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The behavior of horizontally glued laminated beams using rubber wood
Construction and Building Materials 55 (2014) 398–405
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The behavior of horizontally glued laminated beams using rubber wood Yashida Nadir ⇑, Praveen Nagarajan Department of Civil Engineering, National Institute of Technology, Calicut, Kerala 673601, India
h i g h l i g h t s Evaluated the flexural properties of solid rubber wood and laminated rubber wood. If laminated wood could be suggested as a replacement to the solid wood. Evaluated laminated wood flexural properties with different lamina thickness and jointed lamina. Evaluated joint efficiency, wood adhesive bond strength, durability of the adhesive bond. Rubber wood is found suitable for laminated and structural wood products.
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Article history: Received 27 July 2013 Received in revised form 3 January 2014 Accepted 6 January 2014 Available online 13 February 2014 Keywords: Engineered wood products Glued laminated wood Flexural properties Finger joint Joint efficiency Wood adhesive bond Wood failure percentage Delamination
a b s t r a c t The process of manufacturing engineered wood products (EWP) is an effective technique for reducing or eliminating the negative properties of solid wood materials and for obtaining high performance materials. However, there is a dearth of information regarding the studies in glued laminated timber, one among the EWP, in India. Thus, this paper describes an experimental program which examines the flexural properties of horizontally glued laminated timber utilizing rubber wood, a sustainable, plantation grown timber in the country. The experimental test program involved the fabrication and testing in flexure of horizontally glued laminated rubber wood using polyvinyl acetate adhesive, with different lamina thickness and jointed laminas. The study also evaluated the joint efficiency, wood adhesive bond strength and the durability of the adhesive bond. The test results obtained show that the comparison of flexural properties between solid wood and horizontally glued laminated wood have no significant difference. Lamina thickness does not make any statistically significant difference in the flexural properties. Laminated beam and jointed laminated beam with the same lamina thickness have no significant difference in the flexural properties. The wood adhesive bond strength and the wood failure percentage obtained are appreciable. The experimental results obtained and a comparison with code provisions verifies the suitability of the wood species for composite products. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Engineered wood is a composite material alternative to solid wood, consisting of wood and adhesives, and it is available in several varieties; the most commonly used types of structural EWP are: laminated veneer lumber (LVL), glued-laminated timber (glulam), composite I beam, cross-laminated timber (CLT) and parallel strand lumber (PSL). Western, European and South East Asian countries are extensively using timber, particularly the engineered wood products (EWP) both for structural and non-structural purposes [1]. Glulam is made from sawn lumber lamina arranged in horizontal layers using glue, with the grains parallel to the length of the member. Taking the benefit of end jointing of smaller timber ⇑ Corresponding author. Tel.: +91 9446787298; fax: +91 0495 2287250. E-mail addresses: [email protected] (Y. Nadir), [email protected] (P. Nagarajan). http://dx.doi.org/10.1016/j.conbuildmat.2014.01.032 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
pieces, glulam can be made to any indefinitely long beam offering larger size capability than solid wood [2]. Structural glulam members have been widely used in developed countries particularly in America, Europe and Japan. These members are used in straight or curved form in numerous construction applications including sport complexes, commercial buildings, churches and residential houses [3]. In horizontally glued laminated timber load is applied perpendicular to the glue surface. The properties of laminated wood materials are affected by the type of wood, the defects it contain, thickness of layers, number of layers, type of glue used and the compression force used during pressing [4]. The characteristics of glued laminated beams using plantation timbers, African wood and Mangium have been studied by Evalina et al. [5] and found satisfactory performance as per JAS 234:2003. The performance of Acacia Mangium glulam beam was evaluated by Indah et al. [6] with different lamina thickness and found the flexural properties of glulam beam with 20 mm thick
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laminas having the lower values compared to 15 mm and 10 mm thick laminas. Bending strength properties of glued laminated timber from selected Malaysian hardwood timber were studied by Wan et al. [7] and compared the values obtained with JAS 234:2003 and found to be satisfactory. Timber, the traditional construction material was used widely in India for structural works in the earlier days. The less use now is may be of the non-availability of large diameter logs. To supplement the imported timber from countries having intensively managed forestry plantations, low grade/less girth underutilized trees available domestically was gaining significance [8]. Rubber wood, a by-product of a crop grown for latex production is being used and marketed in many applications, substituting higher value and less available traditional hardwoods such as teak in Asian countries. India is the fourth largest producer of natural rubber in the world, 89% of which is produced in Kerala. As is the case of all building materials, it is necessary to define the technical performance of the materials produced with laminates clearly, in order to be able to use them in buildings. Otherwise the material cannot hold up the conditions to which it is subjected [9]. In India the EWP such as the plywood products, particle board, and fiber board are used widely for non- structural works. Glulam is not a commercially established or available product in India. The product is developed for the present study on research interest. This study informs the utilization of rubber wood as glulam material. The objectives of this study included the fabrication and testing in flexure of horizontally glued laminated rubber wood and comparing the properties with solid beam. The effect of lamina thickness and jointed lamina in the flexural properties were investigated. The efficiency of the finger joint strength in tension and bending was evaluated. Also the wood adhesive bond strength and the durability of the adhesive bond were obtained. The test results were checked with code provisions so as to verify the suitability of rubber wood for composite products. As rubber wood is a sustainable timber leading to sustainable construction the authors proposed a thought of value addition of the material with this study. The results of this research can be very useful in realizing the potential usage of rubber wood in laminated products. Furthermore, it would be beneficial to carry out new research studies related to the same topic, but with different l/d ratios, different wood species, mixed wood, different glue types, contributing to the literature on this subject, opening a new area of research focus in the country. 2. Materials and methods 2.1. Manufacture and testing of glulam for its flexural properties Rubber wood (Hevea brasiliensis) used was having an average density of 605 kg/ m3 and an average moisture content of 10%, achieved in the conditions of 27 ± 2 °C temperature and relative humidity 65 ± 5%. As the timber was obtained after the latex unproductivity, the age is above 20 years. The commercial adhesive poly vinyl acetate (PVAc) was used in the fabrication of the laminated beams. Laminas having major defects were avoided and those having minor defects were placed in the neutral zone of the laminated beam. In the case of solid beam, specimens with defects such as knots and spiral grain were included for the study so as to demonstrate the real case. The PVAc adhesive used was with hardener DORUS R 7357 added in the ratio of 15% instructed by the manufacturer, applying on the specimens with the adhesive coverage of 250–300 g/m2. Adhesive was spread on both the surfaces of the laminas using the roller spreader. A nail gun was used to hammer small nails into the ends of the laminated materials so that the glued layers would not slip during pressing. Specimens were pressed with a pressure of 3 MPa applied for a period of 3 h at room temperature, the minimum specified pressure for laminated specimens is 1 MPa [10]. The specimens were then kept undisturbed for one week, conditioned in a room at a temperature of 27 ± 2 °C and relative humidity 65 ± 5% before testing. The dimension of the beam specimen was 900 ⁄ 60 ⁄ 40 mm (Fig. 1). The dimension of the beam was prepared so that the ratio of shear span (av) and the depth of the beam was in the range 4–6 as specified
