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

Chapter: Appendix A - Review of AASHTO T322 and Recent Proposed Changes

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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
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Page 34
Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
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Page 35
Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
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Page 36
Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
Page 40
Page 41
Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
Page 42
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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
×
Page 43
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Suggested Citation:"Appendix A - Review of AASHTO T322 and Recent Proposed Changes." National Academies of Sciences, Engineering, and Medicine. 2004. Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/13775.
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A-1 INTRODUCTION The primary purpose of this appendix is to summarize the procedures for performing the indirect tension (IDT) creep and strength test and the methods for analyzing the subsequent data, as described in AASHTO T322, Standard Method of Test for Determining the Creep Compliance and Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Test Device. This appendix also includes recent suggested modifications to this standard, which have occurred during the course of NCHRP Projects 1-37A and 9-19. This information is crit- ical to understanding the current form of the IDT test system and changes likely to occur over the next few years. This appendix includes a summary of AASHTO T322, a section on modifications to AASHTO T322 recommended during NCHRP Projects 1-37A and 9-19, a section on related research, a section discussing the results of this review and presenting various findings, a section presenting conclusions and recommendations, and a list of references. This appen- dix is intended to provide detailed background information supporting the findings, conclusions, and recommendations presented in the body of the NCHRP 9-29 Phase III final report. However, an attempt has also been made to make this suitable as a stand-alone document. AASHTO T322 AASHTO T322 consists of 17 sections: 1. Scope 2. Referenced Documents 3. Terminology 4. Summary of Method 5. Significance and Use 6. Apparatus 7. Hazards 8. Standardization 9. Sampling 10. Specimen Preparation and Preliminary Determinations 11. Tensile Creep/Strength Testing (Thermal Cracking Analysis) 12. Tensile Strength Testing (Fatigue Cracking Analysis) 13. Calculations 14. Report 15. Precision and Bias 16. Keywords 17. References Many of these sections are only of nominal significance and will not be discussed here, including sections 1, 2, 3, 5, 7, 15, 16, and 17. The sections below address the most significant parts of the specification in sequence. AASHTO T322 Sections Section 4. Summary of Method The Summary of Method in Section 4 presents a good intro- ductory description of the test procedure: 4.1 This standard describes two procedures. For one proce- dure, the tensile creep and tensile strength are determined on the same specimen for thermal cracking analyses, and for the other procedure the tensile strength is determined separately for fatigue cracking analyses. 4.2 The tensile creep is determined by applying a static load of fixed magnitude along the diametral axis of a specimen. The horizontal and vertical deformations measured near the center of the specimen are used to calculate a tensile creep compliance as a function of time. Loads are selected to keep horizontal strains in the linear viscoelastic range (typically below a horizontal strain of 500 × 10−6 mm/mm) during the creep test. By measuring both horizontal and vertical defor- mations in regions where the stresses are relatively constant and away from the localized non-linear effects induced by the steel loading strips, Poisson’s ratio can be more accu- rately determined. Creep compliance is sensitive to Poisson’s ratio measurements. 4.3 The tensile strength is determined immediately after deter- mining the tensile creep or separately by applying a constant rate of vertical deformation (or ram movement) to failure. The most important features of this test system are the indirect tensile test geometry, the use of both compliance and strength tests, the assumption of linear viscoelastic behavior, and the determination of not only creep compliance but also of Poisson’s ratio during the IDT creep test. One of the most important issues concerning the IDT creep and strength test is that of test geometry. The IDT geometry was originally selected for use in low-temperature character- ization of asphalt concrete mixtures during the Strategic Highway Research Program (SHRP) primarily because the specimen preparation methods available at that time did not include ways of making specimens suited for uniaxial mea- surement of creep compliance, relaxation modulus, or strength. The simple performance tests developed as part of NCHRP Project 9-19 and the characterization methods developed for use in conjunction with the pavement design guide devel- oped in NCHRP Project 1-37A require 100-mm diameter by 150-mm high specimens to be used in uniaxial testing. Therefore, this obstacle to uniaxial testing at low temperature no longer exists. This would also potentially allow the use of relaxation modulus tests, rather than creep tests, which APPENDIX A REVIEW OF AASHTO T322 AND RECENT PROPOSED CHANGES

would eliminate the need to calculate the relaxation modulus from the creep compliance. However, relaxation tests have not been widely performed on asphalt concrete mixtures; and, for practical purposes, the creep test should probably be retained regardless of test geometry. Phase III of NCHRP Project 9-29 included an evaluation of the possible use of uniaxial creep testing as the standard low-temperature test for asphalt concrete. Because the uniaxial test can produce compliance data in the same exact format as the IDT test, there would be no need for major changes in the Superpave thermal cracking program. Examination of the equipment requirements for the dynamic modulus master curve equipment as developed earlier in NCHRP Project 9-29 indicates that this equipment should have both the load capacity and transducer resolution for properly performing the creep test on asphalt concrete at low temperatures; this evaluation is described in detail in Appen- dix B of this report and summarized in Chapter 2 of the body of the report. The maximum load capacity of 22.5 kN (5 kips) is, however, too low for performing uniaxial tensile strength tests, which would require a maximum load of 70 kN (16 kips) to ensure that all or almost all mixtures could be tested to failure. Therefore, it is suggested that mixture ten- sile strength normally be determined using either the IDT or uniaxial tensile strength tests on a high-capacity static test machine separate from the dynamic modulus master curve device. However, it should also be possible to perform both creep and strength tests on a single, high-performance servo- hydraulic system, as long as all equipment requirements are met. Such a system would, however, likely be somewhat more expensive than the standard dynamic modulus master curve system. A new, draft test procedure should be written for performing uniaxial creep tests, based upon AASHTO T322 and the specifications developed for the simple per- formance tests and related procedures as part of NCHRP Project 9-29. The issue of linearity is of great practical importance. Intu- itively, it should be expected that asphalt concrete at low tem- peratures should behave in a linear manner through loading approaching the point of failure because of the high stiffness of asphalt concrete under these conditions and the very low strains. It is, however, important to verify that the loads used in the IDT test are appropriate—as high as possible, to ensure large deflections and good repeatability, while still remaining in the linear viscoelastic region. AASHTO T322 calls for a maximum strain of 500 × 10−6 mm/mm, or 0.05 percent. This value is consistent with work performed by Mehta and Chris- tensen (A1), who reported that deviations from linearity began to occur at the same strain level of 0.05 percent. This aspect of AASHTO T322 probably does not need revision. The final general issue in the IDT test procedure is whether it is truly necessary to determine Poisson’s ratio when charac- terizing the mechanical behavior of HMA at low temperature. Poisson’s ratio represents the ratio of lateral to axial deforma- tion under uniaxial loading. It is theoretically necessary to know Poisson’s ratio when performing stress analyses in two or three dimensions. However, in performing simple, one- A-2 dimensional stress analyses, such as those used in the Super- pave thermal cracking analysis, Poisson’s ratio is not needed. Furthermore, for most materials, Poisson’s ratio falls between about 0.2 and 0.5. For asphalt concrete, Huang (A2) states that values typically fall in a narrower range, from 0.3 to 0.4. Huang goes on to state “Because Poisson’s ratio has a relatively small effect on pavement responses, it is custom- ary to assume a reasonable value for use in design, rather than to determine it from actual tests.” (A2) It appears as though determination of Poisson’s ratio is not critical to the prediction of low-temperature cracking, again suggesting that perhaps uniaxial creep tests could provide the needed data more simply and more directly than the IDT creep test. Section 6. Apparatus There are six main components to the IDT test system: • Axial loading device, • Load measuring device, • Deformation measuring device(s), • Environmental chamber, • Control and data acquisition system, and • Specimen loading frame (test fixture). This section of AASHTO T322 provides specifications for each of these subsystems, which are summarized in Table A-1 below. The term “test fixture” is used here rather than “load- ing frame” to describe the device that holds the IDT specimen in place and transfers the load from the testing device to the specimen, as loading frame is an ambiguous term that could be confused with the loading system. In order to evaluate these specifications, it is necessary to examine the possible range of responses for HMA at low temperature and also to understand what ranges and sensitivities are possible and practical for the systems in question. The following paragraphs address these issues. In SHRP Report A-357, the developers of the IDT creep and strength testing procedure present data for a range of mixtures (A3). These show a typical range in compliance values of about 3 × 10−11 Pa−1 to 4 × 10−9 Pa−1. Because the linear range for HMA occurs at strains less than or equal to 0.05 percent, the maximum applied tensile stresses corresponding to these compliance values range from 125 kPa to 17 MPa. Based upon the relationship σt = 2P/πtD, the axial loads corresponding to these tensile stresses are 1.5 and 200 kN, respectively, for a specimen 50 mm thick and 150 mm in diameter. However, another consideration is the maximum load that can be applied without failing a specimen. The lowest tensile strength, σt, reported in SHRP A-357 (A3) was 1.3 MPa; the highest was 4.3 MPa. The corresponding load, P, for these tensile strengths can be calculated as P = σtπtD/2, where t and D are the speci- men thickness and diameter, respectively. The calculated loads based on tensile failure are between 15 and 51 kN for a 50-mm-thick specimen. Limiting the load to one-half that required to cause failure and allowing for specimens up to 100 mm in thickness, the anticipated maximum load is then

