selection of modified binder systems that remedy specific pavement performance gaps.

27 Figure 8 Strategy to Achieve Key Products (after 5) Economic Assessment and Implementation Performance-Based Specification P ro du ct D ev el op m en t Second –Stage Field Asphalt-Aggregate Mixture Test Development for First-Stage Validation of Asphalt Binder Asphalt-Aggregate Mixture Test Development for Asphalt-Aggregate Mix Analysis System Asphalt-Aggregate Interaction Performance-Based Physical Tests Composition Characterization Materials Selection Adoption Validation July 1991 Mar 1993 July 1990 Jan 1988 Conceptualization Definition Jan. ’88 Jan. ’89 Jan. ’90 Jan. ’91 Jan. ’92 Jan. ‘93

28 Contract A-005 (Performance Models and Validation of Test Results): Validate relationships between asphalt binder and asphalt-aggregate mixture properties and pavement performance (second-stage validation). On the basis of documented field performance data, establish criteria, limits and requirements that may be used for asphalt binder and asphalt-aggregate mixture specifications. Develop performance prediction models incorporating the properties of asphalt binders and asphalt-aggregate mixtures Contracts A-001 (Asphalt Experimental Design, Coordination and Control of Materials) and A- 006 (Performance-Based Specifications for Asphalt-Aggregate Mixtures): Prepare model, performance-based specifications for asphalt binders and asphalt-aggregate mixtures, respectively, using the validated results of contracts A-002A, A-003A, A-003B, A-004 and A- 005. Furthermore, the A-001 contractor was responsible for technical direction, leadership and coordination of the asphalt program. 4.2.2 Materials Reference Library The magnitude and breadth of the asphalt program required major endeavors through broad and complex research efforts. For the research to be both meaningful and effective, all the asphalt researchers would have to use the same materials. Accordingly, the A-001 contractor developed and operated the Materials Reference Library (MRL) containing sufficient quantities of asphalts and aggregates for use by the asphalt researchers through the entire 5½-year program. (4) In fact, this library of materials is still in existence and use today (although it has been moved from its original location in Austin, Texas, to Reno, Nevada). Researchers still request samples of the original SHRP asphalts for various projects. This allows current researchers to build on the work done during and since SHRP. Other materials (aggregates, sub grade materials, mixtures, etc.) from LTPP (Long-Term Pavement Performance) projects across the country are also stored in the MRL and available to researchers upon request and with FHWA approval. 4.2.2.1 Asphalt Selection Process The basic premise of the selection process for the asphalts was that the performance of asphalt pavements is directly influenced by the physicochemical properties of the asphalt cement. Thus, asphalt cements were deliberately chosen to create an MRL containing currently available asphalt cements representing a wide range of field performance histories, crude oil sources, refinery practices, and physical and chemical properties. Thirty-two asphalt cements were selected, sampled and stored in the MRL. The geographic distribution of the refineries from which asphalts were sampled is shown in Figure 9. Eight of the asphalts were selected as having sufficiently diverse performance histories, chemical and physical properties to warrant their being designated as the core or common asphalts in the asphalt program. The core asphalts were to be tested in every experiment in the asphalt program to permit a systematic analysis and correlation of the data obtained in the various contracts and parts of the program.

29 4.2.2.2 Aggregate Selection Process A similar approach was employed in the selection of the aggregates. The aggregates were chosen based on known chemical, physical, geologic and petrographic properties as these properties related to perceived performance in asphalt-aggregate mixes. The geographic distribution of the eleven aggregates selected is shown in Figure 10. Figure 9 Geographical distribution of asphalt sampled for the Materials Reference Library

30 Figure 10 Geographical distribution of aggregates sampled for the Materials Reference Library

31 March 1993 January 1991 January 1990 October 1987 Statistically-designed lab experiments to identify property-performance relationships Development and validation of property-performance relationships in mixes Validation with field results Performance-Based Specifications 4.2.3 Validation and Analysis of Research Data A central problem of the asphalt research program was how best to translate the large volumes of research results generated by more than twenty contractors into a coherent set of performance-based specifications. The following narrative outlines the validation strategy employed. 4.2.3.1 Overview of Validation The process of validation was viewed as a pyramid (Figure 11) with the validated, performance-based specifications at the pinnacle. Individual experiments conducted in each of the contracts form the base. These experiments were to be statistically designed. Higher up on the pyramid, research results from all the different experiments in the program were to be evaluated and combined in different ways to select a consistent set of relationships between material properties and performance that may be a suitable basis for specifications. The highest level of the pyramid was the validation process that would occur in two stages in contracts A-003A and A-005. Promising relationships selected on the basis of laboratory test results were to be tested against field data. Ideally, this validation process would be conducted with well-controlled, long-term field experiments such as those of the LTPP Specific Pavement Study (SPS) series. The tight schedule for the asphalt research program, however, precluded complete reliance upon a long-term program such as LTPP. Rather, the best- Figure 11 Data treatment pyramid (after 5)

