Study on the mechanical properties of plant fiber reinforced concrete: the relationship between incorporation methods and performance

Plant fiber-reinforced concrete is gaining increasing attention due to its potential to enhance sustainability and improve mechanical properties. Among various plant fibers, reed fibers have shown promise in reinforcing concrete. However, the impact of different fiber addition sequences on the mechanical performance of concrete is not yet fully understood. This study investigates the effect of different addition sequences of reed fibers on the mechanical properties of concrete. Ordinary Portland cement was used as the base material, incorporating 0.5% by volume of 20 mm-long reed fibers. Four different addition sequences were designed: dry fiber pre-mix, dry fiber post-addition, surface-dried saturated fiber pre-mix, and surface-dried saturated fiber post-addition. Compressive strength, splitting tensile strength, and flexural strength tests were performed to evaluate the mechanical properties and strength development of concrete. The results indicate that the surface-dried saturated fiber post-addition method exhibited the optimal mechanical performance. This method improves fiber-cement particle interaction during the mixing stage, leading to a more stable interfacial bond and higher strength in compressive, flexural, and splitting tensile tests. Based on the experimental results, this study identifies surface-dried saturated post-addition as the most suitable addition sequence for reed fiber reinforced concrete, offering valuable insights for optimizing production processes and enhancing the performance of plant fiber reinforced concrete.

1 Introduction

Concrete, as one of the most widely used building materials in the world, plays a vital role in construction engineering due to its excellent compressive strength and cost-effectiveness (Qin et al., 2024; Shafig et al., 2018). Whether used in infrastructure construction, residential and commercial buildings, or transportation facilities, concrete has long been the primary material for modern construction due to its good performance and ease of construction (Lu, 2023). However, despite its excellent compressive strength, traditional concrete still has some limitations, especially in terms of tensile and flexural properties (Mujalli et al., 2022; Li and Li, 2019). Traditional concrete is highly brittle and prone to cracking when subjected to external loads, environmental changes, or chemical corrosion (Li and Li, 2019; Aslani and Nejadi, 2012; Windisch, 2021). These cracks not only affect the appearance and structural stability of concrete but also may lead to a decrease in its long-term durability (Muthukumarana et al., 2023). To address these issues, many researchers have focused on improving concrete performance through various reinforcement techniques, particularly enhancing its crack resistance, ductility, and impact resistance.

Fiber-reinforced concrete is an emerging concrete enhancement technology developed in recent years (Zhang et al., 2023; Tan et al., 2025; Mahmood et al., 2021). By incorporating different types of fibers (such as steel fibers, polymer fibers, plant fibers, etc.) into concrete, it can significantly improve the crack resistance, tensile strength, flexural strength, and impact resistance of concrete, while also enhancing its toughness, durability, and seismic performance. Compared to traditional reinforced concrete, fiber-reinforced concrete not only performs excellently in terms of tensile and flexural strength but also effectively inhibits the propagation of cracks, significantly improving the crack resistance of concrete. This material shows remarkable advantages, particularly in applications involving seismic resistance, impact resistance, and high-strength performance (Ahmed, 2023; Afroughsabet et al., 2016; Yoo and Banthia, 2017). In recent years, fiber-reinforced concrete has been widely applied in modern construction and infrastructure projects, especially in areas where enhanced ductility and crack resistance are required, demonstrating promising application prospects.

However, despite the certain applications of steel fibers, polymer fibers, and other plant fibers in the field of concrete reinforcement, they still have some drawbacks (Patil, 2025; Abbass et al., 2018). Steel fibers are typically costly and require special handling during processing and transportation, leading to an overall increase in costs (Aidarov et al., 2022). While polymer fibers offer good corrosion resistance and lightweight properties, their poor high-temperature performance limits their application in high-temperature environments (Ünverdi et al., 2025; Liu et al., 2025; Ulu et al., 2022). Some plant fibers (Ah et al., 2022; Ikhine et al., 2025; Jahami et al., 2024), such as bamboo fibers, have excellent renewability and environmental friendliness, but their mechanical properties, interface bonding, and fiber dispersion are poor, which affects their effectiveness in concrete (Noori et al., 2021; Li et al., 2025; Beskopylny et al., 2025). Therefore, the development of alternative materials with superior performance and cost-effectiveness has become a key research direction for enhancing the properties of concrete.

