Diffusion characteristics of surface treatment emulsion in aged asphalt mortar of porous asphalt mixture through SEM/EDS analysis

Spaying surface treatment (ST) asphalt emulsion can be an effective preventive maintenance method to alleviate the ravelling problem of porous asphalt (PA) pavement. The applied ST emulsion may gradually diffuse in and soften the aged asphalt mortar film in PA, which however still lacks quantitative evidence. This study aims to quantify the diffusion behaviour of new ST emulsion residue in aged asphalt mortar of PA within 28 days of curing. The Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS) was utilised to capture the spatial distribution of ST emulsion residue within PA under different curing temperatures and durations. Titanium dioxide was selected as a tracer for ST emulsion. Diffusion degree (Dd), derived from the counting of Ti and S elements, was proposed to quantify the diffusion depth of ST residue in aged mortar. It was found that the SEM/EDS is an effective microscopic method to directly monitor the diffusion phenomenon at the mixture scale. The diffusion rate at 60oC is about 5 times faster than that at 25oC, indicating that spraying ST emulsion in hot weather can improve its diffusion efficiency. Besides, the existence of fine aggregates in mortar may increase the diffusion path due to the hindrance effect.