# Development of a Novel Concrete Curing Method Using Induction Heating System
## 3 Properties of Concrete after Curing
### Results and Discussion
#### 3.2.2 Compressive Strength Results  
The compressive strength results of concretes subjected to different curing methods are given in Table 3. In this Table, the term \(f_{15h}\) represents the compressive strength results of the concrete measured at 15 h, immediately after curing. The term \(f_{c}\)' represents the specified design concrete strength at 28 days, applicable only to ATC specimens, and this strength considered as a benchmark to compare the strength of concretes under different curing regimes. The strength, \(f_{15h}\), was normalized with respect to the same age strength of concrete cured at the ambient temperature, \(f_{ATC}\), to account for the different concrete strengths in each variable, represented as \(f_{15h}/f_{ATC}\). The strength, \(f_{15h}\), is normalized with respect to \(f_{c}\)' as well, and is represented as \(f_{15h}/f_{c}\)' in Table 3.  
All the specimens cured in low-temperature chambers (LTC specimens) exhibited strength values of 0.2-0.4 MPa at 15 h, which are equivalent to approximately 2-3% of the strength of concrete cured at ambient temperature and of the same age. [PERSON] [4] indicated that hydration of cement occurs even at a very low temperature of approximately \(-10\)\({}^{\circ}\)C. However, if concrete that has not been set is permitted to freeze, its setting and hardening are delayed because of the unavailability of water for chemical reactions. In this study, LTC and IHC specimens were stored in the 0 \({}^{\circ}\)C chamber immediately after concrete casting and, therefore, it appears that the hydration reaction did not occur. This result also verified the results obtained by examining the temperature of concrete in the chamber discussed in Figure 9, namely that the inner temperature of the concrete cylinder gradually decreased to the temperature of the test chamber. Furthermore, a comparison (shown in  
Figure 9: Temperature distributions of the concrete measured during induction heat curing: (**a**) cylinder with dimensions \(\phi\)100 \(\times\) 200 mm; (**b**) cylinder with dimensions \(\phi\)150 \(\times\) 300 mm; (**c**) cylinder with dimensions \(\phi\)300 \(\times\) 300 mm; (**d**) infrared thermal camera image of the cylinder with \(\phi\)150 mm. Figure 10) of the surface textures of concretes cured under four different conditions, at the time of testing (15 h), also demonstrates suspended and delayed setting and hydration of concrete at around the water freezing temperature. ACI Committee 306 [2] recommends the provision of appropriate protection immediately after the placement of concrete in cold weather to prevent early age freezing. They recommended that the temperature of concrete that is newly placed in cold weather should be kept close to 13 \({}^{\circ}\)C for cross-sectional sizes of concrete less than 300 mm. Most of the IHC specimens cured in a low-temperature chamber and applied with steel form heating using an induction system exhibited compressive strength values exceeding 20 MPa even at a very early age of 15 h, except for the marginally lower strength values of 18.9 and 19.8 MPa for the specimens S4-8h-50 and S4-5h-60, respectively. Furthermore, the strength values are 1.65-2.33 times higher than those of the ambient-cured specimens at the same age, and attained 34-54% of the specified 28-day strength values. Furthermore, the compressive strength results of IHC specimens are comparable to the results of HTC specimens cured in a high-temperature chamber, although the IHC specimens were cured in a chamber with a significantly lower temperature than that for HTC specimens. These results demonstrate the efficiency of the use of an induction heating system for curing concrete to develop early age strength in cold weather conditions. The normalized compressive strengths, \(f_{15h}/f_{ATC}\) and \(f_{15h}/f_{C}\)1 listed in Table 3, of concretes cured under different curing conditions are compared in Figure 11 to investigate the effects of each variable on the strength development at a very early age.