STRUCTURAL BEHAVIOR OF GLUED LAMINATED GUADUA BAMBOO AS A CONSTRUCTION MATERIAL

Juan Correal1, Fernando Ramirez2, Soffy Gonzalez3, Jessica Camacho4

ABSTRACT: Currently, there is a global need to have sustainable materials in order to ensure resources for present and future generations. Besides timber, bamboo can be an interesting sustainable material since it has high strength to weight ratio, relative low cost and fast growing rate. According to the International Network for Bamboo and Rattan, there are about 1.250 species of bamboo all over the world but only 20 of them are considered suitable for construction. A giant specie of bamboo called Guadua Angustifolia kunt has been used as a construction material in Colombia for more than 20 years. Even though, round guadua has been used in construction of structures with relative success, one of the problems is the variability of its geometry, mechanical properties and anatomical composition that makes this material difficult to characterize and prevents its use in large structures. Glued Laminated Guadua Bamboo (GLG) is considered as an excellent alternative; since few exploratory studies indicated that it has mechanical properties are as good as the best structural woods in Colombia.

The Universidad de los Andes in Bogotá, Colombia is developing a comprehensive study about the performance of GLG as a structural material. Due to the lack of research on mechanical behavior of adhesives on GLG, the present study was focused first on determine the best type of adhesive as well as its optimum amount. Once this stage of research was done, a detail mechanical and physical characterization was performed. Finally, the mechanical properties of GLG are compared to Andean structural woods.

KEYWORDS: Bamboo, Glued Laminated Guadua, Mechanical Properties, Physical Properties.

1 INTRODUCTION 1234 Bamboo is a group of woody plants that belong to the grass Poaceae family and Bambusoideae subfamily. The giant bamboo is the largest member of the grass family and according to Liese [1] this is the fastest growing plant in the world. The rate of growing varies between

1 Juan F. Correal, Director of Integrated Lab and Assistant Professor of Civil and Environmental Engineering Dept., Universidad de Los Andes, Bogotá, Colombia, Cra. 1 Este # 19A-40. Email: jcorreal@uniandes.edu.co. 2 Fernando Ramirez, Associate Professor of Civil and Environmental Engineering Dept., Universidad de Los Andes, Bogotá, Colombia, Cra. 1 Este # 19A-40. Email: framirez@uniandes.edu.co. 3 Soffy Gonzalez, Graduate Assistant of Civil and Environmental Engineering Dept., Universidad de Los Andes, Bogotá, Colombia, Cra. 1 Este # 19A-40. Email: sj.gonzalez41@uniandes.edu.co. 4 Jessica Camacho, Undergraduate Assistant of Civil and Environmental Engineering Dept., Universidad de Los Andes, Bogotá, Colombia, Cra. 1 Este # 19A-40. Email: jf.camacho49@uniandes.edu.co.

20 cm to 100 cm per day and it depends on local soil and climate conditions. The full height of 15 to 30 meters can be reached in about 4 months. Bamboo has been used widely in the daily life of people for: handcraft, raw material for paper baskets and even as a vegetable. Treated bamboo is a strong material that has been used in some countries like China and Japan to make houses. More recently, Bamboo is used for concrete formwork, scaffolding and housing materials and even as a substitute for steel reinforcing rods in concrete construction among others. Also flooring companies are attempting to popularize bamboo floors made of small bamboo pieces which are steamed, flattened, glued together, finished, and cut. Taking into account, that now days there is a global need to have sustainable materials that reduce topical forest pressure, Bamboo has emerged as a promising alternative raw material for eco-friendly and sustainable construction due to its fast growth rate, short rotation age, and high strength. In America there is about 600 species of bamboo (of 1.250 species around the world) grow from the south of the United States to the north of Chile and Argentina but only 20 of them are considered suitable for construction. A giant specie of bamboo called Guadua Angustifolia kunt has been used as a construction material in Colombia for more than 20 years. Even though, round

Guadua has been used in construction of structures with relative success, one of the problems is the variability of its geometry, mechanical properties and anatomical composition that makes this material difficult to characterize and prevents its use in large structures. Glued Laminated Guadua Bamboo (GLG) is considered as an excellent alternative; since some exploratory studies [2-4] indicated that it has mechanical properties as good as the best structural woods in Colombia. The Universidad de los Andes in Bogotá, Colombia is developing a comprehensive study about the performance of GLG as a structural material. Due to the lack of research on mechanical behavior of adhesives on GLG, the present study was focused first on determine the best type of adhesive as well as its optimum amount. After completion of this stage of research, a detail mechanical and physical characterization was performed. This paper present preliminary mechanical and physical characterization of GLG based on the adhesive type and optimum adhesive spread rate found in the first stage of this research. Also, a comparison of mechanical properties of GLG with Andean structural woods is established.

