Unexposed-face temperature criteria in fire resistance tests: A reappraisal

Fire resistance tests for barriers, e.g., walls or floor/ceiling assemblies, utilize unexposed-face temperature criteria as one of the means of determining whether the specimen passes the test. The purpose of a temperature limit on the unexposed face is to prevent fire propagation which might occur by igniting adjacent combustibles in the space not yet fire-involved. The actual criteria date to 1918, with only trivial changes have been made subsequently. Knowledge gained in the intervening decades allows a much more realistic assessment of the potential for ignition. The current literature has been reviewed and the results indicate that the criteria used is excessively conservative. A recommendation is made that 400 °C represents a conservative bound to experimental data on all materials which could see potential use in any facilities that are not dedicated to the storage of hazardous goods (the existing fire-resistance standards are not intended for the protection of hazardous-goods occupancies, in any case). The 400 °C is a fixed value and not a temperature-rise value. Under the existing system, the criteria were two-fold: a limit on the average temperature rise, and a limit on the single-point (worst case) temperature rise. Since the values traditionally used were excessively conservative, the higher single-point limit at least served the function of making the entire criterion somewhat less excessive. This will not be needed if a criterion value is adopted which is more consistent with valid experimental data. Thus, it is recommended that a 400 °C temperature limit be adopted, applicable to any thermocouple location, and that a second, average value not be utilized. The review also indicates that the technique for making the unexposed-face measurement, i.e., the thermocouple+pad assembly, is better conceived in the ISO 834 test, thus it is recommended that this same technique be adopted in ASTM E 119.