The Potential of Sisal Fiber as an Eco-Friendly Concrete Reinforcement in East Nusa Tenggara-Indonesia

Rapid infrastructure development in regions like North Central Timor or Timor Tengah Utara, TTU (in Indonesia), NTT Province - Indonesia demands high-quality, economical, and sustainable building materials. Concrete is the primary choice in construction due to its ease of shaping, high compressive strength, and relatively low cost when using local raw materials [1]. However, conventional concrete has fundamental weaknesses, namely low tensile strength and ductility, so it requires reinforcement, usually steel, to withstand tensile forces [2]. Innovations in concrete technology continue to develop to overcome these limitations, one of which is the addition of fiber, known as fiber concrete. The addition of fiber aims to improve the mechanical properties of concrete, especially tensile strength and ductility, and provide superior post-cracking energy absorption capabilities.

Developing sustainable construction materials is a priority to reduce environmental impacts and efficiently utilize local natural resources. One promising approach is using natural fibers as reinforcing materials in concrete. Various studies have investigated natural fibers such as hemp [3-5], kenaf [6], banana [7, 8], and coconut fiber [9-11], showing significant potential. Natural fibers improve the brittle properties of cementitious composites, inhibit microcrack propagation, and enhance overall durability. Each type of fiber has unique characteristics that influence its performance in the concrete matrix. Adding natural fibers fundamentally improves the brittle structure of cementitious composites, providing enhanced ductility and superior post-cracking energy absorption capacity. This mechanism effectively inhibits the initiation and propagation of microcracks, thus preventing sudden failure and enhancing the overall durability of the composite. Among various natural fibers, sisal fiber (Agave sisalana) stands out for its strength, durability, and ability to grow in dry or degraded lands [12, 13]. Sisal fiber is abundant in dry climates such as East Nusa Tenggara, particularly in TTU Regency, but has not been optimally utilized [14]. This situation creates an excellent opportunity to integrate sisal fiber into the local construction industry, which improves material quality and supports the local community's economic development.

On the other hand, improving the mechanical properties of fiber-reinforced concrete must be examined at the macro and microstructural levels, particularly in the interaction between the fibers and the cement matrix. Microscopy, particularly the scanning electron microscopy (SEM) method, provides essential evidence for this analysis [15, 16]. SEM has become a crucial technique in materials research for investigating the interactions between fibers and the matrix. This type of microstructural analysis allows researchers to visualize the fiber-matrix interface zone, known as the interfacial transition zone (ITZ), at the micrometer scale. The ITZ is a critical area prone to microcracking due to its brittleness, and a strong interface between the fibers and the cement matrix is vital for effective stress transfer. SEM findings indicate that sisal fiber-reinforced concrete exhibits a denser microstructure, increasing its strength and durability.

In this regard, several studies have confirmed sisal fibre's important role in improving concrete's mechanical properties. However, there is significant variation in results between studies, depending on factors such as the quality of the base concrete and the fiber treatment. A study by Buttignol et al. [17] showed that in Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC), fiber acts as a ductility-enhancing agent, not a compressive strength enhancer. In contrast, in this study, which used normal grade concrete (K-125), sisal fiber acted as a primary reinforcing agent, significantly increasing the compressive strength to meet structural standards. Furthermore, a study by Eid et al. [18] reported that chemical treatments, such as alkali treatment, can substantially increase the compressive strength (up to 45.4%).

In contrast, this study used local sisal fiber without chemical treatment, so the results reflect the baseline performance of sisal fiber in TTU. This highlights the potential that can be achieved through pretreatment. In addition, a decrease in concrete workability with fiber addition, as measured by a slump test, is also a common finding. A study by Thomas and Jose [13] explained that this decrease was caused by the hydrophilic nature of the fiber which absorbs water. However, this water absorption can be a beneficial mechanism, namely internal curing, which helps strength development and reduces shrinkage cracks later on, especially in dry environments such as NTT.