Reed fiber, as a rapidly growing and abundant plant, has gradually become a research focus in the field of concrete reinforcement (Wu et al., 2023; Jiménez-Espada et al., 2021; Ramadan et al., 2023). The advantages of reed fiber lie not only in its low cost, environmental friendliness, and renewability, but also in its high tensile strength, strong crack resistance, and good toughness (Koosha et al., 2024; Shon et al., 2019; Suárez et al., 2023). With the increasing demand for green building materials, the application of plant fibers in concrete has been continuously promoted, especially the potential of reed fiber in enhancing the mechanical properties and durability of concrete (Wang et al., 2024). Although research on the application of reed fiber in concrete is still in its preliminary stages, existing preliminary experimental results show that reed fiber holds promise in improving concrete’s crack resistance, flexural strength, and ductility, offering significant application prospects (Koosha et al., 2024; Shon et al., 2019).

Badagliacco et al. found that the appropriate incorporation of reed fiber can significantly improve the crack resistance, flexural strength, ductility, and durability of concrete (Badagliacco et al., 2020; Caponetto et al., 2023). Reed fiber not only effectively prevents the propagation of microcracks in concrete but also maintains good stability under environmental conditions such as high temperature and high humidity (Badagliacco et al., 2020; Caponetto et al., 2023). Additionally, the renewability and low carbon emission characteristics of reed fiber make it an ideal material that aligns with the goals of green building and sustainable development (Badagliacco et al., 2020; Caponetto et al., 2023). With the increasing demand for sustainable building materials, the application of reed fiber in concrete holds great promise, especially in hot and humid environments, where it can enhance the weather resistance and corrosion resistance of concrete, thereby extending the service life of building structures.

From a sustainability perspective, the use of reed fibers may provide additional environmental benefits beyond mechanical enhancement. Reed is a rapidly renewable biomass resource that can be locally sourced and potentially valorized from routine harvesting or management activities, thereby offering a potential pathway to reduce reliance on energy-intensive conventional fibers (Suárez et al., 2023; Faruk et al., 2014). While a full life-cycle assessment was outside the scope of the present work, we include this LCA-oriented remark to contextualize the significance of reed-fiber utilization; future work will incorporate LCA-oriented metrics such as CO₂ footprint and embodied energy to quantify the environmental implications alongside mechanical performance (Manso-Morato et al., 2024).

Although plant fibers have significant advantages in enhancing the properties of concrete, their incorporation methods and processes still exhibit considerable variability, especially the addition method of reed fibers, which has not been extensively and thoroughly studied (Warsi et al., 2023; Wang et al., 2021; Özkılıç et al., 2023). Existing incorporation methods often face issues such as poor fiber dispersion and insufficient interfacial bonding between fibers and the cement matrix. These problems limit the reinforcing effect of fibers in concrete, thereby affecting the overall performance of the material. Therefore, optimizing the fiber addition sequence to enhance the bonding between fibers and the matrix and improve their dispersion has become a key issue in both research and practice for improving the performance of plant fiber reinforced concrete.

This study designs four different fiber addition sequences and systematically analyzes the effect of these sequences on concrete performance through mechanical property tests, including compressive strength, flexural strength, and splitting tensile strength. A comparison of the four addition sequences will reveal the key role of addition sequence in fiber dispersion, interfacial bonding, and the mechanical properties of concrete. The findings will provide theoretical guidance for the process optimization of plant fiber reinforced concrete and offer practical insights for the production processes of green building materials.

2 Materials and methods

2.1 Materials

Reed is widely available and was mechanically processed into fiber segments with a length of 20 mm. The processed reed fibers had a density of approximately 0.9 g/cm³. The water absorption of the untreated fibers was 66.3%, which increased to 133% after 3% NaOH treatment. To modify the fiber surface and enhance fiber–matrix interfacial interaction, a 3% NaOH solution was used as a common alkali treatment. After treatment, the fibers were thoroughly rinsed to remove residual solution and then oven-dried to constant mass. The dried fibers were stored in sealed bags prior to mixing. The fiber preparation procedure and representative SEM morphology after treatment are presented in Figures 1, 2. Given that this study targets concrete-level performance, the treatment effect is quantified here mainly in terms of water absorption and SEM-observed surface morphology.

