# Influence of the Microwave Heating Time on the Self-Healing Properties of Asphal Mixtures
## 1 Introduction  
Asphal mixtures are composite materials made of aggregates and bitumen. They are widely used in the pavement industry around the world. However, mechanical resistance and durability of asphalt mixtures can be reduced due to their permanent exposure to traffic loading and environmental conditions [1]. Ageing [2], moisture damage [3] and thermal cracking [4] are the main factors that affect the durability of asphalt pavements. With the aim of improving the behaviour of flexible pavements against these factors, different types of fibres can be added to the asphalt mixtures [5], such as: cellulose and mineral fibres [6]; polymeric fibres like polypropylene and polyester [7]; and steel wool and other waste fibres [8]. In particular, due to the higher tensile strength of metallic fibres compared with asphalt mixtures they can improve the tensile resistance and cohesive force when added to the mixture [5]. Additionally, these fibres can help to prevent the origin and propagation of cracks [9]. Also, asphalt mixtures with metallic fibres are known for presenting improved resistance, fatigue and ductility characteristics [10]. Therefore, asphalt mixtures reinforced with metallic fibres can have good resistance to ageing, moisture damage and cracking [11]. Moreover, metallic fibres can be added to asphalt mixtures with the aim of increasing their thermal and electrical conductivity [12]. Hence, metallic fibres have been used to develop new flexible pavements with energetic [13] and self-healing purposes via heating processes by electromagnetic induction [12; 14; 15; 16; 17] or microwave radiation [18; 19; 20]. It is known that asphalt pavements have self-healing properties when they reach high temperatures during summer, when the cracks in the road can be closed by themselves [12]. This is due to the fact that the bitumen viscosity is related with the temperature, and consequently when the bitumen reaches a temperature threshold (between 30 \({}^{\circ}\)C and 70 \({}^{\circ}\)C) it can flow by capillarity through the micro-cracks opened in the pavement [21]. There are currently different heating technologies that can accelerate the healing process of asphalt mixtures. One of them is the electromagnetic induction heating [22]. This method consists on adding ferrous materials (like metallic fibres) to the asphalt mixture and exposing it to an alternating electromagnetic field with a frequency of Kilohertz [23], using an induction heating device. This process induces an electrical current in the ferrous particles increasing their temperature due to the Joules principle. Thus, the thermal energy disperses in the asphalt mixture and increases the temperature of the bitumen [24]. A second technique is microwave heating [17], where asphalt mixtures are exposed to an alternating electromagnetic field with a frequency in the order of Megahertz [20], using an electromagnetic radiation device like a microwave. Microwave heating affects the bitumen, producing a change in the orientation of polar molecules, which results in internal friction and an increase of the mixture temperature [25]. Additionally, when ferrous materials are added to the mixtures, they can reflect the microwave radiation and accelerate the temperature increase procedure [25]. In this context, metallic fibres can be used to increase the heating rates of asphalt mixtures considering that they can absorb more thermal energy than the aggregates and bitumen [19]. For all the above, fibres addition can have influence on the mechanical and self-healing properties of the new asphalt mixtures, although it is still not clear how this influence is produced. The main objective of this study is to evaluate the influence of the microwave heating time on the self-healing behaviour of asphalt mixtures. With this purpose, four different asphalt mixtures have been studied under laboratory conditions, using the same aggregates gradation and bitumen content, but varying the amount of fibres added to the mixture: 2%, 4%, 6% and 8% by total volume of bitumen.