A-3 50 kN. However, to ensure good loading system performance, the capacity of the loading system should be about double the anticipated maximum load, giving a maximum capacity of 100 kN, agreeing nearly exactly with the 98 kN given in AASHTO T322. In evaluating the required sensitivity of the loading system, the worst-case situation is for the lowest anticipated load. Because it would be undesirable to approach nonlinearity, in some cases the applied loads might be somewhat less than the estimated minimum load of 1.5 kN, say 1 kN. To calibrate to this load level, ASTM E4 requires a resolution that is 1/100th of the minimum load level or a resolution of 10 N, which is significantly larger (poorer) than the 5 N resolution require- ment given in AASHTO T322. Consideration should be given to changing the required resolution for the IDT loading sys- tem to 10 kN; this would likely reduce the cost of the equip- ment required to perform the test. Addressing the requirements for the required displace- ment rate is more complicated. Because linearity requires a maximum strain of 0.05 percent, this represents the maxi- mum horizontal strain during the IDT creep test. In a creep test, the load is applied very quickly during the initial part of the test, typically within a period of not more than one second. The condition requiring the highest loading rate is for very stiff materials at low temperature, because in this case the behavior is nearly elastic and most of the specimen deformation will occur during the initial application of the load. Therefore, in order to calculate the vertical deforma- tion, an applied load of 98 kN and a specimen compliance of 3 × 10−11 Pa−1 can be assumed. Two useful equations relat- ing load, Poisson’s ratio, and horizontal and vertical defor- mations are given in ASTM D 4123, Standard Test Method for Indirect Tension Test for Resilient Modulus of Bituminous Mixtures: ν = −3 59 0 27 2. . ( )∆ ∆H V A- E P t H= +( )ν 0 27 1. ( )∆ A- where E = modulus, MPa (inverse of the creep compliance D); P = applied load, N; ν = Poisson’s ratio; t = specimen thickness, mm; ∆H = total horizontal deformation, mm; and ∆V = total vertical deformation, mm. Because these equations are based upon conditions of plane stress, which is a simplification of the actual three-dimensional state of stress during an IDT test, they should be considered approximate. However, they should be accurate enough for the purposes of estimating the required loading rates and trans- ducer sensitivities. Rearranging Equation A-1, replacing ∆H with ∆V/5.38 (from Equation A-2 for ν = 0.40): Assuming a Poisson’s ratio of 0.4, for the given conditions of P = 50 kN and D = 3 × 10−11 Pa−1, the maximum expected vertical deformation for the IDT creep test is 0.10 mm. If this is to be applied during a maximum ramp time of one second, the maximum expected displacement rate is then 0.10 mm/s, or 6 mm/min. However, to ensure that the system has adequate reserve capacity for good control of the loading rate, a higher maximum displacement rate is desirable, say 12 mm/min. This corresponds exactly with the lowest displacement rate given in AASHTO T322. It is not clear why a range is specified for the displacement rate; there is no reason to arbitrarily limit the maximum displacement rate for the IDT system. It is rec- ommended that the required displacement rate for IDT test systems be given as at least 12 mm/min. Evaluation of the requirements for the IDT load cell follow directly from the previous discussion. The maximum applied load is 50 kN, and the load cell should have a maximum capac- ity substantially higher than the maximum expected load to avoid overloading and potentially damaging the transducer. Therefore, the load cell should have a maximum capacity of at ∆V P Et= +( )5 38 0 27 3. . ( ) A-ν Component General Requirements Range Sensitivity Axial loading device Shall provide a constant load 98 kN maximum load; Displacement rate between 12 and 75 mm/min 5 N minimum Load measuring device Electronic load cell 98 kN minimum capacity 5 N minimum Deformation measuring device(s) Four linear variable differential transducers (LVDTs) 0.25 mm minimum 0.125 µm minimum Environmental chamber Temperature control only; large enough to perform test and condition 3 specimens -30 to +30 °C Control to ±0.2 °C Control and data acquisition system Shall digitally record load and deformation during test 1 to 20 Hz sampling rate 16-bit A/D board required Test fixture As described in ASTM D4123 (diametral resilient modulus testing) N/A 2 kg maximum frictional resistance TABLE A-1 AASHTO T322 specifications for IDT apparatus

