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7 TABLE 3 IDT creep and strength specimen requirements 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 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 eight 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. therefore, change according to the binder grade used. The In general, most of the recommended modifications to relationship between binder stiffness and mixture stiffness is AASHTO T322 are minor and not controversial; therefore, not 1:1; a given change in binder stiffness will produce a they should be easy to implement. The suggested revisions somewhat lower change in mixture stiffness. It is suggested contained in this report have been forwarded to the task that the current test temperatures of 0, -10, and -20°C be force responsible for recommending revisions to this test maintained for mixtures made using PG XX-22 and PG method to AASHTO. XX-28 binders. For PG XX-16 and PG XX-10 binders or mixtures that have been severely age-hardened, the recom- EXPERIENCE WITH THE IDT TEST AT mended test temperatures should be -10, 0, and +10°C. For THE REGIONAL SUPERPAVE CENTERS PG XX-34 binders (or softer), the recommended test temper- atures should be -30, -20, and -10°C. In the late 1990s, IDT test systems were procured by the A related problem with the current version of AASHTO FHWA for four of the five Regional Superpave Centers. T322 is that the test conditions must be determined through These test systems were to be used for further evaluation of a trial-and-error procedure. A load is applied to the speci- the IDT creep and strength test procedures. The systems were men; if the resulting strains fall outside the allowable range, unusual in that they were closed-loop electro-mechanical sys- the test is aborted, the specimen is allowed to recover for tems, rather than the much more traditional closed-loop servo- 5 minutes, and the test is then repeated at an adjusted load hydraulic systems usually used for IDT tests and other similar level. No suggestions are given concerning what the appro- procedures. Unfortunately, these test systems were plagued priate applied loads should be for different combinations with more or less constant hardware and software problems. of mixture types and test conditions. Given the suggested These problems were not the results of any inherent flaws in revised protocol above, it is possible to provide guidelines the basic concepts underlying the IDT creep and strength for the applied load, as listed in Table 4. The specimen would tests, but were the result of typical problems in first-article initially be tested using the initial applied load listed in the prototypes, exacerbated by a lack of technical support by the second column of Table 4. If the resulting deformations are too vendor and limited support funds available for performing small or too large, the test should be aborted, the specimen needed modifications to the IDT systems. This experience has, allowed to recover, and the test repeated using the alternative however, made it unlikely that similar electro-mechanical sys- loads listed in the third column of Table 4. tems can be effectively implemented for use in IDT testing in the near future. One aspect of the experience among the Superpave Cen- TABLE 4 Guidelines for applied load in the IDT creep test ters that should be given consideration is their abandonment of using Linear Variable Differential Transformers (LVDTs) Initial Test Temperature Applied during the IDT strength test to determine the exact moment Load Other Possible Applied Loads of failure. In a standard IDT strength test, the precise (kN) (kN) Lowest 40 Deformation < 0.01 mm: 80 moment of failure, and hence the "true" tensile strength, is Deformation > 0.02 mm: 20, 10 difficult to determine, because the specimen fails very grad- Intermediate 10 Deformation < 0.01 mm: 20, 40 ually and continues to carry substantial load even after large Deformation > 0.02 mm: 5, 2 Highest 5 Deformation < 0.01 mm: 10, 20 cracks appear. During SHRP, the suggested solution to this Deformation > 0.02 mm: 2, 1 problem was to use the horizontal and vertical LVDTs to