# 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
Figure 11a compares the normalized compressive strengths at 15 h that applied varying maximum curing temperatures. For the HTC specimens, the compressive strength ratio continuously increased with increasing maximum curing temperature. The compressive strength ratio of the IHC specimens also increased by 7% with increasing the maximum curing temperatures from 60 \({}^{\circ}\)C to 70 \({}^{\circ}\)C, but further increase in the compressive strength ratio was not observed for curing temperatures above 70 \({}^{\circ}\)C. Furthermore, the compressive strength ratio of the IHC specimen at 15 h to its specified compressive strength, \(f_{15h}/f_{C}^{\prime}\), also increased up to the maximum curing temperature of 70 \({}^{\circ}\)C and further increase was not significant for above this. The limited test results of this study indicate that the maximum curing temperature of approximately 70 \({}^{\circ}\)C is reasonable for the early-age strength development of induction heat-cured concrete. Footnote 1: \(f_{15h}\): compressive strength at an early age (15 h) considering different curing conditions. \(f_{15h}/f_{ATC}\): compressive strength at 28 days obtained by testing concrete cured at ambient temperature. \(f_{15h}/f_{ATC}\): compressive strength ratio of concrete at 15 h normalized to the strength of concrete cured at ambient temperature and of the same age. \(f_{15h}/f_{C}^{\prime}\): compressive strength ratio of concrete at 15 h normalized to the strength of concrete cured at ambient temperature and of the same age. Figure 11b compares the normalized compressive strengths at 15 h obtained by applying varying durations of maximum curing time. It is noted that the maximum curing temperatures for all the specimens in Figure 11b were 70 \({}^{\circ}\)C. Both IHC and HTC specimens, which were supplied with additional heat for curing, exhibited higher compressive strength ratios at an early age with increasing the curing period. This result is consistent with the  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Specimen** & \multicolumn{3}{c}{**ATC**} & \multicolumn{3}{c}{**HTC**} & \multicolumn{3}{c}{**HTC**} & \multicolumn{3}{c}{**IHC**} \\ \hline  & \(f_{15h}\)1 [FOOTNOTE:1]Footnote 1: \(f_{15h}\): compressive strength at an early age (15 h) considering different curing conditions. \(f_{15h}\): specified compressive strength at 28 days obtained by testing concrete cured at ambient temperature. \(f_{15h}/f_{ATC}\): compressive strength ratio of concrete at 15 h normalized to the strength of concrete cured at ambient temperature and of the same age. \(f_{15h}/f_{C}^{\prime}\): compressive strength ratio of concrete at 15 h normalized to the strength of concrete cured at ambient temperature and of the same age. & \(f_{15h}/f_{C}^{\prime}\): compressive strength ratio of concrete at 15 h normalized to the strength of concrete cured at ambient temperature and of the same age. maturity theory, which explains that the strength gain by concrete is a function of time and temperature [2,4]. The maturity of concrete is generally defined as \(\Sigma\)(time interval \(\times\) temperature). The effects of maturity on concrete strength cured by the induction heating system can be clearly shown by Figure 11c. Each pair of specimens together in Figure 11c have similar maturity (i.e., S4-8h-50 and S4-5h-60 has similar maturity, while S4-7h-70 and S4-5h-80 has similar maturity). The graphs in red on the right-hand side represent the compressive strength values of induction heat-cured concretes at 15 h, while the graphs in blue on the left-hand side represent compressive strength ratios of the same specimens compared with the strength values of the ATC specimens, \(f_{15h}/f_{ATC}\). The results of Figure 11c demonstrate that the early age strength of induction heat-cured concrete is proportion to the value of the maturity factor.