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31 Mixture Workability Test. Initially, the workability tests of testing the 13 binders mixed with the baseline mixture. were to be performed with each of the binders and the base- This experiment examined the effect of the main experimen- line mixture containing granite aggregate. After several early tal factors, binder type and compaction temperature, on workability test trials, a decision was made to change from the changes in the lab compacted density and shear resistance of granite mix to a standard 0.6-mm silica sand to reduce raw a single mixture. The second, smaller compaction experiment data noise that could possibly have been due to binding of ag- consisted of testing four unique binders mixed with four gregate particles between the paddle tips and the workability other mix designs. Selection of the four unique binders was bowl. An asphalt content of 5.1% was used for the sand mix- based on materials that provided a range of binder properties. tures used in the workability tests. The additional mix designs include different aggregate types, The sand-binder mixtures were short-term oven aged at particle shapes, gradations, and RAP contents. The results 180°C (356°F), then transferred to the NCAT workability de- from this experiment provide an indication of the range of vice to measure the resistance to shearing in the loose state as compaction temperatures necessary to provide good com- it cools to approximately 120°C (248°F). The workability de- pactability for different mix types. All mix compaction sam- vice is a large fixed bucket with a two-pronged paddle that is ples were mixed at 150°C (302°F), then short-term oven aged rotated at a constant speed. Twenty-kilogram mix samples were and compacted in an SGC at four temperatures: 110°C, prepared and then dumped into the heated workability bucket. 130°C, 150°C, and 170°C (230°F, 266°F, 302°F, and 338°F). The torque required to turn the paddle through the binder- Indirect Tensile Creep Compliance/Strength Tests. Since sand mix as it cooled in the bucket was automatically recorded indirect tensile creep compliance and strength are highly every second along with the binder-sand temperature. dependent on binder properties, this test was used to assess The raw torque data was filtered to remove extraneous in- how compaction temperatures affect tensile properties of two strument noise in a three-step process using Microsoft Excel. mixtures. Specimens were prepared with each of the binders A 60-second running average and standard deviation of the mixed in the baseline mix design and tested in accordance with raw torque values were calculated in the first step. Then, at AASHTO T 322 and the recommendations from NCHRP each second, a moving range was calculated as the 60-second Report 530 (51). The effect of compaction temperature on average torque plus and minus the 60-second standard devia- compliance and strength was analyzed. tion. Finally, raw torque data outside of this 1-sigma range were removed. The filtered torque versus temperature data were Summary of Research Plan plotted, and a second-order polynomial equation was fitted to the data with a least-squares regression. An example of the The experimental plan involved testing and analysis of a workability test data is shown in Figure 18. variety of binders to characterize their visco-elastic responses Mix Compaction Tests. Two compaction experiments over the range of temperatures for typical field operations. were conducted. The main compaction experiment consisted The testing utilized DSR and rotational viscosity equipment 350 300 Torque, in-lb 250 200 150 y = 0.0004x 3 - 0.1377x 2 + 14.446x - 131.38 R2 = 0.8169 100 100 110 120 130 140 150 160 170 180 190 Temperature, °C Figure 18. Example results from a workability test, torque versus temperature.
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32 common to Superpave binder laboratories. Binders were also predicted mixing and compaction temperatures from the can- analyzed with regard to their potential for emissions and ther- didate binder tests. The study recommends a new method for mal degradation. establishing mixing and compaction temperatures based on Mixture tests were performed to analyze how temperature the simplest method that provides the best correlation statis- and different binders affect aggregate coating, mix workabil- tics. A small validation experiment with an independent set ity, shear resistance during laboratory compaction, and me- of binders from a range of crude sources and with a range chanical properties. The results from these mix tests were used of properties and modification types was used to verify the to identify appropriate mixing and compaction temperatures selected method as a viable new procedure for determining for each binder. These temperatures were then correlated to mixing and compaction temperatures of asphalt binders.