Improving the Durability and Service Life of Wooden Components in Outdoor Applications: the French Approach

Transcription

1 Improving the Durability and Service Life of Wooden Components in Outdoor Applications: the French Approach Magdalena Kutnik 1 Marc Jequel 2 Serge Le Nevé 3 T XX [Theme number, e.g. T 21] ABSTRACT The European Standard EN 335 gives general definitions of Use Classes in different service situations and recommendations for their application to solid wood and wood-based products.accurate assignment of a wooden commodity to a particular Use Class is often critical in actual practice, especially in the case of outdoor applications. Exposing an unprotected (untreated) wooden commodity to environmental factors, such as high humidity and biological agents, may result in its premature degradation. The main objective of the French project Durability of Wooden Components is to highlight how improving the design of wooden structures meant for outdoor applications can reduce the impact of local moisture conditions, and thus also the wood s susceptibility to fungal decay. Field experiments are currently being conducted in four different climatic areas on wooden commodities manufactured using six different untreated timber species and both traditional and improved designs. The ongoing research is strongly related to the desire of the French standardization committee to bring into compatibility different approaches to the service life of wood that are applied according to the current French and European standards and technical guidelines. As an outcome of this study, the range of timber species that could be used without any wood preservative treatment will be hopefully widened and the lifespan (or service life) of outdoor wooden commodities will be extended. KEYWORDS Wood durability, service life, use classes 1 Technological Institute FCBA, Laboratory of Biology, Bordeaux, FRANCE, magdalena.kutnik@fcba.fr 2 Technological Institute FCBA, Laboratory of Biology, Bordeaux, FRANCE, marc.jequel@fcba.fr 3 Technological Institute FCBA, Laboratory of Biology, Bordeaux, FRANCE, serge.leneve@fcba.fr

2 Authors 1 INTRODUCTION Because of its inherent chemical structure, wood is susceptible to biological decay. Its natural durabilityto biological agents varies depending on the timber species, geographical origin, age and growth conditions of the tree, and the presence of heartwood or sapwood and their relative proportions. The service life of wooden commodities, which means how long a product is expected to perform under specific environmental conditions, depends on many factors, which include both the material's inherent characteristics and environmental factors. Exposing wooden commodities to harsh outdoor conditions such as rain, wind and sun highly increases the risks of the material being damaged by biological organisms such as xylophageous insects and decay fungi. Therefore, proper design and protection of wooden products for outdoor use is crucial to ensure the best service life for them as expected by the market and final users. The main biological agents that may damage wood when used outdoors under the conditions of Use Classes 3 and 4 (see the European standard EN for the definitions) are: moulds and stains which are wood colonising fungi that do not cause decay but aesthetically damage the wood, thus lessening the commercial value of the product decay fungi, divided into brown rot, white rot and soft rot fungi, which cause severe mass and strength losses to wood xylophageous insects and termites, which feed on different wood compounds (starch, cellulose) and thus cause significant damage Biological organisms usually attack the outer layers (sapwood), which constitute the non-durable portion of wood. Unlike sapwood, heartwood exhibits natural durability, which may vary from "highly durable" (Durability Class 1, see the European standard EN 350-2) to "non-durable" (Durability Class 5), depending on the timber species and its growing conditions. In order to ensure the best service life, non-durable wood must be treated according to a carefully chosen wood preservation procedure (surface application or impregnation with biocidal products, wood modification, physical protection, etc.). The choice of a particular wood protection technology should be dictated by the wood's natural durability (= the durability class), its susceptibility to preservative treatments (= the impregnability) and its exposure to environmental parameters (= the use class). However, assigning a wooden commodity to a specific Use Class is often difficult and controversial. Additionally, current knowledge about what the reference lifespan (or life in service ) of outdoor timber structures should be is still limited. As a consequence, biological damage (mainly fungal decay) is frequently reported, chiefly caused by inappropriate use of building materials, poor design and bad maintenance generating water traps and increasing the moisture content of the wood. This is particularly true about such common wooden commodities used outdoors as decking, cladding, log houses, and exterior carpentry, which are at constant risk of being prematurely damaged. The idea for the "Durability of Wooden Components" research project arose from the combined desire of the French public authorities and of wood industry professionals to optimize today's quasisystematic use of biocidal preservatives meant for wood products used outdoors in response to the general bad knowledge regarding proper design and use and design, and to value the natural durability of selected French timber species. The general aim of this project is to improve the life expectancy of commodities made with untreated wood and used outdoors under different conditions of exposure and weathering. 2 EXPERIMENTAL SET UP The current test methods used to evaluate the durability of wood species do not very well correspond with the ways of evaluating service life of finished wooden products, therefore more work is needed to improve and refine these methods. Field tests are more time-consuming compared to laboratory tests, but they render results which more closely reflect real-life condtitions (Nilsson and Edlund 1995, Brischke and Rapp 2010). The aim of the experimental phase of the project, initiated in 2009, is to work out an experimental protocol which will make it possible to estimate, in terms of service life, 2 XII DBMC, Porto, PORTUGAL, 2011

