On the importance of aggregate mineralogy in asphalt pavement distress

Abstract

Isolated and circular rust-coloured stains, joined by white or light-yellow areas, were observed on the asphaltic surface of several roads in Romania. They appear as a fine powder on the surface of bitumen-coated aggregates in the pavement wearing course. In some cases, the encountered stains were accompanied by water. A progressive aggregate stripping process occurs, which leads to potholes affecting the pavement. Asphalt mixtures are composite materials consisting of a mineral skeleton (natural aggregates and filler) mixed with a bituminous binder, following a recipe and using a specific technology. Other materials may also be added, to improve certain characteristics. Asphalt mixture samples from different affected locations were collected and investigated. No direct connection was found between the observed stains and the road cross section arrangement or the existence of ditches or drains. Microscopic analyses and X-ray diffractions were performed on the extracted asphalt mixture samples. The conducted research has emphasised degradation processes affecting natural aggregate particles in the asphalt mixture. Secondary minerals are formed following an aggregate carbonation reaction in the presence of carbon dioxide enriched water from atmospheric precipitations. Calcium carbonate (calcite), formed as the result of degradation of aggregate carbonates, was identified as the main component of the observed light-coloured areas. It is accompanied by a thin layer of iron hydroxides coating the surrounded aggregates, therefore the rust-coloured stains. It was also found that the presence of iron sulfide (i.e. pyrite, marcasite) into aggregates lead to the formation of sulfuric acid in the presence of water, which accelerates the asphalt mixture degradation processes. The obtained results demonstrate the importance of quality control during asphalt mixture production, with an emphasis on aggregate composition, mineralogy and coating.