# Mechanical Behavior of Steel Fiber-Reinforced Lightweight Concrete Exposed to High Temperatures
## 3 Test Results and Discussions  
### Visual and Ultrasonic Inspection of Heated Specimens  
The color changes of all kinds of concrete were basically the same after experiencing different heating history: the concrete color was light grey at ambient temperature, while the surface concrete color became dark grey at 200 \({}^{\circ}\)C, pink grey or yellow grey at 400 \({}^{\circ}\)C, brownish grey at 600 \({}^{\circ}\)C and whitish-grey at 800 \({}^{\circ}\)C, as presented in Figure 4a. At 200 and 400 \({}^{\circ}\)C, there are no significant cracks on the surfaces. However, at 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, there were many irregular cracks on the surface of plain concrete specimens, especially for NWC and SLWC specimens. This may be explained by the different thermal expansion of cement mortar and coarse aggregate. There were a steep temperature gradient and a high internal steam pressure in the specimen at high temperatures. It should be noted that the specimen color does not change significantly, but the number and width of the crack obviously decrease when steel fibers were added. For steel fiber-reinforced ALWC and SLWC specimens, tiny cracks could be observed with naked eyes only after 800 \({}^{\circ}\)C. The comparative investigation of the addition of steel fiber is presented in Figure 4b,c. These improvements were due to the fact that steel fibers bridging among cracks can inhibit the developments of cracks [28,34]. Furthermore, the thermal conductivity of steel fiber was higher than plain concrete, making heat transfer more uniform. This helped to reduce the water vapor pressure [35] and the propagation of cracks in the concrete. Figure 5 presents the relative and absolute values of the residual UPVs of the studied concrete at different elevated temperatures. All the absolute values presented in this study are the average values of the three specimens in each series. The relative value was defined as the ratio between the absolute values obtained after being exposed to high temperatures and room temperature. The same definition of absolute value and relative value are adopted in the following figures. The coefficient of variation of the UPV tests were between 0.2% and 4.2%. It can be observed from Figure 5a that no obvious changes can be observed for ultrasonic pulse velocity when 1% hooked end or crimped steel fiber dosage is added into the concrete. Figure 5b presents the relative residual UPVs of the studied concrete after different elevated temperatures. For all the concrete specimens, the UPV results showed a sudden drop in velocity at 400 \({}^{\circ}\)C, which means that the matrix of the concrete experiences considerable damage after exposure to 400 \({}^{\circ}\)C. For the NWC and SLWC specimens, the rate  
Figure 3: Physical and mechanical tests: (**a**) Ultrasonic pulse velocity (UPV) test; (**b**) splitting tension test; (**c**) axial compression test. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Concrete Type**} & \multicolumn{2}{c}{**Oven Dried**} & \multirow{2}{*}{\(f_{c}\)**/MPa**} & \multirow{2}{*}{\(f_{sp}\)**/MPa**} & \multirow{2}{*}{**UPV/(m/s)**} & \multirow{2}{*}{**E\({}_{c}\)/GPa**} \\  & **Density/(kg/m\({}^{3}\))** & & & & \\ \hline NWC & 2361 & 39.36 & 3.72 & 4496 & 28.19 \\ ALWC & 1752 & 48.73 & 3.33 & 4016 & 18.28 \\ CF-AL & 1859 & 49.71 & 4.17 & 4090 & 17.51 \\ HF-AL & 1862 & 51.38 & 4.73 & 4098 & 18.68 \\ SLWC & 1888 & 43.24 & 3.83 & 4253 & 20.25 \\ CF-SL & 2007 & 45.87 & 4.11 & 4268 & 20.27 \\ HF-SL & 1954 & 48.58 & 4.96 & 4270 & 20.43 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Physical and mechanical characteristics of various mixtures at 28 days. of reduction in UPV values at 400-800 \({}^{\circ}\)C was much higher than that at 200 \({}^{\circ}\)C. Furthermore, ALWC and steel fiber-reinforced ALWC exhibited the best performance of all the studied concretes.