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Understanding the durability of shell CCA treated decking

University of British Columbia

CCA has been the dominant preservatives used in North America representing 70 % of total treated wood, and 70 % of this volume has been used for above ground construction. Due to a high portion of heartwood in Canadian softwood, which is hard to treat with chemical solutions, much of CCA treated wood placed in service had limited penetration (less than 5 mm). Those materials were expected to fail early because untreated check surfaces were often exposed during weathering. However, decay has been seldom observed in these checks even after many years of exposure. The hypothesis worked on in this thesis was that minor amounts of mobile CCA preservative components redistribute during weathering into checks, and that this 'surface treatment' prevents fungal spores arriving in checks from germinating and causing decay. A substantial amount of copper was found on the exposed check and end-cut surface in exposed wood after only four months of exposure, and substantial amounts of all CCA components were detected in long term exposed materials. Among the components, copper was considered to be the most important in protecting untreated exposed surfaces since the amount of copper (0.26 mg/g wood) found at a check surface in service prevented fungal colonization through basidiospore germination in a laboratory test, even for a copper tolerant fungus. In contrast a higher amount of arsenic (0.42 mg/g wood) than found at a check surface failed to prevent fungal colonization through basidiospore germination in a laboratory test. Since exposed untreated surfaces were heartwood, the effect of heartwood on development of decay through basidiospore germination was examined in this research. Decay of western hemlock heartwood resulted from fungal colonization through basidiospore germination and weight loss of heartwood was not different from that of western hemlock sapwood in the lab test. This indicates that untreated heartwood (non durable) may have a minor contribution to the good performance of shell CCA treated decking. Despite the role of mobile chemicals, eventual decay of shell CCA treated decking was observed after long term performance (over 15 years). G. sepiarium was the major decay fungus isolated from CCA treated decking. Decay was associated with a check at the early stage of decay, indicating that a check provide the access avenue for fungi. However, visual decay was not observed in most boards collected and the percent frequency of basidiomycetes from the inner parts of the boards was less than 5 %. On the other hand, fungi that can cause soft rot were dominant in CCA treated decking over all exposure periods although soft rot damage was found on only 21 year old material. The high percent frequency of soft-rot fungi over all exposure periods may indicate that the final decay of shell CCA treated decking could be by soft-rot fungi although the process would take much longer than for ground contact material. A thorough understanding of shell CCA treated decking has an important implication for the development of an alternative preservative for above ground application. The work in this thesis has shown that mobile chemicals protected untreated exposed surfaces from fungal attack through basidiospore germination. Therefore, chemical mobility is essential for the good performance in the case of shell treated wood exposed above ground, and this characteristic should be considered for an alternative preservative.

Text

2009-12-16

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http://hdl.handle.net/2429/16883

Forestry

University of British Columbia

UBCV

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