in ASTM D198-09 [11] for the evaluation of flexural properties (bending strength and modulus of elasticity). The span to depth (l/d) ratio of the adopted specimen size corresponds to 14. The static bending test was conducted in accordance with the third-point loading method. Laminated specimens were prepared with different lamina thickness and also with finger jointed laminas to have a comparison with solid beam. The jointing technique favors laminated beam of any size and is particularly useful for rubber wood, where lengthy plies on seasoning gets warped. Accordingly a comparison is made between the flexural properties of the jointed laminated beam with unjointed laminated and solid beam. The placing of joints in the laminas should be staggered, not coinciding with those above or below and the distance between the joints depends on the lengths of the input material [12]. The finger joint used in this study has a length of 12 mm, pitch of 3.6 mm and tip width of 0.7 mm. The joints were placed in a staggered manner in the beams. The joints were placed outside the maximum moment zone in the outer tension lamina and also placed at maximum spacing. In three specimens only, the joints were placed inside the maximum moment zone to verify the strength. The preparation plan of the test specimens for the study is shown in the Fig. 2. The laminated timber beam specimens comprise four groups, as explained in Fig. 2, three groups of varying lamina thickness and the fourth group with jointed laminas. Ten specimens were tested from each group. A third-point loading static bending test was carried out, where the distance between the two loading points and the distance between the right and the left fulcrums was the same. The loading rate was 3 mm/min. Modulus of elasticity (MOE) and the modulus of rupture (MOR) were calculated by the following formula:
MOE ¼
MOR ¼
23PL3 3
108bh D
Pmax L bh
2
where P is the load within the proportional limit (N), L is the span of beam between the supports (mm), D is the mid span deflection (mm), Pmax is the maximum or the ultimate load (N), b is the width of the beam (mm) and h is the depth of the beam (mm).
2.2. Joint efficiency In much of the literature on finger joints, strength is expressed as a percentage of the strength of a piece of clear, unjointed wood of the same species and is referred to as the joint efficiency [13]. In this study the efficiency of the finger joint used for laminating was found in bending and tension as a basic data. For the joint efficiency testing, the finger joint was placed at the center of the specimen shown in Fig. 3(a and b). Small clear specimens were taken; dimension and testing were done as per ASTM D143-09 [14].
2.3. Wood adhesive bond strength The evaluation of the adhesive bond was done by finding the shear strength of the adhesive bond by conducting the block shear test using the ASTM D905-08e1 tool shown in the Fig. 4. The specimen dimension is shown in Fig. 5(a). The form and dimension of the specimen used for this test is adopted as per ASTM D90508e1 [15]. Since an adhesive transfers stress from one substrate to another through shear, shear test is commonly used to evaluate the bond performance of adhesive joints. Block shear test such as ASTM D905 is commonly used to evaluate mechanical properties of adhesive joints [16]. From the shear test, in addition to shear strength, percent wood failure on the fracture surface is also measured. During the test, the machine loading speed was adjusted to 5 mm/min; the maximum load (Pmax) at the moment of breaking was measured. The longitudinal direction of the wood was parallel to the loading direction during the test. Shear resistance was calculated by the following equation Pmax/A, where Pmax is the maximum load at the moment of breaking (N) and A is the surface area glued (mm2).
60 mm av
av 830 mm
40 mm
Fig. 1. Dimension of beam test specimen.
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TEST SPECIMENS FOR FLEXURAL STUDY
LAMINATED TIMBER
SOLID TIMBER
• •
3 LAYERED 20 mm THICK LAMINA
• •
4 LAYERED 15 mm THICK LAMINA
• •
• •
4 LAYERED 15 mm THICK JOINTED LAMINA
6 LAYERED 10 mm THICK LAMINA
Fig. 2. Preparation plan for the test specimens with each group having 10 replications: 3 layered with a thickness of 20 mm are glued; 4 layered with a thickness of 15 mm are glued; 6 layered with a thickness of 10 mm are glued; 4 layered 15 mm thick finger jointed laminas are glued.
(a) (a)
(b)
(b) Fig. 5. (a) Dimension of test specimen (b) wood failure.
Fig. 3. (a) Tension test of finger joint (b) bending test of finger joint. the adhesive bond was evaluated by measuring the delamination of the glue bond by conducting alternate wet and dry test. The form and dimension of the specimens taken for this study was the same as that used for block shear test. The specimen shall be submerged in boiling water for 4 h, cooled in water at room temperature and then dried for 20 h at 63 ± 3 °C in an oven. This completes the first cycle. The cycle shall be repeated making a total test period of 5 days and the data is recorded [17]. A feeler gauge of 0.125 mm thick was used for measuring the delamination.
3. Results and discussions 3.1. Evaluation of flexural properties of the specimens
Fig. 4. ASTM D905 tool.
2.4. Delamination test of the adhesive bond For the effective bonding and load transfer, the adhesives should be strong enough to withstand the swelling and shrinkage stresses caused by the movement of wood by wetting and drying, in the presence of moisture and heat. The durability of
The values obtained for the flexural properties of solid wood, laminated wood having different lamina thickness, jointed laminated wood are given in Table 1. Comparisons of the MOR of various groups showed that the highest mean of 70.35 MPa was exhibited by 20 mm thick laminated beam followed by solid wood with 69.14 MPa. 10 mm thick laminated beam has mean MOR of 67.08 MPa, whereas 15 mm thick laminated and 15 mm thick jointed laminated exhibited mean MORs of 62.94 MPa and 55.10 MPa respectively. A one-way analysis of variance (ANOVA) test was carried out at a 5 percent significance level, to assess whether statistically significant difference existed between the means of the flexural properties of various groups. The ANOVA performed showed, MOR between groups had significant difference at the 0.05 level, but bending MOE had no significant difference between groups. The multiple comparisons of mean MORs between groups showed that lamina thickness
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Y. Nadir, P. Nagarajan / Construction and Building Materials 55 (2014) 398–405 Table 1 Results of flexural properties of the specimens. Source of variation
Samples
Mean a
MOR
Group Group Group Group Group
S A B C J
69.14 70.35 62.94 67.08 55.10
MOE
Group Group Group Group Group
S A B C J
10447.81 9996.87 9949.99 9512.35 9583.65
a a ab ab b
Min
Max
Std.Dev
55.60 49.44 55.56 54.44 33.92
80.06 81.11 72.22 83.33 68.94
8.44 10.52 7.09 11.20 12.89
Group S-solid wood
8938.77 8455.42 8834.02 8329.22 7319.62
11677.31 12620.03 10588.20 12115.23 10727.02
977.44 1710.26 573.34 1630.08 1064.15
B-15 mm thick lamina
A-20 mm thick lamina
C-10 mm thick lamina J-15 mm thick jointed lamina
a Mean value of 10 samples in each group are recorded in MPa and the same small letters within a column are not statistically different at the 0.05 level by the Tukey test of ANOVA.
