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 commercial 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 center, under cyclic relative humidity (RH) was investigated. The specimens were preconditioned to equilibrium under 65 percent RH at 75 degrees F (23.9 degrees C) and then divided into two groups. The first group 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 (35 degrees C). Identical concentrated loads, equivalent to 16.5 percent of the ultimate static bending strength evaluated at 65 percent RH and 75 degrees F for the weakest group among these six OSB products, were applied to each specimen in both groups for 794 hours followed by a 286-hour recovery after the load was removed. Results indicated that even though the structural wood composite panel products were manufactured under the same product standard, and given equivalent engineering performance ratings commercially, a difference in creep performance does exist. Among the six OSB products, panels made of southern pine flakes and bonded with only liquid phenol-formaldehyde (LPF) or bonded with powder phenol-formaldehyde (PPF) in the face and isocyanate resin (ISO) in the core showed much better creep resistance than those made of aspen flakes and bonded with only PPF or bonded with only ISO or bonded with LPF in the face and ISO in the core. Noticeable differences in the creep resistance among the southern pine groups were not observed. However, among the aspen groups, ISO-bonded products had the lowest creep and PPF-bonded products showed the highest. Under identical cyclical RH conditions of 65 <----> 95 percent, a considerable temperature effect on the creep resistance was observed in the groups exposed to the high-temperature environment.