The two contributions to the density of any material are its mass and its volume. FRMs are complex in that both of these contributions are changing during a fire exposure. As exemplified in Figures 1 and 2, most FRMs will lose mass in a monotonic fashion during a high temperature or fire exposure, due to some combination of dehydration, decarbonation, and decomposition of organic compounds. Their volume, however, may either increase or decrease. An increase in volume may be observed as the solid network supporting the FRM expands with increasing temperature or more dramatically when an intumescent coating foams during thermal degradation. A decrease in volume may be observed as shrinkage accompanies the mass loss from this solid network.
Figure 1: Example thermogravimetric results for a gypsum-based spray-applied FRM with a nominal heating rate of 5 ºC/min. Results are for two nominally identical ≈50 mg replicates. Maximum observed coefficient of variation (COV) for mass loss between the two replicate samples is 0.9 %.
Mass loss can be quantified using thermogravimetric analysis (TGA), as described in ASTM E1131.2 Of course, the results will vary with the programmed heating rate, sample size, and sample environment. As shown in Figure 1, spray-applied FRMs may lose as much as 25 % of their initial mass during exposure to 800 ºC. This mass loss also provides critical input for calculating the enthalpies of reaction for the in-place FRM. Once a set of reactions is hypothesized, the standard heats of reactions may be calculated and normalized by the measured mass loss to calculate the enthalpy change for the in-place material, as will be demonstrated later in this paper.
Volume changes (thermal expansion) can be measured using a dilatometer (ASTM E228) or interferometry (ASTM E289).2 High temperature measurements (e.g., > 600 ºC) are often complicated by the large dimensional changes that may be experienced in FRMs, along with their generally fragile nature. In addition, spray-applied materials are inherently anisotropic and may thus exhibit different coefficients of thermal expansion in the in-plane and through-thickness dimensions. Typically, the density at any given temperature is calculated as the ratio of the measured mass at that temperature to the measured volume at that temperature.
Figure 2: Example thermogravimetric results for a portland cement-based spray-applied FRM with a heating rate of 5 ºC/min. Results are given for two nominally identical ≈50 mg replicates. Maximum observed COV for mass loss between the two replicate samples is 0.4 %.