Regarding sustainability and recycled aggregates, Lyu et al. [19] investigated the effect of sisal fiber on concrete made with recycled aggregates (RAC) in the context of sustainability. This study introduced a significant variable absent in the TTU study: the type of aggregate used. While the TTU study utilized locally sourced natural aggregates, Lyu et al. [19] focused on recycled aggregates, representing a step towards more sustainable construction practices. Although specific data from these studies are limited, they illustrate the broader context of sisal fiber research, which combines natural and recycled materials to develop environmentally friendly construction materials.

This paper specifically examines the feasibility and impact of adding sisal fiber sourced from Eban Village, TTU, on the compressive strength of normal-grade concrete. Unlike previous studies on other concrete products, such as paving blocks [14], this study employs standard cube specimens to assess compressive strength. By concentrating on sisal fiber sourced locally from TTU, this research contributes uniquely to the literature, as the characteristics of the fiber can vary based on geographic location and growing conditions. Additionally, this study addresses technical questions related to improving concrete quality and emphasizes the potential for utilizing underexplored natural resources in the area, aligning with sustainable development goals.

3.1 Research method

This research utilized a quantitative experimental approach conducted at the civil engineering materials testing laboratory of the Kupang State Polytechnic. All testing procedures, including material preparation, mix design, specimen fabrication, and concrete mechanical property testing, were performed meticulously and in accordance with the Indonesian National Standard (INS) or SNI (in Indonesia) and relevant international standards. This adherence ensured the accuracy, validity, and reproducibility of the results. The controlled laboratory environment allowed for close monitoring of testing conditions, minimizing the influence of unwanted external variables.

3.2 Materials used

The primary materials used in the manufacture of concrete test specimens include:

Cement: Type I Portland cement is the concrete mix's binder.

Fine aggregate: Natural sand sourced from the Takari area of NTT is utilized. Before use, the fine aggregate undergoes testing to determine its properties, including sieve analysis by SNI 03-1968-1990 and specific gravity and water absorption tests following SNI 03-1970-1990. The fine aggregate is verified to be clean and free from organic matter and other hazardous substances.

Coarse aggregate: Natural crushed stone is used as the coarse aggregate. Its characteristics are assessed through sieve analysis based on SNI 03-1968-1990 and specific gravity and water absorption tests by SNI 03-1969-1990. The coarse aggregate is confirmed to meet the required gradation and durability standards.

Water: The water used for mixing the concrete and curing the specimens is clean and complies with water quality standards for concrete, as specified by ASTM C1602/C1602M. It is ensured to be free from oil, acids, alkalis, salts, organic materials, and any other substances that could harm the concrete or reinforcement.

Sisal fiber (Agave sisalana): Sisal fiber is incorporated as additional reinforcement. This fiber comes from sisal plants that grow abundantly in Eban Village, TTU Regency, NTT Province. The fiber extraction process is conducted traditionally by soaking (retting) to soften the outer skin, followed by manual raking to separate the fibers without causing damage. After extraction, the fibers are cleaned, dried, and cut to a uniform length of 30 mm to ensure a consistent distribution within the concrete mixture. Figure 2 shows sisal fiber in wet condition (Figure 2(a)) and in dry condition (Figure 2(b)).

3.3 Mix design

The concrete grade planned for this study is K-125, equivalent to a characteristic compressive strength (fc') of 9.8 MPa. The concrete mix composition refers to SNI 7394-2008 for regular concrete. SNI 7394-2008 establishes various concrete quality classifications based on its characteristic fc’ in Mega Pascals (MPa) or K value (kg/cm²). Table 1 shows the mix proportions for 1 m³ of concrete: cement = 276 kg, sand = 828 kg, gravel = 1012 kg, and water = 215 L, with a water-cement ratio (w/c) 0.78.

(a) Wet condition            

(b) Dry condition

Figure 2. Sisal fiber

Table 1. Concrete mix composition based on SNI 7394-2008 [22] 

The test specimens used in this study were cube-shaped with dimensions of 15 cm × 15 cm × 15 cm. The nominal volume of one concrete cube is 0.003375 m³. Based on the proportions for 1 m³ of concrete, the material requirements for one cube of the test specimen are calculated as follows:

• Cement: 276 kg/m³ × 0.003375 m³ = 0.9315 kg
• Sand: 828 kg/m³ × 0.003375 m³ = 2.7945 kg
• Gravel: 1012 kg/m³ × 0.003375 m³ = 3.4155 kg
• Water: 215 L/m³ × 0.003375 m³ = 0.7256 L

The percentage of sisal fiber added to the concrete mix is calculated based on the weight of the cement, namely 0% (as a reference/control mix), 0.5%, 1%, 1.5%, and 2%. For each variation of fiber percentage, 5 test specimens were made, so 25 specimens were prepared for the compressive strength test. Table 2 details the sisal fiber requirements for five cubes of test specimens in each variation, with the total cement weight for five cubes being 4.6575 kg.

