Sustainable outdoor use of larch wood through classification of durability by means of innovative measurement methods

In today's wood industry rapid and safe determination of characteristics such as wood density, bending strength or branchiness is state of the art. However, there is presently no industrially applicable method for a quick evaluation and classification of "natural durability". Natural durability of wood is defined by its resistance against fungal or insect attack, which is partly caused by toxic ingredients in the heartwood. Natural durability of wood is a fundamental concern in outdoor uses. It is the goal of this project to develop innovative technologies that determine natural durability of larch wood quickly and simply, which facilitates the use of chemically untreated timber. The latter goal will also help to make the sustainable produced wood more competitive against other products. Own research results give qualified reasons for a technical realisation in the near future. With state-of-the-art methods a sound basis is compiled, which is required to set the conditions for a successful industrial implementation.

Wood in exterior use is permanently exposed to weather influences (UV-radiation, dehydration, rain etc.), which directly attacks the surface. Furthermore, the associated changes of the wood moisture enable successful infestation by decay fungi or insects.

Goals of this study:

• Knowledge of the variability of the natural durability of larch wood used in Austria; including forest locations and proveniences.
• Research on the relationships between natural durability, heartwood colour and extractive content of larch heartwood from Austria as well as imported larch.
• Effects of "disturbance factors" such as branchiness, compression wood; air humidity, temperature and type of surface finish.
• Study of the efficiency of different recording devices, with various operating conditions (off-line, on-line).
• Optimization and evaluation of acquired spectra and models.
• Verification of the new technology under practice conditions.
• First steps for further conversion in subsequent projects.

Four industrial partners, who have been processing native and imported larch in different ways, helped to set the requirements for this basic research:

• Recording of visual and near-Infrared spectra using spectral cameras on lumber (native and Siberian Larch).
• Performing fungal decay tests.
• Wood-chemical investigations and application of spectroscopic methods to acquire spectra in the infrared range.
• Statistic analysis: calibration models and verification.

The methodology of the project is concentrated on a combination of several optical-spectroscopic measuring procedures, connected with extractive determination (wet-chemical), accelerated weathering as well as fungal decay tests. As opto-spectroscopic methods spectral cameras with wavelength coverage from 400nm to 1100nm are used (also to 1750nm) as well as Fourier-transformed infrared-spectrometer with ranges between 1000 2000nm.

The involved research partners represent high expertise in the area of spectral wood analysis. With the outlined infrared-spectrophotometry spectra from wood surfaces may be taken in non-contact mode using fibre optical probes, with completion of measurements within one minute.

The theoretical basis goes back to hydrogen-stretching oscillations in reflection spectra. These signals contain an abundance of chemical and physical information, which may quickly indicate the natural durability status of a given piece of wood. Spectral cameras have a slightly limited scanning ranges, however, they have advantages such as mobility or easy availability. An optimal calibration between different wood constituents, and/or with fungal mass loss data may be achievable. Wood decay tests are done according to the European standards EN 113 und EN 330.

A successful outcome of this project, which is developing a reliable method to forecast the naturally durability, would strongly help to use more larch wood outdoors. Besides cladding other applications are e.g. timber bridges or balconies also need higher levels of natural durability. With the use of more wood in the future an important contribution to the CO2 problem is made. The pre-fabricated houses manufactures would have sound arguments to encourage more wood used as a contribution to reducing net CO2 emission.