Effect of Moisture on Flexural Properties of Clear Southern Pine Plywood

Flexural properties (MOE, MOR, and FSPL) of clear southern pine plywood were obtained at the following MCs: 4, 7, 10, 13, 16, 19 percent, and water-soaked condition. The relationships between MC and flexural properties were established for three-ply, four-ply, and five-ply southern pine plywood. Two moisture content strength adjustment formulas were examined and compared with experimental results. One was a linear formula, strength = a – b (MC), and the other was an exponential formula, Log (strength) = a – b (MC). It was found that these two formulas fitted experimental data equally well. It is recommended that the simple linear formula be used for moisture-flexure adjustment of southern pine plywood. The magnitude of reduction was approximately the same for all plywood constructions. In general, MOE, MOR, and FSPL of southern pine plywood was reduced 2, 3, and 4 percent (base on the strength value at 12% MC), respectively, in response to a 1 percent increase in MC. Flexural properties obtained at any MC level should be adjusted first to 12 percent MC, then to the desired MC, according to the adjustments percentages mentioned above. Adjustments of flexural properties of plywood for MC changes were valid between 7 and 28 percent MC.

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