Performance Specifications for Asphalt Mixtures (2016)

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3 on performance-based properties. As the SHRP research pro- gressed it became apparent that testing and analysis for the performance predictions would be too complex for many routine projects. Therefore, a simple empirical design method (Level 1) was developed as the base or entry level mix design. After the SHRP initiative, when the performance-based tests and models were not implemented, the base mix design method specified in AASHTO M 323 became known as Superpave (AASHTO 2004). As a result, as implemented Superpave is based on consensus mixture properties such as air voids, voids in mineral aggregate (VMA), etc. The key components of Superpave Level 1 and the decisions made about them are as follows: compaction was to be done with a gyratory com- pactor; air voids calculated using the theoretical maximum specific gravity and the test specimen’s bulk specific gravity; VMA calculated using aggregate bulk specific gravity; voids filled with asphalt (VFA) and aggregate gradation using con- trol points and a restricted zone (that was later removed from the specification); coarse aggregate angularity expressed as crushed faces; and, fine aggregate angularity to control the percentage of natural sand. Today’s mix design technology (Superpave) represents an evolution of ideas that been evaluated through the years. The limitation of Superpave (as well as the previous methods of Marshall and Hveem) is its inability to measure expected performance; specifically, to predict rut resistance, fatigue cracking, low temperature cracking, asphalt binder aging, or resistance to moisture damage. Instead, all three methods use surrogate properties to control performance properties. For example, rutting is controlled by the aggregate properties (e.g., crushed faces on coarse aggregate and fine aggregate angularity on fine aggregate) and the volumetric properties (e.g., air voids and VFA). Fatigue cracking is controlled by the asphalt content, while low temperature cracking is con- trolled by the low temperature grade of the asphalt binder and the asphalt binder content. The mixture aging is a function of asphalt content, and moisture damage depends on asphalt binder content and the bond strength of asphalt-aggregate interface enhanced by antistrip agents. It is recognized that these are properties of the mix design. In service, the performance of the mixture is controlled by rutting (influenced by traffic characteristics, high temper- ature environment, and in-place density); fatigue cracking (controlled by pavement deflection and weather, in as much as it controls deflection); low temperature cracking (induced This chapter introduces background information and high- lights the objectives, organization, and key definitions used in the report. The synthesis summarizes a collection of available literature on performance specifications for asphalt mixtures. There was a particular emphasis on reviewing the types of performance testing used to support the specifications related to nontraditional asphalt mixtures that use a variety of pro- duction techniques or mixture additives. Transportation agen- cies were surveyed to determine their current and future use of performance specifications for asphalt mixtures, includ- ing the contents and basis for acceptance of asphalt mixtures using performance specifications, test program implemen- tation and research efforts underway, development of contract provisions and pay factors, and the benefits and challenges to implementation. The survey and interview processes used in generating information in the synthesis are also described. The focus of the synthesis was on plant-produced asphalt mixtures and did not consider preservation and maintenance type mixtures; however, if an agency provided information on such mixtures, the comments have been included in the summary response tables. Background Superpave Mix design The Superpave mix design was developed as part of the Stra- tegic Highway Research Program (SHRP) that occurred from 1987 to 1993. The objective was to develop a performance- based asphalt binder specification, a performance-based asphalt mixture specification, and a mix design system. The asphalt binder performance-graded (PG) specification used today is the result of that research. The attempt to develop a performance-based mix speci- fication was less successful. Although the research program developed performance tests for asphalt mixture and models to predict mixture response (stress, strain, etc.) and to predict mixture performance (rutting, fatigue cracking, thermal crack- ing) the system ended up being too difficult to implement and was never used by state departments of transportation (DOTs). The Superpave mixture design system developed during SHRP had three levels of increasing complexity, referred to as Level 1, Level 2, and Level 3 mix design. Performance- based mixture tests were to be used in the Level 2 and Level 3 designs. Originally, Superpave was to be developed solely chapter one IntroductIon

