The room temperature thermal conductivities and heat capacities measured by the transient plane source method for the four FRMs are provided in Table 1. For these four inorganic FRMs, there is only about 30 % variability amongst their room temperature thermal properties. As stated earlier, the room temperature heat capacity was utilized to determine an effective thermal conductivity as a function of temperature for the various slug calorimeter results. For example, slug calorimeter results for FRM A are presented in Figure 2. In the two graphs in Figure 2, results for three consecutive heating/cooling cycles are presented. For the first two heating curves, the furnace temperature (setpoint) was 538 ºC after 45 min, 704 ºC after 70 min, 843 ºC after 90 min, 927 ºC after 105 min, and 1010 ºC after 2 h. For the third heating curve, the furnace temperature was raised linearly and much more gradually from room temperature to 600 ºC in 4 h, and then held constant for a period of time at 600 ºC, while the slug temperature gradually approached this value as well. All cooling curves were generated by simply turning off the furnace and monitoring the temperatures during its cooling back to room temperature.
Clearly, it takes a finite amount of time for the heat transfer to/from the slug to reach a pseudo-steady state during either initial heating or the transition from heating to cooling mode. Thus, the low temperature (< 400 ºC) thermal conductivities for the heating curves and the higher temperature (> 450 ºC) ones for the cooling curves should not be considered as providing useful effective thermal conductivity values, as the steady-state conditions assumed in the development of equation (1) have not yet been achieved [7]
Figure 2: Effective thermal conductivity results for FRM A for multiple heating (top) and cooling (bottom)cycles in the slug calorimeter.
Still, the slug calorimeter may be utilized to provide estimates of the effective thermal conductivity from about 40 ºC to 700 ºC by overlaying the heating (≥ 400 ºC) and cooling curves (< 400 ºC) from the second heating/cooling cycle, for example [7]. Furthermore, by comparing the first and second heating curves, the influences of reactions and mass transport of reaction gases on the measured effective thermal conductivity can be examined. All four of the FRMs investigated in this study lose a finite mass, consisting mostly of water of hydration, during a high temperature exposure.
While the initial decomposition of the hydrates and transformation from bound water to steam is endothermic (indicated by regions where the first heating cycle effective k falls below that of the second heating cycle), the subsequent (temperature and pressure driven) mass transport of the steam (and other hot reaction gases) towards the central steel slug appears as an "exothermic" component (indicated by the first heating cycle k exceeding the values determined from the second heating cycle). Generally, the "exothermic" behavior observed between 100 ºC and 200 ºC would correspond to the mass transport of free water (steam), while that observed after about 400 ºC would correspond to water released from dehydration (of calcium sulfate dihydrate or hydrates of the calcium silicates).
Because these reactions generally go to completion during the first heating cycle, the effective k values for the three cooling curves basically overlap one another. As shown in Figure 3, the one exception to this is found for FRM B where the first cooling curve differs significantly from the second and third ones. This FRM contained extruded polystyrene beads as its lightweight filler and when these beads decompose during the first heating cycle, they leave behind a series of relatively large pores that will influence both the thermal conductivity and dimensional stability of the FRM during subsequent heating and cooling cycles
Figure 3: Effective thermal conductivity results for FRM B for multiple heating (top) and cooling (bottom) cycles in the slug calorimeter.
The effective thermal conductivity values determined at lower temperatures during the cooling cycles using the slug calorimeter can be compared to the values measured at room temperature using the transient plane source method. Results for the four different FRMs presented in Figures 2 to 5 indicate a favorable comparison. The transient plane source method values are generally slightly higher than those determined using the slug calorimeter, as might be expected due to the higher water contents in the original FRM specimens relative to those remaining in the companion specimens following a high temperature exposure and subsequent cooling.
Figure 4: Effective thermal conductivity results for FRM C for multiple heating (top) and cooling (bottom) cycles in the slug calorimeter.
As indicated earlier in Table 1, the variability between room temperature thermal conductivities determined for the four FRMs examined in this study was only on the order of 30 %. However, as indicated in Figures 2 to 5, at higher temperatures, this variability is much greater, approaching a value of 100 % when comparing the effective k values from the second heating curves at about 600 ºC. Since it is these higher temperature effective thermal conductivities that will be of critical importance to the FRMs providing a thermal barrier and protecting the (steel) substrates during an actual fire, this study highlights the necessity of determining the thermal properties of FRMs over their expected in-use temperature range, and not just at room temperature.
Figure 5: Effective thermal conductivity results for FRM D for multiple heating (top) and cooling (bottom) cycles in the slug calorimeter.
In the present study, results for inorganic FRMs that undergo minimal dimensional changes during fire exposure have been presented. The extension to intumescent (organic or inorganic) materials that expand greatly (up to 40X) during a fire exposure will require modifications to the presented methodology. The transient plane source method can still be applied to determine the room temperature thermal conductivity and volumetric heat capacity, using the thin film module of the Hot Disk system, for example. For intumescent coatings, the slug calorimeter experimental setup has already been modified to contain retaining plates with a central square hole to allow for the (one-dimensional) expansion of the intumescent during the high temperature exposure [7] However, to properly interpret the slug calorimeter results, detailed knowledge of these dimensional changes, as well as the energy transfer, during the test will be required.
Options include using a (infrared) camera to monitor the expansion of the FRM in-situ or developing a (contact) sensor that can be placed in the experimental setup to provide continuous feedback on the "average thicknesses" of the twin FRM specimens.