2 MATERIALS AND PRODUCTION METHOD

2.1 MATERIALS

The main parts of the Guadua, the rhizomes and the culms are shown in Figure 1 a). The height of the culm varies from 20 to 30 meters and it can be separated into five parts each of them from 4 to 5 meters of length which are: cepa, basa, sobrebasa, varillón and copo. The structure of the culm consisted of a cylindrical wall with a diaphragm nodal region is shown in Figure 1 b). The internode at the Cepa is about 10 cm and increases at the Varrillón to 40 cm. Guadua culms diameter varies from 10 to 18 cm at the Cepa and 5 to 10 cm at Varillón and its thickness is 0.5 cm at Varillón and 2 cm at Cepa. The age of the mature Guadua is difficult to estimate. So far there are not studies about the optimal age that give optimal strength. Nevertheless and based on the color of the culm and past experience, the age of the culms for construction is selected between 3 to 4 year old. For the currently study, three to four year old Guadua bamboo culms with and average base diameter of 12 cm to 16 cm were obtained from Caidedonia-Valle in Colombia located at 1400 meters in elevation. In order to have culms with some kind of regular cross section, only the three bottoms parts (cepa, basa, sobrebasa) were taken from the Guadua bamboo (Figure 1(a)). The length of each part (cepa, basa, sobrebasa) was approximately 5 meters. The average thickness of the culm walls varied from 0.7 cm to 2.9 cm (Figure 1(b)). In order to ship the Guadua culms to the warehouse of the factory, each culm was cut into 2 to 3 meters long pieces. The following adhesives were used in the fabrication process of GLG: Polymer 216 FE L (Fenol-Resorcinol-Formaldehyde Adhesive) Polymer 103 (Melamine Urea

Adhesive), MUF 1242 (Melamine Urea Formaldehyde Adhesive) and 50% Polymer 216 FE L with 50 % Polymer 103.

(a) Guadua plant

(b) Guadua culm parts

Figure 1: General parts of guadua

2.2 PRODUCTION METHOD

The manufacture of the laminated Guadua was performed in Colguadua Ltda factory. The culms of 2m to 3m long were cut again into 1m to 1.5m in order to have straight pieces. Each piece is split in the radial direction into proper number of slices and the node sections are removed. The quasi-flattened Guadua slices are passed through a grinding machine to remove the inner and outer layers. These slices are then immersed in a chemical solution to protect bamboo against insects attack, and then dried in an oven at 80 oC to reach an average moisture content of 5%. Once the slices are dried, their four faces are polished with a machine to

flatten their surfaces obtaining Guadua laminae. Each Guadua lamina is about 7mm to 10 mm thick, 20mm to 25 mm wide, and 1 m to 1.5 m long. All laminae are impregnated with adhesive resin along the narrow face (Figures 2(a) and 2(b)) and stacked to form Guadua sheeting. A hot press at 100 oC with a lateral pressure of 1.2 MPa is applied to the laminae. Once the adhesive is cured, the Guadua sheets are glued together by the wide faces in order to form boards in a hot press at a pressure of 2 MPa for 15 minutes at 100 oC.

(a) Glued Laminated Guadua Manufacture

(b) Local directions

Figure 2: Fabrication process of glued laminated guadua

3 EXPERIMENTAL PROGRAM

3.1 ADHESIVE CALIBRATION

The test procedures selected for the adhesive calibration program were static bending and glued line shear. Five samples of each type (adhesive and spread rate) were tested on a MTS Universal Testing Machine in the Material Lab at the Universidad de Los Andes in Bogotá, Colombia. Due to the lack of glued laminated bamboo standards tests, the specifications given by ASTM D1037 (2006) and D143 (2007) were used for glued line shear and the static bending tests, respectively. The adhesive calibration program consisted of two stages. The objective of stage I was to determine the best adhesive from the point of view of its strength. For this stage four types of adhesive (Urea-Formaldehyde (UF), Melamine-Formaldehyde (MF), Melamine-Urea-Formaldehyde (MUF), and mixture of 50% of UF and 50% MF) and two adhesive spread rate applied to the narrow (200 g/m2 and 250 g/m2) and wide (400 g/m2 and 450 g/m2) faces of the laminae (Figure 2b) were selected. These spread rates were based on the adhesive

manufacturer specifications and recommendations. Once the best type of adhesive is selected based on stage I results, stage II start to estimate the optimal spread rate. The spread rate used along the narrow faces was half of that used on the wide faces. The amount of adhesive on wide faces in g/m2 was 260, 280, 300, 400 and 450.

