Mechanical properties of fire-retardant-treated plywood after cyclic temperature exposure

Some fire-retardant-treated (FRT) plywood used as roof sheathing has demonstrated loss of strength in service conditions. Research at the Forest Products Laboratory is aimed toward identifying the failure mechanisms of FRT wood, the chemicals that contribute to wood degradation, the temperature levels at which degradation occurs, the influence of moisture content (MC), and the correlation between these factors and the rate of degradation. To date, these efforts have led to the development of constant temperature test regimes for both plywood and lumber (ASTM D 5516-1994 and D 5664-1995, respectively). The objective of the research addressed here is the determination of how well constant temperature cycles relate to actual cyclic in-service temperature fluctuations. In this study, a special grade of southern pine plywood was treated with two fire-retardant chemicals and subjected to a cyclic temperature variation from room temperature to 65?C at two targeted MC levels, 6 and 12 percent. Results indicate that modulus of elasticity (MOE) and modulus of rupture (MOR) were relatively unchanged. The MOR values indicated a slight negative trend at both targeted MC levels, although the MOR values for the 12 percent specimens were slightly less than the MOR values for the 6 percent specimens. The most affected strength property was work to maximum load. The treated material showed a greater tendency for degrade than did the untreated material. In comparison with data from a plywood study using 65?C and 12 percent MC at constant temperature exposure, cyclic exposure appears to be less severe than constant temperature exposure. However, uncertainty regarding the internal temperature of the plywood could account for the difference in severity.

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