How a certain material will perform during a fire is incredibly important as it can both save lives and minimize damage to structures. Concrete is considered an excellent fire resistive material, as it maintains its structural properties during a fire and has a low thermal conductivity. The measurement of the thermal properties of concrete at high temperatures is important to understanding how it behaves during a fire. Jansson (2004) tested the ability of the Hot Disk Transient Plane Source thermal conductivity system to measure the thermal properties of concrete at high temperatures in order to facilitate fire performance testing. This work details the ability of the Hot Disk TPS to accurately measure thermal conductivity at high temperature, in addition to illustrating how it can be applied in the field of fire protection.
Figure 1. Concrete has excellent fire resistive capabilities, a concrete structure will often remaining standing during a fire.1
Jansson (2004) created concrete samples by mixing self compacting concrete with polypropylene for reinforcement. The thermal conductivity, thermal diffusivity, and specific heat of dried concrete samples were determined during a single test using the Hot Disk TPS system. The TPS system uses a thin nickel foil sensor encased in either Kapton or Mica depending on the temperature of the test. This sensor acts as both the heating element and the temperature sensor, and calculates the thermal conductivity by measuring the change in resistance of the sensor during a measurement. In this project, identical concrete samples were sandwiched on either side of a Hot Disk sensor, and the thermal properties were recorded between a temperature range of 20 °C to 600°C (Figure 2). Data points were taken at 20, 90, 110, 200, 500 and 600 °C as the temperature rose, and again at 500, 200 and 20°C as the samples cooled. Measurements were repeated three times each.
In addition to the measurements performed with the Hot Disk TPS, specific heat was also measured using a modified differential scanning calorimeter (MDSC) in order to verify the results obtained through the TPS. Fire performance testing was then performed to determine the temperature at various depths in the concrete samples relative to temperature and time using ISO-834 exposure. These results were then compared against those collected by entering data from the Hot Disk TPS measurements into a 1-D finite element calculation to predict the same results.
Figure 2. Example of a high temperature testing set up using a Hot Disk sensor. The sensor and the sample can be installed in a furnace, and heat proof cables relay the information to the TPS.
Thermal Properties and Performance of Concrete at High Temperatures
Thermal conductivity and diffusivity results collected between 20°C and 600°C correlated well with the authors predictions and the scientific literature (Figure 3). Thermal conductivity dropped as the temperature increased, and was stable during the cooling period (Figure 3). Thermal diffusivity dropped during heating, and rose slightly during cooling (Figure 3). Specific heat rose alongside the temperature, and values obtained with the Hot Disk TPS corresponded well to those collected with the MDSC (Figure 4). Jansson (2004) also found an excellent correlation between the results of the experimental fire performance testing and the values generated by the 1D finite element calculation using the TPS data. The temperature curves are shown in Figure 5. Jansson noted that there was some discrepancy between the values around 100°C, which he attributed to moisture movement or pressure dependent boiling points.
Figure 3. Thermal conductivity and thermal diffusivity results collected by Jansson (2004) between temperatures of 20°C and 600°C using the Hot Disk TPS.
Figure 4. Specific heat values for concrete at elevated temperatures collected by Jansson (2004) using the Hot Disk TPS and a MDSC.
Figure 5. Internal temperature of concrete at various depths (mm) during experimental fire performance testing (Tm) and predictive analysis (Tc) collected by Jansson (2004).
Knowledge of how materials behave at elevated temperatures is incredibly important to designing buildings that will withstand a fire long enough for safe evacuations to occur. This project illustrates that the Hot Disk TPS is not only capable of accurate thermal property measurement at high temperatures, but that the data it produces is able to used in a wider range of fire performance predictions.