The influence of age on retention and fixation of CCA in pressure-treated Kenyan-grown Eycalyptus saligna: Summary of findings
The present study investigates the influence of age on retention and fixation of copper-chrome-arsenate (CCA) in Kenyan-grown Eucalyptus saligna. Samples for the study were obtained from sound trees aged 4, 8, 12, 16, 20 and 24 years.
Test samples were radially cut from round material to represent total sapwood and heartwood for each age class, and measured 50mm in thickness, 100mm long, with widths varying according to the radii of the pieces. The samples were air-dried to an average of 15% moisture content, end-sealed, and treated at a commercial pressure treatment plant with 6% CCA-C (oxide type). They were subsequently conditioned for 6 weeks, air-dried to 15% MC, leached in running tap water for 9 days, re-dried to 15% MC, retentions calculated and results analysed. The results indicated a decreasing trend in retention with increasing age, but
substantially higher fixation of the preservative was achieved in samples from older than younger trees. Samples from 4-year old trees with wider sapwood bands achieved consistently higher initial retentions (31.4Kg/M3) compared to those from 24-year old trees with narrower sapwood bands (9.4Kg/M3). However, losses through leaching were higher in samples from younger trees, 14.5Kg/M3 (46.2%) in 4-year old trees, compared to 0.8Kg/M3 (8.5%) in 24-year trees. It was also noted that both retention
(31.4 -28.6Kg/M3), and percentage losses of CCA through leaching (46.2-41.6%), were higher in samples from trees between 4 and 8 years, but markedly decreased in samples from trees of between 20 and 24 years, 12.8-9.4Kg/M3 and 10.2-8.5% respectively. Fixation of CCA proved to be low in samples from younger trees, 53.8% and 58.4% for 4 and 8-year old trees respectively, but substantially higher, 89.8% and 91.5%, in samples from 20 and 24-year old trees. Treated eucalyptus poles and posts in the country are invariably from younger trees, and the recommended CCA ground contact retention of 18.0Kg/M3 for pressure treatment may not be adequate.
Keywords: Age, E. saligna, CCA, retention, fixation, leaching, heartwood, sapwood