The main 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 project is divided into two phases; the pilot study reported here is the first phase of the project. The objective of this phase of the work was to develop an appropriate test method.
The Moisture Management in Exterior Wall Systems 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 to 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 developed which was anticipated to provide suitable strength loss data within the constraints of the funding available. The proposed method will subject sheathing samples to various combinations of temperature and humidity and to repeated inoculations with small amounts of a wood-rotting fungus to simulate natural infection. The samples will be monitored first using non-destructive testing and then destructively tested when a change in strength properties is detected. The result is a two-stage test at a range of temperatures and humidity levels, giving a measurement of time to strength loss.
This report is the final one on a pilot study to develop, refine and verify the proposed test method. "Method B" of ASTM 3043 (ASTM, 1999) was evaluated to determine if it was appropriate. The test monitored the bending stiffness and bending failure strength of oriented strand board samples, using a 4-point flexure test. The pilot study exposed samples to 20°C and 99.9% relative humidity and monitored bending stiffness loss as a proxy for bending failure strength loss. When there appeared to be a loss in bending stiffness, samples were destructively tested for bending failure strength loss.
Under the pilot test conditions, samples reached equilibrium moisture content within one month and were rapidly infected with xerophilic moulds. These died off within three to four months to be replaced by an unknown basidiomycete, first noticed after about 11 months. Bending strength loss occurred slowly and was first evident at approximately eight months. Presumably the basidiomycete had been active at this stage, but growth had not progressed to the stage of being noticeable to the naked eye. Only minimal strength loss had occurred after one year in test. Unfortunately, an electrical fault outside the safety cutoff caused the conditioning chamber to overheat after 15.5 months, killing the fungi growing on the samples and thereby terminating the test.
Based on results in the pilot study, recommendations have been made concerning moisture control and general operation of the environmental chambers. Recommendations have also been made regarding the disposition of samples in future tests, the time required for conditioning prior to inoculation and the actual bending stiffness and bending failure strength test procedures. The inoculation protocol should be continued but provision should also be made for more uniform natural infection of the test specimens.