32 available information, running the gamut from reliable data from controlled field experiments to personal observations by experienced engineers, would have to be identified, assessed and combined in an accelerated validation process in order to reach the pinnacle of the pyramid within the 5½-year program. Each level of the pyramid would require a different set of analytical techniques and assumptions. At each stage in the process, a different mix of deductive and inductive reasoning would be needed. The successful development of performance-based specifications would require the validation of binder and mix properties identified as important determinants of pavement performance. 4.2.3.2 First- and Second-Stage Validation Validation of the asphalt program results would be a two-stage process coordinated between contracts A-003A and A-005. The first stage (contract A-003A) would confirm that variation of asphalt binder properties identified as probable, significant determinants of pavement performance caused reasonable, meaningful changes in the relevant performance characteristics of asphalt-aggregate mixes. The second stage of the validation (Contract A-005) would establish the degree of correlation between the asphalt binder properties shown to significantly affect performance- related characteristics of asphalt-aggregate mixes and pavement performance, and provide data upon which to set the specification limits for the relevant properties selected to control performance. The basis for a successful validation process would be the use of statistically sound experiment designs. All major contractors would be required to establish statistically sound designs for all major experiments. Additionally, for the asphalt program to be successful, there had to be a mechanism to allow all the researchers to merge, correlate and draw statistically valid inferences from the data collected from the various studies. These requirements would be satisfied in two ways. First, all researchers participating in the program would employ the same materials, essentially the 32 asphalt cements and 11 aggregates contained in the MRL. Inherent in the MRL selection process was the assumption that the 32 MRL asphalts spanned the range of performance expected from the full set of asphalts available then and in the future in the United States and Canada. Second, the research studies in the asphalt program would be organized as experiments selected to test hypotheses and accomplished according to basic statistical procedures and sound experiment designs. The experiments would be designed to validate relationships among test variables, to calibrate and validate test procedures and equipment, and to establish specification variables or criteria. The validation at the core of the asphalt program would be founded upon a series of well- designed experiments. These experiments were expected to identify important relationships between asphalt binder properties and predicted field performance and to provide the first-stage laboratory validation that these relationships translate into significant variation in the corresponding properties of asphalt-aggregate mixes. The more difficult question was how to demonstrate that the binder property was truly predictive of field performance. The first-stage validation would show only that the binder property was correlated with a mix property. The second stage of the validation process would consist of a mathematical correlation of the candidate binder and mix properties with

33 performance data gathered from both full-scale pavement test facilities such as the FHWA Accelerated Load Facility (ALF) and in situ field pavements studies, typified by the SHRP LTPP General and Specific Pavement Study (GPS and SPS) pavement sections. The first stage of the validation process was looked upon as an inductive process since it would not provide conclusive grounds for the truth of the conclusion that relationships existed between binder properties and pavement performance, but rather afforded some support for it. The second stage of the process, however, would be a deductive process in that it would provide conclusive grounds for this conclusion. It would rely upon the correlation of binder and mix properties with actual field performance to demonstrate the soundness of the inferred relationships between properties and performance. In practical terms, the ultimate predictive value of the binder or mix property would be tempered by several factors including the number of field pavement sections utilized in the validation and the degree to which the field pavement sections represented controlled experiments. Performance data from LTPP SPS sections was preferable to GPS section data since the SPS sections were being constructed as controlled experiments. Both were preferable to data gathered from a random assembly of uncontrolled field pavement sections. It was acknowledged, however, that if insufficient field data were available from existing pavements, performance data from other sources would have to be employed, e.g. historical projects that were extensively described in the literature and/or interviews with experienced materials engineers who could provide information concerning asphalt properties and pavement performance, etc. In summary, the validation in the laboratory of candidate properties for incorporation in performance-based specifications would be principally an inductive process. It would be aided by the existence of complete data sets from well-designed, controlled experiments, but could not conclusively prove perceived relationships to pavement performance. By contrast, the conclusive selection of a final suite of properties actually used in the specifications and their limits would require a deductive validation process that would likely employ a mix of statistical data treatment, judgment, interpretation and intuition to compensate for a lack of long-term performance analysis and the possible need to employ incomplete or poor quality performance data. In the end, this approach worked reasonably well for the stage 1 validation of the binder specification. It was less successful for the mix specification. In that case, the sequencing of the contracts, time constraints and lack of performance data (from controlled field experiments or full-scale accelerated testing) made the completion of the validation process virtually impossible. Flow charts illustrating the integration of work products from the various contracts to develop the performance-based specifications are shown in Figures 12 and 13.