Figure 1

Figure 1. Process diagram of reed fiber.

Figure 2

Figure 2. Microscopic view of reed fiber (a) Untreated reed fiber (b) Treated reed fiber.

The cement used in the experiments was P.O 42.5R ordinary Portland cement. The coarse aggregate consisted of graded crushed stone with particle sizes ranging from 5 to 20 mm, while the fine aggregate was natural river sand with a fineness modulus of 2.73.

2.2 Experimental design

This study is based on reed fibers with a length of 20 mm and a volume fraction of 0.5%, and it designed four different addition sequences along with a control group to investigate the effect of reed fibers on the properties of concrete. The specific design of each experimental group is as follows.

1. Dry Pre-mix Method: Dry reed fibers are pre-mixed with cement to ensure full contact between the fibers and cement at the initial stage. Subsequently, sand, aggregates, and water are added and mixed in the usual sequence to form concrete. This method is used to study the bonding effect between dry fibers and the cement matrix. For convenience in subsequent discussions, this method is referred to as S1.

2. Dry Post-addition Method: Cement, sand, and water are first mixed to form a paste, then dry fibers are added and the mixture is continued to be stirred, followed by the addition of coarse aggregates. This method simulates the actual construction process and aims to investigate the impact of post-adding dry fibers on concrete performance. For convenience in subsequent discussions, this method is referred to as S2.

3. Surface-Dried Saturated Pre-mix Method: Reed fibers were pre-wetted to a saturated condition by soaking them in water for 24 h at room temperature to ensure complete saturation. After soaking, the fibers were drained on filter paper to remove excess water and air-dried for approximately 30 min to reach a surface-dried condition. The fibers were considered to have reached the surface-dried saturated (SSD) state when the mass change between two consecutive measurements was less than 0.1%. After draining and blotting, the fibers were weighed to verify the SSD state and were pre-mixed with cement. The absorbed water was not included in the total mixing water, and the added water was adjusted accordingly in the concrete mix design. For convenience in subsequent discussions, this method is referred to as S3.

4. Surface-Dried Saturated Post-addition Method: Cement, sand, and water are first thoroughly mixed, followed by the addition of surface-dried saturated fibers and continued stirring, and finally, coarse aggregates are added. This method is used to study the effect of adding wet fibers at the later stage of concrete mixing, particularly the impact on fiber distribution and interfacial bonding. For convenience in subsequent discussions, this method is referred to as S4.

5. Plain Concrete (No Reed Fiber Added): As the control group, a conventional concrete mix without reed fibers is used to provide baseline data for evaluating the effect of fibers on concrete performance. For convenience in subsequent discussions, this method is referred to as PC.

The detailed parameters of the concrete ratio are shown in Table 1. Water denotes the added mixing water. For S1-S2, an extra 5.99 kg/m³ was added to compensate for dry-fiber absorption based on the measured absorption and fiber dosage. For S3-S4, the fibers were in a surface-dried saturated condition and therefore no additional water compensation was required.

Table 1

Table 1. Concrete mix ratio (unit: kg/m³).

2.3 Experimental method

As shown in Figure 3, compressive strength tests were conducted using a 2000 kN electro-hydraulic servo universal testing machine. The specimen dimensions were 100 mm × 100 mm × 100 mm. Loading was applied under load control at a constant loading rate of 5 kN/s. The maximum failure load was recorded at ultimate load, and the compressive strength was calculated accordingly.

Figure 3

Figure 3. Experimental device (a) Compressive strength experimental (b) flexural strength experimental (c) Splitting tensile strength experimental.

Flexural strength tests were performed using a three-point bending method with beam specimens of 100 mm × 100 mm × 400 mm and a support span of 300 mm. The load was applied at mid-span under load control at a constant loading rate of 0.5 kN/s. Loading continued until fracture or through-crack formation, and the failure load was recorded to calculate the flexural strength.

Splitting tensile strength tests were conducted using the standard splitting test method using the same testing machine. The specimen dimensions were 150 mm × 150 mm × 150 mm. Loading was applied under load control at a constant loading rate of 1.125 kN/s. The failure load was recorded and used to calculate the splitting tensile strength.

