Effect of Pelletized Coconut Fibre on the Compressive Strength of Foamed Concrete

Foamed concrete is a controlled low density ranging from 400kg/m to 1800kg/m, and hence suitable for the construction of buildings and infrastructures. The uniqueness of foamed concrete is does not use aggregates in order to retain low density. Foamed concrete contains only cement, sand, water and foam agent. Therefore, the consumption of cement is higher in producing a good quality and strength of foamed concrete. Without the present of aggregates, the compressive strength of foamed concrete can only achieve as high as 15MPa. Therefore, this study aims to introduce the pelletized coconut fibre aggregate to reduce the consumption of cement but able to enhance the compressive strength. In the experimental study, forty-five (45) cube samples of foamed concrete with density 1600kg/m were prepared with different volume fractions of pelletized coconut fibre aggregate. All cube samples were tested using the compression test to obtain compressive strength. The results showed that the compressive strength of foamed concrete containing 5%, 10%, 15% and 20% of pelletized coconut fibre aggregate are 9.6MPa, 11.4MPa, 14.6MPa and 13.4MPa respectively. It is in fact higher than the controlled foamed concrete that only achieves 9MPa. It is found that the pelletized coconut fibre aggregate indicates a good potential to enhance the compressive strength of foamed concrete.


Introduction
Foamed concrete has received high attention in the construction industry due to its advantageous of lightness and versatility material.The primary characteristic of foamed concrete is cement based mortar that consist at least 20% of air voids.Therefore, foamed concrete has low density, good strength and reliable toughness at minimal consumption of aggregate.Typically, foamed concrete can achieve dry density in the range of 400kg/m 3 to 1600kg/m 3 with compressive strength around 1MPa to 15MPa [1,2].The tensile strength of foamed concrete is directly proportion of its compression strength and lower approximately ten times.Meanwhile, the fracture energy of foamed concrete is around 15N/m to 35N/m [3][4][5].Although a comparison with normal concrete shows that the strength of foamed concrete is slightly lower.Still, there are demands on the application of foamed concrete as construction material and structural element.
a Corresponding author : rizuan@uthm.edu.mySince the strength of foamed concrete is lower than normal concrete, many attempts have been made to enhance the ability of foamed concrete in carrying meaningful actions.Such attempts include the utilization of supplementary cementitious materials such as rice husk ash, palm oil ash, artificial fibres and natural reinforcements.The fascination toward green and sustainable material in construction industry has prompted the use of natural reinforcements such as coconut and kenaf fibres.Moreover, foamed concrete containing natural reinforcement is an attractive option.Normally, in concrete, steel or polypropylene fibre is used to increase the strength, but it can also increase the weight and cost of the concrete.Therefore, the application of natural fibre is one of the alternatives due to high availability and environmental friendly.

Concrete Containing Coconut Fibre
In recent years, the use of coconut fibre as reinforcement on concrete has received high attention because of renewability, low density, high specific properties and low cost.High specific properties include the strength, lightness, stiffness, impact resistance and flexibility [6].Moreover, coconut fibre in concrete also has become an interesting application towards sustainable construction.Based on an experimental study by Ruben and Braker [7], it was found that concrete containing coconut fibre not just bring several improvement in concrete characteristics but also reduces environment pollution factors.Savastano and Agopyan [8] reported that the combination of short coconut fibres with ordinary Portland cement has presented a significant increase in toughness.Several factors such as properties, volume fraction, distribution and orientation of fibre are identified to be influenced the physical and mechanical properties of concrete.However, the volume fraction appears to be the most dominant factor in influencing the strength of concrete.
The application of concrete containing coconut fibre as structural elements was study by Ali [9].The study on the behaviour of coconut fibre-concrete columns reveals that non-visible degradation occur prior to the cracking of the column.The use of coconut fibre in cement has increased the flexural strength and consequence improves the crack resistance.Coconut fibre alters the behaviour of concrete when cracks occur across the structure and thus can provide a residual strength of the concrete [10].Therefore, concrete containing coconut fibre exhibits same performance as that of conventional reinforced concrete produced by steel fibre [11].In addition, the compressive of strength is also improved up to a certain percentage.Baruah and Talkudar [12] investigated the mechanical properties of concrete containing coconut fibre with different volume fractions ranging from 0.5% to 2%.From the obtained results, it is found that 1.5% of volume fraction of coconut fibre produce optimum strength with increment of 14.66%.A summary of studies about concrete containing coconut fibre is shown in Table 1.IConCEES 2015 3 Experimental Study