least 100 kN. The sensitivity of the load cell should at least match the sensitivity of the loading system, determined to be 10 N rather than the 5 N listed in AASHTO T322. The first issue concerning specimen deformation measure- ment that should be addressed is the type of transducer to be used. Currently, AASHTO T322 requires the use of LVDTs. Although LVDTs are widely used in this type of test, there are other types of transducers that have been used with this sys- tem with success, including strain-gage–based clip-on gages. The specification should not specify the type of transducer to be used, only the required level of performance in terms of gage length, range, and sensitivity. The maximum deflection to be measured during an IDT creep test will occur in the vertical direction. Based upon equations given in SHRP Report A-357 (A3), the vertical strain measured during an IDT test can be nearly twice the horizontal strain, which is limited to 0.05 percent. Therefore, the maximum expected deflection during a typical test would be 0.001 × 38 mm, or 0.04 mm. This range would however be extremely difficult to work with in setting up and execut- ing a test. Current requirements in AASHTO T322 are for a minimum LVDT range of 0.25 mm; commercially available IDT equipment used at Advanced Asphalt Technologies, LLC, uses displacement transducers with an overall range of 2.5 mm, which include a software window of 0.25 mm that is enabled after initial specimen set up. This is an effective system that should be considered in the next generation of HMA low-temperature testing equipment. Evaluation of the required sensitivity of the deformation transducers for the IDT is straightforward. Only the case of horizontal deflections needs to be addressed, because these will always be significantly smaller than vertical deflections and so represent the critical situation. Linearity constraints, as discussed previously, limit horizontal strains during the IDT creep to 0.05 percent. However, it is impossible to deter- mine test conditions a priori so that strains are always close to this limit; therefore, a realistic strain at the end of the test would be 0.025 percent. Also, it must be kept in mind that this is the strain at the end of a typical IDT creep test; the strain at the start of collection of data can be as much as five times less than this, or 0.005 percent (50 parts per million). Given the standard gage length of 38 mm, this represents a min- imum expected deflection of 1.9 µm. To maintain a reasonable resolution under this worst-case situation of about 5 percent, would require a transducer sensitivity of 0.1 µm (4 µin.). The current specifications for the temperature chamber in AASHTO T322 require a range of −30 to +30°C, with a con- trol sensitivity of ±0.2°C. Examining typical IDT creep data, a temperature control sensitivity of ±0.2°C translates to a max- imum potential error in creep compliance of about 3 percent. This appears reasonable; however, a temperature chamber with this level of control sensitivity would be prohibitively expensive. A more realistic requirement for sensitivity would be the one already established for the simple performance tests, ±0.5°C. This could lead to maximum potential compli- ance errors of about 8 percent, though the error in most cases would be smaller because of the cyclic nature of temperature A-4 control systems and the relatively large thermal mass of the IDT specimens. Furthermore, as with the requirements for the temperature chamber to be used with the simple perfor- mance tests, ambient conditions should be given under which the specification should be met—15 to 27°C. There is no need to require the IDT chamber to have a range extending to 30°C; this means that the system must have a substantial heating system in order to control temperatures at ambient temperatures and above, increasing the complexity and cost of the chamber. The required temperature range for the cham- ber should be narrowed to −30 to 10°C under the given ambient conditions. The requirements for system control and data acquisition are largely acceptable but could be slightly improved. The use of a personal computer in the control and data acquisition system should be explicitly required. On the other hand, the required sensitivity of the data acquisition system could be more effectively stated to be consistent with the required sen- sitivity of the various transducers. The manner in which this is achieved should be left to the equipment supplier. The test fixture is specified to meet the requirements of ASTM D4123, which is a standard test method for diame- tral resilient modulus testing. It is suggested that a separate, smaller frame be used to help meet the requirements of this specification. A maximum frictional resistance of 2 kg is also specified in AASHTO T322. If the minimum applied load is 1 kN, as discussed previously, the maximum frictional resis- tance should be no more than about 2 percent of this, or 20 N. This is in very close agreement to the 2 kg frictional resistance in the current specification. However, as frictional resis- tance is a force, AASHTO T322 should be revised to specify the maximum frictional resistance in Newtons rather than kilograms. A simple procedure should be given for evaluat- ing the frictional resistance of the test fixture. ASTM D4123 requires stainless steel loading strips one-half inch wide, with a curvature matching that of the IDT specimen. Generally, load applications to materials such as asphalt concrete must include some provisions for distributing the load evenly over the test specimen and avoiding stress concentrations and eccentricities. These issues are not addressed by the current requirements of ASTM D4123. The curvature of the loading strips, though nominally addressing the geometry of the spec- imen, may in fact cause more problems than it solves, because this could increase stress concentrations and eccentric loading unless the curvature and alignment of the specimen exactly match that of the loading strips. A more conventional approach would be to use flat, neoprene loading strips, one-half inch thick by one-half inch wide. These strips would be compliant enough to assume the shape of the IDT specimen regardless of irregularities and would greatly reduce the potential for stress concentrations and eccentricities. A summary of the suggested revised specifications for the IDT apparatus to be used in conjunction with AASHTO T322 is given in Table A-2. Many of the changes are slight, for example, giving the maximum range of the loading device and load cell as 100 kN rather than the “soft” metric value of 98 kN. The sensitivity of the loading device and load

A-5 cell is decreased, while the sensitivity of the deformation measuring devices has been increased. The suggested use of flat neoprene loading strips—rather than curved, stainless steel strips—should be evaluated in the laboratory testing portion of Phase III of NCHRP 9-29. 8. Standardization The requirements for calibration and verification of the IDT test system in the current version of AASHTO T322 are somewhat vague. AASHTO T322 includes the following requirements: • The testing system shall be calibrated prior to initial use and at least once a year thereafter. • The temperature control in the environmental cham- ber and all transducers used in the IDT system shall be verified (no frequency given). • If the results of any verification are not satisfactory, appropriate actions shall be taken to correct the response of the transducer(s) in question. Accurate execution of the IDT creep test requires that all transducers in the test system be calibrated and operating prop- erly. The calibration requirements should be more detailed, referring to appropriate ASTM standards (ASTM E4 for load and ASTM D 6027 for deflection and specimen deformation). The verification procedure and required hardware for verifica- tion should also be more detailed. IDT systems should include a proving ring for load verification and a verification system for checking the transducers, such as a calibration block with a very sensitive micrometer. A standard specimen, 10 mm thick by 150 mm in diameter, made of 6061 T6 aluminum alloy, should also be supplied with the IDT system. Such a specimen would provide an effective stiffness similar to that of a typical asphalt concrete specimen at −30 to −20°C and would exhibit stable properties with E = 69 GPa and ν = 0.33. Furthermore, the thinness of the specimen should pro- duce conditions approaching that of plane stress, simplifying the analysis and providing additional certainty in the results of the verification. A full system calibration frequency of once every year is probably adequate. A confidence check using the aluminum standard should be performed every time the sys- tem is used. Verification of the load cell and LVDTs should be required when the confidence check fails, at least once per month when the system is being used, and prior to begin- ning tests if the system has not been used for more than 30 days. 9. Sampling This section probably needs little or no revision. Currently, specimen preparation according to either AASHTO T312 (Superpave gyratory compactor) or AASHTO PP3 (rolling wheel compactor) is permitted. Consideration should be given to requiring gyratory compaction only, in order to reduce vari- ability and promote reproducibility in IDT creep and strength tests. The current specification states that if cores from road- ways are to be tested, they should be taken following proce- dures given in ASTM D5361. 10. Specimen Preparation and Preliminary Determinations Requirements for specimen diameter and thickness are not critical to the results of the IDT creep and strength test; how- ever, some revisions in this section of AASHTO T322 are needed. One critical point is the smoothness and parallelism of the specimen faces; currently, AASHTO T322 only states Component General Requirements Range Sensitivity 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) 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 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 sensitivity 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 A-2 Proposed revised AASHTO T322 specifications for the IDT apparatus

