## 3 Results and Discussion
### Chloride Ion Penetration Resistance and Chloride Ion Diffusion  
Table 4 presents the average value of charge passed obtained from the ASTM C 1202 test procedure. Regardless of the fiber content, the amount of charge passed decreased as the volume proportion of polyolefin fiber increased. It should be noted that there were some cases of test termination due to the excessive current flow from the specimen. These cases were more evident from specimens with higher steel fiber content (lower polyolefin fiber content).  
Figure 7: The 28-day flexural toughness of hybrid fiber reinforced base concrete.  
The higher charge passed values observed here were directly related to the electrical conductivity of steel fiber [10]. This is because (1) the same base concrete mix design was used, (2) the increase in charge passed is solely related to the increase in the amount of steel fiber in the base concrete, and (3) the average charge passed value of base concrete with 100% polyolefin fiber did not show clear differences with the increase in fiber volume. This proves that the rapid ASTM C 1202 test procedure, which uses 60 V voltage difference, cannot be applied to evaluate the chloride ion penetration resistance of concrete that contains material with higher electrical conductivity.  
To complement the results of the ASTM C 1202 test method, the NT-Build 492 test procedure was performed, and the results are presented in Figure 8. As mentioned earlier, the setting of the measurement voltage for the NT-Build 492 test was determined by the initial current value (Table 3) of the base concrete. The test was performed at higher voltage with 0.54% fiber content. As the fiber content increased from 0.54% to 1.08%, the initial current from the base concrete increased, and thus the test had to be performed at relatively lower voltage. As a result, the NT-Build 492 test procedure is much less affected by the presence of conductive material in the concrete because a lower voltage can be used.  
Figure 8 shows the chloride ion diffusivity of the base concrete with hybrid fibers. When the total fiber amount was 0.54%, the chloride ion diffusivity gradually decreased as the volume proportion of polyolefin fiber increased. On the contrary, in the case of a fiber content at 1.08 vol%, as the volume proportion of polyolefin fiber increased, the chloride ion diffusivity generally increased. The base concrete with 100% polyolefin fiber showed relatively higher chloride ion diffusivity than the other base concrete specimens. This seems to be associated with the agglomeration or arrangement of fibers within the specimen.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Fiber Content**} & \multicolumn{4}{c}{**Volume Proportion of Polyolefin Fiber in Hybrid Fiber**} \\ \cline{2-6}  & **0\%** & **25\%** & **50\%** & **75\%** & **100\%** \\ \hline
0.54 vol\% & 5635 & 5128 & 4641 & 3305 & 2546 \\ \hline
1.08 vol\% & unmeasurable & 8226 & 7748 & 4793 & 2345 \\ \hline \hline \end{tabular}
\end{table}
Table 4: The average charge passed values of base concrete with hybrid fiber (in coulomb).  
Figure 8: Chloride ion diffusivity of base concrete with hybrid fiber.  
Overall, the chloride ion diffusion coefficient values ranged from 2.67 to 6.30 \(\times\) 10\({}^{-12}\) m\({}^{2}\)/s, indicating a significant degree of salt resistance when using the combination.