Self-healing of open cracks in asphalt mastic Álvaro García ⇑ Delft University of Technology, Faculty of Civil Engineering and Geosciences, Micromechanics Laboratory (MICROLAB), Stevinweg 1, 2628 CN Delft, The Netherlands Empa Swiss Federal Laboratories for Materials Testing & Research, CH-8600 Duebendorf, Switzerland article info Article history: Received 23 August 2010 Received in revised form 1 September 2011 Accepted 5 September 2011 Available online 17 September 2011 Keywords: Self-healing Asphalt mastic Activation energy CT-Scan Capillarity abstract Asphalt mastic is a self-healing material. Cracks may develop in the asphalt mixture as a result of differ- ent factors, such as repeated traffic loads or freeze–thaw cycles. But, once a crack is open in the pavement, it starts healing and, if it has enough time to complete the process, it can even close completely. Asphalt mastic self-healing properties are directly linked to temperature and to the rest periods it can be classi- fied as a thermally induced self-healing material. Moreover, it is well known that the speed at which this phenomenon occurs is higher with the increase of temperature. In this paper, it is explained that the changes in the self-healing rates with temperature can be related by means of the Arrhenius equation. For that, an apparent activation energy for healing is needed. To calculate it, a series of asphalt mastic beams were broken and healed at different temperatures and the time when the recovery is complete was used to calculate the activation energy. Moreover, capillary flow through the crack was hypothesized as the main cause for healing. Finally, to have a visual prove of this theory, the healing process in one of the asphalt beams was examined through CT-Scan tests. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Self-healing materials are inspired in nature. They are artificial counterparts of a tree, which can heal damage to its trunk by itself, or a cut in a finger, which seemingly spontaneously stops bleeding [1]. These materials have the ability of repairing themselves when suffering mechanically or thermally induced damage. Moreover, healing can occur autonomously [2,3] or be activated by external stimuli [4–8] (e.g., heat) once or multiple times. Otherwise, it is well known that asphalt is a self-healing material. And, as its heal- ing properties are directly linked to temperature [9,10] and to the rest periods [11–20] it can be classified as a thermally induced self-healing material. Cracks may develop in the asphalt mixture as a result of different factors, such as repeated traffic loads or freeze–thaw cycles [21]. However, once a crack is open in the pave- ment, it starts healing and, if it has enough time to complete the process, it can even close completely. Different recent studies about asphalt self-healing include the work of [9–20,22–27]. Thus, understanding the healing characteristics of an asphalt binder allows the optimization of the use of asphalt mixtures in an eco- nomical way. Although healing in asphalt has a great importance in the performance of asphalt mixtures, limited work has been done to fully understand its mechanisms. Different authors [15,18,19,28] proposed that two main types of healing exist in asphalt mixtures: adhesive healing at the asphalt aggregate interface and cohesive healing within the viscoelastic asphalt binder. Additionally, Little and Bhasin [29] explain that when both faces of the crack are in con- tact (this process is contributed by the van der Waals forces [30]), the diffusion of molecules from one face to the other starts. This will happen, until the crack has completely disappeared and the repaired material has the level of strength of the original one. It is clear that healing depends on the surface energy of the material (wetting, to put both faces of the crack in contact) and on the interdiffusion and randomization capacity of the molecules from one face to the other [29]. The problem is that, in a fresh open crack, it is improbable that its faces match perfectly. There will always be empty spaces in between, sometimes so wide that wet- ting cannot occur. In spite of this, healing continue happening: in the field, if a rest period is allowed and enough energy is given to the system (the pavement is at an appropriate temperature), visible cracks can disappear. Healing of open cracks in asphalt pavements is something that cannot be fully explained with the current theories. For this reason, the main objective of this paper is to investigate the healing mechanisms of asphalt mastic. 2. Methodology and materials 2.1. Materials Asphalt mortar specimens were prepared with a fixed sand- bitumen ratio. Three different sizes (0.120–0.250, 0.250–1.0, 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.09.009 ⇑ Address: Empa Swiss Federal Laboratories for Materials Testing & Research, CH- 8600 Duebendorf, Switzerland. Tel.: +41 0587654313. E-mail address: alvaro.garcia@empa.ch Fuel 93 (2012) 264–272 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel