Mechanical properties of concrete reinforced with Elaeis Guineensis midrib fibres

Abstract

The use of natural fibres in concrete manufacturing has garnered global attention as sustainable building materials are sought after to enhance performance, mitigate resource depletion, and minimise environmental impacts. This research investigates the physical and mechanical behaviours of Elaeis Guineensis midrib fibres (EGMF)-based concrete, comparing untreated and treated EGMF variants. EGMF samples were treated with 1–5% of Ca(OH)₂ solution at 1% increments for 6, 12, and 24 h, and the tensile strengths were examined. Physical (slump) and mechanical (compressive strength, split tensile strength, flexural strength, bond, and rupture modulus) tests were conducted on 20 MPa characteristic strength, with EGMF replacing the total concrete volume at 0.5-2%, and tested after curing for 7, 28, and 56 days. The results demonstrated an optimum tensile strength for EGMF at 4% and 24 h of Ca(OH)₂ solution and treatment. The slump decreased by 1–5% as EGMF dosages increased from 0 to 2%. Incorporating treated EGMF enhances the compressive, split tensile, and flexural strengths of concrete at optimum contents of 0.5%, 0.5%, and 1%. After 28 days of curing, the bond strength and slab’s modulus of rupture of treated EGMF-based concrete outperformed those of untreated EGMF by 25.93% and 9.79%. Hence, the use of treated EGMF as reinforcement in concrete manufacturing enhances its mechanical properties, promoting the production of high-quality concrete and facilitating sustainable, cleaner production.

Introduction

Managing environmental concerns and addressing infrastructure deficits are among the challenges facing the growing global population. The world’s population is projected to rise from the current 7.6 billion to 8.6 billion by 2030, 9.8 billion by 2050, and 11.2 billion by 2100¹. The demands of a rapidly expanding global population and the growing need for higher living standards, which in turn increase the requirements for infrastructural development, pose a threat to the long-term resilience of many natural systems². Building and construction materials, such as concrete, steel, and aggregates, will be used more because of this growing demand, leading to the depletion of non-renewable natural resources. These global challenges can be addressed through recycling, which promotes a circular economy while preserving natural resources and environmental systems. As a result, palm oil manufacturing byproducts have enormous potential as valuable, sustainable natural resources for the construction sector.

Over the past three decades, considerable research has focused on recycling locally available waste materials to produce sustainable building materials. It is imperative to explore more economical and sustainable building materials for emerging nations that currently face severe infrastructure deficiencies. Securing affordable housing is nearly impossible for low- and middle-income earners in many countries due to high inflation, increased rural-to-urban migration, rapid population growth, and difficulties obtaining housing loans. For example, there is an estimated shortage of more than 20 million housing units in Nigeria and India³.

Elaeis Guineensis, a species of palm commonly just called oil palm but also sometimes African oil palm, has been identified as one of the fastest-growing plantations, primarily in hot climates with temperatures between 23 °C and 33 °C and abundant annual rainfall (1780–2280 mm), such as in Indonesia, Malaysia, Thailand, Colombia, Nigeria, and other countries⁴. Over time, there has been a general increase in palm oil production, occurring practically every year. In 2023–2024, as shown in Fig. 1, Indonesia and Malaysia accounted for an average of 56% and 26%, respectively, of the palm oil needed to meet global demand⁵. The production is expected to increase to meet growing global demand for palm oil, which will also lead to a rise in waste from palm oil mills⁶. Oil production from palm fruits is approximately 20%, with the remaining 80% of the fruit discarded as waste⁷. Waste from palm oil mills, which accounts for around 28% of the total palm oil output, has the largest byproduct composition. This is followed by empty fruit bunch, fibre, palm kernel, and palm shell, with compositions of 23%, 15%, 7%, and 6%⁸. A significant portion of the world’s annual solid waste production originates from the agricultural sector. Over the past 50 years, biomass generation has increased by more than 300%, and this trend is predicted to continue as the global population surpasses 10 billion by 2050⁹. In some countries, such as Indonesia and Malaysia, the waste byproducts from the palm oil sector are among the largest. Most of these wastes are either burnt in the open field or indiscriminately dumped in landfill sites, posing environmental degradation and damage to ecosystems.

Fig. 1.

Global production of palm oil⁵.

