Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt (2004) / Chapter Skim
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Appendix A - Review of AASHTO T322 and Recent Proposed Changes
Appendix A - Review of AASHTO T322 and Recent Proposed Changes
Pages 31-44
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From page 31... ...
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.
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From page 32... ...
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.
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From page 33... ...
( ) 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)
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From page 34... ...
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.
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From page 35... ...
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)
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From page 36... ...
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 temperatures used in the IDT creep and strength test should, therefore, change according to the binder grade used.
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From page 37... ...
The IDT strength test should, therefore, be performed at a vertical displacement 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.
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From page 38... ...
This is especially pertinent 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 performing 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 compliance test would simplify implementation activities and potentially reduce the cost of equipment and training for laboratories wishing to have the capability of performing both procedures.
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From page 39... ...
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 temperatures. That issue can be best addressed through experimental testing to compare creep compliance data at low temperatures determined using both procedures.
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From page 40... ...
Because the Superpave thermal 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 Contract DTFH61-95-C-00100, later NCHRP Project 9-19)
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From page 41... ...
. 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.
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From page 42... ...
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 Superpave thermal cracking software (A8)
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From page 43... ...
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.
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From page 44... ...
• 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 attaching LVDTs or using a uniaxial test geometry. • An improved procedure for either calculating or measuring the coefficient of thermal contraction of asphalt concrete mixtures has been developed and is presented in Chapter 2 of this report.
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