34 Figure 12 Strategy to achieve performance-based asphalt binder specification (5) A-005 Develop and validate models A-005 Validate A-002A with field data (Phase 2) A-002A/A-003B Identify Composition Quantify Composition Develop Physical Tests Correlate Composition and Physical Properties A-004 Validate tests for modified binder A-003A Validate A-002A with simulative large scale lab tests (Phase 1) A-001 Develop binder specification and protocols

35 Note: ALT = Accelerated Laboratory Tests 4.2.4 Mid-course Assessment At the August 1990 “Mid-course Assessment” meeting (3), SHRP Executive Director Damian Kulash asked the 400+ attendees – representatives of state highway agencies, industry, and research organizations – to help SHRP look with fresh eyes at each part of the program and to decide where best to concentrate efforts to get the most out of the research. Three workshops were held for the asphalt program: binder specification, mix specification and validation. For the binder and mix specification workshops, participants considered the following: hard products, gaps in product development, top priority research, potential economic impacts, and routes and barriers to implementation. For the validation techniques, workshop participants considered the methodology, sources of field data, schedule and alternate approaches. Some of the key recommendations from the asphalt workshops, summarized herein, are included for several reasons. They allow one to compare and contrast what was envisioned in 1990 with what finally emerged in 1993. Also, they set the stage for implementation and the post-SHRP asphalt research agenda, as stated in the report on the mid-course assessment (3).  The emphasis in the asphalt research should continue to be on identification of the underlying chemical basis for permanent deformation, fatigue cracking, low-temperature cracking, moisture sensitivity, and aging. Figure 13 Strategy to achieve performance-based asphalt-aggregate mix specification (5) A-003A/A-003B Develop ALT for AAMAS A-003A Validate with ALT A-005 Develop and validate models A-001/A0006 Develop mix spec and protocol A-005 Validate A-003A with field data A-004 Validate tests for modified mix

36  Tests of physical properties referenced in the binder specification should have a sound correlation with the underlying chemical properties of the asphalt. There should be a balance between chemical and physical tests.  Consideration should be given to the inclusion of traffic levels in the binder specification.  The user-producer group concept was proposed as part of the implementation process in the pre-1993 period.  While an aggregate specification was beyond the scope of the program, exploration of the effects of the surface chemistry and porosity of the aggregate on adhesion and moisture sensitivity should receive continued emphasis.  The Asphalt-Aggregate Mix Analysis System (AAMAS) and specification methodology should be kept as simple and practical as possible.  AAMAS would require a link between lab mix design and plant production.  Field tests for quality assurance and quality control should be identified or developed.  The effect of large aggregates should be investigated thoroughly in relation to the development of the AAMAS and the mix specification, particularly in relation to the accelerated laboratory tests being conducted under contract A-003A.  Provisions to assure the workability of the mix should be included in the specification.  SHRP should consider the operational impact of new specifications on both centralized and decentralized design practices.  For the second-stage validation, the A-005 contractor should consider the following sources of data: state projects and test tracks; Asphalt Institute field studies; FHWA and Department of Defense experimental projects; and accelerated loading facilities.  The adoption into practice of the performance-based binder specification and the mix specification would have significant economic impact on the state highway agencies, hot-mix producers and contractors, asphalt refiners, and other components of the industry, in terms of capital equipment purchases, new personnel and training requirements, changes in operations, changes in crude oil sources, etc. Serious efforts to quantify these impacts to aid implementation of the specifications should begin immediately.  A comprehensive training program must be launched as early as possible. To ensure compatibility between the binder and mix specifications, the A-006 contract responsibilities were folded into the A-001 contract shortly after the “Mid-course Assessment.” The following sections include more detailed discussions of the individual contracts including hypotheses, people and products.

