## 3 Results and Discussion
### Freeze-Thaw Resistance  
It is known that in the concrete subjected to freeze-thaw cycles, microcracks are induced and concrete deterioration is accelerated, because the expansion of the water inside concrete due to freezing increases the tensile stress [20; 21]. In this study, an exposure test was conducted according to ASTM C 666 to examine the freeze-thaw resistance of the four concrete mixes. The frost resistance of each concrete mix was evaluated by measuring the RDME, surface electrical resistivity, and compressive strength of concrete in predetermined freeze-thaw cycles. After the dynamic modulus of elasticity was measured for each freeze-thaw cycle to evaluate the durability performance of the concrete mix subjected to freeze-thaw cycles, the RDME was obtained using Equation (1), which is plotted in Figure 6. The measurement results showed that the RDME of OPC concrete (reference mix) was significantly lower than those of the other three concrete mixes after 150 cycles, and reached ~55.4% after 240 cycles. Thereafter, it was not possible to measure the dynamic modulus of elasticity of OPC concrete because the specimen was disintegrated due to the excessive expansive pressure caused by freezing and thawing. The RDME of concrete with 30% WS and 10% AS became ~78.2% after 300 cycles. For WS40 and WS35A505 concretes, the RDME values were larger than 88% after 300 freeze-thaw cycles, suggesting excellent frost resistances. In general, when concrete is exposed to a freeze-thaw environment, tensile stress is generated due to ice formation in the capillary pores. The entrained air in concrete relieves this stress caused by ice formation [22; 23]. Therefore, reducing the capillary pores in concrete and introducing entrained air are very important for improving the freeze-thaw resistance of concrete. For slag-containing concrete that contains entrained air (approximately 6-7%), the freeze-thaw resistance increases because the capillary pores are reduced, and the expansion stress is relieved due to the latent hydraulic property [21; 24; 25]. Moreover, concrete containing AS exhibits the effect of relieving the tensile stress caused by ice formation, which is not observed in OPC concrete because the dense structure caused by the gradual generation of C-S-H reduces the capillary pores [26]. These experimental  
Figure 6: RDME values of concrete mixes exposed to freeze–thaw cycles. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Mix. Code**} & **Air Content** & **Specific Surface Area** & **Spacing Factor** \\  & **(\%)** & **(mm2/mm3)** & **(mm)** \\ \hline OPC & 2.9 & 21.8 & 0.345 \\ WS40 & 2.3 & 39.1 & 0.264 \\ WS35 AS05 & 2.6 & 34.8 & 0.284 \\ WS30 AS10 & 2.1 & 33.5 & 0.288 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Air-void analyses results of hardened concrete. results agree with those of [PERSON] et al. [21], who experimentally examined the frost resistance of concrete containing latent hydraulic binders (i.e., WS and AS). Figure 7 shows the correlations between the DF of concrete and the results of air-void analyses (Table 5); the influence of the spacing factor and specific surface area on the DF can be clearly observed. In other words, the DF of the concrete subjected to freeze-thaw cycles increased as the spacing factor increased, but an opposite tendency was observed between DF and specific surface area [27]. For OPC concrete, which was significantly damaged by freeze-thaw cycles, in particular, DF was \(\sim\)44.3% and the concrete exhibited a larger spacing factor and smaller specific surface area than those of the blended cement concrete. Figure 8 shows the measured surface electrical resistivity of the concrete subjected to 0, 90, 210, and 300 freeze-thaw cycles. Regardless of the concrete mix, the surface electrical resistivity tended to decrease as the number of freeze-thaw cycles increased.