Compared to a composite reinforced with glass fibres, the use of natural fibres reduces weight, cost, and energy consumption by 10%, 5%, and 80%, respectively¹⁰. Benefits of natural (plant) fibres include reduced carbon footprint, low cost, light weight, durability, enhanced thermal and acoustic insulation, high recyclability, biodegradability, and non-toxicity to the environment¹¹. However, there are disadvantages to using plant fibres in concrete. Studies have also demonstrated that these issues can be resolved by pre-treating the fibres before adding them. The pretreatment methods include chemical treatments, ozonolysis, and physicochemical treatments¹¹.

Except for the leaflet ribs, often referred to as oil palm broom fibres, oil palm biomass has garnered significant research interest. Furthermore, with the exception of bamboo culms^12,13, plant-based materials such as shells, husks, and fibres have only recently been used as longitudinal reinforcement in concrete^14,15. Elaeis Guineensis midrib fibre (EGMF) is a potential fibre with over 90 times larger than other oil palm fibres¹⁶. A relevant study examined the physical and mechanical properties of broom fibres from oil palm leaflets. The findings showed that the fibres have specific gravity, tail and cap diameters of 0.45–0.84, 0.20, and 4 mm, respectively, and a maximum tensile strength of 900 MPa¹⁷. Mechanical properties of short discrete oil palm broom fibre of 50 mm length were conducted on C30 MPa at 0.5-4% of total aggregate volume. The experimental findings revealed improvements in flexural and split tensile strengths at 0.5-1% oil palm broom fibre inclusion after 28 days of curing. Moreover, the post-yield behaviour of oil palm broom fibre-based concrete demonstrated improved energy absorption capacity of 70% and 320% after 112 days of curing, indicating potential applications in affordable, earthquake-resistant buildings¹⁸. Another study investigated the effects of three types of treatments (lime treatment, silanisation, and hot water) on the performance of oil palm broom fibre-based concrete. The oil palm broom fibre samples were limesed and silanised for 30–2880 min using 2–10% NaOH solution at 2% increments and 1–3% triethoxyvinylsilane, respectively. For hot water treatment, the samples were subjected to 15–120 min of hot water at 100 °C. The results indicate improvements of 60% and 65% in tensile strength and modulus of elasticity, respectively. The findings revealed that optimum performance was achieved in a 6% NaOH solution at 2880 min at ambient temperature, 3% silane for 1440 min at ambient temperature, and in hot water at 100 °C for 30 min, with the fibre’s tensile strength rising by more than 30%¹⁹. While silanization showed an average decrease in water absorption of 4%, limezation and hot-water treatment showed increases of 40% and 9%, respectively¹⁹.

The incorporation of oil palm shells at a volume fraction of 0.3% of the aggregate reduces the density of the foamed concrete and improves its compressive and split tensile strengths by 23.38% and 37.78%, respectively, after 28 days of curing, compared to the control mix²⁰. Similarly, the lightweight foam concrete incorporating 15% oil palm shells as aggregate replacement enhances fire resistance by 23.50% at 15 min, compared to the control sample²¹. The modulus of elasticity (MoE) and modulus of rupture (MoR) of fibreboard produced from untreated empty fruit bunch (EFB) at a resin content of 8% were 2042 MPa and 24.2 MPa, compared to 2142 MPa and 28.20 MPa for those treated with a NaOH solution²². Likewise, the MoE and MoR of the fibreboards manufactured from the middle portion of mature oil palm fronds at a constant of 12–15% resin were 950.84 MPa and 12.46 MPa, respectively, compared to the top portion, with 842.29 MPa and 11.67 MPa, and the bottom part, with 989.96 MPa and 16.03 MPa, respectively²³.

Despite the availability of information on the engineering properties and potential uses of fibres derived from various oil palm trees in the literature, studies on EGMF are scarce. The pretreatment before use, especially in a low-cost solution, the portion used, and its replacement as a percentage of the total concrete volume have not been explored. Therefore, these reasons motivate this study.

This study is important because, with a significant reliance on fossil fuels, the manufacturing of synthetic fibres uses more than ten times the energy compared to the extraction of natural fibres, resulting in a greater environmental impact, including CO₂ emissions²⁴. Furthermore, the high costs of conventional materials, arising from the mining of iron ore for steel production in reinforced concrete, pose a persistent affordability challenge in developing countries. Hence, EGMF are examples of abundant materials (natural vegetable fibres) that can be repurposed to improve concrete properties while addressing environmental challenges.

