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27 CHAPTER 3 Results and Analysis of Equipment Effects Experiment 3.1 Introduction 3.2 Dynamic Modulus The equipment effects experiment was designed to inves- Dynamic modulus data were collected with each machine tigate differences in dynamic modulus and flow number test beginning at the lowest temperature and proceeding to the data from SPTs built by the three suppliers selected for NCHRP highest. At each temperature, the testing proceeded from the Project 9-29. The experiment was designed as a full factorial highest frequency to the lowest. At the highest temperature, where four independent specimens of the dense-graded mix- the unconfined tests were performed before the confined ture were tested in each device. This experimental design is tests. Initial graphical review of the data revealed several conveniently analyzed using standard analysis of variance problems that required equipment modifications to be made techniques. by the manufacturers as discussed below. The basic design for the equipment effects experiment was repeated for selected testing conditions. The testing condi- 3.2.1 Equipment Modifications tions were selected to examine the range of capabilities of the equipment. For the dynamic modulus test, unconfined tests 126.96.36.199 MDTS were conducted for 10 combinations of temperature and fre- Dynamic modulus data initially collected with the MDTS quency. Confined tests were conducted only at high temper- equipment were consistently 30 percent lower than that col- ature using four frequencies. Table 31 summarizes the testing lected with the other machines. Several possible causes were conditions used in the dynamic modulus test. The responses investigated. This investigation led to the conclusion that the considered in the analysis of variance were the dynamic mod- lower dynamic moduli were the result of the size of the gauge ulus and phase angle. points used with the MDTS equipment. The gauge points used Flow number tests were conducted for unconfined and with this equipment exceed the size given in the specification. confined conditions. Table 32 summarizes the testing condi- Apparently, the dynamic modulus test is sensitive to the size tions used. The responses considered in the analysis were the of the glued gauge point, with larger gauge points resulting measured permanent strain for each load cycle, and the flow in shorter effective gauge lengths and lower modulus values. number for the unconfined tests. Flow did not occur in the The MDTS gauge points were reduced in size by grinding some confined tests. of the material from the top and bottom, and the dynamic To minimize variability associated with specimen fabrica- modulus tests were repeated. The modulus values at low and tion and testing, all specimens were fabricated by the same moderate temperatures improved. However, at high tempera- technician, then grouped to obtain the same average air tures, there was not sufficient contact area to resist the moment void contents for specimens tested in the three machines. caused by the spring force in the LVDT, and the gauge points Table 33 and Table 34 summarize the air void contents for were pried off of the specimen by the LVDT spring force. the specimens used in the dynamic modulus and flow num- Based on these observations, MDTS decided to completely ber testing, respectively. The same experienced technician redesign the specimen-mounted LVDT system. The re- performed all of the tests. Tests with the IPC equipment were designed system uses an LVDT in a holder that is magnetically performed at the Turner-Fairbank Highway Research Center. attached to the gauge points on the specimen. With this sys- Tests with the ITC and MDTS equipment were performed tem the moment caused by the LVDT spring force is signifi- at AAT. cantly reduced. The dynamic modulus tests were repeated