MATERIALS AND METHODS

Materials investigations
Coconut fibres: Coconut fibres were collected from shop that came from Sri Lanka. It was obtained after the oil extraction in the factory from the outer periphery of the coconut fruit. The shells were then washed properly and air dried for five days under ambient temperature and later graded in accordance with the ASTM C330 (2009). Fibres were chopped with sharp scissors maintaining a length from 15 to 35 mm. Chopped fibres were oven dried at 80°C for 5 h and used desiccators for cooling. Chopped fibres were used to determine the length, diameter, thickness, natural humidity, water absorption capacity and density of fibres.

Aggregates: The coarse aggregate form crushed granite was collected from igneous origin. The particle size used ranges between 5 to 20 mm. River sand as fine aggregate was used to mix the concrete according to the ASTM Standard C33 (2006). All particles passing through ASTM sieve No. 4 aperture 4.75 mm but retained on sieve No. 230, aperture 63 μm.

Cement and water: Ordinary Portland cement type whose properties confirm into the requirement of ASTM Type I was used for mixing of concrete and the water was collected from the laboratory stand post.

Preparation of the test specimens: Concrete cubes sizes of 100x100x100 mm and prisms having dimension of 100x100x300 mm were cast for both plain and coconut fibre reinforced concrete for the determination of different properties of concrete. The mix proportion of 1:2:3 by the weight of ordinary Portland cement, river sand, crushed stone and coconut fibre were used to cast the specimens. The water/cement ratio was used 0.4 for the mix. The water/cement ratio was maintained constant at all different volume percentage of fibres. Adhesive named as seal frost also used for quick setting of concrete. 70 grams seal frost was used per 1 kg cement. In preparing the specimens, at first, sand and cement were properly mixed in the machine and then crushed stones were added. In case of fibre reinforced concrete, fibres were also added in the mixture known as premix method. All ingredients were mixed properly with use of concrete mixture machine. Fresh concrete workability was investigated immediately after the final mixing of the concrete using slump test. To avoid void, hammer and vibrator was used for compaction. The cubes and prisms were cast by filling each mould in three layers; each layer had been compacted normally with 25 blows from a steel rod of 16 mm diameter before the next layer was poured and for prism vibrator was used. Slump values were achieved 54 and 38 mm for plain and coconut fibre concrete, respectively, which representing high and medium workability. All specimens were left in the moulds for 24 h to set under ambient temperature. They were removed from the mould and transferred into a curing tank. The curing temperature was 30±2°C. The concrete mixes and the specimens were prepared in accordance with the provisions of ASTM C330 (2009), ASTM C469, (1987) and BS 8110-1 standards.

Compressive strength test: Two types of universal testing machine were used to determine the compressive strength of coconut fibre reinforced concrete. Avery Denison testing machine is manufactured in the United Kingdom. The rate of loading of this machine is 10-3000 kN min^-1. Experiments were done using this machine with a loading rate of 136 KN min^-1. Once in a year, calibration is needed for this machine. Dartec testing machine was also used to determine the compressive strength of coconut fibre reinforced concrete. This machine can plot automatically a graph of load vs. remote. From the given graph or data, it is possible to make stress- strain graph of a specific sample. The loading capacity of this machine is up to 500 KN. The compressive rate of this machine is from 0.00015 to 2.0 mm sec^-1. The experiment has been done using this machine with a compressive rate of 0.00015 mm sec^-1. Compressive strength test of plain and coconut fibre reinforced concrete was carried out to find out the ultimate failure load, compressive strength, number of cracks and it’s length and width and finally to observe the stress-strain relation. Static loading test were done with use of DARTEC testing machine. Experimental compressive strength test was done in the laboratory as shown in Fig. 1.

