**1. 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 freeze-thaw 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 [1,2]: (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.

## **2. 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 [3–7]. 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) [2].

**Figure 1.** Mechanomutable asphalt materials.

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 (Table 1). 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.


**Table 1.** Ferromagnetic materials used in the construction of asphalt materials.