2.4 Qualitative evaluation of fiber dispersion

Fiber dispersion was qualitatively evaluated using the fracture surfaces of flexural beam specimens after testing. For each mix, three beams were examined (n = 3). A four-level dispersion grade (0–3) was assigned based on the presence and size of visible fiber clusters: 0 (uniform, no visible clusters), 1 (minor clustering, maximum cluster diameter<5 mm), 2 (moderate clustering, 5–10 mm and/or local fiber-rich zones), and 3 (severe clustering, >10 mm clusters or continuous fiber-rich bands associated with local weak zones). Two investigators independently scored all images; when the difference was≥2 grades, the image was re-evaluated to reach a consensus. The specimen-level grade was calculated as the average of the two regions, and the mix-level grade was reported as mean ± standard deviation.

2.5 Scanning electron microscopy

SEM was used to examine the fiber–matrix interface and the interfacial transition zone (ITZ). At 28 days, representative specimens were selected from each mixture, and small fragments were taken from the interior region. The fragments were dried to constant mass, mounted on aluminum stubs, and coated with a conductive layer prior to observation. For each mixture, at least three specimens were examined, and multiple fields of view were recorded for each specimen.

3 Results and discussions

3.1 Compressive strength

As shown in Figure 4, concrete specimens incorporating reed fibers exhibited some fine cracks on the surface after compression, but the specimens maintained good structural integrity after failure. In contrast, the PC concrete specimens exhibited more extensive fragmentation after compression. This phenomenon indicates that reed fibers effectively inhibited crack propagation through their bridging effect. The improved mechanical performance may be related to enhanced interfacial contact and crack-bridging efficiency, consistent with SEM observations showing reduced interfacial voids and more continuous hydration-product coverage around the fibers, which enhanced the toughness of the concrete, allowing it to effectively limit further crack extension during loading failure. The presence of fibers dispersed the stress effectively through the bridging action, preventing brittle fracture in localized areas and maintaining the overall integrity of the concrete specimens. This not only improved the crack resistance of the concrete but also significantly enhanced its ductility during failure through the fiber bridging effect.

Figure 4

Figure 4. The failure mode of the test block after the compression test.

As shown in Figure 5, all concrete specimens incorporating reed fibers exhibited relatively good early compressive strength, especially S1, S2, and S4, which showed strength values higher than the PC mix. The compressive strengths of S1, S2, and S4 increased by 17.98%–9.08%, 14.54%–2.32%, and 14.76%–0.63%, respectively, at 3 and 7 days of curing. These results indicate that the appropriate incorporation of reed fibers does not negatively affect the early strength of concrete and can, to some extent, enhance its crack resistance and toughness. As the curing time increased, the strength of PC slightly surpassed that of S1 and S4, with improvements of 5.63% and 7.75%, respectively. This suggests that plain concrete may achieve better strength development during long-term hydration. However, S1 still maintained relatively high compressive strength among all the fiber-incorporated groups, indicating that the dry pre-mix method can enhance the bonding between the fibers and the cement matrix to some extent. In contrast, S3 exhibited consistently lower compressive strength and a slower later-age strength development, showing a decrease of 37.77% compared to PC. This behavior is associated with the SSD pre-mix procedure, where the fibers—already containing absorbed water—are introduced at the early mixing stage and may create a locally water-rich environment around the fibers, resulting in a more porous ITZ and less compact hydration product accumulation at the fiber–matrix interface. This interpretation is consistent with the SEM observations in Figure 11c, which show limited C–S–H gel coverage and evident pores at the interface in S3. The porous ITZ weakens stress transfer and contributes to the reduced compressive strength and delayed strength gain of S3.

Figure 5

Figure 5. Comparison of compressive strength of concrete at different curing ages.

3.2 Flexural strength

As shown in Figures 6, 7, the flexural strength test results indicate that all concrete specimens incorporating reed fibers exhibited cracks during the failure process, but no fracture occurred, demonstrating good flexural performance and toughness. Compared to the PC mix, the fiber-incorporated concrete specimens showed a significant advantage in inhibiting crack propagation, with a slower crack growth rate and better overall integrity, displaying optimal flexural strength. This phenomenon suggests that the enhancement in flexural strength is not only due to the bridging effect of the fibers but also closely related to the uniform distribution of the fibers, the effectiveness of the interfacial bonding, and the hydration reaction of the cement matrix. Reed fibers improve flexural performance by providing crack-bridging and post-cracking stress redistribution, thereby delaying rapid crack growth and increasing ductility. SEM images qualitatively suggest a more compact interfacial morphology and improved fiber–matrix contact, which may support load transfer.