3 Short Title the performance in a real-life situations of a range of currently used wooden commodities made of different timber species, This will involve quantifying the impact of material, climate, exposure, and design on the expected service life of selected outdoor wooden components. 2.1 The timber species Six timber species with different levels of natural durability which are of economic importance for the construction industry in France were chosen for the study. Only the heartwood (or at least the most inner parts of the sawn logs when heartwood was not possible to distinguish from sapwood) were used. One softwood species Norway spruce (Picea abies) and one hardwood species poplar (Populus sp.) - were specifically chosen because they are non-durable and thus subject to attacks by insects and fungal decay (durability class 5, i.e. non-durable according to the European Standard EN 350-2). Including non-durable species in the experiment will allow us to evaluate the impact of design on fungal decay s kinetics after a short period of time (2-3 years), which would certainly not be the case with durable species. It should be noted, however, that in reality spruce and poplar are rarely used without any preservative treatment for outdoor applications. Three softwood species Douglas fir (Pseudotsuga menziesii), Maritime pine (Pinus pinaster) and larch (Larix decidua) are commonly used in manufacturing outdoor commodities, mainly claddings and terrace deckings. Their natural fungal durability ranges from 3 (moderate durability) to 4 (low durability), and all three are resistant to wood boring beetles (class D) but susceptible to termite attacks (termite class S). The last species, a hardwood one oak (Quercus robur) - is classified as durable to fungal decay (class 2), durable to wood boring beetles (D) and moderately durable to termites (M). 2.2 The experimental sites Climatic parameters such as heat, rain, wind and UV radiation strongly affect the esthetic durability and susceptibility to fungal decay of wood used for outdoor purposes. While the test set-up may be identical, climate conditions vary from one trial to another. In order to compare the lifespan of wooden commodities under different climatic conditions, four experimental sites were selected in France as follows (see Fig. 1): Montpellier - mediterranean, Charrey sur Saône - continental, Bordeaux - oceanic, and Kourou (French Guyana) - tropical. Prior to initiating this experiment, climatic data were collected over the previous ten years at the selected sites and then analyzed. Average number of days per year with precipitation > 1 mm, as recorded between 1971 and ROYAUME UNI Dry (<100 days) BELGIQUE Medium ( days) Wet (> 150 days) LA MANCHE LUXEMBOURG ALLEMAGNE Charrey SUISSE AUTRICHE OCEAN ATLANTIQUE Bordeaux Kourou Guyana ITALIE ESPAGNE Kilomètres Montpellier MER MEDITER RANEE Figure 1: Locations and climatic data from the experimental sites XII DBMC, Porto, PORTUGAL,

4 Authors 2.3 The wooden commodities Deficiencies in current practices used in designing outdoor commodities made of wood often result in the wood's excessive or abnormal moistening of wood. Wood products can contain zones where rainwater may accumulate, stagnate in a quasi-permanent way, and generate so-called spots of insalubrity," which are generally places where biological (particularly fungal) attacks occur. Mistakes made at the conception stage often lead to a switch of the in-service situation of the wood: wooden elements initially meant for Use Class 3 are finally exposed to a level of biological risk that is higher than expected in this Use Class, such as soft rot fungi which develop more frequently on wood in Use Class 4. As a result, in situations where wooden components are not in ground contact but may permanently accumulate water due to their design or surface deposits, it may be necessary to consider that these situations are equivalent to contact with the ground or fresh water and thus require a higher level of natural or preservative-based protection. To estimate the decay potential of different wood species under various exposure situations, various wooden commodities typically meant for outdoor use were manufactured. Identical sets of commodities were made with spruce and poplar (the less durable species) on one side (set of commodities No. 1) and with oak, larch, Douglas fir and maritime pine (the more durable species) on the other side (set of commodities No. 2) (Fig. 2). Set No. 1 (spruce and poplar) includes: - A horizontal structure made of six elements (decks) that are fixed on concrete blocks; - A house-like metallic structure with different vertical (clads, logs, posts) and inclined elements (posts) attached to it by screws. The orientation of the two sides of the structure was specifically chosen in order to have one side severely exposed to wind-driven rain and one side with less severe exposure. Both sides are comprised of exactly the same wooden elements. Set No. 2 (oak, larch, Douglas fir and maritime pine) includes: - A horizontal structure made of two elements (decks) that are fixed on concrete blocks; - A wall-like metallic structure with different vertical (logs and posts) elements attached to it by screws. The chosen orientation maximizes the exposure of the wooden elements to wind-driven rain. A total of two No. 1 sets and four No. 2 sets were exposed on each of the four selected experimental fields. Set of commodities n 1 Low exposure Set of commodities n 2 Severe exposure x 2 (spruce and poplar) Severe exposure x 4 (oak, larch, Douglas fir and maritime pine) Figure 2: general presentation of the test devices installed on each experimental site 4 XII DBMC, Porto, PORTUGAL, 2011

