Nanoscale Micromechanical Properties of Allophane-Modified Asphalt Binders Characterized by Peak Force QNM Atomic Force Microscopy

Abstract

Atomic Force Microscopy (AFM) in Peak Force Quantitative Nanomechanical Mapping (PF-QNM) mode was used to evaluate the micromechanical behavior and morphological evolution of conventional and allophane-modified asphalt under medium-and long-term aging. Topographic analysis revealed the presence of “bee structures” in all samples, whose morphology evolved significantly: fresh binders exhibited larger and deeper structures, with roughness values between 16–22 nm, while aged samples showed smaller and smoothed structures, with roughness values close to 3–4 nm, indicating a process of microstructural homogenization. Allophane increased the initial roughness (~38%) and moderated its evolution during aging. The nanomechanical results showed that aging increases stiffness, adhesion, dissipation, and indentation, reflecting a transition toward more viscoelastic behavior. However, the allophane-modified binders showed a smaller change in stiffness, achieving a 6.55% reduction compared to conventional ones during prolonged aging. Additionally, allophane consistently reduced adhesion (Δ = 0.52–0.73 nN), indentation, and energy dissipation, with differences of up to 68.83 eV, indicating an improvement in elastic recovery. Overall, allophane acts as an effective modifier that stabilizes the micromechanical response, reduces susceptibility to aging, and improves the performance of the asphalt binder at the nanoscale.