This research addresses these gaps by investigating the workability and mechanical properties of EGMF-based concrete, comparing untreated and treated EGMF variants. EGMF samples were treated with 1–5% of Ca(OH)₂ solution at 1% increments for 6, 12, and 24 h, and the tensile strengths were determined. The physical and mechanical tests were conducted on a 20 MPa characteristic strength, with EGMF replacing 0.5-2% of the total concrete volume. The tests were performed after 7, 28, and 56 days of curing. The findings would present a valuable opportunity to demonstrate the potential use of EGMF as a raw material in concrete production, while also addressing the environmental issues associated with its disposal.

Materials and methods

Materials

All materials used, such as 42.5 R Portland limestone cement (PLC), Elaeis Guineensis (oil palm) midrib fibres, calcium hydroxide (Ca(OH)₂), fine aggregates (FA) of ≤ 4/75 mm size, and coarse aggregates (CA) of 12. 50 mm size, were sourced locally. All EGMF were dried to a moisture content of less than 10% to optimise their physical characteristics for processing and to prevent fungal growth and deterioration²⁵. This moisture content enhances bonding between the fibres and other matrix materials, ensuring greater stability and preventing dimensional changes. Every extracted rib (broom) was thoroughly inspected to make sure there were no defects or damage. The specific gravity of cement was determined in accordance with the British Standard (BS EN 196)²⁶. The specific gravity and water absorption of the aggregate and EGMF samples were determined according to the BS EN procedures, using the pycnometer method (a glass flask with a stopper)²⁷. The results of the physical properties of concrete constituents and EGMF are presented in Tables 1 and 2, respectively.

Table 1.

Physical properties of materials used.

Materials	Specific gravity	Water absorption (%)
PLC	3.15	-
Coarse aggregate	2.76	0.70
Fine aggregate	2.52	1.70

Table 2.

Physical properties of EGMF.

Materials	Treated EGMF	Untreated EGMF
Total EGMF length (mm)	150	150
Cross-sectional diameter (mm)	2.0	2.0
Bulb length (mm)	20	20
EGMF gauge length (mm)	110	110
Specific gravity	0.97	0.88
Water absorption (%)	41.50	46.20

The particle size distributions of aggregates were carried out in accordance with the provisions of BS EN 12,620²⁸. Figure 2 shows the grading of FA and CA used at the upper limit (UL) and the lower limit (LL).

Fig. 2.

Grading of aggregates used.

Experimental methods

Pretreatment of EGMF

EGMF samples were chemically treated by immersing the fibres in 1–5% Ca(OH)₂ solutions for 6, 12, and 24 h. The reasons for these selections can be attributed to the fact that treatment with low concentrations of alkali solution (1–5%) is generally found to remove sufficient hemicellulose and impurities from natural fibres without degrading cellulose^29,30, thereby improving fibre–matrix adhesion, surface roughness, and mechanical properties³¹. Natural fibres treatment with an alkali solution for 6–24 h is a standard approach used to optimise the treatment process. Treatment of natural fibres with a lime solution for 6, 12, and 24 h provides a reference point for the damage threshold and determines the upper limit of the treatment window before loss of strength becomes dominant^32–34.

The solutions were continuously stirred to maintain uniform exposure and to avoid Ca(OH)₂ sedimentation². After this process, the fibres were air-dried to attain a saturated surface-dry condition and to reduce their intrinsic water absorption, while preserving their lightweight characteristics². Notably, not all pretreatments exhibit positive outcomes, as a pretreatment’s efficacy depends on several variables, including the method, material properties, and chemical concentration². After air-drying, the fibres were stored in sealed plastic bags to prevent moisture exposure. The maximum tensile forces at break were recorded for the tested fibres. Based on the test analysis, the pretreatment concentration and duration that yielded the highest tensile strength for EGMF were adopted for concrete reinforcement.

Fig. 3.

Materials showing (a) untreated EGMF, (b) Ca(OH)₂ solution preparation, and (c) EGMF in Ca(OH)₂ solutions.

Tensile strength of EGMF

This strength was measured using a UTM, Model H10KS, per ASTM standard at a displacement rate of 5 mm/minute³⁵. Prior to testing, the sample’s gauge length (110 mm) was end-dipped in epoxy glue for 72 h to prevent mechanical damage. Each test took an average of 2 min.

