## 2 Materials and Methods
### Experimental Plan and Mix Proportions  
Table 2 shows the experimental plan. For the concrete strength, the concrete of C24 was referenced, which is most commonly used at construction sites. For C24 (330 P) that used OPC, the W/C was set to 0.50 and the unit weight of cement was set to 330 kg/m\({}^{2}\). In general, the unit weight of cement is increased to secure the early strength of concrete at construction sites. In this study, C27 (characteristic value of concrete = 27 MPa) and C30 (characteristic value of concrete = 30 MPa) were selected, which increased the unit weight of cement. The unit weight of cement of C27 (350P) and C30 (380P) was 350 and 380 kg/m\({}^{2}\), respectively. In addition, HFS_OPC and EPC, which increased the fineness and SO\({}_{3}\) content of cement, were additionally selected and examined to improve concrete strength considering the economic efficiency of concrete mixtures. For such concrete mixtures, the unit weight of cement was set as 330 kg/m\({}^{2}\) \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Series**} & **Mix** & \multirow{2}{*}{**W/C**} & **Cement** & **Unit Weight** & \multirow{2}{*}{**Chemical**} & **Curing** & \multirow{2}{*}{**Evaluation Item**} \\  & **ID.** & & & **Type** & **of Cement** & **Admixture** & & (°C) \\ \hline \multirow{4}{*}{I} & 330P & 0.50 & OPC & 330 & & & Chamber & \# Slump (mm) \\  & 350P & 0.47 & & 350 & PC & (13 °C) & \# Air contents (\%) \\  & 380P & 0.43 & & 380 & & & \# Compressive strength (MPa) \\  & 330P & & OPC & 330 & PC (1) & - & - & Cylinder Mold (\(2\)\(100\times 200\)) \\ \cline{1-1} \cline{2-7}  & 330 HFS & 0.50 & HPS & 330 & PC (2) & - & -18, 24, and 7.2 h \\ \cline{1-1}  & 330 HFS, ePC & & HPS & 330 & ePC (2) & (20 °C) & \# Temperature history (°C) \\ \cline{1-1} \cline{2-7}  & 330 EP & & EPC & 330 & PC & & and Maturity (D-h) \\ \hline \hline \end{tabular}  
* PC: Polycarboxylic superplasticizer-based type admixture. \({}^{(2)}\) ePC: early Polycarboxylic superplasticizer with triethanolamine (TEA).  
\end{table} 
Table 2: Experimental plan. Figure 1: Gradation sieve analysis test result for the aggregates: (**a**) Fine aggregates and (**b**) coarse aggregates. similar to that of C24. HFS (330P) increased the fineness and SO\({}_{3}\) content of OPC, and it is effective in developing the early strength [14; 15]. However, as it has limitations in developing the early strength of concrete, ePC (early polycarboxylic superplasticizer with TEA) was added to HFS [23]. To identify the effect of the curing temperature on concrete strength, chamber (13 \({}^{\circ}\)C) and water curing (20 \({}^{\circ}\)C) were examined. The slump (mm) and air content (%) of concrete were evaluated to ensure that they could be used onsite. As for hardened properties, a cylindrical mold (\(\varnothing\)100 mm \(\times\) 200 mm) was fabricated, and its concrete strength was measured at 18 h, 24 h, and 72 h. In addition, the temperature history (\({}^{\circ}\)C) was measured, and the maturity (D-h) was calculated based on the temperature history. The time required to remove the vertical formwork on concrete was estimated at the curing temperature by examining the correlation between the concrete strength and maturity. The formwork removal time on concrete is specified in each country's regulations. However, its decision criteria differ. This study referred to the Asian (including South Korea and Japan) criteria that present a detailed demolding strength value [24; 25]. The strength criterion for the formwork removal of concrete was set as 5 MPa. Table 3 shows the mixing proportions on concrete. For the mixing proportions on concrete, their corresponding practical values were used as reference, and the unit weight of water was 165 kg/m\({}^{2}\). For the fine aggregates, crushed sand and washed sea sand were mixed at a volume ratio of 6:4. For the fresh properties on concrete, the slump was set to 180 \(\pm\) 25 mm and air content was \(4.5\pm 1.5\%\), respectively, to apply workability in the construction site.