# Influence of Cracking on Oxygen Transport in UHPFRC Using Stainless Steel Sensors
## 4 Conclusions  
Several conclusions can be drawn from this study:  
* The stainless steel voltammetric sensor used in this work and the Id.O\({}_{2}\) parameter were able to detect variations in the availability of oxygen and, thus, detect differences in size of cracks produced in reinforced concrete in the range of 0.1-0.3 mm, as well as smaller cracks in the range of microns. This sensor was also able to detect differences in crack pattern for a same concrete type, depending on the zone that is measured. The study of the sensitivity of the sensor at different locations in UHPFRCs shows that oxygen availability decreased while going towards the upper part of the beam, as the crack width was thinner.
* In uncracked conditions, HPC and UHPFRCs improved the air permeability coefficient by three orders of magnitude, and the absorption and porosity to water by one order of magnitude. For cracked conditions, the air permeability coefficient for cracked concrete has been estimated through a correlation model. The obtained results show that cracked UHPFRCs improved 65-80% the values of uncracked HPC and 98-99% the values of uncracked SQC. This improvement is produced by the compacity of UHPFRCs matrices and their diffused micro-crack pattern.
* Oxygen availability, as measured in UHPFRC and UHPFRC-n with the stainless steel sensor, indicates that cracked UHPFRCs registered improvements in the Id.O\({}_{2}\) parameter in the range of 20-30% comparing to uncracked HPC and 50-60% when comparing to uncracked SQC.
* Increasing the damage level or strain in HPC beams increased the presence of oxygen in 5% for LS and 20% for HS when compared to the uncracked level. In the case of UHPFRCs, the beams cracked to LS conditions showed the same oxygen availability than uncracked beams. However, the beams cracked at the HS level experienced increments between 11 and 19% for UHPFRC and UHPFRC-n.  
* This work demonstrates that multi-cracking patterns are more beneficial in terms of air transport than single crack patterns for a comparable strain level, especially for small strain levels, and that using these concretes in construction will potentially produce high durability structures with an extended service life, even after suffering cracks.  
Future studies that can be derived from this work may focus on establishing mathematical models of cracking and oxygen presence in concrete with variable fibers content, as well as in evaluating the internal tortuosity and crack width variation in the interior of concrete by means of the availability of oxygen in different areas, or while analyzing the effects of this different oxygen availability in the corrosion processes produced.  
Conceptualization, P.S. and M.V; methodology, P.S., A.M.-I; formal analysis, A.M.-I. and M.R.-F; investigation, A.M.-I., E.J.M.-A. and J.L.-F.; resources, P.S. and M.V.; writing--review and editing, M.R.-F and A.M.-I; supervision, P.S. and M.V.; project administration, M.R.-F; funding acquisition, A.M.-I., M.V., P.S. and M.R.-F. All authors have read and agreed to the published version of the manuscript.  
The authors would like to express their gratitude to the Spanish Ministry of Science and Innovation for the pre-doctoral scholarship granted to [PERSON] (FPU 16/00723), to the Universitat Politecnica de Valencia for the pre-doctoral scholarship granted to [PERSON] (FPI-UPV-2018), and the European Union's Horizon 2020 ReSHELience project (Grant Agreement No. 760824).  
The authors declare no conflict of interest.