37 4.3 BINDER-RELATED RESEARCH Much of the research phase was spent exploring binder chemistry. This section outlines the guiding philosophy behind the research; the people, contracts and hypotheses employed in the work; and the eventual evolution of the binder specification. 4.3.1 Guiding Philosophy As noted in the Blue Book and reiterated in the Brown Book, there were five guiding principles or objectives related to the SHRP Asphalt Research Program and four of them were directly related to asphalt binder. They were as follows: 1. Identify and describe asphalt properties with a specific interest in the chemical and physical properties of asphalt cements and their interrelationships. The goal would be to correlate the chemical and physical characteristics of binders. 2. Develop improved testing and measuring systems for asphalt binders. 3. Establish the association between asphalt binder and pavement performance. 4. Develop an asphalt binder model that reflects the complex molecular structure of asphalt cement. These guiding principles were used throughout the conduct of the research as benchmarks by the coordination contractor and the researchers to maintain focus. Furthermore, the guiding principles evolved into working hypotheses and models employed by the researchers, as seen in the following sections. 4.3.2 Hypotheses and Models Employed in the Binder Research A thorough treatment of the hypotheses and models employed in the binder research is found elsewhere (5). The discussion in the following sections is intended to provide a brief overview. 4.3.2.1 Contract A-002A Binder Characterization and Evaluation (5-9) Contract A-002A was led by Claine Peterson and Ray Robertson of Western Research Institute and Dave Anderson of Pennsylvania State University. It was the basis for the conceptualization and development of the asphalt binder performance-based specification. Also, this contract was the primary source of data used to generate the binder specification. The work was divided into three major tasks with the following objectives: 1. Identify and quantify the chemical, compositional factors in asphalt that significantly influence physical properties and the performance of asphalt-aggregate systems. 2. Develop new and improved techniques for measuring the physical properties of asphalt. 3. Develop standardized test methods for asphalt or modified asphalt which satisfy requirements of AASHTO and ASTM and which could be employed to specify and accept binders for use with performance-based specifications.

38 Chemical Composition and Performance of Asphalts Hypothesizing that the chemical composition determined its physical (rheological) properties, the focus of the research was on the separation of asphalt into chemically distinct fractions. The asphalts were separated into five chemically distinct factors – neutral, weak acid, strong acid, weak base and strong base – by ion exchange chromatography (IEC). The results demonstrated that the strong acid fraction was the viscosity-building component in the asphalt and controlled its temperature susceptibility. Also, the data suggested that the strong acid fraction governs adhesion and water sensitivity through the interaction of polar functional groups and aromatic ring structures with aggregate surfaces. Finally, the data suggested that specific molecular entities in the strong acid fraction linked together into an elastic network, the structure of which affected the load and thermally-induced stresses that caused fatigue and low- temperature cracking, respectively. Physical Properties and Performance of Asphalts The selection of the most appropriate physical properties that merit characterization was driven by the distress modes (permanent deformation, fatigue cracking, thermal cracking, aging, moisture sensitivity and adhesion) encountered during the service life of the pavement. The behavioral modes that relate to the distress factors were rheology (stiffness), fracture, stress- strain characterization, tensile strength, asphalt-aggregate adhesion (debonding) and oxidative hardening. The physical property data should be developed from correlation with chemical, compositional properties since asphalt chemical structures varied with temperature and applied stress, and therefore so did their apparent molecular weights. Consequently, the general approach was to employ the concepts of physical chemistry, tempered with engineering judgment, by considering the physical (rheological) properties of asphalts as being directly related to their chemical properties. This approach is shown schematically in Figure 14. Noting that asphalt is a viscoelastic material, the rheological behavior would depend on the loading time of the external force. Accordingly, shear susceptibility (complex flow) and temperature susceptibility (stiffness properties) were hypothesized as being reflective of component interactions of asphalts. Finally, the researchers hypothesized that in the case of sophisticated chemical tests, surrogate physical tests would be developed to mimic the physicochemical parameters being evaluated. These physical tests would yield results in fundamental engineering units (stress and strain) to provide a sound link between standardized tests and field performance. 4.3.2.2 Contract A-004 Asphalt Modification (5, 9) Since asphalt cements with optimum properties could not be obtained from all crude oils by conventional refining processes or blending practices, Contract A-004, led by David Rowlett of Southwestern Laboratories, focused on asphalt modification.