that the specimen sides should be “smooth” and “parallel.” The specimen requirements given in Table A-3 are partly based upon those developed for the First Article Equipment Specifications for the Simple Performance Test System devel- oped earlier during NCHRP Project 9-29 and should help ensure good test results with the IDT creep and strength pro- cedure (A4). The required specimen diameter in Table A-3 has been given as 150 to 154 mm, rather than the 150 ± 9 mm given in AASHTO T322, to maintain consistency with the requirements of the simple performance test. The specimen thickness requirement has also been changed slightly, given as 40 to 60 mm, rather than as 38 to 50 mm as in AASHTO T322. This change is suggested to provide a “hard” metric specification and also to allow some margin for error in pro- ducing 50-mm-thick specimens, which are considered stan- dard for the IDT test. Specimen parallelism is specified through the use of the standard deviation of the thickness, which is limited to less than 1.0 mm, which corresponds to a 2s limit of about 1.2 degrees, similar to the 1 degree require- ment for the simple performance test. Another requirement of this section is to determine the bulk specific gravity of the specimen following AASHTO T166, with the caveat that high-absorption specimens should be tested using an impermeable plastic film rather than a paraf- fin coating as specified in AASHTO T166. This requirement is necessary to ensure that the surfaces are clean so that the LVDT gage points can be properly glued to the specimen. There is also a statement here that if direct immersion is used to determine the bulk specific gravity, the specimen must then be dried to a constant weight prior to fastening of the LVDT gage points. In the interest of ensuring consistent bulk specific gravity measurements and also to ensure rapid and consistent specimen preparation, it is suggested that this part of AASHTO T322 be revised to require that all bulk specific gravity measurements be made using imper- meable plastic rather than the saturated surface-dry method or the paraffin coating technique. A-6 11. Tensile Creep/Strength Testing (Thermal Cracking Analysis) Several changes are needed within this section of AASHTO T322. First, the suggested test temperatures for the creep pro- cedure are 0, −10, and −20°C. Because of the variability in binder grades and the resulting low-temperature properties of asphalt concrete, some specimens are extremely stiff at −20°C, while others may be too compliant at 0°C. The test tempera- tures used in the IDT creep and strength test should, therefore, change according to the binder grade used. The relationship between binder stiffness and mixture stiffness is not 1:1; a given change in binder stiffness will produce a somewhat lower change in mixture stiffness. Therefore, it is not neces- sary or advisable to link IDT test temperatures directly to low- temperature binder grade. It is suggested that the current test temperatures of 0, −10, and −20°C be maintained for mixtures made using PG XX-28 and PG XX-22 binders. For PG XX-16 and XX-10 binders, or mixtures that have been severely age- hardened, the recommended test temperatures should be −10, 0, and +10°C. For PG XX-34 binders (or softer), the recom- mended test temperatures should be −30, −20, and −10°C. A practical problem with the current version of AASHTO T322 is that the test conditions are to be determined using a trial-and-error procedure. A load is applied to the specimen and, if the resulting strains fall outside the allowable range, the test is aborted, the specimen is allowed to recover for 5 minutes, and the test is then repeated at an adjusted load level. No suggestions are given concerning what the appropri- ate applied loads should be for different combinations of mix- ture types and test conditions. Given the suggested revised protocol recommended above, Table A-4 presents guidelines for the applied load. These guidelines are based upon typical ranges for asphalt concrete modulus under the conditions likely under the pro- posed protocol. The maximum allowed deformation cor- responds to the maximum allowable horizontal strain for Item Specification Remarks Average diameter 150 to 154 mm See Note 1 Standard deviation of diameter 1.0 mm See Note 1 Average thickness 40 to 60 mm See Note 2 Standard deviation of thickness 1.0 mm See Note 2 Smoothness 0.3 mm See Note 3 Table A-3 Notes: 1. Measure the diameter at the center and third points of the test specimen along axes that are 90 degrees apart. Record each of the six measurements to the nearest 1 mm. Calculate the average and the standard deviation of the six measurements. The standard deviation shall be less than 1.0 mm. The average diameter, reported to the nearest 1 mm, shall be used in all material property calculations. 2. Measure the thickness of the specimen to the nearest 1 mm at 8 equally spaced points along the circumference of the specimen, using a pair of calipers or other similar device. Calculate and report the average thickness to the nearest 1 mm. The standard deviation of the specimen thickness shall be less than 1.0 mm. The average thickness shall be used in all material property calculations. 3. Check this requirement using a straight edge and feeler gauges. TABLE A-3 IDT creep and strength specimen requirements

A-7 linearity, 0.05 percent, rounded up from 0.019 to 0.02 mm. The lower limit represents one-half this value, which is nec- essary to ensure adequate resolution of the deformation data during the test. In the final version of the IDT software, it might be possible to provide a utility that estimates the spec- imen compliance from the binder grade (or bending beam rheometer test data) and mixture composition and uses this information to calculate the initial load for the test. Additional software controls could be designed to monitor the progress of the test and make adjustments to the applied load as needed. Another important issue in executing the IDT creep and strength test is the temperature and rate for IDT strength test- ing. In the original conception of the IDT procedure and in the current version of AASHTO T322, the strength test was to be performed at the same three temperatures as the creep test—typically, −20, −10, and 0°C. However, partly because of the irregular relationship between temperature and tensile strength and probably to make the entire test procedure more efficient, most laboratories perform the strength test at −10°C only. The specified loading rate in AASHTO T322 for the strength test is 12.5 mm/min. The assumption in this approach to testing is that the IDT strength test should be performed quickly, to eliminate time dependency from the result. How- ever, because the strength of HMA, like modulus or compli- ance, is time and temperature dependent, an effort should be made to make the time and temperature conditions for the IDT strength test at least approximately representative of what occurs in the field during low-temperature cracking events. The analysis of a suitable loading rate for the IDT strength test can only be done in an approximate manner, but should help obtain reasonable test conditions. Examination of thermal stress development curves shows that at cooling rates of 5°C/hr, most of the tensile stress in the mixture is generated during the last two hours of cooling. This representative load- ing time agrees with the 2-hour effective loading time used in most limiting stiffness approaches to controlling thermal crack- ing (A5). However, it is suggested that the IDT strength test be performed at the middle creep test temperature, which is 12 to 18°C higher than the minimum binder grading tempera- ture. Considering that the actual cracking temperature should generally be several degrees below the grading temperature, the IDT strength test would normally be performed at about 15 to 21°C above the anticipated cracking temperature. Typ- ically, shift factors for asphalt binders at low temperature vary −0.2 log shift factors per °C (A6). Therefore, the fail- ure time for an IDT strength test roughly equivalent to the 2-hour failure time during a thermal cracking event would be 7200 s / [10(−0.2)(−18)] = 1.8 s. A typical failure strength for asphalt concrete at low temperature would be 3 MPa (A3, A5, A7 ). Because for diametral loading, σx = 2 P/πtD, the corre- sponding vertical load for a typical IDT strength would be 35 kN. Using a typical low-temperature asphalt concrete modulus value of 14 GPa, a Poisson’s ratio of 0.4, and a spec- imen thickness of 50 mm, Equation A-3 can be used to esti- mate the vertical displacement at failure for a typical IDT strength test as 0.18 mm. Because the estimated equivalent failure time was found to be 1.8 sec, the loading rate for the IDT strength test should be 0.1 mm/sec. The IDT strength test should, therefore, be performed at a vertical displace- ment rate of approximately 0.1 mm/sec or 6 mm/min, which is somewhat slower than the 12.5 mm/min currently specified in AASHTO T322. Considering the approximate nature of this analysis and the fact that the Superpave thermal cracking model has been calibrated using strength data collected at 12.5 mm/min, no change to the strength test loading rate in AASHTO T322 is recommended. The specimen conditioning time given in AASHTO T322 is 3 hours ±1 hour. Three hours is probably an acceptable time for temperature equilibration, but the range of ±1 hour is prob- ably too large given the potential for possible physical hard- ening of the specimen at low temperatures. It is suggested that this range be reduced to ±0.5 hours. AASHTO T322 should also include an alternate approach using a dummy specimen with an embedded temperature sensor, which could be used to provide additional assurance of proper specimen equilibration. If the dummy specimen is used, the test should be completed within 1 hour of reaching equilibration. Some mention should be made here of the possibility for steric hardening under con- tinued storage at low temperatures, so engineers and techni- cians have some understanding of the reason for this limitation and the possible consequences if it is ignored. This section of AASHTO T322 also states that the test should not begin until the chamber is within ±0.2°C of the target temperature. As discussed previously, this requirement is too stringent; the allowable temperature range should be increased to ±0.5°C. 12. Tensile Strength Testing (Fatigue Cracking Analysis) Tensile strength is not required information for the fatigue analysis to be used in the pavement design guide developed Test Temperature Initial Applied Load (kN) Other Possible Applied Loads (kN) Lowest 40 Deformation < 0.01 mm: 80 Deformation > 0.02 mm: 20, 10 Intermediate 10 Deformation < 0.01 mm: 20, 40 Deformation > 0.02 mm: 5, 2 Highest 5 Deformation < 0.01 mm: 10, 20 Deformation > 0.02 mm: 2, 1 TABLE A-4 Guidelines for applied load in the IDT creep test

