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Air-cured banana-fibre-reinforced cement composites
Cement & Concrete Composites 16 (1994) 3-8 O 1994 CS1RO
ELSEVIER
Air-Cured Banana-Fibre-Reinforced Cement Composites W. H. Zhu, B. C. Tobias Department of Mechanical Engineering, Victoria University of Technology, Footscray, Australia
R. S. P. Coutts & G. Langfors Fibre Composites Group, Division of Forest Products, CSIRO, Clayton, Australia (Received 11 May 1993; accepted 16 August 1993)
softwood pulp fibres (Pinus radiata (D. Don)) can replace asbestos fibres in commercial, autoclaved, fibre-reinforced cement products manufactured by the Hatschek process. ~ Air-cured P. radiata wood-fibre-reinforced cement composites have yielded materials with flexural strengths close to 30 MPa and fracture toughness values of approximately 2 kJm- 2, with a density of about 1.6 gcm - 3 (when the composite contains 8% fibre by mass).-' Banana fibre is a natural leaf fibre with an average fibre length of 2-01 mm, a diameter of 18-30 urn, a tensile strength of 700-800 MPa and a density of about 1.5 gcm--~, which is very close to that of P. radiata (2-6 mm in length, 13-45/am diameter, 500-900 MPa tensile strength, and 1.5 gcm -3 density). 3 In India about 1.5 million acres of land is cultivated with banana plantations, which yield about 3 x 105 tons of fibre. 4 In this study banana fibres were extracted from the pseudo-stem of a commonly used banana plant (Cavendish variety), which is predominantly found in tropical parts of Australia. The stalks are usually left to decompose on the ground after the fruit has been harvested. Banana is a large tropical plant widely cultivated for its nourishing fruits. In Australia they are extensively grown in Queensland and northem New South Wales (NSW). The banana plant is classified as genus Musa of the family Musaceal, comprising more than 30 distinct species and at least 100 subspecies. 5 The genus is divided into two broad groups namely Sumusa with edible fruits and Physicals with inedible fruits. Cavendish and Lady finger, the varieties grown in NSW and Queensland, are of the Sumusa group. The plant has a tree-like appearance, a trunk-like stalk
Abstract
In this paper techniques for the fabrication of banana-fibre-cement composites are reported. The physical and mechanical properties of air-cured fibre-reinforced cement composites, containing banana reinforcement prepared by several different methods are discussed. It was found that kraft pulped banana fibre, at a loading of between 8 and 167/o by mass, resulted in composites with flexural strengths in excess of 20 MPa. At a fibre loading of 14% by mass, the flexural strength is 24"92 MPa and the fracture toughness value is 1.74 kJm -2, properties adequate for the production of commercially viable building materials. Thus, in countries like India, China, and within South East Asia, where there is a lack of softwood fibre (the natural fibre most preferred as an alternative to asbestos), but in which banana fibre occurs in abundance, a viable alternative fibre resource is available for commercial consideration. Keywords: Fibre composites, natural fibres, banana fibre, fracture toughness, pulping of banana plant, flexural strength, building materials, renewable resource. INTRODUCTION Increasing attention should be paid to natural fibres with a view to conserving energy, by the use of such renewable resources, while at the same time noting their lack of health hazards so frequently associated with the use of fibres such as asbestos, glass and carbon. It has been shown that 3
4
W H. Zhu, B. ('. lobias, R. S. 1'. Coutts, G. l.anKqors
which, however, contains no woody material. They are propagated from the stem of the matured banana plants, and grow from 3 to 9 m~'. The stalk, which ranges in diameter from 200 to 370 ram, consists of layers of overlapping leafstalk surrounding a hollow core. At the end of each stalk is a dark-green oblong leaf, measuring about 3600 by 600 ram. The stalk contains long multi-celled fibres extending length-wise through the pulpy tissues of long leaves or leaf-stems. About 24% of the pseudo-stem is crude fibre. Preliminary studies carried out at CSIRO, Australia on the performance of banana fibre as a source of reinforcement for autoclaved cement mortar, air-cured cement and air-cured plaster indicated that such composites, containing 8% fibre by mass, had mechanical and physical properties adequate for building applications? This paper describes a systematic examination carried out on the mechanical and physical property relationships of air-cured cement reinforced with banana fibres prepared by four different methods.
dried material was treated in two ways: kraft pulping (see pulp 2) and processing through a Wiley Mill No. 1, using 6.35 ram, or 3"18 mm screens.
EXPERIMENTAL WORK
Matrix The matrix material was prepared from ordinary Portland cement.
Extracted fibre bundles The traditional process of obtaining 'so-called" fibre is one of mechanical scraping. This, in fact, results in fibrous strands which are bundles of fibres. Much published scientific data refer to natural fibre properties when in fact they are reporting the properties associated with fibre strands. The banana stalk was supplied from a banana plantation in Woolgoolga (NSW, Australia). The leaves were separated from the pseudo-stem (stalk). The fibrous material was extracted from the stalk by manual scraping to provide strands over 1 m in length. These strands were dried and then used for fibre preparation by the kraft pulping method (see pulp 1 ). Small fibre bundles were also prepared from stalks by cutting them into manageable lengths and widths (300-500 mm long and 50-100 mm width), then processing them through a Bauer Refiner using defibrator plates with gaps of 1.270, 0.254 and 0.127 mm, respectively. The pithy material was removed using a Sommerville Screen (2.00 mm aperature) thus separating out the fibre bundles. The yield after extraction was 22.04% on an oven-dry basis. The mechanical refiner produced the fibrous material from the plant in a far more efficient and economical manner than traditional scraping. The
Pulped fibre The kraft chemical pulping process was used to obtain individual fibres. The conditions for pulping extracted fibre were as follows: 9% active alkali (as Na_,O), 25% sulphidity, 7:1 liquor to sample ratio, 75 min to temperature and 2 h cook at 170°C. For pulp 1 the yield was 63"58% pulp on an oven-dried sample basis and the pulp had a kappa number of 33.6. Figure 1 indicates that banana fibre (for pulp 1) has an average length of 2.01 mm (measured on a KAJAANI FS-200). Fibres were modified in the laboratory using a Valley Beater. Kraft pulps were prepared from fibre strands produced manually by scraping from raw stalk (pulp 1), from Bauer refined stalk which was screened and dried before pulping (pulp 2) and from raw stalk processed directly in a pulping vessel (pulp 3).