39 Chemical Composition and Performance of Modified Asphalts Logically, the working hypotheses employed in the area of modified binders were similar conceptually to those discussed for unmodified asphalts. Additionally, a working concept was advanced to investigate the molecular forces which produce an elastic network (entanglement) within modified asphalts. Theoretically, extensive branching of the asphalt molecules would decrease viscosity at low temperatures due to molecular motion of the functional end groups which are active at low temperatures. Similarly, extensive branching would increase viscosity at high temperatures and would introduce significant entanglement. Performance-Related Physical Properties of Modified Asphalts Close interaction and cooperation were required between the A-002A and A-004 contractors. The tests identified for unmodified asphalts would be employed with modified asphalts, if feasible. The objective, however, was to distinguish between those tests which simply characterized the presence of modifiers in asphalt from those which provided results that reflect the influence of the modifiers on the pavement performance factors. 4.3.2.3 Contract A-003B Fundamental Properties of Asphalt-Aggregate Interaction (5, 9, 10) Led by Christine Curtis at Auburn University, Contract A-003B was tasked with providing fundamental information on the following:  chemical nature of the asphalt-aggregate bond;  chemistry and morphology of the aggregate;  aggregate-induced asphalt chemistry; and  changes in asphalt chemistry due to selective absorption and adsorption. Figure 14 Relationship of asphalt organic and physical chemistry to asphalt physical properties (5)

40 It was envisioned that these fundamental results would provide a direct link between the asphalt- aggregate chemistry and the pavement performance properties in terms of fundamental engineering properties as measured by accelerated laboratory test procedures. Chemical Composition and Model Conceptualization The model investigated considered interactions between the asphalt and aggregate surfaces occurring in three zones or regions as shown in Figure 15. Molecules absorbed within the pores of the aggregate constitute the absorbed region. Those molecules attached directly to the aggregate surface are considered as the interface region. Molecules that are structured near the interface but not attached to the aggregate surface are considered as the interphase region. The bulk asphalt lies beyond the interphase region. Molecular structuring, which is often induced by aggregate chemistry, occurs in the asphalt at the interface and in the interphase regions. The researchers hypothesized that this structuring had a definite effect on the chemistry of the asphalt-aggregate mix and subsequently on the pavement performance characteristics. Furthermore, it was hypothesized that the asphalt absorbed within the pore space of the aggregate had different chemical and physical properties than the bulk asphalt such that selective absorption would occur; i.e., selective absorption of the highly polar molecules led to a situation in which the absorbed asphalt had a substantially different composition than the asphalt film. The net result was that the actual effective asphalt film coating the aggregate had a composition, and properties, which were different from the bulk asphalt. Performance-Related Test Methods to Measure Asphalt-Aggregate Interactions The approach pursued in contract A-003B was similar to those of A-002A and A-004 in that the rheological properties (viscoelastic, complex behavior) were explained in chemical terms by molecular association. Similarly, mechanical deformation (e.g., shear flow) was characterized as breaking or altering the intermolecular structure. Figure 15 Asphalt-aggregate model illustrating interphase and interface regions (5)