Figure 6

Figure 6. The failure mode of the test block after the bending test.

Figure 7

Figure 7. Fiber dispersion in reed fiber concrete mixes.

As shown in Figure 8, the concrete incorporating reed fibers exhibited a significant improvement in flexural strength, particularly in the S4 group, which showed the largest increase in flexural strength among all addition methods. At 28 days, the flexural strength of the S4 group was 56.68% higher than that of the PC group, indicating that the SSD post-addition method effectively optimized fiber dispersion and enhanced the interfacial bonding with the cement matrix. Through SSD treatment, the fiber surface remained adequately moist, ensuring uniform distribution of the fibers in the concrete, which improved the interfacial bonding, enhanced the toughness, and crack resistance of the concrete. This bonding mechanism allowed the fibers to effectively disperse external stresses, suppress crack propagation, and ultimately significantly improve the flexural performance.

Figure 8

Figure 8. The flexural strength after 28 days of standard curing.

The improvement in flexural strength for S1 was relatively small, with an increase of 17.2% compared to the PC. This may be due to the insufficient interfacial bonding formed during the initial contact of dry fibers with the cement matrix, resulting in a weaker interface between the fibers and the cement matrix, which affected the concrete’s crack resistance and toughness. Especially when the fibers are not uniformly dispersed, cracks tend to propagate along these weak interfaces, weakening the fibers’ bridging effect and stress dispersion function. Therefore, although fibers can provide some enhancement in the compressive direction, the ineffective bonding interfaces limited the bridging effect of the fibers in the flexural test, restricting the improvement in flexural strength.

S2 showed a moderate improvement in flexural strength, with an increase of 18.64% compared to the PC. This was mainly due to the use of dry fibers in S2, which absorbed water when in contact with the mortar, altering the water-to-binder ratio and affecting the progress of the cement hydration reaction, thereby influencing the flexural performance of the concrete. Additionally, the poor dispersion of dry fibers led to uneven distribution in the mortar, which could result in fiber agglomeration and the formation of local weak zones. This uneven fiber distribution limited the bridging effect of the fibers, reducing the stress dispersion during crack propagation and further limiting the improvement in flexural strength. Consistent with this interpretation, the qualitative dispersion grades (Table 2; Figure 7) indicate more pronounced clustering in the mixes showing limited flexural improvement, supporting the role of fiber agglomeration and associated local weak zones in controlling flexural performance.

Table 2

Table 2. Summary of qualitative fiber dispersion grade for each concrete mixture.

S3 exhibited the lowest improvement in flexural strength, with an increase of 7.22% compared to the PC. The wet fibers treated with SSD may have already absorbed some water during mixing, and this water reacted preferentially with the cement, leading to the prioritization of cement hydration, which impacted the full progress of the cement hydration reaction. Incomplete hydration of the cement, especially in the later stages, limited the full bonding between the cement matrix and other components, resulting in slower strength gain in S3 in the later stages.

3.3 Splitting tensile strength

As shown in Figure 9, the PC specimens exhibited significant brittle failure, with cracks propagating through the entire specimen, splitting it into two-halves, which displayed lower tensile strength and poor toughness. Although the specimens of S1, S2, S3, and S4 showed visible cracks after the test, they maintained good overall integrity without fragmentation. The incorporation of reed fibers effectively restrained crack propagation and improved the tensile-related response and toughness of the concrete. The enhanced ductility can be attributed to fiber crack-bridging and post-cracking stress redistribution; meanwhile, SEM images qualitatively indicate improved fiber–matrix contact and a denser ITZ, which may facilitate load transfer and delay crack coalescence.

Figure 9

Figure 9. The failure mode of the test block after splitting tensile test.

As shown in Figure 10, concrete incorporating reed fibers generally exhibited an improvement in splitting tensile strength, especially S2, which showed a 51.3% higher splitting tensile strength than the PC mix. This indicates that the dry post-addition method is beneficial for splitting tensile performance. A plausible explanation is that adding dry fibers after the initial mortar mixing stage promotes fiber participation in crack-bridging and stress redistribution during splitting, thereby delaying crack localization and improving specimen integrity. In addition, the SEM observation for S2 (Figure 11b) shows partial hydration-product coverage on the fiber surface with remaining interfacial porosity, which qualitatively suggests improved fiber–matrix contact that may facilitate load transfer.