Fabrication of composite products The composites were fabricated in the laboratory from a fibre-matrix-water slurry of approximately 20% solids. First the pulp was mixed for 2 min in the water, then the cement was added into the mixture with stirring, 5 min later, the mixture was poured into an evacuable casting box 125 x 125 mm and distributed over the screen. At
10"[
"
I 8-
About16%[_tnes~
-1 =
7
5-
~2 .ft.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Fibre L,cnglh (ram)
Fig. I. tion.
Banana fibre
4
5
,
(pulp 1) length population distribu-
Air-cured banana-fibre-reinforced cement composites
the same time the vacuum pump was switched on and water was drawn off until the pad appeared dry on the surface. It was then flattened carefully with a tamper. Then a vacuum of 90 kPa (gauge) was applied for 2 min. The pad was removed from the filter screen. The pad and screen were stored between two steel plates and the procedure repeated until a stack of two to four pads had been prepared. The stack of pads was pressed for 5 rain at a pressure of 3.2 MPa. Initially the load was applied slowly to prevent damage to the pads. The preparation was completed within 1 h of starting the mix. After pressing, the pads were carefully removed from the screens then stacked flat in a sealed plastic bag for 7 days. Then the pads were taken from the bag and air-cured for a further 21 days before testing.
Test methods Specimens were cut with a diamond saw to specified dimensions and stored under standard conditions of 50 + 5% relative humidity and 23 + 2°C for 5 days prior to testing the mechanical properties. Rectangular strips, of uniform but varying thickness (measured with calipers), approximately 125 × 40 mm were used for flexural strength, fracture energy and elastic modulus tests. A threepoint bend configuration was employed. The modulus of rupture (MOR) according to ASTMD790 was measured as
5
where m is the slope of the load-deflection curve during elastic deformation (Nm-i), and L, b, d are described as above. A span of 100 mm and a deflection rate of 0"5 mm min-' was used on an Instron Testing Machine (Model 11 ! 4). The results were collected by automatic data collecting equipment on the testing machine, and analysed by means of a program developed for routine flexural testing of cellulosic fibre-cement. The fracture energy was calculated from the area under the load-deflection curve. The fracture toughness is the fracture energy divided by the specimen cross-sectional area. For the purpose of this paper, fracture toughness is given as fracture toughness = fracture energy/bd The comparison of fracture toughness is strictly valid only for specimens of the same thickness. Water absorption values and density measurements were obtained by the methods laid down in ASTM-C220-75. The specimens were oven dried at 105°C for 24 h to obtain the dry weight, then soaked in water for 48 h with excess water being removed by a cloth before weighing to test water absorption values and density. In all cases, at least six samples were tested for MOR, MOE, fracture toughness, density and water absorption. Both means and standard deviations have been included for all properties measured.
MOR--- 3PL (Nm_2) 2bd z
RESULT AND DISCUSSION
where P is the maximum load recorded during test (N), L is the specimen span (m), b is the specimen width (m), and d is the specimen depth (m). The modulus of elasticity in flexure (MOE) according to ASTM-D790 was measured as mL 3 MOE = 4bd3 (Nm- 2)
A preliminary study of composites, containing 8% banana fibre by mass was carried out. Table 1 shows that the samples made from pulped extracted banana fibre (pulps 1 and 2) were worthy of further consideration. These samples had superior flexural strength (average 22 MPa) to the other banana-reinforced materials when compared with P. radiata-reinforced samples. There-
Table 1. Properties of composites fabricated in trials
Reinforcingfibre
Pulped raw banana stalk Chopped (1/4") banana Chopped (1/8") banana Pulped extracted banana
P. radiata
MOR (MPa)
Fracture toughness (kJm- 9
Density (gcm-~)
Water absorption ~%)
13-13 + 0.38 16.01 + 1 - 0 5 15"59 + 0"76 21.93:1:2.88 30"30 + 1-90
0.25 + 0.04 0.66:1:0.03 0"43 + 0"02 0.62 + 0.13 1"93 + 0"25
1"61 + 0.02 1.49:1:0.03 1"45 + 0"03 1.62:1:0.04 1"55 + 0"03
21.22:1:0.16 21.68:1:0-67 20.52 + 1.40 19.73:1:0.75 21.10 + 0.86
Reference
Pulp 3 Unpulped Unpulped Pulps 1 and 2 Ref. 2
6
W.H. Zhu, B. ('. 7bbias, R. S. P. Coutts, G. Lang]'ors
Table 2. Properties of banana-fibre-reinforced cement
Fibre (w %)
MOR (MPa)
MOE (GPa)
Fracture toughness (k.lm - -')
Water absorption (%)
l)ensio, (gcm - :)
9-13 5. 1"06 12"955-0"62 15"38 + 0"91 19' 10 5- 2-23 21"66 5- 2"47 26"58 5- 1-78
14-54 + 1-79 13-22__+ 1"53 11-42 -+ 0'75 11"49 _+0"97 10' 19 _+ 1'41 10"32 _+0"96
0"04 __+0"01 0"12_+0"01 0'24 -+ 0'04 0'40 _+0'07 0'56 _+0"07 0'82 _+0"07
15'57 5- 0-26 16"21 __+0-23 18"42 :t: 0-58 19'28 _+0"40 20'26 _+0"72 21'38 _+0"58
1'88 __+0-02 1"805-0"01 1'72 5. 0"03 1'68 _+0'02 1'65 _+0"03 1-60 + 0"03
14' 17 + 1"83 17"70 + 2'25 19'38 + 2"20 22'20 5- 2"34 23'42 + 1"97 23'86 5- 2"35 24'92 5- 2"11 24'52 5. 2"63
15"73 +__1"80 12-64 -+ 1-10 10"69 __+1"43 9'36 -+ 1'35 7'74 ____0'91 7' 17 ____0'97 5'92 __ 0'20 5'66 + 0'65
O' 11 + 0"01 0'21 ____0"01 0'35 ____0-08 0"68 + 0"07 0"95 __+0-14 1"35 __+0"19 1'74 +__0-22 1"79 + O' 18
14'59 __+2"00 16"99 + 0-97 17"71 __+0"84 19"20 +__0"31 21'94 __+0"64 23' 1(1 + 1"08 24"79 + 1"80 25'72 + 0"73