41 4.3.3 Evolution of Binder Specification Had the SHRP Asphalt Program evolved as originally envisioned, the binder specifications would be based on the chemical composition of asphalt and common laboratory testing terminology would include IEC and FTIR instead of the now-familiar BBR, DSR and PAV. The asphalt program began with an intensive laboratory investigation to relate the chemical and physical properties of asphalt to the behavior of asphalt mixes and pavement performance. The interest in a chemically based binder specification was still keen, as evidenced by statements made following the 1990 mid-course assessment (3): “The emphasis in the asphalt research should continue to be on identification of the underlying chemical basis for permanent deformation, fatigue cracking, low- temperature cracking, moisture sensitivity, and aging. Tests of physical properties referenced in the binder specification should have a sound correlation with the underlying chemical properties of the asphalt. There should be a balance between chemical and physical tests.” As the A-002A binder studies progressed, the researchers concluded that because each crude source contained unique and complex chemistry, measuring physical properties (fundamental engineering properties) was a much more effective and practical approach to predict performance. Quite simply, connecting chemical properties to pavement performance was “a bridge too far.” 4.3.3.1 “Strawman,” Supporting Tests and Criteria As noted previously, the ultimate responsibility for developing a performance-based asphalt binder specification was that of the A-001 contractor. Integrating the work done by the A-002A and A-002B contractors, this activity was led by Tom Kennedy of the University of Texas at Austin. Essentially the specification required the selection of the grade to be based on the temperature regimen to which the pavement will be exposed (both high and low temperatures). A decision was made to provide a “strawman” specification to public and private stakeholders interested in asphalt binder to inform them of the thinking of the research team and to obtain their feedback early in the development process. In all, approximately fifteen versions of the specification were developed and modified during the process as new information became available from both researchers and stakeholders. Examples of early editions of the strawman specifications are shown in Figures 16 and 17. Some have described the use of the strawman specification as a stroke of genius. Industry’s initial response was somewhat less complimentary. Some in industry were skeptical. Others were downright incensed. In fact, one notably vocal individual is reported to have said it was “positively ridiculous” to proffer a specification when the research had not been completed.

42 “Strawman” Specification for Asphalt Binders Graded at 0°C (32 F°) and 80°C (176°F) for aged binders Property Rheology Index*. 0°C (32°F) AB 21-20 AB 30-20 AB 40-20 AB 11-10 AB 15-10 AB 20-10 AB 6-5 AB 7.5-5 AB 10-5 AB 3-2.5 AB 4-2.5 AB 5-2.5 2100±210 3000±300 4000±400 1100±110 1500±150 2000±200 600±60 750±75 1000±100 300±30 400±40 500±50 Rheology Index*, 80°C (176°F) 2000±200 1000±100 500±50 250±25 Nitrogen Factor** a ± for all grades Acid Factor**, max b ± for all grades Healing Factor***, min c ± for all grades Viscosity, 135°C (275°F), Ca, max 600 ± for all grades Flash Index. °C (F) min d (d’) e (e’) f (f’) g (g’) * Related to low-temperature cracking and permanent deformation. Test is conducted on aged binders. Binders are aged using low-temperature, high oxygen pressure test simulating 5 years of service life. ** Nitrogen factor and acid factor are related to moisture damage and are optional for regions without moisture damage problems or if the asphalt is modified. A surrogate test on the asphalt mixture can be substituted. *** Related to fatigue cracking. Figure 16 Example of Early “Strawman” Binder Specification

43 Aged Asphalt Binder Grades AB 1- AB 2- AB 3- AB 4- 1 2 3 1 2 3 1 2 3 1 2 3 Highest mean monthly temperature °F <80 80-90 90-100 >100 Lowest anticipated temperature °F <-20 -10 to -20 >-10 <-20 -10 to -20 >-10 <-20 -10 to -20 >-10 <-20 -10 to -20 >-10 Temperature dependency Low-Temperature Cracking Low-temperature stiffness at -10°F, psi (Bending Beam Test, SHRP B001) Permanent Deformation Dynamic stiffness at 140°F (Indentation Test, SHRP B002), psi Fatigue Cracking Cycles to failure at 77°F (Bending Beam Fatigue Test, SHRP B003), min Healing index at 77°F (Microcrack Healing Test, SHRP B004), min Aging Mass change (TFOT or RTFOT, AASHTO Test,), max., % Low-temperature stiffness SHRP B001 at -10°F max, psi After POV aging (POV Aging Test, SHRP B005) at temperature of, °F 120 120 120 140 140 140 160 160 160 180 180 180 Water Sensitivity Bond strength at 90°F (Blister Test, SHRP B006), min, psi Adhesion Bond strength at 32°F (Modified Blister Test, SHRP B006M), min, psi Constructability Kinematic viscosity at 275°F test (ASTM D2170), max cSt 1500 1500 1500 1500 Safety Flash point (COC Flash Point, ASTM D92), max, °F 450 450 450 450 Figure 17 Example of Early Strawman Binder Specification

