The mechanical properties of lightweight foamed concrete incorporated with different proportions of oil palm shell

Malaysia is an eminent exporter nation of palm oil, which generated 19.14 million tonnes of the resource in solely year 2020. Nevertheless, massive amount of oil palm shell has been generated as the by product from the manufacturing of palm oil. Over the past decades, numerous researches have ascertained the possibilities of converting the oil palm shell to something beneficial, for instance its adoption as lightweight aggregate in construction industry. The objective of this study is to investigate the influence of oil palm shell proportions (0%, 10%, 20%, 30%, 40% and 50%) on the mechanical properties of oil palm shell lightweight foamed concrete (OPSLFC). Compressive strength, splitting tensile strength and flexural strength tests were conducted to evaluate the mechanical performance of OPSLFC possessing various oil palm shell proportions. Generally, the strength properties improve as the oil palm shell replacement level increases. In comparison between OPSLFC-0 and OPSLFC-50 samples, the increment percentages of compressive strength, splitting tensile strength and flexural strength are 18.89 % (10.48 MPa to 12.46 MPa), 22.28 % (1.84 MPa to 2.25 MPa) and 26.74 % (7.03 MPa to 8.91 MPa), respectively. Furthermore, the employment of oil palm shell enhances the flexural strength of the OPSLFC remarkably.


Introduction
In this industrialized and globalized century, Malaysia has been one of the largest exporter of natural resources. According to the statistics published in the recent years, Malaysia is the second largest exporter of palm oil, which is about 19.14 million tonnes [1]. However, the massive production of such resource also contributes to the generation of biomass wastes, as this sector has accounted for 85.5% of the total production of biomass wastes in Malaysia [2]. Abundant oil palm shell is produced as the waste from the manufacturing of palm oil and waste disposal is always a main problem in this industry. Therefore, to convert the produced waste to something beneficial, numerous researches have been conducted over the past decades to investigate its applications in various industries. In construction industry, the effort to incorporate oil palm shell into the concrete mix as a replacement to coarse of fine aggregate materials has been initiated in Malaysia in 1984 [3]. The lightweight nature of oil palm shell, which possesses the density of about 1100 to 1400 kg/m 3 , makes it one of the suitable replacement materials in the production of lightweight concrete. The utilisation of oil palm shells as lightweight aggregates does not only minimize the waste disposal to the landfills, but it is also a way to relieve the depletion issue of natural resource.
Generally, air-entraining agent, known as foam is introduced to the fresh concrete mix to lower down the fresh density of the concrete. However, excessive amount of foam lead to a massive deterioration in the mechanical properties of the concrete. Thus, a definite proportion pf lightweight material such as oil palm shell can be amalgamated into concrete, in order to reduce the foam amount required to decrease the concrete fresh density to the targeted one. Therefore, the aim of this study is to exploit the potential of oil palm shell to be employed as one of the concrete components in lightweight foamed concrete, in the production of lightweight foamed concrete with satisfactory mechanical properties, which include compressive strength, splitting tensile strength and flexural strength. Furthermore, another objective of this study is to investigate the effect of oil palm shell proportion on the mechanical performance of lightweight foamed concrete with the density of 1500 kg/m 3 . The crushed oil palm shell will be adopted to substitute the fine aggregate in the concrete at different proportions (0%, 10%, 20%, 30%, 40% and 50%).

Research significance
Due to the continuous depletion of raw materials in earth crust, the use of industrial byproduct as a substitution material in construction materials has been a new trend over the past decades. Oil palm shell which is plentifully generated in oil palm industry plays an excellent role as the replacement material in concrete. The lightweight nature of oil palm shell makes it a suitable component in producing concrete for different lightweight applications. Besides, such feature also helps to reduce the transportation cost as well as construction cost associated with the foundation and structural design, as the dead load is much smaller than the conventional concrete. Moreover, the less dense oil palm shell also possesses better thermal and acoustic insulating properties than the aggregates. It implies that the inclusion of oil palm shell in replacement of the aggregate materials improves the insulating properties of the concrete [4].
In fact, oil palm shell concrete is usually granted with small proportion of fibre such as synthetic fibre, glass fibre, natural fibre and organic fibre to enhance the ductility and the performance of concrete under loading stage. Fibres generally restrain the widening of the concrete opening and retard the propagation of cracks [5]. Therefore, the ultimate failure of the concrete is temporized and it results in higher ultimate strength of concrete. Nevertheless, fibres are usually costly and it is not financially efficient to manufacture fibre reinforced concrete in massive volume for general construction applications. Therefore, the novelty of this study is to ascertain whether the incorporation of oil palm shell solely, in the concrete of is able to produce lightweight foamed concrete with sufficient strength to fulfil distinct lightweight applications.

