Effect of Prefreezing on the Perpendicular-To-Grain Creep of First-Dried Redwood

The effect of prefreezing on dimensional change was determined for 0.200-inch-thick flatsawn and quartersawn samples loaded perpendicular to the grain during drying at either 100?F or 160?F. Prefreezing reduced shrinkage and compression set for samples tested at both temperatures, but the greatest reductions were for the flatsawn samples tested at 160?F. The reductions developed in total during the removal of free water. For flatsawn samples there was a transient increase in tension set during this period, suggesting that the decreases in compressive set and shrinkage were due at least in part to increased tensile creep. The decrease in dimensional change for prefrozen shrinkage samples was totally accounted for by the elimination of the compressive set that in nonfrozen samples was associated with free water removal. Maximum moisture content (MC) of the test samples was 90 percent. This fact, plus others summarized from past research on redwood pre-freezing, led the authors to conclude that prefreezing produces its dimensional effects during free water removal by some mechanism(s) other than the seeding of saturated cells with air bubbles and the attendant elimination of Tiemann type liquid tension stress. The results show that reduced board shrinkage and the elimination of collapse in prefrozen redwood lumber are attributable to modified creep and set behavior during free water removal.

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