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

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

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

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

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

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B-6 TABLE B-2 Summary of requirements for dynamic modulus test equipment Requirements for Dynamic Modulus Test Equipment for Generating Master Curves for Structural Design Item 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 2 % of specified value load accuracy 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 Controlled from personal computer and capable of running dynamic modulus acquisition test and analyzing resulting data as specified design for low-temperature IDT creep testing should also in Appendix A, determination of the required transducer have the capability of performing the IDT strength test. How- resolution should be based upon a maximum strain of about ever, if this is not practical, the strength test could be per- 0.025 percent and a strain during the initial stages of the formed on a separate, stand-alone system design specifically creep test of about one-fifth this value, or 0.005 percent. For for high-capacity static testing. the gage length of 70 mm, this translates to a deformation of The requirements for contact load and static load accuracy 0.0035 mm, or 3.5 m. For a maximum error of about 5 per- appear to be acceptable. Determining the required loading rate cent, the required resolution would then be 0.2 m (7 in). This requires some analysis. As noted in Appendix A for the IDT requirement is precisely the same as that established for the test, the most extreme requirements for loading rate occur at dynamic modulus test and therefore need not be changed. low temperatures, where the asphalt concrete is behaving elas- The requirements for error should also be appropriate for tically and therefore will deform very quickly. Assuming a low-temperature testing. The need for a system that can be compliance of 3 10-11 Pa-1 (2 10-7 in2/lb) and an applied rapidly attached and zeroed during testing also remains the stress of 2.2 MPa (320 lb/in2), the resulting strain would be same. The requirements for the axial strain transducers for 6.6 10-5, which for a 150-mm-high uniaxial specimen would low-temperature creep testing are identical to those already translate to a deflection of 0.010 mm. Assuming that the max- established for the dynamic modulus master curve test. imum load should be reached in one second, this would trans- The miscellaneous requirements for the test system are late to a loading rate of 0.6 mm/min; allowing for adequate equally applicable to the low-temperature creep and strength reserve capacity in the system, the required loading rate would tests. Therefore, to adapt the dynamic modulus master curve be 1.2 mm/min (0.047 in/min). This rate is quite slow and test system to low-temperature creep and strength testing, should be well within the capability of the dynamic modulus either in a uniaxial or diametral mode, only two changes are test equipment. This required loading rate is ten times lower needed: (1) the maximum static capacity of the system must than the 12 mm/min (0.47 in/min) rate required for IDT testing be 100 kN (22 kips) and (2) the system must be capable of and demonstrates the greater efficiency of uniaxial loading loading at a rate of at least 12 mm/min. as compared with diametral loading. Although the increased static capacity required for low- The gage length and range for the axial strain transduc- temperature creep and strength testing is substantial, it greatly ers appear to be appropriate. As with the IDT test discussed increases the flexibility and capability of the dynamic modu-

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B-7 lus master curve test system. Also, as mentioned, because it is A combined dynamic modulus/low-temperature IDT static capacity, rather than dynamic, the increased cost should creep and strength test system should be recommended not be large. The accessories required for low-temperature by NCHRP and FHWA IDT tests should be included in the low-temperature creep and strength system option. APPENDIX B REFERENCES B1. Lytton, R. L., J. Uzan, E. G. Fernando, R. Roque, D. Hiltunen, CONCLUSIONS AND RECOMMENDATIONS S. Stoffels, "Development and Validation of Performance Prediction Models and Specifications for Asphalt Binders A thorough review of the low-temperature creep and and Paving Mixtures," Report SHRP-A-357, Washington D.C.: strength test procedures and equipment was performed and has Strategic Highway Research Program, National Research Coun- been presented in Appendix A for the IDT test and this appen- cil, 1993. dix for uniaxial tests. Based upon this review, the following B2. The Asphalt Institute, Superpave Asphalt Mixture Analysis, conclusions and recommendations are made: Course Notebook, National Asphalt Training Center II, Wash- ington, DC: Federal Highway Administration, Office of Tech- Several minor refinements in the IDT equipment speci- nology Applications, May, 1996. B3. Witczak, M. W., R. Roque, D. R. Hiltunen, and W. G. Buttlar, fication are needed; revised equipment requirements are "Modification and Re-Calibration of Superpave Thermal discussed in detail in Appendix A of this report and are Cracking Model," NCHRP 9-19 Project Report, Arizona State summarized in Table B-1 of this appendix. University Department of Civil And Environmental Engineer- The dynamic modulus master curve test equipment ing, Tempe, Arizona, December 2000. needed for HMA characterization in the pavement design B4. Janoo, V., T. Pellinen, D. Christensen, H. Von Quintus, "Eval- guide developed in NCHRP 1-37A is capable of properly uation of the Low-Temperature Cracking Model in Superpave," performing low-temperature uniaxial creep tests with Draft Report to the Federal Highway Administration, Contract only minor modification. DTFH61-95-C-00100, undated (ca. 1997). A significant increase in static loading capacity is need B5. Christensen, D. W., "Analysis of Creep Data for Indirect Tension Test on Asphalt Concrete," Journal of the Association of Asphalt in the dynamic modulus master curve test system in Paving Technologists, Vol. 67, 1998, pp. 458492. order to perform IDT strength tests at low temperature. B6. Witczak, M. W., Kaloush, K., Pellinen, T., El-Basyouny, M., If necessary, strength tests could be performed on a and Von Quintus, H., "Simple Performance Test for Superpave separate system designed specifically for high-capacity Mix Design," NCHRP Report 465, Transportation Research static testing. Board, Washington, D.C., 2002.