Experimental program
The experimental program in this study comprises preparation of materials, production of OPSLFC samples with different mix proportions and laboratory testing on the samples.

Materials
Materials used to produce oil palm shell lightweight foamed concrete (OPSLFC) in this study consisted of ordinary Portland cement (OPC), fine aggregate, water, crushed oil palm shell and air-entraining agent.
Under ASTM C150 type 1, the OPC with a specific gravity (SG) of 3.15 complying the Malaysian standard of MS 522: Part 1: 2003 was used in this study. The OPC particles were sieved through 600 μm sieve to ensure that the particles were not clumped together which would affect the quality and homogeneity of the cement during the mixing phase. The sieved OPC was then stored into an airtight container to minimize the hydration of cement by the surrounding air moisture.
The sand was employed as the fine aggregate. Firstly, the sand particles were sieved through 4.75 mm sieve and the sieved sand particles were oven-dried at the temperature of 105 °C for at least 24 h to remove the unnecessary moisture content in the sand. The ovendried sand was then sieved through 600 μm in accordance with ASTM C778 standard and eventually, stored in an airtight container.
In this study, SikaAER@ -50/50 was used as the air-entraining agent. Its application in this study was to reduce the density of the resultant concrete in order to produce OPSLFC with the density of 1500kg/m 3 . The ratio of air-entraining agent to water applied was 1:20 to produce foam with the density of 45kg/m 3 .
The crushed oil palm shells were used to substitute conventional fine aggregates in this study. Initially, oil palm shells were sieved through a 10 mm sieve to isolate those having size greater than 10 mm. Prior to the mixing process, oil palm shells were immersed in the water for 24 h. Owing to their high water retention property, oil palm shells were air-dried for approximately 3 h to attain saturated surface-dry state to prevent the addition of reduction of free water to the concrete mix. The specifications of oil palm shells are displayed in Table 1. Potable water from the water tap supplied by Syarikat Bekalan Air Selangor Sdn. Bhd. (SYABAS) was utilized in mixing and curing processes. The water was used at room temperature, which ranged from 28 °C ± 1 °C for both mixing and curing process. Excessive impurities in the mixing water can negatively affect the hydration of cement and the properties of fresh and hardened concrete.

Mix proportion
The mix proportion of the control sample (OPSLFC-0) was set at 1.00:1.00:0.55 (cement:sand:water) and oil palm shell was not added into it. Then, the air-entraining agent, or simply known as foam, was introduced to the fresh concrete mix to lower down its fresh density to 1500 kg/m 3 . On the contrary, 10%, 20%, 30%, 40% and 50% of oil palm shells were respectively incorporated into samples OPSLFC-10, OPSLFC-20, OPSLFC-30, OPSLFC-40 and OPSLFC-50 to substitute fine aggregate in volume fraction. The overall mix proportions of different samples are summarized in Table 2.

Laboratory test on the sample
Three tests were carried out to accomplish the objectives of the study namely, compressive strength test, splitting tensile strength as well as flexural strength.
Concrete compression test was performed in accordance with the standard BS EN 12390-3 (2002). The concrete cube specimen with a dimension of 100 mm X 100 mm X 100 mm was fabricated to fulfil the requirement of this test. The mass of each concrete cube specimen was measured and then tested in the AD 300/EL Digital Readout 3000 kN concrete compression testing machine.
Concrete cylinder test denotes assessment of splitting tensile strength of concrete by applying a diametric compressive force along the length of a concrete cylindrical specimen until failure happens in accordance with ASTM C496. Concrete cylinder with standard dimension of 200 mm in height and 100 mm in diameter was used. The concrete cylindrical specimens was tested in the AD 300/EL Digital Readout 3000 kN concrete.
Flexural strength test was implemented by using concrete rectangular specimen with a dimension of 160 mm X 40 mm X 40 mm. The procedures and set-up of the test were fully complied to the instruction and guidelines specified in ASTM C293 standard.