3.2. Evaluation of finger joint efficiency The joint efficiency was found in bending and tension, given in Table 2. The joint efficiency in the tension test obtained was lower than that in bending test. The percentage of tensile strength of clear wood obtained with the finger jointed connection was 39.31% and for bending strength it was 68.27%. Among the two test modes, bending and tension, the tension test was generally considered more critical and exhibit smaller values than the bending test. This might be due to the fact that the glue line in tension was more uniformly stressed than in bending and the tension is the weakest direction for glue bonds [18]. The efficiency of a joint can vary depending on the quality of the wood, whether the stress applied is in tension, bending or compression; the finger geometrical parameters and also the adhesive used [19]. The most popularly available finger geometry in the region was used for this study. Glulam which is used in a variety of structural applications is commonly employed in beams whose main rupture mode is caused by the stress in the lower member usually beginning in
Table 2 Joint efficiency of the finger joint profile used in the laminas. Mean
Min
Max
Std.Dev
in MPa 85.63a 58.46 68.27
72.18 49.30
104.44 70.11
11.59 6.84
Tension strength parallel to grain (TS) in MPa Control specimen (CS) 82.72 55.41 PVAc FJ 32.52 23.90 Joint efficiency (%) (JE) 39.31
116.21 44.53
20.42 6.47
Static bending strength (MOR) Control specimen (CS) PVAc FJ Joint efficiency (%) (JE)
a
FJ Average value of 8 specimens was recorded. JE ¼ CS 100.
Table 3 Shear resistance of wood-adhesive bond and control specimen. Adhesive
Shear strength
Wood failure percentage (WFP) (%)
PVAc
8.59a (1.09)
99.7 (0.95)
Solid wood (Control specimen)
13.45 (1.45)
a Average value of shear strength in MPa and WFP in % of 10 samples and standard deviation in parenthesis.
Load in KN
had no statistically significant effect. Also comparison of mean MOR between laminated and jointed laminated beams of the same lamina thickness had no statistically significant difference. The mean MORs of solid wood and laminated wood (unjointed) having different lamina thickness showed no statistically significant difference. But the multiple comparisons of mean MORs between groups showed that statistically significant difference existed between the mean MOR of 15 mm jointed laminated beam and 20 mm thick laminated beam and also with solid wood. ANOVA performed indicates laminating and also jointed laminating has found to have no statistically significant effect on MOE in bending. This research study on the flexural properties and the inferences was made on small scale specimens. It should be noted that further experimental study is needed to investigate the size effect of the specimens on the strength predictions.
16 14 12 10 8 6 4 2 0
A-20 mm laminated beam B-15 mm laminated beam C S J A B
0
10
20
30
C-10 mm laminated beam J- Jointed laminated beam S- Solid wood beam - Ultimate/Failure load point (Pu)
Deflection in mm Fig. 6. Load deflection curves in bending of one beam specimen from each group.
the finger joint of the outer tension lamina. Tensile tests are therefore the most critical and appropriate type of test to evaluate this factor [20], even though the bending test is considered as the most convenient and practical test for an extensive preliminary study of finger joints [13]. 3.3. Evaluation of adhesive bond strength The results of the adhesive bond strength (dry stage) in compression and the wood failure percentage are shown in Table 3. The average value of the shear strength of the adhesive bond was approximately 64% of the average value of the shear strength of solid wood, with appreciable wood failure percentage. Wood failure percentage is usually taken as an indication of the strength of the glue bond, with a higher percent wood failure indicating that the glue bonds are stronger than the wood itself. As per the standards for structural glued laminated timber by JAS 234:2003, the minimum value of adhesive bond strength is 5.4 MPa and the wood failure percentage 60%. The obtained tests results of this study shown in Table 3 met this requirement. 3.4. Evaluation of durability of adhesive bond The length of the delamination on both the end grain surfaces divided by the total length of bond line was the measure of
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3.5. Load-deflection curves
Load in KN
Pu
The load deflection curves in bending of some specimens are shown in Fig. 6. The deflections in these curves are based on mid-span deflection. Each curve comprises linear and nonlinear parts. The load at proportional limit (LP), was tabulated as shown in Fig. 7 to find the contribution of the nonlinear part on the load deflection curve. On an average, in solid and laminated beam specimens, LP is 70% of the ultimate load (Pu) in static bending test. In the jointed laminated beams this average value is rising to 78%. But the ductility or the nonlinear part could be increased by increasing the spacing of the finger joints in the outer tension lamina.
LP LP=70 % Pu
Deflection in mm Fig. 7. The load at proportional limit in the load–deflection curve in bending.
3.6. Comparison of experimental and theoretical results delamination. The percentage delamination of the lapped specimen at the first cycle of boil–dry was noted as 22.08%. ASTM D1101-97a [21] and JAS234:2003 specifies the percentage delamination not more than 5% for structural glued laminated timber. But the used adhesive PVAc for this study is generally not recommended for joints under high humidity and high temperature [22,23].
Table 4 Comparison of experimental and theoretical ultimate load values.
