# Ultrasonic Pulse-Echo Signals for Quantitative Assessment of Reinforced Concrete Anomalies
The current commercial UPE devices have enabled the rapid acquisition of shear-wave echoes of concrete. The attenuation by ultrasonic scattering depends on the ratio of the wavelength to the diameter/dimension of the scatterer. Some studies demonstrated that the scattering attenuation of shear waves is less significant at frequencies lower than 50 kHz for commonly used aggregate of size 15 mm in concrete. As such, by using current UPE devices, concrete can be treated as a homogeneous medium with a single constant shear-wave velocity, and the standard SAFT is employed in the UPE equipment to detect voids, imperfections, and defects in concrete structures [22; 23; 24; 25; 26; 27; 28; 29; 30]. Ref. [31] used a focusing method (TFM) and SAFT concepts for enhancing multimodal 2D imaging of concrete structures. Ref. [32] used M-distance and linear discriminant analysis to classify the imagines of SAFT. The standard SAFT's results can be further improved by introducing a few developments, such as calibrating the sagging in the resulting images due to the long wavelength of the pulse [33; 34], the extended SAFT methods and applications [35; 36; 37; 38; 39; 40; 41; 42], the AI-basedalgorithms applications [43; 44], and ultrasonic image by using both linear and non-linear wave properties [45]. Two-time indices are called \(t_{1}\) and \(t_{2}\) in the ultrasonic transmission, reflection, and receiving process. By precisely analyzing the designated timings, we can effectively discern reflections originating from the front surface and any imperfections (Figure 1). This pulse type is instrumental in generating an A-scan that meets rigorous quality standards. The data derived from this scan are the time differential \(t_{f}\), which is represented as \(t_{f}=t_{2}-t_{1}\) and commonly known as the ultrasonic time-of-flight (TOF). Knowing the propagation speed of bulk waves in the material, the TOF is invaluable in determining the location of the defect. TOF-based methods have proven effective in detecting, locating, and sizing faults in ultrasonic nondestructive testing and evaluation [7; 11]. However, echo signals in UPC testing often contain significant noise. The intensity of this noise can vary depending on the medium's properties and the distance of ultrasonic wave transmission, resulting in low signal-to-noise ratios and peak variations in the time domain. These factors pose a challenge in accurately locating and extracting peak amplitudes from defect echo signals. Various signal processing techniques enhance the detection capabilities of ultrasonic NDT applications, where signals are often mixed with noise. The ultrasonic signals should be denoised, which is feasible using short-time Fourier transform, wavelet transform, improved wavelet transforms, adaptive filtering, and empirical mode decomposition (EMD) [46; 47; 48; 49; 50; 51]. Each technique has its strengths and weaknesses, which are considered when selecting the appropriate method for the given application. Originating from concrete defect echoes in UPC testing, the resulting intense noise makes identifying and quantifying defect echoes difficult. Therefore, the conventional averaging and filtering techniques could be more helpful for pulse-echo noise reduction. Denoising assists with filtering the noise components and retaining the original signal's details. The most used denoising algorithm thus far is the wavelet threshold algorithm. However, the wavelet threshold denoising has a severe drawback. It requires appropriate wavelet base, threshold, and decomposition level values, thus allowing for different parameters to affect the denoising performance remarkably. Empirical mode decomposition (EMD) works for non-linear, stochastic, and non-stationary signal processing. The EMD denoising algorithm does not require presetting the base and decomposition level and is more adaptable than the wavelet threshold algorithm. Furthermore, the EMD denoising algorithm can result in high-frequency resolution.