Fig. 1:	Experimental compressive strength test

Scanning electron microscope (SEM): The scanning electron microscope was used to determine the thickness and cross section of fibres. SEM is capable to produce high resolution images of a sample surface. A SEM can resolve much smaller feature than a standard microscope, down to nearly 2 nanometres. In a classical Scanning Electron Microscope (SEM), electrons are thermionically emitted from a tungsten or lanthanum hexaboride (LaB6) cathode and are accelerated towards an anode alternatively; electrons can be emitted via Field Emission (FE). Tungsten is used because it has the highest melting point and lowest vapour pressure of all metals, thereby allowing it to be heated for electron emission. When the primary electron beam interacts with the sample, the electrons lose energy by repeated scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to around 5 μm in to the surface.

Tensile strength test of coconut fibre: To determine the tensile strength of coconut fibre, Hounsfield tensile testing machine was used. This machine can plot automatically load vs. extension graph. From given data, the stress and strain to failure of coconut fibres can be made. Tensile strength test of coconut fibre was carried out to investigate the behaviour of fibre under tensile load, maximum extension of fibres under tensile load and breaking point. Hounsfield tensile testing machine was used to carry out this experiment. After complete testing, this machine can plot automatically a graph Load vs. extension. From the load-extension data, the value of stress and strain can be obtained as follows:

Four different thicknesses of fibres were tested under tensile strength test. Fibres fitted in the jaw of the tensile testing machine with gauge length (clear fibre length from one jaw to another) of 35 mm. After tightly fitted with the jaw, the machine started to go on the progress of test. Generally jaws are pulled the fibre in two opposite direction. The fibre breaks down automatically when reached in its ultimate expansion. The point where the fibre breaks down is known as breaking point. All tests were done with a speed of 15 mm min^-1.

RESULTS AND DISCUSSION

Physical structure of coconut fibre: Scanning Electron Micrograph (SEM) was used to observe the physical formation of coconut fibres. Figure 2-4 are showing the SEM photograph of bunch of fibres, surface of fibres and cross-section of fibres respectively.

Fig. 2:	Scanning electron micrograph showing bunch of coir fibre (x50)

Fig. 3:	Scanning electron micrograph showing single strand of coir fibre (x250)

Fig. 4:	Scanning electron micrograph showing cross section of coir fibre (x500)

From Fig. 2, it has been observed that the fibre generally looked at an open eyes is eventually attached with 5/6 single fibres. Left hand side of Fig. 2 showed the single fibre, which is 5 to 6 times less than other fibres. It has also been observed that the surface of coconut fibre is not smooth refer to Fig. 3. So in a matrix, it can make strong bond with other materials. Cross section of fibre has clearly shown a hole in the middle of it and small holes around it as illustrated in Fig. 4. It is roughly estimated 15-20% holes exists compared with a single cross section of fibre.

Physical properties of fibre
Length of coconut fibre: Generally, the natural lengths of coconut fibres are from 60-230 mm. The lengths of fibres were measured using steel ruler and 30 pieces were randomly chosen to find out the length of coconut fibre. However, in this study chopped coconut fibres used with size of 15-35 mm.

Diameter of coconut fibre: To determine the diameter of coconut fibre, micrometre was used with precision of 0.01 mm. It has been observed that diameter of coconut fibre is from 0.17-0.24 mm.

Natural humidity of coconut fibre: To determine the natural humidity, fibres were at first open air-dried for 5 days and then the same fibres were dried in an oven at 80°C for 5 h. The weights of fibres were measured using electronic bench scale with a precision of 0.01 g. The natural humidity “H” was calculated using Eq. 1 and found the natural humidity of coconut fibre is 12.2%. It can be seen that humidity percentages are nearly similar for different types of coconut fibre samples:

(1)

where, W_d and W_O are the weight of air-dried and oven dried fibres, respectively.

Water absorption: During mixing and drying of matrix, the fibres absorb water and expand. The swelling of the fibres pushes away the concrete, at least at the micro-level. Then at the end of the drying process, the fibres lose the moisture and shrink back almost to their original dimensions leaving very fine voids around themselves. The water absorption capacity “W” was calculated using Eq. 2:

(2)

where, W_sw and W_ad are the weight of soaked fibres in drinking water and weight of air dried fibres, respectively. The measurements were carried out at 24 h intervals for 7 days. Experimental data has shown that the maximum water absorption of the coir fibre occurs during the first 24 h and until increase up to 120 h.