4 by weather-related factors such as low temperature and cool- ing cycles); mixture aging (due to high temperature environ- ment factors such as extreme temperatures and duration of hot weather, and the in-place density); and moisture damage (affected by in-place density and by traffic, in which the load pulses play a role). asphalt Mixture Performance tests SHRP sought to identify and develop test methods for prop- erties that could be used to predict pavement response and thereby expected pavement performance. In October 1990, a group of senior members from FHWA, state DOTs, and industry made a technology tour of Europe, where they were introduced to mixture performance tests developed at the Lab- oratoire Centrale des Ponts et Chausees (LCPC). LCPC had developed a wheel-tracking test to measure rutting susceptibil- ity and a trapezoidal fatigue test to measure fatigue cracking potential. These test methods, or similar variants, were sug- gested for the SHRP program, but were discounted as being nonfundamental. During the late 1990s and early 2000s, the European Union harmonized specifications for asphalt mixture design. The har- monized specifications (Execution Des Assisses des Chausees Couches de Liaison et Couches de Roulement 2008) adopted a hierarchal approach to mix design, which defined various levels to include: • Level 0—Aggregate gradation and asphalt content. • Level 1—Volumetric properties of gyratory compacted plus moisture damage test. • Level 2—Tests from Level 1 plus wheel tracking rut test. • Level 3—Tests from Level 2 plus dynamic modulus. • Level 4—Tests from Level 3 plus fatigue testing. As Superpave was adopted in North America during the late 1990s and early 2000s there was increasing interest in adoption of tests that were targeted for specific asphalt mix- ture distresses. During the 1980s, rutting was recognized as a major national issue and the interest in a performance test based on rut testers began to emerge. Moisture damage has remained an issue of great interest in some parts of the coun- try and the confidence in the Tensile Strength Ratio (TSR) test adopted as part of Superpave was low; thus, alternative tests were identified in lieu of TSR. Following the implementation of Superpave mix design, the occurrence of rutting in asphalt pavements has been signif- icantly less common. More recently cracking, predominantly top-down cracking, has become more common and as a result there has been increased interest in the development and use of cracking tests. This synthesis seeks to identify performance tests being used by DOTs. An awareness of materials variability, coupled with advances in testing, increased use of statistical concepts, and improved understanding of materials behavior led researchers to pur- sue the following complementary lines of inquiry over the past several decades: • Performance specifications—How can we develop and implement more performance-oriented construction specifications that would support the use of acceptance parameters and pay adjustments that are more indicative of how the finished product will perform over time? How can the appropriate warranty period be determined? How can we work with contractors to invest in their product beyond the limits of the contract? • Performance-based mixture designs—How can a performance-based approach to developing and testing mixture designs be incorporated that will provide for satisfactory pavement performance (i.e., optimal bal- ance between rutting and cracking resistance) over a wide range of service conditions and source materials? The results of such research are being used or piloted by several transportation agencies to improve the long-term per- formance and cost-effectiveness of their asphalt pavements. PerforMance SPecIfIcatIonS for aSPhalt PaveMentS Much of the research related to performance specifications for asphalt pavement has focused on the implementation of pay adjustment systems to address the expected future performance of the in-place pavement. A common payment approach, as incorporated in today’s quality assurance (QA) specifications, involves statistically based sampling and test- ing plans that consider the measured variability of the product to determine pay adjustment factors. A more rational approach, as promoted in performance-related specifications (PRS), uses predictive models to assign pay adjustments based on the dif- ference between the as-designed and as-constructed life-cycle cost of the pavement. Quality assurance Specifications The high construction and materials variability observed in the AASHTO Road Test suggested that the traditional pre- scriptive specifications used by highway agencies at the time could not adequately control the construction process. This realization led to the development and implementation of so-called QA specifications, which addressed the issues of testing and test variability, sample size, lot size, estimates of the total population, percentage within limits, and pay adjustment factors. Over time, QA specifications gained widespread accep- tance as an improved method for determining the contrac- tor’s degree of compliance with specification limits. Various surveys of state DOTs, conducted in 1997, 2000, 2002, and

