Durability of thermally modified sapwood and heartwood of Scots pine and Norway spruce in the modified double layer test

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1 Wood Material Science & Engineering ISSN: (Print) (Online) Journal homepage: Durability of thermally modified sapwood and heartwood of Scots pine and Norway spruce in the modified double layer test Sini Metsä-Kortelainen & Hannu Viitanen To cite this article: Sini Metsä-Kortelainen & Hannu Viitanen (2017) Durability of thermally modified sapwood and heartwood of Scots pine and Norway spruce in the modified double layer test, Wood Material Science & Engineering, 12:3, , DOI: / To link to this article: Published online: 26 Aug Submit your article to this journal Article views: 194 View related articles View Crossmark data Citing articles: 1 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 23 December 2017, At: 00:40

2 Wood Material Science & Engineering, 2017 Vol. 12, No. 3, , ORIGINAL ARTICLE Durability of thermally modified sapwood and heartwood of Scots pine and Norway spruce in the modified double layer test SINI METSÄ-KORTELAINEN & HANNU VIITANEN VTT Technical Research Centre of Finland, FI VTT, Finland Abstract In the present study, durability of untreated and thermally modified sapwood and heartwood of Scots pine and Norway spruce was examined using a modified double layer test. Base layer samples were partly on contact with ground where exposure conditions were harder than that in a double layer test above the ground. The base layer on ground contact gave results already after one year of exposure in Finnish climate, but the top layer of a double layer test element simulated more the situation of decking exposure. Significant differences in durability and moisture content (MC) between the wood materials were detected after six years of exposure in the field. Thermally modified pine heartwood performed very well in all layers of the test element and only minor signs of decay were found in some of the base samples. Both sapwood and heartwood of thermally modified spruce were suffering only slight amounts of decay while thermally modified pine sapwood was slightly or moderately decayed. Untreated sapwood samples of pine and spruce were severely decayed or reached failure rating after six years in the field. Untreated heartwood samples performed clearly better. The highest MCs were measured from untreated and thermally modified pine samples. Thermal modification increased significantly the durability and decreased the MC values of all wood materials. Keywords: Biological durability, decay resistance, ground contact, heartwood, Norway spruce, sapwood, Scots pine, thermal modification. Introduction Thermal modification of wood is nowadays a commercial method to enhance the performance of wood in demanding applications. The Finnish ThermoWood process is based on heating the wood material for a few hours at high temperatures ( C) at atmospheric pressure using water vapour. The production of thermally modified timber (TMT) has grown year after year and the main wood species for the industrial-scale thermal modification are Scots pine and Norway spruce. The properties of thermally modified wood have been studied widely. Thermal modification has been found to improve the biological durability of wood in many laboratory tests (Viitanen et al. 1994, Tjeerdsma et al. 2000, Kamdem et al. 2002, Boonstra et al. 2007, Metsä-Kortelainen and Viitanen 2009, Calonego et al. 2010, Tripathi et al. 2014). There have been reported some reasons for the enhanced biological durability like the changes in the chemical composition of the wood which makes the wood more difficult for fungi to attack (Kotilainen 2000, Sivonen et al. 2003, Hakkou et al. 2006, Mburu et al. 2006, Lekounougou et al. 2009). The degradation of the wood components in thermal modification has resulted also in decreased hygroscopicity, which limits the shrinking and swelling as well as absorption of water which may also be conducive to diminished fungal growth (Tjeerdsma et al. 1998, Bekhta and Niemz 2003, Metsä-Kortelainen et al. 2006, Korkut and Bektaʂ 2008, Almeida et al. 2009, Herajärvi 2009, Metsä-Kortelainen and Viitanen 2012). In addition, thermally modified wood has been shown to be less susceptible to discolouring organisms than untreated wood, and especially coated thermally modified wood has been found to have good weather resistance which is dependent on the coating type (Jämsä et al. 2000, Petrič et al. Correspondence: S. Metsä-Kortelainen, VTT Technical Research Centre of Finland, PO Box 1000, FI VTT, Finland. Tel: sini.metsa-kortelainen@vtt.fi (Received 5 May 2015; accepted 9 June 2015) 2015 Taylor & Francis

