A Review of the Contribution of Mechanomutable Asphalt Materials Towards Addressing the Upcoming Challenges of Asphalt Pavements

In the coming years, asphalt materials will face significant challenges due to the demand for smart multifunctional materials in transportation infrastructures, designed under sustainability criteria. Asphalt pavements will not only have to contribute towards the provision of an adequate surface for the transportation of different types of vehicles, but will need to do so considering the increased loads that they will have to support, as well as the extreme weather conditions resulting from climate change. These pavements will also need the capacity to interact with autonomous vehicles and provide information to the users and maintenance agencies regarding traffic data or performance levels. This paper describes how mechanomutable asphalt materials (MAMs) could enhance the properties of asphalt materials, enabling their use as a solution for smart infrastructures.


Introduction
Over the years, engineers have worked on developing solutions to meet the needs of society, including housing, basic services and communications. In the area of transport infrastructure, there is an identifiable process of development, which began with Roman roads and continually evolved to produce the improved paved roads that allow for the driving of motorized vehicles. These roads were later transformed into the high-capacity roads (the roads that we know today) that have recently begun to incorporate more sustainable and smarter materials and practices of building and design.
Therefore, roads are beginning to be regarded as more than simply a means of transporting goods and services between cities; they are required to provide extra value for the investments made to build them and the spaces that they use. In particular, new trends have emerged in the design and production of advanced materials with the capacity to overcome the challenges associated with recent technological advances, as well as the effects of extreme weather resulting from climate change. One example is the implementation of methodologies for evaluating the environmental impact of traditional processes of production and construction, and for the definition of strategies to make these processes cleaner and more sustainable (e.g., low temperature manufacturing technologies, the use of waste as a substitute for natural resources and LCA studies). A further example is the design of new materials capable of dealing with extreme temperature changes and the associated problems, including permanent deformations produced at high temperatures, aging generated by high exposure to UV rays and chemical pollutants, fatigue cracking produced by freezethaw cycles and thermal cracking caused by the retraction of the material at low temperatures.
In general terms, this new generation of roads could be built using resilient and smart materials that have the capacity to extend the life of the pavement, improve sustainability, reduce maintenance costs and improve road safety, in addition to avoiding the traffic disruptions that lead to significant economic losses and, therefore, the slower development of countries. Additionally, these roads could contribute towards meeting the requirements established by recent technological advances in the autonomous vehicle industry.
Therefore, some of the main characteristics of the materials developed to be included in this new generation of roads include the capacity to self-heal the cracks developed as a result of traffic-induced fatigue, to show more visible colors at night in order to enhance road safety, to register the level of pressure and strain experienced by the effect of vehicle loads, to capture energy from the environment (wind, solar, hydraulic) in order to feed other components of the traffic system, to communicate with autonomous vehicles to assist in their guidance, as well as to send traffic signals to generate an integral system of communications.
Included among this new generation of materials are mechanomutable asphalt materials (MAMs), which are composed of a bituminous matrix with magnetically susceptible materials activated by the effect of external magnetic fields, if needed. MAMs can be used in three main potential applications [1,6]: (1) the mechanical control of the modulus of the bituminous matrix, to improve the performance of specific areas of the road; (2) the generation of thermal changes in order to propose alternative solutions for road maintenance and (3) the construction of encoded roads to assist self-guided vehicle systems.
The present paper presents a general description of MAMs, the mechanisms associated with their functioning and use, as well as the specific areas in which they could be most conveniently placed, in order to consider their use as a potential solution for the construction of smart pavements.

Mechanomutable asphalt materials
Mechanomutable asphalt materials can be defined as a bituminous matrix modified with ferromagnetic materials, which can modify mechanical performance using the effect of magnetic fields produced in permanent magnets [2][3][4][5]7]. As shown in Figure 1, this concept can be extended to thermomutable materials due to the thermal changes produced by the effect of magnetic fields generated in induction coils, which can also allow for the construction of encoded roads for smart cities (communication with self-driving vehicles, cars, scooters, bikes people with limited mobility, traffic signals and for collecting traffic count data, among other uses) [6].

2nd. Thermal changes 1st. Mechanical changes
Control of temperature increases to produce: Self-healing (mechanical recovery produced by the flow of the binder with increments in temperature). Clearing of snow from the surface of the road.

Message Emission/Reception
Control of the stiffness increases/decreases to manage road performance in response to permanent deformations and fatigue cracking.