Figure 10

Figure 10. Splitting tensile strength after 28 days of standard curing.

Figure 11

Figure 11. Microscopic view of reed fiber reinforced concrete (a) S1 (b) S2 (c) S3 (d) S4.

Although S1 demonstrated excellent compressive strength, it performed poorly in the splitting tensile strength test, with tensile strength even lower than that of the PC mix, showing a 3.6% decrease compared to PC. This is closely related to the poor fiber dispersion caused by the dry pre-mix method. During the dry pre-mixing process, the fibers were not evenly distributed in the concrete matrix, resulting in a weak interfacial bond between the fibers and the cement matrix, preventing the fibers from fully performing their bridging effect. The fibers did not form a tight interfacial bond with the matrix, and as a result, in the tensile test, the fibers were unable to disperse enough stress and could not effectively inhibit crack propagation. This weak interfacial bond and uneven fiber dispersion limited the improvement in tensile strength, even causing it to be lower than that of the unreinforced PC mix. This inversion (higher compressive strength but lower splitting tensile strength) can be explained by the different governing mechanisms in compression and tension. Under splitting tension, load transfer and crack bridging rely strongly on fiber dispersion and fiber–matrix adhesion; any weak or porous ITZ promotes interfacial debonding and premature fiber pull-out, thus reducing tensile resistance. In the present study, the qualitative dispersion grade indicates noticeable clustering in S1 (Table 2), and the SEM observation in Figure 11a shows a relatively loose fiber–matrix interface with evident pores and limited C–S–H gel coverage, suggesting a porous ITZ and weaker adhesion. These features may facilitate crack propagation along the interface in splitting tension and limit the effective bridging contribution. In contrast, compressive strength is mainly governed by the matrix load-bearing capacity and is less sensitive to interfacial debonding; fibers can still restrain microcrack growth and improve the integrity of the specimen under compression, which helps explain why S1 can maintain higher compressive strength despite reduced tensile performance.

S3 exhibited a 6.84% improvement in tensile strength compared to PC. The SSD-treated wet reed fibers, when pre-mixed with cement powder, reacted preferentially with the cement during mixing and participated in the cement hydration process early on. This preferential cement hydration affected the full reaction between the cement and other components, resulting in incomplete cement hydration and limiting the amount of water available for hydration reactions with other components. This incomplete hydration weakened the strength development of the cement matrix, leading to a smaller increase in splitting tensile strength for S3 and failing to significantly improve the tensile properties of the concrete.

The tensile strength of S4 was slightly lower than that of S2, but still significantly higher than PC, with a 30.7% increase. By adding the SSD-treated wet fibers into the already formed mortar, S4 ensured uniform fiber distribution in the concrete and effectively avoided the negative impact of fiber agglomeration on tensile properties. Compared to S2, the fibers in S4 were more evenly dispersed within the cement matrix, reducing the unevenness of the interfacial transition zone, thereby enhancing its overall tensile strength. Additionally, the SSD-treated wet fibers formed a tighter interfacial bond with the cement matrix, improving the fiber bridging effect and facilitating the full cement hydration process. The interfacial bond between the wet fibers and the cement matrix was significantly strengthened, allowing the fibers to effectively disperse externally applied stress, inhibiting the rapid propagation of cracks, and improving the concrete’s tensile strength.

3.4 Micro interface analysis

As shown in Figure 11, the effect of different addition sequences on the microstructure of concrete can be observed qualitatively. The S1 image shows a relatively loose bond between the fibers and the cement matrix, with obvious pores on the fiber surface and limited coverage of C-S-H gel, indicating weak bonding between the fibers and the matrix. This structure may lead to insufficient mechanical properties of the concrete, particularly in terms of tensile strength and toughness, which may be poor.

In the S2 image, the fiber surface is partially coated by hydration products. Although the coverage of C-S-H gel has improved, the porosity remains high, indicating that the bond strength between the fibers and the matrix still has room for improvement. In this case, the bonding between the fibers and the matrix has not reached an ideal state, and there are still some structural defects, which may affect the long-term performance of the concrete.

