1. NCHRP Projects 9-25 & 9-31: Findings Related to Surface Cracking FHWA Mixture ETG Washington, DC February 2003 Advanced Asphalt Technologies, LLC “Engineering Services for the Asphalt Industry”

2. Fatigue Testing and Analysis Continuum damage approach Normalization to reduce or eliminate effect of mixture stiffness Produces single continuum damage fatigue constant “C**2” characteristic of intrinsic fatigue resistance Independent of stress- vs. strain-control

3. Effective Binder Content

4. WesTrack Cracking Vs. Vbe

5. Mixture Stiffness Increased mixture stiffness decreases strains Decreased strains increases fatigue life HOWEVER…

6. Binder Viscosity & Healing Healing more rapid with low viscosity binders Generally, fatigue resistance will decrease with increased high- temperature binder grade

7. Healing and Binder Viscosity (after Little et al., 1999)

8. Age Hardening |G*| aged / |G*| unaged = 1.76  0.0037(% LS) + 0.028 (VTM)  0.068 (Agg. S.A.) R 2 = 74 %

9. Age Hardening: Aggregate Type and Permeability Limestone fines appears to significantly retard age hardening Age-hardening also decreases with decreasing permeability –Increasing aggregate surface area –Decreasing air voids

10. Summary: Factors Affecting Fatigue Cracking Effective binder content Structural stiffness Mixture stiffness Binder viscosity & healing Mixture age hardening

11. Ways to Increase Fatigue Life Increase binder content Decrease binder viscosity Increase pavement thickness Use limestone/hydrated lime fines and/or filler

12. Controlling Rut Resistance Rut resistance a function of –Binder grade –N-design –VMA relative to aggregate fineness To maintain rut resistance with increased binder, increase proportion of fines/mineral filler

13. Mix Resistivity

14. Recommendations Use 9.5 and 12.5-mm mixtures for wearing courses Increase binder 0.5 % by weight Maintain filler/binder ratio Avoid unnecessary grade bumping Use limestone/hydrated lime fines and mineral filler

15. What Can 0.5 % Change in Binder Content Do?