Fig. 5:	Water absorption capacity of coconut fibre

After 120 h, the fibre get into fully saturated condition and this state continue for last as illustrated in Fig. 5.

Density of fibre: Density of each fibre is an important parameter. For composite materials, density of fibre has a significant effect. The weight of fibre in a composite matrix is depends on the density of fibre. The density of coconut fibre, ρ_f was calculated using Eq. 3 and found the density of coconut fibre is 1.18 g cm^-3. It was observed that densities of coconut fibres are almost same for different type’s samples:

(3)

where, m_f is the mass of fibre, m_w is the mass of water, m_w* is mass of water reduced by fibre volume, ρ_w is the density of water.

Mechanical properties of fibre
Tensile strength of coconut fibre: Tensile strength test of coconut fibre was carried out to investigate the behaviour of fibre under tensile load, maximum extension of fibres under tensile load and breaking point. Hounsfield tensile testing machine was used to carry out this experiment. Four different thickness of fibre were taken and got the following results as mentioned in Table 1. Fig. 6 and 7 show the load versus extension of coconut fibres for samples 2 and 3, respectively.

Experimental data has shown that the average strength of coir fibre is 19.51 MPa and strain to failure is 2.83. It was also observed that the strength and strain to failure is not depended on area of fibre. The more area of fibre is not always given the more strength and strain to failure.

Fig. 6:	Tensile strength test of fibre (sample 2)

Fig. 7:	Tensile strength test of fibre (sample 3)

Fig. 8:	Density of plain and coconut fibre reinforced concrete

Table 1:	Stress and strain to failure of coconut fibre

Fig. 9:	Strength of plain and fibre reinforced concrete

Mainly the strength of fibre depends on its and chemical composition, process of fibre separation, treatment, humidity, temperature etc.

Density of plain and coconut fibre reinforced concrete: The weight and volume of plain and fibre reinforced concrete was measured before the compressive strength test. The obtained weight of each concrete was divided with its volume to find out the density. Figure 8 is presenting the density of plain and fibre reinforced concrete. Experimental result has shown the density of plain concrete is more than the fibre reinforced concrete. It is also observed that the density of concrete has decreased with increase in the volume of coconut fibre in the conventional coarse aggregate as illustrated in Fig. 8.

Comparison the properties of plain and fibre reinforced concrete: Plain and fibre reinforced concrete was loaded under static loading, to find out the ultimate compressive strength of different fibre volume concrete as shown in Fig. 9. The numbers, length and width of cracks were also measured for both plain and coconut fibre reinforced concrete after loaded with static loading as shown in Fig. 10 to 12. Also, the stress-strain relationship and modulus of elasticity of plain and coconut fibre reinforced concrete are shown in Fig. 13 and 14, respectively.

It was observed from the test results (Fig. 9) that the compressive strength decreased gradually due to the increase of fibre volume percentage in conventional concrete. For the conventional concrete with 0% fibre volume have the highest compressive strength values for the specified mix ratio. In case of crack nos., length and width of fibre reinforced concrete shown optimistic behaviour compared with plain concrete. It is concluded that the concrete strength depends on the strength, stiffness and density of coarse aggregates. Generally, lower the density causes lower the strength. Increased volume percentage of coconut fibre lower the density of concrete and hence, giving less compressive strength.

The numbers, length and width of cracks were measured for both plain and coconut fibre reinforced concrete after loaded with static loading. Cracks were counted in those surface where developed more than other surfaces. Experimental data has shown the development of crack is less in fibre reinforced concrete. Seven percent fibre volume reinforced concrete has developed least nos. of crack among rest fibre volume reinforced concrete. Figure 10 shows the nos. of crack of plain concrete and the different fibre volume percentage concrete. Plain concrete has developed more cracks compared with fibre reinforced concrete. Generally, in concrete cracks occur when the stress reaches the modulus of rupture of the concrete. For all the specimens, the crack appeared in the middle height position of the concrete cubes. The vertical pattern of cracks indicates that they were flexural cracks. It can be seen from test that crack spacing of plain concrete was highest then the different fibre volume percentage of reinforced concrete.

