# Influence of Cracking on Oxygen Transport in UHPFRC Using Stainless Steel Sensors
## Materials and Methods
### Methodology and Experimental Program in Cracked State
#### 2.3.3 Pre-Cracking Procedure  
A nonstandard test methodology was developed to generate a controlled crack state in a reinforced beam, based on a methodology that was previously proposed in [25]. After 28 days, the beams were  
Figure 4: Molds and stainless steel sensors placed before and after pouring concrete.  
Figure 3: Diagram showing the three sensors levels and four sectors of the study.  
pre-cracked while using a four-point bending test to produce cracks in the middle span (Figure 5 left). Before starting the test, two rows of small disks were glued to one side of each sample to measure the strain variation with a Demountable Mechanical Strain Gauge (DEMEC) before, during, and after loading (Figure 5).The procedure for controlling crack width was to calculate the strain at six intervals separated by a distance of 100 mm and, from these values, to calculate the average strain (\(\varepsilon\)). The intervals between points 1-5 will be used to calculate the strain value at the depth of the stainless steel sensor L (\(\varepsilon_{\text{L}}\)) and points 6-10 will be used to calculate the same parameter at the depth of sensor M (\(\varepsilon_{\text{M}}\)). An LVDT was also placed in the middle span to measure the vertical displacement during the test.  
The beams were pre-cracked at two damage levels, named low strain (LS) and high strain (HS) levels. Some beams remained unloaded as the reference beams state (Ref). The two strain levels are defined as:  
* Low Strain level (LS): average strain \(\varepsilon_{\text{M}\text{v}\text{g}}\) at middle height of the beam around 0.5%o as measured in loaded conditions. This level represents a condition inside the service limit state, in which a crack has been produced and it has been closed afterwards. This level produced one or two localized cracks of widths below 0.1 mm in traditional concretes and a series of very closed micro-crack of 10-15 microns in UHPFRCs at the reinforcement level.
* High Strain level (HS): average strain \(\varepsilon_{\text{L}\text{v}\text{g}}\) at the reinforcement level between 1%o and 2%o as measured after unloading. The condition used in practice was when the averaged strain of the three intervals exceeded the value of 1.5%o, since strain was measured between three intervals. This level represents a condition beyond the service limit state, with yielded or almost yielded rebars. This level produced localized cracks between 0.2-0.3 mm in traditional concretes and multiple micro-cracks of 25-50 microns in UHPFRCs at the reinforcement level. This cracking level represents a highly demanding condition, which is common for concrete structures in aggressive environments.  
Figure 6 shows a comparison of the average strain values that were obtained at the lower row (\(\varepsilon_{\text{L}}\)) and at the medium row (\(\varepsilon_{\text{M}}\)) for all of the beams tested. The points of all the concrete types are clearly aligned following a linear trend, where the strain obtained at the bottom row is 1.78 times the strain obtained at the middle row. One of the points obtained at the HS level suffered higher than targeted strains due to the difficulties in controlling the process around the yielding point of the rebar.  
Figure 5: Four-point bending test setup (**left**) and Demountable Mechanical Strain Gauge (DEMEC) points location (**right**).