Comparison of experimental and theoretical ultimate load values Failure load (KN) Theoretical Laminated beam
ft
Experimental (average)
Laminated beam
14.88
12.02
Jointed laminated beam
7.82
9.91
The experimental results of this study show no effect for lamina thickness and no significant difference in the flexural properties between solid wood and laminated wood as per Table 1. So for the theoretical failure load calculation in this study, a laminated beam could be considered same as a solid beam with the assumptions of perfect bonding, constant modulus of elasticity for the individual laminas and the adhesive property same as laminas [24]. The theoretical analysis was done using the bending theory; to compare the ultimate load values of the laminated specimens with the experimental results obtained. Ultimate load (Failure load) in the simply supported condition of the beam was calculated shown in Table 4, using the bending equation M ¼ fz, f is the static bending strength of the wood (MOR) in MPa (Table 2), Z is the section modulus in mm2. The experimental average value of the ultimate load for laminated beams was showing a lower value than the theoretically obtained, may be because the experimental average value has included different types of failure modes. The ultimate load calculation of finger jointed laminated beam in this study was theoretically predicted using the method proposed by walter [25], which assumes that the failure always occurs at a finger joint in the extreme tension lamina. The theoretical model by walter predicted the strength of any beam size based on the strength of finger joints. The model also
Table 5 Types of failure during loading on the failure limit. Types of failure
Number of occurrences Solid
Glulam Unjointed
Tension failure—Crack initiating from the bottom and spreading to the top laminas with high wood failure
2
Sliding mode of failure—The fractured line passed partly through the wood and majority through the glue line surface. Also the fracture happened through the entire width of the beam
Jointed
20
6
Failure of finger joint—Failure initiated at the finger joint and spread to the top laminas with high wood failure
8
Compression failure due to buckling just beneath the bearing plate of loading
3
2
3
Splintering tension
2
1
2
Description of failures [26]
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assumed that the modulus of elasticity between the various laminas was not significantly different. This experimental study tested 10 finger jointed laminated beams, only 3 beams are with finger joints inside the maximum moment zone in the outer tension lamina. But all the beams with finger joints inside and also outside the maximum moment zone failed due to the finger joint failure, occurred at the outer tension lamina. Except two beams with finger joints outside the maximum moment zone in the lower tension lamina, failed by slope of grain failure. The number of finger joints in the outer tension lamina of all the tested beams were two. As finger joints appeared to be the critical factor, prediction of the strength of the jointed laminated beam by walter model may be reasonable. Knowing the strength of finger joints in bending, the coefficient of variation on that strength, the ratio between bending strength and tensile strength of finger joints, the number of finger joints and the number of laminates, the modulus of rupture of the jointed laminated beam could be predicted using walter model. Using these available datas of this study, the mean MOR of the finger jointed laminated beam obtained was 43.43 MPa. The experimentally obtained value was 55.10 MPa. The finger joints were symmetrically placed in the beam, accordingly a balanced laminate was assumed, the ultimate load was calculated shown in Table 4, using the bending equation with the obtained values of MORs.
Fig. 11. Slope of grain failure.
3.7. Failure description The failure descriptions of the solid and laminated beams are given in Table 5. The predominant mode of failure is the tension
Fig. 12. Splintering tension failure.
Fig. 8. Typical illustration of the tension failure and destruction of the wood of two specimens.
Fig. 9. Typical illustration of the sliding mode failure and destruction of the wood of two specimens.
Fig. 10. Typical illustration of the finger joint failure and destruction of the wood of two specimens.
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Table 6 Min permissible stress limits in MPa in three groups of structural timbers as per IS 883 and the permissible stress limits of rubber wood in MPa. Strength character
Location
Group A
Group B
Group C
Rubber wood
Bending and tension along the grain Shear along grain Compression parallel to grain Modulus of elasticity (1000)
Inside All locations Inside All locations
18 1.5 11.7 12.6
12 0.91 7.8 9.8
8.5 0.7 4.9 5.6
17.13 1.92 10.34 7.9
failure in laminated beams with the crack starting from bottom lamina and spreading to the top. The crack lines are with sufficient wood failure (Fig. 8). In perfectly bonded beams, free from other defects the tension failure is the probable mode. The sliding failure also happened in some beams, the crack line passed through wood and mostly through the adhesive surface, showing the glue failure on that surface (Fig. 9). In the jointed laminated beam the crack initiated from the finger joint and propagated to the top (Fig. 10). No sliding failure happened in the jointed laminated specimens. The sliding mode of failure (delamination failure) in a laminated beam may occur when there is a defectively glued area in the specimen so that the crack starts from the defected zone propagates through the adhesive surface, but when a very strong bonded area comes, it gets passed through the wood. The defectively glued area happens in the beams because of several reasons; the warped samples when laminated may not get properly interlocked, another is the non-uniform spread or absence of the glue on any point in the surface of the laminas. When the pressure application is delayed, also if the amount of pressure is more or less than required, add up the chances of slip failure. The tested beam is with smaller l/d ratio, so that the contribution from shear deformation is not less contributing to the chance of shear failure [27]. As finger jointing is done by taking small pieces of wood, there will be no warped samples, resulting in better bonding and no case of slip failure. The specimens having the outer tension laminas with the finger joints spaced at farthest positions have shown the better load carrying capacity than the specimens with finger joints near or inside the maximum moment region. In the specimens having the finger joints at the extreme spacing, the critical parameter of the failure was the defect in the wood, in the tested samples it was the slope of grain. Solid wood specimens tested, exhibited most of the different types of characteristic failure patterns of wood beams. Figs. 11 and 12 shows slope of grain failure and splintering tension in solid wood. 3.8. Comparison of the permissible stress limits of rubber wood with IS Standards The permissible stress limits of small clear wood specimens of rubber wood was found and compared with the minimum permissible stress limits of three groups of structural timbers specified in IS 883 [28] given in Table 6. The strength values are tented to group A, but regarding the lower stiffness values it can be determined in group C. 4. Conclusions Statistical analysis shows the flexural properties of solid wood and laminated wood as having no significant difference. Lamina thickness does not make any statistically significant difference in the flexural properties. Laminated beam and jointed laminated beam with the same lamina thickness have no significant difference in the flexural properties. The wood adhesive bond strength and the wood failure percentage are appreciable. The experimental results obtained and a comparison with code provisions verifies the suitability of the wood species for composite products.
As rubber wood is a by-product and is a sustainable resource, this study could be well-thought-out as a value addition to the material. It may also be possible to use this material as the core layers of laminated elements and to combine with high density materials in the outer layers, for its strength enhancement and as a safeguard to its durability. This adds up a further scope of this study. With the new generation chemicals and the advanced strength enhancement techniques, research studies and also initiatives from the industry would be supportive for the effective utilization of this plenteous resource.
Acknowledgements The authors thank the managerial staffs Uma Sankar, Aniprabha Nair, Jayamon and all the workers of India Wood, RRII, Kottayam for the great help and support extended for the fabrication of the specimens needed for the study.
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Y. Nadir, P. Nagarajan / Construction and Building Materials 55 (2014) 398–405 [17] Indian Standards Institution. Performance requirements of adhesives for structural laminated wood products for use under exterior exposure conditions. IS 9188-1979, Indian Standard Institution, New Delhi, 1980. [18] Ayarkwa J, Hirashima Y, Sasaki Y, Ando K. Effect of glue type on flexural and tensile properties of finger-jointed tropical African hardwoods. Forest Prod J 2000;50(10):59–68. [19] Ozcifici Ayhan, Yapici Fatih. Structural performance of the finger-jointed strength of some wood species with different joint configurations. Constr Build Mater 2008;22:1543–50. [20] Martinez ME, Calil CJ. Statistical design and orthogonal polynomial model to estimate the tensile fatigue strength of wooden finger joints. Int J Fatigue 2003;25:237–43. [21] American Society for Testing and Materials. Standard test methods for integrity of adhesive joints in structural laminated wood products for exterior use. ASTM D 1101-97a (Reapproved 2006). ASTM, West Conshohocken, Pa, 2006. [22] Charles R. Frihart, Christopher G. Hunt. Adhesives with Wood Materials Bond Formation and Performance. Wood Handbook-Wood as an engineering
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[24]
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material. Centennial Edition, Forest Products Laboratory, US Department of Agriculture Forest Service, general technical report; FPL-GTR-190, 2010. p.508 Petrie Edward M. Improving the moisture and heat resistance of PVAc wood adhesives. Special Chem adhesives and sealants, Formulation, bulletin April 25, 2007. Sepehr Rahili Khorasan. Finite element simulations of glulam beams with natural cracks. Master’s Degree Thesis, Blekinge Institute of Technology, Sweden, 2012. Walter MG Burdzik. Finger joint strength-A laminated beam strength predictor. Research Note, Southern African Forestry Journal, No. 178. March 1997. Vanerek J, Hradil P. Evaluation of timber beams reinforced with FRP fabrics. Asia-Pacific conference on FRP in structures, 2007 (APFIS 2007). Baird JA, Ozelton EC. Timber designers’ manual. 2nd ed. London: Granada Publishing Ltd.; 1984. Indian Standards Institution. Design of structural timber in building-Code of practice. IS 883-1994, Bureau of Indian Standards, New Delhi, 1994.
