# Electrochemical Diagnostics of Sprayed Fiber-Reinforced Concrete Corrosion
## Laboratory Tests  
### Test Specimens and Methodology  
For the test, 12 slab specimens were prepared to estimate the probability of corrosion and corrosion activity of the reinforcement (A), 12 rectangular specimens to determine the shrinkage (B), and 6 cube specimens to determine compressive strength (C). In each group, half of the specimens were made of plain concrete (PC), and the other half were made of fiber-reinforced concrete with randomly dispersed steel micro-reinforcement fibers (SFRC). All specimens were collected according to standard [44] from the same concrete mix; 1 m\({}^{3}\) of the mix contained the following ingredients: CEM I cement--384 kg; sand--680 kg; gravel 2/8--600 kg; gravel 8/16--650 kg; water--160 L, and plasticizer (0.6%). The \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Criteria for Assessing the Reinforcement Corrosion Risk Level**} \\ \hline Reinforcement stationary potential, & \(>\)\(-\)200 & 5\% probability of corrosion \\ E\({}_{\text{st}}\) [mV] & \(-\)350\(--\)200 & 50\% probability of corrosion \\  & \(<\)\(-\)350 & 95\% probability of corrosion \\ Concrete cover resistivity, & \(\geq\)20 & low probability of corrosion \\ \(\Theta\) [k\(\Omega\)\(\mathrm{cm}\)] & 10–20 & medium probability of corrosion \\  & \(\leq\)10 & high probability of corrosion \\  & \(<\)0.5 & no corrosion activity is expected \\ Corrosion current density, & 0.5-2.0 & negligible corrosion activity \\  & 2.0-5.0 & low corrosion activity \\  & 5.0–15.0 & moderate corrosion activity \\  & \(>\)15.0 & high corrosion activity \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Criteria for assessing the reinforcement corrosion risk level [40,43]. Figure 5: Diagram of connection of GP-5000 GalvaPulseTM with diagnosed reinforcement: (**a**) sketch [own study on the basis of Reference [43]]; (**b**) measurement on a specimen. water/cement ratio was 0.43. In the SFRC specimens, the concrete mix was reinforced with steel BauMix 60/1 fibers making up 1% vol. of the mixture; the parameters of the fibers were as follows: fiber length l\({}_{\mathrm{w}}\) = 60 mm; fiber diameter \(\emptyset\) = 1.0 mm; shape--hooked-end (Figure 6). This type of fibers was chosen due to their widespread use. In addition, longer fibers seemed more exposed to corrosion. The content of fibers in the mixture (1%) was chosen for its efficiency as well as the workability of the mixture and its use as a spraying concrete. The specimens for the estimation of corrosion (A specimens, including A1-PC specimens and A2-SFRC specimens) had a surface area of 210 \(\times\) 228 mm and a thickness of 100 mm. In each specimen, two parallel ribbed \(\Phi\)8 mm BST 500 steel rods were cast (Figure 7). The rods had equal 70 mm clearance from the edge of the specimen. There were no transverse rods. The concrete cover thickness was 25 mm. The measurements of the estimation of corrosion probability on the specimens surface, and the corrosion activity of the reinforcement, were performed on A specimens in two stages: stage I--preliminary measurements (reference measurements) taken after casting the specimens; stage II--measurements after 120 freeze--thaw cycles of specimens in 3% NaCl solution (the specimens were immersed in the NaCl solution so that the chloride ions could penetrate into the concrete cover in the direction of reinforcement, which was supposed to simulate the processes taking place in reality). The measurements were taken with the use of a galvanostatic pulse and the GP-5000 GalvaPulseTM measurement kit. The measurements of all relevant parameters, i.e., the reinforcement stationary potential (Est) and the concrete cover resistivity (\(\Theta\)) to estimate the probability of corrosion on the analyzed surface, and the corrosion current density (i\({}_{\mathrm{cor}}\)) to determine the corrosion activity of the rod, were taken for each rod in each specimen at two measurement points spaced evenly in 70 mm intervals along the axis of the rod. Figure 6: Fibers used for test specimens.