Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt (2004)

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B-1 INTRODUCTION The purpose of this appendix is to present a detailed review of equipment requirements for low-temperature creep and strength testing of asphalt concrete mixtures. During the Strate- gic Highway Research Program (SHRP), procedures were developed for characterizing the mechanical behavior of asphalt concrete at low temperature and using the resulting data in a rational analysis to provide reasonably accurate pre- dictions of thermal cracking. The test procedures developed were the indirect tension (IDT) creep and strength tests sum- marized in AASHTO T322. Since the conclusion of SHRP, these procedures and the required equipment have not been fully evaluated, refined, and implemented. Implementation activities were attempted through the FHWA Regional Super- pave Centers, but were unsuccessful, largely due to problems associated with the specific IDT test system purchased for use by the Superpave Centers. In the meantime, development of new uniaxial test methods for use in the Superpave simple performance tests and in characterizing asphalt concrete mix- tures as required by the pavement design guide developed in NCHRP Project 1-37A have provided engineers with an attractive alternative to the IDT creep and strength procedure. This appendix focuses on an evaluation of the possible use of the dynamic modulus test equipment to perform both the IDT and uniaxial creep and strength tests at low temperature. Following this introduction, a substantial background sec- tion is presented, in which the essentials of low-temperature cracking are presented, along with a discussion of the develop- ment of the Superpave IDT creep and strength test procedures and the more recently developed simple performance and dynamic modulus tests. This is followed by a detailed review of the equipment requirements for both procedures. Specific recommendations for revising the IDT creep and strength equipment requirements are summarized. The dynamic modu- lus test equipment—the version required for master curve development for structural pavement design—was reviewed to determine the changes needed for performing low-temperature creep and strength tests. It was concluded that this version of the dynamic modulus test equipment should require only slight modifications to perform low-temperature creep and strength tests, in either a uniaxial or diametral geometry. The NCHRP Project 9-29 Phase III Interim report included the recommendation that the low-temperature creep and strength testing required for the Superpave thermal cracking model should primarily be performed using uniaxial testing performed on the dynamic modulus master curve equipment as required in the pavement design guide developed in NCHRP Project 1-37A. However, as is made clear throughout this report, the presence of anisotropy in the creep compliance of asphalt concrete mixtures measured at low temperatures strongly suggests that the IDT test should be retained as the standard procedure, though some relatively minor revisions are needed in this method. The reader should keep this in mind while reading this appendix and the comparison of the IDT and uniaxial test geometries. BACKGROUND In order to fully appreciate the various issues surrounding appropriate equipment for performing low-temperature creep and strength tests on asphalt concrete, it is essential to under- stand the basics of low-temperature cracking. It is also useful to know the history of the development of the IDT creep and strength tests. Furthermore, recent development of uniaxial test procedures and equipment for use in the Superpave sim- ple performance tests and in the dynamic modulus test needed for asphalt concrete characterization in the pavement design guide developed in NCHRP Project 1-37A make a uniaxial creep and strength test at low temperatures a possible alterna- tive to the IDT procedure. In the sections below, information is presented to provide the reader with background needed to understand these and other important issues surrounding low- temperature testing of asphalt concrete mixtures. Low-Temperature Cracking Low-temperature cracking, also referred to as thermal cracking, occurs in flexible pavements during rapid tempera- ture drops in the winter months in temperate and sub-Arctic regions. Like most materials, the volume of asphalt concrete changes with changes in temperature—when it cools down, it contracts, and when it warms up, it expands. In an actual pave- ment, the asphalt concrete is prevented from moving, because there are normally no joints in flexible pavement systems. Therefore, when an asphalt concrete pavement is rapidly cooled, it develops substantial tensile stresses. This situation is worsened by the temperature-dependent nature of asphalt con- crete; not only does it contract upon cooling, but its modulus increases, and its strain capacity decreases. Therefore, when an asphalt concrete pavement is subjected to rapid cooling at low temperatures, it becomes more brittle while at the same time developing substantial thermal stresses in tension. This combination of conditions is the primary cause of thermal cracking in asphalt concrete pavements. In severe low-temperature events, cracking can be cata- strophic, occurring explosively and resulting in the immediate development of transverse cracks. These cracks are typically APPENDIX B EQUIPMENT CONFIGURATIONS FOR CREEP AND STRENGTH TESTING OF HOT MIX ASPHALT CONCRETE AT LOW TEMPERATURES

