## 3 Results and Discussion  
### Residual Mechanical Properties of NC and LC after High Temperature Exposure  
Figure 5 shows the compressive strength of NC and LC after high temperature exposure. At 100 \({}^{\circ}\)C, NC41 and NC33 showed a residual compressive strength of approximately 0.91, which was slightly higher for LC41 and LC33 at approximately 0.96. At a W/B ratio of 0.28, the residual compressive strength was relatively lower compared to the other W/B ratios, resulting in 0.86 for NC28 and 0.89 for LC28. For all specimens, strength degradation was observed until 200 \({}^{\circ}\)C, which slightly increased at 200-300 \({}^{\circ}\)C. The strength for all specimens increased by approximately 3%, except for NC28, which can be attributed to the influence of the forced hydration reaction caused by the evaporation of free water inside the specimen at high temperature and the influence of the stress and vapor pressure caused by the thermal expansion of the aggregates [21; 22; 23]. Above 300 \({}^{\circ}\)C, the strength continuously decreased, leading to a rapid strength loss above 400 \({}^{\circ}\)C. This was because of the severe matrix damage caused by the decomposition of C-S-H, the main component of hardened cement. Above 500 \({}^{\circ}\)C, LC exhibited a higher residual compressive strength than NC at all levels. NC41, NC33, and NC28 showed residual compressive strengths of 0.52, 0.56, and 0.70, respectively, while LC41, LC33, and LC28 exhibited 0.59, 0.64, and 0.73, respectively. At 700 \({}^{\circ}\)C, the residual compressive strengths of NC and LC showed the largest difference. NC41, NC33, and NC28 showed residual compressive strengths of 0.14, 0.32, and 0.41, respectively, while LC41, LC33, and LC28 exhibited 0.38, 0.44, and 0.56, respectively. It appears that the relatively small thermal expansion after high temperature exposure caused by the porosity of the coal-ash aggregate  
Figure 5: Residual compressive strength of normal concrete (NC) and lightweight concrete (LC) after high temperature exposure. \begin{table}
\begin{tabular}{c c c c} \hline
**Test Items** & **Test Method** & **Equation (1)** & **Equation (2)** \\ \hline Residual compressive strength & ASTM C39/C39M & \(V_{p}=\frac{1}{3}\) & \(\mathrm{M}_{w}=\frac{\mathrm{M}_{B}}{\mathrm{M}_{T}}\) \\ Residual modulus of elasticity & ASTM C469 & \(V_{p}\): ultrasonic pulse velocity(m/s) & \(\mathrm{M}_{w}\): residual mechanical properties \\ Residual ultrasonic pulse & ASTM C597 & \(L\): distance(m) & \(\mathrm{M}_{B}\): mechanical properties at room temperature \\ velocity & & \(t\): time(s) & \(\mathrm{M}_{T}\): mechanical properties at high temperature \\ \hline \end{tabular}
\end{table} 
Table 5: Testing of residual mechanical properties. mixed in LC improved the stress at the paste-aggregate interface [24; 25]. Microanalysis has shown that the stress at the paste-lightweight aggregate interface is stronger compared to that at the paste-normal aggregate interface [26]. Figure 6 shows the residual ultrasonic pulse velocity of NC and LC after high temperature exposure. Unlike the residual compressive strength, the residual ultrasonic pulse velocity showed a tendency to linearly decreased as the temperature increased. At 100 \({}^{\circ}\)C, the residual ultrasonic pulse velocity was approximately 0.97% at all levels, showing a higher residual rate compared to the residual compressive strength. At 200 \({}^{\circ}\)C, the residual ultrasonic pulse velocities were found to be 0.83 and 0.86 for NC41 and LC41, respectively, 0.88 and 0.85 for NC33 and LC33, respectively, and 0.90 for both NC28 and LC28. At 300 \({}^{\circ}\)C, they were 0.75 and 0.79 for NC41 and LC41, respectively, 0.76 and 0.80 for NC33 and LC33, respectively, and 0.89 and 0.90 for NC28 and LC28, respectively. At temperatures higher than 300 \({}^{\circ}\)C, LC exhibited higher residual ultrasonic pulse velocities than NC. As the temperature increased, the difference in the residual ultrasonic pulse velocity between NC and LC became larger.