THE ROLE OF GRADE AND THICKNESS IN THE DEGRADATION OF FIRE-RETARDANT-TREATED PLYWOOD

In some cases, fire-retardant-treated plywood used since 1980 for roof sheathing has rapidly degraded and failed, apparently because of ther mally induced acid hydrolysis. This study sought to determine whether plywood grade or thickness influences the manner in which fire-retarda nt treatment (FRT) and subsequent high-temperature exposure affects th e strength properties of plywood. The effects of FRT were evaluated on two thicknesses and three commercial grades of southern pine plywood as well as plywood constructed from nearly defect-free N-grade veneer. Specimens were treated with monoammonium phosphate (MAP), then subjec ted to exposure at 66 degrees C (150 degrees F) and 75 percent relativ e humidity for either 30, 60, or 90 days, Modulus of rupture (MOR) wor k to maximum load (WML), and modulus of elasticity (MOE) of the specim ens were evaluated. Results suggest that the rate of plywood degrade r esulting from FRT, redrying, and subsequent high-temperature exposure is largely independent of plywood quality or grade. Although the initi al strength loss caused by FRT and redrying appeared greater for the t hinner plywood, degrade during subsequent temperature exposure appeare d to be independent of plywood thickness. Thus, it appears that findin gs from previous studies on thermal degrade using high quality, N-grad e plywood are readily applicable to commercial grades and thicknesses. Evaluation of the effects of knots and voids on MOR revealed that the se defects an only partially responsible for the difference in bending strength among specimens.