1-66 ± 0'05 1"64 + O"10 1"63 + 0"06 1-59 ± 0"02 1"54 5- 0"05 1-50 __+0'03 1'45 -+ 0"02 1"41 + 0"(}3
I. l'ulp I
0-5 2 4 6 8 10
2. I'ulp 2 2 4 6 8 10 12 14 16
fore, a series of air-cured cement composites containing between 0.5 and 16% by mass of kraft pulped banana fibre were fabricated. The mechanical and physical properties of these composites are listed in Table 2.
30 "
-
-
-
T
l;~
f6
25-
a.
20-
?. =
Flexural strength Composites prepared from banana fibre from pulped raw stalk (pulp 3) were inferior to those made from fibre bundles (unpulped chopped fibre, see Table 1 ) or pulped fibre strands (pulps 1 and 2). This may be due to the presence of pithy material, parenchyma cells etc. in the raw stalk. The flexural strength values of the pulped fibre (pulps 1 and 2) composites are reported in Table 2. Both pulps produce similar values of flexural strength at any given fibre content (within the range 2-10% fibre by mass) and so the average value is presented in the graph of flexural strength versus fibre content shown in Fig. 2. In Fig. 2 the error bars encompass all experimental points resulting from composites made from both pulps 1 and 2. It can be seen that as the fibre content is increased the flexural strength increases up to a maximum average value of about 25 MPa, at 12% fibre by mass, after which the strength starts to fall away due to ineffective mixing of the fibre. This behaviour is similar to that of air-cured P. radiata softwood pulp fibre reinforced cement, 2 the fibre most used in commercially manufactured products) At 8% by mass of banana fibre the value of MOR was only about 22 MPa, about 72% of softwood reinforced material (30.3 MPa). At 12% by
15-
~
10-
5
o
~
a
g
h
lb
Fibre Content (wt
Fig. 2.
%)
h
Effect of b a n a n a fibre content on flexural strength
(MOR).
mass the MOR value of the banana fibre composite had increased to 25 MPa, this is > 90% of softwood product at 12% content (27"6 MPa). This change in maximum flexural strength with change in fibre content could be associated with the fabrication of the composite and/or the aspect ratio of the respective fibres. At low fibre content the longer P. radiata fibres are able to bridge cracks more effectively than the shorter banana fibres, whereas, at high fibre loadings the shorter fibres are more readily dispersed in a homogeneous manner (less 'bailing' of the fibres) throughout the composite material and so can arrest cracks more effectively.
Air-cured banana-fibre-reinforced cement composites
Fracture toughness The fracture toughness of cellulosic fibre cement composites has been considered to be primarily dependent upon two micromechanics: fibre fracture and fibre pullout. The aspect ratio is often considered to be one of the major factors (see Table 3), along with fibre volume fraction, which affect the flexural strength and fracture toughness of a composite. 7,8 As the aspect ratio increases, a higher value of fracture toughness, due to fibre pullout, would be expected, providing the fibres do not clump within the matrix during mixing. Earlier research studies of NZ flax (Phormium tenax), 9 abaca '° and softwood 2 fibre-reinforced cementicious composite materials showed little variation in MOR of the composites, but a noticeable variation in fracture toughness values when composites of the same fibre content were under examination. NZ flax was compared with P. radiata as a reinforcement (at the same fibre loading) for autoclaved cement mortar, although, the aspect ratio is approximately twice that of softwood fibres (see Table 3), the observed fracture toughness was only half that of the composites containing softwood fibres? Abaca fibre reinforced air-cured cement also demonstrated that fibres, with over four times the aspect ratio of P. radiata generated fracture toughness values similar to those noted for P. radiata-fibre-reinforced air-cured cement. '(~ In the present study, even though banana fibre has a similar aspect ratio to P. radiata, the flexural strength and fracture toughness of the bananafibre-reinforced cement are decidedly lower in value at a fibre loading of 8% by mass. This would suggest that the effect of lid ratio proposed in the general theory of fibre reinforcement of composites 7 may not be as important in natural-fibrereinforced composite (see Table 3). This behaviour could be due to a number of parameters including the tensile strength of the fibres or their ability to bond to the matrix material and/ or to each other. The bonding can in turn be affected by fibre diameter, conformability of the
7
fibre, surface condition of the fibre and the number of fibres present in a given volume of material. Although Table 3 provides approximate values of lid one cannot be certain of real values present in the pulps used to prepare the composites. In composite theory the application of aspect ratio to explain fibre pullout has relevance; however, when the fibre is hollow as in natural fibres the ratio of length to cross-sectional area is more significant in fibre pullout. This approach is the subject of a quantitative study taking place in these laboratories at the present time. The fracture toughness increased as the banana fibre content was increased. The highest fibre loadings studied (16% by mass) gave values of 1.79 kJm 2 approximately 45-50 times greater than that of the matrix material. Figure 3 shows the effect of fibre content on fracture toughness.
