Evaluation of Air Voids in Reinstatement Materials for Footways

Abstract

The specification for reinstatement of openings in highways in the UK prescribes the materials that may be used and sets out the fundamental requirements for compaction. The preferred mixture for the reinstatement of footways is AC 6 complying with EN 13108-1. Meeting in situ air voids requirements for this mixture is not always achievable in practice due to, among others, the variability of plant produced mixtures, compaction conditions, surcharge and test variability. In this work, two standard AC 6 mixtures, a coarse graded and a fine graded, have been evaluated along with an alternative AC 4 mixture designed to give low air voids with minimum compaction effort. The mixtures were compacted in the laboratory at different temperatures and compaction efforts. Air voids were determined using standard methods. X-ray computed tomography (CT) was also used for measuring the internal structure of the asphalt specimens. Results showed large differences in air voids between the two standard AC 6 mixtures. Considerably lower voids were obtained for the AC 4 mixture. CT scanning showed better compactability of the AC 4 mixture as seen by the air voids distribution with specimen thickness. Also, the AC 6 coarse mixture had a relatively small number of large air voids whereas the AC 6 fine mixture had a relative large number of small air voids. The AC 4 material, on the other hand, had the smallest air voids and their number was small compared to the AC 6 fine mixture but similar to that of the AC 6 coarse mixture.