Mix design

The ACI mix design method was adopted³⁶, utilising a 25–50 mm slump, a maximum crushed aggregate size of 12.5 mm, a water-to-cement ratio of 0.57, and a specified characteristic strength of 20 MPa at 28 days of curing. EGMF were cut into 50 mm lengths using a pair of metallic scissors, and randomly incorporated into the total concrete volume at 0.5%, 1%, 1.5%, and 2.0% levels. The mixing was carried out adequately until a homogeneous mixture was achieved. The samples were removed from the moulds after 24 h and immersed in a water-curing tank (23 ± 5 °C and 65 ± 5% RH) for 7, 28, and 56 days³⁷. Table 3 shows the details of the mix design proportions.

Table 3.

Mix proportions.

EGMF (%)	EGMF (kg/m3)	PLC (kg/m3)	FA (kg/m3)	CA (kg/m3)	Water (kg/m3)
0	0	350	857	912	199
0.5	11.59	350	857	912	199
1.0	23.18	350	857	912	199
1.5	34.77	350	857	912	199
2.0	46.36	350	857	912	199

Experimental tests

Workability

Slump measures the consistency and workability of concrete. It indicates the ease with which the concrete can be mixed, placed, and compacted. This test was carried out on freshly mixed concrete in accordance with BS EN 12350-2³⁸, using a 300 mm high slump cone. The slump cone was cleaned and lubricated with engine oil and then placed standing upright with the large end facing the base plate. The mould, as indicated in Fig. 4, was filled with concrete in three layers, with each layer receiving 25 strokes of the tamping rod. After the third layer had been struck, the surplus concrete was scraped off, and the top layer was levelled with a hand trowel. After this, the cone was slowly lifted off the wet concrete using handles attached to the cone, allowing the wet concrete to slump. The cone was set aside, and a straight edge was mounted on its top. The distance between the slump and the top of the cone was measured to the nearest 5 mm as the slump value of the concrete.

Fig. 4.

Slump testing.

Mechanical properties

Compressive strength

Compressive strength was measured on concrete specimens on a UTM at a displacement control rate of 3 mm/minute. Concrete 100 × 200 mm cylindrical specimens, as shown in Fig. 5, were subjected to compressive strength tests in accordance with BS EN 12390-3³⁹ after curing for 7, 28, and 56 days. The specimen was removed from the water and wiped to remove excess moisture from its surface. The sample was positioned so that the load was applied perpendicularly to the casting direction, centrally in the testing machine. When necessary, the packing strips and pieces were positioned along the top and bottom loading planes. The maximum load at failure was recorded, and the compressive strength, f_c, in N/mm², is given by Eq. (1).

1

Fig. 5.

Concrete specimens for compressive and split tensile strengths tests.

where P is the maximum load (in N), and A_c is the specimen’s cross-sectional area (in mm²).

Split tensile strength

Splitting tensile test was performed on 100 × 200 mm cylindrical specimens (see Fig. 5). The test was conducted in accordance with the requirements of BS EN 12390-6⁴⁰. The splitting tensile strength (f_t, in MPa) was calculated using Eq. (2).

2

where P is the maximum failure load (in N), l is the sample height (in mm), and d is the sample diameter (in mm).

Flexural strength

The flexural strength test, as displayed in Fig. 6, was conducted on a 100 mm x 100 mm x 500 mm beam subjected to third-point loading in accordance with BS EN 12390-5⁴¹. The flexural strength (f, in MPa) is determined by the relationship illustrated in Eq. (3).

3

Fig. 6.

Flexural strength test.

where L, b, and d are the sample length, width, and depth (in mm), respectively.

Direct pull-out (bond) strength

This test, as shown in Fig. 7, was conducted on a 100 × 200 mm concrete cylinder using a UTM equipped with a 3 kN load cell, operating in displacement control at a rate of 5 mm/minute. EGMF samples were prepared by gauging them to 150 mm and dipping them into epoxy glue. Moreover, water absorption was minimised by wrapping the samples with a cello tape to a length of 50 mm from the free end. The cello tape was then lubricated with petroleum jelly to prevent the EGMF samples from adhering to the concrete. To prevent the fibres from getting soaked, the samples were immersed so that the water above the concrete was about 5 mm deep. Equation (4) illustrates the bond strength (u, in MPa) under the assumption of a uniform bond:

Fig. 7.

