Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
Testing modified wood and natural durability in use class 3 with the block-test approach Antje Pfeffer1, Andreas Krause2, Holger Militz3 1
Wood Biology and Wood Products, Burckhardt-Institute, Georg-August-University, 37077 Göttingen, Büsgenweg 4, Germany e-mail:
[email protected] 2
e-mail:
[email protected] 3
e-mail:
[email protected]
Keywords: Block test, outside exposure, dynamic MOE
ABSTRACT The block-test was designed as part of the assessment methodology for testing the behaviour of natural and modified wood used under use class 3 conditions. The test was developed to expose the wood close to the ground to an environment with high humidity and high biological activity. The decay of the samples was assessed with the so called pick-test and the determination of the Modulus of Elasticity (dynamic MOE). After 5 years, the feeder stakes and untreated references failed. MOE-measurements showed deterioration at earlier stages than both visual assessments of the samples and the evaluation with the pick-test. The results showed that the block-test gives opportunities for testing the durability of wood out of ground contact under use class 3 conditions. The determination of strength loss by dynamic MOE provides additional information to the pick-test and visual assessments of wood particularly in early stages of decay.
INTRODUCTION The durability of wood in exterior use is limited due to climatic factors and wood destroying organisms. Within the last years, new non-biocidal treatments, like wood modification systems, were developed to improve the biological resistance of wood. First short-time information about resistance against wood destroying fungi can be given by laboratory tests like EN 113 or CEN/TS 15083-1. But for final valuable information about the resistance of untreated and modified wood against wood destroying fungi, natural outside exposure is necessary. The EN 252 is a test standard for the outside ground contact situation. Today, this standard is widely used and accepted in Europe and most of the tests with ground contact are based on this standard. However most applications for untreated and modified wood are without ground contact in use class 3 situations. The existing test methodology follows the CEN/TS 12037. The test setup bears some disadvantages since the samples are complicated to prepare due to their overlapping areas. Moreover the sample size is too large particularly if the samples have to be prepared from sapwood (Sailer et al. 1999). The sample size is also not optimised for evaluations with non-destructive methods, like the determination of the dynamic modulus of elasticity. Furthermore, the test takes much to long before a biological attack occurs under many European climatic conditions, because of the rather low wood moisture contents over longer periods (Militz and Bloom 2000). Therefore other methods are needed to predict the durability under use 77
Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
class 3 conditions. Rapp and Augusta (2004) developed the so-named “double layer test method” and research is on-going to evaluate its suitability for testing wood preservatives. The development of the block test was started in 2002. The test was designed as part of the assessment methodology for testing the behaviour of natural and modified wood used in use class 3. The test was developed to expose the wood close to the ground, hence to an environment with high humidity and high biological activity. The objectives of the test were: fast colonisation by fungi based on the positioning of the samples non-destructive evaluation while the test runs significant results after a relatively short time (5 years) simple preparation of the samples and test setup Between 2001 and 2008, 50 blocks were prepared with different wood species and with treated and modified wood. The present study describes the test setup and results of the first blocks after 6 years outdoor exposure of natural and modified wood.
EXPERIMENTAL METHOD General test setup Wood stakes with a size of 20 x 30 x 300 mm³ are used. The stakes are arranged to blocks and placed on concrete with a pebble surface to avoid direct contact of the wood to concrete, mainly to avoid the impact of the concrete pH-values. Each block was surrounded by a rack covered with a water permeable textile grid to protect the samples against fast drying by airflow and direct sunlight. One block includes 40 wood samples consisting of: 20 test samples (modified samples or wood species under test) 10 untreated references, Scots pine sapwood (Pinus sylvestris) or beech (Fagus sylvatica) depending on the wood species under test 10 feeder stakes, spruce (Picea abies) Untreated samples, feeder stakes and treated samples are arranged alternately in the block to increase the risk of a biological attack. An overview about the positioning of the samples in the block is given in Fig. 1, the test setup is shown in Fig. 2.