Modulus of elasticity The modulus of elasticity in flexure has been studied. As fibre content increased from 2 to 16% by mass the modulus of elasticity fell from 14 GPa down to about 6 GPa. This behaviour is depicted in Fig. 4 where the change in flexural modulus is plotted against change in fibre content by mass. The decrease in stiffness of the composites as the fibre content increases is expected in that the fibre has a lower modulus compared to the unreinforced matrix material.
Water absorption and density The densities of the cellulose fibre reinforced cement composites are shown in Table 2 and are seen to vary from values of approximately 1.7 gcm-3 at a fibre content of 2% by mass to about
2.5-
2-
Ls-
Table 3.
P r o p e r t i e s o f cellulosic fibres
Fibre
Length Fibre Aspect (mm) diameter ratio
Ref.
O,m) S o f t w o o d (P. radiata) Banana N e w Z e a l a n d flax Abaca
~ 3"0 ~ 2.0 ~2.7 ,~7-0
~ 30 -~ 23 ~- 14 ~ 16
~ 100 10 '~ 90 T h i s study ~ 190 10 ~ 4 4 0 10
0.5-
~
~
k
nb
f2
f4
f6
Fibre Conz©n!(wl %)
Fig. 3. ness.
Effect o f b a n a n a fibre c o n t e n t o n f r a c t u r e
tough-
8
W.H. Zhu, B. C 7obias, R. S. P. Courts, G. Langfors
CONCLUSIONS
i
104
-!t
~
8
Fibre Content (..,vt%)
Fig. 4. Effect of banana fibre contents on modulus of elasticity (MOE).
-1.8
-1.7
Pulped banana fibre (Cavendish) is a satisfactory fibre for incorporation into a cement matrix. At a loading in the range of 8-16% by mass, flexural strengths in excess of 20 MPa (as high as those found in some synthetic fibres reinforced cements, and up to 90% that of pulped softwood fibre-cement materials). At a fibre loading of 14% by mass, the flexural strength is about 25 MPa and the fracture toughness value is 1-74 kJm-2. These mechanical properties coupled with a density of 1.45 gcm- -~and a water absorption of < 25% by mass make such materials suitable for use as building materials.
REFERENCES 1. Coutts, R. S. P., From Forest to Factory to Fabrication. Keynote address. In Proceedings of the Fourth RILEM
-1.6
2. -1.5 I~
3. -I.4
14
tO
2
4
6
8
10
12
14
16
1.3
Fibre content (wl %)
Fig. 5. Water absorption and density versus banana fibre content.
4. 5. 6. 7.
1.4 gcm- 3 at a fibre content of 16% by mass. Over the same fibre range the water absorption properties increase from 16 to 26% by mass. The relationship between water absorption and density is illustrated in Fig. 5. This relationship is dependent upon void volume within the material, which has a dominant effect on both density and water absorption.
8. 9. 10.
International Symposium, Fibre Reinforced Cement and Concrete, ed. R. N. Swamy & F. N. Spon. London, 1992, pp. 31-47. Coutts, R. S. P. & Warden, P. G., Air-cured, wood pulp, fibre cement composites. J. Mater. Sci. Left., 4 (1985) 117-19. Coutts, R. S. P., Banana fibres as reinforcement for building products. J. Mater. Sci. Lea., 9 (1990) 1235. Kulkarni, A. G., Satyanarayana, K. G., Rohatgi, P. K. & Kalyani Vijayan, Mechanical properties of banana fibres (Musa sepientum). J. Mater. Sci., 18 (1983) 2290-6. Van Loesecke, H. W., Banana: Chemistry, Physiology and Technology, Economic Crops (Vol. 1 ). lnterscience Publishers Inc., 1950. Mickles, N., Banana. In Collins Encyclopedia (Vol. 3). Macmillan Educational Company, 1990. Hannant, D. J., Fibre Cements and Fibre Concretes. John Wiley & Sons, Chichester, UK, 1978, Chapter 3. Coutts, R. S. P. & Kightly, P., Bonding in wood fibre-cement composites. J. Mater. Sci.. 19 (1984) 3355-9. Coutts, R. S. P., Flax fibre as a reinforcement in cement mortars. The Int. J. Cement Comp. Lightweight Concrete, 5 (1983) 257-62. Coutts, R. S. P. & Warden, P. G., Air-cured Abaca reinforced cement composite. The Int. J. Cement Comp. Lightweight Concrete, 9 (1987) 69-73.