Direct pull-out (bond) strength test.

where P_max, d_b, and L_b represent the maximum pull-out load (in N), diameter of the fibre cross-section (in mm), and embedded fibre length (in mm), respectively.

Slab’s modulus of rupture

The nine specimens were used to test the slab’s modulus of rupture. These specimens consisted of three unreinforced (control), three reinforced with untreated EGMF meshes, and three reinforced with treated EGMF meshes. The meshes were made by cutting fibres into 200 mm lengths and placing them longitudinally and transversely at 20 mm intervals. The mesh was fixed at all intersections with annealed steel wire to prevent distortion or displacement during casting. This test, as shown in Fig. 8, was conducted on 225 mm x 225 mm x 75 mm slab specimens in accordance with BS EN 12390-5⁴¹, and tested after curing for 28 days. The modulus of rupture (MOR) was calculated by Eq. (5).

5

Fig. 8.

Slab’s modulus of rupture testing.

Results and discussion

Tensile strength of EGMF

Figure 9 provides insights into the tensile strength of EGMF under different pretreatment times (6–24 h) and lime concentrations (1–5%). From Fig. 9, it is evident that the tensile strength of EGMF is influenced by pretreatment time and lime solution. At 6 h, there was a gradual increase in tensile strength as the Ca(OH)₂ concentration increased from 1% to 5%, with values ranging from 469.72 MPa to 795.25 MPa, signifying a 40.93% increase in tensile strength. A similar trend was observed at the 12-hour treatment, with tensile strength increasing from 483.53 to 872.85 MPa across the same concentration doses, indicating a 44.60% increase in tensile strength as the lime solution increased from 1% to 5%. Optimum tensile strengths were achieved at 24 h, ranging from 745.39 to 998.79 MPa, as the lime concentration increased from 1% to 5%. This indicates a 25.37% increment. The improved tensile strength of oil palm midrib fibres treated with Ca(OH)₂ solutions for 24 h can be attributed to the removal of the weak amorphous component, which increases the cellulose crystallinity and improves the fibre-matrix interaction, while maintaining the rigid cellulose backbone^30,31. These findings support a related study, which confirmed that alkali-treated oil palm broom fibres used as reinforcements for laterite-based roof tiles had a tensile strength of more than 900 MPa⁴².

Fig. 9.

Tensile strength of EGMF at various Ca(OH)₂ solutions and treatment times.

The tensile strength increased with increasing lime solution concentration up to 4%, then slightly decreased at 5% for all pretreatment times. This indicates that a 4% Ca(OH)₂ solution is optimal across all conditions and pretreatment times tested. This performance can be attributed to the fact that 4% Ca(OH)₂ offers the optimal level of non-cellulosic impurity removal and enhances the fibre-matrix structure without degrading cellulose. However, a 5% Ca(OH)₂ represents over-treatment, where excessive cellulose degradation, increased fibre fibrillation, and detrimental microstructural changes occur, resulting in reduced tensile strength³³. Overall, the maximum tensile strength of EGMF was achieved with a 4% Ca(OH)₂ pretreatment solution at 24 h.

Slump

Figure 10 shows the slump of treated EGMF-based concrete at 4% of Ca(OH)₂ solution. The slump, as shown in Fig. 10, decreased with increasing EGMF content, indicating reduced workability and consistency at higher fibre content. There was a 10–44% decrease in slump as the EGMF content increased from 0.5% to 2% in the concrete mixes, compared to the concrete sample without EGMF. The reduction in slump indicates that the concrete becomes less fluid and more difficult to work with as the EGMF proportion increases. The increase in water demand for EGMF-based concrete is primarily due to the EGMF’s increased surface area. Fibres introduce mechanical interlocking and entanglement between the fibres and the aggregate particles. This hinders the flow of the paste, restricting the smooth movement of the particles. Moreover, fibres are high surface area-to-volume ratio materials that demand increased surface area cement paste bond and reduce flow or ‘slump’^29,32. Ultimately, these slump ranges can be applied in footings, foundations, and heavily reinforced sections where high strength and low workability are needed. Superplasticisers can be used to enhance the flow characteristics without raising the w/c ratio.

Fig. 10.

Slumps of concrete reinforced with various percentages of EGMF.

