Effect of pressure drying and pressure steaming on warp and stiffness of radiata pine lumber

Matched 100- by 40-mm sawn lumber was dried in three kiln runs: run 1: dry 160/100?C, steam 100/100?C (high-temperature (HT) control); run 2: dry 160/100?C, pressure steam 130/130?C; run 3: pressure dry 160/130?C, pressure steam 130/130?C. Times for pressure drying and normal HT schedules were similar but pressure steaming required only 1 hour compared to 3 hours at normal settings. The pressure treatments caused noticeable darkening but less residual stresses. Compared to the HT control, normal HT drying/pressure steaming gave 25 percent lower twist and approximately one-third less rejection for twist in excess of pre-set limits. Combined pressure drying/pressure steaming gave no improvement in warp as compared to normal HT drying/pressure steaming. Machine stress grading indicated that the material subjected to both pressure drying and pressure steaming was approximately 12 percent stiffer compared to the HT control material. However, small-clear results indicated that the material subjected to both pressure drying and pressure steaming was approximately 8 percent less stiff. Mean bending strength and mean compression strength were not significantly affected. In conclusion, pressure treatments are unlikely to have a noticeable effect on the strength of structural timber. The variation in the structural timber properties due to defects such as knots, sloping grain, etc., should mask any effect caused by the pressure treatments. It is concluded that pressure steaming after normal HT drying will significantly improve drying quality and the process could be easily implemented commercially.

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