Evaluation of strength properties of fire-retardant treated wood using the 1983 National Forest Products Association protocol

As part of a research program on treatments for wood, a new testing protocol developed by the National Forest Products Association was evaluated using a single proprietary fire-retardant formulation. Small, clear samples of southern yellow pine, Douglas-fir, and white spruce were tested for modulus of rupture (MOR), work to maximum load (WML), modulus of elasticity (MOE), specific gravity (SG), shear parallel to grain, compression parallel to grain, and tension parallel to grain. Ratios of treated to untreated properties were calculated and Student’s t-tests were computed for all properties within each species. Treated samples were significantly weaker than control samples for WML and tension strength. Strength loss in MOR was significant except for spruce samples. Differences between treated and control samples were not significant for MOE, shear strength, and compression strength. Attempts to account for property reductions by fire-retardant retention level and thermal degradation effects due to redrying were generally unsuccessful. The variation in reduction for different properties demonstrates that design reductions should be based on the property in question. The need for additional work is also discussed.

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