5 most recently in 2005, have demonstrated the increasing use of QA specifications over the last 20 years. The study by Elmore et al. (1997) included a survey of state DOTs to determine which items are being controlled by these speci- fication types, what parameters are measured, how pay fac- tors are determined, and who conducts the testing (agency or contractor laboratories). All 19 of the responding agencies reported that they use QA specifications for hot mix asphalt (HMA). Nine of the 19 agencies indicated that they have a pay incentive included in the specifications, while 11 have disincentives, clauses that could result in a pay reduction or require rework of the completed pavement. The parameters that were most frequently reported as being used for deter- mining pay factors were asphalt content (by all 11 states with pay factors), in-place density (by 11 states), gradation (by eight states), VMA (by five states), and laboratory-compacted density (by four states). In a relatively short period, use of QA specifications spread to several other DOTs, as captured in the findings from surveys conducted in 2000 (FHWA 2001) and 2001 (Ksaibati and Butts 2003). In the FHWA survey (2001), many DOTs (including the District of Columbia and Puerto Rico) responded that they either implemented a QA specification (21 states) or were in the process of developing one at that time (14 states). The response rate shifted a year later when the University of Wyoming survey reported that 90% (39 states) of the respond- ing DOTs had implemented a QA specification for asphalt pavement (Ksaibati and Butts 2003). Research related to QA specifications also focused on identifying the appropriate quality measures on which to base acceptance of the asphalt mixtures and/or in-place pavements. NCHRP Synthesis 346, which focused on various QA pro- grams used by state DOTs, provided some insight into the quality measures used for HMA (Hughes 2005). A survey conducted in support of this research found that the measures used in QA testing programs circa 2005 included asphalt con- tent (reportedly used by 40 DOTs), gradation (by 43 DOTs), and compaction (by 28 DOTs). Other reported measures included volumetric properties, ride quality, thickness, and moisture content. Performance-related Specifications Although widely accepted as an improved method for deter- mining compliance with specification limits, QA specifica- tions still did not necessarily address product performance, as they were largely based on quality measures that were not directly tied to the performance of the asphalt mixture or the in-place pavement. Moreover, pay factors were often arbi- trarily combined into a composite payment factor that did not necessarily relate to the reduced or enhanced value of the as-built pavement. To address such limitations, research began to focus on the development of enhanced QA specifications, referred to as PRS, which would more directly relate quality measures to long-term performance. PRS are often referred to as the next generation of QA specifications, as they attempt to use predictive models to assign rational pay adjustments based on the difference between the as-designed and as-constructed life-cycle cost of the pavement. Some of the original research related to the development of a prototype PRS for asphalt mixtures was conducted at the WesTrack site. WesTrack was an experimental test road facility located near Fallon, Nevada, sponsored by FHWA (Epps et al. 2002a, b). The primary project for WesTrack was entitled “Accelerated Field Test of Performance-Related Specifications for Hot-Mix Asphalt Construction,” which listed two primary objectives: (1) to provide data to support the continued development of PRS and PRS software for HMA construction by examining how deviations in materials and construction properties (e.g., asphalt content and degree of compaction) affect long-term pavement performance, and (2) to provide field verification of the Superpave mix design procedures developed through the original SHRP Asphalt Research Program. The testing for this project included three experimental variables: asphalt content, air void content, and aggregate gradation. The results, which were summarized in terms of rut depths and percentage of the wheel path areas con- taining fatigue cracking, were used to develop simple empiri- cal relationships for performance prediction to support a PRS. Subsequent research to refine PRS models and software led to the development of the Quality-Related Specification Software (QRSS). QRSS is a stand-alone program that calcu- lates the predicted performance of an HMA pavement from the volumetric and materials properties of the as-designed HMA and compares it with that of the as-built pavement calculated from the contractor’s lot or sub-lot quality con- trol data. It computes a Predicted Life Difference (PLD) based on fatigue, rutting, and thermal cracking that can be used to reward and/or penalize contractors for their product (Moulthrop and Witczak 2011). Although PRS have seen only limited use to date potential future enhancements, such as the development and incorpo- ration of more timely and reliable test methods and related criteria—particularly if consistent with the work being per- formed to advance performance-based mixture designs and mechanistic-empirical pavement structural design—could increase confidence in the predictive capabilities of the under- lying performance models and make owners and industry more amendable to wider application of PRS. PerforMance-BaSed MIxture deSIgn For an asphalt mix to perform well in the field it must provide adequate resistance to the various distresses com- monly associated with flexible pavement failure such as rut- ting, fatigue cracking, and thermal cracking. Conventional

