# Smart Materials for Green(er) Cities, a Short Review
## 2 Smart Materials for Green(er) Cities
### Smart Concrete
The IDT is connected to a dipole antenna via the pins. An electromagnetic (EM) pulse sent by a reader to the sensor is converted into an acoustic wave by the IDT. This wave propagates on the surface and is reflected back to the IDT by the reflectors (each reflector reflects a fraction of the incident wave). The IDT retransforms these reflected waves into electromagnetic echoes, which are transmitted back to the reader by the sensor antenna. The whole system works like radar: an EM pulse is emitted, followed a few microseconds later by a series of echoes. However, the acoustic velocity of surface waves is highly dependent on the temperature of the piezoelectric chip, as well as its state of deformation. The position of the echoes in time will therefore also vary depending on the temperature and state of deformation at the sensor. Time-domain tracking of the echo position therefore makes it possible to measure the temperature and state of deformation at the SAW sensor location, wirelessly and without embedded electronics, up to a distance of several tens of centimeters. In this configuration, the return signal can also be seen as a bar code. It is made up of a number of peaks (echoes) whose sequence can be made unique among millions of possible sequences. This makes it possible to identify the sensor (SAW ID tags). As SAW sensors can be integrated into a wide range of dielectric materials, they are an ideal solution for making concrete \"smart\". In the remainder of this Section 2.2 (Smart Concrete), we present some of our most recent results in the field of SAW ID-Tags sensors integrated into concrete structures for precise temperature monitoring in the maturation phase. Beyond this specific application, the results demonstrate the strong applicative potential of SAW sensor technology in the broadest sense (including deformation, corrosion and moisture sensors) in the field of smart concrete. Measuring the temperature within concrete immediately after pouring and throughout the maturation phase can provide a wealth of useful information for civil engineering contractors. As concrete stiffens, it emits heat. The temperature rises significantly over hours or even days before slowly dropping and stabilizing at room temperature. The temperature rise is greatest at the center of the concrete element. The temperature profile, over several days, enables experts to verify that the concrete solidification phase proceeds smoothly and as expected. This enables them to certify the quality of the manufacturedconcrete products. However, it also makes it possible to detect any problems that may arise over the long term and to remedy them in good time. It is therefore a very interesting preventive SHM solution. Figure 1 shows the results of our own tests in this field. An ID-Tag SAW temperature sensor is shown in Figure 1a. The sensor is marketed by the Austrian company sersideon (www.sensideon.com, accessed on 10 August 2023). It operates in the ISM Band at 2.45 GHz. We manufactured (using 3D-printing) a protective casing in polymer material, into which we slipped the sensor (see Figure 1b). The aim was to avoid direct contact between the concrete and the antenna so as not to overly degrade the antenna's electromagnetic properties. The housing also serves to protect the sensor and its antenna during casting. This is indeed a fairly brutal operation. Stones in concrete can hit the sensor quite hard. The sensor can also be damaged by the series of vibrations applied after casting for concrete compaction purposes (see Figure 1c). We integrated the sensor into a concrete cube measuring 10 cm on each side (see Figure 1d). We then interrogated the sensor (using a reader also supplied by sersideon) at regular intervals over several days. The main measurement results are shown in Figure 2. The maximum interrogation distance in air is around 60 cm (see Figure 2a). Beyond this distance, temperature measurement is no longer possible.