spaced at 3 to 10 meters and usually run from one-half to com- pletely across the pavement. Although crack-widths are often initially quite small, thermal cracks will gradually widen, allowing water and dirt to enter the crack. After several years, thermal cracks can lead to serious pavement distress. Thermal cracking can also occur through a fatigue mechanism. In this case, individual low-temperature events are not severe enough to create stresses in excess of the tensile strength of the pavement but are high enough so that accumulated dam- age over months or years will eventually cause transverse cracks to develop. Figure B-1 is a photograph of typical low- temperature cracking. The primary factor contributing to low-temperature crack- ing is the use of asphalt binders that are too stiff for a given climate. Recent experience suggests that the Superpave per- formance grading of binders, when properly applied, has greatly reduced the potential for thermal cracking in asphalt concrete pavements. However, other factors besides binder grade will affect the low-temperature properties of an asphalt concrete mixture, including binder content, air void content, aggregate gradation and type, pavement thickness, type and thickness of the pavement subbase, and the type of the under- lying subgrade. In order to obtain the most reliable evalua- tion of the resistance of an asphalt concrete mixture to low- temperature cracking, a rational procedure for testing and analysis of the mixture is needed that takes into account most of these factors. The SHRP IDT Creep and Strength Tests During SHRP, low-temperature cracking was identified as one of the major forms of distress in asphalt concrete pave- ments. A concerted effort was made to develop an effective mechanics-based approach to evaluate the resistance of asphalt concrete mixtures to this form of damage; the IDT creep and strength tests were the result (B1). In these tests, a thin, circu- lar specimen of asphalt concrete is loaded across its diameter to determine its mechanical properties at low temperatures. A typical specimen is 50-mm thick and 150-mm in diameter and is prepared by sawing a thin section out of a standard speci- men prepared using a gyratory compactor. Pavement cores B-2 can also be used to make specimens for this procedure. In the creep test, constant stress loading is used to determine the compliance of the mixture at −20, −10, and 0°C, usually over a period of 100 seconds. In the strength test, the specimen is loaded at a constant rate of 12.5 mm/min (0.5 in/min) until the specimen fails in tension. This test is usually performed at −10°C. The IDT test geometry was selected, rather than a simpler uniaxial test, because at the time laboratory specimens for testing asphalt concrete mixtures were generally 100-mm in diameter and no more than 100-mm high, and usually shorter. Preparing an appropriate specimen for uniaxial test- ing from this type of compacted sample would be difficult or impossible (B1). Furthermore, the general procedures and equipment for performing uniaxial tests on asphalt concrete were not well developed, whereas the IDT test geometry had been widely used in a number of procedures. An additional advantage of the IDT geometry is that thin field cores can be easily tested. The SHRP research team therefore decided to use the IDT geometry for the SHRP thermal-cracking tests (B1). Figure B-2 is a sketch of an instrumented IDT test specimen (B2). The Superpave thermal cracking computer model is quite complex, and a detailed description is beyond the scope of this appendix. It will only be briefly summarized here; the inter- ested reader should refer to Witczak et al. (B3), a recent report providing up-to-date, detailed information on this computer program. There are several steps in the analysis of data gath- ered using the IDT creep and strength test: data evaluation and averaging; compliance calculation; master curve construction; calculation of relaxation modulus; stress calculation; and cracking prediction. In the Superpave thermal cracking com- puter program, these steps are implemented through a number of subroutines that model various aspects of the problem, such as environmental effects, pavement response, and pave- ment distress. A special procedure is used in the strength test to determine the exact moment of failure. This involves mon- itoring the specimen deflection during testing and defining the moment of failure as the point at which the difference between the vertical and horizontal deformations reaches a peak. Cal- culation of compliance using the IDT system is somewhat complicated by the three-dimensional state of stress that exists Figure B-1. Typical low-temperature cracking in an asphalt concrete pavement. Figure B-2. Sketch of an instrumented IDT test specimen. SOURCE: The Asphalt Institute (B3).

