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36 Table 44. Initial repeatability estimates for the dynamic modulus and flow number. Test Parameter s CV d2s Dynamic Modulus NA 11.6% 32 % Dynamic Modulus Phase Angle 1.2 degrees NA 3.3 degrees Unconfined Flow Flow Number 119 cycles* NA 320 cycles* Number * For a material with a flow number of 1,000. limit 95 percent of the time. The d2s limit is determined using measuring equipment to counteract the LVDT spring force Equation 2. and minimize unwanted movement of the glued gauge points. Finally the MDTS device required a complete redesign of the d 2s = 1.960 2 s (2) specimen-mounted deformation measuring system. Significant equipment effects were detected in the dynamic where: modulus testing. For low stiffness dynamic modulus measure- ments, below about 500 MPa, the dynamic modulus meas- d2s = difference two standard deviation limit ured with the MDTS equipment was significantly lower and s = standard deviation of the test the dynamic modulus measured with the ITC equipment was significantly higher. The equipment effect was approximately When the standard deviation of the test varies with test result, 20 percent while the testing error was only approximately as it does of the dynamic modulus, the coefficient of variation 12 percent. One possible cause for this difference at low stiff- is used in place of the standard deviation in Equation 2. ness levels is calibration of the low range of the load cell. The Table 44 summarizes estimates of single laboratory re- applied load levels for low stiffness dynamic modulus tests are peatability for the dynamic modulus and flow number tests. very low, 0.5 percent or less of the capacity of the load cell of Two properly conducted tests on the same material in the the machine. The manufacturer-supplied load cell calibrations same laboratory should not result in differences in the dy- were not verified prior to the equipment effects experiment. namic modulus greater than 32 percent or differences in the The calibration of the temperature sensor and the deformation phase angle greater than 3.3 degrees. The repeatability of measuring equipment was verified using independent NIST the flow number is likely to depend on the magnitude of the traceable standards immediately before conducting the equip- flow number and whether the test is confined or uncon- ment effects experiment. For various combinations of tem- fined. For unconfined flow number tests on materials with perature and frequency, significant differences in phase angles a flow number of approximately 1,000, two properly con- were also detected. The trend that was evident in phase angle ducted tests on the same material in the same laboratory data was that the IPC equipment produces the highest phase should not result in differences in the flow number of more angles and the MDTS equipment produces the lowest phase than 320 cycles. angles. The difference between these two machines averaged over the range of data collected was 1.5 degrees. The testing error for the phase angle is 1.4 degrees. 3.5 Summary Significant equipment effects were not detected in the flow The equipment effects experiment provided the opportunity number tests. Although there was a statistical difference in the to compare dynamic modulus and repeated load permanent early portion of the permanent deformation in the unconfined deformation test data collected on the same mixture using tests, its magnitude was not of engineering significance. equipment from the three manufacturers. The dynamic Variability of the dynamic modulus data collected during modulus portion of this testing revealed flaws with each de- this Phase of NCHRP Project 9-29 is similar to that collected vice that were resolved by the respective manufacturer during during Phase II. The use of the new flow number algorithm the experiment. The ITC device required modification of the developed at ASU appears to reduce variability in the com- control software to control low frequency dynamic modulus puted flow number. This algorithm provides a more precise testing at high temperatures. The IPC device required the method for computing the derivative and inflection point in addition of springs to the specimen-mounted deformation repeated load permanent deformation curves.