3 130 S. Metsä-Kortelainen & H. Viitanen 2006, Kocaefe et al. 2007, Frühwald et al. 2008). The negative effects of thermal modification are brittleness and reduced strength properties which are the reasons thermally modified wood is not recommended for load-bearing constructions (Viitaniemi 1997, Esteves et al. 2007, Shi et al. 2007, Korkut et al. 2008, Majano-Majano et al. 2010, Metsä-Kortelainen and Viitanen 2010, Lekounougou and Kocaefe 2014). The intensity of thermal modification, which usually depends on the thermal modification temperature and time, has a significant effect on the appearance and biological and physical properties of the wood. The level of changes in wood properties increases by higher modification temperature and longer time. There are also other factors affecting the properties of thermally modified wood such as wood species, sapwood/heartwood proportion, quality, moisture content (MC) and the method of thermal modification used. (Mitchell 1988, Welzbacher et al. 2007, Metsä-Kortelainen 2011). In our previous studies in laboratory, great differences between sapwood and heartwood of untreated and thermally modified Scots pine and Norway spruce in decay resistance and water absorption were detected (Metsä-Kortelainen et al. 2006, Metsä-Kortelainen and Viitanen 2009). Especially the results of thermally modified Scots pine heartwood showed high biological durability and low hygroscopicity. Boonstra et al. (2007) also found thermally modified heartwood of Scots pine to be more durable against fungal attack than sapwood. In addition, differences between heartwood and sapwood have been reported by Esteves et al. (2014) who studied the durability of sapwood and heartwood of thermally modified Pinus pinaster against Rhodonia placenta. The heartwood reached a good durability already at a low thermal modification temperature of 190 C while the durability of sapwood had not improved until thermal modification was carried out at higher temperatures (200 C). Thermally modified wood is widely used in exterior applications where especially increased biological durability and dimensional stability are considered to be advantages compared with untreated wood materials. Therefore not only assessments in laboratories but also exterior trials in ground and above-ground situations are valuable in prediction of service life of wood products in different use conditions. In our last study, the effect of thermal modification temperature on the durability of Scots pine and Norway spruce was investigated using a lapjoint test in field and the results showed a significant increase in durability of modified samples and a clear correlation between the modification temperature and the biological durability during exposure of nine years. The results after nine years of exposure in the field had also a good correlation with mass losses in a laboratory test with brown-rot fungi (Metsä-Kortelainen et al. 2011). Welzbacher and Rapp (2007) studied the decay resistance of thermally modified wood originating from four different European industrial thermal modification processes in field tests and concluded that thermally modified materials showed a significantly improved natural durability compared to untreated Scots pine sapwood controls after 5.5 years of exposure. In the study of Larsson Brelid and Edlund (2013), the biological resistance of several untreated and modified wood products were investigated using field test methods. The results showed that thermal modification of Scots pine and Norway spruce slowed down the decaying process; however the test samples were highly decayed after a long time of 11 years in the field but still in slightly better condition than the untreated references. It is well known that heartwood of many wood species is naturally more resistant to attack by decay organisms than the sapwood. Differences between sapwood and heartwood of Scots pine and Norway spruce samples in durability have been studied in laboratory and field tests in or out of ground contact (Augusta and Rapp 2003, Metsä-Kortelainen and Viitanen 2009). Generally heartwood stands in better condition for longer periods than the sapwood especially with pine. In the present study, the performance of samples made of untreated and thermally modified (Thermo-D class) sapwood and heartwood of Scots pine and Norway spruce was investigated on and out of ground contact. The main interest was to study the interaction of thermal modification and sapwood/heartwood. The principles of horizontal double layer test were followed; however some samples were placed also directly on the ground (Rapp and Augusta 2004). The MC and the amount of decay were inspected after one, two, three and six years in the field. Material and methods Wood material Industrial partners selected approximately 5m-long kiln-dried boards consisting mainly sapwood or heartwood of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) in A second selection of the test material was performed at VTT. Altogether 140 boards with high sapwood or heartwood content were selected for the tests. Each board was cut into two 2.5m-long pieces of which one piece was thermally modified and the other piece was used as a reference material. Thermal