Bituminous matrix
Magnetically susceptible materials Magnetic fields Effects: Table 1 displays the materials with ferromagnetic properties that have been used until now in the construction of asphalt materials. These can be summarized as: steel slag, steel grit, ferrite fillers, fibers (carbon, steel, steel wood and carbon nanofibers), magnetite (tailings and powered), carbonyl iron powder, graphene, graphite, carbon nanotubes and carbon black. Depending on the type of material, these can serve as total or partial substitutes for the fine/coarse aggregate. The main parameters evaluated in previous studies are associated with the typical distresses developed in the material during practice, but without the application of magnetic fields (Error! Reference source not found.). These are: cracking (dynamic and quasi-static tests to represent fatigue and fracture), permanent deformation (Marshall stability and rutting) and moisture damage (stiffness loss).
More recent studies (Table 1) have also evaluated the changes produced by magnetic fields in terms of dynamic modulus values, phase angle and the healing capacity of these materials (understood as the mechanical recovery of the material following the application of heat and resting periods), as well as susceptibility to detection by magnetic field sensors, a characteristic that makes these materials a viable option for the encoding of the roads that is needed for the autonomous vehicle industry.

Mechanomutable asphalt materials as improved materials for constructing pavements
Mechanomutable asphalt materials were born from the principle of the functioning of the magneto-rheological (MR) fluids applied to asphalt binders [3,4]. As shown in Error! Reference source not found., the characteristics of magneto-rheological fluids provide a medium (in this case a bituminous matrix) in which to magnetically suspend active particles during the absence of magnetic fields. However, when magnetic fields are present, these materials have the capacity to behave as a quasi-solid material, increasing the apparent viscosity of the material [35,36], modifying its rheological behavior from Newtonian to viscoelastic [37] and impeding the movement of the magnetically active particles, which produce an internal stress tensor in the mixture [4]. Research studies that have specifically focused on mechanomutable asphalt materials have confirmed that, at the binder scale (Error! Reference source not found., MAB1, MAB2, MAB3), the higher intensity of the magnetic field and higher quantities of magnetically active particles can increase the modulus and decrease the phase angle of the mechanomutable binder [4]. Related studies that have simulated the dynamic demands of traffic (creep and recovery tests) have revealed that the cumulative deformations in these materials are due to a decrease in their propensity to deform rather than an increase in their recovery capacity (which is more strongly influenced by the penetration grade of the bitumen) [3]. In order to scale those findings to practice, later studies at the mortar scale (Error! Reference source not found., MAM1) confirmed the increase in modulus values, depending on the increase in the intensity of the magnetic field and the quantity of magnetically susceptible materials [5]. Finally, studies regarding the piezoresistivity of electrically conductive asphalt base composites reveal that there is a correlation between this parameter and the stress-strain responses of the pavement, which could be useful for weighing, traffic monitoring, border monitoring and structural vibration control [39,40]. Therefore, the aforementioned capacities of these materials point to their potential use in the construction of airport runways, the high loading areas of airports, ports and parking lots, bus stops and exclusive lanes for heavy traffic.

Mechanomutable asphalt materials for improving road safety and service conditions
Mechanomutable asphalt materials are electroconductive; thus, they have the capacity to produce and conduct Foucault currents generated from the effect of magnetic fields (Figure 4). In turn, these currents are the source of the energy losses transformed into heat through the Joule effect. Thus, mechanomutable asphalt materials can also be regarded as thermomutable asphalt materials, which makes them ideal for two types of road maintenance operations: 1) clearing ice and snow from the surface of the road (Error! Reference source not found.), and 2) healing the cracks produced from the dynamic loads of traffic, temperature and moisture changes (Error! Reference source not found.).
To achieve the first type of maintenance activity, the temperature of the electroconductive layer is increased with the goal to heat up the whole pavement system though conduction. This has the effect of melting possible snow and ice on the road and increasing user safety. For the second maintenance activity, a magnetic field is applied to the MAM to produce a controlled increase in temperatures between 30°C and 70°C, via induction [19,41] and microwaves [19], which is the interval of temperatures between which the binder flows as a Newtonian fluid and is then able to seal potential cracks in the pavement [22].
To achieve both aforementioned maintenance operations, an electroconductive smart layer can be placed inside the bituminous layers of the pavement. As shown Error! Reference source not found., the surface course would be applied on top to protect vehicle tires from possible damage caused by magnetically responsive materials in the mechanomutable asphalt layer; these materials, such as steel fibers obtained from end-of-life tires, typically have sharp edges. This surface layer would be an ultra-thin layer, to ensure the structural performance of the pavement and resist the temperature increase produced in the electroconductive layer by the activation of a magnetic field.  Error! Reference source not found. describes recent strategies for managing snow-and icerelated maintenance activities on road pavements. These strategies have considered the use of electroconductive materials, as in this study, as well as other alternative systems. For example, the use of infrared heating, hydronic heating systems and embedded heating wires to melt the snow and ice in various public areas, such as pedestrian walkways, emergency entrances, loading dock ramps, hotel lobby entrances, bridge decks, sidewalks and pavements. These snow-melting systems are each composed of an energy source (which can be electric, geothermal and/or magnetic), heat exchanging elements, sensors (for measuring actual weather conditions) and a system control. The power consumption of these strategies varies depending on the current development of the system, which demonstrates that these solutions still require further research in order to optimize their performance and, finally, bring their use into practice.
Given that hazardous weather conditions can increase the susceptibility of traffic accidents, especially with snowfall being a particularly high risk factor in road accidents [46], these solutions can be used as environmental-friendly and smart alternative strategies for traditional "de-icing salts". While these salts have been shown to be economically accessible and effective, they affect the roadside soil, underground and surface water and vegetation integrity [47,48], as well as the longterm structural performance of the road infrastructure and vehicle health [49].  [30,52,57] With respect to the second type of maintenance operation, with the aim to transform roads into more resilient structures through self-healing, recent research efforts have designed and evaluated the use of encapsulated healing agents (oily rejuvenators) [42][43][44] and the effect of resting periods [45]. As it stands, this area is somewhat newer and has undergone less research.