feeder stakes treated samples / wood species under test untreated references
Figure 1: Positioning of samples in a block
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Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
Figure 2: Test setup of the block-test, left side: covered blocks; right side: block with visible fruiting bodies
Wood samples All samples were numbered and climatised at 20°C / 65% RH before outdoor exposure. The wood samples in the blocks were untreated and modified by Melamine treatment. The Melamine samples were vacuum pressure treated with a Melamine-solution with a concentration of 10%. An overview about the wood samples in the exposed and analysed blocks is given in Table 1. Blocks 1
2
3
Wood species
Table 1: Overview of the samples Treatments
Spruce Scots pine sapwood Scots pine heartwood Spruce Beech Oak Spruce Scots pine sapwood Scots pine sapwood
untreated feeder stakes untreated references untreated untreated feeder stakes untreated references untreated untreated feeder stakes untreated references Melamine treated
Beginning of outdoor exposure 2002
2002
2003
Evaluation of the wood samples The dry mass of the samples and the dynamic MOE were determined before exposure. Prior to measuring the MOE the samples were soaked in water for 72 hours in order to get a wood moisture content above the fibre saturation point. They were submerged (held down by weights) in tap-water in plastic boxes, separated according wood species and treatments. The measuring device was a GRINDOSONIC MK 4-1 (J.W. Lemmens N.V., Belgium). By measuring the MOE a light tap with a small hammer on the middle of the sample initiates the vibration energy into the sample and a transducer in contact with the sample quantifies the resulting vibration. The MOE calculation was based on the following formula, derived by Hearmon (1966), shown in Eq. 1. I 4 × π ² × L4 × f ² × ρ × A ⎛ ⎞ (1) ×⎜L + × Kl ⎟ dyn.MOE = 4 L² × A ml × I ⎠ ⎝ I A f r L Kl ml
= = = = = = =
moment of inertia [mm4] area of the cross section [mm²] frequency [kHz] density [g/mm³] length [mm] 49.48 4.72
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Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
During outdoor exposure, twice a year (Spring and Autumn) the samples were visually evaluated for fungal attack (fungal mycelia, blue stain and mould) and weighed. The surface of the samples was assessed with a sharp pointed knife to reveal softened areas on all surfaces. The evaluation criteria of the so called “pick-test” were adopted according to the guidelines of EN 252 (1989). The decay rating of the pick-test is shown in Table 2. The MOE was determined as described above. Rating
Description
0
Sound
1
Slight attack
2
Moderate attack
3
Severe attack
4
Failure
Table 2: Decay rating of the pick-test Definition No evidence of decay. Any change of colour without softening has to be rated as 0. Visible signs of decay, but very limited in intensity or distribution: changes which only reveal themselves externally by very superficial degradation, softening of the wood being the most common symptom, to an apparent depth in the order of one millimetre Clear changes to a moderate extent of decay according to the apparent symptoms: changes which reveal themselves by softening of the wood to a depth of approximately 2 to 3 millimeters over more than 1 cm² Severe attack: marked decay in the wood to a depth of more than 5mm or 3-5 millimetres over a wide surface (more than 20 cm2) Impact failure of the stake
RESULTS AND DISCUSSION
The outside weathering of BLOCK 1 started in November 2002. The first evaluation was after 12 months. The results until May 2008 (after 66 months) are shown in Fig. 3. average feeder stakes
average references Scots pine sapwood
average feeder stakes
average Scots pine heartwood
0
6
12
18
24
30
36
average references Scots pine sapwood
average Scots pine heartwood
12000
Time [months] 42
48
54
60
66
0
Classification
1
2
Dyn. MOE [N/mm²]
10000
8000
6000 3
4000 4
0
6
12
18
24
30
36
Time [months]
42
48
54
60
66
Figure 3: Evaluation pick-test and dyn. MOE, mean values of BLOCK 1
The results of the pick-test showed for all sample types a mean decay rating lower than 1 within the first 30 months. After 42 months, the biological activity increased rapidly especially in case of the feeder stakes and the reference material. At the end of 2007 (after 60 months), the first reference stakes and feeder stakes failed. The MOE decreased during the entire evaluation period. The feeder stakes showed the highest loss of MOE.
