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35 5.1 Dynamic Modulus Master Curves A methodology was developed to construct dynamic mod- ulus master curves for pavement structural design using an abbreviated testing protocol. In this approach, the limiting maximum modulus is estimated from mixture volumetric properties and a limiting binder modulus of 145,000 psi. When a reasonable estimate of the limiting maximum mod- ulus is available, it is not necessary to perform dynamic mod- ulus testing at the lowest temperature included in AASHTO TP62. Eliminating the low temperature testing offers three advantages. First, the cost of environment control capabilities is substantially less. Second, smaller, less expensive actuators can be used since the load required for dynamic modulus testing depends on the stiffness of the material that increases with decreasing temperature. Third, testing below 32°F is difficult and more variable due to potential icing of the in- strumentation. Using the abbreviated dynamic modulus methodology and the SPT, it is possible for highway agencies to routinely collect dynamic modulus data for the MEPDG. A recommended standard practice was developed to imple- ment the abbreviated dynamic modulus protocol, and is included in Appendix A. This standard practice provides rec- ommended testing temperatures and frequencies. It also describes how to fit the dynamic modulus master curve to the measured data and to compute input data for Level 1 analysis in the MEPDG. 5.2 Simple Performance Test Systems The Simple Performance Test System Specification was modified to specify a device capable of performing the three simple performance tests and developing dynamic modulus master curves using the abbreviated testing protocol. The revised equipment specification is presented in Appendix B. The first article SPTs purchased in Phase II of the project were upgraded to meet the revised specification. Two new devices meeting the revised specification were purchased in Phase IV of the project. SPTs meeting the revised specifica- tion currently are available from three sources: Interlaken Technology Corporation, IPC Global, and Medical Device Testing Services. The three devices are very similar. All are relatively small, bottom-loading, servo-hydraulic machines with automated testing chambers that serve as a confining pressure cell and temperature control chamber. The primary differences are in the hardware and software used for tem- perature control, the user friendliness of the equipment, and the operational details of the control software. Hopefully competition generated by these suppliers will lead to im- provements to the equipment. 5.3 Simple Performance Test Specimen Fabrication Test specimen preparation for the SPT is a multi-step process. First, tall gyratory specimens must be prepared to an air void content that is 1 to 2 percent higher than the desired air void content of the test specimen. Next, the 100 mm (4 in.) diameter test specimen must be cored from the larger gyratory specimen. Finally, the test specimen is cut to the ap- propriate length by sawing approximately 12.5 mm (1/2 in.) from each end of the specimen. A recommended standard practice for SPT specimen fab- rication was prepared. This standard practice is included in Appendix A. It addresses each step of the fabrication process in detail, and includes two important appendices that provide additional guidance for preparing SPT specimens. The first is a procedure for obtaining the target air void content for spec- imens from mixtures that the technician is not familiar with. The second appendix provides a method for evaluating the uniformity of air void contents within SPT test specimens. C H A P T E R 5 Conclusions and Recommendations
In evaluating the specimen preparation process, it was determined that an automated system for coring and sawing the specimens would be beneficial to the future implementa- tion of the SPT. Such a system would reduce the amount of skilled labor needed to prepare test specimens. It also would minimize the potential for errors in the coring and sawing operations that result in specimen rejection due to noncom- pliance with the SPT specimen dimensional tolerances. A pro- totype automated coring and sawing system, called FlexPrepâ¢, was developed by Shedworks, Inc. in NCHRP 9-29. The equip- ment specification that the FlexPrep⢠was designed to meet is included in Appendix C. The development of this equipment proved to be more difficult than the SPT systems, requiring approximately five years to complete. The machine is capable of preparing SPT specimens in less than 15 minutes with little technician intervention. While the FlexPrep⢠is a promising prototype, additional development work must be completed before production models of the design can be made available. 36