in NCHRP Project 1-37A. Therefore, there is no longer a need for this section in AASHTO T322. 13. Calculations This section of AASHTO T322 describes in detail the pro- cedure for organizing data and calculating creep compliance and Poisson’s ratio. The procedure for data collection is not explained; the specification should provide information con- cerning the standard structure for data files, including times at which data should be collected, and what properties should be reported and in what format. A key issue in this section of AASHTO T322 is the data trimming process, in which arrays of data are collected representing six cases: two sides for each of three specimens. The highest and lowest values are some- what arbitrarily discarded, and the remaining four arrays are used to estimate average values. This procedure was appar- ently needed because of the high variability in IDT data dur- ing early versions of the test. There are several problems with this approach. As the hardware and procedures used in this procedure have been improved, the quality of the data has also improved, to the point where the data trimming might in most cases represent an unnecessary discarding of otherwise useful and perfectly accurate data. On the other hand, it is con- ceivable that in some cases perhaps only one or as many as three data arrays might be faulty. An alternate approach is sug- gested to ensure that the quality of IDT creep data is acceptable: • Data for each test should be analyzed as the test is run, to ensure that they are of good quality. The IDT software should automatically verify that load and deformation data are reasonable and produce sensible results. If not, the operator should be informed that the test data gener- ated were of poor quality, and the test should be repeated. If an additional test fails, the specimen should be dis- carded and only two specimens used in the analysis. • Upon completion of the test and analysis of the data, the creep compliance, m-values, and Poisson’s ratio values for each specimen should be compiled, and the average and standard deviation reported for the complete set of tests. The software should notify the operator if the val- ues appear unusual or otherwise of poor quality. Analyzing the data in this way would ensure that if an indi- vidual test is suspect, it is repeated immediately rather than waiting until all tests are completed to evaluate the data and realizing that there were one or more suspect test results. Fur- thermore, analyzing the replicate specimens separately and reporting statistics on these data allows the technician and/or engineer to evaluate the overall quality of the data and the repeatability of the results. This is particularly important in situations where the IDT procedure is being used to compare two different mixtures. For example, without appropriate test statistics, it would be very difficult to evaluate if a difference of 20 percent in the creep compliance of two such mixtures represents a statistically significant difference. A-8 The details of the calculations presented in AASHTO T322 have been modified somewhat over the past 6 years and so will not be discussed in this appendix. This section of the specifi- cation needs to be edited to ensure that it represents the latest version of the calculation procedure as developed during NCHRP Project 9-19 (A8). 14. Report This section of AASHTO T322 is straightforward but does need some revision. Because the reporting of creep compli- ance is relatively complicated, the standard format for such a report should be given here, including the times at which test results are to be reported and the properties to be included in the report. This section should also include information con- cerning the standard format for input into the pavement design guide developed in NCHRP Project 1-37A for analyzing ther- mal cracking. There are references to the Superpave software in this section of the specification that should be deleted. 15. Precision and Bias This section currently contains no information. Although some limited information is now available, it probably is not extensive enough to include in a precision and bias statement. Perhaps a note could be included here giving preliminary estimates of the precision of the IDT creep and strength tests. AASHTO T322 Summary There are a number of important issues concerning AASHTO T322. The most fundamental issue is whether the low-temperature creep compliance of asphalt concrete should be determined using the IDT geometry or whether a uniaxial creep test should be used. This is especially perti- nent as the simple performance tests being developed as part of NCHRP Project 9-19 are uniaxial tests, and, as a result, in a few years, equipment for preparing specimens and per- forming uniaxial creep tests should be commercially available at a reasonable cost. Using the same test geometry for both the simple performance tests and the low-temperature creep com- pliance test would simplify implementation activities and potentially reduce the cost of equipment and training for lab- oratories wishing to have the capability of performing both procedures. Various other relatively minor issues have been identified in the review of AASHTO T322. Some of the existing require- ments for the loading system, environmental chamber, and load and deformation transducers should be revised; suggested changes were presented previously in Table A-2. Existing requirements for IDT specimen dimensions are largely sub- jective. Specific requirements for specimen dimensions and uniformity were given in Table A-3 and were based upon requirements developed for use in conjunction with the sim- ple performance test. The current test protocol involves test-