6 volumetric mixture design systems, however, provide only limited insight into such behavior, which has driven research- ers and practitioners to explore the development of more performance-based mixture designs and related test methods that could be used to optimize the often competing perfor- mance needs of a pavement (e.g., adequate resistance to both rutting and cracking) to meet the unique characteristics of a given project or application. One of the unmet goals of the original SHRP Asphalt Research Program was to develop such a performance-based mixture specification with supporting test methods and equip- ment. Although the resulting Superpave system led to sweep- ing changes in the design, selection, testing, and specification of asphalt materials, it still largely relies on surrogate proper- ties and empirical relationships to control the performance of the mixture. To address this limitation, some DOTs have attempted to supplement their conventional volumetric cri- teria with more performance-based testing conducted to establish the mixture’s resistance to common distresses. For example, wheel-tracking tests and the use of stiffer binders have been used to help prevent placement of rut-susceptible mixtures. However, without a timely and reliable method to evaluate fatigue performance, such measures can lead to increases in early cracking, a growing concern given today’s increasing use of reclaimed asphalt pavement (RAP), recycled asphalt shingles (RAS), and other non conventional modified mixtures that are often stiffer than those used in the past. Performance-based mixture testing would more readily allow for the evaluation and inclusion of such locally avail- able and/or innovative materials for which limited empiri- cal data are available. This is of particular relevance today given recent initiatives to bring more additives to asphalt mixtures in the interest of environmental stewardship and/ or fiscal responsibility. A number of additives to asphalt mix- tures, such as RAP, RAS, and recycled tires such as ground tire rubber (GTR), have been explored to varying degrees. However, the impact of these additives on the flexible pave- ment’s long-term performance is relatively unknown without extensive field validation. For example, the use of asphalt roofing shingles in asphalt mixtures may improve the performance of the pavement, due to the increased stiffness compared with standard asphalt mix- ture designs (Calrecycle 2006). In addition, roofing shingles tend to improve the pavement’s resistance to rutting, stability, compaction, rideability and index, and decreases temperature susceptibility. According to Calrecycle (2006), as of July 2006, the following DOTs were reported to permit the use of shin- gles in asphalt pavement, and to allow a certain percentage of shingles that may replace a portion of aggregate: Georgia (5%, manufacturing scrap only), Maryland (5%, manufactur- ing scrap only), Michigan and Minnesota (5%, manufacturing scrap only), Missouri (5% max), New Jersey (5%, manufac- turing scrap only), North Carolina (5%, manufacturing scrap only), Ohio (allows a certain percentage as listed in speci- fication), and Indiana (5%, manufacturing scrap only). The Calrecycle research also reported that past research con- ducted by the Florida DOT indicated that shingles can com- prise 15% of the aggregate portion and still perform to the levels expected from standard HMA mixtures. Development and implementation of practical and timely performance tests would allow pavement designers to better understand the expected behavior of these modified asphalt mixtures and allow for the tailoring of specific material require- ments (e.g., stiffness, rutting, and cracking properties) to meet the needs of a given project. SyntheSIS oBjectIve The objective of this synthesis is to provide state DOTs, Canadian provincial ministries of transportation (MOTs), and other public agencies with information on how to more effectively implement performance specifications and per- formance testing for asphalt mixtures. The scope of this synthesis focused on performance tests used in conjunction with volumetric properties for specifying both traditional and nontraditional plant-produced asphalt mixtures. Perfor- mance tests are intended to provide information to assist in extending service life by guiding material selection (i.e., asphalt binder and aggregate) and proportions (i.e., asphalt content and gradations). The results