Modeling the influence of thermal modification on the electrical conductivity of wood

A model has been developed aiming at the description of the effect of thermal modification on the electrical conductivity of wood. The intention was to calculate the moisture content (MC) of thermally modified timber (TMT) through the parameters electrical resistance R, wood temperature T, and CIE Lab color data, which are known to correlate well with the intensity of a heat treatment. Samples of Norway spruce (Picea abies Karst.) and beech (Fagus sylvatica L.) samples were thermally modified in laboratory scale at 11 different heat treatment intensities and the resistance characteristics of the samples were determined. Within the hygroscopic range, a linear relationship between the resistance characteristics and the mass loss (ML) through the heat treatment was established. Based on this, a model was developed to calculate MC from R, T, and ML. To validate this model, color values of 15 different TMTs from industrial production were determined for estimation of their ML and fed into the model. MC of the 15 arbitrarily heat-treated TMTs was calculated with an accuracy of ± 3.5% within the hygroscopic range. The material-specific resistance characteristics based on experimental data led to an accuracy of ± 2.5%. © 2014 Walter de Gruyter GmbH, Berlin/Boston

Performance of thermally modified timber (TMT) in outdoor application - durability, abrasion and optical appearance

Thermally modified timber (TMT) is increasingly offered in Europe as an alternative to preservative treated timber. TMT durability, infield tests as well as its moisture sorption behaviour in facade application was determined so as to consider its suitability for outdoor use. Additionally, abrasion and crack-formation of TMT deckings were examined and the optical appearance of a TMT facade was evaluated after 5 years of service. After 7.5 years exposure in ground contact, the various TMT materials tested were classed as "slightly durable" to "not durable" whereas the classification in above ground exposure was "very durable" to "moderately, durable", which was in line with the reduced moisture sorption of TMT in weathered application. Moreover, the TMT-decking showed less abrasion and crack-formation compared to references, though the TMT facade revealed considerable discoloration by weathering. Hence, the suitability of TMT for above ground use is suggested, but a surface treatment is obligatory if discoloration is objectionable

Performance of thermally modified timber (TMT) in outdoor application – durability, abrasion and optical appearance

Thermally modified timber (TMT) is increasingly offered in Europe as an alternative to preservative treated timber. TMT durability in field tests as well as its moisture sorption behaviour in façade application was determined so as to consider its suitability for outdoor use. Additionally, abrasion and crack-formation of TMT deckings were examined and the optical appearance of a TMT façade was evaluated after 5 years of service. After 7.5 years exposure in ground contact, the various TMT materials tested were classed as “slightly durable” to “not durable” whereas the classification in above ground exposure as “very durable” to “moderately durable”, which was in line with the reduced moisture sorption of TMT in weathered application. Moreover, the TMT-decking showed less abrasion and crack-formation compared to references, though the TMT façade revealed considerable discoloration by weathering. Hence, the suitability of TMT for above ground use is suggested, but a surface treatment is obligatory if discoloration is objectionable

Statistical analysis of durability tests, part 2: principles of time-to-failure and application on field test data

Service life prediction is an important topic in wood research, especially with regard to the Construction Products Regulation (CPR). Both laboratory tests as well as in-service performance testing is therefore essential, in combination with proper monitoring and analysis tools. A crucial concept is variability and the incorporation of variability in tests and analysis. In this paper we focus on the use of probability density functions (pdf) to describe time-to-failure in field tests as such build further on the pdf fitting as described in part I of this publication. The statistical approach elaborated here, shows that statistically sound results can be obtained even if data are censored, i.e. when for some samples the time-to-failure criterion has not been reached yet or when the exact date of failure is not known e.g. due to non-continuous monitoring, which is normally the case for field testing. Different wood species and treatments exposed in an EN 252 set-up are compared for the test site Hamburg, Germany, and illustrate how a reference product has a longer time-to-failure than e.g. pine sapwood, and shows the sharp failure of the latter. The comparison of double layer samples at different sites resulted in the calculation of acceleration factors compared to the Hamburg site. Also, acceleration factors for shaded and non-shaded set-up were calculated. Seemingly, such an analysis is valuable to rate different sites and to compare different set-ups. It is, however, important that objective and frequent monitoring is aimed at to improve the reliability of analysis and assessment of wood durability

Wood natural durability testing under laboratory conditions: results from a round-robin test

In Europe, the durability of wood against wood-destroying basidiomycetes is tested according to CEN/TS 15083-1 (Durability of wood and wood-based products—determination of the natural durability of solid wood against wood-destroying fungi, test methods—part 1: basidiomycetes, 2005). Existing experience with this standard is quite heterogeneous wherefore six research institutions teamed up and established a new round-robin trial. Fagus sylvatica, Quercus robur, Robinia pseudoacacia as well as sap- and heartwood of Pinus sylvestris, were tested against Coniophora puteana and Trametes versicolor without any pre-treatment, with pre-leaching (EN 84) and with 6 months natural weathering of the specimens. Durability classification revealed differences between test laboratories and depended on pre-treatment and respective statistical measures applied

Determination of the natural durability of solid wood against wood-destroying fungi: a European round-robin test

In Europe the durability of wood against wood-destroying basidiomycetes is tested according to CEN/TS 15083-1 (2005). Hitherto existing experience with this standard is quite heterogeneous and results from previous round-robin tests have stayed unreported or have been reported incompletely. In particular the need for natural pre-weathering of the test specimens to allow potential detoxification of the material is discussed in-depth. Six European research institutions teamed up and established a new round-robin trial. The durability of Scots pine sap- and heartwood (Pinus sylvestris L.), European beech (Fagus sylvatica L.), English oak (Quercus robur L.) and Black locust (Robinia pseudoacacia L.) against Coniophora puteana and Trametes versicolor have been evaluated without any pre-treatment, with pre-leaching (EN 84) and with 6 months natural weathering of the specimens. The durability classification revealed significant differences between test laboratories (up to four durability classes). Furthermore durability was depending on the pre-treatment and the respective statistical measures used. Natural pre-weathering led to an aligned durability classification between some test laboratories, but with some exceptions. A general conclusion about the impact of a pre-treatment on the durability classification was not achieved, wherefore it was neither urgently recommended nor disapproved

Statistical analysis of durability tests, part 1: principles of distribution fitting and application on laboratory tests

Service life prediction is an important topic in wood research, especially with regard to the Construction Products Regulation (CPR). Both laboratory tests as well as in-service performance testing are therefore essential in combination with proper monitoring and analysis tools. A crucial concept is variability in testing and analysis, especially for a biological material such as wood. The larger the sample size the more representative this is for the entire population, yet the number of specimens is often limited by a financial upper limit. Therefore it is essential to use the sub-optimal amount of data and assess as accurately as possible the characteristic under study. In this paper we focus on the use of probability density functions (pdf), also known as distributions. The principles and guidelines for pdf fitting will be explored as well as the use of confidence intervals. The theoretical concepts will be applied on mass loss data. Intra- and interspecies variability but also inter-laboratory variability is illustrated. Therefore the analysis of test results of a round-robin as described in Brischke and co-workers (2013) will be illustrated as well as the analysis of lab tests performed at Woodlab-UGent according to CEN/TS 15083-1 (2005). A validation procedure, as part of a future updated standard, can be useful to erase inter-laboratory differences. Furthermore, the use of a reference wood species can also be an option as a benchmark to compare other species rather than using ‘absolute’ testing resulting in a ranking based on median values. In Part 2 of this paper we will then further use the concepts of pdf fitting for time-to-failure analysis of field test data