Air Void Analyses on Asphalt Specimens Using Plane Section Preparation and Image Analysis

Air voids in asphalt mixes significantly influence the material behavior and durability. Depending on their sizes, shapes, connectivity and distribution, they allow the inflow of water and oxygen, leading to oxidation aging and moisture damage. Additionally, they provide space for permanent deformation. The failure mechanisms are not yet known in every detail. A major cause for this is the lack of a suitable analytical method for air void structures. Still, today, the usual practice is to determine only a single total air void content for every asphalt layer by weighing and measuring methods, which do not distinguish between different appearances of air void structures and exclude layer boundaries. Image analysis is a suitable tool to gain further structural information as it “opens” and “illustrates” the air void structure across all layer boundaries. In the area of research, different approaches have been tested within the last years. Most of them rely on image acquisition by computer tomography (CT). In addition to the high expenses for this technology, the resolutions of typical CT-systems are currently too low for detailed scans of entire asphalt specimens with common dimensions (e.g., drill cores, 150 mm diameter). Furthermore, the distinction between air voids, bitumen, and aggregates is often fuzzy. As a practicable alternative, plane sections of asphalt specimens can be prepared and scanned by conventional flatbed scanners. In this case, the contrast between binder and aggregates and air voids is achieved by impregnation with pigmented resins. First attempts are known from Switzerland and Denmark, but they were limited in specimen size and image resolution. This paper presents recent advancements in the preparation, scan, and analysis of large asphalt plane sections. By using contemporary preparation and computing tools, it is now comparatively simple to get a detailed insight into the air void structure of large asphalt specimens. Furthermore, some examples are given to illustrate the potential of this enhanced examination method. Damage assessment, mixture design, and research can profit from the additional structure information.