B-3 during diametral loading. In the Superpave IDT creep and strength tests, the distribution of stress and strain within the IDT specimen is modeled through a series of semi-empirical equa- tions based upon the results of three-dimensional finite element analyses. This approach provides more accurate results than the simpler and more widely used approach of applying a simple plane stress analysis to the IDT loading geometry. Soon after the conclusion of SHRP, numerous problems were identified in many of the tests and computer programs developed during SHRP, including the IDT test and thermal cracking program. These problems were well documented in a report by Janoo and his associates (B4). Over the next sev- eral years, Witczak et al. made a substantial effort to improve the test and the associated analyses and computer program. As documented in the report published by this group (B3), the IDT creep and strength test and the Superpave thermal cracking program now appear to be reasonably reliable and accurate. Problems with Electromechanical IDT Systems Used at the Regional Superpave Centers In 1996, the Federal Highway Administration established five Regional Superpave Centers, to assist with the implemen- tation of the Superpave technology. The Superpave Centers generally represented cooperative ventures between the host- ing state highway department and a state-run or state-related research university. Most of the Superpave Centers were given IDT creep and strength test systems designed and manufac- tured by Instron Corporation. These systems were unique in that they were closed-loop electromechanical (“screw”) test machines; most closed-loop test systems are servo-hydraulic. It was believed that these systems would potentially be less expensive to purchase and operate and also easier and safer to operate, especially in a state highway or contractor’s laboratory that might lack experienced test engineers. Unfortunately, these systems were plagued with a wide range of hardware and software problems and a lack of customer support. There were frequent problems with mal- functioning of the LVDTs used to measure IDT deformation and the conditioners used in conjunction with these transduc- ers. Part of this problem was related to the practice of keep- ing LVDTs mounted on the specimens during strength tests, which frequently damaged the LVDTs, sometimes enough so that the LVDT was completely nonfunctional, but often times only slightly, so that it was not clear that the LVDT was dam- aged and not functioning properly. For this reason, a procedure is needed to estimate the “corrected” IDT strength from that determined without use of LVDTs. Another source of prob- lems was the placement of some of the LVDT conditioning circuits inside the environmental chamber, which subjected these electronics to frost and moisture. The manufacturer (Instron Corporation) explained that the nature of the bid doc- uments required them to design the system in a less than ideal manner and indicated that given more flexibility in their choice of transducer type, they could have produced a significantly more reliable system. The software supplied with these systems was inflexible and difficult to operate, and frequently crashed. The latter problem was probably caused by insufficient memory in the computer systems supplied with these test systems. Some engineers at the Superpave Centers complained that the ramp times required to reach specified loads for the creep tests were too long, though experience at the Northeast Center was that this was a software problem and not due to limitations in the capability of the electromechanical loading system. Because of the numerous problems encountered by the various Superpave Centers in operating these systems, only one—The Northeast Center—performed IDT tests on a regu- lar basis using this equipment; recently, the Northcentral Superpave Center also began using their system. The quality of the data produced at the Northeast Center was, however, marginal and testing was continued mostly in an effort to gain experience with this system. The Northeast Center did pub- lish one research paper on analysis of the IDT creep test (B5), which was essentially a detailed explanation of a simplified version of Roque and Hiltunen’s analysis (B1), suitable for use in estimating thermal cracking temperatures using IDT creep and strength data. Although ruggedness testing with the IDT systems was planned, because of the frequent and serious problems with the Instron IDT system, significant progress was never made on this task. The frustrating experience within the Superpave Centers with the Instron IDT system has probably created a situa- tion in which it would be inadvisable to continue to pro- mote electromechanical systems for use in IDT creep and strength testing. The likely market size for this test is prob- ably perceived as too small to motivate any equipment man- ufacturer to provide significant custom engineering, design, and support for the IDT test system. The most practical ap- proach for pavement engineers is, therefore, to use off-the- shelf test systems to perform the test, with a minimum of specially machined accessories. For this reason, suppliers of the frequency-sweep