A number of factors affecting leaching of CCA from treated wood have been variously investigated and discussed. However, the possible effects of moisture movement in treated timbers on leaching have not received attention yet. The present study examines and compares moisture uptake and volumetric swelling in CCA-treated and untreated sapwood of Eucalyptus saligna, and tries to establish a correlation between the mechanics of moisture movement, swelling and shrinking, and leaching. Small samples of E. saligna sapwood were air-dried to 12% MC, pressure-treated with CCA-C (oxide) at a commercial treatment plant, conditioned for 28 days, leached in static water, re-dried to 12% MC and average retentions (8.2 Kg/m3) calculated on a weight-gain basis. The samples were divided into two batches, including untreated controls, oven-dried to constant weight and weights recorded. One batch of samples (including controls) was soaked in de-ionised water until water-saturated, and moisture uptake calculated on a weight-gain basis. The second batch, including controls, was measured to calculate oven-dry volumes. They were then similarly water-saturated, removed, measured and volumetric swelling computed. Water-saturation increased the MC of the treated samples to 67.69% on average, compared to 89.29% in untreated samples. Volumetric swelling in treated samples was similarly lower (6.19%) than in untreated controls (6.59%). However, both moisture uptake and volumetric swelling were significant in treated samples. Exposure to conditions of alternating wetting and drying, as in soils, with intermittent moisture uptake and release, and resulting swelling and shrinking may accelerate removal of CCA from treated timbers, and allow faster microbial colonisation and degradation of the wood. High leaching of CCA and early microbial attack, especially bacterial and softrot fungal attack, is common in CCA-treated timbers exposed to wet soils, where a higher volume of treated timber is normally used. It is probable that leaching of CCA from timbers exposed to soils is accelerated by movement in the wood caused by intermittent moisture uptake, swelling and shrinking.

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