## 2 Experimental Procedure
### Test Methods and Instrumentation  
To measure the temperature at different depths, K-type thermocouples (the highest measuring temperature can reach 1000 \({}^{\circ}\)C) were embedded in the slab specimen, and they were placed at depths of 0, 10, 30, 50, and 150 mm from the heated surface. In addition, at the center point, 100 mm above the center point, and 100 mm below the center point of the slab specimen to be poured, the depths were 10, 30, and 50 mm from the heated surface (as shown in Figure 3a). A thin steel tube with a diameter of 1.6 mm and a length of 200 mm was used. Referring to the pore pressure measurement method of [PERSON] and Horiguchi [46], a copper ring (12 mm in inner diameter and 3 mm in length) was welded to the edge of the end of these thin steel tubes, and the copper ring was filled with a porous sinter metal plate (as shown in Figure 3b). The plate could prevent the cement matrix from flowing back to the thin steel pipe when pouring the specimen so that the pore pressure at these positions could be measured correctly.  
The heating of the concrete slab was made by a self-designed high-temperature electric furnace. The concrete slab was heated by one-way heating, and the pore pressure at different depths (10, 30, and 50 mm) of the heated surface and the temperature at the corresponding depth were measured (as shown in Figure 4). Before the test, the side of the slab specimen in the thickness direction was placed in a high-temperature furnace, and the surrounding of the specimen was wrapped in fireproof cotton to ensure that the concrete slab specimen was heated in one direction. Referring to the pore pressure measurement method of [PERSON] and Horiguchi [46], the inside of the pressure tube was filled with silicone oil. Therefore, the pore pressure could be transmitted smoothly, and the pressure gauge was then connected to the data acquisition junction box instruNET 100.  
Figure 4: Specimen installation and sensor configuration.  
Figure 3: Scheme of the experimental setup: (**a**) arrangement of thermocouples and pressure gauges in the slab specimens; (**b**) structural diagram of the pressure pipe.  
On the other hand, thermocouples were placed at depths of 0, 10, 30, 50, and 150 mm from the heated surface to measure the temperature changes inside the specimen and the furnace, and the data were captured by the data acquisition system (GW Instruments). The high-temperature test used two different heating rates (as shown in Figure 5) until the maximum target temperature was reached. The duration of the 600 \({}^{\circ}\)C target temperature was 1 h, and the duration of the 800 \({}^{\circ}\)C target temperature was 2 h. The specimen was then cooled naturally to room temperature. During the test, the pore pressure and temperature were recorded at the same time, and the sampling rate was one instance of data acquisition every 10 s. Under the planned eight test conditions, each group of concrete slabs had only one specimen.  
A 500 kN MTS servo controlled universal testing machine (Eden Prairie, MN, USA) with a measurement accuracy of \(\pm\)1% of the measurement range was used for the compressive strength test. The compressive strength test was carried out in accordance with the CNS 1232 specification. During the test, the cylindrical specimen was placed in the center of the compression shaft of the testing machine for loading, and the compression rate was between 0.14 and 0.34 MPa/sec until the specimen was destroyed. The compressive strength was obtained by dividing the ultimate load measured by the testing machine by the cross-sectional area of the cylindrical specimen, and the average value of three specimens was taken as the compressive strength of the group of cylindrical specimens.