Experimental Investigation on the Mechanical Properties of ...

DUET Journal 37 Volume 5, Issue 1, June 2019

Experimental Investigation on the Mechanical Properties of Bamboo in Bangladesh

Md. Mozammel Hoque1*, Md. Niamul Islam2, Kazi Abu Manjur2 and Mohammad Anis1

1Department of Civil Engineering, Dhaka University of Engineering & Technology, Gazipur, Bangladesh 2Development Design and Management, Dhaka, Bangladesh

ABSTRACT

This study investigates the mechanical properties of bamboo grown in Bangladesh experimentally. The reliability of props, scaffolding systems and low-cost houses, made of bamboos, largely depends on the mechanical properties: compressive strength, tensile strength, bending strength and modulus of elasticity of bamboo culms. A parametric study on the mechanical properties of bamboo with different parameters such as wall thickness (t), length of bamboo culms (L), outer diameter (D) and moisture content (green and dried bamboo) have been investigated. The compressive strength was found to vary 18.5 MPa to 54 MPa. No specific relationship could be observed between the compressive strength, wall thickness and length of bamboo. However, the average compressive strength was found to decrease with increasing the number of nodes of the specimens. Larger values of tensile strength and modulus of elasticity have been observed for green bamboo as compared to dried ones. The range for tensile strength as observed is 48 MPa and 112 MPa for dried and green bamboo respectively. In addition, the bending strength was found about 29 MPa to 34 MPa. It was also revealed that the compressive elastic modulus ranges from around 0.3 GPa to 8.8 GPa. On the contrary, the tension modulus was found the lowest at 0.16 GPa.

*Corresponding author’s email: [email protected]

1. INTRoduCTIoN

Bamboo is a fast growing natural fiberous plant largely used in engineering construction not only in Bangladesh but also in many other developing countries over the world. It is considered as a composite material because of varying constituents of cellulose and lignin matrix. Bamboo consists of roots, culms and leaves. Culms are the main portion used for numerous purposes. The culms could be in hollow or solid cylindrical shape with varying wall thickness / diameters that change along its length, age and region of origins. The outer part of the culms is stronger than the inner surface and the fibers are oriented along the length of the culms ([1]- [3]). It has a myriad of applications from the ancient period to modern age. In many ornamental works, bamboo is being satisfactorily used for instance, gardening, fencing, and making furniture, handicrafts, mats, baskets, trays, lamp shades, caps, pulp and so on. Bamboo is also used for manufacturing of papers, fiberboards, animal fodder and even as foods (edible shoots). Because of fast, abundant and growing characteristics, it can be used as a sustainable construction material. Interestingly, bamboo can be used as an alternative material of steel in concrete for making low cost buildings in many developing countries like India, Bangladesh and so on due to availability and reliable reinforcing materials ([5]-[6]). Bamboo is grown in many countries such as China, Malaysia, India,

Bangladesh, Indonesia, Thailand, Philippines, Sri-Lanka, Zimbabwe and Africa. It is also used in making furniture [2]. Moreover, bamboos are commonly and largely used as the props and scaffolding system at the construction site in Bangladesh, India and parts of Asia (Fig. 1). It can carry substantial number of compressive loads within reasonable cost than steel props. Additionally, many researchers have already showed that bamboos can be used in structural concrete and polymer composites (bamboo fibers) as the reinforcement and able to enhance the mechanical properties remarkably ([4], [7]-[12]). Study shows that 25% bamboo fibers addition in bamboo-epoxy fiber composites increases the tensile and flexural strength about three times of its original strength. In contrast, the improvement was found approximately 1.5 times in hardness of epoxy composites when content of bamboo fibers was 30% [3]. The experimental investigation, conducted on the mechanical and physical properties of accelerated aged bamboos, showed remarkable increment in mechanical properties and density both reference and accelerated aged bamboos with the height of the culms. However, volumetric shrinkage was inverted [4]. The mechanical and physical properties of bamboo vary with the region of origins, weather, environmental locations, and soil conditions [13]. Mechanical properties of different bamboo species are available in the literature. However, properties of bamboo originating in Bangladesh are still unavailable which is important to investigate for

ExpErimEntal invEstigation on thE mEchanical propErtiEs of BamBoo in BangladEsh


Fig. 1: Photographs of bamboo scaffolding used for high-rise construction

proper design of bamboo props and scaffoldings due to its widespread usages. The reliability of props or scaffolding made of bamboos depend on the mechanical properties. Therefore, present paper aims to explore the mechanical properties of bamboo available in Bangladesh. The compressive and tensile strength of both green and dried bamboo were investigated. Likewise, bending strength and modulus of elasticity were also studied. Additionally, the changes in mechanical properties of bamboo culms of different lengths and wall thickness were determined.