Experimental data has shown the length of crack is less in fibre reinforced concrete compared with plain concrete. There is a linear relationship between length of crack and nature of loading. For all concrete, the more length of crack has been found with 80% of static load. Therefore, coir fibre reinforced concrete can limit the length of crack. Length of cracks in plain and fibre reinforced concrete are shown in Fig. 11.

Experimental data has shown the width of crack is less in fibre reinforced concrete compared with plain concrete as shown in Fig. 12. In case of 1% fibre volume concrete, the width of crack is decreased with the decrease of loading. On the other hand, the width of crack in 3, 5 and 7% showed different result in the static loading. It has been clearly revealed the width of plain concrete is more than all fibre volume content. So, coir fibre reinforced concrete can limit the width of crack. Moreover, closed spaced cracks or more number of cracks, leads to smaller crack width. The reason for this behaviour is that the crack spacing is a function of both the tensile strength and the bond strength of the concrete. The decrease in the tensile strength of concrete is due to the decrease in its strength for the contribution of fibre volume percentage then the decrease in the bond strength of concrete. When the different fibre volume percentage added on the conventional concrete, thus the crack position a shorter distance is required for the tensile force in the fibre to be retransferred to the surrounding concrete, which implies shorter crack spacing.

Used the Load (KN) vs. remote (mm) graph and data, stress-strain curve for both plain and fibre reinforced concrete were plotted. Figure 13 presents the stress-strain relation of plain and fibre reinforced concrete.

Fig. 10:	No. of cracks developed in plain and fibre reinforced concrete

Fig. 11:	Length of cracks in plain and fibre reinforced concrete

Fig. 12:	Width of cracks in plain and fibre reinforced concrete

It is observed that plain concrete exhibits higher stress than the fibre volume mixed concrete. Consequently, fibre volume concrete shows higher strain values than the conventional concrete. It’s implies that fibre volume concrete cannot resist greater amount of load and show brittle failure at the ultimate stages of loading.

Fig. 13:	Stress-strain relationship of plain and fibre reinforced concrete

Fig. 14:	Modulus of elasticity of plain and fibre reinforced concrete

Modulus of elasticity is the slope of a stress-strain curve. Stress-strain curves often are not straight-line plots, indicating that the modulus is changing with the amount of strain. In this case the initial slope usually is used as the modulus. Modulus of elasticity is also called as Young’s modulus. The tangent of stress- strain curve of plain and coir fibre reinforced were calculated to determine the modulus of elasticity. Figure 14 presents the modulus of elasticity of plain and fibre reinforced concrete. Experimental result has also shown modulus of elasticity is slightly higher in plain concrete compared with fibre reinforced concrete. It is remarkable that 3% fibre volume concrete has shown more modulus of elasticity rather than 1% fibre volume concrete. This is mainly attributed to the less stiffness value of the coconut fibre compared to gravel. The development of E values of concrete is influenced by the type of coarse aggregate, type of cement, w/c ratio of the mix, aggregate size and curing age (Alexander and Milne, 1995). Generally, the modulus of elasticity of concrete depends on the stiffness of coarse aggregate. Also, the interfacial zone between the aggregates and paste and the elastic properties of component materials influence the modulus of elasticity of concrete.

CONCLUSION

This study has presented the results of an experimental program investigating the physical and mechanical properties of reinforced concrete incorporating different volume percentage of coconut fibre. Based on the experimental results and observations, the following conclusions can be stated:

ACKNOWLEDGMENTS

This study was conducted at the Heavy Structures Laboratory, School of Computing, IT and Engineering, University of East London, UK and School of Civil Engineering, Linton University College, Legenda Education Group, Malaysia and the authors would like to thank the technicians in the laboratory for providing assistance in specimen fabrication and testing.