Service life prediction of wood and wood products
Background and need for service life prediction
A unified and harmonised system for performance classification and specification of wood and wood-based products in Europe is still lacking. At the same time the need for such a system becomes more and more obvious. Users and customers have a strong interest in reliable information on the performance of wooden products to be expected. They are the decision makers on the market and will particularly decide, if wood-based products or other substitute building materials will be applied.
For the user the only valuable product information needs to base on performance characterisation. To determine the relative protective effectiveness of a wood preservative, as it is done according to different European standards, e.g. EN 113 (1996), ENV 807 (2001), is not very helpful for the user. For potential customers it is essential to assess the possibility that a product will meet his requirements (i.e. the desired service life). For this purpose ideally reliable service life estimation is needed, but at least a product specification by means of performance classes. Furthermore, such a system should not be limited for a particular group of wood products only, but universally applicable for all wood-based products:
(a) wood treated with classical wood preservatives
(b) wood treated with new organic preservatives
(c) natural durable wood
(d) modified wood
(e) wood-based composites.
Secondly, the need for performance classification emanates from the European Construction Products Directive, CPD (1988) on the approximation of laws, regulations and administrative provisions of the member states relating to construction products. The CPD requires in particular "products fit for an intended use", which can be translated as a sufficient performance over a certain time period – the time period of the intended use.
Thus, for the wood industry as well as for wood scientists two important requests derive from the CPD: 1. Deliver suitable test methods to assess the performance over time of wood-based products, and 2. Establish a Europe-wide harmonized classification and specification system.
Wood moisture content and wood temperature – Quantification of key factors
Numerous factors have a direct or indirect influence on wood decay and thus on its service life to be expected. Hereby wood moisture content and wood temperature act as key factors. To quantify the impact of these two decay factors a measurement system was developed to allow automatic recordings in the field as well as on real buildings in service. The system was applied in field trials at 30 different sites in Europe and the United States and worked reliably over a period of more than 8 years.
Long-term moisture recordings were carried out on different objects and within field studies: On the pedestrian timber bridge in Essing, Bavaria, which showed significant brown rot decay due to poor protection by design, moisture induced decay risk was detected in dependence of orientation, height above ground and contact to metal fasteners, which inhibited the re-drying of wooden components.
Service life prediction on the basis of dose-response functions
The mathematical basis for service life prediction of wooden components was developed in terms of dose-response functions derived from field test data recorded at 27 different European test sites during 8 years.
Besides the annual decay assessment of all wood specimens moisture content and temperature were recorded daily. The dose was calculated as a combination from daily values of both parameters, which was most highly correlated with corresponding decay ratings. A computer-assisted optimization led to one sigmoid function for all test sites.
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