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A study was undertaken to obtain increased knowledge of the relationship between moisture content of gypsum plaster and its fire endurance. Specimens 2-ft square were prepared in nominal thicknesses of 3, 1 1/2, 3/4, and 3/8 in. They were conditioned in air at 23 C and 50 per cent relative humidity for periods ranging from 1 to 259 days. Each specimen was exposed to fire, controlled to produce the furnace time-temperature curve defined in ASTM Methods E 119. The fire endurance was taken as the elapsed time to a limiting temperature rise of 139 C on the unexposed surface. The relationships between fire endurance and each of several factors—age, thickness, weight loss, and moisture content—are presented. It is shown that endurance did not change significantly during the initial aging period, despite appreciable change in moisture content. The initial period was followed by an intermediate period of aging marked by more significant changes in endurance. Finally there was an extended period of small changes in endurance. It was shown that endurance was affected by fairly small changes in the moisture content of gypsum plaster as equilibrium was approached.
Ryan, J. V.
Physicist, National Bureau of Standards, Washington, D. C.,