Nanoscale Evaluation of Moisture Damage in Polymer Modified Asphalts

The moisture damage in polymer modified asphalts has been studied for decades, yet the effects of chemical functional groups on moisture sensitivity are not known. In this study, a nanoscale experiment is conducted to measure these effects in terms of adhesive/cohesive forces using an atomic force microscopy (AFM). A base asphalt binder and two polymers such as styrene-butadiene (SB) and styrene-butadiene-styrene (SBS) modified asphalts are used to prepare an AFM sample on glass substrates. The AFM samples are conditioned under dry and wet conditions. In the AFM, these samples are probed by silicon nitrite $\left({\text{Si}}_3{\text{N}}_4\right)$ , carboxyl $\left(\text{COOH}\right)$ , methyl $\left({\text{CH}}_3\right)$ , and hydroxyl $\left(\text{OH}\right)$ functionalized AFM tips and nanoscale pull-off or adhesion/cohesion forces between asphalt and tip molecules are measured. Based on the ratio of wet to dry adhesion/cohesion forces, it is shown that the polymer modification makes binders less susceptible to moisture damage. Among the four tips, the $\text{COOH}$ tip shows almost no difference in adhesion forces between wet and dry samples. Using $\text{OH}$ tips, it is shown that the cohesion in SBS modified wet asphalt samples is significantly higher than the cohesion in SB modified wet asphalt samples. The ${\text{Si}}_3{\text{N}}_4$ tip shows higher adhesion in SB modified wet samples than that in the SBS polymer modified wet samples. Based on the adhesion/cohesion force, 3% polymer is found to be the optimum for both SB and SBS polymers. In addition, this study determines the methodology for sample preparation and testing parameters for nanoscale AFM testing on asphalt binders.