## 2 Experimental Investigation
### Materials  
The cement used in this study was Type I ordinary Portland cement (OPC) from H Company (Sokcho, Republic of Korea). In this study, standard sand in Hyangho-ri, Jumunjin-eup, Gangwon-do, was used to make homogeneous mortar. Standard sand is a material containing natural round particles and 98% or more silicon dioxide (SiO\({}_{2}\)) as specified by KSL ISO 679 [36], with an average particle diameter of 0.5 mm or less. The specific gravity and water absorption of the fine aggregate in the dry saturated state were 2.65 and 0.1%, respectively. The physical properties and particle size of the fine aggregate are shown in Table 2. A polycarboxylate-type superplasticizer (SP), a domestic product (Sika Korea Central, Co., Ltd., Gunsan, Republic of Korea), was used as chemical admixture for fluidity control of ECM. The used dosage of SP admixture was based on the mass of cement, an appropriate amount that satisfies the required ECM fluidity within the range of 0.5-1.0%. The SF used in this study is a hook-style product of bundle type with a length of 30 mm and a diameter of 0.5 mm produced by K company in the Republic of Korea, and the CF is a product (T700 SC) of D company in Japan, with a length of 6 mm and a diameter of 7 \(\upmu\)m. In the case of CF, a polyacrylonitrile (PAN)-based material having a carbon content of 92% or more, an electrical resistivity of 1.6 \(\times\) 10\({}^{-3}\)\(\Omega\).cm and a thermal conductivity of 6.4 W/m-K was used. The physical and mechanical properties of the conductive CF and SF used in this study are shown in Table 3, and these were provided by the manufacturers.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Factors** & **Levels** \\ \hline Fiber volume fractions (\%) & 0.25, 0.5, 0.75, 1.0, 1.25 \\ Applied voltages (V) & DC 10, 20, 30 \\ Electrodes spacings (mm) & 40, 120 \\ Infrared thermal camera (FLIR T630 sc) \\ Universal testing machine (UTM) \\ SEM observation & TGA analysis \\ \hline \hline \end{tabular}
\end{table}
Table 1: Experimental plan of heating performance considered in this study.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Size** (mm)** & **Unit Weight** (kgf/m\({}^{3}\))** & **Density** (g/cm\({}^{3}\))** & **Absorption** (\%)** & **Amount of Passing 0.3 mm** & **Fineness Modulus (FM)** \\ \hline
2\(\leq\) & 1490 & 2.65 & 0.1 & 3.0 & 2.40 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physical properties and grain size of fine aggregate.  
Figure 1 is a picture of conductive CF and SF enlarged with SEM imaging. Figure 1a shows a SEM picture of the surface shape of CF taken at a magnification of 10,000 times. The diameter of the CF is about 7.07 \(\upmu\)m and the surface is very smooth, showing very poor interfacial adhesion with the cement matrix. On the other hand, Figure 1b shows a SEM picture of the surface shape of SF taken at a magnification of 150 times. The diameter of SF is about 512.37 \(\upmu\)m and the surface looks somewhat rough, presumably forming a surface texture that is advantageous for interfacial adhesion to the cement matrix. Since CF and SF were mixed in the same weight ratio in this study, a large number of micro-sized fibers distributed in the specimen could be more effective for exhibiting higher heating performance than a small number of macro-sized fibers distributed in the specimen, when considering the difference in density between the two fibers.