2. MECHANICAL PRoPERTIES

Mechanical property is the most important part for designing of any durable and reliable materials. Without knowing the proper mechanical properties, reliable props or scaffolding systems made of bamboos are difficult to obtain. The mechanical properties of bamboos are determined from physical, chemical and mechanical point of view. It includes stress, strain, compressive strength, tensile strength, bending strength and modulus of elasticity. In this study, the mechanical properties were calculated from destructive tests of bamboo specimens. In addition, some physical properties of bamboo culms i.e. the wall thickness and cross-sectional area were also measured during the test.

2.1 Compressive Strength

Compressive strength of bamboo is one of the critical engineering properties for designing effective and reliable props or scaffolding system. Compressive strength test was conducted using a Universal Testing Machine (UTM) on different lengths of bamboo culms. The height of each specimen was taken as the distance between two nodes of a culm. In addition, a group of specimens was also chosen designated as SC-1 and SC-2 whose height was around two times the external diameter of the bamboo culms.

2.2 Tensile Strength

Tensile strength of bamboo was determined from the tensile test of the dried and green bamboo strips. To prepare the strips, bamboo culms were taken without nodes and divided along its length as shown in Fig. 2. Subsequently, the two ends of the strips were gripped by the upper and lower head of the UTM and tensile force were applied along the length of the strips until failure. The failure loads and the corresponding deformations were recorded and tensile strength was calculated.

2.3 Bending Strength

Bending strength of bamboo was determined according to the experimental test conducted by Chung and Yu [14]. Each bending test specimen was supported over a clear span of 525 mm and three-point bending load was applied until failure the specimen.

2.4 Modulus of Elasticity

Elastic modulus of bamboos was determined from both the specimens tested under compressive and tensile forces. To determine the compressive elastic modulus, the stress-strain data were used from the same the specimens that were derived from compressive strength test. However, like compressive modulus, modulus of elasticity under tension was also calculated from the tested data obtained from tensile specimen test.

3. SPECIMEN PREPARATIoN ANd TESTING

This section presents specimen preparation, identification and testing procedure of bamboo specimens.

Fig. 2: Photographs of bamboo strips



3.1 Specimen Preparation

The specimens for strength tests were prepared from green bamboos of different lengths. In this regard, locally available bamboos in Bangladesh were collected. The length of bamboo culms varies with bamboo types and even in a bamboo. And wall thickness of culms in a bamboo are also variable which may result differential strength properties. Therefore, it is difficult to select the specimen height. However, in this study the specimen height was chosen approximately twice the diameter of the bamboo culm for compressive strength testing and designated as SC-1 and SC-2. In addition, compressive strength with variable specimen height was also studied. The specimen heights were considered based on the number of nodes present in the specimens. A total 13 specimens of different heights, divided into two groups (9 specimens for 1st group + 4 specimens for 2nd group), were collected for compressive strength test.

Specimens of first group is identified as SXN, where the first character “S” represents specimen, second character “X” carries number of nodes and the third character “N” denotes the specimen number. Table I represents detail of the specimens of first group. However, the specimens of second group were designated as WN-Y. The characters “WN” and “Y” stand for respectively without nodes, and specimen numbers.

3.2 Specimen Testing

All specimens were tested using the universal testing machine (Torsee’s universal testing machine) of capacity 200 kN, MFG NO.: 20650. Specimens were placed under the lower cross head of UTM and compressive load was applied axially on the tip of the specimens until failure as shown in Fig. 3 (a). The axial shortening was measured with an interval of 2kN load for every

specimen by a deflection meter placed on the top of the machine foundation as shown in Fig. 3(a). Stress and strain were determined from axially applied loads and the corresponding shortenings. For tensile testing, both ends of the strips were gripped by the upper and lower cross head of UTM as shown in Fig. 3 (b).

Bending test of the specimen was performed by placing it on the simple supports at two ends of the specimen. A deflection meter was attached as shown in the Fig. 4 to measure the deflection of the specimen from its initial position. Bending strength was calculated from the equation 1.

bMSZ

= (1)

Failure modes of all the specimens during each test were observed. The physical appearances e.g. wall thickness, outer and inner diameter of the culms, was measured before testing of each specimen.

