# Recent Advancements in Non-Destructive Testing Techniques for Structural Health Monitoring
## 2 Sweep Frequency Technique  
The sweep frequency technique uses electromagnetic waves. Electromagnetic (EM) waves are defined as the energy that propagates through space at the speed of light [5]. EM waves are formed of oscillating electric and magnetic fields where the magnetic field is oriented orthogonal to the electric field. The electromagnetic frequency range can be divided into separate bands, with low-frequency (long wavelengths) radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the high-frequency (short wavelengths). The EM waves in each of those bands have different characteristics, namely how they are produced, interaction with measured material, and their practical applications [5,6]. The radio and microwaves are actively investigated for the purpose of detection of various anomalies in materials under test [7]. Microwaves are EM waves with wavelengths ranging from one meter to one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz [8,9]. The benefit of this part of the EM spectrum is it is none ionizing and providing high depth penetration through dielectric materials offering great potential to be commercialized and applied across several industrial sectors. This technique has been broadly used for water and wastewater quality control [10,11], detection of substances deposited on various materials [12,13], cultural heritage [14] and healthcare applications [15]. The Sweep Frequency Technique (SFT) is a microwave-based method that was investigated and proposed as a non-destructive measurement approach for structural health monitoring (SHM). The proposed SFT technique consists of two wideband horn antennas (transmitter and receiver), Vector Network Analyzer (Rohde & Schwarz ZVL13) and PC with in-house developed LabVIEW graphical user interface (GUI). The antennas are mounted on a bespoke frame that provides an ability to adjust the angle between the antennas and control the distance between the system and measured material under test (MUT). The antennas are connected to the vector network analyzer (VNA), which is controlled via the GUI. The SFT system operates in the frequency domain and is set to sweep frequency from 2 GHz to 13 GHz. Measurements of the system are provided by S-parameters (S\({}_{21}\)), i.e., the incident wave is generated by the VNA and sent to the transmitter antenna via coaxial cable. The reflected signal from the MUT is captured via the receiver antenna and analyzedby the LabVIEW GUI in real-time. The dimensions of the bespoke frame with antennas and block diagram of the SFT system is presented in Figure 1.  
The SFT was used for monitoring the excess moisture content caused by waterproof membrane defects [17; 18] and chloride level [16] in concrete structures. [PERSON] et al. [18] has first reported the use of the SFT for the detection of water infiltration for concrete roof structures. In the study, researchers replicated the concrete flat roof designs according to Malaysian standards and ran a series of experiments to monitor water infiltration, rebar location and membrane fault detection. The initial experimental work was undertaken with two flat roof specimens with and without reinforcement rebars. The obtained results demonstrated significant shifts in both frequencies (from 3.8 GHz to 4 GHz) and amplitude (from \(-\)38 dBm to \(-\)24 dBm) of the reflected signal from specimens with and without rebar, respectively. The changes were caused by more microwave energy is being able to penetrate through the specimen without rebar, i.e., microwaves are not able to penetrate through steel rebar, so more energy is reflected in the case of the reinforced specimen. The second study was carried out to detect membrane failure on concrete roof structures. Two concrete specimens (with and without rebar) were used to test the technique.