Stresses Develop after Redrying

 

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

Problem

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 Bulb0F Wet Bulb 0F EMC% Time
126 95 5.5 6 days
140 110 5.8 5 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%.

Conditioning
The following conditioning schedule was applied.

Phase Dry Bulb 0F Wet Bulb 0F EMC% Time
Warm Up 170 140 5.7 2 hours
1 170 149 7.6 6 hours
II 170 156 9.6 28 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.

 

MC%
A = From Dry Storage Shed 9 – 11
B = After Redrying ~5 ½
C = After Conditioning ~6 3/4

 

(September 1991)