Fig. 3: Specimen testing (a) compressive strength, (b) tensile strength

Table I: Specimen details and compressive strength

Sample IdSXN

Length (mm)

Number of Nodes

Outer Diameter

(mm)

Inner Diameter

(mm)

Wall Thickness

(mm)

Compressive Strength (MPa)

Average Strength (MPa)

Ec (GPa)

S4N1 7564

76.66 39.05 37.61 30.234.7

8.80S4N2 730 76.29 32.9 43.39 26.3 2.85S4N3 727 72.97 32.5 40.47 35 4.34S3N1 725

365 37.8 27.23 47.3

42.278.50

S3N2 685 61.2 42.55 18.65 42.4 7.60S3N3 680 68.4 34.3 34.1 37.1 3.00S2N1 362

262.26 39.1 26.16 54.18

49.792.08

S2N2 315 53.4 34.45 17.95 38.8 4.56S2N3 282 70.05 33.9 36.15 47.4 2.30



4. RESuLTS ANd dISCuSSIoNS

Compressive strength was calculated at the end of the test from the obtained test data. Stress and strain were determined for all the specimens. No distinct relationship between wall thickness and compressive strength were found. Compressive strength was obtained ranges from 18.39 MPa to 54.18 MPa. The maximum compressive strength, 54.18 MPa was achieved for the noded specimens Table I. On the contrary, node-less specimen showed the lowest compressive strength in Table II.

4.1 Effect of Parameters

Length: length effect of bamboo on compressive strength was varying. Specimen with four nodes: specimens S4N1 and S4N3 show a downward trend of compressive strength with increasing the length without considering the specimen S4N2. However, the amplitude of compressive strength was about 26.3 MPa to 35 MPa. Specimens S3N1, S3N2 and S3N3 exhibited the compressive strength on rising with increasing the length of bamboos. Specimens with two nodes: specimens with two nodes also showed the similar trends in compressive strength except specimen S2N2 38.8 MPa. Specimen S2N1 showed the maximum compressive strength 54.18 MPa.

Specimens with No Nodes: Table II illustrates the variation in compressive strength of the specimens having no nodes. These specimens also showed waving values of compressive strength with the length.

Effect of Length: For noded specimens, Compressive strength was obtained unaffected due to changes in wall thickness. On the other hand, the compressive strength was found increasing with increasing the wall thickness when specimens are nodelss. Table II represents the compressive strength of nodeless specimens. In addition, an increase in average compressive strength was seen with decreasing the number of nodes.

4.2 Wall Thickness

Before testing the specimen’s physical appearance was observed and measured. The overall wall thickness of bamboo specimens was obtained ranges from 17.95 mm to 45.94 mm.

4.3 Modulus of Elasticity

Modulus of elasticity of bamboo was determined from the stress-strain values of tested specimens both under tension and compression. In this study, modulus of elasticity for compressive load was obtained ranges from 0.30 GPa to 8.8 GPa. Modulus of elasticity was observed similar to the previous study accomplished by Chang and Yu, but different from Purwito [15]. Modulus of elasticity was found varying along the length of the bamboo due to the presence of nodes and varying wall thickness in Fig. 8 where N4, N3, N2 and N0 represent four nodes, three nodes, two nodes and no node respectively. Specimens with nodes showed largest modulus of elasticity whereas, lower values were found for the specimens without nodes, Table I and Table II.

However, modulus of elasticity under tensile loads, green bamboo revealed higher values than the specimens of dried bamboo. Table V represents the results of modulus of elasticity of tensile forces.

4.4 Tensile Strength

Tensile strength of both green and dried bamboo was investigated. Dried bamboo showed the lowest tensile strength 48 MPa (STD-2) whereas, the maximum tensile strength, 112MPa (STG-5) was achieved for the specimens

Fig. 4: Bending testing of bamboo specimen

Table II: Specimen details and compressive strengthSample

IdSXN

Length (mm)

Number of Nodes

outer diameter

(mm)

Inner diameter

(mm)

Wall Thickness

(mm)

Compressive Strength (MPa)

Average Strength (MPa)

Ec (GPa)

WN-1 155

0

68.85 22.91 45.94 22.89

20.90

0.30

WN-2 257 68.55 26.72 41.83 22.42 0.50

WN-3 305 69.77 41.60 28.17 19.88 1.00

WN-4 181 63.02 40.22 22.80 18.39 1.10



collected from green bamboo. Table III demonstrates the detail results of tensile strength.