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Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
The outside weathering of BLOCK 2 also started in November 2002. The first evaluation was after 12 months. The results until May 2008 (after 66 months) are shown in Fig. 4. average feeder stakes
average references Beech
average Oak
average feeder stakes
Time [months] 0
6
12
18
24
30
36
42
48
54
60
average references Beech
average Oak
12000
66
0
Dyn. MOE [N/mm²]
10000
Classification
1
2
3
8000 6000 4000 2000 0
4
0
6
12
18
24
30
36
42
Time [months]
48
54
60
66
Figure 4: Evaluation pick-test and dyn. MOE, mean values of BLOCK 2
After 24 months, the evaluation of the pick-test still showed a decay rating of 0. After 30 months the decay rating increased very rapidly. After 54 months, the first reference stakes and feeder stakes failed. The MOE started to decrease after 12 months during the whole evaluation period. After 54 months, the references revealed a rather high strength loss. The outside weathering of BLOCK 3 started in November 2003. The first evaluation was after 12 months. The results until May 2008 (after 54 months) are shown in Fig. 5. average feeder stakes
average references Scots Pine sapwood
average Melamine
average feeder stakes
Time [months] 0
6
12
18
24
30
36
average references Scots Pine sapwood
average Melamine
12000 42
48
54
60
66
11000
0
Classification
1
2
Dyn. MOE [N/mm²]
10000 9000 8000 7000 6000 5000 4000 3
3000 2000
4
0
6
12
18
24
30
36
Time [months]
42
48
54
60
66
Figure 5: Evaluation pick- test and dyn. MOE, mean values of BLOCK 3
The decay of the samples in BLOCK 3 occurred within the first evaluation period of 12 months. The pick-test showed a high increase of decay between 30 and 36 months for all samples. The MOE indicated a more constant decrease over the whole period. The Melamine-treated samples showed no relevant decrease of the MOE compared to reference stakes and feeder stakes. The pick-test results and the MOE of the Melamine-treated samples strongly differed. After 54 months, the samples were in class 2 according the pick-test, but did not show pronounced strength loss. In summary, after 5 and 6 years outdoor exposure a biological attack of the samples, specifically feeder stakes and untreated reference stakes, is clearly visible. The results for both evaluation criteria differ particularly in early evaluation stages and on Melamine treated wood. The evaluation of the pick-test shows in the beginning no or only low indication of decay and after a
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Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
longer period the indication of decay increased very rapidly. Measurements of dynamic MOE show deterioration earlier than visual inspection of the samples and evaluation with the pick-test. The effect of fungal decay must not necessarily become visible on the surface in early stages, but can cause significant losses in strength (Tsoumis 1993). These results corresponds with findings of other authors in case of field tests in ground contact and soil bed tests (Grinda and Göller 2005, Machek et al. 2001). The different results in case of the Melamine treated samples between both methods depend on the different evaluation systems. With the pick-test only the surface is evaluated while with the MOE the whole sample is considered. The investigated Melamine treated wood is more brittle on the surface than unmodified wood (Götz et al. 2006), which can have an influence of the evaluation. The percentage loss of MOE for all blocks during an evaluation period of 54 months is shown in Fig. 6. The strength loss of the feeder stakes differs between the blocks. The values in the third block are much higher than in BLOCK 1 and 2. The reference Scots pine sapwood stakes showed similar results. The strength loss is much higher in BLOCK 3 than in BLOCK 1. These results indicate that the rate of decay differs between the blocks, because of the different performances in the blocks for fungal growth. One possible reason will mainly relate to the different starting time of each block tested. BLOCK 1 and 2 started in 2002 and BLOCK 3 in 2003. Scots pine heartwood and the Melamine treated samples have the lowest strength loss of all samples after 54 months. Scots pine heartwood samples showed a better resistance than oak samples. But a direct comparison was not possible, because of their location in different blocks with different values of decay. The evaluation of results of many more blocks is running to see if the first results can be validated in case of different values of fungal decay in different blocks and between heartwood of Scots pine and oak and between modified and untreated samples. BLOCK 1
BLOCK 2
BLOCK 3
120
Loss dyn. MOE [%]
100 80 60 40 20 0 feeder stakes
references Scots pine sapwood
Melamine
references Beech
Oak
Scots pine heartwood
Figure 6: Loss of dynamic MOE until May 2008
