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Rethinking PP Timing Strategies

Discussing the implementation of tiered cracking specifications to reduce age-induced surface damage, focusing on optimized timing strategies based on material properties. Research needs, evolving PG specifications, and impact on asphalt supply are explored.

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Rethinking PP Timing Strategies

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  1. Rethinking PP Timing Strategies Tiered Superpave Specifications to Reduce Age-Induced Surface Damage Emulsion Task Force Indianapolis June 13, 2019 Gayle King

  2. OutlineTiered Cracking Specifications • Background • PP Roadmap • PG & Superpave Implementation • Increasing demand for high RAP mixes (9-58) • Evolving PG specifications for cracking (9-59;-60) • G* max; Minimum ΔTc (or equivalent); G-R • Impact on asphalt supply and use of RAP • Questions: • Can seals limit age-induced surface damage? • Should seals be part of a tiered approach that pavement designers use to limit surface damage?

  3. PP RoadmapResearch Needs: Materials Materials Research Need Statement • Optimize PP timing strategies based upon in-place material properties • Estimated cost: $10 million • Can early seals reduce aging rates near the pavement surface? Probably yes! • Pooled Fund Study: AI: M. Anderson, Blankenship, King • Supported by: MN, MD, OH, WI, TX, FHWA, MLRRB • CTAA 2016: Early seals slow evolution of aging: G-R, LAS • AAPT 2016: DC(T) testing, including early seal effectiveness • Agencies now chip seal new rural AC pavements. SD,MN

  4. PG Grade BumpingTiered Quality Standards for Rutting Premise: Mixes require stiffer binders at high summertime temperatures when subjected to heavy, slow moving traffic. Tiers: G*/sin δ MSCR Standard: M320 @ Th; M332: Jnr ≤ 4.0 Heavy: M320 @ Th+6C; M332: Jnr ≤ 2.0 Very Heavy: M320 @ Th+12C; M332: Jnr ≤ 1.0 Extreme: M320 @ Th+18C; M332: Jnr ≤ 0.5 To Meet Spec: add more polymer/additive

  5. Recycling/RejuvenatorsHigh RAP/RAS Problem: RAP mixtures prone to early surface damage. NCHRP 9-58 • TX A&M High RAP/RAS study for TXDOT • Softer virgin AC with PAV20 ΔTc of -4.5 • RAP had much lower ΔTc • Sections with various recycling agents • Surface damage: < 1 year • Loving County, TX • 20% RAP; ΔTc < -10 after construction • Remove and replace in 4 months

  6. Evolving PG Specifications for CrackingFailure Parameters and Early Implementation • New surface damage failure parameters • Black Space parameters: ΔTc, G-R, R-value • Failure stress and strain: ABCD, Dent, S-Dent • Fatigue: LAS • Physical Hardening: Hesp approach, ΔTc • Agencies implement stopgap specs (REOB) • BBR: max G* and min ΔTc • UT, FL, NV, Northeast • DENT, Isothermal Hardening • Ontario

  7. Evolving PG Specifications for Cracking Impact on asphalt supply and use of RAP • Black Space parameters (ΔTc, R-value) are inherent crude quality parameters related to molecular motion at low pavement temperatures. • Poor AC quality (low ΔTc) is difficult to restore, even with the best rejuvenators. • Aging, REOB & wax negatively impact ΔTc. • RAP quality matters! • Polymers and other anti-rutting additives may reduce ΔTc, but rarely increase it.

  8. A tiered quality specification for crackingMaking Preservation Part of the Solution Where to set threshold values for ΔTc? Strict limits eliminate some asphalt sources and greatly restrict RAP use Less stringent limits shorten pavement surface life. Solution: Tiered quality standards for ΔTc Impose strict limits for high volume surface mixes Adopt intermediate limits for binder/base course and low volume surfaces Pavement Preservation option: Accept a third lower PG quality standard for base mixes, or for rural surface mixes (mill & fill) only if an air-impermeable seal is placed on the surface

  9. Tiered Cracking SpecificationsWhat’s missing? Research Needs: • Establish appropriate ΔTc threshold values for traffic, climate, and pavement layer • Modify LTPPBind grade selection • Field research and models which tie effectiveness of early pavement seals to limit aging and/or moisture related damage Implementation/supply issues: • Ease grade proliferation by tying tiered rutting grades to tiered cracking specs??

  10. Tiered Cracking SpecificationsWhere to start? Organize invited workshop, to include: • Researchers for cracking specifications • FAA & AASHTO Pooled-Fund Studies: AI • NCHRP 9-58: TTI, UNH, UNR • NCHRP 9-59: AAT, NCAT • NCHRP 9-60: WRI, AAT, NCAT • TRB/NCHRP: Ed Harrigan • Stakeholder representation • ETF • Binder & Mix ETGs ?? • Agencies/Universities implementing ΔTc: UT, FL, NV

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