Asphalt Pavements – Kim (Ed) © 2014 Taylor & Francis Group, London, ISBN 978-1-138-02693-3 1233 Evolution of bubble size distribution during foam bitumen formation and decay Biruk W. Hailesilassie Division of Highway and Railway Engineering, Infrastructure Engineering, School of Architecture and the Built Environment, Royal Institute of Technology (KTH), Stockholm, Sweden Philipp Schuetz & Iwan Jerjen Laboratory for Electronics/Metrology/Reliability, EMPA, Swiss Federal Laboratories for Material Science and Technology, Duebendorf, Switzerland Andrea Bieder Ammann Schweiz AG, Langenthal, Switzerland Martin Hugener Road Engineering/Sealing Components, EMPA, Swiss Federal Laboratories for Material Science and Technology, Duebendorf, Switzerland Manfred N. Partl Division of Highway and Railway Engineering, KTH Stockholm, Stockholm, Sweden Road Engineering/Sealing Components, EMPA, Swiss Federal Laboratories for Material Science and Technology, Duebendorf, Switzerland ABSTRACT: The warm asphalt mixture process using foam asphalt technology allows mixing and compaction at lower temperature. Nevertheless the higher air void content and incomplete coating of large aggregates are issues that need improvement to reach the proper- ties of hot mix asphalt. In order to improve the understanding and characterization of the bitumen foam, X-ray radiography was used to investigate the formation and decay of bitu- men foam in 2D representation. Image segmentation analysis was used to determine the foam bubble size distribution as a function of time. The impact of water content on the process has been studied for two penetration grade bitumen. The water content showed consider- able influence on the foam quality in terms of expansion ratio and bubble size distribution. Increasing the water content in the foaming process leads to a quicker collapse of the bubbles and favors coalescence of individual bubbles. Keywords: Foamed bitumen, expansion ratio, half-life, X-ray radiography, image segmenta- tion analysis 1 INTRODUCTION Foamed asphalt technology, which allows lower mixing temperatures, has been used success- fully in many countries. Energy saving, decreased emissions of fumes and odor, extending the paving season, compaction aid for stiffer mixes … etc., are main advantages of the foam asphalt (warm mix asphalt) over the conventional hot mix asphalt. Foam asphalt produced at ambi- ent temperature needs improvement to reach the properties of hot mix asphalt for heavy duty roads. Higher void content and incomplete coatings of large aggregates are the main reasons for its lower performance. Foam bitumen is a mixture of air, water and bitumen. It is produced through the injection of small quantities of water typically 1–6.0% regarding the mass of the