Compressive strength

Compressive strength is a critical parameter in assessing a concrete’s ability to withstand axial loads or pressure. Figure 11 presents the compressive strength of concrete reinforced with different percentages of EGMF for 7, 28, and 56 days of curing. The results demonstrated that adding EGMF decreased the concrete’s early age (7-day) strength by approximately 0.20% to 18% at EGMF levels ranging from 0.50% to 2%. However, the incorporation of 0.5-1% EGMF into the concrete mix improved the concrete’s later-age strengths (at 28 and 56 days) by 3%. The early-age loss in compressive strength can be attributed to EGMF’s high water absorption and inadequate internal curing, resulting in incomplete cement hydration and reduced strength. However, as curing progresses over longer periods, it becomes a beneficial effect, as the water initially absorbed by EGMF is gradually released back into the matrix, allowing increased hydration, fibre-matrix bonding, and strength⁴³.

Fig. 11.

Compressive strengths of concrete reinforced with various percentages of EGMF.

After 1% of EGMF incorporation, the compressive strengths, at 1.5-2% EGMF contents, declined by 13–17% and 11–15% after curing for 28 and 56 days, respectively. These findings suggest that higher percentages of EGMF negatively affect the compressive strength of concrete. This reduction in compressive strength may be attributed to various factors, including the disruption of the concrete matrix by the fibres or inadequate bonding between the fibres and the cementitious matrix⁴⁴. The reduction in compressive strength may also be attributed to the comparatively low transverse rigidity of EGMF, thereby creating weak planes within the composite matrix². The increased porosity of the matrix, resulting from the dimensional instability of EGMF, may also contribute to the decrease in compressive strength¹⁸. The addition of pineapple leaf fibre showed a similar pattern, with the reduction being exacerbated by higher fibre contents⁴⁵. Similarly, a prior study showed that adding palm fibres reduces the compressive strength of oil palm shell concrete due to weaker fibre-matrix bonding⁴⁶. Ultimately, this study recommends an optimal EGMF content of 1% of the total concrete volume. This can be used for a general-purpose concrete suitable for light structural, residential (low-rise buildings), and general building applications. It is suitable for most uses where moderate strength (light loading condition) is required, and cost is a consideration.

Split tensile strength

Split tensile strength is crucial for assessing a material’s ability to resist tensile stresses, which tend to pull the material apart. Figure 12 indicates the split tensile strength of concrete reinforced with EGMF at various percentages. Unlike the impact of EGMF on the early age compressive strength of concrete at 7 days, as shown in Fig. 11, the results, as depicted in Fig. 12, revealed that adding EGMF to concrete at 0.5-1% improves the early age (7-day) split tensile strength of concrete by 4%. However, the strength declined by 6–15% as EGMF contents increased from 1.5% to 2%, compared to the control sample. This suggests that incorporating a small amount of EGMF (up to 1%) into the concrete mix can minimally improve its tensile strength. Furthermore, the addition of 0.5-1% EGMF into the concrete mix, as indicated in Fig. 12, enhanced the concrete’s later-age strengths by 3–9% and 3–7% after curing for 28 and 56 days, respectively. But after 1% of EGMF addition, the split tensile strengths, at 1.5-2% EGMF contents, declined by 7–30% and 10–30% after 28 and 56 days of curing, respectively. These findings suggest that higher EGMF content (above 1%) negatively affects concrete tensile strength. This reduction in tensile strength at higher EGMF content may be attributed to various factors, including the presence of fibres that act as stress concentrators or disruptors within the concrete matrix⁴⁷.

Fig. 12.

Split tensile strengths of concrete reinforced with various percentages of EGMF.

Building on earlier studies, adding 0.50% and 1% of untreated oil palm broom fibre to the concrete mix increases its tensile strength by 3.50% and 1.20%, respectively¹⁸. According to a similar study, adding 2% palm fibre to the oil palm shell concrete mix increases its tensile strength by about 11%, compared to the concrete mix without fibre⁴⁸. Therefore, this improvement highlights the potential of EGMF as a sustainable option for resisting tensile stresses.

