English Summary

Introduction

This report is written by the Danish Technological Institute, Wood Technology on the basis of their research project "Documentation of Wood Protection by Design", which included both laboratory and full-scale field tests with a link to wooden noise barriers. The laboratory tests were conducted in co-operation with the Danish Building Research Institute.

Background

Wood is a popular building material because it is friendly to the environment, i.e. wood is CO₂ neutral and renewable. Adding chemicals to preserve wood is in conflict with this "clean" image but on the other hand a reasonable durability is requested by consumers and the construction industry.

Furthermore, in recent years environmental issues have caused the Danish authorities, Danish Environmental Protection Agency, to introduce stricter regulations for wood treated against biodegradation. E.g. the production and use of arsenic-containing wood inclusive CCA treated wood has been prohibited.

Therefore, there is a need to develop or rediscover alternative preservation methods. An alternative method is to protect wood by designing outdoor constructions in such a way that the wood does not accumulate moisture. This is called wood protection by design.

No complete documentation has yet been prepared on the effectiveness of wood protection by design nor on the natural durability of wood above ground. A research project has been carried out to document the effectiveness of these measures.

Laboratory Tests

Documentation of service life of wood and wood constructions requires long term natural exposure but in this project accelerated testing was used. The effect of the design parameters was measured by exposing them to a significantly harder climate in a driving rain chamber and two climate simulators than they would normally be exposed to.

• In a driving rain chamber the protecting effect of different types of painted drip caps was tested. The test parameters were: slope, depth and design of the drip notch.
• In the climate simulator tests were made to evaluate a number of selected test parameters. The test specimen dimensions were 19 x 85 x 361 mm. The samples were exposed to artificial ageing in the form of cyclic exposure to heat, UV radiation, rain, frost and thaw. Six wood species as well as various surface treatments and orientations etc. were tested.
• An exterior wall unit measuring 2900 x 3200 mm with bevel siding made out of larch (European and Siberian origins) and WRC (Danish and Canadian origins) was tested in a large climate simulator. In order to test various window details and ways of building-in windows, two windows were fixed in the unit. The wall unit was tested for six months in climate cycles.

Field Test

However, not all design details are suitable for laboratory testing and, in general, the results from laboratory tests also have to be verified. Therefore, full-scale field tests with the same test parameters were conducted.

A rack with two walls measuring 2500 x 15000 mm facing north and south was constructed on the Danish Technological Institute’s field test area (12° east, 55° north). The walls were finished in February, 1998, and test readings will continue as long as they provide valuable information.

Wooden Noise Barriers

The Danish Road Directorate has been advised in connection with the construction of wooden noise barriers to use non-impregnated wood and wood protection by design. Subsequently, approximately 8 km wooden noise barriers have been constructed at various locations in 1997-98.

The Danish Technological Institute has been involved in the construction of all these noise barriers and the results from the research project have been incorporated. However, some of the noise barriers are used as real life situation field tests as different wood species, end sealing products, surface treatments, and fixing methods have been used in order to prove which are the most suitable for the purpose.

• Wood species/wood types. In general, heartwood is more durable than sapwood. Heartwood of WRC, larch, douglas fir, pine, oak and spruce showed properties which make them suitable as alternatives to chemical treated wood in some above ground situations.
• Orientation/kerfing. Turning the inside face (face near the pith of the log) against the weathering and/or kerfing of the wood on the outside face (face further from the pith of the log) will reduce cracks and scratches of the surface.
• End sealing. The absorption of water is much faster through the end surfaces than through the side surfaces. Using suitable end sealing products prove to reduce water absorption.
• Vertical/horizontal exposure. It is not yet possible to determine which orientation of a siding, horizontal or vertical, provides the longest service life.
• Surface treatment. Rainwater combined with sun will cause wood to decay. Using oils makes the wood resistant to water absorption and decreases the rate of deterioration. However, the effect is dependent on maintenance.
• Fixing method. Different methods of fixing wood are used. Nails, screws and gun nails made from galvanised iron cause rain water run-off stains. It is not yet possible to determine the durability of the different fixings.
• Overhang. Deep overhangs on buildings protect the wood sidings/panels from the rain. The importance of the depth of the overhang in relation to service life is not yet established.
• Design of drip caps. Different designs of drip caps are used, e.g. above and under windows, some more efficient than others. The slope of a drip cap should have an inclination of 1:5 or more and the drip cap should be notched on the lower surface.

Follow-up

In order to get the full benefit of the project the field test will be evaluated systematically in the future.