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37 Several modifications to the SPT equipment specification, the SPT test methods, and the equipment supplied by the three manufacturers were made as a result of the ruggedness and equipment effects experiments. These modifications are discussed below. 4.1 SPT Equipment Specification Modifications The ruggedness and equipment effects experiments con- firmed that the SPT equipment specifications developed in NCHRP Project 9-29 are appropriate. The ruggedness test- ing demonstrated that the level of control required by the SPT equipment specifications provides precise data for the dynamic modulus and flow number tests. Because the flow number and flow time tests are very similar, this conclusion can also be extended to the flow time test. The equipment effects experiment demonstrated that there is little differ- ence in dynamic modulus and flow number data collected with equipment meeting the SPT equipment specification supplied by three manufacturers. Significant differences in dynamic modulus were detected only for tests resulting in modulus values below about 500 MPa. Differences in flow number test data between machines from different manufacturers were confined to the early portion of the permanent deformation curve and were not of engineering significance. Three SPT equipment specification changes were identified by the by the ruggedness and equipment effects experiments. First more precise control of strain is needed in confined dynamic modulus tests compared to unconfined tests. Based on the ruggedness testing, strain in confined dynamic modulus tests should be controlled to within ±15 μstrain of the 100 μstrain target. The SPT equipment specifications included a strain control tolerance of ±25 μstrain. The tolerance was reduced to within ±15 μstrain in the final version of the SPT specification. Second, the equipment effects experiment identified that the length of the gauge point in the direction of the strain measurement had a significant effect on the measured dynamic modulus. In the final version of the SPT equipment specification, a maximum dimension for the gauge point in this direction was added. Finally, the new flow number algorithm developed at ASU using the Franken model produced reduced variability in flow number test re- sults with the SPT. The final version of the SPT equipment specification was modified to include flow number computa- tions based on the Franken model. The final version of the SPT equipment specification is included as Appendix E. 4.2 SPT Test Methods Modifications Two changes to the SPT test methods were also made as a result of the ruggedness and equipment effects experiments. First, the ruggedness testing clearly showed that the flow number test results were significantly affected by the type of end friction reducer used. The SPT test methods were revised to specify the use of greased latex membranes friction reduc- ers in the flow number test. A standard method for preparing the greased latex membranes was also added. The second change to the SPT test methods was the addition of a check on the direction of the drift in the dynamic modulus test as a data quality indicator. During the equipment effects experi- ment, it was discovered that the spring force of the LVDTs could result in drift that tended to move the gauge points fur- ther apart. The drift compensation included in the dynamic modulus computations is not intended to remove this form of drift; therefore, a significant error in the dynamic modulus can result if the gauge points move apart during the test. The data quality check that was added is to accept only data where the drift is in the same direction as the applied load. A stan- dard test method for conducting dynamic modulus and flow number tests in the form of an AASHTO standard is included in Appendix F. C H A P T E R 4 Conclusions
4.3 Manufacturer Modifications Each manufacturer made modifications to their equipment during the ruggedness and equipment effects experiments. The ITC equipment could not accurately control the loading rate for the 0.01 Hz tests at high temperatures. ITC modified the control software to use a different control algorithm for low frequency loading. The spring force in the LVDTs of the IPC equipment moved the gauge points apart at high tem- peratures. IPC designed a set of springs to counter the LVDT spring force. With these springs, the IPC specimen-mounted deformation measuring system can be used to higher tem- peratures without experiencing the gauge point drift problem. Finally poor performance of the specimen-mounted deforma- tion measuring system for the MDTS equipment resulted in complete redesign of this component of the equipment. 38