5 Short Title Identical sets of experimental devices have been so far installed in Montpellier, Charrey sur Saône, and Bordeaux. Installation is still under way in Guyana. 2.4 The selected designs For each family of commodities, different designs were selected in order to progressively decrease the risk of water being entrapped in the wood. Decks: Horizontal structures are regarded as the most severely affected by weathering because of the possibility of rain-water accumulation and stagnation on the wood's surface. Six different designs were chosen, the main differences between them being the thickness (22 or 30 mm) and the width (5 or 12 cm) of the boards, their shape (plain or slope-shaped), the way of screwing them on the joists (top or bottom screwing), and the number of wood-to-wood contact zones generating water traps (direct contact between boards and joists or insertion of nylon or rubber joints, boards overhanging the joists or not). Six different deck units (squares of 1 m x 1 m) were made of spruce and six of poplar, only two (with the worst and the best designs, as presented in Fig. 3) with the four other species. Traditional design Water accumulation on the surface & water-traps Boards = thin (2,2 cm), wide (12 cm) and plain Top screwing One central joist Boards not overhanging the joists Improved design rubber joins Facilitated water elimination bottom screwing Boards = thin (2,2 cm), wide (12 cm) and slope shaped Bottom screwing Nylon or rubber joints Fixed on bridging joists Figure 3: examples of traditional and improved designs that have been chosen to represent the worstcase and the best-case scenario of exposure for terrace decking. XII DBMC, Porto, PORTUGAL,

6 Authors Cladding: facade elements were built after choosing four different designs, the main differences between them being the thickness (20 or 30 mm) of the test boards, the way of assembling and screwing them on the battens (tongue and groove boards with visible nails or lap joint boards with hidden nails), and the orientation of the wood's fibers (three horizontal and one vertical cladding). Each selected design was applied to manufactured cladding units of 1m x 1m. Eight facade elements were made of spruce and eight of poplar. The same set of elements (four elements made of spruce and four elements made of poplar) was installed in situations that either allow or prevent their direct exposure to driven rain. The end-grain of the vertically exposed boards was protected from rainwater by stainless steel sheets. Log walls: two different designs were chosen, the main differences being the shape of the logs and thus their ability to facilitate water drainage. Identical sets of log walls were installed in situations that either allow or prevent their direct exposure to driven rain. Two different walls 2 meters high and 50 cm wide were made with each of the timber species selected for the study. Posts: the durability of wooden posts is being tested along their incline (vertical or semi-horizontal with a 10% slope) and their thickness (from 3 to 25 cm). The initial assumption is that thicker posts may be more sensitive to deformation that can generate shrinks and cracks and thus water-traps. The end-grain of the posts was protected from rainwater by stainless steel sheets. Posts connected to beams: different kinds of joinery are being tested which are representative of traditional carpentry: the mortise and tenon joint, which has been used for centuries around the world to connect wooden elements, mainly when the adjoining pieces connect at an angle of 90, and the cross lap joint, which occurs in the middle of two elements being at right angles to each other. Two innovative types of joint are also being tested, both including the use of stainless steel connectors which reduce the wood-to-wood contact zones and allow for efficient water drainage from the two connected elements. The posts were screwed to the metallic frames. Their end-grain was protected from rainwater by stainless steel sheets. 3 EVALUATION OF THE BEHAVIOUR OF THE WOODEN COMPONENTS WITH REGARD TO BIOLOGICAL AND PHYSICAL DEGRADATION The test specimens are exposed in different configurations producing a decay risk corresponding either to Use Class 3 or 4. The cumulated impact of the type of wood, exposure conditions and effectiveness of different protective measures by design on the service life of wooden components can be quantified through the quotation of biological and physical degradation. The progress of molds, blue stain, fungal decay as well as UV aging and the appearance of shrinks, cracks, swelling and all kinds of mechanical defects is monitored once a year, starting in Because differences between exposure sites, wood species and different designs are expected to begin to manifest themselves only after a couple of years, the experiment will be conducted over a period of ten years. The specimens are evaluated yearly by rating the extent and distribution of molds and blue stain according to XP X (1999) as: 0 (0% of the sapwood), 1 (<5%), 2 (6 to 25%), 3 (26 to 50%) or 4 (>50%), and of decay according to EN 252 (1989) as: 0 (sound), 1 (slight attack), 2 (moderate attack), 3 (severe attack), or 4 (failure) After the first year of exposure, all the specimens were rated 4 for blue stain and 0 for decay in Bordeaux and Charrey. No significant differences in termes of durability were found between the different species or design details. However, big differences were reported with regard to the mechanical behaviour of the exposed wood, swelling and shrinking strongly affecting the wooden 6 XII DBMC, Porto, PORTUGAL, 2011

