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”

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

Effective Binder Content

WesTrack Cracking Vs. Vbe

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

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

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

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

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

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

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

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

Mix Resistivity

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

What Can 0.5 % Change in Binder Content Do?