Redwood Durability in NZ – Can pure culture laboratory tests predict outdoor service life

IRG/WP 17-20610

D O’Callahan, C Chittenden, J van der Waals, D Meason, T Singh

Coast Redwood (Sequoia sempervirens) is an alternative species in New Zealand to the large scale Pinus radiata plantations. Redwood is classified as moderately durable but its durability is extremely variable. In order to reduce this variability in future stock, understanding growth conditions, clone, and tree age effects are important however this involves many samples and is not suitable for assessment in field trials where tests take many years to complete. Therefore laboratory trials have been used to assess the multitude of parameters. But how reliable are these in assessing in-service durability? In this study we have compared the data from EN113 laboratory tests with in-ground and above ground data from trees of similar ages and growth sites to understand the correlation between laboratory and field data along with natural durability classification methods. The European standard for natural durability has recently been updated and the method in which durability class is calculated in the laboratory has changed therefore in this paper we have calculated durability class for laboratory samples using both EN350:1994 and EN350:2016 methods as a comparison. For field trials we have used the Australasian standard durability classifications (AS5604:2003) and the European standard classifications (EN350:2016) to calculate durability class. It was found that durability classifications of laboratory samples that were subjected to leaching (EN84:1989) before exposure correlated well with in-ground field tests but that there were differences in durability classification between classification methods with the laboratory classifications being slightly more durable using the new EN350 method compared to the old method. Similarly the Australasian standard gave higher classifications in general than the European standard for in-ground tests. The durability rating of non-leached blocks related well to above ground exposure trials even though above ground tests were subjected to rain and UV exposure. The effect of extractive wash off would need to be studied to get a true correlation. It was deemed important to include leached and non-leached laboratory samples wherever possible to understand the range of durability of each wood species. It is not recommended to rely totally on laboratory data although laboratory tests are beneficial for screening lots of different elements.

Keywords: Redwood, Sequoia sempervirens, natural durability, laboratory, field tests

Conference: 17-06-04/08 Ghent, Belgium

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