Flexural strength

Flexural strength is a crucial parameter in assessing a material’s ability to withstand bending or flexural stresses. Figure 13 presents the results of the flexural strength of concrete reinforced with different percentages of EGMF at 28 and 56 days of curing. The results demonstrated an improvement in flexural strength with the addition of EGMF to the concrete mix. For instance, there were 2%, 6%, 3%, and 2% increases in flexural strength at 0.5%, 1%, 1.5%, and 2% EGMF, respectively, after 28 days of curing, compared to the sample without EGMF. Similar trends were observed after 56 days of curing, with increases of 2%, 6%, 2%, and 1% in flexural strength at 0.5%, 1%, 1.5%, and 2% EGMF, respectively. Interestingly, at 1% EGMF, a 6% increase in flexural strength was observed at 28 and 56 days of curing compared to the control sample (mix without EGMF). This implies that a moderate percentage of EGMF reinforcement (1% optimal) can improve concrete’s flexural strength, possibly due to the reinforcing action of the matrix fibre. The improvement can be attributed to the EGMF’s high internal adhesion, which produces a densely cohesive matrix, thereby increasing its ability to support loads under bending².

Fig. 13.

Flexural strengths of concrete reinforced with various percentages of EGMF.

These results corroborate other relevant studies that reported a higher flexural capacity of concrete reinforced with natural fibres. For example, a 1% bamboo fibre-based concrete achieved a flexural strength of 6.80 MPa, indicating an approximately 31% improvement, compared to the sample without bamboo fibre⁴⁹. Another study found an improvement in the flexural strength of oil palm shell-based concrete reinforced with pineapple leaf fibre⁴⁵. A 13% improvement in flexural strength was observed when 1% treated hemp fibre was used as a reinforcing agent in concrete⁵⁰. Therefore, this study suggests that adding 1% EGMF to the concrete mix is ideal, highlighting its advantages as a reinforcement and demonstrating its potential as a natural fibre to improve concrete’s flexural performance.

Bond strength

Bond strength refers to the adhesion between the reinforcement (e.g., EGMF) and the surrounding concrete matrix, which is crucial for ensuring structural integrity and load-bearing capacity. Figure 14 presents the bond strength of treated and untreated EGMF reinforced concrete at two curing days of 28 and 56. The results indicated that concrete reinforced with treated EGMF exhibited higher bond strength than its untreated counterpart. At 28 and 56 days of curing, the bond strength of treated EGMF-based concrete, as shown in Fig. 14, was approximately 26% higher than that of the untreated EGMF-based concrete. This indicates that the treatment applied to the fibres has positively influenced the bond strength between the fibres and the concrete matrix. The surface of oil palm midrib fibres is rough and irregular. This increases friction and mechanical interlock with the adjacent cement paste, thereby improving bonding. During hydration, the fibres–matrix interface becomes denser partly due to internal water supplied by the fibres that absorbed water prior to mixing. The enhanced adhesion improves bond behaviour⁵¹. The results, as shown in Fig. 14, suggest that the treatment applied to the EGMF enhances their compatibility with the concrete matrix, resulting in improved adhesion and bonding². A related study further stated that the confinement effect enables natural fibres to act as micro-reinforcements of the concrete matrix. This significantly reduces the risk of splitting cracks under pullout or shear stresses, which are typical failure modes at the fibres-matrix interface⁵². Ultimately, concrete reinforced with treated EGMF improves bond strength by enhancing interfacial friction, confining the matrix, and redistributing tensile stresses, which are critical under bond conditions.

Fig. 14.

Bond strengths of concrete reinforced with treated and untreated EGMF.

Modulus of rupture (MoR)

MoR measures a material’s resistance to bending or flexural stresses, indicating its strength under applied loads. Figure 15 illustrates the MoR of concrete slabs reinforced with untreated and treated EGMF. The MoR, as displayed in Fig. 15, was 2.60 MPa for the control concrete slab produced without EGMF. This indicates the concrete’s initial flexural strength before adding any fibres. The MoR values of the treated and untreated EGMF reinforced concrete slabs were found to be higher than those of the control concrete slab. The untreated and treated EGMF reinforced concrete slabs yielded about 12% and 21% higher MoR values, respectively, than the control concrete slab. These results indicate that adding either treated or untreated EGMF as reinforcement to the concrete mix improves the MoR of the concrete slab. Interestingly, the MoR of the treated EGMF-based concrete slab was about 10% higher than that of the untreated EGMF-based concrete slab. This suggests that the treatment applied to the EGMF further improves the MoR. The increase in MoR values suggests that EGMF reinforcement effectively increases the concrete’s resistance to bending (flexural) stresses. These results confirm the findings of a similar study, where the MoR of fibreboard produced from untreated EFB at a resin content of 8% was approximately 14% lower than that of the treated samples with a NaOH solution²².

