# Contactless Ultrasonic Wavefield Imaging to Visualize Near-Surface Damage in Concrete Elements
## 4 Discussion  
The measured ultrasonic wavefield data seen in Figure 4 demonstrate that near-surface concrete damage introduced by mechanical impacts set up distinct non-propagating oscillatory fields nearby the impact locations. Such non-propagating oscillatory fields can be considered multiply reflected wave events from multiple scatterers (e.g., cracks) that are densely gathered within a localized region  
Figure 8: Extracted non-propagating oscillatory fields: (**a**) Case 0 (pristine), (**b**) Case 1 (D1 only), (**c**) Case 2 (D1 and D2), (**d**) Case 3 (D1 to D3) and (**e**) Case 4 (D1 to D4). The wavefield images each represent a 250 mm (horizontal) \(\times\) 49 mm (vertical) scan area. \(\mathcal{N}^{\prime}\) indicates impact damage locations.  
Figure 9: Normalized non-propagating wave energy maps \(E_{NPO}^{N}(x,y)\): (**a**) Case 0 (pristine), (**b**) Case 1 (D1 only), (**c**) Case 2 (D1 and D2), (**d**) Case 3 (D1 to D3) and (**e**) Case 4 (D1 to D4). Each scan area represents a 250 mm (horizontal) \(\times\) 49 mm (vertical) scan area.  
of a medium [19]. Considering the strong oscillatory responses observed nearby the impact locations, especially D1 and D2, the damage is presumed to be present in the form of multiple scatterers (presumably cracks) densely gathered nearby the impact points rather than by surface pits. If the impact damage was present in the form of surface pits, the scattered wavefields would have been in the form of propagating waves (e.g., back-scattered surface waves or radiated surface waves from the pits).  
The localized nature of the non-propagating oscillatory fields causes magnitude spectra broadening in the wavenumber domain as shown in Figure 5. Because of their localized nature, the non-propagating oscillatory fields enable localization (or visualization) of concrete damage within a scan area. The normalized non-propagating wave energy maps shown in Figure 9 demonstrate the potential of the proposed approach to detect and to visualize near-surface concrete damage.  
We assert that the presented approach is insensitive to small variations of signal processing parameters, in particular the nature of the filtering masks \(M_{FP}\) and \(M_{NPO}\). As long as \(M_{FP}\) suitably covers the wavenumber range where most forward propagating wave components are present, the filtered responses and the corresponding damage visualization maps should not vary significantly.  
In the experiments, we introduced near-surface concrete damage using mechanical impacts. Impact-based damage is suitable for laboratory investigations. However, these artificial damage types differ from real-world concrete damage (e.g., those caused by deterioration mechanisms) in damage morphology, geometric connectivity, and dimension. Such discrepancies in geometric features of damage may cause important discrepancies in scattered wavefields. Further research on ultrasonic wavefield imaging applied to damaged concrete by realistic deterioration mechanism is needed. We are currently addressing this issue by conducting experiments on concrete samples subjected to realistic damage-promoting environments including that caused by alkali-silica reaction and freezing/thawing cycles.  
The wavefield imaging hardware presented in this study should be further improved for industrial or field applications. Also, quantitative damage characterization (e.g., crack sizing) should be carried out for in-depth characterization of detected damage. We set these topics as our future work.