The S3 image shows a weak bond between the fibers and the cement matrix, with very limited coverage of C-S-H gel, appearing only in a few areas and unevenly distributed. The porosity is quite evident, suggesting poor interfacial bonding between the fibers and the matrix, which is likely associated with the mechanical properties of the concrete.

The S4 image displays the best bonding between the fibers and the cement matrix. The fiber surface is completely covered by C-S-H gel, the porosity is further reduced, and the ITZ appears more compact. The bond strength between the fibers and the matrix reaches its optimal state, indicating that the S4 addition sequence contributes to the enhancement of the overall mechanical properties of the concrete, particularly key indicators such as compressive strength and flexural strength.

Although this study primarily examines mechanical performance, the interfacial characteristics observed in Figure 11 are commonly associated with durability-related transport processes in cementitious materials. Accordingly, the next section discusses literature-based durability implications of the addition sequence and proposes targeted durability tests for validation in future work.

3.5 Durability implications and future work (literature-based)

3.5.1 Durability considerations for plant-fiber-reinforced concrete

Although this study focuses on mechanical performance, the long-term durability of plant-fiber-reinforced concretes is critical for practical applications. Lignocellulosic fibers are intrinsically moisture-sensitive and may undergo swelling–shrinkage under wet–dry exposure, which can induce interfacial microcracking and progressive debonding, thereby reducing crack-bridging efficiency and mechanical retention over time (Akinyemi and Omoniyi, 2020). From a transport perspective, the incorporation of plant fibers may increase water absorption and capillary uptake due to the hydrophilic and porous nature of the fibers and potential changes in pore connectivity, which can further amplify moisture-related deterioration under service conditions (Camargo et al., 2020). In addition, the highly alkaline cementitious environment may cause chemical degradation of fiber constituents and gradual mineralization, where calcium-rich products precipitate within or around the fibers, increasing brittleness and weakening the fiber–matrix interaction (Addis et al., 2022).

It should be noted that the NaOH treatment adopted in this work is widely used to remove surface impurities and modify fiber surface characteristics, which may benefit fiber–matrix interaction. Nevertheless, durability still needs to be verified systematically under relevant exposure conditions (Moshi et al., 2020).

3.5.2 Implications of addition sequence for durability

Based on the above mechanisms and the microstructural observations in this study, durability is expected to be influenced by the addition sequence through its effects on fiber dispersion and the compactness of the ITZ. Literature reports that moisture uptake can reduce the tensile strength of lignocellulosic fibers by up to about 40% and decrease the elastic modulus by roughly 20%–30%; moreover, moisture-induced swelling followed by drying shrinkage may generate interfacial stresses that promote microcracking and debonding. In cementitious environments, the precipitation of calcium-rich hydration products within or around the fiber structure may further cause fiber mineralization and embrittlement, weakening long-term crack-bridging efficiency (Akinyemi and Omoniyi, 2020; Addis et al., 2022).

In the present study, the saturation state is therefore expected to affect early interfacial development by altering swelling history and local water availability at the fiber surface. Dry fibers may absorb mixing water and swell in situ, which can disturb the fresh paste around the fibers and create interfacial gaps or weak zones before a stable hydration shell develops. This tendency is consistent with the SEM features in Figure 11, where S1 shows relatively loose fiber–matrix contact, visible interfacial voids, and only localized C–S–H coverage on the fiber surface. By contrast, SSD fibers have largely completed moisture-induced swelling prior to mixing, which may reduce additional swelling within the matrix and help maintain a more stable interfacial geometry during early hydration. When SSD fibers are introduced after the mortar phase has formed, as in S4, fiber dispersion is improved and the interface develops with more continuous hydration-product coverage and a denser ITZ, as observed in Figure 11. Conversely, S3 exhibits limited and non-uniform hydration-product coverage with more pronounced interfacial pores, suggesting that the combination of pre-mixing SSD fibers and the evolving paste structure may still lead to a less compact interface, which is expected to be less favorable for moisture-related durability.

Accordingly, the denser ITZ and more continuous hydration-product coverage observed for S4 are expected to reduce interfacial porosity and limit moisture transport pathways, which may be beneficial for moisture-related durability by reducing water ingress and slowing interfacial degradation. In contrast, more porous interfacial features, as observed for S3, may increase moisture sensitivity and accelerate interfacial degradation under wet–dry or alkaline exposure. These statements represent literature-based expectations supported by qualitative SEM observations and should be validated quantitatively.

