Ferric Oxide Adhesion Promoters for Water Resistant Asphalt Pavements

Additives are often necessary to prevent the loss of strength of asphalt paving materials when they are exposed to moisture. It has been known for many years that naturally-occurring aggregates which contain iron oxides are resistant to the phenomenon of water stripping in asphalt pavements. In this investigation Pyrex brand glass was chosen as a model substrate for silicate-type aggregates in order to determine the effect of iron compounds on the adhesion of bitumen to glass in the presence of water. It was found that both iron naphthenate and iron oxide were equally effective in maintaining adhesion in the presence of moisture. Two independent methods were employed to measure the wet adhesion-peel testing and contact angle measurements. Both methods confirmed that iron is an essential element for prolonged retention of wet strength adhesion. It is postulated that iron oxides react with acidic components in asphalt to form organometallic complexes which subsequently bind to the glass surface and form strong attachments which are resistant to hydrolysis. Iron compounds were equally effective with other substrates such as limestone and traprock, indicating equally good performance with other types of aggregates. Calcium additives in the form of lime did not confer wet adhesion. It is suggested that the known effectiveness of lime as an antistripping agent may be attributed to other factors such as reaction with polar groups in the asphalt, rendering it less hydrophilic. A boiling water test is described for assessing whether a particular asphalt mix may be susceptible to eventual loss of interfacial adhesion when exposed to water.