of this synthesis are intended to benefit government agencies, researchers, and the road-building industry in providing guidance on making better use of recycled materials, while also providing better performing and more cost-effective asphalt mixtures for spe- cific applications. The report will help state and provincial materials engineers, construction, and design engineers, along with other transportation managers to better understand the state of the practice, challenges, and gaps in existing knowl- edge with respect to performance specifications for asphalt mixtures. Other aspects of this topic that are explored in this study include: • Agency mix design specifications for plant-produced HMA or warm mix asphalt (WMA); • Agency use of performance testing and how performance specifications were developed; • Agency specification criteria (e.g., project selection criteria and testing done internally by agency staff or by external entities); • QA procedures and supporting information; and • Performance testing time, equipment availability, and benefit-to-cost analysis. Various efforts have been made in recent years by some states and other public agencies to address the application of performance testing in making better use of pavement distress prediction models for achieving longer pavement

7 service lives. There is a need to evaluate these efforts and obtain examples of practices that are reported to be effective in order to facilitate the exchange of information and to help other states. The synthesis also includes suggestions for future research based on existing gaps identified through the literature review, survey, and agency interviews. Study aPProach A multifaceted approach was taken to document the various efforts that have been made in recent years by some states and other agencies toward the development and implemen- tation of performance specifications for asphalt mixtures. The approach to this synthesis included a literature review of federal, state, international, and regional research, and a survey of state, provincial, and other transportation agencies. In addition, detailed interviews with state and other agencies were conducted as suggested by the analysis of the survey responses. The following sections provide more detail on each step in the approach. literature review A number of resources were consulted including the Trans- port Research International Documentation (TRID), Inter- net and web searches, FHWA and DOT internal reports, journal publications, conference proceedings, transportation agency specifications and standards, and resources of pro- fessional associations. A comprehensive literature review of sources both in the United States and internationally was used to establish current practice and emerging trends related to the use of performance specifications on asphalt paving projects. Survey of State and other transportation agencies The survey consisted of 40 questions and was sent to mem- bers of the AASHTO Research Advisory Committee with a recommendation for distribution through the DOT research director’s office to the DOT materials engineers to complete the survey. The survey was sent to contacts in each of the state DOTs; Washington, D.C.; 11 Canadian MOTs; five ports; three cities; two counties; and two turnpike authorities. Ninety percent (45/50) of all DOTs responded to this syn- thesis survey. The survey questions and results are included in Appendix A of this report and the full list of respondents is provided in Appendix B. Ten Canadian MOTs, the Dis- trict of Columbia, two counties, and one city also provided responses to the survey and their responses are indicated separately within the text and in the tables of the appendix. The maps in Figures 1 and 2 show the agencies in the United States and Canada, respectively, that responded to the survey, FIGURE 1 Location of U.S. agencies that responded to the survey, were interviewed, and were selected as case example agencies. : DOT interviewed and selected as case example : DOT interviewed: DOT or LPA responded to survey Map source: diymaps.net - Clark County Orange - County

8 specific agencies that were interviewed, and indicates the agencies that were ultimately selected to serve as the case examples, which will be presented in chapter four. Interviews with transportation Practitioners in case example agencies Based on the results of the survey responses, 11 states and one local agency were selected for additional data gathering on practices used related to the use of performance specifications for asphalt paving mixtures. A number of criteria were con- sidered in the selection of these agencies to be interviewed; however, priority was given to agencies that provided detailed responses based on (1) agencies that reported they have sub- stantially moved into the development or use of performance specifications; (2) geographical distribution of states in order to reflect a national perspective and varying climatic condi- tions; and (3) variety of paving program sizes. It was critical to include agencies in which performance testing has been imple- mented into specifications for an extended