With respect to freeze–thaw exposure, the expected performance is governed by pore structure and degree of saturation. Improved crack control and bridging may help delay microcrack coalescence, whereas increased moisture uptake and higher saturation can intensify freeze–thaw damage. Therefore, mixes with a denser ITZ and reduced moisture transport are expected to be more favorable under freeze–thaw cycling, while mixes exhibiting higher interfacial porosity and moisture sensitivity may be more vulnerable (Wei et al., 2024).

3.5.3 Recommended durability validation in future work

Future work will therefore conduct durability-oriented verification, including: (1) wet–dry cycling to quantify mass change and residual mechanical performance (compressive, flexural, and splitting tensile strength retention); (2) alkaline immersion aging to assess interfacial stability and post-aging strength retention; (3) transport-related measurements, such as water absorption and sorptivity, to characterize moisture ingress; and (4) freeze–thaw cycling to evaluate durability under cold-climate exposure. Post-aging microstructural characterization will then be performed to relate durability performance to the evolution of the fiber–matrix interface and the ITZ, thereby enabling application-dependent optimization of the addition sequence.

4 Conclusion

This study systematically tested the compressive strength, flexural strength, and splitting tensile strength of concrete specimens incorporating 0.5% volume fraction reed fibers with different addition sequences, compared to plain concrete (PC). The results indicate that the appropriate incorporation of reed fibers can significantly improve the mechanical properties of concrete, and the addition sequence plays an important role in the final performance of the concrete.

In terms of compressive strength, the concrete specimens with reed fibers exhibited similar or even slightly higher early strength compared to plain concrete, demonstrating that the incorporation of fibers, under appropriate and reasonable mixing conditions, does not impair the early strength of concrete.

The fibers, through their bridging effect and interfacial bonding with the cement matrix, help to effectively inhibit crack propagation, enhancing the toughness and overall integrity of the concrete.

Overall, the surface-dried saturated fiber post-addition method (S4) provided the best balanced improvement, particularly in flexural and splitting tensile performance, and is therefore recommended when crack resistance and toughness under tensile/flexural loading are the primary design objectives. However, the optimal addition sequence is application-dependent: the dry fiber pre-mix method (S1) showed comparatively stronger compressive performance and may be preferable for compression-dominated applications, whereas the dry fiber post-addition method (S2) yielded the highest splitting tensile strength and may be considered when tensile resistance is prioritized. Accordingly, S4 can be regarded as the preferred option for comprehensive performance, while S1 or S2 may be selected when a specific mechanical index governs the design.

This study not only confirms the potential of reed fibers as a natural, renewable, and low-cost reinforcement material in concrete but also provides important experimental evidence for the process optimization of plant fiber reinforced concrete.

Given that this study focuses on a single fiber volume and length combination, future research will further explore combinations of different fiber volumes, lengths, and surface treatments, as well as conduct long-term durability tests, to more comprehensively evaluate the applicability and long-term performance of reed fiber reinforced concrete in practical engineering applications.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

JH: Visualization, Investigation, Data curation, Resources, Software, Writing – review and editing, Formal Analysis, Writing – original draft, Methodology, Conceptualization. YZ: Software, Data curation, Writing – original draft, Investigation. ZY: Software, Data curation, Writing – original draft. XM: Formal Analysis, Resources, Writing – review and editing, Software, Investigation, Visualization, Data curation, Funding acquisition, Validation, Conceptualization, Supervision, Project administration, Methodology, Writing – original draft.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research was supported by Fujian Provincial Department of Science and Technology (grant number: 2023J011046), Wuyi University (grant number: YJ202216 & KC2024005S), the Department of Education, Fujian Province (grant number: JAT220379), and State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University (grant number: SKHL2117). The authors declared that they have no conflicts of interest to this research.

Acknowledgements

The author gratefully acknowledges the strong support provided by the university, whose academic atmosphere, teaching resources, and research facilities created a solid foundation for this work. Special thanks are extended to the faculty members for their patient guidance and insightful suggestions throughout the study, as well as to the administrative staff for their kind assistance in coordinating various academic affairs. The support and encouragement from classmates and friends at the university also played an important role in the successful completion of this research.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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