Table 2. Sisal fiber requirements per 5 cubes of test specimen

3.4 Test specimen preparation

The process of preparing test specimens involves mixing the concrete mixture and forming the specimens, which is carried out through the following steps:

1. Material dosing: All materials (cement, sand, gravel, water, and sisal fiber) are accurately measured according to the specified proportions in the mix design.
2. Mixing the mixture:

• Cement and sand are placed in a concrete mixer and mixed until they form a homogeneous blend.
• Coarse aggregate (crushed stone) is added, and the mixer is rotated until all components are thoroughly combined.
• Sisal fiber is gradually introduced into the mixer while it rotates, then gradually adding water until all the water is incorporated.
• The mixer is rotated for approximately 10 minutes to ensure all components are evenly and homogeneously mixed. The mixer is occasionally stopped and turned throughout this process to maintain the mixture's homogeneity.

3. Workability testing: Once the mixture is homogeneous, a slump test is performed on a portion of it to assess workability. During the slump test, the mixer continues to rotate to maintain the homogeneity of the remaining mixture. The required slump value for this study is between 25 and 75 mm, according to standards for pavements and floor slabs.
4. Casting of test specimens: The homogenized concrete mixture is cast into molds measuring 15 cm × 15 cm × 15 cm. Compaction is performed according to standards (e.g., by tamping or vibration) to eliminate air voids and ensure optimal density.
5. Curing: All test specimens are cured using a water immersion (water curing) immediately after casting, continuing until 28 days after testing. This curing method follows SNI 03-2834-2000 to ensure optimal cement hydration, allowing the concrete to achieve its design strength.

3.5 Concrete compressive strength testing

The testing procedure adheres to the SNI 03-1974-1990 or ASTM C39/C39M standards and follows these steps:

1. Sample preparation: The concrete cubes are removed from the soaking bath and then air-dried or wiped dry. Each concrete sample's weight is measured and recorded. A visual inspection is conducted to identify any defects in the test specimen.
2. Sample placement: The concrete sample is placed centrally on the pressure plate of the compression testing machine, which can be a Universal Testing Machine or a Compression Testing Machine.
3. Load application: The testing machine is powered on, and the compressive load is applied continuously and smoothly, without shock, at a constant loading rate (e.g., 0.25 ± 0.05 MPa/s, as recommended by ASTM C39/C39M) [23], until the test specimen fails.
4. Recording results: The maximum load reached at the time of failure is recorded. The compressive strength of the concrete is calculated by dividing the maximum load by the cross-sectional area of the test specimen. Additionally, the crack pattern and type of failure are documented.

Research consistently shows that adding sisal fiber positively impacts the compressive strength of concrete. Concrete without sisal fiber only achieved a compressive strength of 7.64 MPa, which falls short of the K-125 quality standard of 9.8 MPa. However, with the incorporation of sisal fiber, the compressive strength significantly increased, reaching 9.47 MPa at 0.5% fiber content, 12.44 MPa at 1% fiber, 11.93 MPa at 1.5% fiber, and peaking at 12.56 MPa at 2% fiber. This increase suggests that sisal fiber is a bridging mechanism within the concrete matrix. When microcracks develop under compressive loading, the sisal fiber can bridge these cracks, delay their propagation and distribute stress more evenly throughout the material. This increase results in concrete's ability to withstand loads before experiencing brittle failure due to compressive strength. Overall, these findings demonstrate that incorporating sisal fiber significantly enhances the compressive strength of concrete. Notably, concrete reinforced with sisal fiber at 0.5% or greater not only meets but surpasses the K-125 quality standard, something that standard concrete without fiber fails to achieve. This underscores the practical potential of utilizing local sisal fiber to produce concrete with superior structural qualities.