44 4.4 ASPHALT-AGGREGATE MIX RELATED RESEARCH At the onset of SHRP, specifications assured only that the asphalt binder would respond in a predictable, consistent manner during plant production and placement. There was, however, no minimum level of pavement performance warranted, or even intended, in any but a peripheral sense. Similarly, there were no mix specifications directly linked to pavement performance. Thus, a second major objective of the asphalt program was to develop a performance-based mix specification and supporting test protocols. This would also provide a means to verify the asphalt binder specifications being developed. In addition to the results produced though the SHRP contracts, the researchers were to consider the findings from related NCHRP projects 09-6(1), Asphalt-Aggregate Mixture Analysis System (AAMAS), and 10-26A, Performance-Related Specifications for Hot-Mix Asphalt. 4.4.1 Guiding Philosophy As with the binder specification, the mix specification was to accommodate both unmodified and modified binders and consider the six performance factors of low-temperature and fatigue cracking, permanent deformation, moisture sensitivity, aging and adhesion in conjunction with the effects of environmental conditions and traffic. Also, like the binder specification, a “strawman,” Figure 18, was developed to focus the research, generate input from users and producers, and “to bring a sense of reality” to the end-products. As shown in Figure 19, the four environmental regimes defined by LTPP were included initially with the understanding that the regions might be further subdivided as the specification evolved and was adopted by the states. It is instructive to note the features of this initial asphalt-aggregate mix specification as it allows a comparison to what emerged upon the conclusion of the research. The specification addressed the following: • A minimum number of traffic levels in terms of 18 kip ESALs were included in the initial specification with the ultimate goal of considering the possible interaction between traffic and environment. • Conditioning procedures to address mix aging and moisture sensitivity were also envisioned. For aging of the loose mix, a modification of the rolling thin film oven test, forced draft oven, and high pressure aging vessel were suggested. For moisture sensitivity a triaxial compression type cell for measuring stiffness was proposed. Measuring permeability was also a possibility. • To assess rutting potential, cylindrical specimens would be subjected to a vertical axial stress and to a repeated shear stress. • For the two forms of low-temperature cracking (single drop in temperature and thermal fatigue), a thermal stress-restrained beam specimen test was envisioned. • To capture the fatigue behavior of both thick and thin pavement layers, several tests were proposed: flexural beam, an axial push-pull, or some combination of tests which might serve as a surrogate.

45 • Although there was significant money and effort devoted to fundamental research on aggregate properties that affect adhesion and absorption, there were no provisions to address the more routine but critical factors which affect hot-mix asphalt performance; e.g., physical/mechanical properties of aggregate and aggregate gradation. Accordingly, the narrative in 4.5.6, The Delphi Story, is presented to describe how these critical but heretofore neglected elements of aggregate properties were addressed in the asphalt program. The importance of gradation was also recognized as evidenced by the initial requirements for VMA (voids in the mineral aggregate) and avoidance of the “restricted zone,” shown in Figure 20.

46 Climatic Zone Wet-No Freeze Dry-No Freeze Wet-Freeze Dry-Freeze Highest mean monthly temperature, °F 90-100 >100 90-100 >100 90-100 >100 90-100 >100 Lowest anticipated temperature, °F -10 to -20 >-10 -10 to -20 >-10 -10 to -20 >-10 -10 to -20 >-10 -10 to -20 >-10 -10 to -20 >-10 -10 to -20 >-10 -10 to -20 >-10 Traffic Level1 L M H L M H L M H L M H L M H L M H L M H L M H L M H L M H L M H L M H L M H L M H L M H L M H Low-Temperature Cracking Stress at cracking, psi Temperature at Cracking, °F (Thermal Stress-Restrained Tensile Test, SHRP M001) Thermally-Induced Fatigue Cracking Cycles to Failure, Nf (Thermal Stress-Restrained Tensile Test, SHRP M001) Permanent Deformation Strain/cycle at 104 °F (Triaxial Compression-Repeated Shear Stress Test, SHRP M002) Fatigue Cracking Cycles to failure at 68°F, Nf (Beam Fatigue Test, SHRP M003) Short-Term Aging Stiffness aging index (Mixture Rolling Thin Film Oven Test, SHRP M004) Long-Term Aging Stiffness aging index (POV Aging Test, SHRP M005) Water Sensitivity Minimum retained stiffness, psi (Repeated Load-Triaxial Water Conditioning Test, SHRP M006) Figure 18 Strawman Specification for Asphalt-Aggregate Mixes