The standard deviation of the MOE measurements was relatively high. This was caused by the failure of stakes. At a high decay rate there was no determination of the MOE possible anymore (from approximately 60% strength loss). These stakes were ranked with a MOE of 0. A second reason could be the influence of the location of the decay and defects. The location of an attack in the stakes is important for the outcome of the measurement (Machek and Militz 2004). Furthermore there are differences in fungal decay between the outer and inner layers of samples 82
Cost Action E37 Final Conference in Bordeaux 2008 Socio-economic perspectives of treated wood for the common European market
in the block. The samples in the first layer on top of the block have lower wood moisture content than samples in the inner layers (Weigenand 2005). The faster drying of these samples should be considered in the future, evaluations according different layers can be helpful to obtain more detailed results. CONCLUSIONS
All presented data are preliminary data and not completed. The data of the other outside exposed blocks will have to be evaluated to confirm the first presented results. The evaluation is running and will be presented in the near future. The presented results showed a considerable decay when using block testing. After 5 years some of the feeder stakes and untreated references already totally fail. Therefore this test setup can be proposed an alternative for untreated and modified wood to existing outside exposure tests in use class 3. Measurements of the dynamic MOE show deterioration in earlier stages than with visual inspection of the samples and evaluation with the-pick-test. However, the evaluation of MOE needs more sophisticated equipment and experienced staff to perform the measurements. Nevertheless, the determination of strength loss could be a good addition to the pick test and visual assessments of wood particularly in early stages of decay. Both evaluation methods show benefits and drawbacks. More investigations are necessary to clarify the influence on the MOE of the different forms of fungal decay (decay in interior parts of the stake or on their surface) and the way of the evaluation of failed stakes to reduce the high standard deviations in the measurements. Furthermore the evaluation with the pick test has to be more accurate mainly when considering the different surface properties of modified wood compare to untreated wood. Therefore both evaluation methods should be used as complementary methods in the described test setup. Further studies are necessary to clarify the differences in fungal decay between outside and inside stakes in the block and the influence of leachable components out of treated samples on feeder stakes.
REFERENCES
CEN/TS 12037 (2004) Field test method for determining the relative protective effectiveness of a wood preservative exposed out of ground contact – Horizontal lap-joint method. CEN/TS 15083-1 (2005) Determination of the natural durability of solid wood against wood destroying fungi, test methods – Part 1: Basidiomycetes. EN 113 (1996) Wood preservatives – Method of testfor determining the protective effectiveness against wood destroying basidiomycetes – Determination of toxic values. EN 252 (1989) Field test method for determining the relative protective effectiveness of a wood preservative in ground contact. Götz, M., Illner, M., Krause, A., Militz, H., Schmid, J., Schwarz, B., Stetter, K. (2006) Einheimisches, dimensionsstabilisiertes Holz im Fenster- und Fassadenbau. di-sta Forschungsvorhaben, Abschlussbericht.
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Grinda, M., Göller, S. (2005) Some experiences with stake tests at BAM test fields and in the BAM fungus cellar. Part 1: Comparison of results of visual assessments and determinations of static Modulus of Elasticity (MOE). International Research Group on Wood Preservation, Doc. Nr.: 05/20319 Hearmon, R.F.S. (1966) Vibration testing of wood. Forests Product Journal, 16, 29-39 Machek, L., Militz, H., Sierra-Alvarez, R. (2001) The use of an acoustic technique to assess wood decay in laboratory soil bed tests. Wood Science and Technology, 34, 467-472 Machek, L., Militz, H. (2004) The influence of the location of a wood defect on the Modulus Of Elasticity determination in wood durability testing. International Research Group on Wood Preservation, Doc. Nr.: 04-20287 Militz, H., Bloom, C.J. (2000) The use of Lap-joints in natural durability testing: moisture content development during 36 months outside exposure trials. International Research Group on Wood Preservation, Doc. Nr.: 00-20217 Rapp, A.O., Augusta, U. (2004) The full guideline for the “double layer test method” – A field test method for determining the durability of wood out of ground. International Research Group on Wood Preservation, Doc. Nr.: 04-20290 Sailer, M., Rapp, A.O., Peek, R.-D., Nurmi, A., Beckers, E.P.J. (1999) Interim balance after 20 months of Lap-joint exposure. International Research Group on Wood Preservation, Doc. Nr.: 99-20164 Tsoumis G. (1993): Degradation of wood. Science and technology of wood: structure, properties and utilisation, 213-233 Weigenand, O. (2005): Block-test – Climatic influences. Unpublished.
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