3.2 MECHANICAL AND PHYSICAL CHARACTERIZATION

The mechanical tests performed were: compression and tension parallel and perpendicular to grain, and shear parallel to grain, and bending. Since there are no standards developed for glued laminated bamboo, ASTM D 143-94 [5] were used to perform mechanical tests. All the tests were conducted on a MTS Universal Testing Machine at the Materials Lab at the Universidad de Los Andes in Bogotá, Colombia. Temperature, moisture content, and relative humidity were recorded for all specimens. Besides density, radial, tangent and longitudinal toughness and contraction were the physical properties determined in this study following the ASTM D 2395-97 [6]. Twenty samples were used on each mechanical and physical test. Test procedures of the mechanical and physical properties are summarized as follows: 3.1.1 Compression parallel to grain The specimens were 50 mm by 50 mm in section and 200 mm in length as shown in Figure 3 was used. A continuous compressive load with a 0.6mm/min load rate was applied. The load-displacement curve was recorded and the elasticity modulus (MOE), proportional limit stress, and ultimate stress were determined.

Figure 3: Compression parallel to grain specimen and test setup

3.1.2 Compression perpendicular to grain The specimens were 50 by 50 mm in section and 150 mm in length. A MTS load frame with a 50 mm width bearing metal plate was used to apply a continuous compressive load with a 0.3mm/min load rate. The load was applied until a deformation equal to 5% of the specimen thickness was reached, and the stress at that point was calculated. The proportional limit stress was determined.

3.1.3 Tension parallel to grain Figure 4 shows the test setup. The load was applied continuously throughout the test at a rate of 0.9 mm/min. The load-displacement curve was recorded and the elasticity modulus (MOE), proportional limit stress, and ultimate stress were determined.

Figure 4: Tension parallel to grain test setup 3.1.4 Tension perpendicular to grain Figure 5 shows the dimensions of the tensile test specimen. The load was applied continuously throughout the test at a 2.5 mm/min rate of the movable crosshead. Ultimate tensile stress was calculated.

Figure 5: Specimen of tensile perpendicular to grain test 3.1.5 Flexural strength The specimens were 25 by 25 mm in section and 410 mm in length. The test setup is shown in Figure 5. The load was applied at the center of a 350 mm span with a 2.5mm/min load rate. The failure load was recorded and the rupture module (MOR) was calculated.

Figure 6: Flexural Strength test setup 3.1.6 Shear strength parallel to grain Dimensions of the test specimen, as well as the test setup are shown in Figure 6. The load was applied continuously throughout the test at a 0.6 mm/min rate. Ultimate shear stress was calculated.

Figure 6: Specimen of shear strength parallel to grain test 3.1.6 Hardness The specimens were 50 by 50 mm in section and 150 mm in length. The load was applied continuously throughout the test at a rate of motion of the movable crosshead of 6 mm/min. The load at which the ball penetrated to one half its diameter was recorded. 3.1.7 Specific gravity and shrinkage in volume Test specimens were regular in shape and had rectangular cross sections in order to enable the determination of volume through linear measurement. Both specific gravity and shrinkage-in volume values were obtained from each specimen at about 12% moisture content of the samples in the oven-dried condition.

4 RESULTS AND DISCUSSION A total of two hundred samples were tested during the adhesive calibration program. A comparison of the bond shear strength for different types of adhesives using the spread rate specified by the manufacturer (stage I) is shown in Figure 7. Although the specimens with MUF adhesive exhibited slightly higher values of bond shear strength, there were no significant differences in this value among the four adhesives used. Moreover, the amount of the adhesive applied on the wide and narrow faces did not affect the value of bond shear strength. Similar behavior was observed for the MOE and MOR obtained from the bending tests with different type of adhesives and spread rates. Failure of the substrate (Guadua) was observed in all the specimens of glued line shear and bending tests. It seems that the spread rate specified by the adhesive manufacturer was on the conservative side producing failure the substrate. Since, all the adhesives behave well from the strength point of view and taking into account strength, durability and cost, the 50% UF +50% MF was the adhesive selected from the stage I. The optimum amount of adhesive spread rate at wide/narrow faces of 300/150 g/m2 was selected since lower adhesive spread rate of 280/140 and 260/130 g/m2 presented a decrease in bond shear strength of 27%.

