PREDICTION OF WOOD DECAY IN THE EXTERIOR WALL OF WOODEN HOUSES BY THE DETERIORATION OF NAILS Prediction of decay by nails

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1 PREDICTION OF WOOD DECAY IN THE EXTERIOR WALL OF WOODEN HOUSES BY THE DETERIORATION OF NAILS Prediction of decay by nails H. IMAMURA Wood Technological Association of Japan, Tokyo, Japan M. KIGUCHI Forestry and Forest Products Research Institute, Tsukuba, Japan Durability of Building Materials and Components 8. (1999) Edited by M.A. Lacasse and D.J. Vanier. Institute for Research in Construction, Ottawa ON, K1A 0R6, Canada, pp National Research Council Canada 1999 Abstract The deterioration of nails was graded using a five-rank numerical rating. Nails, moisture content and the decay of wood were examined in some mortal-wall wooden houses in central Japan. The wood was mostly in decay when the nail rating was above about four and the moisture content above about 20 %. Nailed wood specimens kept in several conditioning rooms for four years showed that the nail rating was about three in wood with a moisture content of 20 %. A nail rating curve against the service life was obtained the examination of 524 nails in mortal-walls. This curve could be used as a standard service life curve of nails in mortar-walls in central Japan. The curve obtained by connecting the points at rating three in four years and rating four in 34 years seems to be a wood decay warning line in terms of high moisture content. Shear tests for deteriorated nailed joints showed that the load was expressed as a function of the slip and the amount of rust on the nail. Keywords: bevel siding, decay, mortar-wall, nail, rust, service life, wooden house 1 Introduction Since the durability of a wooden house depends largely on its joints, nail durability is important especially for the bearing walls (Soshiroda 1950, Hirai and Wakashima 1996). Numerous wooden houses were destroyed during the Hanshin- Awaji (Kobe) Earthquake in 1995 (Murakami et al. 1996). Although most of the houses were old and their construction weak against for big earthquakes, their joint parts had been damaged by wood decay or termites (Hijikuro 1995). Thus the inspection of wooden houses is very important, and easy an inspection method is

2 strongly in demand. In this study, the deterioration of nails was used as a sensor for the prediction of wood decay. Most iron nails used for wooden houses are standard products and prescribed by the JIS (Japan Industrial Standard), and their quality is stable. The exterior walls of wooden houses in Japan are mainly covered with lathmortar for protection from fires. In this study, the deterioration of nailed joints was investigated mainly from mortar-walls, and environmental corrosive factors were assessed by the deterioration of nails in the bevel siding walls (Imamura et al. 1987). 2 The relationship between the deterioration of nails and wood decay 2.1 Rating of nail deterioration A five rank numerical rating scale to grade the deterioration of nails as shown in Table 1 was established. A relationship between the change in diameter d (%) and the rating of the nail (D) was found as shown in eq.1 and Fig. 1 (Imamura et al. 1983a). d=0.586d 2 (1 D 5) (1) The length of the CN50 nails was 50 mm. The change in diameter was the average of the changes at the neck, middle and near the point. The relationship between the amount of rust r (% in weight) and the rating of the CN50 nails (D) was obtained as follows. r = D2 (1 D 4.5) (2) 2.2 Effective diameter of nails Morris reported that, in the shearing tests of nails with diameter d, the value of K/d at slip δ = 0.6 (mm) and the load P (N) was stable regardless of d, where K was P/δ (Morris and Gajiar 1973). In fact, however, K/d increased with the d value. From Morris s data, eq. (3) was obtained as follows (Imamura et al. 1983b). K = d 1.57 (3) Table 1: The rating of the deterioration of nails

3 Fig. 1: Relationship between the rate of diameter change and nail rating Fig. 2: Shear test specimen of nailed joint Table 2: Residual percentage of effective diameter of nails Fig. 3: Load of the deteriorated nail (32mm) of the shear specimen with the sound wood A shear test of nails (32 mm length) taken from 30-year-old wooden bevel siding was carried out with the specimen shown in Fig. 2. The effective diameter of the nail d was obtained from K at δ (0.6 mm) in the curves in Fig. 3. The diameter obtained from eq.(3) was 1.85 and the one obtained from direct measurement was Table 2 shows the residual percentage of the effective diameter. 2.3 Deterioration of wood in contact with rusty nails Fig. 4 shows the differential thermal analysis curves of the wood powder (Western hemlock: Tsuga heterophylla) (Ohkuro and Imamura 1988). The samples were taken from the part adjacent to the nails (A), the part where the rust penetrated (B), and the sound part (C). The main peaks of the curves differ in temperature, and