A-9 ing at three temperatures (−20, −10, and 0°C) regardless of the binder grade used. This sometimes results in marginal data for one of the temperatures, where the compliance of the specimen was either too high or too low to be of value in the analysis. A more efficient system would be to link the creep compliance test temperatures to the low-temperature binder grade used in making the asphalt concrete. This would ensure that the creep data would almost always be in the desired range. Statistical analyses should be provided in calculating compliance data, so that the technician or engineer running the test can immediately evaluate the quality of the data and repeat the test if needed. In general, most of the required modifications in AASHTO T322 are minor, other than the fundamental issue of whether the IDT test is the most efficient method for determining the creep compliance of asphalt concrete mixtures at low temper- atures. That issue can be best addressed through experimental testing to compare creep compliance data at low temperatures determined using both procedures. Provided that the results of such testing suggest that the IDT test be retained, the sug- gested modifications in AASHTO T322 could be easily made and should not be controversial. If the laboratory testing supports the use of uniaxial compression in low-temperature creep tests, then a new standard would have to be developed, although much of it could be borrowed from AASHTO T322 and from existing proposed standards for the simple perfor- mance tests. RECENT RELATED CHANGES TO THE IDT TEST PROCEDURE, EQUIPMENT, AND ANALYSIS The IDT creep and strength procedure was developed dur- ing SHRP, which took place 10 to 15 years ago. Since the conclusion of SHRP, there have been numerous substantial changes in the test procedure, equipment, and analysis meth- ods used in performing the IDT creep and strength test and interpreting the resulting data. The following subsections of the report discuss the various changes that have occurred, organized more or less chronologically: post-SHRP devel- opments, IDT research at the Superpave Regional Centers, and modifications during NCHRP Projects 1-37A and 9-19. Post-SHRP Developments in the IDT Procedure The modifications in the IDT test and analysis procedure in the first several years following completion of SHRP primar- ily involved improvements in the methods used to calculate creep compliance and Poisson’s ratio from load and deflection data. During SHRP, finite element analyses performed on the IDT test geometry indicated that the simple, plane stress analy- sis typically used in the past to analyze the results of the test can produce substantial errors. These errors result from two sources: horizontal and vertical bulging of the specimen and nonuniform strains across the vertical and horizontal diame- ter. Correction factors were developed for use in a cumber- some, iterative calculation of creep compliance and Poisson’s ratio (A3). Within 2 years of the completion of SHRP, a sim- plified procedure was developed for accounting for nonideal conditions during the IDT test (A7). An empirical set of equa- tions was developed based upon the results of the finite element analysis, which avoided the iterative procedure in calculating compliance and Poisson’s ratio. A second area of modification occurred in the manner in which calculated creep compliance data are used to generate a master curve, providing creep compliance data at a selected reference temperature (−20°C in this case) over a wide range of loading times. In producing such master curves, use is made of time-temperature superposition, which essentially involves shifting log compliance-log time functions determined at several temperatures along the log time axis until a single func- tion is created. Often this procedure is done visually, which leads to substantial differences in results generated by differ- ent engineers; and it also requires substantial overlap among the compliance curves for best results. During SHRP, creep tests were performed for 1,000 seconds, which generally pro- duced good overlap of data. Details of the procedure used to develop compliance master curves were not provided in the final SHRP reports, but later publications provided such infor- mation. Also, at the conclusion of SHRP, it was decided that the length of the IDT creep test should be reduced from 1,000 to 100 seconds to shorten the test time required to complete the test. This unfortunately meant that the compliance curves determined at the three test temperatures often provided lit- tle or no overlap for developing the master compliance curve. This required development of new algorithms for extrapolat- ing the master curve and shifting the resulting data. IDT Research at the Superpave Regional Centers A third area of research, unfortunately of limited scope, occurred under the auspices of the Regional Superpave Cen- ters established by the FHWA in 1995–96. There were five such regional centers throughout the country: the Northeast Superpave Center, located at the Pennsylvania Transportation Institute of the Pennsylvania State University; the Southeast Superpave Center, located at the National Center for Asphalt Technology at Auburn University; The Northcentral Super- pave Center, associated with the Indiana Department of Trans- portation and Purdue University; the Southcentral Superpave Center at the University of Texas; and the West Coast Super- pave Center, which was divided between the University of California at Berkeley and the University of Nevada at Reno. All of the Superpave Centers, except for the West Coast Center, were given IDT creep and strength test systems de- signed and manufactured by Instron Corporation. These systems were unique in that they were closed-loop electro- mechanical (“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 high- way 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 cus- tomer support. There were frequent problems with malfunc- tioning LVDTs used to measure IDT deformation and with the conditioners used in conjunction with these transducers. Part of this problem was related to the practice of keeping LVDTs mounted on the specimens during strength tests, which fre- quently damaged the LVDTs. Sometimes the LVDT was dam- aged enough to be completely nonfunctional, but often times it was only slightly damaged, so that it was not clear that the LVDT was not functioning properly. 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 explained that the nature of the bid documents 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 possibly 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 limitation and not a limitation of the capability of the electromechanical system. Because of the numerous problems encountered by the var- ious Superpave Centers in operating these systems, only one— the Northeast Center—performed IDT tests on a regular basis using this equipment; recently, the Northcentral Center also began using their system. The quality of the data produced at the Northeast Center was, however, marginal, and testing was continued only in an effort to gain experience with this system. The Northeast Center did publish one research paper on analy- sis of the IDT creep test, which was essentially a detailed explanation of a simplified version of Roque and Hiltunen’s analysis (A3), suitable for use in estimating thermal cracking temperatures using IDT creep and strength data (A9). In general, it appears that most of the problems encoun- tered in the IDT systems used within the Superpave Centers could have been corrected, given an adequate investment of time and money by the manufacturer and/or the Superpave Centers. Many of the problems were relatively minor ones dealing with the LVDTs and conditioners or were related to the software and were not fundamental problems with the test system. Unfortunately, this experience has probably created a situation in which it would be politically inadvisable to continue to promote electromechanical systems for use in IDT creep and strength testing. The likely market for this test is probably too small to motivate any equipment manufacturer to provide significant custom engineering design and support for the IDT test system. The most practical approach for pave- ment engineers is, therefore, to use off-the-shelf test systems to perform the test, with a minimum of specially machined accessories. One potentially effective approach, for example, would be to encourage suppliers of the frequency-sweep equipment to be used in characterizing mixtures for the pave- A-10 ment design guide developed in NCHRP 1-37A to include as an option the necessary capacity, hardware, and software for performing the IDT creep test, perhaps in combination with uniaxial tensile strength. It is even possible that uniaxial creep tests would provide data equivalent to that provided by the IDT procedure, which would mean that the same specimens could be used throughout the testing needed for flexible pave- ment design work. This is an issue that should be addressed in the laboratory testing to be done as part of Phase III of NCHRP Project 9-29. One aspect of the experience among the Superpave Cen- ters that should be given consideration is their abandonment of using LVDTs during the IDT strength test to determine the exact moment of failure. In a standard IDT strength test, the precise moment of failure, and hence the “true” tensile strength, is difficult to determine, because the specimen fails very gradually and continues to carry substantial load even after large cracks appear. During SHRP, the suggested solu- tion to this problem was to use the horizontal and vertical LVDTs to monitor horizontal and vertical deflections during the strength test. The point of failure is defined as occurring when the difference between the vertical and horizontal defor- mations reaches a maximum. This is the procedure included in AASHTO T322. Unfortunately, as explained previously, keeping LVDTs in place during the strength test often results in damage or destruction to these sensitive and expensive transducers. Engineers within the Superpave Centers agreed that for practical reasons, the IDT strength test should be done without LVDTs and the strength based only upon the maximum load. Although the AASHTO T322 procedure is probably more accurate, it appears that it is impractical, and damage to the LVDTs as a result of this procedure could actu- ally reduce the overall reliability of the IDT creep and strength tests. In any case, the IDT strength test is only an approxi- mation of the “true” tensile strength, and there is no reason to suspect that the refinement included in AASHTO T322 provides a more accurate result. For example, it is quite pos- sible that IDT tensile strengths are in general lower than uni- axial tensile strengths. Because the AASHTO T322 “cor- rection” actually results in lower IDT strengths, this would actually increase the error inherent in the test. The relationship between IDT strength and uniaxial tensile strength should be evaluated experimentally by testing a range of mixtures using both procedures. If necessary, empirical relationships can be developed among apparent IDT strength, the “corrected” strength as used in the Superpave thermal cracking program, and uniaxial tensile strength. Because the Superpave ther- mal cracking program was designed to use “corrected” IDT strength as input, care must be taken to provide test data equivalent to that produced using this procedure. Modification of the IDT Procedure During NCHRP Projects 1-37A and 9-19 One of the early work elements in the Superpave Support and Performance Models Management Project (FHWA Con- tract DTFH61-95-C-00100, later NCHRP Project 9-19) was

A-11 an evaluation of the Superpave low-temperature cracking model. A report on this work element was compiled that doc- umented numerous problems in the original SHRP thermal cracking model (A10). Most of these problems were in the computer program used to analyze the data and predict ther- mal cracking and have been addressed in recent modifica- tions of the program. However, some suggestions made in this report were not incorporated into later versions of the Superpave thermal cracking model. One important issue raised in the report by Janoo and col- leagues was the determination of the coefficient of thermal contraction, α (A10). The value of α has an extremely strong effect on the cracking temperature of asphalt concrete, and an accurate value for this parameter is essential to developing accurate predictions for low-temperature cracking. It is prob- ably of equal importance to obtaining accurate measurements of compliance and strength. In the original SHRP procedure, α was to be estimated based upon mixture composition (A3). The accuracy of this procedure, however, was never verified. Kwanda and Stoffels actually measured the coefficient of thermal contraction of the mixtures used in developing the SHRP low-temperature cracking test procedures and models and found very poor correlation between the predicted and measured values of α (A11). Mehta et al. later presented a procedure based upon Kwanda and Stoffel’s, in which α was measured using the instrumentation used in the IDT creep test (A12). However, the accuracy of this procedure has not been fully evaluated. Also, Janoo and associates (A10) pointed out that the coefficient of thermal contraction of asphalt cement binders and asphalt concretes is not constant, but varies with temperature. Typically, α is relatively constant at high temper- atures, but begins to reduce as temperature is lowered, reach- ing a value at lower temperatures which is substantially lower than that at high temperatures (A10). However, assum- ing a binder α-value typical for temperatures above the glass transition is a conservative approach. Furthermore, mixture α-values measured by Kwanda and Stoffels (A11) suggest that in the temperature range of −20 to 0°C this assumption appears to be reasonable, as discussed in Chapter 2 of this report. The change in the coefficient of thermal contraction of mix- tures with temperature is due entirely to the properties of the binder, as the value of α for aggregates is constant and inde- pendent of temperature. Furthermore, it should be kept in mind that the value of α for asphalt binders is much greater than for aggregates, and as a result, the coefficient of thermal contrac- tion for mixtures is mostly a function of the binder properties. Thus, if the value of α for mixtures is to be estimated, a typi- cal value for α for the aggregate can probably be assumed, and what is then critical is assuming the correct relationship between α and temperature for the selected binder. Using these assumptions, a simple and reasonably accurate equation for estimating the coefficient of thermal contraction for mixtures has been developed as part of Phase III of NCHRP Project 9-29 and is presented at the end of Chapter 2 of this report. Another important suggestion made by Janoo and his coauthors (A10) was to increase the time of the creep test to 1,000 seconds, rather than 100, to simplify the procedure used in developing the master curve, and also to improve the reliability of the results. Recent improvements in the algo- rithms used to develop master curves from IDT creep data have probably addressed this problem. Although Janoo and associates indicated that the procedure used to estimate relax- ation modulus from creep compliance seemed to work well, they suggested that perhaps a better approach would be to measure relaxation modulus directly, using a constant rate of strain test. However, recent research in which constant rate of strain tests were performed on asphalt concrete has clearly shown that the strain rate in these tests is difficult to control, and the results are, therefore, difficult to analyze and interpret. At this time, as the general approach and analysis method appear to work well, there is no reason to consider this sug- gestion further. A final serious, pertinent issue raised by Janoo and his associates (A10) was the inadequate incorporation of tensile strength in the model. Although the original intent in SHRP was to use tensile strength data at −20, −10, and 0°C, this apparently proved impractical. Later versions of the Superpave low-temperature cracking model used only tensile strength at −10°C. As pointed out in the report by Janoo and colleagues, the tensile strength of asphalt concrete increases with decreasing temperature, up to a certain point, after which the tensile strength begins to decrease slowly (A10). Although this would appear to create a significant problem in the Superpave thermal cracking model, the tensile strength data are in fact used only to estimate the fracture parameter, A, from an empirical equation. Because this equation was devel- oped based upon −10°C IDT strength data, altering the data used as input would result in substantial errors in the proce- dure. Because the thermal cracking model has been calibrated based upon IDT strength data at −10°C, this approach should continue to be used. Partly in response to the report by Janoo and colleagues, Witczak et al. (A8) made a considerable effort to refine the thermal cracking program. The simple errors identified in the program were corrected. Minor refinements were made in the data reduction module. For example, the calculation of compliance is based upon using “trimmed” means of deflec- tions, which for the IDT test generally means averaging the data from four transducers after discarding the lowest and highest transducer outputs. This procedure originally did not properly handle LVDTs that were erroneously providing no output; improvements in the data reduction procedure han- dled this situation appropriately and apparently provide the operator with some indication of overall data quality, though the nature of this information is not yet clear. Equations for making corrections for bulging and for nonuniform distribu- tion of stress and strain across the IDT specimen were empir- ically simplified into forms that allowed direct calculation of the factors, rather than iterative calculations as initially required (A8). Significant improvements were made in the procedure used in developing master compliance curves from IDT creep data during NCHRP Project 9-19 (A8). In developing a master curve, compliance data at several temperatures is shifted with

