# Ultrasonic Pulse-Echo Signals for Quantitative Assessment of Reinforced Concrete Anomalies
## 2 Experimental Setup  
Voids are popular defects that occur in concrete due to many reasons, such as poor consolidation of the concrete or other construction errors. Expanded polystyrene (EPS)foam has been widely used to represent the artificial voids, as it has the same dielectric constant (1.02-1.04) as air [55; 56; 57; 58]. The low density of foam allows for the ultrasonic waves to be reflected in a similar manner to the presence of air. In this research, the testing slabs (The Ultrasound Tomographer called MIRA, manufactured by Germann Instruments (Evanston, IL, USA) was used to conduct the tests) as shown in Figure 2a, which is same as the one in [35]. A specimen of reinforced concrete (RC) slab was designed with two layers of #5 steel rebars with a spacing of 203 mm and artificial defects. The specimen included a pair of foam voids to simulate embedded void scenarios. The dimensions of the RC deck slab specimens were precisely measured to be 114.3 cm in width, 121.9 cm in length, and 17.8 cm in thickness. The artificial voids, depicted as V1 and V2 in Figure 2a, were created using rectangular prisms of expanded polystyrene (EPS) foam, which possessed the same dielectric properties as the surrounding air. The artificial void was 102 \(\times\) 30 mm (width \(\times\) thickness), and the top surface of the foam was placed 83 mm down from the top surface of the deck specimen. This approach allowed for us to accurately simulate the behavior of the voids in the slab and evaluate their impact on the overall performance of the structure. Ready-mix concrete was used from in-transit mixers for casting the RC slab, which was subsequently cured and tested under laboratory conditions. A shear-wave pulse velocity of 2450 m/s was assumed. The specimen was made from normal concrete with a concrete strength of 27 MPa, and small-sized elements less than 10 mm to avoid wave scattering by the aggregates were adopted for the experiment.  
Figure 2b depicts the commercial UPC device, an ultrasonic low-frequency, shear-wave tomography device that rapidly images the subsurface concrete condition. The device boasts 48 dry-point-contact (DPC) transmitting and receiving transducers with ceramic wear-resistant tips arranged in a matrix. Its antenna array comprises 12 channels, C1 through C12, at a longitudinal spacing of 30 mm, with each channel featuring four transducers at a transverse spacing of 25 mm. Each transducer can transmit and receive low-frequency (55 kHz) shear waves.  
Utilizing DPC transducers, the device mentioned above can provide a consistent level of impact and wavefront penetration for diagnostics up to 3 ft deep while dealing with concrete surface textures. The device uses a shear-wave pulse velocity of 2450 m/s for concrete for the SAFT reconstruction. Figure 3 illustrates the fundamental principles of the UPE device.  
Figure 2: (a) Reinforced concrete deck slab specimen before concrete placement; (b) front panel of the UPC device and 48 low-frequency dry-point-contact transducers. Reproduced from [35], licensed under CC BY 4.0 (Deed—Attribution 4.0 International—Creative Commons).