Performance of thermally modified wood during 14 years of outdoor exposure

Abstract

The long-term outdoor performance of thermally modified timber (TMT) from four different industrial heat treatment processes was investigated. Graveyard tests (in-ground exposure, use class (UC) 4) and horizontal double layer tests (above ground, UC 3.2) with and without artificial shading were performed. Results from field tests after 14 years of exposure are compared with results from short-term laboratory tests with pure cultures of wood-destroying basidiomycetes and terrestrial microcosms (TMC). The thermally modified material used in the study was: Plato wood (Norway spruce, Picea abies, NL), Thermowood (Scots pine, Pinus sylvestris, FI), New Option Wood (NOW, Maritime pine, Pinus maritima, F), and oil-heat treated (OHT) wood (Scots pine, DE). In addition, Scots pine sapwood and heartwood, Douglas fir (Pseudotsuga menziesii), Sessile oak (Quercus petraea), and CCB treated Scots pine sapwood (0.7 and 2.8% CCB) were examined as control and reference materials. Based on results from laboratory standard tests with brown and white rot causing basidiomycetes all thermally modified materials were classified as durable to slightly durable (durability class (DC) 2–4). In TMC tests with unsterile soil against soft rot and other soil-inhabiting micro-organisms all TMT were classified as durable (DC 2). Consequently, all TMT showed remarkably increased durability compared to the untreated controls (DC 5) and reference species (DC 3–5). In contrast, according to the results of the graveyard tests all four TMTs were classified less durable (DC 4) as well as Douglas fir and Sessile oak. Also exposure above ground in horizontal double layers led to a significantly worse durability compared to the one calculated on the basis of lab test results: Plato, Thermowood, and NOW were classified as ‘less durable’ (DC 4), OHT performed significantly better. Since not all specimens had failed its durability was preliminarily classified as ‘very durable’ (DC 1). On the basis of 14 years outdoor testing under different exposure conditions durability of four industrially produced TMTs turned out to be clearly less than lab test results indicated. Neither in-ground nor above ground performance met the expectations based on testing with basidiomycete cultures and TMCs. Therefore, short-term durability test methods need improvements regarding their predictive capability for modified wood. In summary, the use of TMT in ground as well as under severe above ground conditions cannot be recommended based on the findings from this field test study.