4.5 Bending Strength

Bending strength of specimen BS1 and BS2 was found 29.16 Mpa and 33.80 respectively in Table IV. The minimum value of bending strength was achieved similar to those values as explored by Chung and Yu [14]. However, the maximum bending strength for Bangladeshi available bamboo was extreme lower than that studied by Chung and Yu [14].

4.6 Failure Modes

The failure of the specimens was observed in the same nature as it was found by Chung and Yu [14], i.e end bearing and splitting. Under compressive loads, failure in end bearings and buckling were observed for the specimens having nodes. During tensile test, local crushing and tearing were found at the two ends of the specimens in Fig. 5. The Fig. 6 represents the node-less specimen failed by end bearing. Specimens with nodes exhibited buckling in many situations when length was comparatively larger. The presence of nodes in bamboo retarded the specimens to be splitted during failure. On the other hand, splitting failure was appeared for the specimens without nodes Fig. 7.

Fig. 5: Failure of bamboo strip under tensile force

Fig. 6: Failure of specimen in end bearing (compressive force)

Fig. 7: Splitting failure of bamboo specimen

Table III: Tensile strength testingSample

IdLength (mm) B (mm) d

(mm)area (mm)

Tensile Strength (MPa)

STG-1 255 20.48 11.53 236.13 94

STD-2 215 21.70 13.73 297.94 48STG-3 254 19.63 11.76 230.85 102STD-4 212 18.86 12.86 242.54 57STG-5 257 20.3 10.35 210.11 112STD-6 211 20.33 13.43 273.03 48

Table IV: Result of bending strength test

Sample Id

Span Length (mm)

outer dia.

(mm)

Inner dia.

(mm)

Wall Thick (mm)

Crushing load (kN)

Bending Strength (MPa)

BS1 533 68.54 35.75 32.79 6.40 29.16BS2 533 70.90 41.77 29.12 7.80 33.80

Table V: Elastic modulus under tensile force

Sample Id L (mm) B (mm) d (mm)Tensile

Strength (MPa)

Et (GPa)

STG-1 255 20.48 11.53 94 0.61STG-2 215 21.70 13.73 48 0.16STG-3 254 19.63 11.76 102 0.70STG-4 212 18.86 12.86 57 0.17STG-5 257 20.30 10.35 112 0.81STG-6 211 20.33 13.43 48 0.18

Table VI: Compressive strength and Elastic modulus under compression

Sample Id L (mm) d (mm) t (mm) Fc (MPa) Ec (GPa)

SC-1 133 66.70 41.05 23.77 0.45SC-2 136 68.30 45.75 22.18 0.64



Fig. 8: Variation in Elastic Modulus under compressive load with length

4.7 Stress-Strain Properties

Figure 9 illustrate representative stress strain curve of the bamboo specimens under compressive loads. The maximum strain against compressive loads was found 0.003 (mm/mm), for specimen having two nodes. The minimum strain 0.005 (mm/mm) was noticed for specimen S4N2 with four nodes.

Figure 10 shows the tensile stress-strain curve obtained from dried bamboo strips. The overall strain was recorded from 0.005 to 0.25 (mm/mm).

Fig. 9: Stress-strain curve for compressive force (Sample S4N3)

Fig. 10: Stress-strain curve under tensile load (Sample STG-3)

5. CoNCLuSIoN

The aim of this investigation is to find the mechanical properties of bamboos that commonly used in Bangladesh. Destructive tests namely compressive strength, tensile strength, bending strength and modulus of elasticity of bamboos were executed on bamboo specimens. The outcomes were compared to find the variation in mechanical properties against length and wall thickness of bamboo culms. Finally, the following conclusions may be drawn from this experimental study.

i. The maximum and minimum compressive strength were 54.18 MPa and 18.39 MPa for specimens S2N1 and WN-4 respectively.

ii. No exact relationship was determined between wall thickness, specimen length and compressive strength. However, specimens without nodes exhibited an increase in compressive strength with rising the wall thickness.

iii. Compressive strength was noticed increasing with increasing the wall thickness for node-less specimens.

iv. The average compressive strength was decreasing with increasing the number of nodes of bamboos.

v. Tensile stress was found a range from 48 MPa to 112 MPa. Green bamboos exhibited improved tensile strength and Young’s modulus than dried bamboos.

vi. Mechanical properties of Bangladeshi available bamboos were found lower than that of the bamboo of other regions.

ACKNoWLEdGMENT

The authors would like to acknowledge their gratitude to Development Design and Management for providing all sorts of financial assistances.

REFERENCES

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