ELSEVIER
Air-Cured Banana-Fibre-Reinforced Cement Composites W. H. Zhu, B. C. Tobias Department of Mechanical Engineering, Victoria University of Technology, Footscray, Australia
R. S. P. Coutts & G. Langfors Fibre Composites Group, Division of Forest Products, CSIRO, Clayton, Australia (Received 11 May 1993; accepted 16 August 1993)
softwood pulp fibres (Pinus radiata (D. Don)) can replace asbestos fibres in commercial, autoclaved, fibre-reinforced cement products manufactured by the Hatschek process. ~ Air-cured P. radiata wood-fibre-reinforced cement composites have yielded materials with flexural strengths close to 30 MPa and fracture toughness values of approximately 2 kJm- 2, with a density of about 1.6 gcm - 3 (when the composite contains 8% fibre by mass).-' Banana fibre is a natural leaf fibre with an average fibre length of 2-01 mm, a diameter of 18-30 urn, a tensile strength of 700-800 MPa and a density of about 1.5 gcm--~, which is very close to that of P. radiata (2-6 mm in length, 13-45/am diameter, 500-900 MPa tensile strength, and 1.5 gcm -3 density). 3 In India about 1.5 million acres of land is cultivated with banana plantations, which yield about 3 x 105 tons of fibre. 4 In this study banana fibres were extracted from the pseudo-stem of a commonly used banana plant (Cavendish variety), which is predominantly found in tropical parts of Australia. The stalks are usually left to decompose on the ground after the fruit has been harvested. Banana is a large tropical plant widely cultivated for its nourishing fruits. In Australia they are extensively grown in Queensland and northem New South Wales (NSW). The banana plant is classified as genus Musa of the family Musaceal, comprising more than 30 distinct species and at least 100 subspecies. 5 The genus is divided into two broad groups namely Sumusa with edible fruits and Physicals with inedible fruits. Cavendish and Lady finger, the varieties grown in NSW and Queensland, are of the Sumusa group. The plant has a tree-like appearance, a trunk-like stalk
Abstract
In this paper techniques for the fabrication of banana-fibre-cement composites are reported. The physical and mechanical properties of air-cured fibre-reinforced cement composites, containing banana reinforcement prepared by several different methods are discussed. It was found that kraft pulped banana fibre, at a loading of between 8 and 167/o by mass, resulted in composites with flexural strengths in excess of 20 MPa. At a fibre loading of 14% by mass, the flexural strength is 24"92 MPa and the fracture toughness value is 1.74 kJm -2, properties adequate for the production of commercially viable building materials. Thus, in countries like India, China, and within South East Asia, where there is a lack of softwood fibre (the natural fibre most preferred as an alternative to asbestos), but in which banana fibre occurs in abundance, a viable alternative fibre resource is available for commercial consideration. Keywords: Fibre composites, natural fibres, banana fibre, fracture toughness, pulping of banana plant, flexural strength, building materials, renewable resource. INTRODUCTION Increasing attention should be paid to natural fibres with a view to conserving energy, by the use of such renewable resources, while at the same time noting their lack of health hazards so frequently associated with the use of fibres such as asbestos, glass and carbon. It has been shown that 3
4
W H. Zhu, B. ('. lobias, R. S. 1'. Coutts, G. l.anKqors
which, however, contains no woody material. They are propagated from the stem of the matured banana plants, and grow from 3 to 9 m~'. The stalk, which ranges in diameter from 200 to 370 ram, consists of layers of overlapping leafstalk surrounding a hollow core. At the end of each stalk is a dark-green oblong leaf, measuring about 3600 by 600 ram. The stalk contains long multi-celled fibres extending length-wise through the pulpy tissues of long leaves or leaf-stems. About 24% of the pseudo-stem is crude fibre. Preliminary studies carried out at CSIRO, Australia on the performance of banana fibre as a source of reinforcement for autoclaved cement mortar, air-cured cement and air-cured plaster indicated that such composites, containing 8% fibre by mass, had mechanical and physical properties adequate for building applications? This paper describes a systematic examination carried out on the mechanical and physical property relationships of air-cured cement reinforced with banana fibres prepared by four different methods.
dried material was treated in two ways: kraft pulping (see pulp 2) and processing through a Wiley Mill No. 1, using 6.35 ram, or 3"18 mm screens.
EXPERIMENTAL WORK
Matrix The matrix material was prepared from ordinary Portland cement.
Extracted fibre bundles The traditional process of obtaining 'so-called" fibre is one of mechanical scraping. This, in fact, results in fibrous strands which are bundles of fibres. Much published scientific data refer to natural fibre properties when in fact they are reporting the properties associated with fibre strands. The banana stalk was supplied from a banana plantation in Woolgoolga (NSW, Australia). The leaves were separated from the pseudo-stem (stalk). The fibrous material was extracted from the stalk by manual scraping to provide strands over 1 m in length. These strands were dried and then used for fibre preparation by the kraft pulping method (see pulp 1 ). Small fibre bundles were also prepared from stalks by cutting them into manageable lengths and widths (300-500 mm long and 50-100 mm width), then processing them through a Bauer Refiner using defibrator plates with gaps of 1.270, 0.254 and 0.127 mm, respectively. The pithy material was removed using a Sommerville Screen (2.00 mm aperature) thus separating out the fibre bundles. The yield after extraction was 22.04% on an oven-dry basis. The mechanical refiner produced the fibrous material from the plant in a far more efficient and economical manner than traditional scraping. The
Pulped fibre The kraft chemical pulping process was used to obtain individual fibres. The conditions for pulping extracted fibre were as follows: 9% active alkali (as Na_,O), 25% sulphidity, 7:1 liquor to sample ratio, 75 min to temperature and 2 h cook at 170°C. For pulp 1 the yield was 63"58% pulp on an oven-dried sample basis and the pulp had a kappa number of 33.6. Figure 1 indicates that banana fibre (for pulp 1) has an average length of 2.01 mm (measured on a KAJAANI FS-200). Fibres were modified in the laboratory using a Valley Beater. Kraft pulps were prepared from fibre strands produced manually by scraping from raw stalk (pulp 1), from Bauer refined stalk which was screened and dried before pulping (pulp 2) and from raw stalk processed directly in a pulping vessel (pulp 3).
