Deterioration of Dowel Bearing Properties of Timber Due to Fungal Attacks

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1 Deterioration of Dowel Bearing Properties of Timber Due to Fungal Attacks Ali Awaludin Dpt. Civil and Environmental Engineering, Universitas Gadjah Mada, Grafika Street #2, Sleman, Yogyakarta J.P. Gentur Sutapa Faculty of Forestry, Universitas Gadjah Mada, Yogyakarta Kei Sawata Faculty of Agriculture, Hokkaido University, Sapporo Tomonori Azuma, Mitsunori Mori Forest Products Research Institute, Hokkaido Research Organization, Asahikawa Abstract The effect of wood decay on dowel bearing properties of Melia azedarach, Swietenia mahagoni and Pterospermum javanicum was investigated in this study. Wood decay due to fungal attacks, Schizophyllum commune Fr, was simulated for six and twelve months. A half-hole test configuration was implemented on the wood bearing specimens and decay was initiated only at the contact area between wood specimen and dowel, other surfaces of the wood specimens were sealed. Steel dowel of 12 mm in diameter was used in the bearing strength test. The results showed that decay caused an increase of moisture content and decrease of oven-dry weight of the wood specimens. After one year decay period the greatest mass loss or decrease of oven-dry weight was found in Melia azedarach (7.86%) followed by Swietenia mahagoni (5.9%) and Pterospermum javanicum (2.25%). Wood-decaying fungus deteriorated the bearing strength of those wood species. Average decrease of bearing strength after six and twelve months of decay period was 2% and % (for Melia azedarach); 18% and 38% (for Swietenia mahagoni); 1% and 29% (for Pterospermum javanicum). In addition, bearing stiffness of the wood specimens decreased after experiencing decay though several samples indicated this deterioration fairly. Introduction As an organic material wood or wood-based composites may undergo biological deterioration during its service period 1

2 especially in aggressive environment. Fungus and termites are examples of some organisms that are frequently encountered in Southeast Asian regions. Decay due to fungal attacks cause wood fibers become soft and losing its mechanical strength as shown in Fig 1, while termite attacks cause rapid loss of wood fiber amount. In general those biological attacks deteriorate both stiffness and strength properties of the wood material. Many previous studies used percentage of mass loss over the initial mass of the wood material as an indicator of strength degradation in laboratory scale [1-]. Figure 1. Photos of wood decay due to fungal attacks Information about strength and stiffness degradation of wood material due to fungal attacks is required for a good service life prediction of timber structures. As structural performance of timber structures greatly depends on the performance of their elements and joints, study about the influence of wood decay on shear resistance of timber connections is necessary. In this study, effect of wood decay due to fungal attacks was evaluated on dowel bearing parallel-to-grain specimens. Dowel bearing properties are required to estimate the lateral resistance timber joint using dowel-type fasteners such as bolts and nails. Materials and Methods In the case of wood specimens exposed to wood-decaying fungus, wood decay was assumed occur at the dowel lead hole. The specimens were coated with epoxy resin except for half dowel lead hole were sterilized and placed in plastic container containing the fungus culture of Schizophyllum commune Fr cultured on potato dextrose agar [5]. The incubation was performed at 26 o C and 98% of relative humidity for 6 and 12 months period. The part of samples was dried at 6 o C for 8 hr and the other samples were immediately tested after the decay procedure. The range of the moisture content for clear samples and samples exposed to wood-decaying fungus was % and %, respectively. Dowel bearing strength tests parallel to the grain according to ASTM D576 were conducted as shown in Figure 3. Tests were carried out at a constant rate of 1. ~ 1.5 2

3 mm/min and the diameter of steel dowel was 12 mm. Dowel bearing strength is defined as the applied load divided by the dowel bearing area, which is the dowel diameter multiplied by wood thickness. f, 1 28 u x xf, ; greater than 28% MC (1b) where f e,e is the estimated dowel bearing strength, u e is the moisture content, u d is the moisture content of the air-dried wood (12%), r is the change in the value of f e /f e,mc=12% for 1% change in MC, and f e,d is the dowel bearing at 12% MC. Moreover, Sawata et al [] found that r equaled to 3 ~ 3.7 gave a good agreement between experimental bearing strength data and the prediction considering four different wood species. In this study, the value of r was taken as Figure 2. Specimens after six months exposed to wood-decaying fungus Load Comparison between test results of the clear specimens and specimens exposed to wood-decaying fungus was carried out at moisture content (MC) of 12%. Thus experimental dowel bearing strength was normalized to a moisture content of 12% according to this following equation []. D = 8 mm D = 8 mm f, 1 u u x xf, ; less than 28% MC (1a) Figure 3. Dowel bearing test of wood sample (D is dowel diameter, 12 mm) 3