Figure 7: Bond shear strength for different types of adhesives and spread rate.

An average of 17.7 oC, 10.5 %, and 56.8% of temperature, moisture content and relative humidity were recorded at the moment of the tests. Table 1 presented the 5th percentiles of the results of the mechanical properties of GLG and the corresponding values of the structural wood according to Colombian Seismic Regulations NSR-1998 [7]. The compression parallel to grain test showed a combination of crush with buckling failure for most of the specimens. The 5% percentile value of the ultimate stress for CPAG is about two times more than the best Colombian wood (type A). The failure mode of the compression perpendicular to grain (CPEG) test was crushing of the material. Relatively low value of the 5th

percentile was achieved in CPEG test compare to Colombian wood. Failure of the substrate (Guadua) was observed in all the specimens of the tension perpendicular to grain (TPG) test. The glue-line shear test specimens failed in the interlaminate adhesive as expected. Bending test resulted in two main failure types: tension with compression and horizontal shear as shown in Figure 8 a) and b), respectively.

(a) Tension and compression failure

(b) Horizontal shear failure

Table 1: Mechanical Strength Values of Glued Laminated Guadua and NSR-98 Wood Grades Based on Table 1, all the values of the mechanical properties of GLG are higher than those of the best structural wood grade (A). In most of the cases, the mechanical properties of GLG are almost twice the values for all NSR-1998 [7] wood grades. In contrast, average density of GLG is about the same than that of structural wood grade A, since average density of GLG is 730 kg/m3 whereas average density of wood grade A is 710 kg/m3. Regarding toughness and contraction in GLG, an average toughness of 5832 N, and radial, tangent and longitudinal contraction of 4.03%, 3.48% and 0.33% were determined. Since fibers in bamboo are running along the bamboo culm, less contraction is expected in the longitudinal direction.

5 CONCLUSIONS Based on the preliminary results of this research, the following conclusions are drawn for Glued Laminated Guadua Bamboo: Although the specimens with MUF adhesive exhibited slightly higher values of bond shear strength, there were no significant differences in this value among the four adhesives used on this research, with the spread rate specified by the adhesive manufacturer. Bond shear strength is a good indicator of the optimal amount of adhesive to be used in any glued laminated material. The optimal amount of adhesive is reached when bond shear strength is close to the strength of the substrate. Particularly for GLG, 300gr/m2 of adhesive applied on the wide faces and 150gr/m2 along the narrow faces is the optimum adhesive spread rate. Taking into account that the density of GLG is about the same that the best structural Colombian wood (grade A) and the mechanical properties of GLG are almost double than those of wood grade A, GLG can be a suitable material for construction and design of structural elements

ACKNOWLEDGEMENT The research presented in this paper is sponsored by the Ministry of Agriculture and Rural Development of Colombia (Contract No 030-2007M3307-920-07), Universidad de los Andes and Colguadua Ltda. Thanks are to the staff of the Center of Research in Materials and Civil Works (CIMOC) and the Structural Lab Models at the Universidad de Los Andes in Bogotá, Colombia for their help and support.

REFERENCES [1] Liese, W.: Research on bamboo. Wood Science and

Technology, 21, pp. 189-209. 1987. [2] Lopez L, Correal J.: Exploratory study of the glued

laminated bamboo Guadua angustifolia as a structural material. Maderas-Ciencia y Tecnología, 11 (3): 171-182. 2009

[3] Duran, L.: Estudio de Guadua Laminada y su Aplicación al Sistema Tensegrity, Thesis Work in Architecture, Universidad Nacional de Colombia sede Bogotá, 2003.

[4] Vanegas, G.: Guadua Laminada Investigación, Experimentación y Aplicación, Thesis Work in Architecture, Universidad Nacional de Colombia sede Bogotá, 2003.

[5] ASTM-D143-94, Standard Methods of Testing Small Clear Specimens of Timber, Sección 4, Vol 4.1wood, July 1997.

[6] ASTM-D2395-97, Standard Test Methods for Specific Gravity of Wood and Wood-Based Material, Sección 4, Vol 4.1wood, July 1997.

[7] Asociación Colombiana de Ingeniería Sísmica, Normas Colombianas de diseño sismorresistente (NSR-98), Asociación colombiana de Ingeniería Sísmica, Bogotá, Colombia, 1997.