equipment to be used in characterizing mixtures for the pavement design guide developed in NCHRP Project 1-37A should be encouraged to include as an option the necessary capacity, hardware, and software for perform- ing low-temperature creep and strength tests using the IDT procedure. Although it has some practical advantages, uniaxial test- ing does not provide data equivalent to that produced with the IDT test. The IDT strength test should however be per- formed without LVDTs, and the apparent strength calculated using the maximum load. The “true” IDT strength should then be adjusted using the empirical relationship given in the body of this report as Equation 8. NCHRP Projects 1-37A and 9-19 During the past 5 years, much effort has been made at improving the standard test procedures and analysis meth- ods used to design asphalt concrete mixtures and pavements. This effort has progressed on several fronts. In NCHRP

Project 9-19 work has continued on developing and refining test methods and models for use in a comprehensive and accu- rate version of Superpave. A similar effort has been made under NCHRP Project 1-37A in selecting test methods and procedures for pavement structural design; but, in this case, a more conservative approach has been used in order to ensure that the resulting procedures are highly robust and reliable and suitable for use by practicing engineers. A third related effort has been in the development of simple performance tests for use in conjunction with Superpave volumetric mix design, performed under NCHRP Project 9-19. There has been significant articulation among these efforts, so that there is consistency among many of the proposed test procedures. At this time, the specific procedure to be used for the simple performance tests has not been finalized, but the test geometry has been; a 150-mm-high by 100-mm-diameter specimen will be used, which is to be prepared by coring and sawing a 170-mm-high by 150-mm-diameter gyratory speci- men. All candidate simple performance tests involve uniaxial testing. This same geometry is also to be used in the dynamic modulus test to be used in the mixture characterization needed for the pavement design guide developed in NCHRP Proj- ect 1-37A. Furthermore, many of the tests being performed in developing advanced models for eventual incorporation into the comprehensive Superpave pavement modeling system also involve this same test geometry. Figure B-3 is a schematic B-4 of an instrumented specimen for dynamic modulus testing, using the same uniaxial geometry as proposed for the various candidate simple performance tests (B6). Equipment specifications for the simple performance tests and the dynamic modulus master curve test were developed during NCHRP Project 9-19 and refined during NCHRP Proj- ect 9-29. First article devices have been manufactured and evaluated. It is likely that within several years, many laborato- ries will have the capability of performing the simple perfor- mance tests and the dynamic modulus test as required by the pavement design guide developed in NCHRP Project 1-37A. An initial review of the specifications for the dynamic modulus master curve test device (presented later in this appendix) has indicated that with only slight modifica- tions, it could be used to perform low-temperature creep and strength tests for use in the Superpave thermal cracking model. Many private and public laboratories would be well- served by having the ability to perform not only the simple performance tests and the dynamic modulus master curve procedure using a single piece of equipment but also the low- temperature creep and strength test. This would have many advantages: • Cost savings on purchase of test equipment; • Cost savings on purchase of specimen preparation equip- ment and test accessories; Figure B-3. Schematic of dynamic modulus test. SOURCE: Witczak et al. (B6).

B-5 • Cost savings on training engineers and technicians to prepare specimens and perform tests; • Greater reliability of data due to greater experience with a single test geometry and test device; and • Greater flexibility in scheduling testing, if more than one device is needed in a lab. The decision during SHRP to use the IDT test geometry rather than a uniaxial test was made mostly because the equipment and procedures for preparing and testing uniaxial specimens did not exist at that time (B1). Although this situ- ation has changed with the ongoing development and imple- mentation of uniaxial tests as part of NCHRP Project 9-29 and related efforts, the IDT should be retained as the stan- dard method for low-temperature characterization of asphalt concrete because IDT tests and uniaxial tests simply do not provide equivalent results. In the following section of this appendix, recommended improvements for the IDT creep and strength test are summarized. Current draft specifications for the dynamic modulus master curve test equipment are presented and evaluated with respect to performing both uni- axial and diametral tests at low temperature; suggestions are also made concerning the use of the dynamic modulus mas- ter curve test equipment for determining creep compliance at low temperatures. REVIEW OF IDT CREEP AND STRENGTH EQUIPMENT The procedure