respect to the time (horizontal) axis to form a single curve rep- resenting creep compliance as a function of time. In analyzing IDT data, the reference temperature is usually −20°C. To form the IDT master curve, the creep data at −10 and 0°C are shifted to form a unified curve with the data at −20°C. This shifting is equivalent to dividing the actual loading times for a given test by a constant called the shift factor, a(T). Figure A-1 is a sketch showing graphically the construction of a master compliance curve from IDT data. Although the construction of a master curve is not difficult, producing master curves in a standardized manner can be difficult, especially if the data are noisy or otherwise non- ideal. Often, experienced engineers will develop master curves graphically, using a trial-and-error procedure involving sub- stantial judgment. In order to make use of a master curve within a computer program, this process must be implemented through a series of algorithms, which apply logic and math- ematics rather than judgment and experience to automati- cally generate a master curve. It is essential that such a pro- cedure be robust and repeatable. That is, such an algorithm should, from a similar set of data, produce a comparable mas- ter curve, even with a substantial amount of variation in the data. Another problem in generating master curves is that, ideally, the compliance curves at each temperature should overlap slightly in order to produce the most accurate master curve. However, the current IDT creep testing protocol does not always produce compliance curves with such overlap. An effective automated procedure must, therefore, also address this shortcoming. The initial algorithms used in generating master curves from IDT creep data were not always effective, resulting in sub- stantial errors in the shift factors, which in turn produced errors in the calculation of thermal stresses and the resulting cracking. Buttlar and Roque addressed this problem in the development of a computer program called MASTER, which was designed to reliably generate master curves from IDT creep data even when substantial noise was present or when the data did not overlap. The details of the algorithms used in this program are described in detail in a NCHRP Project 9-19 report (A8). In summary, MASTER functions by considering a full range of ideal and nonideal situations, evaluating an IDT data set to determine what potential problems are present, and then implementing an effective algorithm for shifting the creep A-12 curves to generate a master compliance curve. The NCHRP Project 9-19 report also includes the results of an evaluation of MASTER. This program appears to work effectively in reliably producing effective creep curves. The only potential problem at this point appears to be with the shift factors. In MASTER, shift factors are determined individually for each set of temperatures; there is no assumed function (exponen- tial, Arhennius, etc.) used to fit shift factors as a function of temperature. For very stiff mixtures, because of the very small slope in the creep compliance data, shift factors at the lowest temperatures can become unreliable. Although it is not fully explained in recent NCHRP Project 9-19 reports, it appears that in order to evaluate shift factors at temperatures other than those used in IDT testing, a polynomial is fit to the calculated shift factors and is then used to interpolate or extrapolate shift factors at other temperatures. This proce- dure can potentially produce substantial errors, though such errors should be infrequent and should only occur with poor- quality data. This potential shortcoming in MASTER could be avoided by two changes: (1) linking the IDT test temperature to the low-temperature binder grade used, so that excessively low compliance values are avoided, and (2) using a linear fit to the log a(T)-temperature data. Using IDT test temperatures related to the binder grade would also tend to produce much better quality data in general, as this protocol would tend to result in compliance data in the ideal range for the test sys- tem. It would also simplify the test procedure, as the response of different mixtures would tend to be similar regardless of the binder used, so that it will be easier for the technician per- forming the test to establish appropriate stress levels. Summary of Recent Changes in the IDT Procedure The current version of the IDT test and analysis procedure have been substantially improved and have addressed many of the shortcomings found immediately after the conclusion of SHRP. The following changes have been incorporated into the most recent version of the IDT test procedure and Super- pave thermal cracking software (A8): • Simplified formulas have been developed for making cor- rection factors for specimen bulging and non-uniform stress and strain distribution across the specimen; • The initial portion of data analysis, which involves devel- oping a “trimmed” mean for the response of a given set of specimens, has been enhanced to avoid problems that occurred when a transducer was not responding and also to provide the user an overall indication of the quality of the data being analyzed; • The procedure used to shift the individual compliance curves to form a master compliance curve has been sub- stantially improved and is more robust and produces reasonable and repeatable master curves even for non- ideal data; Log Time, s Log D(t) -20 C -10 C 0 C -log a(0 C) -log a(-10 C) Figure A-1. Schematic of master curve construction from IDT data.