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The behavior of horizontally glued laminated beams using rubber wood Yashida Nadir ⇑, Praveen Nagarajan Department of Civil Engineering, National Institute of Technology, Calicut, Kerala 673601, India
h i g h l i g h t s Evaluated the flexural properties of solid rubber wood and laminated rubber wood. If laminated wood could be suggested as a replacement to the solid wood. Evaluated laminated wood flexural properties with different lamina thickness and jointed lamina. Evaluated joint efficiency, wood adhesive bond strength, durability of the adhesive bond. Rubber wood is found suitable for laminated and structural wood products.
a r t i c l e
i n f o
Article history: Received 27 July 2013 Received in revised form 3 January 2014 Accepted 6 January 2014 Available online 13 February 2014 Keywords: Engineered wood products Glued laminated wood Flexural properties Finger joint Joint efficiency Wood adhesive bond Wood failure percentage Delamination
a b s t r a c t The process of manufacturing engineered wood products (EWP) is an effective technique for reducing or eliminating the negative properties of solid wood materials and for obtaining high performance materials. However, there is a dearth of information regarding the studies in glued laminated timber, one among the EWP, in India. Thus, this paper describes an experimental program which examines the flexural properties of horizontally glued laminated timber utilizing rubber wood, a sustainable, plantation grown timber in the country. The experimental test program involved the fabrication and testing in flexure of horizontally glued laminated rubber wood using polyvinyl acetate adhesive, with different lamina thickness and jointed laminas. The study also evaluated the joint efficiency, wood adhesive bond strength and the durability of the adhesive bond. The test results obtained show that the comparison of flexural properties between solid wood and horizontally glued laminated wood have no significant difference. Lamina thickness does not make any statistically significant difference in the flexural properties. Laminated beam and jointed laminated beam with the same lamina thickness have no significant difference in the flexural properties. The wood adhesive bond strength and the wood failure percentage obtained are appreciable. The experimental results obtained and a comparison with code provisions verifies the suitability of the wood species for composite products. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Engineered wood is a composite material alternative to solid wood, consisting of wood and adhesives, and it is available in several varieties; the most commonly used types of structural EWP are: laminated veneer lumber (LVL), glued-laminated timber (glulam), composite I beam, cross-laminated timber (CLT) and parallel strand lumber (PSL). Western, European and South East Asian countries are extensively using timber, particularly the engineered wood products (EWP) both for structural and non-structural purposes [1]. Glulam is made from sawn lumber lamina arranged in horizontal layers using glue, with the grains parallel to the length of the member. Taking the benefit of end jointing of smaller timber ⇑ Corresponding author. Tel.: +91 9446787298; fax: +91 0495 2287250. E-mail addresses: [email protected] (Y. Nadir), [email protected] (P. Nagarajan). http://dx.doi.org/10.1016/j.conbuildmat.2014.01.032 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
pieces, glulam can be made to any indefinitely long beam offering larger size capability than solid wood [2]. Structural glulam members have been widely used in developed countries particularly in America, Europe and Japan. These members are used in straight or curved form in numerous construction applications including sport complexes, commercial buildings, churches and residential houses [3]. In horizontally glued laminated timber load is applied perpendicular to the glue surface. The properties of laminated wood materials are affected by the type of wood, the defects it contain, thickness of layers, number of layers, type of glue used and the compression force used during pressing [4]. The characteristics of glued laminated beams using plantation timbers, African wood and Mangium have been studied by Evalina et al. [5] and found satisfactory performance as per JAS 234:2003. The performance of Acacia Mangium glulam beam was evaluated by Indah et al. [6] with different lamina thickness and found the flexural properties of glulam beam with 20 mm thick
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laminas having the lower values compared to 15 mm and 10 mm thick laminas. Bending strength properties of glued laminated timber from selected Malaysian hardwood timber were studied by Wan et al. [7] and compared the values obtained with JAS 234:2003 and found to be satisfactory. Timber, the traditional construction material was used widely in India for structural works in the earlier days. The less use now is may be of the non-availability of large diameter logs. To supplement the imported timber from countries having intensively managed forestry plantations, low grade/less girth underutilized trees available domestically was gaining significance [8]. Rubber wood, a by-product of a crop grown for latex production is being used and marketed in many applications, substituting higher value and less available traditional hardwoods such as teak in Asian countries. India is the fourth largest producer of natural rubber in the world, 89% of which is produced in Kerala. As is the case of all building materials, it is necessary to define the technical performance of the materials produced with laminates clearly, in order to be able to use them in buildings. Otherwise the material cannot hold up the conditions to which it is subjected [9]. In India the EWP such as the plywood products, particle board, and fiber board are used widely for non- structural works. Glulam is not a commercially established or available product in India. The product is developed for the present study on research interest. This study informs the utilization of rubber wood as glulam material. The objectives of this study included the fabrication and testing in flexure of horizontally glued laminated rubber wood and comparing the properties with solid beam. The effect of lamina thickness and jointed lamina in the flexural properties were investigated. The efficiency of the finger joint strength in tension and bending was evaluated. Also the wood adhesive bond strength and the durability of the adhesive bond were obtained. The test results were checked with code provisions so as to verify the suitability of rubber wood for composite products. As rubber wood is a sustainable timber leading to sustainable construction the authors proposed a thought of value addition of the material with this study. The results of this research can be very useful in realizing the potential usage of rubber wood in laminated products. Furthermore, it would be beneficial to carry out new research studies related to the same topic, but with different l/d ratios, different wood species, mixed wood, different glue types, contributing to the literature on this subject, opening a new area of research focus in the country. 