Effect of Moisture on the Flexural Properties of Commercial Oriented Strandboards1

Moisture content (MC), thickness swelling (TS), and bending properties of five commercial oriente 1 strandboards (OSBs) made of two wood species were measured. The measurements were made after specimens were conditioned to equilibrium at 35, 55, 75, 85, 95% relative humidity (RH) and 24°C'. It was shown that bending modulus of elasticity (MOE) and modulus of rupture (MOR) decreasej with increases in board MC. Both MOE and MOR followed a linear relation with MC over the give1 MC change. Thickness swelling of these OSBs was shown to have a large nonrecoverable componen , which mainly occurred with MC increases above the 8 to 10% level. For an MC change from 4 t~ 24%, the combined effect of increased MC and TS led to an average MOE loss of 72% in the parallc 1 direction and 83% in the perpendicular direction; and to an average MOR loss of 58% in the parallc 1 direction and 67% in the perpendicular direction. Predictive equations expressing the bending MOEI MOR and thickness swelling as a function of MC and MOE/MOR loss as a function of thickness swelling were established for various products.