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23 C H A P T E R 5 Conclusions The performance-based mix design procedure for PFC proposed in NCHRP Project 01-55, âPerformance-Based Mix Design of Porous Friction Courses,â includes performance tests to be conducted during the mix design process to evaluate the resistance of a PFC mixture to raveling, moisture susceptibility, and asphalt draindown (with optional tests for cracking and rutting) while maintaining an air void structure that would provide water drainage from the surface (Watson et al., 2018). At the conclusion of NCHRP Project 01-55, NCHRP Project 20-44/Task 18 was initiated with the goals of assisting SHAs in implementing the proposed performance-based mix design procedure, verifying if the thresholds proposed in the procedure could be achieved, and refining the procedure if needed. Several state highway agencies have reviewed and made changes to their specifications and mix design procedures based on results of recent research. Changes made by GDOT, SCDOT, ALDOT, and TDOT to their specifications were reviewed and used as case studies in this report. The following observations can be drawn based on the specification review. ï· Agencies have typically developed their own PFC mix design procedures that incorporate parts of the AASHTO PP 77 or ASTM D7064 design procedures or the NAPA IS-115 guidelines. ï· Examples of changes to state specifications based on recent research results to improve PFC mixture durability include: o Adding gradation limits for 9.5-mm PFC mixtures as this finer aggregate gradation appears more durable while still maintaining good permeability. o Adding a Cantabro loss requirement of 15 percent or less as the Cantabro test is found to be an adequate indicator of PFC mixture durability. o Specifying less-tracking hot applied tack coat or PG tack with a higher application rate to provide a better interlayer bond between PFC and the underlying surface. ï· In addition, since rutting and cracking are not primary distresses for PFC mixtures, most SHAs do not have a plan to implement rutting and cracking tests, such as HWT and I-FIT, for their PFC mix design procedures. The research team later worked with GDOT, SCDOT and ALDOT to (1) demonstrate how PFC mix designs can be evaluated based on the proposed performance-based mix design procedure and (2) test the plant-produced mixtures from three PFC construction projects to determine if the plant-produced mixtures can meet the thresholds in the proposed mix design procedure. The following observations can be drawn based on the results of the demonstration projects. ï· The thresholds proposed in the performance-based mix design procedure can be achieved based on the mix designs provided by the three state highway agencies. ï· The plant-produced mixtures had lower asphalt contents than the respective optimum asphalt contents in the mix designs. The binder content was 0.3 percent lower in the projects in South Carolina and Alabama and 0.8 percent lower for the project in Georgia. ï· For the two PFC construction projects with granite aggregate materials in Georgia and South Carolina, the gradations of the plant mixtures were similar to those in the mix designs. Thus, the air voids of PMLC
24 specimens were also similar to those of the LMLC specimens. However, since the binder contents in the plant mixtures were lower, the Cantabro losses of the PMLC specimens were higher than those of the LMLC specimens. The Cantabro loss of the PMLC specimens in the GDOT project was higher than the proposed maximum Cantabro loss of 20 percent as the binder content in the plant mixture was 0.8 percent lower than the optimum binder content. ï· For the construction project in Alabama, there may be aggregate breakdown during project construction, leading to changes in the plant mix gradation and lowering the air voids of PMLC specimens by approximately 3.0 percent below those of LMLC specimens. Even though the binder content of the plant mixture was 0.3 percent lower than the optimum binder content, the changes in the gradation and air voids (i.e., by 3.0 percent) had higher impacts on the Cantabro loss of PMLC specimens, which was only 5.7 percent, compared to 17.2 percent for LMLC specimens. ï· For the construction project in Alabama, the extended silo storage and haul time at elevated temperatures significantly influenced the Cantabro loss, adversely affecting mixture durability in the field. In summary, the recent changes in specifications made by the four state highway agencies can improve the PFC mix durability as was seen in the PFC construction project in South Carolina. The PFC mixture used in the project was based on a 9.5-mm PFC mix design with a maximum Cantabro loss threshold of 15 percent. In addition, the Cantabro test can also be used as an acceptance test of plant mix as it is sensitive to changes in binder content, and it should be implemented with a minimum air void requirement (i.e., 15 percent based on the Corelok method) to minimize the effect of air voids on the Cantabro test results and to provide adequate field permeability. A maximum Cantabro loss of 15 percent was achievable for both lab-mixed and plant-produced specimens with the mimimum air void requirement (15 percent). Finally, extended silo storage and haul time at elevated temperatures can have adverse effects on PFC mix durability, especially for absorptive aggregates. In this situation, warm mix asphalt may be used to reduce the effects.