# Development of a Novel Concrete Curing Method Using Induction Heating System
## 3 Properties of Concrete after Curing
### Results and Discussion  
#### 3.2.1 Temperature Distribution during Curing  
Typical temperature distributions of concrete that were measured during induction heat curing are illustrated in Figure 9. Specifically, Figure 9a-c show the temperature distributions of the cylinders with diameters of 100 mm (SS4-5h-70), 150 mm (S4-5h-70), and 300 mm (LS4-5h-70), respectively. In Figure 9, the thermocouple RT represents the room temperature of the test chamber and this data demonstrates that a low temperature of approximately 0 \({}^{\circ}\)C was maintained in the test chamber to simulate cold weather concrete conditions. It is noted that the slight increase in the temperature of the test chamber every 6 h (hump of RT in Figure 9) is owing to the defrosting cycle of the chamber to prevent freezing. The thermocouple LT in Figure 9 represents the temperature measured at the center of the cross-section of a cylinder in the test chamber without heat curing. At the beginning of the tests, the temperature was approximately 20-24 \({}^{\circ}\)C because the concrete was cast at ambient temperature. Then, the inner temperature of the concrete cylinder gradually decreased to a temperature similar to that of the test chamber at around 4 to 6 h, maintained until the end of testing. This result indicates that the inner temperature of concrete could be reduced to a very low value according to the surrounding temperature, particularly in winter, although hydration heat occurred in the concrete. Therefore, frost is likely to occur in concrete cast in the winter season if additional heat curing is not applied.  
The cylindrical specimens of relatively small sizes, both \(\phi\)100 \(\times\) 200 mm and \(\phi\)150 \(\times\) 300 mm, exhibited quite similar temperatures at the inner surface of steel form and the center of concrete, as shown in Figure 9a,b. The temperature at the top surface of the concrete cylinder of dimensions \(\phi\)150 \(\times\) 300 mm, which was measured using infrared thermal imaging (see Figure 9d), was quite uniform as well through the entire cross-section. This result consistent with the measurements obtained using the thermocouples. It is noted that the temperature at the top surface of the concrete is marginally lower than that inside the concrete, because the top surface is adjacent to the low-temperature air (approximately 0 \({}^{\circ}\)C). However, in the case of the cylindrical specimen with a diameter of 300 mm, the temperature at the inner surface of the steel form tended to increase faster than the temperature at the center of the concrete and the temperature differences in the maximum temperature regions were approximately 10-17 \({}^{\circ}\)C. Furthermore, the target temperature could not be achieved in the interior of this specimen. These results indicate that the heat generated in the steel form was efficiently transferred to the interior of the concrete in the case of small specimens and, therefore, the entire cross-section of small-sized concrete structures can be heated uniformly by applying an induction heating system. Meanwhile, the heat generated in the steel forms could not be transferred fully to the interior of the concrete when the specimen size increased, resulting in temperature differences between the interior and surface of the concrete. Nevertheless, the measured temperature differences of 10-17 \({}^{\circ}\)were not significant compared to the temperature difference required to cause cracking of concrete (exceed 20 \({}^{\circ}\)C) as suggested by FitzGibbon [23].