Mechanomutable asphalt materials for the guidance of autonomous vehicles
According to the levels established by the US National Highways Traffic Safety Administration (NHTSA), there are five levels, ranging from zero (no automation) to four (full self-driving automation) for classifying the autonomy of vehicles [58]. Due to the fact that the majority of the new conventional vehicles have one or more specific control functions that are automated, it can be assumed that Level 1 and Level 2 are now being implemented in real life. Ongoing research by the major vehicle manufacturers is being carried out with the aim of reaching Level 3, where the drivers can concede full control of the safety-critical functions of the vehicle under certain traffic or environmental conditions. Finally, some researchers [6,59,60] consider that to fully achieve Level 3 and Level 4, that is, full self-driving automation, it will be essential to incorporate the necessary infrastructure.
If the benefits of assigning a multifunctional character to the road are also considered, the use of mechanomutable asphalt materials can be extended to encoding the road [6] (Figure 6). Encoding the road entails assigning, alongside it, a type of language that is easily read by magnetic field sensors, with the aim of processing this signal so that it can be converted into specific functions for the vehicles and the rest of the traffic signals in the road infrastructure. Therefore, studies [6] in this field have addressed this issue by evaluating the performance of MAMs to allow for encoded roads, with the aim of establishing design methodologies and guidelines for their real-life implementation.
Error! Reference source not found. shows the potential areas of application, such as tunnels (where the GPS signal can be weak) and intersections (where the interaction can also involve traffic signals, pedestrians and personal mobility vehicles). In addition, it is important to note that, with the emerging concept of smart cities, the mentioned benefits of MAMs would greatly benefit and be used in harmony with already existing smart systems. These other systems include smart sensors (both in the road and on vehicles and traffic signals), artificial intelligence (based on machine learning), image processing, big data and computer vision, to develop better traffic management systems and manage the road network in real time [61][62][63][64][65][66] (Error! Reference source not found.). Figure 7. Schema of a novel traffic management system using encoded roads.

Conclusions
The aim of this paper was to describe how mechanomutable asphalt materials (obtained by modifying conventional asphalt materials with magnetically susceptible particles) could contribute towards meeting the challenges faced by asphalt roads. The main conclusions can be summarized as follows: -The first potential application in which MAMs aim to improve the mechanical performance of asphalt pavements showed that the magnetically susceptible materials used in their manufacture tended to be aligned to the forces of activated magnetic fields. This movement improves the mechanical properties of these materials, which depends on the temperature, the amount of magnetically susceptible materials and the intensity of the magnetic field. The mechanisms of action involved in the process are associated with: (1) the development of an internal structure in the material at high temperatures and (2) the generation of a stress field inside the bituminous matrix at low temperatures; -In the case of the second application, the concept of MAMs can be extended to their use as thermomutable materials. The electrical properties of the magnetically susceptible materials used in their manufacture allow for the production of parasite currents which produce energy losses and increments in the temperature of the material and the surrounding layers. This thermal capacity could be taken into account when proposing new strategies for improving the safety and service conditions of the road; -With regard to the third application, the development of a better means of transportation means that the road infrastructure should be prepared to respond to the associated advances in technology, and mechanomutable asphalt materials provide a tool for developing roads that can be used for the guidance of autonomous vehicles; -Finally, it should be noted that further research is required with regard to the development of these materials. In particular, there is a need is to replicate the results obtained in the laboratory in a real-scale study. More results are needed to provide inputs for validating numerical models, which can be useful in the parametrization-and, therefore, the extrapolation-of the uses of MAMs in practice. This will allow for validating and adjusting the tested designs, a step that is necessary for the future implementation of MAMs as smart materials that can be used in the construction of smart pavements.