and equipment for performing IDT creep and strength tests are described in detail in AASHTO T322, Standard Method of Test for Determining the Creep Com- pliance and Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Test Device. This standard is reviewed in detailed in Appendix A of this report. A summary of the sug- gested revised specifications for the IDT apparatus to be used in conjunction with AASHTO T322 is given in Table B-1; the interested reader should refer to Appendix A for the details of the evaluation. The changes proposed in Table B-1 are not substantial and should not be difficult to implement. USE OF DYNAMIC MODULUS MASTER CURVE TEST EQUIPMENT FOR LOW-TEMPERATURE CREEP AND STRENGTH TESTING A similar approach to the analysis presented in Appendix A for the IDT creep and strength test is presented below for uniaxial testing of asphalt concrete at low temperatures. Table B-2 is a summary of the requirements for a testing sys- tem to perform the dynamic modulus master curve testing required for the pavement design guide developed in NCHRP Project 1-37A. The sections that follow present an analysis of modifications required to use this equipment for IDT creep and strength tests at low temperature. The temperature requirements for the dynamic modu- lus master curve test system should be expanded for low- temperature testing from −10 to −30°C. The specified accuracy of ±0.5°C is adequate. The load capacity for the dynamic modulus master curve test system is given as 22.5 kN (5.0 kips) in dynamic mode. This capacity is adequate for low-temperature creep testing, as it would allow the application of creep stresses up 2.8 MPa (410 lb/in2). Considering that the tensile strength of asphalt concrete at low temperature ranges from about 1.3 to 4.3 MPa (190 to 630 lb/in2), this should be more than adequate for creep testing. However, the load capacity must be increased for tensile strength testing. Doubling the typical maximum tensile strength of 4.3 MPa (630 lb/in2) and rounding, the required load capacity of the system would be 70 kN (16 kips). Although this represents a tripling of the load capacity, this additional capacity is for static loading, which is a less stringent condition than for dynamic loading. If possible, equipment Component General Requirements Range Resolution Axial loading device Shall provide a constant load 100 kN maximum load; Maximum displacement rate of at least 12 mm/min 10 N or better Load measuring device Electronic load cell 100 kN minimum capacity 10 N or better Deformation measuring device(s) Four displacement transducers (LVDTs or equivalent) 0.1 mm minimum 0.1 µm or better Environmental chamber Temperature control only; large enough to perform test and condition 3 specimens –30 to +10 °C under ambient conditions of 15 to 27 °C Control accuracy to ±0.5 °C Control and data acquisition system System shall be operated with the use of a personal computer and shall digitally record load and deformation during test 1 to 20 Hz sampling rate Consistent with required resolution of all system transducers Test fixture As described in ASTM D4123 (diametral resilient modulus testing), but with flat neoprene loading strips 12-mm thick by 12- mm wide. N/A 20 N maximum frictional resistance TABLE B-1 Proposed revised AASHTO T322 specifications for the IDT apparatus

design for low-temperature IDT creep testing should also have the capability of performing the IDT strength test. How- ever, if this is not practical, the strength test could be per- formed on a separate, stand-alone system design specifically for high-capacity static testing. The requirements for contact load and static load accuracy appear to be acceptable. Determining the required loading rate requires some analysis. As noted in Appendix A for the IDT test, the most extreme requirements for loading rate occur at low temperatures, where the asphalt concrete is behaving elas- tically and therefore will deform very quickly. Assuming a compliance of 3 × 10−11 Pa−1 (2 × 10−7 in2/lb) and an applied stress of 2.2 MPa (320 lb/in2), the resulting strain would be 6.6 × 10−5, which for a 150-mm-high uniaxial specimen would translate to a deflection of 0.010 mm. Assuming that the max- imum load should be reached in one second, this would trans- late to a loading rate of 0.6 mm/min; allowing for adequate reserve capacity in the system, the required loading rate would be 1.2 mm/min (0.047 in/min). This rate is quite slow and should be well within the capability of the dynamic modulus test equipment. This required loading rate is ten times lower than the 12 mm/min (0.47 in/min) rate required for IDT testing and demonstrates the greater efficiency of uniaxial loading as compared with diametral loading. The gage length and range for the axial strain transduc- ers appear to be appropriate. As with the IDT test discussed B-6 in Appendix A, determination of the required transducer resolution should be based upon a maximum strain of about 0.025 percent and a strain during the initial stages of the creep test of about one-fifth this value, or 0.005 percent. For the gage length of 70 mm, this translates to a deformation of 0.0035 mm, or 3.5 µm. For a maximum error of about 5 per- cent, the required resolution would