2. Materials and methods 2.1. Manufacture and testing of glulam for its flexural properties Rubber wood (Hevea brasiliensis) used was having an average density of 605 kg/ m3 and an average moisture content of 10%, achieved in the conditions of 27 ± 2 °C temperature and relative humidity 65 ± 5%. As the timber was obtained after the latex unproductivity, the age is above 20 years. The commercial adhesive poly vinyl acetate (PVAc) was used in the fabrication of the laminated beams. Laminas having major defects were avoided and those having minor defects were placed in the neutral zone of the laminated beam. In the case of solid beam, specimens with defects such as knots and spiral grain were included for the study so as to demonstrate the real case. The PVAc adhesive used was with hardener DORUS R 7357 added in the ratio of 15% instructed by the manufacturer, applying on the specimens with the adhesive coverage of 250–300 g/m2. Adhesive was spread on both the surfaces of the laminas using the roller spreader. A nail gun was used to hammer small nails into the ends of the laminated materials so that the glued layers would not slip during pressing. Specimens were pressed with a pressure of 3 MPa applied for a period of 3 h at room temperature, the minimum specified pressure for laminated specimens is 1 MPa [10]. The specimens were then kept undisturbed for one week, conditioned in a room at a temperature of 27 ± 2 °C and relative humidity 65 ± 5% before testing. The dimension of the beam specimen was 900 ⁄ 60 ⁄ 40 mm (Fig. 1). The dimension of the beam was prepared so that the ratio of shear span (av) and the depth of the beam was in the range 4–6 as specified
in ASTM D198-09 [11] for the evaluation of flexural properties (bending strength and modulus of elasticity). The span to depth (l/d) ratio of the adopted specimen size corresponds to 14. The static bending test was conducted in accordance with the third-point loading method. Laminated specimens were prepared with different lamina thickness and also with finger jointed laminas to have a comparison with solid beam. The jointing technique favors laminated beam of any size and is particularly useful for rubber wood, where lengthy plies on seasoning gets warped. Accordingly a comparison is made between the flexural properties of the jointed laminated beam with unjointed laminated and solid beam. The placing of joints in the laminas should be staggered, not coinciding with those above or below and the distance between the joints depends on the lengths of the input material [12]. The finger joint used in this study has a length of 12 mm, pitch of 3.6 mm and tip width of 0.7 mm. The joints were placed in a staggered manner in the beams. The joints were placed outside the maximum moment zone in the outer tension lamina and also placed at maximum spacing. In three specimens only, the joints were placed inside the maximum moment zone to verify the strength. The preparation plan of the test specimens for the study is shown in the Fig. 2. The laminated timber beam specimens comprise four groups, as explained in Fig. 2, three groups of varying lamina thickness and the fourth group with jointed laminas. Ten specimens were tested from each group. A third-point loading static bending test was carried out, where the distance between the two loading points and the distance between the right and the left fulcrums was the same. The loading rate was 3 mm/min. Modulus of elasticity (MOE) and the modulus of rupture (MOR) were calculated by the following formula:
MOE ¼
MOR ¼
23PL3 3
108bh D
Pmax L bh
2
where P is the load within the proportional limit (N), L is the span of beam between the supports (mm), D is the mid span deflection (mm), Pmax is the maximum or the ultimate load (N), b is the width of the beam (mm) and h is the depth of the beam (mm).
2.2. Joint efficiency In much of the literature on finger joints, strength is expressed as a percentage of the strength of a piece of clear, unjointed wood of the same species and is referred to as the joint efficiency [13]. In this study the efficiency of the finger joint used for laminating was found in bending and tension as a basic data. For the joint efficiency testing, the finger joint was placed at the center of the specimen shown in Fig. 3(a and b). Small clear specimens were taken; dimension and testing were done as per ASTM D143-09 [14].
2.3. Wood adhesive bond strength The evaluation of the adhesive bond was done by finding the shear strength of the adhesive bond by conducting the block shear test using the ASTM D905-08e1 tool shown in the Fig. 4. The specimen dimension is shown in Fig. 5(a). The form and dimension of the specimen used for this test is adopted as per ASTM D90508e1 [15]. Since an adhesive transfers stress from one substrate to another through shear, shear test is commonly used to evaluate the bond performance of adhesive joints. Block shear test such as ASTM D905 is commonly used to evaluate mechanical properties of adhesive joints [16]. From the shear test, in addition to shear strength, percent wood failure on the fracture surface is also measured. During the test, the machine loading speed was adjusted to 5 mm/min; the maximum load (Pmax) at the moment of breaking was measured. The longitudinal direction of the wood was parallel to the loading direction during the test. Shear resistance was calculated by the following equation Pmax/A, where Pmax is the maximum load at the moment of breaking (N) and A is the surface area glued (mm2).
60 mm av
av 830 mm
40 mm
Fig. 1. Dimension of beam test specimen.
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TEST SPECIMENS FOR FLEXURAL STUDY
LAMINATED TIMBER
SOLID TIMBER
• •
3 LAYERED 20 mm THICK LAMINA
• •
4 LAYERED 15 mm THICK LAMINA
• •
• •
4 LAYERED 15 mm THICK JOINTED LAMINA
6 LAYERED 10 mm THICK LAMINA
Fig. 2. Preparation plan for the test specimens with each group having 10 replications: 3 layered with a thickness of 20 mm are glued; 4 layered with a thickness of 15 mm are glued; 6 layered with a thickness of 10 mm are glued; 4 layered 15 mm thick finger jointed laminas are glued.
(a) (a)
(b)
(b) Fig. 5. (a) Dimension of test specimen (b) wood failure.
Fig. 3. (a) Tension test of finger joint (b) bending test of finger joint. the adhesive bond was evaluated by measuring the delamination of the glue bond by conducting alternate wet and dry test. The form and dimension of the specimens taken for this study was the same as that used for block shear test. The specimen shall be submerged in boiling water for 4 h, cooled in water at room temperature and then dried for 20 h at 63 ± 3 °C in an oven. This completes the first cycle. The cycle shall be repeated making a total test period of 5 days and the data is recorded [17]. A feeler gauge of 0.125 mm thick was used for measuring the delamination.
3. Results and discussions 3.1. Evaluation of flexural properties of the specimens
Fig. 4. ASTM D905 tool.
