One of the action items that came out of the initial FEMA study 18 of the collapse of the World Trade Center was that the durability of FRMs is a little-considered but critically important component of their long term performance. In response to this, Underwriters Laboratories, along with the FRM industry and end users, are developing a draft standard to assess the durability of FRMs19, based on their existing evaluations of intumescent coatings for outdoor use. The basic procedure is to expose the FRM to some aging environment and then verify through thermal exposure (fire) testing that the performance of the aged material is at least equivalent to a specified percentage of that of the original material. Performance is generally assessed in terms of the time that it takes a steel (duct) pipe protected with the FRM to reach a specific temperature (typically 538 ºC) when exposed to a standard temperature rise curve "fire environment". In developing these durability exposures, care must be taken that the exposure conditions are both reasonable and applicable to the various classes of spray-applied FRMs. For example, as shown in Figure 10, the current practice of exposing intumescents to a temperature of 70 ºC for 270 days can result in considerable mass loss for other types of spray-applied FRMs, even for much shorter exposures of two to three months (particularly those based on gypsum binders). Since the loss of water due to dehydration during a fire exposure is one of the mechanisms by which these materials "insulate" the steel substrate, it would be expected that these "aged" materials would exhibit an inferior performance in comparison to their original counterparts. But, is it the material performance or the aging conditions that should be called into question? When moisture is added to the aging exposures, the degradation may become even greater for conventional fibrous insulating materials.20
Figure 10: Mass loss (fraction) upon long term (two month to three month) oven exposure to different temperatures for gypsum-based (FRM-D) and portland cement-based (FRM-E) FRMs (sample size of ≈3 g).