A-13 • Most or all of the minor problems (“bugs”) in the original SHRP computer program have been corrected; and • The entire program has been recalibrated with an ex- panded data set, which includes the original mixtures and pavements used during SHRP and additional ma- terials and pavements from the Canadian SHRP program. Potential problems that have not been addressed include potentially inadequate characterization of the coefficient of thermal contraction and use of LVDTs during the IDT strength test, which often results in damage to the LVDTs, which can then result in the collection of faulty data for subsequent creep and strength tests. DISCUSSION AND FINDINGS The review of the original IDT strength and creep test and data analysis methods and subsequent modifications and related research indicate that the current procedure and analy- sis are much improved over the original SHRP version and should in most cases provide reliable results. A number of minor changes in AASHTO T322 have been suggested to improve the specifications for the IDT equipment and pro- cedure. Many of the problems pointed out in the report by Janoo and colleagues (A10) have either been effectively addressed or are no longer pertinent. One issue that requires additional attention is the characterization of the coefficient of thermal contraction. Although Witczak and his associates apparently believe that the equation for estimating α is reason- ably accurate (A8), research by Kwanda and Stoffels suggests otherwise (A11). A simple and reasonably accurate equation for estimating the coefficient of thermal contraction for asphalt concrete mixtures has been developed as part of Phase III of NCHRP Project 9-29 and is presented in Chapter 2 of this report. More reliable data and more consistent results from subse- quent analysis of these data can probably be obtained by using an IDT testing protocol in which the test temperatures are linked to the low-temperature binder grade used in the asphalt concrete. This would ensure that the compliance values for a given mixture would be either within or close to an ideal range for measurement and subsequent analysis. In order to simplify implementation, it is suggested that the basic test protocol of testing at −20, −10, and 0°C be maintained for PG XX-22 and PG XX-28 binders. For PG XX-16 binders (and harder), the test temperatures should be −10, 0, and +10°C. For PG XX-34 binders (and softer), the test temperatures should be −30, −20, and −10°C. Furthermore, it is suggested that for severely aged mixtures (either from pavement cores or from an accelerated laboratory aging procedure), the test temperatures be increased by 10°C. Tensile strength tests should be performed at the mid- dle test temperature, usually −10°C. For some mixtures, use of the Prony series to characterize the creep compliance of asphalt concrete mixtures can poten- tially cause problems in that the Prony series predicts rapidly increasing compliance when extended to longer reduced times than those for which the model was fitted. This problem is ana- lyzed in detail in Chapter 2 of this report. It is most likely to occur for unusually stiff mixtures, and so using the adjustable test temperature protocol described previously would help to reduce or eliminate this problem. If necessary, the Superpave thermal cracking program should be modified to provide a power-law extrapolation of the compliance data to reduced times well beyond those used to fit the master curve, to ensure that this problem does not occur. The use of the LVDTs to determine the precise moment of failure in the IDT strength test must be abandoned; it results in damage to the LVDTs that can then create severe problems in data quality in subsequent IDT creep and strength tests. Empir- ical relationships should be established between IDT strengths determined in AASHTO T322 and (a) those based upon max- imum load during the IDT test and (b) those determined using a direct tension test with a 100-mm diameter by 150-mm high specimen, as will be used in the proposed Superpave simple performance tests. This will simplify the IDT test and allow engineers to use a test procedure consistent with what will probably become standard test procedures and geometries in the future. Because the barriers that existed during SHRP to developing procedures for uniaxial tests at low temperatures no longer exist and because such uniaxial tests will become standard pro- cedures in the near future, it is suggested that uniaxial creep and strength become the standard test method for low-temperature characterization of asphalt concrete mixtures. However, the IDT procedure as currently used should be retained for use on field cores. Laboratory testing should be performed to evaluate the relationship between data produced using uniaxial and IDT procedures and to develop empirical corrections if necessary. CONCLUSIONS AND RECOMMENDATIONS Based upon a review of AASHTO T322, and related papers and reports documenting changes in the IDT creep and strength test procedures and analysis, the following conclusions and recommendations are made: • A number of minor changes in AASHTO T322 have been suggested and should be made in the next version of the standard. • The proposed specification for the dynamic modulus master curve test equipment, as developed during NCHRP 9-29, should be revised to include optional requirements for equipment intended to perform not only the dynamic modulus test but also uniaxial creep tests and IDT creep tests at low temperature. • Mixture tensile strength at low temperatures should be determined using either the current IDT procedure or uniaxial tensile strength. Normally, these tests should be performed on a large, static test system separate from the dynamic modulus master curve/low-temperature creep system. However, all tests could be performed on a single high-performance system if desired.

• A draft specification should be developed for uniaxial creep and strength testing at low temperatures, based upon AASHTO T322 and the specifications for the dynamic modulus master curve test equipment as developed as part of NCHRP Project 9-29. • The relationship between uniaxial compliance and IDT compliance at low temperature should be experimentally evaluated, and empirical equations developed for estimat- ing IDT compliance from uniaxial compliance should be developed if needed. • Empirical relationships between the SHRP “corrected” IDT strength, the uncorrected IDT strength, and uniaxial tensile strength should be developed so that strength tests can be performed using the IDT geometry without attach- ing LVDTs or using a uniaxial test geometry. • An improved procedure for either calculating or mea- suring the coefficient of thermal contraction of asphalt concrete mixtures has been developed and is presented in Chapter 2 of this report. • Test temperatures for low-temperature creep tests should vary according to the binder grade. PG XX-22 and PG XX-28 binders should be tested at −20, −10, and 0°C; PG XX-16 binders should be tested at −10, 0, and +10°C; PG XX-34 binders should be tested at −30, −20, and −10°C. Test temperatures for severely aged mixtures should be increased 10°C above these temperatures. Tensile strength tests should be performed at the middle creep test temperature. APPENDIX A REFERENCES A1. Mehta, Y. A., and D. W. Christensen, “Determination of the Linear Viscoelastic Limits of Asphalt Concrete at Low and Intermediate Temperatures,” Journal of the Association of Asphalt Paving Technologists, Vol. 69, 2000, pp. 281-312. A2. Huang, Y. H., Pavement Analysis and Design, Englewood Cliffs, N.J.: Prentice-Hall, Inc., 1993, p. 366. A3. Lytton, R. L., J. Uzan, E. G. Fernando, R. Roque, D. Hiltunen, S. Stoffels, “Development and Validation of Performance Pre- diction Models and Specifications for Asphalt Binders and A-14 Paving Mixtures,” Report SHRP-A-357, Washington D.C.: Strategic Highway Research Program, National Research Council, 1993. A4. Bonaquist, R. F., D. W. Christensen, and W. Stump, “Simple Performance Tester for Superpave Mix Design: First Article Development and Evaluation,” NCHRP Report 513, Trans- portation Research Board, National Research Council, Wash- ington, D.C., 2003, 54 pp. A5. Anderson, D. A., D. W. Christensen, R. Dongre, M. G. Sharma, J. Runt, and P. Jordhal, Asphalt Behavior at Low Service Tem- perature, Report FHWA-RD-88-078, Final Report to the FHWA, Springfield, VA: National Technical Information Ser- vice, March 1990, 337 pp. A6. Christensen, D. W., and D. A. Anderson, “Interpretation of Dynamic Mechanical Test Data for Paving Grade Asphalts,” Journal of the Association of Asphalt Paving Technologists, Vol. 61, 1992, pp. 67–98. A7. Buttlar, W. G., and R. Roque, “Development and Evaluation of the Strategic Highway Research Program Measurement and Analysis System for Indirect Tensile Testing at Low Temperature,” Transportation Research Record No. 1454: Asphalt Concrete Mixture Design and Performance, Wash- ington, D.C.: National Academy Press, 1994, pp. 163–171. A8. 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. A9. Christensen, D. W., “Analysis of Creep Data for Indirect Ten- sion Test on Asphalt Concrete,” Journal of the Association of Asphalt Paving Technologists, Vol. 67, 1998, pp. 458–492. A10. Janoo, V., T. Pellinen, D. Christensen, H. Von Quintus, “Eval- uation of the Low-Temperature Cracking Model in Super- pave,” Draft Report to the Federal Highway Administration, Contract DTFH61-95-C-00100, undated (ca. 1997). A11. Kwanda, F. D., and S. Stoffels, “Determination of the Coeffi- cient of Thermal Contraction of Asphalt Concrete Using the Resistance Strain Gage Technique,” Journal of the Association of Asphalt Paving Technologists, Vol. 65, 1996, pp. 73–92. A12. Mehta, Y., S. Stoffels, and D. W. Christensen, “Determina- tion of Coefficient of Thermal Contraction of Asphalt Con- crete Using Indirect Tensile Test Hardware,” Journal of the Association of Asphalt Paving Technologists, Vol. 68, 1999, pp. 349–367.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 530: Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot-Mix Asphalt evaluates the use of the indirect tensile creep and strength test procedures in American Association of State Highway and Transportation Officials Standard Method of Test T322-03 in mixture and structural design methods for hot-mix asphalt.

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