2.4. Delamination test of the adhesive bond For the effective bonding and load transfer, the adhesives should be strong enough to withstand the swelling and shrinkage stresses caused by the movement of wood by wetting and drying, in the presence of moisture and heat. The durability of
The values obtained for the flexural properties of solid wood, laminated wood having different lamina thickness, jointed laminated wood are given in Table 1. Comparisons of the MOR of various groups showed that the highest mean of 70.35 MPa was exhibited by 20 mm thick laminated beam followed by solid wood with 69.14 MPa. 10 mm thick laminated beam has mean MOR of 67.08 MPa, whereas 15 mm thick laminated and 15 mm thick jointed laminated exhibited mean MORs of 62.94 MPa and 55.10 MPa respectively. A one-way analysis of variance (ANOVA) test was carried out at a 5 percent significance level, to assess whether statistically significant difference existed between the means of the flexural properties of various groups. The ANOVA performed showed, MOR between groups had significant difference at the 0.05 level, but bending MOE had no significant difference between groups. The multiple comparisons of mean MORs between groups showed that lamina thickness
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Y. Nadir, P. Nagarajan / Construction and Building Materials 55 (2014) 398–405 Table 1 Results of flexural properties of the specimens. Source of variation
Samples
Mean a
MOR
Group Group Group Group Group
S A B C J
69.14 70.35 62.94 67.08 55.10
MOE
Group Group Group Group Group
S A B C J
10447.81 9996.87 9949.99 9512.35 9583.65
a a ab ab b
Min
Max
Std.Dev
55.60 49.44 55.56 54.44 33.92
80.06 81.11 72.22 83.33 68.94
8.44 10.52 7.09 11.20 12.89
Group S-solid wood
8938.77 8455.42 8834.02 8329.22 7319.62
11677.31 12620.03 10588.20 12115.23 10727.02
977.44 1710.26 573.34 1630.08 1064.15
B-15 mm thick lamina
A-20 mm thick lamina
C-10 mm thick lamina J-15 mm thick jointed lamina
a Mean value of 10 samples in each group are recorded in MPa and the same small letters within a column are not statistically different at the 0.05 level by the Tukey test of ANOVA.
3.2. Evaluation of finger joint efficiency The joint efficiency was found in bending and tension, given in Table 2. The joint efficiency in the tension test obtained was lower than that in bending test. The percentage of tensile strength of clear wood obtained with the finger jointed connection was 39.31% and for bending strength it was 68.27%. Among the two test modes, bending and tension, the tension test was generally considered more critical and exhibit smaller values than the bending test. This might be due to the fact that the glue line in tension was more uniformly stressed than in bending and the tension is the weakest direction for glue bonds [18]. The efficiency of a joint can vary depending on the quality of the wood, whether the stress applied is in tension, bending or compression; the finger geometrical parameters and also the adhesive used [19]. The most popularly available finger geometry in the region was used for this study. Glulam which is used in a variety of structural applications is commonly employed in beams whose main rupture mode is caused by the stress in the lower member usually beginning in
Table 2 Joint efficiency of the finger joint profile used in the laminas. Mean
Min
Max
Std.Dev
in MPa 85.63a 58.46 68.27
72.18 49.30
104.44 70.11
11.59 6.84
Tension strength parallel to grain (TS) in MPa Control specimen (CS) 82.72 55.41 PVAc FJ 32.52 23.90 Joint efficiency (%) (JE) 39.31
116.21 44.53
20.42 6.47
Static bending strength (MOR) Control specimen (CS) PVAc FJ Joint efficiency (%) (JE)
a
FJ Average value of 8 specimens was recorded. JE ¼ CS 100.
Table 3 Shear resistance of wood-adhesive bond and control specimen. Adhesive
Shear strength
Wood failure percentage (WFP) (%)
PVAc
8.59a (1.09)
99.7 (0.95)
Solid wood (Control specimen)
13.45 (1.45)
a Average value of shear strength in MPa and WFP in % of 10 samples and standard deviation in parenthesis.
Load in KN
had no statistically significant effect. Also comparison of mean MOR between laminated and jointed laminated beams of the same lamina thickness had no statistically significant difference. The mean MORs of solid wood and laminated wood (unjointed) having different lamina thickness showed no statistically significant difference. But the multiple comparisons of mean MORs between groups showed that statistically significant difference existed between the mean MOR of 15 mm jointed laminated beam and 20 mm thick laminated beam and also with solid wood. ANOVA performed indicates laminating and also jointed laminating has found to have no statistically significant effect on MOE in bending. This research study on the flexural properties and the inferences was made on small scale specimens. It should be noted that further experimental study is needed to investigate the size effect of the specimens on the strength predictions.
16 14 12 10 8 6 4 2 0
A-20 mm laminated beam B-15 mm laminated beam C S J A B
0
10
20
30
C-10 mm laminated beam J- Jointed laminated beam S- Solid wood beam - Ultimate/Failure load point (Pu)
Deflection in mm Fig. 6. Load deflection curves in bending of one beam specimen from each group.
the finger joint of the outer tension lamina. Tensile tests are therefore the most critical and appropriate type of test to evaluate this factor [20], even though the bending test is considered as the most convenient and practical test for an extensive preliminary study of finger joints [13]. 3.3. Evaluation of adhesive bond strength The results of the adhesive bond strength (dry stage) in compression and the wood failure percentage are shown in Table 3. The average value of the shear strength of the adhesive bond was approximately 64% of the average value of the shear strength of solid wood, with appreciable wood failure percentage. Wood failure percentage is usually taken as an indication of the strength of the glue bond, with a higher percent wood failure indicating that the glue bonds are stronger than the wood itself. As per the standards for structural glued laminated timber by JAS 234:2003, the minimum value of adhesive bond strength is 5.4 MPa and the wood failure percentage 60%. The obtained tests results of this study shown in Table 3 met this requirement. 3.4. Evaluation of durability of adhesive bond The length of the delamination on both the end grain surfaces divided by the total length of bond line was the measure of
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3.5. Load-deflection curves
Load in KN
Pu
The load deflection curves in bending of some specimens are shown in Fig. 6. The deflections in these curves are based on mid-span deflection. Each curve comprises linear and nonlinear parts. The load at proportional limit (LP), was tabulated as shown in Fig. 7 to find the contribution of the nonlinear part on the load deflection curve. On an average, in solid and laminated beam specimens, LP is 70% of the ultimate load (Pu) in static bending test. In the jointed laminated beams this average value is rising to 78%. But the ductility or the nonlinear part could be increased by increasing the spacing of the finger joints in the outer tension lamina.
LP LP=70 % Pu
Deflection in mm Fig. 7. The load at proportional limit in the load–deflection curve in bending.
3.6. Comparison of experimental and theoretical results delamination. The percentage delamination of the lapped specimen at the first cycle of boil–dry was noted as 22.08%. ASTM D1101-97a [21] and JAS234:2003 specifies the percentage delamination not more than 5% for structural glued laminated timber. But the used adhesive PVAc for this study is generally not recommended for joints under high humidity and high temperature [22,23].
Table 4 Comparison of experimental and theoretical ultimate load values.
