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35 CHAPTER 5 Conclusions and Recommendations 5.1 Dynamic Modulus Master Curves The first article SPTs purchased in Phase II of the project were upgraded to meet the revised specification. Two new A methodology was developed to construct dynamic mod- devices meeting the revised specification were purchased in ulus master curves for pavement structural design using an Phase IV of the project. SPTs meeting the revised specifica- abbreviated testing protocol. In this approach, the limiting tion currently are available from three sources: Interlaken maximum modulus is estimated from mixture volumetric Technology Corporation, IPC Global, and Medical Device properties and a limiting binder modulus of 145,000 psi. Testing Services. The three devices are very similar. All are When a reasonable estimate of the limiting maximum mod- relatively small, bottom-loading, servo-hydraulic machines ulus is available, it is not necessary to perform dynamic mod- with automated testing chambers that serve as a confining ulus testing at the lowest temperature included in AASHTO pressure cell and temperature control chamber. The primary TP62. Eliminating the low temperature testing offers three differences are in the hardware and software used for tem- advantages. First, the cost of environment control capabilities perature control, the user friendliness of the equipment, and is substantially less. Second, smaller, less expensive actuators the operational details of the control software. Hopefully can be used since the load required for dynamic modulus competition generated by these suppliers will lead to im- testing depends on the stiffness of the material that increases provements to the equipment. with decreasing temperature. Third, testing below 32F is difficult and more variable due to potential icing of the in- strumentation. Using the abbreviated dynamic modulus 5.3 Simple Performance Test methodology and the SPT, it is possible for highway agencies Specimen Fabrication to routinely collect dynamic modulus data for the MEPDG. Test specimen preparation for the SPT is a multi-step A recommended standard practice was developed to imple- process. First, tall gyratory specimens must be prepared to an ment the abbreviated dynamic modulus protocol, and is air void content that is 1 to 2 percent higher than the desired included in Appendix A. This standard practice provides rec- air void content of the test specimen. Next, the 100 mm ommended testing temperatures and frequencies. It also (4 in.) diameter test specimen must be cored from the larger describes how to fit the dynamic modulus master curve to the gyratory specimen. Finally, the test specimen is cut to the ap- measured data and to compute input data for Level 1 analysis propriate length by sawing approximately 12.5 mm (1/2 in.) in the MEPDG. from each end of the specimen. A recommended standard practice for SPT specimen fab- rication was prepared. This standard practice is included in 5.2 Simple Performance Test Appendix A. It addresses each step of the fabrication process Systems in detail, and includes two important appendices that provide The Simple Performance Test System Specification was additional guidance for preparing SPT specimens. The first is modified to specify a device capable of performing the three a procedure for obtaining the target air void content for spec- simple performance tests and developing dynamic modulus imens from mixtures that the technician is not familiar with. master curves using the abbreviated testing protocol. The The second appendix provides a method for evaluating the revised equipment specification is presented in Appendix B. uniformity of air void contents within SPT test specimens.

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36 In evaluating the specimen preparation process, it was was developed by Shedworks, Inc. in NCHRP 9-29. The equip- determined that an automated system for coring and sawing ment specification that the FlexPrepTM was designed to meet is the specimens would be beneficial to the future implementa- included in Appendix C. The development of this equipment tion of the SPT. Such a system would reduce the amount of proved to be more difficult than the SPT systems, requiring skilled labor needed to prepare test specimens. It also would approximately five years to complete. The machine is capable minimize the potential for errors in the coring and sawing of preparing SPT specimens in less than 15 minutes with little operations that result in specimen rejection due to noncom- technician intervention. While the FlexPrepTM is a promising pliance with the SPT specimen dimensional tolerances. A pro- prototype, additional development work must be completed totype automated coring and sawing system, called FlexPrepTM, before production models of the design can be made available.