4 Durability of sapwood and heartwood of Scots pine and Norway spruce 131 modification of the boards was performed by industry according to Thermo-D class requirements (ThermoWood Handbook 2003). Thermo-D class timber is recommend for use in cladding, outer doors, shutters, environmental constructions, sauna and bathroom furnishing, flooring and garden furniture which are applications where a good natural durability is usually needed. Both modified and untreated boards were planed after thermal modification and 14 boards from each group consisting 35 boards (thermally modified/ untreated, sapwood/heartwood, pine/spruce) were chosen for the modified double layer test. The rest of boards were stored or used in other field tests with glue-laminated posts. In addition, few boards of kiln-dried Western red cedar (WRC) and preservative-treated Scots pine (nordic wood preservation council (NWPC) class A and AB) were used as a reference material. The dimensions and MCs measured from small samples taken from the planed boards using the weighing drying method (room temperature, ambient relative humidity) prior the start of the test are given in Table I. Modified double layer test The target of this study was to compare the biological durability of untreated and thermally modified sapwood and heartwood materials both in partial ground contact and in above-ground conditions and Table I. Dimensions, amounts and MCs of replicate boards used in the modified double layer test. Wood material Dimension (mm) Replicate boards MC (%) Scots pine sapwood, untreated Scots pine sapwood, Thermo-D Scots pine heartwood, untreated Scots pine heartwood, Thermo-D Norway spruce sapwood, untreated Norway spruce sapwood, Thermo-D Norway spruce heartwood, untreated Norway spruce heartwood, Thermo-D WRC Impregnated Scots pine, CCA (NWPC class A) Impregnated Scot pine, CC (NWPC class AB) Note: CCA, copper chromium arsenic. for this reason a modified double layer test was developed. The other target was to develop a research method which would give results after a short exposure period under climate conditions in southern Finland. A modified double layer test element was made up of base boards on the ground and two-layered decking above the ground. The double decking was separated from the base boards with four perpendicularly installed supporting planks sawn from four different boards (Figure 1). Per each wood material, 20 replicate 0.5 m long base samples from five different boards were sawn. The upper layer (later top layer) and lower layer (later middle layer) of the double decking consisted of two replicate 1.0 m long samples from five different boards. One test element consisted of only one type of wood material; however, both preservative-treated materials and WRC composed altogether one test element. The amount of test elements was thereby 9 and all samples were placed outer face downwards there. The test was started at the test field of Otaniemi in Finland in September The condition of all samples was analysed after one, two, three and six years of exposure in the field during autumn. Samples were cleaned of soil and dry leaves before each evaluation. The samples were weighed in the field and the MC was determined based on the calculated dry-weights of the samples before the test. Decay was rated from both faces of the samples according to the evaluation criteria given in Table II. After the last evaluation in the field in 2010, five samples from each test material were randomly selected for more detailed analysis carried out in laboratory. Small specimens were sawn from the end and middle part of the selected field samples. Decay and discoloration were analysed from the crosssections of the specimens according to the scales given in Tables III and IV. Results and discussion Decay evaluations in field Decay ratings of the test materials after one, two, three and six years of exposure in the field are shown in Figure 2. The numbers are average values evaluated from the outer faces of the base samples in direct ground contact. It can be noted that the first signs of decay were found from almost all research materials already after one year of field exposure and especially untreated spruce heartwood samples were suffering from moderate attack of decay. This indicates that the research conditions were very severe. The next years in the field raised

