## 3 Results and Discussion
### Freeze-Thaw Resistance
Before exposure to the freeze-thaw environment, the surface electrical resistivity of OPC concrete was \(\sim\)9.15 Kohm\(\cdot\)cm, whereas that of the three blended cement concrete types was in the range 17.0-18.5 Kohm\(\cdot\)cm, indicating that these structures were relatively denser. Figure 9 compares the surface electrical resistivity losses of all concrete mixes against the number of freeze-thaw cycles. Regardless of the number of freeze-thaw cycles, the resistivity of OPC concrete decreased most significantly. Although the surface electrical resistivity loss of OPC concrete was \(-\)44.8% at 210 cycles, that of WS30 AS10 concrete was \(\sim\)24.5%, which was found to be the lowest. Figure 8: Surface electrical resistivity of concretes exposed to freeze–thaw cycles. Figure 7: Relationship between the durability factor and air-void system of concrete. When concrete is exposed to the freeze-thaw environment, microcracks occur because of the increased porosity of the structure caused by the reduced density, resulting in reduction of the compressive strength [3]. Figure 10 shows the compressive strengths of the four concrete mixes measured before exposure to freeze-thaw cycles and at predetermined cycles. The compressive strength tendency was found to differ for each concrete mix. In particular, after 90 freeze-thaw cycles, the compressive strength of OPC concrete decreased compared with that before exposure to freeze-thaw cycles, but that of the remaining types containing blast furnace slag powder (WS and AS) increased slightly. This is because the increase in the strength of concrete caused by hydration was more significant than the decrease in strength caused by freeze-thaw cycles for the blended cement concrete--this tendency is similar to that observed by [PERSON] et al. [21]. Figure 11 shows the CLS of concrete at each cycle based on the compressive strength of concrete before exposure to freeze-thaw cycles. Regardless of the number of freeze-thaw cycles, the blended cement concrete exhibited negligible CLS (below 20%), indicating excellent frost resistance. Figure 10: Compressive strength of concretes exposed to freeze–thaw cycles. Figure 9: Comparison of surface electrical resistivity loss of concretes. The relationship between durability factor (DF) and compressive strength loss (CSL) is presented in Figure 12. High correlation coefficient value (0.951) for two parameters were obtained, implying that the CSL of the concrete decreased as the DF increased. Overall, it can be said that the measurement of CLS may be a useful method to determine the durability of concrete under freeze-thaw cycles. In fact, the effect of WS on the durable performance of concrete under freeze-thaw cycles was reported by many researchers [28; 29; 30]. For example, [PERSON] et al. studied that the permeability and water absorption of concrete with different replacement levels of WS exposed to a combined interaction of the fatigue load and freeze-thaw cycles, and established a model of the relationship between flexural strength and pore structure [29]. However, little studies carried out to verify influence of AS on the resistance of concrete against repeated freeze-thaw action. Results clearly demonstrated that if there is a proper mix proportion, the application of AS may improve the performance of concrete.