period of time. At least one representative from each of the organizations was interviewed over the phone or, in some cases, by e-mail, to gather input on issues and practices in their state related to the status of performance specifications and performance testing of asphalt mixtures. A listing and sampling of docu- ments obtained as examples of current practice are included in web-only Appendix D. rePort organIzatIon This synthesis report is organized into five chapters. The bal- ance of chapter one presents the report’s structure with brief explanations of each chapter’s content and defines some of the key terms used throughout the report. Chapter two describes asphalt performance testing and types of specifications that use the results of performance testing as documented in published literature of federal, state, international, and regional research. Each section of the chap- ter provides a summary from the literature that focuses on findings related to the types and use of performance speci- fications for asphalt mixtures, performance tests for asphalt mixtures, pay adjustment factors in performance specifica- tions, and research on the advancement of performance spec- ifications for asphalt mixtures. Chapter three presents the results from a survey of all state DOTs, provincial MOTs, and other agencies along with the provision of the survey response rates. It provides the state of the practice in many states on the extent to which performance specifications have been developed and implemented. The sections of this chapter are organized to provide a summary of the findings from the survey related to the basis for accep- tance of asphalt mixtures using performance specifications, contents of performance specifications for asphalt mixtures, and test program implementation. Another section presents the survey findings related to the development of contract : Agency interviewed and selected as case example: MOT or LPA that responded to survey City of Edmon- to Map source: diymaps.net n FIGURE 2 Location of Canadian agencies that responded to the survey, were interviewed, and were selected as case example agencies.

9 provisions and pay factors for asphalt performance specifica- tions. The chapter closes with a presentation of the benefits and challenges to performance testing of asphalt mixtures, as reported by the transportation agencies. The majority of the information presented in this chapter was obtained through surveys of all 50 states and the District of Columbia, with contributions from personnel in Canadian provincial trans- portation agencies, one city, and two counties. Chapter four examines the specific examples of state and local practices related to performance specifications used for the construction of flexible pavements. Detailed inter- views were conducted with a number of states and a local agency and these agencies were selected based on their sur- vey responses. The case examples also summarized practices reported in the following key areas: details on the projects that have used performance specifications and outcomes from these projects; post-construction monitoring of projects that used performance specifications; testing protocols; and responsi- bility for laboratory testing. The majority of the information presented in this chapter was gathered through the detailed interviews with multiple personnel in various agencies or organizations in a number of states and one city that were selected for further study. Chapter five concludes the synthesis with a summary of findings and suggestions for further study. Key findings are summarized in several areas and the documented prac- tices are drawn from the literature review, results of the state agency survey, and interviews. These chapters are fol- lowed by a glossary, a reference section, a bibliography, and four appendices. Appendix A includes a copy of the survey questions along with graphical and tabular presentation of the survey results. Appendix B is a list of the agency sur- vey respondents. Appendix C includes links to resources that were provided by agencies through their survey responses or during the in-depth interviews. Web-only Appendix D pre- sents several sample documents that were offered by agencies as a result of the interviews for sharing as examples. defInItIonS Key definitions related to specification types and key prop- erties and types of asphalt mixtures, as used in the context of this report, are provided here. Additional terms are defined within the context of their relevant sections. A glossary is also included at the end of the report that further defines acronyms and abbreviations used in the report. Specification types The definitions were adapted, as applicable, from the sixth edition of TRB Circular E-C173: Glossary of Transportation Construction Quality Assurance Terms (2013). End result specifications: Specifications that require the contractor to take complete responsibility for supplying a product or an item of construction. The highway agency’s responsibility is to either accept or reject the final product or to apply a pay adjustment commensurate with the degree of compliance with the