Despite the overall increasing trend, a slight decrease in compressive strength was observed at 1.5% fiber (11.93 MPa) before it increased again at 2% fiber (12.56 MPa). This decrease may be attributed to the heavier average weight of sample D (7601.2 grams) compared to sample C (7591.3 grams) (Table 8). This phenomenon suggests that there may be an optimal point for fiber dispersion or that challenges in achieving homogeneity arise at higher fiber concentrations. When fiber is added in excess without proper mixing techniques, it can lead to issues such as fiber balling or increased void content due to a significant reduction in workability, ultimately diminishing the effectiveness of fiber reinforcement. While the study found that sisal fiber did not ball at a 0.5% addition, dispersion challenges might emerge at higher percentages. This could explain the slight drop in performance at 1.5%, followed by improved results at 2%, which may indicate better dispersion or a more prominent reinforcement effect at that level. An important finding of this study is that compressive strength exceeded the K-125 standard (9.8 MPa) across all sisal fiber variations from 0.5% to 2%. This indicates that locally sourced sisal fiber from TTU Regency can effectively produce concrete with structural qualities that meet or surpass standard requirements. This makes it a viable and superior alternative to conventional, fiber-free concrete tested in this study.

Furthermore, using sisal fiber as reinforcement in concrete has been widely researched worldwide. This study aligns with existing literature, which reports improvements in the mechanical properties of concrete due to the addition of sisal fiber. For instance, Lyu et al. [19] and Thomas and Jose [13] also observed that sisal fiber enhances the compressive strength of concrete. Eita and Attia [24] specifically explored how the content and length of sisal fiber affect concrete's compressive and tensile strength, demonstrating that these factors are crucial for performance.

This study makes a unique contribution by validating the performance of sisal fiber sourced from the TTU Region in NTT, Indonesia. While many studies on sisal have been conducted in various parts of the world—such as the case study by Ferraz-Almeida et al. [25] in Brazil, which characterized sisal fiber from different regions—the properties of natural fibers can vary significantly based on climate, soil type, and local processing methods. Therefore, this characterization and validation of sisal fiber from TTU Regency addresses a regional-specific knowledge gap, confirming that the potential of this local resource can be effectively utilized.

For comparison, in a previous study in 2024, Asrial et al. [14] showed an increase in compressive strength by adding 3-7% sisal fiber mixture, confirming that sisal fiber is a very effective reinforcement. The results of the study showed that the compressive strength of paving blocks with the addition of 3%, 5%, and 7% sisal fiber, with an average compressive strength of 11.85 MPa, increased compared to using 8.9 MPa fiber, or an increase in compressive strength of 2.95 MPa was obtained. However, it was emphasized that the maximum addition of sisal fiber was up to 3%, if it exceeded the average compressive strength would decrease. Meanwhile, in this study, with the addition of 0-2% sisal fiber, a higher compressive strength of 12.56 MPa was obtained in cube test specimens. This means adding 2% sisal fiber mixture is the optimal average strength in concrete. In addition, direct comparisons between studies need to consider differences in the type of test specimen (cube vs. cylinder) and the quality of the base concrete. However, both studies showed the positive effects of natural fiber.

Several other studies have explored sisal fibre surface treatments to improve chemical bonding, such as Na₂CO₃ [26], Sodium Hydroxide (NaOH) [27] or combinations of sisal with other materials, such as plastic waste [28], and nano-silica [29], for better performance. This study, which focused on traditionally processed sisal fiber without any special chemical treatment, provides a solid foundation for understanding the performance of local sisal fiber in its “native” condition and paves the way for further research that could explore bonding enhancement methods or material combinations for further optimization. Limitations of this study include the primary focus on compressive strength; testing of other mechanical properties, such as flexural tensile strength and splitting tensile strength, has not been conducted. Furthermore, this study did not include long-term durability analysis or the behavior of sisal fiber concrete under extreme environmental conditions. Future research could expand the scope of mechanical property testing, optimize fiber aspect ratio, evaluate fiber surface treatments, and conduct durability studies [30-32] and life cycle analysis for a more comprehensive sustainability assessment.