7 Short Title elements exposed in Charrey (the colder area). However, after only one year of exposure, the results need to be considered as preliminary. 4 EXPECTED ISSUES FOR THE TIMBER CONSTRUCTION MARKET The French and European standards provide keys to proper selection of timber species (EN 350-2), understanding of the biological risks associated with their use (EN 335) and proper treatment if necessary (EN 599). However, the timber construction industry needs practical guidelines that will explain how to improve the expected service life of wooden products by taking into account parameters such as climate, exposure and design details. Moreover, only informed designers are able to correctly interpret the existing standards and handle complex cases where the implementation of wooden products can turn out delicate. One practical goal of the work being done within the framework of the Durability of Wooden Components project is the publication of two technical guidebooks. The results of the present field experiments will be included in the first guidebook, a collaborative effort of the FCBA and FIBC (Memento «Durability of the Works made with Wood ") and will also be used in another one, which will present «healthy» constructive solutions designed to reduce biological hazards as compared to the current practices. The first guidebook will explain the rules of assigning use classes to wooden products or components of larger structures and provide a review of different durability hazards that are commonly encountered. The second one will review the traditional designs as commonly applied for outdoor commodities and outline some recommended technical practices that architects, engineers, contractors and others can use to create long-lasting wooden structures. These technical rules will be consistent with the technical standards currently being prepared by the French Standardization Office for Wood, which aims to harmonize with one another the recommendations given in the different French standards and building specifications referring to the durability of wooden components. They are be summarized in Fig. 4. Naturally durable species No treatment CONCEPTION PHASE Definition of the required performances Definitionof the expectedservice life Identification of the use class (EN 335) EXECUTIVE PHASE Selectionof the wood species (EN 350-2) Selection of the required treatment Selectionof the adapted design Ensure the aesthetic durability Non durable Fit to the use treatable species class Non durable and not Select another treatable species species Refer to the specifications of EN 599-1, EN et NF B Facilitate the water evacuation Avoid water traps Physical protection - Coating - Maintenance Figure 4: The decisionnal rules for propoer use of wooden components 5 CONCLUSIONS The durability of wood is either natural or a result of appropriate treatment. Whether natural or preservative-based, it needs to be adapted to the end use of particular wooden products, commodities or structures, as their service life depends so strongly on their design and exposure to physical and biological agents. The research project presented here contributes to the knowledge of how to extend the service life of commodities made with untreated wood and increase the quality and value of timber and different wooden products. XII DBMC, Porto, PORTUGAL,

8 Authors ACKNOWLEDGMENTS We are grateful to the French Ministry of Agriculture and Forestry (DGFAR) for the financial support given to the Durability of wooden commodities project. Our thanks also go to France Douglas, CRPF Champagne-Ardenne and ADIB for their help in supplying the wood, and to Lycée du Bois de Mouchard for the great job they made manufacturing and installing the experimental devices. REFERENCES EN (2007) Durability of wood and wood-based products - Definition of use classes Part 2: Application to solid wood EN (1994) Durability of wood and wood-based products - Natural durability of wood Part 2: Guide to natural durability and treatability of selected wood species of importance in Europe XP X (1999) Wood preservatives - Evaluation of fongicide efficacy to temporary wood protectives for green sawn timber Site method. EN (2006) Durability of wood and wood-based products - Effectiveness of preventive wood preservatives as determinated by biological tests Part 1 : Specifications according to use class Brischke, C. & Rapp, A.O. 2010, Service life prediction of wooden components Part 1: Determination of dose response functions for above ground decay, Document IRG/WP International Research Group on Wood Protection, Biarritz, France. Nilsson, T. & Edlund, M.-L. 1995, Laboratory versus field tests for evaluating wood preservatives: A scientific view, Document IRG/WP International Research Group on Wood Preservation, Stockholm. Ross Gobakken, L., Mattson, J. & Alfredsen, G. 2008, In-service performance of wood depends upon the critical in-situ conditions. Case studies, Document IRG/WP International Research Group on Wood Protection, Stockholm. 8 XII DBMC, Porto, PORTUGAL, 2011