Fabrication of composite products The composites were fabricated in the laboratory from a fibre-matrix-water slurry of approximately 20% solids. First the pulp was mixed for 2 min in the water, then the cement was added into the mixture with stirring, 5 min later, the mixture was poured into an evacuable casting box 125 x 125 mm and distributed over the screen. At
10"[
"
I 8-
About16%[_tnes~
-1 =
7
5-
~2 .ft.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Fibre L,cnglh (ram)
Fig. I. tion.
Banana fibre
4
5
,
(pulp 1) length population distribu-
Air-cured banana-fibre-reinforced cement composites
the same time the vacuum pump was switched on and water was drawn off until the pad appeared dry on the surface. It was then flattened carefully with a tamper. Then a vacuum of 90 kPa (gauge) was applied for 2 min. The pad was removed from the filter screen. The pad and screen were stored between two steel plates and the procedure repeated until a stack of two to four pads had been prepared. The stack of pads was pressed for 5 rain at a pressure of 3.2 MPa. Initially the load was applied slowly to prevent damage to the pads. The preparation was completed within 1 h of starting the mix. After pressing, the pads were carefully removed from the screens then stacked flat in a sealed plastic bag for 7 days. Then the pads were taken from the bag and air-cured for a further 21 days before testing.
Test methods Specimens were cut with a diamond saw to specified dimensions and stored under standard conditions of 50 + 5% relative humidity and 23 + 2°C for 5 days prior to testing the mechanical properties. Rectangular strips, of uniform but varying thickness (measured with calipers), approximately 125 × 40 mm were used for flexural strength, fracture energy and elastic modulus tests. A threepoint bend configuration was employed. The modulus of rupture (MOR) according to ASTMD790 was measured as
5
where m is the slope of the load-deflection curve during elastic deformation (Nm-i), and L, b, d are described as above. A span of 100 mm and a deflection rate of 0"5 mm min-' was used on an Instron Testing Machine (Model 11 ! 4). The results were collected by automatic data collecting equipment on the testing machine, and analysed by means of a program developed for routine flexural testing of cellulosic fibre-cement. The fracture energy was calculated from the area under the load-deflection curve. The fracture toughness is the fracture energy divided by the specimen cross-sectional area. For the purpose of this paper, fracture toughness is given as fracture toughness = fracture energy/bd The comparison of fracture toughness is strictly valid only for specimens of the same thickness. Water absorption values and density measurements were obtained by the methods laid down in ASTM-C220-75. The specimens were oven dried at 105°C for 24 h to obtain the dry weight, then soaked in water for 48 h with excess water being removed by a cloth before weighing to test water absorption values and density. In all cases, at least six samples were tested for MOR, MOE, fracture toughness, density and water absorption. Both means and standard deviations have been included for all properties measured.
MOR--- 3PL (Nm_2) 2bd z
RESULT AND DISCUSSION
where P is the maximum load recorded during test (N), L is the specimen span (m), b is the specimen width (m), and d is the specimen depth (m). The modulus of elasticity in flexure (MOE) according to ASTM-D790 was measured as mL 3 MOE = 4bd3 (Nm- 2)
A preliminary study of composites, containing 8% banana fibre by mass was carried out. Table 1 shows that the samples made from pulped extracted banana fibre (pulps 1 and 2) were worthy of further consideration. These samples had superior flexural strength (average 22 MPa) to the other banana-reinforced materials when compared with P. radiata-reinforced samples. There-
Table 1. Properties of composites fabricated in trials
Reinforcingfibre
Pulped raw banana stalk Chopped (1/4") banana Chopped (1/8") banana Pulped extracted banana
P. radiata
MOR (MPa)
Fracture toughness (kJm- 9
Density (gcm-~)
Water absorption ~%)
13-13 + 0.38 16.01 + 1 - 0 5 15"59 + 0"76 21.93:1:2.88 30"30 + 1-90
0.25 + 0.04 0.66:1:0.03 0"43 + 0"02 0.62 + 0.13 1"93 + 0"25
1"61 + 0.02 1.49:1:0.03 1"45 + 0"03 1.62:1:0.04 1"55 + 0"03
21.22:1:0.16 21.68:1:0-67 20.52 + 1.40 19.73:1:0.75 21.10 + 0.86
Reference
Pulp 3 Unpulped Unpulped Pulps 1 and 2 Ref. 2
6
W.H. Zhu, B. ('. 7bbias, R. S. P. Coutts, G. Lang]'ors
Table 2. Properties of banana-fibre-reinforced cement
Fibre (w %)
MOR (MPa)
MOE (GPa)
Fracture toughness (k.lm - -')
Water absorption (%)
l)ensio, (gcm - :)
9-13 5. 1"06 12"955-0"62 15"38 + 0"91 19' 10 5- 2-23 21"66 5- 2"47 26"58 5- 1-78
14-54 + 1-79 13-22__+ 1"53 11-42 -+ 0'75 11"49 _+0"97 10' 19 _+ 1'41 10"32 _+0"96
0"04 __+0"01 0"12_+0"01 0'24 -+ 0'04 0'40 _+0'07 0'56 _+0"07 0'82 _+0"07
15'57 5- 0-26 16"21 __+0-23 18"42 :t: 0-58 19'28 _+0"40 20'26 _+0"72 21'38 _+0"58
1'88 __+0-02 1"805-0"01 1'72 5. 0"03 1'68 _+0'02 1'65 _+0"03 1-60 + 0"03
14' 17 + 1"83 17"70 + 2'25 19'38 + 2"20 22'20 5- 2"34 23'42 + 1"97 23'86 5- 2"35 24'92 5- 2"11 24'52 5. 2"63
15"73 +__1"80 12-64 -+ 1-10 10"69 __+1"43 9'36 -+ 1'35 7'74 ____0'91 7' 17 ____0'97 5'92 __ 0'20 5'66 + 0'65
O' 11 + 0"01 0'21 ____0"01 0'35 ____0-08 0"68 + 0"07 0"95 __+0-14 1"35 __+0"19 1'74 +__0-22 1"79 + O' 18
14'59 __+2"00 16"99 + 0-97 17"71 __+0"84 19"20 +__0"31 21'94 __+0"64 23' 1(1 + 1"08 24"79 + 1"80 25'72 + 0"73