then be 0.2 µm (7 µin). This requirement is precisely the same as that established for the dynamic modulus test and therefore need not be changed. The requirements for error should also be appropriate for low-temperature testing. The need for a system that can be rapidly attached and zeroed during testing also remains the same. The requirements for the axial strain transducers for low-temperature creep testing are identical to those already established for the dynamic modulus master curve test. The miscellaneous requirements for the test system are equally applicable to the low-temperature creep and strength tests. Therefore, to adapt the dynamic modulus master curve test system to low-temperature creep and strength testing, either in a uniaxial or diametral mode, only two changes are needed: (1) the maximum static capacity of the system must be 100 kN (22 kips) and (2) the system must be capable of loading at a rate of at least 12 mm/min. Although the increased static capacity required for low- temperature creep and strength testing is substantial, it greatly increases the flexibility and capability of the dynamic modu- Item Requirements for Dynamic Modulus Test Equipment for Generating Master Curves for Structural Design ENVIRONMENTAL CHAMBER Temperature range −10 to 60 °C Control accuracy To within ±0.5 °C of specified temperature LOADING SYSTEM Dynamic load 22.5 kN (5.0 kips) Contact load 5 % of test load Static load and peak dynamic load accuracy ±2 % of specified value Dynamic load accuracy Maximum standard error of 5 % Loading rate 0.01 to 25 Hz LOAD MEASUREMENT SYSTEM Range Equal to or greater than stall force of loading system actuator Accuracy ±1 % maximum for loads ranging from 2 to 100 % of the machine, when verified in accordance with ASTM E4 Resolution Shall comply with requirements of ASTM E4 AXIAL STRAIN TRANSDUCER Gage length 70 mm nominal Range 1 mm minimum Resolution Equal to or better than 0.0002 mm (7.8 micro-inch) Error 0.0025 mm (0.0001 in) maximum when verified according to ASTM D 6027 Miscellaneous Shall be designed for rapid specimen installation and testing MISCELLANEOUS REQUIREMENTS Confining pressure No Computer control and data acquisition Controlled from personal computer and capable of running dynamic modulus test and analyzing resulting data as specified TABLE B-2 Summary of requirements for dynamic modulus test equipment

B-7 lus master curve test system. Also, as mentioned, because it is static capacity, rather than dynamic, the increased cost should not be large. The accessories required for low-temperature IDT tests should be included in the low-temperature creep and strength system option. CONCLUSIONS AND RECOMMENDATIONS A thorough review of the low-temperature creep and strength test procedures and equipment was performed and has been presented in Appendix A for the IDT test and this appen- dix for uniaxial tests. Based upon this review, the following conclusions and recommendations are made: • Several minor refinements in the IDT equipment speci- fication are needed; revised equipment requirements are discussed in detail in Appendix A of this report and are summarized in Table B-1 of this appendix. • The dynamic modulus master curve test equipment needed for HMA characterization in the pavement design guide developed in NCHRP 1-37A is capable of properly performing low-temperature uniaxial creep tests with only minor modification. • A significant increase in static loading capacity is need in the dynamic modulus master curve test system in order to perform IDT strength tests at low temperature. If necessary, strength tests could be performed on a separate system designed specifically for high-capacity static testing. • A combined dynamic modulus/low-temperature IDT creep and strength test system should be recommended by NCHRP and FHWA APPENDIX B REFERENCES B1. Lytton, R. L., J. Uzan, E. G. Fernando, R. Roque, D. Hiltunen, S. Stoffels, “Development and Validation of Performance Prediction Models and Specifications for Asphalt Binders and Paving Mixtures,” Report SHRP-A-357, Washington D.C.: Strategic Highway Research Program, National Research Coun- cil, 1993. B2. The Asphalt Institute, Superpave Asphalt Mixture Analysis, Course Notebook, National Asphalt Training Center II, Wash- ington, DC: Federal Highway Administration, Office of Tech- nology Applications, May, 1996. B3. Witczak, M. W., R. Roque, D. R. Hiltunen, and W. G. Buttlar, “Modification and Re-Calibration of Superpave Thermal Cracking Model,” NCHRP 9-19 Project Report, Arizona State University Department of Civil And Environmental Engineer- ing, Tempe, Arizona, December 2000. B4. Janoo, V., T. Pellinen, D. Christensen, H. Von Quintus, “Eval- uation of the Low-Temperature Cracking Model in Superpave,” Draft Report to the Federal Highway Administration, Contract DTFH61-95-C-00100, undated (ca. 1997). B5. Christensen, D. W., “Analysis of Creep Data for Indirect Tension Test on Asphalt Concrete,” Journal of the Association of Asphalt Paving Technologists, Vol. 67, 1998, pp. 458–492. B6. Witczak, M. W., Kaloush, K., Pellinen, T., El-Basyouny, M., and Von Quintus, H., “Simple Performance Test for Superpave Mix Design,” NCHRP Report 465, Transportation Research Board, Washington, D.C., 2002.