Effect of nine-year soil contact on physical performance of crude tall oil impregnated, copper salt impregnated, and non-treated Scots pine posts

Crude tall oil (CTO), a side product of sulphate pulping, has promising results in moisture and decay resistance. This article reports the Pilodyn indentation hardness, basic density, and mechanical performance of 600-mm long Scots pine posts after a nine-year soil contact test. The material consisted of round and square-shaped CTO-impregnated posts, supplemented by copper saltimpregnated and untreated reference posts. In total, 240 posts were installed in fertile soil in south-eastern Finland in spring 2010. Posts were harvested in May 2019. At the end of the test, 192 posts were in a condition that allowed the preparation of specimens for density and compression tests. Discs were sawn from the ground level, as well as above and below the ground level to measure the density and deformation under static 45,000 N parallel-to-the-grain compression load. Altogether 80% of the untreated control posts were rejected from further tests due to severe decay. Square-shaped control posts had a slightly higher survival rate than the round ones. After a nine-year soil contact, the performance of CTO impregnated. Scots pine posts were clearly better than that of untreated control posts and comparable to that of copper saltimpregnated posts. The study suggests that CTO has a high potential as a non-biocide wood preservative even in structures in soil contact.