# Recent Advancements in Non-Destructive Testing Techniques for Structural Health Monitoring
## 2 Sweep Frequency Technique
The measurements were undertaken with and without faulty membranes (a hole with 20-30 mm diameter was made in the middle of the membrane) to investigate the changes in microwave signal, namely amplitude and frequency changes. The obtained results demonstrated that membrane failure influenced the microwave signal. The changes were both in the amplitude and frequency of the reflected microwave signal. The proposed technique was also investigated to detect chloride level in reinforced concrete (RC) structures by [PERSON] et al. [16]. In this study, five concrete specimens were made and submerged first in  
Figure 1: (**a**) Sensor dimensions, (**b**) Proposed microwave Transmitter/Receiver setup and (**c**) block diagram of proposed Sweep Frequency system for SHM [16]. tap and then in salted (various concentrations of chloride) water. The aim was to investigate the influence (if there is any) of the presence of chloride on the microwave signal. The captured results demonstrated that the absorption of the microwave energy was lower in specimens with lower chloride concentration owing to the lower conductivity, which in turn influences the microwave signal, namely the amplitude. The obtained data was pre-processed and analyzed using Levenberg-Marquardt algorithm, which demonstrated that the technique can detect chloride level in concrete with R\({}^{2}\) = 0.99 and RMSE = 0.00012 at 5.42 GHz. Microwave sensing technology was also investigated by researchers as an embedded solution for long term monitoring of concrete and masonry structures. A rectangular patch antenna (resonating at 2.45 GHz) and a split ring resonator (SRR) (resonating at 3.4 GHz, 4.7 GHz, 5.7 GHz, 8.7 GHz and 10.6 GHz) were investigated to monitor the excess moisture content in concrete specimens [19] and detect cracks in RC [20], respectively. Both sensor structures were embedded inside the concrete specimens for real-time monitoring. A model for the determination of moisture content and deterioration of concrete was successfully developed and implemented. Moisture content inside concrete specimens and crack development in RC specimens can be studied from the data of their interactions with microwaves. This measurement is provided via S-parameter (reflection coefficient (S\({}_{11}\))), i.e., in the form of complex data. The experimental setup for the detection of moisture content with a rectangular patch antenna is presented in Figure 1(a). The setup consists of a concrete specimen with the embedded antenna, which is connected to Vector Network Analyzer (VNA) and a PC with a bespoke LabVIEW program to control VNA and capture data. The specimen was submerged in water in order to increase the moisture content of the specimen and then was placed on a digital weighing scale to monitor the drying off process. Both specimen weight and microwave signal from the embedded antenna were recorded every 5 min for a period of 48 h. The data analysis presented a strong linear relationship between the weight loss of the specimen and changes in amplitude of the microwave signal from the embedded antenna, with R\({}^{2}\) = 0.98. Figure 1(b) shows the experimental setup for the detection of cracks in RC concrete specimens. The SRR antenna was placed 100 mm from the bottom of the concrete specimen in order to capture the crack propagation in the tensile zone just below the neutral axis of the specimen. In addition, the strain gauges were attached to the tensile reinforcement bars to measure the vertical deflection of the specimens while a load cell was used for the recording of the load increments applied by a calibrated hydraulic 100 kN load actuator. Synchronized data acquisition systems were used for the structural response measurements. The full captured microwave signal (1-10 GHz) was analyzed in order to investigate a  
Figure 2: (**a**) Embedded Rectangular patch antenna setup for moisture detection [19], (**b**) Embedded SRR antenna setup for crack detection [20]. relationship between the crack propagation and the signal.