4 the closer to the rusty nail, the earlier the thermal degradation of the wood occurs. This phenomenon suggests that wood in contact with the rust deteriorated (Marian and Wissing 1960; Farber 1954). 2.4 Deterioration of nails and wood decay Fig. 5 shows the amount of rust on the nail (CN50) in Western hemlock (160 x 80 x 30 mm) conditioned at several humidity levels at 20 C for four years. The amount of rust increases rapidly with a relative humidity of %, where the equilibium moisture content of the wood is about 15 %. In Fig. 6, the amount of rust is 9 to 10 % at the moisture content of 20 %. This amount of rust corresponds to the rating of amount three (III) using eq. (2). Since the deterioration of nails and wood is accelerated by water and temperature (Okamoto and Inoue 1977), there seems to be some connection between the degree of deterioration of the nails and that of the wood. Fig. 6 shows the moisture content of the wood and the rating of nails in the mortar-walls of a 34-year-old house. The moisture content was measured by using an electric resistance type moisture meter. The white circles show that the wood near the nails is without decay; and black circles show the wood with decay. Decay or sound was determined by visual inspection. Decay included some damage caused by termites. Fig. 4: Curves of differential thermal analysis Fig. 5: The equilibrium moisture content ( ) of wood and the amount of rust A: Wood in contact with rust nail (o) of nail in wood which were equilibrated B: Wood colored with nail rust in several RH at 20C for 4 years. C: Wood of a sound part

5 Fig. 6: Relationship between moisture content of wood and the nail rating (o): Wood without decay ( ): Wood with decay By using a statistical method, the discriminant function Z WD is expressed as eq.(4) where W (%) is the moisture content of the wood and D is the rating of the nail. Z WD = 0.03 W 3.76 D (4) According to the discriminant function, the wood is sound when Z WD is positive, and in decay when negative. In Fig. 6, the right side of the dotted line which expresses Z WD is a decay area. The possibility of misdiscrimination is 10.9 %. It is also 10.9%, when the discrimination depends upon the nail rating only. In this case, the discrimination function Z D becomes zero, at a nail rating The discrimination function by the moisture content Z W becomes zero at the moisture content 19% with the possibility of misdiscrimination of 23.6 %. This moisture content value is close to 20 % which is said to be the critical value in wood decay. 2.5 Shearing properties of the deteriorated joint The double shear specimens were made by using CN50 nails, plywood (70 x 120 x 12 mm) and Western hemlock solid wood specimens (70 x 120 x 26 mm, average specific gravity: 0.53). The conditions for the deterioration were that (a) the specimens were kept in a sink for one year where hot water (50 C) flowed beneath the specimens, (b) the specimens were kept in a transparent plastic desiccator for one year which was placed near a sunny window side with water beneath the specimens. The moisture content at the test was % for (a) and % for (b). After the shear test, nails were treated with ferric ammonium citrate for removing rust, and the amount of rust was obtained as weight loss.

6 Fig. 7 shows the load of the deteriorated double shear specimens (solid line) and the specimens made of the sound plywood, Western hemlock and the deteriorated nails (dotted line). Each curve is the average of three specimens. The curves show that the load of the deteriorated specimens was higher than that of the specimens with sound wood in the lower slip region, and the deteriorated wood decreased the load in the higher slip region. The iron rust made the connection tight between the nails and wood mainly effective in the lower slip region, and the deteriorated wood decreases the load in the higher slip region. Fig. 8 shows the relationship between the load and the rating at the slip of 0.9 mm. The load of the deteriorated specimen and that of the specimen with sound wood agree with each other at the slip of 0.9 mm where the effect of the rust and the wood on the load becomes a minimum value. The possibility of estimation of the load at the slip of 0.9 mm from the rating of the nails seems likely to be high regardless of the difference in the specific gravity of the wood. Fig. 7: Load of deteriorated samples Fig. 8: Relationship between the load at (1-3.5) and of sample (1-3.5 ) made 0.9mm slip and the rating (CN50, with the deteriorated nails and the double shear) sound wood o: Deteriorated samples : Samples made with deteriorated nails and sound wood

7 3 Effect of the design and details of the wall One of the most influential factors for the durability of the mortar-wall is a crack in the mortar. Rainwater penetrates through cracks into the wall and deteriorates the nails and wood. Fig. 9 shows the mortar-wall of a house with a crack. The rainwater penetrated through the crack and increased the average nail rating seen on the right side of the floor in Fig. 10. Fig. 11 shows the rating of the nails and the moisture content of the area between the top of the eaves of the first floor and the bottom of the window sash on the second floor in the eastern wall. Penetration of rainwater from the window sash increases the rating and the moisture content. The rainwater permeates the mortar when it is saturated with water. Traces of permeation were found inside the mortar-wall of the northern aspect of the Northwest corner NW (N), but not in the western aspect of the corner NW (W) of the house inspected. The moisture content of the mortar at the NW (N) was higher than NW (W) as shown in Table 3. The influence of the interior corner and the rainwater pipe on the nail deterioration was investigated in the bevel siding wall of a schoolhouse. The average ratings on five to six nails at a height within one meter from the top of the foundation were 4.38, 3.94 and 3.93 for the service years of 21, 25, and 28 years, respectively. This indicates that the factor of environmental circumstances is more influential than the service years. Fig 9: Elevation (Nagoya City) Fig 10: Distribution of the nail rating in the eastern wall Table 3: Moisture content of mortar

