EFFECT OF TEMPERATURE ON THE CREEP OF COMMERCIAL ORIENTED STRANDBOARDS UNDER CYCLIC RELATIVE-HUMIDITY

The effect of temperature on the flexural creep behavior of six commer cial oriented strandboards (OSB products: A, B, C, D, E, and Fl, 23/32 inch (1.826 cm) thick with a span-rating of 24 inches (60.96 cm) on c enter, under cyclic relative humidity (RH) was investigated. The speci mens were preconditioned to equilibrium under 65 percent RH at 75 degr ees F (23.9 degrees C) and then divided into two groups. The first gro up was tested under environmental conditions of RH cyclically changed from 65 to 95 percent on a 96-hour frequency at a constant temperature of 75 degrees F. The second group was tested in the same RH cycle as the first group, but the temperature was maintained at 95 degrees F (3 5 degrees C). Identical concentrated loads, equivalent to 16.5 percent of the ultimate static bending strength evaluated at 65 percent RH an d 75 degrees F for the weakest group among these six OSB products, wer e applied to each specimen in both groups for 794 hours followed by a 286-hour recovery after the load was removed. Results indicated that e ven though the structural wood composite panel products were manufactu red under the same product standard, and given equivalent engineering performance ratings commercially, a difference in creep performance do es exist. Among the six OSB products, panels made of southern pine fla kes and bonded with only liquid phenol-formaldehyde (LPF) or bonded wi th powder phenol-formaldehyde (PPF) in the face and isocyanate resin ( ISO) in the core showed much better creep resistance than those made o f aspen flakes and bonded with only PPF or bonded with only ISO or bon ded with LPF in the face and ISO in the core. Noticeable differences i n the creep resistance among the southern pine groups were not observe d. However, among the aspen groups, ISO-bonded products had the lowest creep and PPF-bonded products showed the highest. Under identical cyc lical RH conditions of 65 <----> 95 percent, a considerable temperatur e effect on the creep resistance was observed in the groups exposed to the high-temperature environment.