## 2 Experimental Section
### Testing the Effects of the Sensor Device on the Concrete  
After the data collection, the concrete columns produced with concrete M1 were used to further investigate the effects of the sensor device on the concrete properties. Two of them were used to verify the effects of the embedded devices on the compressive strength of the concrete and the third one to verify the influence of the device on the distribution of concrete around the sensor.  
To test the compressive strength, two cubes with dimensions 320 mm \(\times\) 320 mm \(\times\) 320 mm were cut from two of the reinforced concrete columns (Figure 6). The cube cut from the upper part of the column contained the two housing devices embedded in the column, while the cube cut from the middle part of the column was fully composed of reinforcement and concrete, without any devices. The cubes were tested 99 days after the casting of the columns. A Wolpert testing machine with a load capacity of 10,000 KN was used, as shown in Figure 7.  
Figure 4: Reinforcement cages of the concrete columns before placing in the formwork. The hardware devices are highlighted.  
Figure 5: Details of the reinforcement cage.  
To investigate possible influences of the presence of the housing device on the distribution of concrete inside the column and possible increase in the air content/voids near the devices, a non-destructive and three-dimensional technique such as micro-computed tomography (micro-CT) was required [36]. A cylindrical core with a diameter of 115 mm and height of 320 mm was drilled from the upper part of the third column produced with concrete M1. The cylinder was composed of two regions: one affected by the presence of the housing device, and another without the device. The cylinder was then scanned using the TESCAN CoreTOM system equipped with a 180 kV X-ray tube and a large 2856 \(\times\) 2856 pixels flat panel detector (see Figure 8). A stack of 4 scans of the core were taken to cover the entire volume, reaching a resolution of 90 \(\upmu\)m. A 1.5 mm Cu filter, 180 kV, and 90 W were required to reduce the noise given by the steel reinforcement of the concrete. A total of 2142 projections with an exposure of 650 ms were captured per scan. The reconstruction of the volume was achieved using the Panthera 1.4.4 software by TECCAN XRE, and the image analysis was carried out using Avizo 2020.3.  
The quantification of the air voids was carried out by separating the reconstructed volume into two regions: region 1, containing the housing device; and region 2, formed by the bulk of the reinforced concrete core. A total of 1520 slices were used for the analysis of region 1, and 1204 for region 2. Firstly, a segmentation of the voids was performed to isolate them from the rest of the core. Afterwards, each void was assigned geometric properties to quantify the total volume per region. Lastly, a distribution of the different sizes of air voids was obtained.  
Figure 6: Schematic of the cutting of the columns to obtain the testing cubes. The circle-Transportation hook.  
Figure 7: Testing machine used for assessing the compressive strength of the cubes.  
This investigation was justified based on preliminary tests that showed a significant influence of the hardware on the distribution of concrete and air voids in the areas surrounding the device when it was embedded in a concrete cube with side length of 150 mm (Figure 9).