Material preparation
Main mix proportion consists of cement, sand, foam agent and water.The density of foamed concrete is 1600 ± 5kg/m 3 .Therefore, water-cement ratio and cement-sand ratio were set up at 0.55 and 0.5 respectively.Table 2 shows the mix proportion of raw materials.The brown coconut coir was used to produce pelletized coconut fibre aggregate.The coconut coir was compressed into bar with diameter approximately 5mm and thin-coated using epoxy.The bar then cut into pellets with size of 10mm to 20mm. Figure 1 shows the pelletized coconut fibre aggregate.Meanwhile, three (3) samples of coconut coir bars were tested under tensile loading to determine the tensile strength and Young's modulus.The results from the test, as shown in Figure 2, gave the average tensile strength and Young's modulus around 3.785MPa and 0.079GPa respectively.

Specimen preparation
In the experimental study, forty-five (45) cube specimens were prepared for the compression test.Thirty-six (36) of the cube specimens are foamed concrete containing different volume fractions of pelletized coconut fibre aggregate at 5%, 10%, 15% and 20%.Another nine (9) cube specimens were cast for the controlled foamed concrete.Table 3 shows the specification of cube specimens.All cube specimens were placed at the ambient condition for the air curing process at 7, 14 and 28 days before the compression test take place.Figure 3 shows the curing process of the cube specimens and the compression test.

Results and Discussion
The compressive strength of foamed concrete containing pelletized coconut fibre aggregate along with dry density and standard deviation are given in Table 4.The average compressive strength of controlled foamed concrete at 28 days is 9.0MPa.In accordance with Jaini et al. [2] and Aldridge [18], the compressive strength of foamed concrete with density of 1600kg/m 3 is in the range of 7.5MPa to 10.0MPa.Therefore, the controlled foamed concrete showed good agreement and convincing compressive strength.Meanwhile, the average compressive strength of foamed concrete containing 5%, 10%, 15% and 20% of pelletized coconut fibre aggregate are 9.6MPa, 11.4MPa, 14.6MPa and 13.4MPa respectively.The pattern of compressive strength due to the volume fraction of pelletized coconut fibre aggregate at 7, 14 and 28 days is shown Figure 4.It can be observed that by increasing the volume fraction of pelletized coconut fibre aggregate on foamed concrete, the compressive strength is escalated significantly.Among the specimens, foamed concrete containing 15% pelletized coconut fibre aggregate showed the highest compressive strength.There is 62.22% of strength increment, as can be referred in Figure 5, compared with that obtained by controlled foamed concrete.Although foamed concrete containing 15% pelletized coconut fibre aggregate has achieved significant increment of compressive strength, however, it is still insufficient for structural application.A comparison with previous studies indicate that the pelletized coconut fibre is sustainable and effective in improving the compressive strength of foamed concrete with high utilization of volume fraction.This condition can contribute to the great saving of cement and consequently promote green concrete in the construction industry.In addition, it is also found that the use of pelletized coconut fibre aggregate tends to reduce the dry density of foamed concrete.

Conclusions
The study on the effect of pelletized coconut fibre aggregate on the strength of foamed concrete was conducted by using the compression test.The results showed that the compressive strength increases significantly until optimum volume fraction of 15%.It is found that the pelletized fibre aggregate indicates a good potential to enhance the compressive strength of foamed concrete.However, other characteristics of fresh and harden concrete need further investigations.

Figure 4 .
Figure 4. Strength development of foamed concrete based on volume fraction of aggregate.

Figure 5 .
Figure 5. Strength increment of foamed concrete based on volume fraction of aggregate.

Table 1 .
Effect of coconut fibre on the strength of concrete.

Table 2 .
Mix proportion of foamed concrete.

Table 4 .
Dry density and compressive strength of cube specimens.