# Development of a Novel Concrete Curing Method Using Induction Heating System
## 3 Properties of Concrete after Curing
### Test Program
#### 3.1.3. Curing  
After concrete casting, the top surface (casting surface) of the all cylinder specimens was immediately covered and sealed with plastic sheets to prevent abrupt water evaporation and rapid setting during curing. Then, each of three cylinder specimens in the same variable was cured under four different curing conditions as listed below and the detailed curing regime is also shown in Figure 4.  
(1) Induction heat curing (IHC): this is considered primarily as a variable in this study. In order to simulate cold weather concrete conditions, concrete specimens were stored in a constant temperature chamber, targeted to maintain 0 \({}^{\circ}\)C, immediately after casting. Meanwhile, steel forms containing concrete were heated using an induction heating system according to the temperature hysteresis, specifically the red line in Figure 4. This curing regime comprised 3 h of ambient temperature after casting, followed by a temperature increase at the rate of 20 \({}^{\circ}\)C/h up to the maximum temperatures of 50-80 \({}^{\circ}\)C. This maximum temperature was maintained for 5 h-9 h and then, the temperature decreased to ambient temperature at the rate of 15 \({}^{\circ}\)C/h. After curing was performed at ambient temperature for 15 h, compression tests were carried out to determine the early age strength. Figure 8a shows the test chamber that simulated cold weather conditions (0 \({}^{\circ}\)C), test specimens, and setup for IHC. An induction heating system with a maximum output electric power of 7 kWh and resonance frequency of 40 kHz was used for the IHC, as illustrated in Figures 1 and 6. According to the FEM simulations and steel form heating test results described in Section 2, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multicolumn{2}{c}{**Max. Curing Temperature**} & \multicolumn{2}{c}{**Metallic Form**} & \multicolumn{2}{c}{**Note**} \\ \cline{2-7}  & \multirow{2}{*}{\(T_{max}\) (\({}^{\circ}\)C)} & **Duration** & \multirow{2}{*}{**Material**} & **Thickness** & **Dimensions** & \\  & & **(Hour)** & & **(mm)** & **(mm \(\times\) mm)** & \\ \hline S4-8h-50 & 50 & 8 & & & \(\phi 150\times 300\) & \\ \hline S4-5h-60 & 60 & 5 & & & \(\phi 150\times 300\) & \\ \hline S4-5h-70 & 70 & 5 & & & \(\phi 150\times 300\) & Effects of curing temperature and duration \\ \hline S4-5h-80 & 80 & 5 & & & \(\phi 150\times 300\) & \\ \hline S4-7h-70 & 70 & 7 & & & \(\phi 150\times 300\) & \\ \hline S4-9h-70 & 70 & 9 & & & \(\phi 150\times 300\) & \\ \hline S4-5h-70 & 70 & 5 & & & \(\phi 100\times 200\) & Effect of specimen size \\ \hline LS4-5h-70 & 70 & 5 & & & \(\phi 300\times 300\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Test variables for evaluating the properties of concrete cured by induction heating. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Mixture**} & \multirow{2}{*}{**w/c (\%)**} & \multirow{2}{*}{**S/a (\%)**} & \multicolumn{4}{c}{**Unit Weight (kg/m\({}^{3}\))**} & \multirow{2}{*}{**SP \({}^{1}\)**} \\ \cline{4-7}  & & & **Water** & **Cement** & **Sand** & **Gravel** & \\ \hline OPC35 & 38.3 & 47.9 & 176 & 460 & 800 & 894 & 0.75–0.90 \\ \hline \hline \end{tabular} \({}^{1}\) SP: Super-plasticizer (wt% to the cement).  
\end{table} 
Table 2: Mix proportion of concrete. coils having three turns with a distance of 75 mm between coils were applied since this arrangement of coils provided the most uniform temperature distributions along the entire height of the steel form. The temperature hysteresis for IHC, shown in Figure 4, is simulated precisely by a temperature controller that operates based on the proportional integral derivative (PID) method. This system controls the output electric power supplied by the power converter, by considering the difference between the target temperature and the temperature measured using a thermocouple located at the mid-height and inner surface of the form in contact with the concrete.