Strength properties of renewable bio-based lightweight foam concrete incorporating of polypropylene fibre

This paper investigates the incorporating of renewable lightweight bio-based aggregate (RLWBBA) in lightweight foam concrete (LWFC). The aim of this research is to incorporate different volume fraction (Vf) of polypropylene (PP) fibre into LWFC to determine the optimum compressive strength and splitting tensile strength. Four different mix was designed containing different percentage of PP replacement (0, 0.1, 0.2 and 0.3%). From the results, the compressive strength of the oil palm shell lightweight foamed concrete with 0.3% of macro polypropylene fibre (OPSLWFC/0.3) had showed the highest compressive strength and splitting tensile strength at 28 days, which are recorded at 4.01 MPa and 0.62 MPa respectively. It also showed the lowest density among all the mix design which is 1152 kg/m3 under demoulded condition. The OPSLWFC/0.3 has increased about 23.38% of 28 days compressive strength and 37.78% of splitting tensile strength compared to the control mix, which contains 0% of fibre proportion. Hence, the findings of this research revealed that the development of environmentally friendly lightweight foamed concrete can be used as an alternative solution for traditional lightweight concrete.


Introduction
There are up to 10 billion tonnes of concrete produced over the world annually [1]. The enormous amount of concrete produced has shown the concrete is one of the most used materials in the construction industry. The recent engineering project is utilising lightweight concrete as one of the materials. Researchers have focused on the development of lightweight concrete in the past few decades to meet the needs of the industry, especially bio-based material concrete. The bio-based aggregate will become the main trend of the development of the lightweight concrete because the normal weight concrete and conventional lightweight concrete give high negative impact to the environment. The biobased aggregate concrete has low embodied carbon compared to the non-bio-based aggregate concrete [2]. From the previous studies and research, the plant aggregates that have been utilize as the bio-based aggregate are hemp, peach shell, wood, bamboo, mussel shell, oil palm shell, and coconut shell. These materials have low density and high porosity, which can fulfil the density of the lightweight concrete, and have the better sound absorption and thermal insulation [3,4]. The advantages of using the bio-based aggregate in the manufacture of concrete are it is environmentally friendly material that can reduce the usage of natural resources, and it can save the construction cost due to its low self-weight [5].
The palm oil manufactory of Malaysia is developed rapidly in past two decades, and it has become the second-largest palm oil producer in the world [6]. It has contributed a lot of agricultural waste to Malaysia because the production of the palm oil will only consume 10% of oil palm fruit, and the rest matter will become waste such as fibre, kernel, husk, fruit bunches, and shell [7,8]. The by-products of palm oil are introduced to the production of concrete to reduce the impact on the environment [9]. From the previous study, the oil palm shell concretes have lower density and strength properties compared to the conventional lightweight concrete. Thus, the researchers have introduced fibre to reinforce the concrete and enhance the strength properties. They have proven that the fibre reinforced concrete has better quality in term of mechanical properties. The strength of the concrete has significantly increased due to the bridging effect of the fibre [10,11,12,13]. In this research, the oil palm shell is utilized as the coarse aggregate and mixing with the macro polypropylene fibre (15 mm length and 0.5 mm diameter) with the volume fraction of 0%, 0.1%, 0.2%, and 0.3%. Thus, the general objective of this research is to determine the strength properties of the oil palm shell lightweight concrete incorporating with 0%, 0.1%, 0.2%, and 0.3% of 15 mm macro polypropylene fibre.

Material and specimens
In this study, the materials used are cement, fine sand, oil palm shell, foaming agent, and fibre.

Cement
In this research, the cement used is Orang Kuat Type I Ordinary Portland Cement, which is accordance with the standard of ASTM: C150/C150M-12. It has 3.14 of the specific gravity.

Fine sand
The aggregate used in this research is fine sand with passing through the 0.6 mm of sieve diameter. The properties of fine sand used are 2.68 of fineness modulus, 2.78 of water absorption, and 0.98 % of maximum normal grain size.

Oil palm shell
The aggregate used in the research is oil palm shell aggregate below 4.75 mm diameter in order to achieve homogeneous mix. The species of the oil palm shell can be found in Malaysia are dura, tenera, and pisifera [7,8]. The blended oil palm shell used in the research, the specific gravity of the blended oil palm shell is between 1.14 and 1.37 [14].

Foaming agent
The synthetic foaming agent has used in this research, which the brand is Sika ® AER 50/50. It is a product from the Sika Kimia Sdn. Bhd. The volume fraction for the dilution is 1 to 20. The foam is produced in a pressurized foaming generator with applied 500 kPa pressure.