4 Results and Discussions Mass loss over the initial mass of the wood specimen for Melia azedarach, Swietenia mahagoni and Pterospermum javanicum after six and twelve months of inoculation period is shown in Fig. (a) ~ (c). All specimens indicated that the mass loss of the samples exposed to wood-decaying fungus for twelve months was much greater than that of the specimens exposed to six months of inoculation period. Mass loss (%) (a) Melia azedarach samples 2 16 Mass loss (%) (c) Pterospermum javanicum samples Figure. Mass loss of wood samples exposed to wood-decaying fungus: Figure clearly show that the mass loss of Pterospermum javanicum wood specimens is the lowest among the three wood species considered here, and the wood specimens of Melia azedarach had the greatest mass loss among all specimens. The average mass loss after six and twelve months of incubation period for the three wood species is as follows: Melia azedarach,.28% and 7.86%; Swietenia mahagoni, 1.73% and 5.9%; Pterospermum javanicum, 1.5% and 2.25%. Mass loss (%) 12 8 (b) Swietenia mahagoni samples

5 Dowel bearing strength (MPa) (a) Melia azedarach samples 6 Bearing stiffnessh (kn/mm3) (a) Melia azedarach samples 16 Dowel bearing strength (MPa) Bearing stiffnessh (kn/mm3) 12 8 (b) Swietenia mahagoni samples (b) Swietenia mahagoni samples 6 16 Dowel bearing strength (MPa) Bearing stiffnessh (kn/mm3) 12 8 (c) Pterospermum javanicum samples (c) Pterospermum javanicum samples Figure 5. Maximum dowel bearing strength of samples exposed to wood-decaying fungus Figure 6. Bearing stiffness of the specimens exposed to wood-decaying fungus: 5

6 Maximum dowel bearing strength parallel-to-grain of wood specimens for each incubation period is shown in Fig. 5 where a significant decrease was observed as incubation period is prolonged. When incubation period was 6 months, compared to clear samples, maximum dowel bearing strengths parallel-to-grain of Melia azedarach, Swietenia mahagoni and Pterospermum javanicum species were 2%, 18% and 1%, respectively. For samples of 12 months incubation period, maximum dowel bearing strengths parallel-to-grain of Melia azedarach, Swietenia mahagoni and Pterospermum javanicum species were %, 33% and 29%, respectively. Figure 6 shows the bearing stiffness of the wood samples, which is defined as the slope of a line passes through points on the dowel bearing curves corresponding to 1% and % of the maximum load. Decrease of bearing stiffness due to fungal attacks is clearly indicated by Swietenia mahagoni wood samples (Fig. 6b) and the specimens exposed to wood-decaying fungus for twelve months had a lower bearing stiffness than that of the wood samples exposed to wood-decaying fungus for six months period. However, this decrease is fairly observed in the other two wood species. Comparing the test results of mass loss and dowel bearing strength shows that bearing strength decrease significantly even for a small amount of mass loss. This study indicates that after twelve months incubation period the mass loss of wood samples is 2.25%-7.86%, causing dowel bearing strength decreases by 29-%. Detection of wood decay due to fungal attacks at early stage is, therefore, necessary to monitor service life and safety of timber connections. Conclusions Dowel bearing strength tests was conducted on three wood species (Melia azedarach, Swietenia mahagoni and Pterospermum javanicum) exposed to wood-decaying fungus, Schizophyllum commune Fr, for six and twelve months incubation period. Reduction of dowel bearing strength caused by the fungus differed according to wood species and incubation period. Among the three wood species considered here, the least bearing strength reduction was observed in Pterospermum javanicum specimens, which is 1% and 29% for six and twelve months incubation period, respectively. A remarkable strength reduction was found in Melia azedarach samples, which is about 2% and %, respectively, for six and twelve months incubation period. As this significant bearing strength reduction was potentially related to a small percentage of mass loss, for instance: % in the case of Pterospermum javanicum samples, detection and monitoring activities at early stage of fungal attacks is most important. References [1] Kent MK, Leichti RJ, Rosowsky D, 6

7 Morrell JJ, 25, Effects of decay on the cyclic properties of nailed connections, J. Materials in Civil Engineering, ASCE, Vol. 17, No. 5, p [2] Machek L, Militz H, Alvarez RS, 21, The use of acoustic technique to asses wood decay in laboratory soil-bed tests, J. Wood Science and Technology, Vol. 3, p [3] Sawata K, Sasaki T, Doi S, Iijima Y, 28a, Effect of decay on shear performance of dowel-type timber joints, J. Wood Science, Vol. 5, No. 5, p [] Sawata K, Takiuchi H, Toda M, Sasaki T, Mori M, 28b, Effect of decay on performance of wood and shear performance of dowel-type joints and nailed joints, Proceeding of the World Conference on Timber Engineering, Miyazaki. (CD-ROM) [5] Badan Sandarisasi Nasional, 26, SNI , Ketahanan kayu dan produk kayu terhadap organisme perusak kayu (Durability test of wood and wood products upon wood-decaying organism). 7