Protection of untreated wood by adjacent CCA-treated wood

Sandwich test units with two treated outer boards and one untreated center board were exposed at several test sites as part of a collaborative evaluation of aboveground test methods for a task force of the American Wood-Preservers’ Association technical committees. Examination of material at a site in Vancouver, British Columbia, revealed that the untreated center boards between outer boards treated with chromated copper arsenate (CCA) were decaying more slowly compared to untreated center boards sandwiched between untreated outer boards or boards treated with 3-iodo-2-propynyl butyl carbamate (IPBC). Nondestructive analysis of the surfaces of the center boards provided estimated copper metal loadings of 0.6 mg/g wood on surfaces in contact with boards at 2.0 kg/m3 CCA and 0.8 mg/g Cu on surfaces in contact with boards at 4.0 kg/ m3 CCA. Arsenic and chromium peaks were not detected. These results suggest that low levels of copper readily migrate during service and such movement can protect untreated wood exposed during checking from colonization by wood-rotting basidiomycetes.

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