47 Figure 19 Environmental Regimes Defined by LTPP

48 Figure 20 Restricted Zone for Aggregate Gradation 4.4.2 Hypotheses and Models Employed in the Mix Research A thorough treatment of the hypotheses and models employed in the asphalt mix research is found elsewhere (5). The discussion in the following sections is intended to provide a brief overview. 4.4.2.1 Contract A-003A Performance-Related Testing and Measuring of Asphalt- Aggregate Interactions and Mixtures This contract was considered a cornerstone of the asphalt program as it was to provide the foundation upon which accelerated performance-related tests would be developed for asphalt-aggregate systems. The fundamental knowledge of mix performance and material component interaction obtained in this research was critical to the development of the performance prediction models and the validation effort. Furthermore, this contract would provide the majority of the research data needed to conceptualize and develop the performance- based specification for asphalt-aggregate mixes. The principal investigator was Carl Monismith of the University of California-Berkeley. Co-principal investigators were Gary Hicks of Oregon State University and Fred Finn of Austin Research Engineers. Given the shortcomings of the empirically-based Marshall and Hveem test methods, the goal was to develop theoretically sound, reliable and reproducible test methods that could be used to characterize asphalt mixes in terms of fundamental engineering properties. These properties would then be used to predict performance under a wide range of in-service conditions. Other factors that were to be considered in the development of these tests were practicality, efficiency and cost.

49 4.4.2.2 Contract A-005 Performance Models and Validation of Test Results Bob Lytton of Texas A&M University and Rey Roque of Pennsylvania State University led this effort. Ideally, the second-stage validation of important relationships between asphalt properties and field performance could be accomplished through a long-term study of controlled field experiments. This approach, however, would require an estimated twenty or more years and was not compatible with SHRP's objective of rapid development of performance-based asphalt specifications. Therefore, this contract was structured to accelerate the validation process through a correlation of the relationships between asphalt properties and field performance, and the predictive performance models expressing these relationships. It was envisioned that statistical treatment of in-place field performance data coupled with sound judgment could be used in place of a long-term experiment. A second and equally important goal was to develop performance prediction models using data from SHRP’s Long-Term Pavement Performance (LTPP) General Pavement Studies (GPS), state highway agencies, FHWA, and accelerated field tests such as the Pennsylvania State University test track and/or the FHWA's Accelerated Loading Facility (ALF). It was essential to the success of the research to formulate relationships between asphalt binder, mix properties and field performance in a manner that realistically accounted for the effects of traffic, the environment, pavement layer geometry and construction. 4.4.2.3 Contract A-006 Performance-Based Specifications for Asphalt-Aggregate Mixtures After the mid-course assessment this research, led by principal investigator Chuck Hughes, was folded into contract A-001. While the research conducted on the mix specification did not lend itself to the development of working hypotheses to guide the work, the starting points were conceptual frameworks generated by SHRP contract A-003A and NCHRP Project 10-26(A). Conceptually, the performance-based specification would incorporate a mix analysis system; performance-related test methods; a modifier evaluation protocol; and specification tolerances for the various performance factors. It was envisioned that the performance-based specification would allow selection of an optimal job mix formula that would provide for satisfactory pavement performance over the wide range of environment, traffic loadings and construction conditions encountered in the United States and Canada. In addition, it would provide a structured method for estimating the probable effects of off-specification paving mixes on short- and long-term pavement performance.

50 4.5 PRODUCTS Superpave (Superior Performing Asphalt Pavements) was the final product of the SHRP Asphalt Program. It was envisioned to be a comprehensive system for the design and analysis of paving mixes to accommodate project-specific performance requirements. Encompassing new material specifications, test methods and equipment, and software, it was developed to address permanent deformation, fatigue cracking, and low-temperature cracking as tempered by aging and moisture sensitivity, and was conceived to be applicable to virgin and recycled, dense- graded hot-mix asphalt, with or without modification. Lastly, it was hoped that it would replace the diverse and numerous material specifications and mix design methods then used by the fifty states with a single system that could provide results tailored to the distinct environmental and traffic conditions found anywhere in the United States and Canada. Specifically, the major products included the following: 1) a performance-based specification for asphalt binders with supporting test methods and equipment; 2) a performance-based mix design system with supporting test methods and equipment; 3) a modifier evaluation protocol; and 4) the Superpave specification, design, and support software. The evolution of the Superpave products was fraught with challenge and debate. The evolution of the name was no less contentious. “What’s in a name?” you ask. Read on for a behind-the- scenes tale of how “Superpave” came to be.