5 132 S. Metsä-Kortelainen & H. Viitanen Figure 1. Schematic description of modified double layer test elements (left) and double layer elements in the field (right). up the decay rates of all wood materials despite some values of thermally modified materials where the decay ratings were higher after three years than after six years. This might have been caused by different weather conditions (wood is softer when wet), human alternation in evaluation and differences in amount of samples (some of the most decayed samples were removed from the field for laboratory analyses after some years of exposure). The only exception was A-class-treated pine where no decay was detected during all the years. Generally, the thermal modification increased significantly the biological durability of all wood materials. Wood materials without thermal modification or other preservations suffered a moderate to severe attack of decay after six years of exposure in the field while the thermally modified ones mostly had only a slight attack of decay. Both untreated pine and Table II. Evaluation scale of decay rate in the field. spruce sapwood materials were most decayed and also the untreated spruce heartwood suffered from decay in direct ground contact. The most durable wood material in direct ground contact was thermally modified pine heartwood in which the level of decay was same as that of AB-class-treated pine after six years. WRC had very similar decay behaviour to thermally modified pine sapwood during all the years. Generally, the results of the untreated base samples are quite well in line with Rapp et al. (2006) who examined the durability of sapwood and heartwood of several wood species in the horizontal double layer test and found large gap between the decay rate of sapwood and heartwood of pine. With spruce materials this kind of gap was not detected. The more specific information on the decay ratings after six years field test is expressed in Figure 3. It can be perceived that the base samples of untreated sapwood materials were most decayed and especially the lower faces which were on direct ground contact were severely attacked or had the Rating Classification Definition of condition 0 No attack No evidence of decay. Any change of colour without softening has to be rated 0 1 Slight attack Visible signs of decay, but of very limited intensity or distribution. Softening of wood to a depth in the order of 1 mm 2 Moderate attack Clear changes to a moderate extent of decay according to the apparent symptoms. Softening of wood to a depth of 1 3 mm (less than 20 cm 2 ) 3 Severe attack Marked decay in the wood to a depth of more than 5 mm or 3 5 mm over a wide surface (more than 20 cm 2 ) 4 Failure Impact failure of the stake Table III. Evaluation scale of decay rate in the laboratory. Rating Classification Definition of condition 0 No attack No evidence of decay 1 Start of attack First visible signs of decay (some spots) 2 Slight attack Slight decayed area on the end grain and surface area <25% of the area 3 Moderate decay Clearly decayed area on the end grain and surface area 25 50% 4 Severe attack Clearly decayed area on the end grain and surface area >50% 5 Complete decayed Clearly decayed area on the end grain and surface area >90 or 100%

6 Durability of sapwood and heartwood of Scots pine and Norway spruce 133 Table IV. Evaluation scale of blue-stain and mould growth. Rating Classification 0 No growth or hyphae 1 Some spot of mould or blue-stain % discoloured surface of total area % discoloured surface of total area % discoloured surface of total area 5 >70% discoloured surface of total area failure rating in most of the cases. Only exception was the pine heartwood which was only moderately attacked. The both untreated sapwood materials had a severe attack also in the upper faces of the base samples; however, the upper faces of untreated heartwood materials were on average only slightly attacked. Thermal modification increased significantly the biological durability of all base samples and the trend was similar also in the middle and top samples. Middle samples constructed the lower layer of the double decking situated under the top samples. The all upper faces of the middle samples were more decayed than the lower faces. A presumable reason for this is that some moisture pockets were formed between the layers of double decking which kept the conditions for decay organisms favourable for longer time periods than in the upper face of the top layer and the lower face of the middle layer. Also the untreated sapwood materials in the middle layer were in notably worse condition than thermally modified and heartwood ones from where only slight amount of decay was observed. The top layers of the double layer elements were in best condition. Only small amounts of decay were detected except in the lower face of the untreated spruce sapwood which was moderately or severely attacked. Thermally modified pine heartwood was in best condition and only slight attack was detected in the lower face of few base samples. Larsson Brelid and Edlund (2013) also found similar results from the ground proximity multiple layer test where the bottom layers were much more heavily degraded, as compared to the top layers. After the exposure of 11 years in Borås, Sweden, the untreated Scots pine and Norway spruce samples in both layers were completely decayed. However, thermal modification increased the durability of both wood species in both layers. The modified samples were moderately or severely attacked in the bottom layer but the top layer samples were only slightly or moderately attacked after a very long exposure period. This confirms that thermal modification significantly increases the service life of the wood materials, especially in out-of-ground conditions of the use class 3 (EN 335-1, 2013). It should also be taken into account that in this study sapwood and heartwood were not separately studied and especially the results of the pine materials could be even better if pure heartwood material would have been used. MC in field exposure During the exposure, the wood materials absorbed different amounts of water from the ground and air which is expressed as an average change in weight in Figure 4. Untreated sapwood of pine absorbed water at the minimum of two times more than the other wood materials. In addition, the weight increase was high with thermally modified pine sapwood and untreated spruce sapwood. The MCs of the wood materials in different layers prior and after the test are shown in Figure 5. The MCs have been determined based on the calculated Figure 2. Decay rates of the base samples after one, two, three and six years of outdoor exposure.