specifications. Performance specifications: Specifications that describe how the finished product should perform over time. For high- ways, performance is typically described in terms of changes in the physical condition of the surface and its response to load, or in terms of the cumulative traffic level that degrades the pavement into a condition that can be defined as failed. Performance-based specifications: Quality assurance specifications that describe the desired levels of fundamental engineering properties (e.g., resilient modulus, creep proper- ties, and fatigue properties) that are predictors of performance and appear in primary prediction relationships (i.e., models that can be used to predict pavement stress, distress, or perfor- mance from combinations of predictors that represent traffic, environmental, roadbed, and structural conditions). Performance-related specifications: Quality assurance specifications that describe the desired levels of key materials and construction quality characteristics that have been found to correlate with fundamental engineering properties that predict performance. These characteristics [e.g., air voids in asphalt concrete (AC) and compressive strength of portland cement concrete (PCC)] are amenable to acceptance testing at the time of construction. QA specifications: A combination of end-result specifica- tions and materials and methods specifications. The contrac- tor is responsible for quality control (QC) (process control), and the highway agency is responsible for acceptance of the product. types and Properties of asphalt Mixtures The following definitions related to key properties and types of asphalt mixtures were adapted primarily from NCHRP or FHWA reports; however, it can be noted that these terms may vary in different states and one must refer to the specific state specifications. Asphalt mixtures: For the purposes of this synthesis, the term “asphalt mixtures” is intended to represent any HMA or WMA that is produced at the plant. This term includes mixtures that have been modified with the use of recycled materials (e.g., RAP, RAS, crumb rubber from recycled tires or GTR) and other additives. This term does not include maintenance mixtures (e.g., cold mix asphalt and seal coats) or in-place recycled mixtures (e.g., hot-in-place recycled asphalt and cold-in-place recycled asphalt). Durability: In the context of asphalt pavement, durability is defined is the ability to withstand wear, pressure, damage, or repeated use over a relatively long period, usually several

10 years or more. It is synonymous with longevity and for asphalt pavement can mean a mixture’s resistance to raveling, rutting, fatigue and thermal cracking, or moisture damage. Durability also relates to the aging of the asphalt binder over time. Dynamic modulus E* : This is the ratio of stress to strain under vibratory conditions (calculated from data obtained from either free or forced vibration tests, in shear, compression, or elongation). It is a property of viscoelastic materials. Fatigue: In materials science, fatigue is the weakening of a material caused by repeatedly applied loads. It is the pro- gressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maxi- mum stress values that cause such damage may be much less than the strength of the material typically quoted as the ulti- mate tensile stress limit or the yield stress limit. For asphalt pavements, fatigue is manifested by the propagation of cracks in the pavement materials over time. High RAP asphalt mixtures: NCHRP Report 752: Improved Mix Design, Evaluation, and Materials Management Prac- tices for Hot Mix Asphalt with High Reclaimed Asphalt Pave- ment Content (West et al. 2013) defined high RAP asphalt mixtures to include RAP content that is greater than 25% and may exceed 50%. As a result, this report generally assumes high RAP to be defined as in NCHRP Report 752; however, it should be recognized that each individual agency’s defi- nition of high RAP may be slightly different in terms of the percentage of RAP content. Rutting: A rut is a depression worn into a roadway result- ing from permanent deformation (strain) in the pavement or subbase material caused by repeated wheel loading. Ruts can also be formed by wear, as from studded snow tires com- mon in cold climate areas. Stiffness: The stiffness of a material, characterized by the elastic or resilient modulus, is a measure of the extent to which pavement resists deformation in response to an applied force. Warm mix asphalt: The FHWA Long Term Pavement Performance program has defined warm mix asphalt in their current WMA SPS-10 experiments as asphalt mixtures pro- duced at either 275°F or less or at 30°F below the HMA production temperature. As a result, this report generally assumes WMA to be defined as in the FHWA program; how- ever, it should be recognized that each individual agency or group of agencies (such as in the case of the Northeast Asphalt User-Producer Group), definition of WMA may be slightly different in terms of the percentage of RAP content. More discussion on the definition of WMA can be found in NCHRP Research Results Digest 374 (2012).