## 3 Materials and Methods
### Inspection  
**Microscopy**: A Dino-lite AM4113T handheld digital microscope was used to capture high-resolution 1.3-megapixel images of the cracked zones at the concrete surface. The surface was scanned considering a wide magnification range (up to 200\(\times\)) and using all 8 LED illumination probes. The images were captured and stored with the Dino-Capture 2.0 software. **Water permeability**: A water basin was developed in-house by means of poly (methyl methacrylate) (PMMA), the design was chosen according to the testing procedure and is illustrated in Figure 3. The inner dimensions of the basin were 300 \(\times\) 100 \(\times\) 80 mm\({}^{3}\). Before testing, the basin was positioned at the bottom side of the beam, centered to the middle zone and sealed with silicone glue. A hole was drilled through the basin top plate, and a measuring tube with an inner diameter of 12 mm was screwed on the basin (Figure 3a). The basin was fully filled with water through the tube at the top (Figure 3a) to ensure enough water pressure was built at the crack face during testing. The tube was aligned vertically before testing (Figure 3b) and the test started when the water in the tube was up to 200 mm high. Starting from this point, the water level drop was tracked by a chronometer. The time at which the water level dropped by 100 mm was recorded and the permeability coefficient (k in m/s) was obtained accordingly [(] (Equation (2)):  
\[\mathrm{k}=\frac{\alpha T}{At}\ \ln\left(\frac{h_{0}}{h_{f}}\right) \tag{2}\]  
Figure 2: Testing setup with AE sensors 1-8 marked in red. where \(\alpha\) is the tube cross-section (m\({}^{2}\)); \(A\) is the water-beam contact area (m\({}^{2}\)); \(T\) is the beam height (m); \(t\) is the measured time (s); and \(h_{0}\) and \(h_{f}\) are the initial and final water levels, respectively (m). **Ultrasonic pulse velocity (UPV)**: The velocity of ultrasonic pulses through the beam was measured using the Controls PULSONIC portable device according to ASTM C597. The transmitter probe was mounted to the beam side as illustrated in Figure 2 and the receiver was mounted at the other end of the sample. This way the emitted pulses travelled through the beam's length and transversely crossed the cracked plane. The pulse had a magnitude of 2500 V and frequency of 54 kHz, recorded in a sampling rate of 10 MHz. The transit time (\(\Delta t\) in s) was measured and the pulse velocity (UPV in m/s) was obtained taking into account the wave propagation distance (\(\Delta\)x) equal to 0.85 m (Equation (3)):  
\[\text{UPV}=\frac{\Delta\text{x}}{\Delta t} \tag{3}\]  
The crack opening was assumed negligible relative to the beam length, therefore it was not considered when defining the wave propagation distance. **Acoustic emission (AE)**: An array of eight AE transducers were mounted on the concrete surface using Vaseline couplant and the transducers were fixed with magnetic holders. The probes had a resonant frequency response at 150 kHz and recorded the damage-emitted signals pre-amplified by 40 dB. An amplitude threshold was set at 35 dB to eliminate the ambient noise and the frequency bandwidth was fixed from 20 kHz to 1 MHz. The waveforms were recorded with a sampling rate of 1 MSPS. The DiSP-24 monitoring device was equipped with a PCI/DSP-4 data acquisition board and PowerPAC software provided by Physical Acoustics. AEWin software was used to process the recorded AE data and the post-processing was completed with Noesis software. Three-dimensional AE events localization analysis was performed considering the AE transducers position (Table 3) and assuming a constant wave propagation velocity equal to 4000 m/s. The velocity was obtained by measuring the transit time between two sensors following Hsu-Nielsen excitation (pencil lead break) [20] at an intact state.