Fatigue Cracking Resistance of Crumb Rubber-Modified Asphalt: Mechanisms, Testing Methods, and Multiscale Performance...

Fatigue cracking is a load-related distress limiting flexible pavement service life. This review evaluates 121 studies on the fatigue resistance of crumb rubber-modified asphalt (CRMA) across binder, mixture, modeling, and field scales. CRMA improves fatigue resistance through rubber-asphalt interaction and the elastic contribution of undissolved particles; adequate swelling, dispersion, and bonding slow crack propagation, whereas weak interaction or aging-related interface weakening promotes localized damage. Optimum rubber contents and particle sizes vary with damage stage, material scale, dosage basis, and processing. The benefit is strain-dependent, greater at 2.5% than 5.0%, and rankings may reverse between amplitudes. Linear viscoelastic parameters poorly characterize progressive damage in highly modified binders; linear amplitude sweep testing with viscoelastic continuum damage analysis is more informative but sensitive to the failure criterion. The benefit narrows with evaluation scale, though not uniformly: binder-level improvements reach 116 times the control at 2.5% strain and 45 times at 5.0%, mixture-scale improvements about 1.4 to 10 times (upper bound provisional), and field performance comparable to or moderately better than modified controls. CRMA is a viable alternative to polymer-modified asphalt under specific conditions; equivalence rather than superiority is defensible. Standardized failure criteria, phase-resolved aging procedures, protocols representing healing, and validated multi-scale relationships are needed.