Impact of lime modification of asphalt and freeze-thaw cycling on the asphalt-aggregate interaction and moisture resistance to moisture damage

Experiments were performed to investigate the impact on the moisture resistance of paving mixtures of the addition of hydrated lime directly to the asphalt prior to mixture preparation. Moisture resistance was evaluated on mixtures prepared using a strategic highway research program asphalt and limestone and granite aggregates by freeze-thaw cycling of the mixtures in water. Other paraineters investigated in addition to hydrated lime that might affect moisture damage were oxidative aging and the addition of a model ketone to the asphalt. After the freeze-thaw cycling experiments, the specimens were subjected to selective solvent extractions to isolate the polar organic materials strongly adsorbed on the aggregate surfaces. Results of the extraction experiments indicate that the composition in the asphalt-aggregate interfacial region may change during repeated freeze-thaw cycling. Oxidative aging improved the resistance of the mixtures to moisture damage. Hydrated lime, when added to the asphalt prior to preparation of the mixtures, dramatically improved mixture resistance to moisture damage for both aggregates. The model ketone containing no additional polar chemical functionality reduced moisture damage resistance, suggesting that the apparent resistance of ketones formed in asphalt to displacement from the aggregate by water reported in previous studies resulted from strongly adsorbed, moisture resistant chemical groups contained on the same component as the ketone functional group. Results from the present study support previous work indicating that carboxylic acids play a major role in determining the moisture sensitivity of pavement mixtures.