## 2 Experimental Procedure
### Mix Proportions of the Concrete  
According to the \"Lightweight aggregate concrete reference guidelines for use and construction technology reference specifications\" [45], the compressive strength of LWAC is divided into twelve grades, ranging from 5 to 60 MPa (i.e., LAC 5 to LAC 60), with a difference of 5 MPa for each grade. In this study, LAC 40 was used, that is, its designed 28-day compressive strength was 40 MPa, and its allowable variation range was 38-42 MPa. To compare the behavior of LWAC and NWC at high temperature, the same strength was used for both. Basically, the design principles of the mix proportions of structural LWAC and NWC are similar. The main difference is that LWAC needs to consider the influence of the density and water absorption of the lightweight coarse aggregate between different particle groups. Since the strength of lightweight aggregates is lower than that of ordinary aggregates, the LWAC used a lower water-cement ratio to achieve the same 28-day compressive strength as NWC. In addition, in order to make the prepared concrete have proper workability, different amounts of superplasticizers were used in the mix design of the LWAC and NWC. After several trial mixes, the mix designs of the LWAC and NWC were established, and they are shown in Table 4. The compressive strength of each age is shown in Table 5.  
\begin{table}
\begin{tabular}{c c} \hline
**Item** & **Content** \\ \hline SiO\({}_{2}\) (\%) & 20.21 \\ Fe\({}_{2}\)O\({}_{3}\) (\%) & 2.97 \\ Al\({}_{2}\)O\({}_{3}\) (\%) & 5.35 \\ CaO (\%) & 60.55 \\ MgO (\%) & 3.94 \\ SO\({}_{3}\) (\%) & 2.51 \\ Specific gravity & 3.15 \\ Specific surface area (cm\({}^{2}\)/g) & 3440 \\ Loss on ignition (\%) & 1.3 \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition and physical properties of the cement used.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Particle Size** & \begin{tabular}{c} **Particle Density** \\ **(kg/m\({}^{3}\))** \\ \end{tabular} & \
\begin{tabular}{c} **Water Absorption (\%)** \\ **30 min** \\ \end{tabular} &
\begin{tabular}{c} **Crushing Strength** \\ **24 h** \\ \end{tabular} \\ \hline
3/4”-4\({}^{\#}\) & \(1410\) & 7.1 & 10.5 & 8.6 \\ \hline \end{tabular}
\end{table}
Table 4: Mix proportions of the concrete mixtures.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Content** \\ \hline SiO\({}_{2}\) (\%) & 20.21 \\ Fe\({}_{2}\)O\({}_{3}\) (\%) & 2.97 \\ Al\({}_{2}\)O\({}_{3}\) (\%) & 5.35 \\ CaO (\%) & 60.55 \\ MgO (\%) & 3.94 \\ SO\({}_{3}\) (\%) & 2.51 \\ Specific gravity & 3.15 \\ Specific surface area (cm\({}^{2}\)/g) & 3440 \\ Loss on ignition (\%) & 1.3 \\ \hline \end{tabular}
\end{table}
Table 2: Physical properties of the aggregates used.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & \begin{tabular}{c} **Specific Gravity** \\ **(SSD)** \\ \end{tabular} & \begin{tabular}{c} **Water Absorption** \\ **(SSD) (\%)** \\ \end{tabular} & \
\begin{tabular}{c} **Dry Rodded Unit** \\ **Weight (kg/m\({}^{3}\))** \\ \end{tabular} &
\begin{tabular}{c} **FM** \\ **FM** \\ \end{tabular} \\ \hline Fine aggregate & 2.59 & 1.60 & - & 2.58 \\ Coarse aggregate & 2.60 & 0.60 & 1451 & - \\ \hline SSD: saturated surface-dry condition; FM: fineness modulus. & & & \\ \hline \end{tabular}
\end{table}
Table 3: Basic properties of the lightweight coarse aggregate used.