Fig. 15.

Modulus of rupture of concrete slabs reinforced with treated and untreated EGMF.

Fibres distribute stress concentrations and ensure that any brittle fracture occurs slowly and in a stepwise manner, thus substantially increasing the apparent modulus of rupture. Furthermore, after cracking, the fibres continue to hold the surrounding matrix together, thereby increasing toughness, energy absorption, and modulus of rupture⁴³. Notably, treated EGMF emerges as a promising material for light-loading applications requiring moderate MoR.

Modelling

Figure 16 shows the correlation between the split tensile and compressive strengths of treated EGMF-based concrete with curing days ranging from 7 to 56 and EGMF dosages ranging from 0% to 2%, respectively. The characteristic cylinder compressive strength for mix design purposes was based on 20 MPa. The performance metrics, as shown in Fig. 16, indicate a strong correlation, as evidenced by the low standard deviation (S = 0.175935) of the data points around the fitted value. Moreover, the coefficient of determination (R²) of 0.903 is closer to 1⁵³.

Fig. 16.

Relationship between the split tensile strength and compressive strength of concrete reinforced with treated EGMF.

EGMF-based concrete presents significant differences from conventional concrete. It is important to state that the performance indicators generated between the split tensile and compressive strengths can be affected by several variables, which interact in complex ways: fibre volume fraction, fibres aspect ratio (length/diameter), water absorption of the fibres, age of specimens, curing conditions, and dispersion and orientation of fibres within the concrete matrix. Finding suitable properties for a concrete experiment by trial and error is a time-consuming and costly task. Modelling is, therefore, a powerful tool for predicting strength as a function of independent variables and for subsequently determining optimal ranges for practical applications. Ultimately, the developed model, as illustrated in Fig. 16, can predict the relationship with 95% confidence interval (CI) and a prediction interval (PI).

Conclusion

This study explored and treated Elaeis Guineensis midrib fibres (EGMF) with 1–5% Ca(OH)₂ solutions for 6, 12, and 24 h. The treated EGMF, which exhibited superior tensile strength performance, was added to the total volume of concrete at a dose of 0.5-2%, and its effects on the mechanical properties of the concrete were investigated. Based on the experimental findings, the following conclusions were drawn:

• Incorporating 0.5-2% of EGMF reduces the concrete’s slump by 10–44%.

• A 24-hour treatment of EGMF with 4% of Ca(OH)₂ solutions results in about 15% and 20% higher tensile strengths compared to 12 and 6 h of treatment, respectively.

• Replacing concrete volume with 0.5%, 0.5%, and 1% optimum contents of treated EGMF enhances its compressive, split tensile, and flexural strengths by 3%, 9%, and 6%, respectively, after curing for 28 days.

• Bond strength and MoR of treated EGMF-based concrete samples outperformed untreated EGMF-based concrete samples by 26% and 10%, respectively.

• The relationship between the split tensile and compressive strengths of EGMF-based concrete yielded a strong correlation, with 90.30% R² at 95% CI and PI.

This study focuses on a novel type of reinforcement material, EGMF, sourced from the fresh central midrib of oil palm broom fibres, in contrast to the widely studied OPBF derived from aged and degraded leaflets. The controlled extraction conditions for EGMF yield consistent geometry and quality, minimising issues such as surface impurities and variability that plague OPBF. By providing a detailed characterisation of EGMF, the research enhances existing knowledge in the field and addresses the performance limitations identified in previous studies of oil palm fibre types, thereby advancing the utilisation of oil palm resources. This research also provides an experimental database on the properties of concrete reinforced with EGMF, thereby improving its mechanical properties.

Limitations and recommendations

This study was limited to the mechanical performance of concrete reinforced with Elaeis Guineensis midrib fibres after 7–56 days of curing. Future studies can investigate the durability, microstructure, and thermal properties of concrete reinforced with EGMF.

Abbreviations

CA

Coarse aggregates

Ca(OH)₂

Calcium hydroxide

EGMF

Elaeis Guineensis midrib fibres

FA

Fine aggregates

MoR

Modulus of rupture

PLC

Portland limestone cement

Author contributions

S.O. conceptualised and designed the study, collected the data, analysed and interpreted the results, S.O, Y.A., M.A., M.A, and S.O. analysed the data with software, and wrote and reviewed the manuscript. All authors have read and agreed to the published version of the manuscript.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.