8 Fig. 11: The nail rating and the moisture content of plywood under the window 4 Prediction of wood decay from nails The change in the rating of nails as service life elapses is fundamentally important for the evaluation of a house. Table 4 shows the number of nails for the rating levels in the mortar-walls and Fig. 12 shows the curve obtained from Table 4. The relationship between the rating D and the years of service (t) is expressed in eq. (5). D = 2.2 log t (5) The black circles in Fig. 12 are the average rating of nails with limited numbers and were not used in obtaining the equation. The following findings are important to make an attempt to predict wood decay from nails. (1) From the inspection of the mortar-wall of the 34-year-old house, the wood decay was found to be prevailing at a nail rating of above about four and moisture content of above about 20 %. (2) The test showed that the nail rating was about three in the wood kept in 20% moisture content for four years. The following assumptions are needed: The moisture content of the wood in the inspected mortar-wall is stable. Namely the moisture content of 20 % is significant, and brings the nail to rating four after 34 years.

9 Fig. 12: Relationship between the nail rating in the exterior wall covered with lath-mortar and service life Fig. 13: Relationship between the nail rating and decay of wood in the exterior wall covered with lath-mortar

10 Based on the above findings and assumptions, a line from rating point three at four years (III at 4 y) to rating point four at 34 years (IV at 34 y) is conceivably a decay warning line of wood in mortar-wall. Fig. 13 shows the deterioration curve of the nail and the decay warning region of the wood. In the inspection of the mortarwall, if the rating of the nail is in the wood decay warning region, the moisture content is above 20 % and the wood seems to be undergoing decay even if it is sound upon the inspection. 5 References Farber, E. (1954) Chemical Deterioration of Wood in the Presence of Iron, Ind. Eng. Chem., Vol. 46, pp Hijikuro, K. (1995) Damage Factors of Wooden Houses by Hanshin Awaji Earthquake 4. Durability of wooden houses, Mokuzai Kogyo (J. Wood Industry), Vol. 50, pp Hirai, T. and Wakashima, Y. (1996) Lateral Resistance and Fracture Modes of Nailed Timber Joints III. Numerical analyses of maximum lateral resistance of nailed timber-plywood joints, Mokuzai Gakkaishi (J. Japan Wood Res, Soc.), Vol. 42, pp Imamura, H., Ohkuro, A., Karasawa, H. and Takagi, J. (1983a) Deterioration of Nail in 34-year Served Exterior Wall Covered with Wire Lath Mortar. Bull. The Forestry and Forest Products Research Institute, No. 322, pp Imamura, H., Kanaya, N., Takagi, J., Ohkuro, A., Karasawa, H. and Chiba, Y. (1983b) Deterioration of Nail in 30-year Served Wooden houses. Bull. The Forestry and Forest Products Research Institute, No. 322, pp Imamura, H., Kiguchi, M. and Ohkuro, A. (1987) The Deterioration Circumstances of Wood Viewed from the Deterioration of Nails in the Exterior Wall of Wooden Houses. Bull. The Forestry and Forest Products Research Institute, No. 345, pp Marian, J. and Wissing, A. (1960) The Chemical and Mechanical Deterioration of Wood in Contact with Iron, Part 1 Mechanical Deterioration, Svensk Papperstidning, Vol. 63, pp Morris, E.N. and Gajiar, S. (1973) Load-displacement Relationship for Nailed Joints with Solid Timber and Plywood Members. J. Inst. Wood Sci., Vol. 9, No. 2. pp Murakami, M., Tahara, M., Fujita, F. and Misawa, F. (1996) Damage of Urban Wooden Houses by Hanshin-Awaji Earthquake. J. Struct. Constr. Engng, No pp 71. Ohkuro, A. and Imamura H. (1988) Deterioration of Wood Used to Houses (II) Thermal Analysis of Wood in Contacting with Nail. Proceedings of 33 rd Annual meeting of the Japan Wood Research Society, pp238. Okamoto, T. and Inoue, K. (1977) in Corrosion and corrosionproofing, Dainippon Tosho, Tokyo, pp Soshiroda, S. (1950) Wood Decay and Preservation. J. Architectural Institute of Japan, No. 41. pp 1-26.