Effect of moisture on the flexural properties of commercial oriented strandboards

Moisture content (MC), thickness swelling (TS), and bending properties of five commercial oriented 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) decreased with increases in board MC. Both MOE and MOR followed a linear relation with MC over the given MC change. Thickness swelling of these OSBs was shown to have a large nonrecoverable component, which mainly occurred with MC increases above the 8 to 10% level. For an MC change from 4 to 24%, the combined effect of increased MC and TS led to an average MOE loss of 72% in the parallel direction and 83% in the perpendicular direction; and to an average MOR loss of 58% in the parallel direction and 67% in the perpendicular direction. Predictive equations expressing the bending MOE/MOR and thickness swelling as a function of MC and MOE/MOR loss as a function of thickness swelling were established for various products