Experimental results
The purpose of this study is to assess the mechanical properties of OPSLFC containing different oil palm shell proportions. The mechanical properties can be broken down into three main strength categories, which are compressive strength, splitting tensile strength as well as flexural strength. The results are displayed and discussed in this section.

Compressive strength test
The relationship between the compressive strength of the OPSLFC and the oil palm shell replacement percentage is depicted in both Table 3 and Figure 1. It is evidently shown that the compressive strength of the OPSLFC increases gradually as the oil palm shell proportion rises. As mentioned in the earlier part, the incorporation of oil palm shell in the concrete will decrease the dependence on the foam utilization to reduce the concrete fresh density to 1500 kg/m 3 . Despite its ultimate lightweight nature, the inclusion of excessive foam in the concrete will adversely affect the strength properties of the concrete. Thus, the compressive strength grows from 10.48 MPa to 12.46 MPa (18.70 %) as the oil palm shell proportion increases from 0 % to 50 %. Another worth mentioning finding is that the increment in the compressive strength is more noticeable as the oil palm shell replacement ratio rises beyond 20 %, as can be clearly seen in Table 3.

Splitting tensile strength
The influence of oil palm shell proportion on the splitting tensile strength of the OPSLFC is displayed in Table 4 and Figure 2. Generally, the splitting tensile strength of the OPSLFC is strengthened (1.84 MPa to 2.25 MPa) as the oil palm shell replacement percentage is increased from 0 % to 50 %. However, a fluctuation in the splitting tensile strength value can be observed in Figure 2. There is a slight decline (1.84 MPa to 1.68 MPa) recorded in the splitting tensile strength of OPSLFC-10, as opposed to OPSLFC-0. Splitting tensile strength of OPSLFC is comparatively lower than the conventional concrete because of the weak bonding between the oil palm shell and cement matrix [6]. The OPSLFC cylindrical specimens tend to split into two halves as the tensile stress is induced along their circular segments. Nevertheless, reducing the foam amount introduced to the OPSLFC by increasing the oil palm shell proportion is proved to be capable of enhancing the splitting tensile strength of the OPSLFC.

Flexural strength test
The relationship between the flexural strength of the OPSLFC and the oil palm shell substitution percentage is presented in Table 5 and graphically illustrated in Figure 3. The gradient of the increment percentage is much steeper in this relationship than those of compressive and splitting tensile strengths. As the oil palm shell replacement level heightens from 0 % to 50 %, the flexural strength of the OPSLFC rises steadily from 7.03 MPa to 8.91 MPa, and 26.74 % of strength increment is reported. This indicates that the incorporation of oil palm shell is more efficient in improving the flexural strength of the OPSLFC. When flexural load is applied on the OPSLFC sample in three-point loading test, the plane of failure is not likely to intersect the crushed oil palm shells. Therefore, the load is transmitted to the surrounding areas of the oil palm shells on the plane of failure, hence enlarging the surface area of the plane of failure.

Conclusion
The study mainly highlights the practicability and potential of incorporating the oil palm shell lightweight aggregates into the concrete to produce OPSLFC which is appropriate for wide range of lightweight construction applications. Based on the obtained data and findings through a series of laboratory tests, several conclusions can be drawn.
The amalgamation of oil palm shell in the OPSLFC will induce a growth in the strength properties of the concrete, which comprise compressive strength, splitting tensile strength and flexural strength. In these three tests, the peak strength values were delivered by the same OPSLFC designation, which is OPSLFC-50. In comparison between OPSLFC-0 and OPSLFC-50, the presented increment percentages of compressive strength, splitting tensile increases. Furthermore, another notable finding is that OPSLFC containing high percentage of oil palm shell performs far better in flexural strength as compared to the conventional lightweight foamed concrete. In other words, the utilization of oil palm shell, to reduce the foam usage dose, is superiorly effective in ameliorating the flexural strength of OPSLFC.