# Smart Materials for Green(er) Cities, a Short Review
## 2 Smart Materials for Green(er) Cities
### Smart Concrete  
Worldwide, the construction of concrete bridges, tunnels and buildings is extremely energy-intensive. We have three main options for reducing this energy consumption as much as possible. The first is, of course, to reduce construction activity altogether. However, this option is unrealistic. A second option is to improve cement production techniques and optimize the construction stages of buildings and major structures to make the construction industry less energy-intensive overall. A great deal of research and development work is being carried out worldwide to achieve this goal [61; 62]. A third option is to extend the service life of reinforced concrete bridges, tunnels and buildings as far as possible. To achieve this, these structures need to be maintained as well as possible. In addition to using self-healing concrete solutions (see Section 2.1), signs of deterioration in structural integrity must be detected as early as possible so that they can be remedied by appropriate maintenance, repair and/or consolidation operations. In the case of bridges and tunnels, early detection of signs of deterioration, or Structural Health Monitoring (SHM), is nowadays mainly carried out using a battery of sensors mounted directly on the concrete surface [63]. These include deformation, temperature, vibration and humidity sensors. These sensors are usually connected to an electronic box for wireless data reading and transmission. These boxes are often equipped with photovoltaic panels to supply them with energy. These solutions are well-known and marketed by many suppliers. However, these systems cannot be used to measure, for example, the temperature or state of deformation inside concrete. These two parameters have to be deduced from surface measurements, which is not ideal. Less mature solutions already exist for measuring temperature or the state of deformation within concrete [64]. Most are based on embedded sensors connected to the exterior via electrical cables. These solutions work well, but the cables pose a number of problems. They require special care when the concrete is poured. They degrade over time in the concrete and can also facilitate water penetration, leading to corrosion of the reinforcing steel bars. Fully integrated solutions also exist to overcome these problems. Here, however, the electronics themselves have to be integrated into the concrete alongside the sensor. This poses long-term problems (including limited lifespan of the embedded electronics, recycling problems due to the presence of electronic components and batteries in the material). To solve these problems, we propose the use of smart solutions in line with the principles mentioned at the start of this article. In other words, we propose to use passive and wireless solutions without embedded electronics. In the remainder of Section 2.2 (Smart Concrete), we will focus on the \"SAW sensor\"-based solution, which meets all the above specifications [65; 66]. SAW sensors are a well-known technology and there is an abundance of literature on the subject [67; 68]. SAW sensors essentially consist of a piezoelectric chip on which interdigital transducers (IDTs) and reflectors are deposited. IDTs and reflectors are both made of a series of very thin metal fingers. The piezoelectric chip is housed in a protective case. The IDT is connected to pins via very thin bonding wires. These pins pass through the housing, connecting the IDT to an external (electrical) signal source. IDTs can then convert an electrical signal into acoustic waves, and vice versa, thanks to the electromechanical coupling properties of the piezoelectric substrate. These acoustic waves, which propagate on the surface, can be reflected by the reflectors. A very interesting SAW sensor configuration is where an IDT and several reflectors are deposited on the surface. Such SAW sensors are usually called \"Delay Lines\". In the past, they have been used for numerous industrial applications, particularly in extreme environments [69; 70].