# Influence of Cracking on Oxygen Transport in UHPFRC Using Stainless Steel Sensors
## 3 Results and Discussion
### Discussion: Oxygen Availability Relation with Porosity and Permeability  
From the experimental campaign 1, the value of the Id.O\({}_{2}\) parameter inside the uncracked beams has been compared to the results of porosity accessible to water, water absorption, and air permeability. Figure 8: Influence of the strain level (damage level) on the presence of oxygen for the concretes studied. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{3}{c}{**\(\Delta\%\) vs. SQC\_Ref**} & \multicolumn{3}{c}{**\(\Delta\%\) vs. HPC\_Ref**} \\ \cline{2-6}  & **HPC** & **UHPFRC** & **UHPFRC-n** & **UHPFRC** & **UHPFRC-n** \\ \hline
**Ref** & 36.05\% & 57.32\% & 57.03\% & 33.26\% & 32.81\% \\
**LS** & 33.10\% & 56.97\% & 57.16\% & 32.71\% & 33.01\% \\
**HS** & 23.24\% & 49.30\% & 52.09\% & 20.72\% & 25.07\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Percentual improvement of Id.O\({}_{2}\) as compared to uncracked SQC\_Ref and HPC\_Ref. The concrete types with higher permeability are those with a higher amount of pores and higher values in the Id.O\({}_{2}\) parameter, since the transport of gaseous agents is easier in porous concretes. Figure 9a,b represent the values of porosity accessible to water and absorption with the Id.O\({}_{2}\) parameter. Because of the relevance of the air permeability coefficient, which is a common parameter with standard procedures for its evaluation in concrete, its correlation with the Id.O\({}_{2}\) parameter has been studied in depth in this section. Two additional concrete types that were analyzed in [17] have been added to this study to increase the robustness of this analysis. Figure 10 displays the resulting curve. The exponential curve adjusted to these points shows very good fitting (R\({}^{2}\) = 0.98). The equation that was obtained through this correlation allows for estimating the air permeability (K) parameters for the four concretes of this study in the three conditions studied (Ref, LS and HS). Using this equation, an estimation of the effective air permeability has been calculated in the different situations analyzed. Figure 11 represents the values of K that were calculated through the equation that was obtained from the correlation in Figure 10 while using the Id.O\({}_{2}\) average values that were obtained (Figure 7). These values are consistent with those that were provided in [24; 26], where they indicate that an UHPFRC will always have in standard conditions K \(<10\times 10^{-19}\) m\({}^{2}\). Figure 10: Correlation between the parameters Id.O\({}_{2}\) and K air permeability coefficient (UNE 83981). Figure 9: (a) Curve Id.O\({}_{2}\) vs porosity accessible to water (UNE 83980) (b) Curve Id.O\({}_{2}\) vs water absorption (UNE 83980). Figure 12a shows the improvement produced in the estimated K parameter in UHPFRC, UHPFRC-n and HPC as compared to SQC in uncracked state (3). The permeability of UHPFRC and UHPFRC-n in uncracked state improves by 99% that of an SQC concrete. For the cracked beams (either at LS or HS levels), the value that was obtained for the estimated K parameter still produced an improvement when compared to uncracked SQC of a similar magnitude. When comparing the percentual increase to uncracked HPC (4), as shown in Figure 12b, the results indicate that UHPFRCs produce improvements of 80% in Ref and LS conditions and around 65-75% in HS conditions. \[\Delta\%vsSQC_Ref=\frac{K_{SQC_Ref}-K_{i\_j}}{K_{SQC_Ref}} \tag{3}\]  
\[\Delta\%vsHPC_Ref=\frac{K_{HPC_Ref}-K_{i\_j}}{K_{HPC_Ref}} \tag{4}\]  
where \(i\) is the concrete type and \(j\) the cracking condition. Figure 11: K coefficients that were estimated through the correlation from Figure 10.  
It is clear that UHPFRC concrete types produced a clear durability improvement in the cracked state compared to HPC, due to their significantly lower air permeability, after the results that were obtained in Figures 11 and 12.