Fibre
The fibre used in this research is macro polypropylene fibre with the diameter of 0.5 mm and aspect ratio (l/d) 30, as displayed in Fig. 1. The specific gravity of the macro polypropylene is 910 kg/m 3 [15]. The fibre that mixes with the mortar must fulfil the standard requirement of ASTM C1116 (2015), fibre reinforced concrete.

Specimen
The specimen used was 100 mm x 100 mm x 100 mm of concrete cube and cylindrical concrete with 100 mm (diameter) x 200 mm (length) [5,12].

Mix proportion
There is total 4 mixes were prepared in this research. The water-cement (w/c) ratio used in the concrete mixing is 0.55. The coarse aggregate (blended oil palm shell) added is 5% of the volume, and the volume fraction of the macro polypropylene fibre is 0%, 0.1%, 0.2%, and 0.3% of the total volume.

Result and discussion
This section is to show the result of the testing method that has been carried out and analyse and discuss the result.

Inverted slump test
The inverted slump test was conducted during the concrete under the mortar state. This is to investigate the workability of each OPSLWFC. Table 2 shows the result of the recorded inverted slump test, which comply with BS EN 1881-102: 1983.   Table 2, the reduction of the inverted slump diameter of OPSLWFC/0.1, OPSLWFC/0.2, OPSLWFC/0.3, which have the 0.1%, 0.2%, and 0.3% volume fraction of macro polypropylene fibre are 8.00%, 12.67%, and 20.00%. respectively compared with the control mix (OPSLWFC). The control mix has the highest slump diameter and the OPSLWFC/0.3 has the lowest slump diameter, and the result showed that increase the volume fraction of macro polypropylene fibre decrease the workability of the mortar. The macro polypropylene absorbs the cement content to wrap around the surface of fibre, and the viscosity of mortar increase. Thus, the workability of concrete decreased [16].

Density
In this research, there are 3 types of the density is determined, which are fresh, demoulded, and oven dry density. The Table 3 shows the density of each mix design. From the result of Table 3, it showed that the OPSLWFC has the highest fresh, demoulded, and oven dry density, while the OPSLWFC/0.3 has the lowest density. The  Fig. 2 shows the decrease in the density when the volume fraction of the macro polypropylene increase. The Fig. 2 clearly showed the density is in decreasing trend with increase the volume fraction of macro polypropylene fibre. The reason of this condition is the macro polypropylene fibre has the lower specific gravity (0.91) than other concrete material and cause the reduction of the overall density of concrete. As the result from Fig. 3, it showed that the compressive strength is increasing significantly when the fibre proportion in concrete specimen increases. The OPSLWFC/0.1, OPSLWFC/0.2, and OPSLWFC/0.3 has increased about 20.46%, 29.77%, 41.40% of 7 days compressive strength and 8.31%, 21.23%, 23.38% of 28 days compressive strength respectively. As the result of slump diameter of the concrete specimen is decreased and the compressive strength of the specimen increased when the fibre proportion increased. It can be predicted the water-cement ratio of in the concrete decreased and cause the workability of concrete decreased and increase the compressive strength. The macro polypropylene fibre absorbed the water from the mortar due the porous space in the fibre. Thus, the compressive strength is affected by the porosity of the fibre. In the other hand, another prediction is the compressive strength increased because the fibre in the concrete help to resist the propagation of crack. It was found that the fibre reinforced concrete specimen has higher compressive strength and become more ductile [1,11]. Besides, it was found that the 28 days compressive strength of OPSLWFC/0.1, OPSLWFC/0.  The result from Fig. 4 showed that the splitting tensile strength is increased when the macro polypropylene fibre proportion increased. The OPSLWFC/0.1, OPSLWFC/0.2, and OPSLWFC/0.3 has increased about 17.14%, 25.71%, 45.71% of 7 days splitting tensile strength and 17.78%, 26.67%, 37.78% of 28 days splitting tensile strength respectively compared to OPSLWFC. The addition of macro polypropylene fibre will increase the splitting tensile strength because macro polypropylene fibre can provide the bridging effect between the cementitious material. The effect causes the stress applied on the concrete specimen transfer to the fibre from the concrete matrix. This leads to the improvement in the tension capacity of the specimen [

Conclusion
From the analysed result, the conclusion can be made that the oil palm shell lightweight foamed concrete incorporating with macro polypropylene fibre (15 mm of length and 0.5mm of diameter) has the lower density and higher strength properties of compressive and splitting tensile strength. The best fibre proportion among 0%, 0.1%, 0.2%, and 0.3% in this research is 0.3% of the macro polypropylene fibre. It has the lowest density, at the same time has the highest compressive and splitting tensile strength. The maximum 28 days compressive and splitting tensile strength are 4.01 MPa and 0.62 MPa, which has increased about 23.38% and 37.78% respectively. Besides, it achieved the minimum requirement of concrete non-load bearing wall panels, which is compressive strength higher than 3.5 MPa [19].