7 134 S. Metsä-Kortelainen & H. Viitanen Figure 3. Decay rates of untreated and thermally modified sapwood and heartwood of pine and spruce in different layers after six years of outdoor exposure. dry-weights of the samples prior to the test. That is the reason why no MC differences exist between the different layers before the test (results from year 2004). The MCs of the untreated wood materials were between 9% and 13% while thermally modified wood materials had significantly lower MCs 5% before the test. The only exception was thermally modified pine sapwood for which the MC was 11.2% in The MC differences between the wood materials and different layers were significantly higher after the test in The MC of the untreated wood materials was higher in the top and middle layers than in the base layer which was partly protected from the rain by the upper layers. The MC of the untreated pine sapwood was very high (62 86%) as that of also the untreated spruce sapwood (33 56%) after the test. In addition, the MC of the Figure 4. Average change in weight of samples after six years of outdoor exposure.

8 Durability of sapwood and heartwood of Scots pine and Norway spruce 135 Figure 5. MC of untreated and thermally modified sapwood and heartwood of pine and spruce in different layers before and after six years of outdoor exposure. thermally modified pine sapwood was very high (39 47%) after the test which actually is well in line with the results from the water absorption test (Metsä- Kortelainen et al. 2006). In other cases the thermal modification significantly decreased the MC values below the critical level of 30% needed for the decay development. The thermally modified pine heartwood was even below the MC of 20% at the end of the test. The MC differences between the sapwood and heartwood of thermally modified spruce were not significant. Laboratory evaluations Results of the detailed laboratory analyses of decay and discoloration of the cross-sectional surfaces of the specimens randomly taken from the field samples are shown in Figure 6. Half of the specimens Figure 6. Decay and discoloration of small specimens sawn from samples exposed six years in the field.

9 136 S. Metsä-Kortelainen & H. Viitanen Figure 7. Overall conditions of small specimens sawn from samples exposed six years in the field. Samples in better condition (left) and more decayed and discoloured (right). were taken from the end and half from the middle parts of the field samples. Generally, the decay and discoloration distinctions between the different parts were quite small. Almost all randomly selected thermally modified wood samples were clear of decay, blue-stain and mould. Only some decay spots were observed from the surface of thermally modified spruce samples. In addition, the AB-classtreated pine was in perfect condition. The untreated wood specimens were significantly more decayed. The most and severely attacked was spruce heartwood and also the both sapwood