1-66 ± 0'05 1"64 + O"10 1"63 + 0"06 1-59 ± 0"02 1"54 5- 0"05 1-50 __+0'03 1'45 -+ 0"02 1"41 + 0"(}3
I. l'ulp I
0-5 2 4 6 8 10
2. I'ulp 2 2 4 6 8 10 12 14 16
fore, a series of air-cured cement composites containing between 0.5 and 16% by mass of kraft pulped banana fibre were fabricated. The mechanical and physical properties of these composites are listed in Table 2.
30 "
-
-
-
T
l;~
f6
25-
a.
20-
?. =
Flexural strength Composites prepared from banana fibre from pulped raw stalk (pulp 3) were inferior to those made from fibre bundles (unpulped chopped fibre, see Table 1 ) or pulped fibre strands (pulps 1 and 2). This may be due to the presence of pithy material, parenchyma cells etc. in the raw stalk. The flexural strength values of the pulped fibre (pulps 1 and 2) composites are reported in Table 2. Both pulps produce similar values of flexural strength at any given fibre content (within the range 2-10% fibre by mass) and so the average value is presented in the graph of flexural strength versus fibre content shown in Fig. 2. In Fig. 2 the error bars encompass all experimental points resulting from composites made from both pulps 1 and 2. It can be seen that as the fibre content is increased the flexural strength increases up to a maximum average value of about 25 MPa, at 12% fibre by mass, after which the strength starts to fall away due to ineffective mixing of the fibre. This behaviour is similar to that of air-cured P. radiata softwood pulp fibre reinforced cement, 2 the fibre most used in commercially manufactured products) At 8% by mass of banana fibre the value of MOR was only about 22 MPa, about 72% of softwood reinforced material (30.3 MPa). At 12% by
15-
~
10-
5
o
~
a
g
h
lb
Fibre Content (wt
Fig. 2.
%)
h
Effect of b a n a n a fibre content on flexural strength
(MOR).
mass the MOR value of the banana fibre composite had increased to 25 MPa, this is > 90% of softwood product at 12% content (27"6 MPa). This change in maximum flexural strength with change in fibre content could be associated with the fabrication of the composite and/or the aspect ratio of the respective fibres. At low fibre content the longer P. radiata fibres are able to bridge cracks more effectively than the shorter banana fibres, whereas, at high fibre loadings the shorter fibres are more readily dispersed in a homogeneous manner (less 'bailing' of the fibres) throughout the composite material and so can arrest cracks more effectively.
Air-cured banana-fibre-reinforced cement composites
Fracture toughness The fracture toughness of cellulosic fibre cement composites has been considered to be primarily dependent upon two micromechanics: fibre fracture and fibre pullout. The aspect ratio is often considered to be one of the major factors (see Table 3), along with fibre volume fraction, which affect the flexural strength and fracture toughness of a composite. 7,8 As the aspect ratio increases, a higher value of fracture toughness, due to fibre pullout, would be expected, providing the fibres do not clump within the matrix during mixing. Earlier research studies of NZ flax (Phormium tenax), 9 abaca '° and softwood 2 fibre-reinforced cementicious composite materials showed little variation in MOR of the composites, but a noticeable variation in fracture toughness values when composites of the same fibre content were under examination. NZ flax was compared with P. radiata as a reinforcement (at the same fibre loading) for autoclaved cement mortar, although, the aspect ratio is approximately twice that of softwood fibres (see Table 3), the observed fracture toughness was only half that of the composites containing softwood fibres? Abaca fibre reinforced air-cured cement also demonstrated that fibres, with over four times the aspect ratio of P. radiata generated fracture toughness values similar to those noted for P. radiata-fibre-reinforced air-cured cement. '(~ In the present study, even though banana fibre has a similar aspect ratio to P. radiata, the flexural strength and fracture toughness of the bananafibre-reinforced cement are decidedly lower in value at a fibre loading of 8% by mass. This would suggest that the effect of lid ratio proposed in the general theory of fibre reinforcement of composites 7 may not be as important in natural-fibrereinforced composite (see Table 3). This behaviour could be due to a number of parameters including the tensile strength of the fibres or their ability to bond to the matrix material and/ or to each other. The bonding can in turn be affected by fibre diameter, conformability of the
7
fibre, surface condition of the fibre and the number of fibres present in a given volume of material. Although Table 3 provides approximate values of lid one cannot be certain of real values present in the pulps used to prepare the composites. In composite theory the application of aspect ratio to explain fibre pullout has relevance; however, when the fibre is hollow as in natural fibres the ratio of length to cross-sectional area is more significant in fibre pullout. This approach is the subject of a quantitative study taking place in these laboratories at the present time. The fracture toughness increased as the banana fibre content was increased. The highest fibre loadings studied (16% by mass) gave values of 1.79 kJm 2 approximately 45-50 times greater than that of the matrix material. Figure 3 shows the effect of fibre content on fracture toughness.
