Critical times, moisture contents and temperatures for detectable strength loss in plywood and oriented strand board : pilot study|IN: Limiting conditions for decay : compilation of reports.
The project objective is to identify the wood-rotting fungi causing decay in Canadian buildings, and to provide data for a numerical model which will provide an indication of the time required for initiation of strength loss in wood-based panels when exposed to a range of moisture contents and temperatures.
The MEWS consortium led by the National Research Council's Institute for Research in Construction is developing a computer model to predict the moisture and temperature conditions within a construction assembly in service. By including a damage function calculation for the various building components, the model can predict the consequences of these conditions in terms of strength loss.
Forintek's role is to develop an experimental protocol that will be universally acceptable in the field of wood science, and generate a data set from which one could derive a damage equation for wood decay as a function of time, temperature and moisture conditions. Discussions have established that strength loss in sheathing is the first priority.
A series of proposed test methods were examined. In consultation with members of the consortium task force, a method was selected which was felt would provide suitable strength loss data within the constraints of the funding available. Sheathing samples will be subjected to various combinations of temperature and humidity and repeatedly inoculating with a wood-rotting fungus to represent natural infection. The samples will be monitored using non-destructive testing and then destructively tested when the first test suggests a strength change. The result is a two-stage test at a range of temperatures and humidity levels, giving a measurement of time to strength loss.
An initial pilot study is concerned with development, refining and verification of the method. "Method B" of ASTM 3043 is being evaluated to determine if it will be appropriate. The test is monitoring the bending stiffness and strength of oriented strand board samples, using a 2-point flexure test. The pilot study is underway, with samples exposed to 20°C and a relative humidity of approximately 96%. Problems involving moisture control in the environmental chambers have been resolved, as have questions around the sample size, the number of test specimens required in the pilot study, the time required for conditioning prior to inoculation and the actual bending and strength test procedures. The inoculation protocol is being evaluated. At this time none of the test pieces have shown significant losses in bending stiffness. A number of test conditions remain to be defined, however, and these will be established at the conclusion of the pilot study.