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5 CHAPTER 2 FINDINGS REVIEW OF AASHTO T322 typical cost-performance characteristics for loading systems, environmental chambers, and transducers. When appropriate, The discussion below is a summary of the most significant consistency with suggested requirements for the simple perfor- findings presented in Appendix A of this report, which presents mance test has been considered in evaluating the requirements a detailed review of AASHTO T322, and recent related work for the IDT system. done as part of NCHRP Projects 9-19 and 1-37A. Readers As an example of the nature of the evaluation performed on interested in details on these topics should refer to this Appen- the requirements summarized in Table 1, consider the speci- dix. These findings and resulting recommended changes to fications for the range for the axial loading device. In SHRP AASHTO T322 to improve the effectiveness and efficiency of Report A-357, the developer of the IDT creep and strength this procedure were forwarded to the task force responsible for testing procedure presents data for a range of mixtures (1). recommending revisions to this test method to AASHTO. These exhibit a range in compliance values of from about AASHTO T322 consists of 17 sections: 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 1. Scope maximum applied tensile stresses corresponding to these 2. Referenced Documents compliance values range from 125 kPa to 17 MPa. Based 3. Terminology upon the relationship t = 2P/tD, the axial loads corre- 4. Summary of Method sponding to these tensile stresses are 1.5 and 200 kN, respec- 5. Significance and Use tively, for a specimen 50 mm thick and 150 mm in diame- 6. Apparatus ter. However, another consideration is the maximum load 7. Hazards that can be applied without a specimen failing. The lowest 8. Standardization tensile strength (t) reported in SHRP A-357 was 1.3 MPa, 9. Sampling and the highest was 4.3 MPa. The corresponding load (P) 10. Specimen Preparation and Preliminary Determinations for these tensile strengths can be calculated as P = ttD/2, 11. Tensile Creep/Strength Testing (Thermal Cracking where t and D are the specimen thickness and diameter, Analysis) respectively. The calculated loads based on tensile failure 12. Tensile Strength Testing (Fatigue Cracking Analysis) are between 15 and 51 kN for a specimen 50 mm thick. Limiting the load to one-half that required to cause failure and 13. Calculations allowing for specimens up to 100 mm in thickness, the antic- 14. Report ipated maximum load is then 50 kN. However, to ensure good 15. Precision and Bias loading system performance, the capacity of the loading 16. Keywords system should be about double the anticipated maximum 17. References load, giving a maximum capacity of 100 kN, agreeing nearly exactly with the 98 kN given in AASHTO T322. Evaluation Many of these sections are only of nominal significance and of the displacement rate is more detailed and is presented in have not been addressed as part of this study. Of special signif- Appendix A. icance are Sections 6. Apparatus, 10. Specimen Preparation, In the previous example, the requirements in AASHTO and 11. Tensile Creep/Strength Testing. The most signifi- T322 for the range of the axial loading device appear to be cant findings of this study on these three critical sections of reasonable. However, the sensitivity requirements appear to AASHTO T322 are summarized below. be too stringent. Consider the worst-case situation, which is The specifications for the IDT creep and strength device, for the lowest anticipated load. Because it would be undesir- as currently given in AASHTO T322, are listed in Table 1. able to approach nonlinearity, in some cases the applied loads A careful review of these requirements has found several might be somewhat less than the estimated minimum load problems with the ranges and sensitivities for the various of 1.5 kN, say 1 kN. To calibrate to this load level, ASTM devices and transducers comprising the IDT test system (see E4 requires a resolution that is 1/100th of the minimum load Appendix A). The evaluations have been based upon typical level or a resolution of 10 N, which is significantly larger properties of HMA at low temperatures, the fundamental (poorer) than the 5-N resolution requirement given in stress-strain relationships for the IDT loading geometry, and AASHTO T322. Consideration should be given to changing

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6 TABLE 1 AASHTO T322 specifications for IDT apparatus Component General Requirements Range Sensitivity Axial loading Shall provide a constant load 98 kN maximum load; 5 N minimum device Displacement rate between 12 and 75 mm/min Load measuring Electronic load cell 98 kN minimum capacity 5 N minimum device Deformation Four linear variable 0.25 mm minimum 0.125 m measuring differential transducers minimum device(s) (LVDTs) Environmental Temperature control only; -30 to +30 C Control to 0.2 chamber large enough to perform test C and condition 3 specimens Control and Shall digitally record load 1 to 20 Hz sampling rate 16-bit A/D board data acquisition and deformation during test required system Test fixture As described in ASTM N/A 2 kg maximum D4123 (diametral resilient frictional modulus testing) resistance the required resolution for the IDT loading system to 10 kN; Requirements for specimen preparation in AASHTO T322 this would likely reduce the cost of the equipment required to are currently vague and should be stated more explicitly to perform the test. ensure good test data. Table 3 and the accompanying notes Various aspects of the equipment specifications, as pre- list suggested requirements for IDT creep and strength spec- sented previously in Table 1, were evaluated in detail. The imens. The values given in this table have largely been based recommended revised specifications for the IDT creep and on requirements for specimen uniformity developed during strength test are given in Table 2. NCHRP Project 9-29 for the simple performance tests (6 ). In many cases, the changes are slight, such as rounding the Maintaining similar requirements for both tests will ensure maximum load to 100 kN rather than 98 kN, or the recom- that technicians are familiar with these standards and that the mended 0.1 m sensitivity for the deformation measuring equipment and techniques needed to produce such specimens devices compared with the original 0.125 m sensitivity. The are available in most laboratories. most substantial changes are probably the less stringent Currently the suggested test temperatures for the creep pro- requirements for the sensitivity of the axial loading device cedure are 0, -10, and -20C. Because of the variability in and load cell (increased to 10 N from 5 N) and the change in binder grades and the resulting low-temperature properties the required range of the environmental chamber from -30 of asphalt concrete, some specimens are extremely stiff at to +30C to -30 to +10C. Both of these changes should help -20C, while others may be too compliant at 0C. The test reduce the cost of the IDT system. temperatures used in the IDT creep and strength tests should, TABLE 2 Proposed revised AASHTO T322 specifications for the IDT apparatus Component General Requirements Range Sensitivity Axial loading Shall provide a constant 100 kN maximum load; 10 N or better device load 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 0.1 mm minimum 0.1 m or better measuring transducers (LVDTs) device(s) Environmental Temperature control only; -30 to +10 C under Control to 0.5 chamber large enough to perform ambient conditions of 15 to C test and condition 3 27 C specimens Control and System shall be operated 1 to 20 Hz sampling rate Consistent with data acquisition with the use of a personal required system computer and shall sensitivity of all digitally record load and system deformation during test transducers Test fixture As described in ASTM N/A 20 N maximum D4123 (diametral resilient frictional modulus testing), but with resistance flat neoprene loading strips 12-mm thick by 12-mm wide.