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Use Category for Preservative-Treated Wood in Northern Regions Containing Permafrost

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The effect of climate change on the loss of Arctic sea ice has attracted global attention.

One important issue is the lowering of permafrost as the soil surface warms.

The most common structural use for timber in the Arctic and Northern Regions where there is permafrost is as pilings to support buildings and as retaining walls to contain large-dimension timbers. Over time, the combination of bacterial and fungal decay due to thermal losses in spaces below the buildings has left many of the untreated piles in need of replacement. The loss of permafrost, coupled with the previous use of untreated timber, has created an urgent need to replace the aging infrastructure.

In addition, there are initiatives to expand commercial activities in the Arctic, increasing the need for new infrastructure in this region. Much of this infrastructure, made up of timber piles and structural timbers, should involve preservative-treated wood.

The treatment of timber for use in structures is standardized in Canada in the CAN/CSA-O80-15 Wood Preservation Standard. CAN/CSA-O80.1-15 Specification of Treated Wood [5] recognizes different Use Categories (UC) for treated wood, ranging from UC1 for wood used indoors to UC5 for treated wood used in the marine environment. The UC that applies to treated wood used in ground contact is UC4. This UC currently applies for all new and replacement infrastructure in the Arctic and Northern Regions, where contact with the ground occurs. However, when considering the risk of decay of timber, no allowance has been made for the influence of temperature.

The purpose of this study was to assess decay in treated wood in regions containing permafrost to inform the parameters of a future UC4 for the Arctic and Northern Canada. This new UC4 would seek to balance the lower risk of decay arising from the low temperatures present in regions containing permafrost with the important need to ensure long-term performance.

Core samples were collected from four structures in regions of the Arctic and Northern Regions Containing Permafrost. The results from the structures examined support the adoption of a creosote retention of 120 kg/m3 and chromated copper arsenate (CCA) retention of 6.4 kg/m3 for preservative-treated wood used in structural applications in regions where permafrost is affecting foundation construction.

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