The Effect of Unidirectional Diffusion and Prefreezing Upon Flexural Creep in Redwood

Minature beams were prepared from redwood heartwood and then dried from the green condition under static load at both 100?F and 150?F. The influences on relative creep (creep deflection divided by elastic deflection) of both prefreezing and controlled diffusion were studied. Unidirectional diffusion was achieved by covering all but the compression or tension face of the beam with aluminum foil. In other tests, drying was prevented by completely encasing the beam in aluminum foil. It was found that moisture loss is a requirement for prefreezing to effect a reduction in creep, and that the mechanism for this reduction appears to reside in the fibers dehydrated while under compressive stress. Both the amount of creep and the reductions due to prefreezing were greatest at 150?F. Due to similarities in the results obtained in the flexural creep tests and kiln-drying studies, it is suggested that the effect of prefreezing upon redwood deformation during drying resides in the amorphous lignin matrix. It is thought that extractives form a part of this matrix and that they normally impart high compressive creep to the matrix during drying at high temperature. This was supported by the fact that cold-water extraction of thin cross sections from a collapsed redwood board recovered less during steam reconditioning than the serially matched, unextracted sections.

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