The individual thermocouple temperatures collected during a single heating/cooling cycle of FRM A are provided in Figure 6. The ASTM E119 standard temperature-time curve1 is shown for comparison to the heating curve achievable in the electric furnace. It is worth noting that during a portion of the heating curve the exterior FRM surface temperatures actually exceed the "ambient" temperature of the furnace, most likely due to enhanced radiation transfer between the Inconel retaining plates and the individual furnace elements. This reinforces the need to center the sandwich specimen in the furnace, so that both sides (specimens) are exposed to nominally the same thermal environment. There is little variability among the three thermocouples mounted in the steel slug, indicating that the assumption that it behaves as an isothermal slug mass is a generally valid one. Since these three thermocouples are also mounted at different depths in the steel plate, a low variability among them also supports the validity of the assumption of one-dimensional heat transfer through the FRM that is critical to the quantitative analysis. In Figure 6, it is clearly observed that even after the furnace is turned off (as indicated by the peak in the furnace and outer FRM temperature curves near 100 min), the temperature in the steel slug continues to rise due to the thermal inertia (lag) of the system. When the interior temperature of the slug exceeds the exterior temperature of the FRM, the direction of the specimen heat flow reverses.
One performance criterion that could be conveniently extracted from the data in Figure 6 is the time necessary for the steel slug to reach some specific temperature, such as 538 ºC for example.1 For the initial heating cycle of FRM A shown in Figure 6, approximately 105 min were required for the steel slug to achieve this temperature. For two subsequent heating cycles, these times were observed to be on the order of 110 min. These results suggest that most of the performance of this particular FRM is achieved via its "low" thermal conductivity and not via the contribution of significant endothermic reactions, as will be discussed in more detail when the computed thermal conductivity curves are presented below.
Figure 6-Temperature vs. time data for one heating/cooling cycle for FRM A specimens.
Equation (7) was applied to the data shown in Figure 6 and the computed thermal conductivity values for five different heating/cooling cycles are provided in Figure 7 in comparison to the previously measured values. One of the testing laboratories reported their results for thermal conductivity to be normally within ± 3 %.6 The first three heating cycles followed the furnace heating curve shown in Figure 6. For the fourth and fifth heating cycles, as outlined above, the furnace temperature was ramped to 600 ºC in 4 h and 16 h, respectively. The cooling curves for the third and fourth cycles are incomplete due to power outages that occurred during the course of these experimental runs. The computed thermal conductivity values agree with the values measured previously using either a hot wire6 (ASTM C11131) or a transient plane source (TPS) method7,13 to within 15 % for temperatures up to 600 ºC. The computed values at temperatures above 600 ºC for the first three heating cycles are clearly higher than those previously measured, which could be due to enhanced heat transfer by radiation in these highly porous fibrous materials14 sandwiched between the "radiating" exterior Inconel plates and the interior steel slug. It is observed that, after the initial transients, the data for the five different cooling curves are all quite similar, indicating that an equilibrium had been reached within the FRM with respect to reactions after the first heating cycle.
A comparison of the heating curves is even more informative. The differences among the heating curves for the first and the two subsequent runs should be indicative of the reactions, etc. occurring in the FRM. In the data in Figure 7, two phenomena appear to be contributing to these differences. First, there is an endothermic contribution, most likely due to dehydration of the hydrated cement component of the FRM. This is indicated by the downward "peaks" in the first heating curve that fall below the nominal thermal conductivity values. These endothermic reactions should reduce the heat flow through the specimen and thus appear as a reduction in the "effective" thermal conductivity. In Figure 7, they are clearly present for mean temperatures between 300 ºC and 400 ºC. Interestly, prior to and beyond this range of temperatures, there appear to be several smaller exothermic peaks relative to the baseline curves. There are at least two possibilities for the cause of these apparent exotherms. One would be the presence of true exothermic reactions within the FRM, such as those that would be generally expected during the decomposition of any organic components within the material. The second (more likely) possibility is that this increase in effective thermal conductivity is due to the convection of superheated steam and gases created during the endothermic dehydration reactions. The particular FRMs evaluated in this preliminary study have a high porosity and a fairly open pore structure so that the steam produced during the dehydration reactions (and its accompanying energy) could be easily driven inward towards the stainless steel plate slug. This phenomenon would appear as an apparent increase in the effective thermal conductivity of the FRM material, and would likely only be present during the initial heating cycle. This same transport of steam/gas, or more specifically the lack thereof, is responsible for the often observed spalling of high-performance (low permeability) concrete during fire exposure.15
Figure 7-Effective thermal conductivity results for FRM A in comparison to measured data.6, 7
In Figure 7, it can be observed that by combining the effective thermal conductivity results produced during a complete heating/cooling cycle such as cycle #2, a much larger temperature range can be covered than that covered by an individual heating or cooling curve. This is due to the transient effects present both during the initial part of the heating curve and during the transition that occurs as the direction of heat flow is reversed during the cooling cycle. Much of the transient effect on heating was removed when the heating rate was slowed to either ≈150 ºC/h or ≈37.5 ºC/h, as illustrated by the results in Figure 7 for heating curves 4 and 5, respectively.
Based on these considerations, the following experimental procedure will be adopted for all future testing of FRMs using this slug calorimeter. The first two heating/cooling cycles will follow the heating curve shown in Figure 6, with natural cooling. Finally, a third heating/cooling cycle will consist of heating the furnace to 600 ºC slowly over the course of 4 h, then holding this temperature until it is also nearly achieved by the slug plate, followed once again by natural cooling. These three sets of curves will be used to characterize the thermal performance of the FRM as presented above.
The testing protocol outlined above was applied to the evaluation of FRM B. For the first two heating curves, times of 104 min and 102 min, respectively, were required for the inner steel slug to reach a temperature of 538 ºC, slightly less than those observed for FRM A above. The effective thermal conductivities computed from equation (7) are provided in Figure 8. The results are quite similar to those in Figure 7 with a few characteristic differences. In comparing the heating curves for the 1st and 2nd cycles in Figure 8, it is observed that the endotherms and exotherms are generally smaller in magnitude in comparison to those observed in Figure 7. This is consistent with the lower mass loss (less reaction) of FRM B relative to FRM A (Figure 5). In addition, there is some indication of an additional high temperature endothermic reaction present for FRM B, possibly due to decarbonation of the carbonated portland cement component of the FRM. Secondly, there is a larger observed difference between the slug calorimeter and the previously measured thermal conductivities at higher temperatures (> 400 ºC) than that observed in Figure 7. Because FRM B has a significantly lower density and higher (open) porosity than FRM A, it would be expected that any increased heat transfer due to radiation between the Inconel plates and the stainless steel slug would be enhanced in this material relative to FRM A. For temperatures below about 400 ºC, where these radiation effects would expected to be much less significant, the agreement between the slug calorimeter effective thermal conductivity values and those previously measured is within 20 %.
Figure 8-Effective thermal conductivity results for FRM B in comparison to measured data.6, 7