Comparison of experimental and theoretical ultimate load values Failure load (KN) Theoretical Laminated beam
ft
Experimental (average)
Laminated beam
14.88
12.02
Jointed laminated beam
7.82
9.91
The experimental results of this study show no effect for lamina thickness and no significant difference in the flexural properties between solid wood and laminated wood as per Table 1. So for the theoretical failure load calculation in this study, a laminated beam could be considered same as a solid beam with the assumptions of perfect bonding, constant modulus of elasticity for the individual laminas and the adhesive property same as laminas [24]. The theoretical analysis was done using the bending theory; to compare the ultimate load values of the laminated specimens with the experimental results obtained. Ultimate load (Failure load) in the simply supported condition of the beam was calculated shown in Table 4, using the bending equation M ¼ fz, f is the static bending strength of the wood (MOR) in MPa (Table 2), Z is the section modulus in mm2. The experimental average value of the ultimate load for laminated beams was showing a lower value than the theoretically obtained, may be because the experimental average value has included different types of failure modes. The ultimate load calculation of finger jointed laminated beam in this study was theoretically predicted using the method proposed by walter [25], which assumes that the failure always occurs at a finger joint in the extreme tension lamina. The theoretical model by walter predicted the strength of any beam size based on the strength of finger joints. The model also
Table 5 Types of failure during loading on the failure limit. Types of failure
Number of occurrences Solid
Glulam Unjointed
Tension failure—Crack initiating from the bottom and spreading to the top laminas with high wood failure
2
Sliding mode of failure—The fractured line passed partly through the wood and majority through the glue line surface. Also the fracture happened through the entire width of the beam
Jointed
20
6
Failure of finger joint—Failure initiated at the finger joint and spread to the top laminas with high wood failure
8
Compression failure due to buckling just beneath the bearing plate of loading
3
2
3
Splintering tension
2
1
2
Description of failures [26]
Y. Nadir, P. Nagarajan / Construction and Building Materials 55 (2014) 398–405
assumed that the modulus of elasticity between the various laminas was not significantly different. This experimental study tested 10 finger jointed laminated beams, only 3 beams are with finger joints inside the maximum moment zone in the outer tension lamina. But all the beams with finger joints inside and also outside the maximum moment zone failed due to the finger joint failure, occurred at the outer tension lamina. Except two beams with finger joints outside the maximum moment zone in the lower tension lamina, failed by slope of grain failure. The number of finger joints in the outer tension lamina of all the tested beams were two. As finger joints appeared to be the critical factor, prediction of the strength of the jointed laminated beam by walter model may be reasonable. Knowing the strength of finger joints in bending, the coefficient of variation on that strength, the ratio between bending strength and tensile strength of finger joints, the number of finger joints and the number of laminates, the modulus of rupture of the jointed laminated beam could be predicted using walter model. Using these available datas of this study, the mean MOR of the finger jointed laminated beam obtained was 43.43 MPa. The experimentally obtained value was 55.10 MPa. The finger joints were symmetrically placed in the beam, accordingly a balanced laminate was assumed, the ultimate load was calculated shown in Table 4, using the bending equation with the obtained values of MORs.
Fig. 11. Slope of grain failure.
3.7. Failure description The failure descriptions of the solid and laminated beams are given in Table 5. The predominant mode of failure is the tension
Fig. 12. Splintering tension failure.
Fig. 8. Typical illustration of the tension failure and destruction of the wood of two specimens.
Fig. 9. Typical illustration of the sliding mode failure and destruction of the wood of two specimens.
Fig. 10. Typical illustration of the finger joint failure and destruction of the wood of two specimens.
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Table 6 Min permissible stress limits in MPa in three groups of structural timbers as per IS 883 and the permissible stress limits of rubber wood in MPa. Strength character
Location
Group A
Group B
Group C
Rubber wood
Bending and tension along the grain Shear along grain Compression parallel to grain Modulus of elasticity (1000)
Inside All locations Inside All locations
18 1.5 11.7 12.6
12 0.91 7.8 9.8
8.5 0.7 4.9 5.6
17.13 1.92 10.34 7.9
failure in laminated beams with the crack starting from bottom lamina and spreading to the top. The crack lines are with sufficient wood failure (Fig. 8). In perfectly bonded beams, free from other defects the tension failure is the probable mode. The sliding failure also happened in some beams, the crack line passed through wood and mostly through the adhesive surface, showing the glue failure on that surface (Fig. 9). In the jointed laminated beam the crack initiated from the finger joint and propagated to the top (Fig. 10). No sliding failure happened in the jointed laminated specimens. The sliding mode of failure (delamination failure) in a laminated beam may occur when there is a defectively glued area in the specimen so that the crack starts from the defected zone propagates through the adhesive surface, but when a very strong bonded area comes, it gets passed through the wood. The defectively glued area happens in the beams because of several reasons; the warped samples when laminated may not get properly interlocked, another is the non-uniform spread or absence of the glue on any point in the surface of the laminas. When the pressure application is delayed, also if the amount of pressure is more or less than required, add up the chances of slip failure. The tested beam is with smaller l/d ratio, so that the contribution from shear deformation is not less contributing to the chance of shear failure [27]. As finger jointing is done by taking small pieces of wood, there will be no warped samples, resulting in better bonding and no case of slip failure. The specimens having the outer tension laminas with the finger joints spaced at farthest positions have shown the better load carrying capacity than the specimens with finger joints near or inside the maximum moment region. In the specimens having the finger joints at the extreme spacing, the critical parameter of the failure was the defect in the wood, in the tested samples it was the slope of grain. Solid wood specimens tested, exhibited most of the different types of characteristic failure patterns of wood beams. Figs. 11 and 12 shows slope of grain failure and splintering tension in solid wood. 3.8. Comparison of the permissible stress limits of rubber wood with IS Standards The permissible stress limits of small clear wood specimens of rubber wood was found and compared with the minimum permissible stress limits of three groups of structural timbers specified in IS 883 [28] given in Table 6. The strength values are tented to group A, but regarding the lower stiffness values it can be determined in group C. 4. Conclusions Statistical analysis shows the flexural properties of solid wood and laminated wood as having no significant difference. Lamina thickness does not make any statistically significant difference in the flexural properties. Laminated beam and jointed laminated beam with the same lamina thickness have no significant difference in the flexural properties. The wood adhesive bond strength and the wood failure percentage are appreciable. The experimental results obtained and a comparison with code provisions verifies the suitability of the wood species for composite products.
As rubber wood is a by-product and is a sustainable resource, this study could be well-thought-out as a value addition to the material. It may also be possible to use this material as the core layers of laminated elements and to combine with high density materials in the outer layers, for its strength enhancement and as a safeguard to its durability. This adds up a further scope of this study. With the new generation chemicals and the advanced strength enhancement techniques, research studies and also initiatives from the industry would be supportive for the effective utilization of this plenteous resource.
Acknowledgements The authors thank the managerial staffs Uma Sankar, Aniprabha Nair, Jayamon and all the workers of India Wood, RRII, Kottayam for the great help and support extended for the fabrication of the specimens needed for the study.
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