# A Concise State-of-the-Art Review of Crack Monitoring Enabled by RFID Technology
## 5 RFID-Based Crack Monitoring Applications for Concrete Materials  
For RFID-based crack monitoring applications, the two most commonly used materials are concrete materials and metal materials. This section mainly introduces how to apply RFID technology to concrete crack monitoring, including monitoring the width and depth of cracks. [PERSON] [34] employed RFID technology to detect cracks in concrete elements by monitoring changes in the resistance of conductive materials coated on the surface of concrete. When the concrete substrate is subjected to pressure, the conductive material \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Advantages** & **Disadvantages** \\ \hline Crack monitoring based on & Low-cost, high-speed & Difficult implementation, \\ chipless RFID technology & printing production. & short transmission distance. \\ \hline Crack monitoring based on & Small size, light weight, low & Short transmission distance, \\ passive RFID technology & cost, and long service life. & usually within a few meters. \\ \hline Crack monitoring based on & Fast transmission rate, small & \\ UHF RFID technology & antenna size, and ability to & Poor penetration, and greatly \\  & read a large number of & affected by the environment in \\  & electronic labels in a & ultra-high-frequency bands. \\  & short time. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Advantages and disadvantages of different RFID crack monitoring technologies. Figure 16: (**a**) Health monitoring of coal mining conveyor belt using UHF RFID-based sensors; (**b**) UHF RFID system-based sensor [33]. Figure 15: Experimental setup for the proposed RFID sensor [32]. is stretched, and the electrical resistance increases. If the concrete substrate is subjected to enough pressure to crack, the conductive material will also crack, and the resistance will increase significantly. [PERSON] [35] focused on the RIFD-based deformation and crack problems of structures, emphasizing the importance of monitoring them to prevent catastrophic failures and reduce maintenance costs, and discussed the application of sensors in the monitoring of structures, including various physical monitoring principles, sensor types, system designs, signal processing methods, and application areas (as shown in Figures 17 and 18). [PERSON] et al. [36] employed RFID-based crack sensing to monitor ultra-high-performance concrete (UHPC) modifications. A crack was simulated on the UHPC sample, and then a commercial low-cost label was obtained (as shown in Figure 19). The proposed RFID sensors can be used to monitor the small cracks that occur during the creation of UHPC to ensure that the capability of modification is maintained. There are also some scholars who carried out research on UHF RFID sensing technology for concrete crack monitoring. [PERSON] et al. [37] introduced a new wireless sensor tag based on passive UHF RFID technology to measure the opening of a crack in concrete materials (as shown in Figures 20 and 21) as a phase shift in the signal backscattered from  
Figure 19: Lab-based system crack detection setup: (**a**) RFID controller and reader; (**b**) RFID specimen, tag, and antenna [34]. Figure 17: Envisaged system layout: an RFID reader wirelessly interrogates [34]. Figure 18: Isolines of (**a**) realized gain; (**b**) phase over mutual distance [34]. RFID sensor tags. The sensor measures the opening of the crack by detecting the phase variation in the signal backscattered from an RFID tag connected to a load sensitive to changes in crack width. The innovation in the sensor design lies in the use of electromagnetic methods to enhance both the communication stability and sensitivity of the RFID tag. [PERSON] et al. [38] discussed the implementation of a passive UHF RFID sensor platform as a measure to prevent cracks in concrete (as shown in Figure 22). Monitoring data are gathered and transmitted to an RFID reader, where information regarding chlorine concentration is processed.