# Influence of Cracking on Oxygen Transport in UHPFRC Using Stainless Steel Sensors
## Materials and Methods
### Methodology and Experimental Program in Cracked State
#### 2.3.4 Voltammetry Analysis to Detect Cracks  
Cyclic voltammetry was used through custom-made fit-for-the-purpose voltammetric sensors once the beams were pre-cracked [17]. These sensors were used to detect the presence and size of cracks through the availability variation of oxygen.  
Briefly, these sensors work in the following way: the electric current response that is obtained during the application of cyclic voltammetry is processed in the potential range associated to the oxygen reduction. In this potential range, the electric current is directly associated to the concentration of O\({}_{2}\) inside of the material [17]. The analysis of the electric response allows for the definition of a parameter, named Id.O\({}_{2}\), which is directly proportional to the ratio between the change in the electric current (logarithmic form) and the change in potential (Id.O\({}_{2}\) = \(\Delta\)logi\(\parallel\)/\(\Delta\)V). Therefore, this parameter is correlated with oxygen availability, as demonstrated in a previous work [17]. Currently, the authors are working on a theoretical model for quantifying the concentration of oxygen from the results that were obtained with stainless steel voltammetry sensors. The results that are presented in this work show an initial experimental study to verify the relation between the Id.O\({}_{2}\) parameter and the transport parameters of concrete in the different cracking states.  
The cyclic voltammetry was used through the Autolab PGSTAT10 equipment, working with three electrodes and while using a saturated calomel electrode (SCE) as the reference electrode. A stainless steel plate sized \(100\times 750\) mm\({}^{2}\) was used as a counter electrode (CE). The CE was l externally coated, but electrical contact between the concrete beam and the plate was ensured through a wet cloth. The sweeping potential applied was in the range between 0.6 V and \(-1.1\) V vs SCE.  
Each measurement was repeated three times to have statistical certainty in each condition. During the test, the temperature and humidity conditions were kept controlled and constant (T = 25 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C, RH \(\approx\) 60%).  
The reference position was the difference in the voltammetric test in each campaign. In the first experimental campaign, the reference electrode was always located in the middle span of the beam, at the height of sensor L. Conversely, in the second experimental campaign, the reference electrode was put in the same position and, additionally, in those spots that are indicated in Figure 3, to study the effect of changing the position of the sensor between the different sectors and heights. The relevance of this study is justified, because the closest area to the sensor is the one that will have more weight in the electrochemical reactions that will be produced on the surface of the sensor (oxygen reduction), closer to the position with the reference electrode.  
Figure 6: Comparison of the strain obtained at the bottom row (\(\varepsilon_{\mathrm{L}}\)) and middle height row (\(\varepsilon_{\mathrm{M}}\)).