Performance-Based Mix Design for Porous Friction Courses (2018)

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122 Findings The following are important findings from this study: • The traditional pie plate test is inadequate to determine how well a PFC mix will perform. • PFC mixes are different from dense-graded mixes and cannot be designed for optimum AC at a specified air void level. • Variations in AC had little to no effect on mixture VMA (Table 23 and Figure 44). • A Cantabro loss of 20% appears to be a reasonable threshold for mix design (Figure 38). • Film thickness does not seem to be a critical property for performance of PFC (Table 23 and Figure 41). • VCA testing was not an indicator of performance and may not be applicable for PFC mixes since they are typically placed in thin lifts. • Permeability is directly related to air voids. • OT results did not rank PFC mixes according to field performance. • An additional 2% BHF reduced Cantabro loss to the equivalent of adding 1% more AC for both the South Carolina and Georgia designs (Figure 62). • Air voids and permeability of the South Carolina mix were still well above acceptable minimum levels, even with added 4% BHF. • Adding 4% BHF increased conditioned ITS by more than 40% (Table 50). • GTR improved resistance to moisture damage (Table 68) and mitigated draindown, but reduced permeability (Table 66) and (Figure 76). • Use of HiMA reduced Cantabro loss and improved rutting resistance (Figure 75 and Figure 77), but reduced permeability (Figure 76). • Binder modified with SBS may provide more cracking resistance than GTR mixes (Table 71). Conclusions This study has shown that PFC mixes can be designed with the same materials and specifica- tions, but produce very different results. Therefore, it is expedient that performance testing be integrated into the PFC mix design procedure. Based on work in this study and information obtained through the literature review, the following conclusions are drawn: • Performance tests must be made an integral part of PFC mix design. • The current 100 m/day permeability criteria is not practical for many PFC mixes, especially at the low end of the air void range. A minimum value of approximately 50 m/day is recommended. However, agencies may set a higher or lower value depending on annual rainfall amounts (or experience). C H A P T E R 8 Conclusions and Recommendations for Future Research

Conclusions and Recommendations for Future Research 123 • Experimental results indicate the P-200 content is optimized between 4% to 6%, with some mixes showing a trend of improvement at even higher levels. Revision of the P-200 gradation range to 2% to 6% passing is proposed. • A minimum Va of 15% is proposed based on the CoreLok method, or a minimum of 17% based on dimensional methods. • The Appendix presents a proposed PFC design procedure in AASHTO format; the procedure is outlined below: 1. Materials Selection a. Select aggregates that meet Superpave specifications for the appropriate traffic category. b. Select binder that is two standard PG grades stiffer than the paving grade typically used for the location. (Some agencies have successful experience with PFC mixes when using only one PG grade stiffer than typically used.) 2. Determine Aggregate Gradation Based on Primary Objective a. For permeability and rutting resistance, design a coarse gradation within agency ranges. b. For noise reduction, design a fine gradation within agency ranges. c. For durability, design with higher binder content and/or higher P-200. 3. Prepare Trial Specimens a. For most aggregates with polymer modifier, use binder at 5.0%, 6.0%, and 7.0% by weight of total mix. b. For GTR modified mixes and mixes with oolitic limestone aggregates, use 6.0%, 7.0%, and 8.0% binder by weight of total mix. c. Prepare two loose mix samples at the middle trial AC to determine Gmm and Gse. d. Prepare 12 samples compacted to 50 gyrations. i. Two at each AC to determine air voids and permeability. ii. Two at each AC to determine Cantabro stone loss. 4. Select Optimum Asphalt Content—Select the lowest binder content which meets all the following criteria: a. Air voids ≥ 15.0%. b. Cantabro Loss ≤ 20%. c. Permeability—meet agency criteria based on annual rainfall (or experience). Minimum recommended permeability is 50 m/day. 5. Conduct Performance Tests a. Moisture susceptibility and cohesion. i. Conditioned ITS ≥ 50 psi (based on performance with PG 76-22; a lower threshold may be needed for softer binders). ii. TSR ≥ 0.70. b. Shear strength (optional) ≥ 125 psi. c. Draindown ≤ 0.3%. d. HWTT (optional). i. PG 64 or lower ≥ 10,000 passes before reaching 0.5 in. rut. ii. PG 70 ≥ 15,000 passes before reaching 0.5 in. rut. iii. PG 76 or higher ≥ 20,000 passes before reaching 0.5 in. rut. e. Cracking resistance (optional)—SCB I-FIT Flexibility Index ≥2 5. 6. Adjust mix, if necessary, to meet recommended design criteria and verify results. Recommendations for Future Research • Further study is needed to validate the PFC mix design method developed through this research, with mixtures placed on field projects. • This study concentrated on three aggregate types: granite, limerock, and traprock, which were all obtained from quarries in the eastern U.S. Validation is needed for aggregate sources in

124 Performance-Based Mix Design of Porous Friction Courses western states such as California, Nevada, and Arizona, and from midwestern states such as Texas and Oklahoma. • There is a need to evaluate field density of PFC mixtures to determine whether mixes are compacted in the field to the density obtained in the laboratory. Based on poor performance of mixes with high air voids used in this study, it is suspected that excessive air voids in field projects will result in lack of cohesion and lead to early raveling. • This study showed that added BHF can improve durability of PFC mixtures. There is a need to determine the feasibility of adding fines to PFC mixtures in the field. • In this study, and several others, the Cantabro stone loss procedure has shown to be a good indicator of resistance to raveling. There is a need to develop a precision statement for conducting this test.