FIRE RETADANT-TREATED WOOD FAILURES

FIRE RETADANT-TREATED WOOD FAILURES

Fire retardant-treated (FRT) wood (lumber and plywood) is an important component of non residential commercial and residential constructions. It has been used successfully in structures exposed to temperatures under 100°F. However, a substantial number of problems and failures have occurred in buildings constructed with FRT wood and, in particular, FRT plywood roof sheathing.

Following are some of the findings of the U.S. Department of Agriculture’s Forest Service Forest Products Laboratory, in Madison, Wisconsin, relative to this issue:

• Most fire retardant (FR) treatments initially reduce wood strength 10 to 25 percent; the magnitude of the reduction varies with the FR treatment and the property being considered. Considered as a whole, the results of several studies suggest clear wood property’ reductions from fire retardants and kiln-drying averaging 13 percent for modulus of rupture (MOR) and five percent for modulus of elasticity (MOE).
• Two classes of chemicals are used commercially as fire retardant treatments for wood: inorganic salts and organic salts. Monoammonium phosphate (MAP), an inorganic salt FR, was a major chemical component used in the FR-treated plywoods now experiencing roof-sheathing failure. Guanvlurea phosphate/boric acid (GUP) and dicyandiamide phosphoric acid formaldehyde (DPF) represent commercial interior and exterior organic FR salts, respectively.

Fire retardants lower pyrolysis temperature and increase the residual weight of the char, compared with untreated wood, thereby reducing the amount of combustible volatiles formed.

• Investigations have shown that field problems resulted from thermally induced acid degradation of wood carbohydrates by the acidic form of the FR and that the relative effects of many FR treatments can be classified by the type of fire retardant employed and the time-temperature combination required to convert the FR formulation into its acidic form.
• Different formulations have had different effects.
• After 160 days of constant exposure to temperatures of 180°F, most fire retardant chemicals caused significant reductions in strength properties in experimental samples. At 130°F, strength reductions for most chemicals were not significant.
• Each fire retardant studied accelerated strength loss, but the inorganic salt monoammonium phosphate (MAP) had a significantly greater effect dian other exterior or interior organic salts studied. MAP-treated plywood is lower in bending strength than untreated plywood at all temperatures.

As temperature increases, the rate of strength degradation is similar between untreated and MAP-treated plywood. Some tests have shown that as relative humidity increased at 170°F, the rate of strength degradation increased; the effect of relative humidity, however, appeared to be less influential than that of temperature.

• Even untreated wood, however, suffered permanent thermal degradation when exposed at 180°F. Treated and untreated material showed similar rates of strength degradation once degradation began.
• Untreated wood is not recommended for use in load-carrying structural applications when environmental temperatures exceed 150°F. Most fire retardants have been shown to reduce the strength and stiffness of treated wood products significantly.
• The combined effect of FR chemicals and temperature is probably greater than would be expected from the sum of the individual effects.

FR-treated wood has been used for nearly 50 years in the United States. Since FR treatments reduce wood strength, strength reduction must be considered in the design process. Additional strength reductions related to thermal degradation recently have been encountered with the use of some FR treatments for plywood roof sheathing. Based on the unacceptable performance of some FR formulations, FR-treated lumber and plywood should not be considered interchangeable commodity items. New test methods are being developed to differentiate among various proprietary commercial FR treatments.

Building inspectors have accepted two nondestructive test methods fo evaluating the residual strength of FR F plywood. One technique applies a bending proof load to a section of material. The other measures the force necessary to extract a probe embedded into a member

Additional information is available from the Forest Products Laboratory, One (iifford Pinchot Drive, Madison, WI 53705-2398.

References

1. LeVan, Susan L., Jerrold FI. Winandy. “Effects of Fire Retardant Treatments on Wood Strength: A Review. Wood and Fiber Science. 1990; 22(1 ):113-131.
2. Ross, Robert J., John Cooper, Zhitong Wang. “In-Place Evaluation of Fire-Retardant-Treated Plywood.” USDA Forest Service, Forest Products Laboratory (Madison. Wise.), Sept. 1991.
3. Winandy, Jerrold E. “Fire-Retardant-Treated Wood: Effects of Elevated Temperature and Guidelines for Design.” USDA Forest Service, undated.
4. “Effects of Fire Retardant Treatments on Strength Properties of Wood: Executive Summary of Workshop, USDA Forest Service laboratory, Apr. 26, 1988.
5. “Effects of Fire Retardant Treatments on Wood Strength at Elevated Temperatures.” Techline. USDA Forest Service Laboratory’, #9009.
6. “Performance of Fire Retardant Treated Wood Under High Temperatures. Techline. USDA Forest Service Laboratory, #9001.