Modulus of elasticity The modulus of elasticity in flexure has been studied. As fibre content increased from 2 to 16% by mass the modulus of elasticity fell from 14 GPa down to about 6 GPa. This behaviour is depicted in Fig. 4 where the change in flexural modulus is plotted against change in fibre content by mass. The decrease in stiffness of the composites as the fibre content increases is expected in that the fibre has a lower modulus compared to the unreinforced matrix material.
Water absorption and density The densities of the cellulose fibre reinforced cement composites are shown in Table 2 and are seen to vary from values of approximately 1.7 gcm-3 at a fibre content of 2% by mass to about
2.5-
2-
Ls-
Table 3.
P r o p e r t i e s o f cellulosic fibres
Fibre
Length Fibre Aspect (mm) diameter ratio
Ref.
O,m) S o f t w o o d (P. radiata) Banana N e w Z e a l a n d flax Abaca
~ 3"0 ~ 2.0 ~2.7 ,~7-0
~ 30 -~ 23 ~- 14 ~ 16
~ 100 10 '~ 90 T h i s study ~ 190 10 ~ 4 4 0 10
0.5-
~
~
k
nb
f2
f4
f6
Fibre Conz©n!(wl %)
Fig. 3. ness.
Effect o f b a n a n a fibre c o n t e n t o n f r a c t u r e
tough-
8
W.H. Zhu, B. C 7obias, R. S. P. Courts, G. Langfors
CONCLUSIONS
i
104
-!t
~
8
Fibre Content (..,vt%)
Fig. 4. Effect of banana fibre contents on modulus of elasticity (MOE).
-1.8
-1.7
Pulped banana fibre (Cavendish) is a satisfactory fibre for incorporation into a cement matrix. At a loading in the range of 8-16% by mass, flexural strengths in excess of 20 MPa (as high as those found in some synthetic fibres reinforced cements, and up to 90% that of pulped softwood fibre-cement materials). At a fibre loading of 14% by mass, the flexural strength is about 25 MPa and the fracture toughness value is 1-74 kJm-2. These mechanical properties coupled with a density of 1.45 gcm- -~and a water absorption of < 25% by mass make such materials suitable for use as building materials.
REFERENCES 1. Coutts, R. S. P., From Forest to Factory to Fabrication. Keynote address. In Proceedings of the Fourth RILEM
-1.6
2. -1.5 I~
3. -I.4
14
tO
2
4
6
8
10
12
14
16
1.3
Fibre content (wl %)
Fig. 5. Water absorption and density versus banana fibre content.
4. 5. 6. 7.
1.4 gcm- 3 at a fibre content of 16% by mass. Over the same fibre range the water absorption properties increase from 16 to 26% by mass. The relationship between water absorption and density is illustrated in Fig. 5. This relationship is dependent upon void volume within the material, which has a dominant effect on both density and water absorption.
8. 9. 10.
International Symposium, Fibre Reinforced Cement and Concrete, ed. R. N. Swamy & F. N. Spon. London, 1992, pp. 31-47. Coutts, R. S. P. & Warden, P. G., Air-cured, wood pulp, fibre cement composites. J. Mater. Sci. Left., 4 (1985) 117-19. Coutts, R. S. P., Banana fibres as reinforcement for building products. J. Mater. Sci. Lea., 9 (1990) 1235. Kulkarni, A. G., Satyanarayana, K. G., Rohatgi, P. K. & Kalyani Vijayan, Mechanical properties of banana fibres (Musa sepientum). J. Mater. Sci., 18 (1983) 2290-6. Van Loesecke, H. W., Banana: Chemistry, Physiology and Technology, Economic Crops (Vol. 1 ). lnterscience Publishers Inc., 1950. Mickles, N., Banana. In Collins Encyclopedia (Vol. 3). Macmillan Educational Company, 1990. Hannant, D. J., Fibre Cements and Fibre Concretes. John Wiley & Sons, Chichester, UK, 1978, Chapter 3. Coutts, R. S. P. & Kightly, P., Bonding in wood fibre-cement composites. J. Mater. Sci.. 19 (1984) 3355-9. Coutts, R. S. P., Flax fibre as a reinforcement in cement mortars. The Int. J. Cement Comp. Lightweight Concrete, 5 (1983) 257-62. Coutts, R. S. P. & Warden, P. G., Air-cured Abaca reinforced cement composite. The Int. J. Cement Comp. Lightweight Concrete, 9 (1987) 69-73.