# Influence of Cracking on Oxygen Transport in UHPFRC Using Stainless Steel Sensors
## 3 Results and Discussion  
### Experimental Campaign 1: Effect of Concrete Type on Cracking through Oxygen Availability  
In this campaign, the absolute average value of the parameter Id.O\({}_{2}\) is compared to the different strain levels (and thus crack sizes) for the four concrete mixes tested. The differences in the Id.O\({}_{2}\) parameter obtained for HPC and UHPFRCs solely depend on the cracking pattern, which is multi-cracking in the case of the UHPFRCs and single-crack pattern with only \(\sim\)2 cracks in HPC, since the strain levels and total crack width obtained were similar for each strain level. Figure 7 compares the average values of the Id.O\({}_{2}\) parameter obtained for each concrete type at the three strain levels (Ref, LS, and HS), as well as the standard deviation. These results show that the Id.O\({}_{2}\) parameter was able to discern between the different levels in oxygen availability of the four concrete types, as well as detect the increase in oxygen presence when cracks or microcracks were produced in the concrete beams. Analyzing the Id.O\({}_{2}\) results that were obtained (Figure 7), SQC in uncracked state has the highest availability of oxygen, followed by HPC, and UHPFRCs obtain the lowest availability. Regarding the damage level (strain level) in HPC, it is clear that increasing the strain from Reference level to LS or HS increases the presence of oxygen (5% for LS and 20% for HS), which is a clear indicator of the presence of cracks in the beam. In UHPFRCs, either this parameter is not sensitive to the LS level or the cracks are not able to transport oxygen. Thus, the LS condition in UHPFRCs was very similar to an uncracked situation in terms of oxygen availability inside the matrix. However, at the HS level, the Id.O\({}_{2}\) parameter experienced increments of 11.5 % for UHPFRC-n and 18.8% for UHPFRC as compared with the value that was obtained for uncracked beams (Ref). Despite the increase in oxygen availability that UHPFRC beams suffered in the HS condition when compared with uncracked UHPFRC beams, they obtained smaller Id.O\({}_{2}\) values than the uncracked HPC, and clearly much smaller than uncracked SQC. These results indicate that cracked UHPFRC might have higher durability than the uncracked HPC. This higher durability would be even higher if the crack levels were maintained similar to LS. It should be highlighted that the variation between beams of a same concrete and pre-cracking level was small, as seen in the standard deviation values, which show the similarity of the pre-cracking levels that were obtained in a specific concrete and the robustness of this parameter to evaluate oxygen availability. The coefficient of variation of the results that were obtained for each pre-cracking condition and the measurement repetitions performed were below 11% in all cases, which is a very good value when considering the heterogeneities of concrete. Figure 8 shows the relation between the individual strain level reached in each beam after unloading and the O\({}_{2}\) presence that was obtained in that beam through the stainless steel sensor. These results show a linear increase in the presence of oxygen when increasing the strain level for HPC,  
Figure 7: Absolute average values Id.O\({}_{2}\) parameter for each analyzed scenario. UHPFRC, and UHPFRC-n, with a very similar slope for the three concrete types. This result confirms the suitability of the stainless steel sensor that was used to evaluate the presence of oxygen in different concrete types, either from a FRC family or from a traditionally reinforced concrete. Additionally, when comparing UHPFRC-n and UHPFRC, it seems that the nano-alumina and crystalline admixtures provided a slight improvement under the HS conditions (seen as a lower oxygen presence for the HS strain levels), despite the extra amount of water that was used in this mix. However, this result would need further studies to be confirmed.