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Stresses Develop after Redrying


Stresses Develop after Redrying
Stephen J. Hanover
Associate Professor and Wood Products Extension Specialist
North Carolina State University
Robert C. Gilmore
Superintendent, Hodges Wood Products Laboratory
North Carolina State University


Properly kiln dried lumber stored, dead packed in an enclosed, unheated shed may pick up moisture. Redrying may be necessary. Drying stresses do develop during redrying and should be relieved.

Test Material
Properly kiln dried and stress relieved 4/4 red oak was placed dead packed in an enclosed, unheated shed. Average initial moisture content was 5.5%. Some checks were observed. After several months, moisture samples showed 9 – 11%. Stress sections showed no stress (Sample A in figure.)

Redrying Schedule
The lumber was stickered and placed into the kiln. The following schedule was used:

Dry Bulb0FWet Bulb 0FEMC%Time
126955.56 days
1401105.85 days

These mild conditions were used for the purpose of reducing the re-opening or deepening of existing checks. The length of time was excessive, but time was not of essence in the test and small moisture content variability was an objective. After redrying, the lumber had stress. (Sample B in Figure). Average moisture content was about 5.5%.

The following conditioning schedule was applied.

PhaseDry Bulb 0FWet Bulb 0FEMC%Time
Warm Up1701405.72 hours
11701497.66 hours
II1701569.628 hours

Total time after set points were reached was 34 hours. Average moisture gain was 1.25%. Stress tests cut while the lumber was hot showed slight outward turning of the prongs as expected. After three days, prongs straightened, showing no stress. (Sample C in figure.) The major reason why conditioning was done in two steps was to avoid too rapid of an initial moisture gain. If such would happen, there may be chances of reverse case hardening, resulting in open checks.

A = From Dry Storage Shed9 – 11
B = After Redrying~5 ½
C = After Conditioning~6 3/4

(September 1991)