10 Durability of sapwood and heartwood of Scots pine and Norway spruce 137 materials were moderately decayed. Pine heartwood was in the best condition from the untreated wood materials as it was only slightly decayed and discoloration was found from 1% to 30% from the cross-sectional area. In other untreated wood materials, 30% or more of the cross-sectional area was under blue-stain or mould. The results are quite well in line with the decay analysis of the base samples in the field. The visual appearance of the laboratory specimens is seen from Figure 7 where there are two examples from each material, one is the worst case and the other one is the best case from each group. In the field evaluations, more decay was found from the thermally modified samples. There might be several reasons for that. In the field both faces of the samples were evaluated thoroughly, however in the laboratory only small specimens taken from field samples were evaluated and some of the decay spots might have been cut off. In addition, Larsson Brelid and Edlund (2013) reported that it is difficult to evaluate thermally modified samples according to the usual method using a knife, as the thermal treatment itself reduces the strength of the material. They concluded that even though the samples had lost strength during the first years of exposure in the field, no biological attack could be detected in the microscopy analysis. In our evaluation the samples were evaluated similarly using a knife in the field which might have been caused more hard evaluation for the thermally treated samples. General remarks on the results According to the round robin tests of COST E 37, the double layer test showed to be too slow for TMT UC3 in Nordic climate conditions as the first signs of decay were detected after three years in the field (Westin et al. 2013). However, the decay ratings in Central and South European tests fields were much higher. The modified double layer test used in this study showed accelerated decaying especially in the layers on ground and the upper layers were simulating the results in the double layer test. The test method used is therefore relevant when it is critical to have results after a short field exposure especially in Nordic climate conditions, where climate conditions are milder for decay development than that in Southern and Middle Europe (Viitanen et al. 2011). In addition, the results support the conclusions of our earlier studies that there are clear differences between the sapwood and heartwood of thermally modified wood materials in decay and moisture exposures (Metsä-Kortelainen et al. 2006, Metsä-Kortelainen and Viitanen 2009). In the present study, the first sign of decay in the lower layer close the ground was found after one year of exposure, which reflects the effectiveness of the test. A less aggressive test, however, is also needed to find out the potential differences between different wood materials and treatments. The upper layer of the boards in the modified double layer test simulated the use condition of use class 3.2 (above ground, prolonged wetting conditions), and difference in weathering and cracking could be evaluated in this layer. Especially the outer surface of sapwood samples in upper layer showed higher cracking than that in heartwood boards. This was caused both by the fibre direction and the MC or water uptake of the wood. The performance and weather resistance of thermally modified heartwood of spruce and pine seemed to be significantly better than that of sapwood of pine and spruce, which also appeared to have lower MC and higher decay resistance. Conclusions Thermally modified pine heartwood performed well during the test and only minor decay was found after six years of exposure in the field. In addition, only slight amount of decay was detected from the thermally modified spruce materials. Only thermally modified pine sapwood samples in the base layer were partly attacked. Signs of decay were found also in WRC on ground contact. The condition of thermally modified materials was in general clearly better than that of untreated control samples, and especially the untreated sapwood control samples in ground contact were heavily decayed after six years of exposure. In the CCA-treated boards, no decay was found during the test. In the AB-classtreated wood, some decay was found from the lower surfaces faced ground, but according to detailed analyses, no decay was found (only colour changes of the surface). The MC of thermally modified wood materials was clearly lower than that of untreated materials. The highest MCs were measured from both untreated and thermally modified pine sapwood materials. The MC of thermally modified pine heartwood was the lowest and also the MCs of thermally modified spruce materials were below the critical level of 30% needed for the decay development at the end of the test. A modified double layer test using samples, which are partly on ground contact, is a harder test method than the double layer test. The bottom layer on ground contact gave results already after one year of exposure, but the top layer of the test element partly simulated the situation in a not-covered decking exposure.

11 138 S. Metsä-Kortelainen & H. Viitanen Acknowledgements International ThermoWood Association is gratefully acknowledged for funding the research. Leena Paajanen, Pentti Ek, Heikki Murto, Antti Nurmi and Saila Jämsä are acknowledged for their work in the sample preparation and the decay tests. Disclosure statement No potential conflict of interest was reported by the authors. References Almeida, G., Brito, J. O. and Perré, P. (2009) Changes in woodwater relationship due to heat treatment assessed on microsamples of three Eucalyptus species. Holzforschung, 63, Augusta, U. and Rapp, A. O. (2003) The natural durability of wood in different use classes (Doc. No. IRG/WP/ ). International Research Group on Wood Preservation. Bekhta, P. and Niemz, P. (2003) Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood. Holzforschung, 57, Boonstra, M. J., van Acker, J., Kegel, E. and Stevens, M. 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