In the most recent edition of the CSA O80 wood preservation standards (1997), retention by assay rather than by gauge was specified for chromated copper arsenate (CCA), ammoniacal copper arsenate (ACA), and creosote. Based, in part, on data from Forintek’s field trials, these assay retentions are lower than the old gauge retentions, and lower than the assay retentions which had been specified in the standards of the American Wood-Preservers’ Association (AWPA). The AWPA independently introduced lower retentions for northerly waters in 1995. Continued testing was needed to confirm that the lower retentions would still provide the service life required from marine structures.
The marine tests covered in this report were set up in 1978 in West Vancouver, BC, and in 1984 at two sites in New Brunswick. Red pine sapwood coupons were pressure-treated with a range of retentions of preservatives that were listed in the standards at the time. They were suspended in the water column on metal racks and inspected once a year until 2000 at West Vancouver and 1997 at the New Brunswick sites. They were given a final inspection in 2004.
At the West Vancouver test site, coupons treated to the recommended assay retention with CCA-C (24 kg/m3) were in good condition after 26 years’ exposure. ACA-treated coupons at the recommended assay retention of 30 kg/m3 had failed as a result of surface degradation by bacteria and fungi. While significant on a thin test coupon, such degradation on a marine pile or timber would have less effect on the strength of the structure. Wrapping the ACA-treated coupons for a period after treatment to simulate the drying rate of large-dimension commodities did not improve performance, nor did using an alternative formulation with a higher proportion of copper. The performance of creosote at above-the-recommended retention was superior to ACA, but significantly inferior to CCA.
Untreated coupons failed in less than a year at West Vancouver and between 2 and 3 years at the New Brunswick sites, Shediac Bridge and Whitehead Island. At Whitehead Island, coupons treated to the recommended assay retentions with CCA and ACA were still performing well after 20 and 17 years’ exposure, respectively (higher retentions of ACA were added 3 years after the experiment was set up). However, at Shediac, while CCA-treated samples treated to 24 kg/m3 remained sound, coupons treated to 30 kg/m3 ACA had deteriorated badly after 10 years in test. In contrast, samples treated to close to the recommended retention with creosote performed better at Shediac Bridge than at Whitehead Island.
Interestingly, the pressure-treated wood has performed better than the test racks. The racks at West Vancouver, made from 3.5 mm thick carbon steel, had moderate corrosion after 10 years and had to be replaced after only 15 years of service.