1.     Introduction

Fiber-reinforced concrete (FRC) offers superior performance compared to conventional concrete due to its composite material composition and structural elements. One of its most notable advantages is the significant enhancement of flexural strength, which varies depending on the type and properties of the fibers used. Although the concept is not entirely new, FRC has been steadily gaining traction in the construction industry [1]. Over the past five decades, FRC has undergone substantial development, resulting in widespread applications and increased market adoption. This progress is attributed to advancements in fiber technology, improvements in cementitious matrices, enhanced fiber–matrix interfacial bonding, and a deeper understanding of composite mechanics [2].

FRC is considered a specialized form of concrete that retains a certain level of toughness even after cracking [3]. The primary function of fibers is not necessarily to increase the concrete’s initial strength but rather to bridge cracks within the matrix as loads are applied, thereby enhancing post-cracking ductility and toughness [4]. The modern use of steel fibers as dispersed reinforcement can be traced back to Romualdi’s pioneering work in 1963 and 1964. Since then, the concept of incorporating dispersed fibers into cement-based materials has evolved significantly, resulting in extensive research publications, doctoral theses, and diverse practical applications in civil and structural engineering projects worldwide [5].

1.1. Improvement of Concrete Performance in Aerodrome Pavements

The operational condition of aerodrome movement areas evolves over their service life, primarily influenced by traffic loads and climatic factors [6]. Historically, most aerodrome pavements were not constructed with reinforced concrete; however, the adoption of reinforced pavement has become increasingly essential due to its critical role in aerodrome infrastructure [7]. To safeguard airfield pavements against accidental overloading, an internationally recognized pavement strength rating system is employed. This system comprises two components: the primary component, which protects against subgrade rutting, and the secondary, which shields asphalt surfaces. The latter is largely empirical, assigning tire pressure limits based on aircraft categories. In 2008, aircraft manufacturers proposed an increase in these limits, which was subsequently approved in 2013 [8].

While airfield pavements experience fewer load repetitions compared to highways, they are subjected to significantly higher gross loads. Pumping is a more common issue in highway pavements than in rigid airfield pavements. Additionally, whereas highway pavement loads often occur close to slab edges, airfield pavement loads are typically concentrated near the center of slabs [9]. Maintenance and rehabilitation of airfield pavements present major logistical and economic challenges, as they require substantial time and can disrupt airport operations, directly impacting airport revenue. Effective pavement asset management demands systematic, precise monitoring of pavement condition over time. Among the various Non-Destructive Testing methods available, the Falling Weight Deflectometer is considered the most representative for evaluating the structural capacity of airfield pavements [10].

1.2. Importance of mechanical properties in aerodrome pavements

The continuous growth of the aviation sector has subjected airport pavements to progressively heavier loads, creating an urgent need for materials with improved mechanical properties and long-term durability [11]. The choice of pavement type is influenced by several factors, including the operational characteristics of the aircraft expected to use the runway, specific operational requirements, particularly in terms of runway reconstruction and prevailing geological conditions [12]. Findings from accelerated traffic simulations on rigid pavement indicate that decreased flexural strength leads to a marked reduction in the crack resistance of Portland cement concrete pavements [13]. Complementary numerical modelling of the stress-strain behavior of strip pavements, using a discrete model of prestressed flight strip slabs with a single plane of symmetry, has provided detailed insights into their structural performance. The resulting nodal displacement profiles, bending moment diagrams, and soil base reaction data confirmed that the pavement structure met both strength and deformability requirements. These results suggest that, with adequate feasibility analysis, material usage in pavement construction could be significantly reduced while maintaining required durability and reliability, even for large, heavy-duty aircraft [14]. Moreover, research has shown that even when compressive strength remains constant, variations in the estimated flexural strength can influence slab thickness. Specifically, a lower inferred flexural strength requires a greater slab thickness, highlighting the critical role of flexural capacity in optimizing pavement design [15].

1.3. Role of basalt fibers in concrete

The naturally low tensile strength of concrete frequently results in abrupt, brittle failure without prior warning. One of the most effective strategies to address this weakness is the addition of fibers, which improves tensile strength and mitigates brittleness. Among the various fiber types, basalt fibers (BFs) have recently attracted considerable interest due to their favorable characteristics-high strength, excellent elasticity, a high elastic modulus, superior thermal stability, and strong chemical resistance [16].

High-modulus micro BFs are particularly effective in controlling microcrack formation because of their dense distribution and strong restraining action, leading to significant improvements in the pre-cracking mechanical performance of ultra-high-performance concrete (UHPC). For instance, UHPC reinforced with 0.3 % micro BFs achieved maximum compressive and flexural strengths of 132.6 MPa and 26.10 MPa, respectively. In contrast, macro BFs primarily enhance post-cracking performance, where fiber pullout dissipates energy and produces a more ductile failure mode. UHPC with 3 % macro BFs recorded peak compressive, first-crack flexural, and post-crack flexural strengths of 151.8 MPa, 24.97 MPa, and 26.32 MPa, respectively [17].

The study [18] identified BF content as the most influential parameter on concrete’s mechanical properties within a dosage range of 0.1–0.3 %, with an optimal value of 0.2 %. Fiber length had little impact on compressive strength in recycled aggregate concrete but enhanced splitting tensile and flexural strengths with longer fibers. BF pellets have also been shown to improve post-crack flexural behavior by reinforcing the cementitious matrix and limiting strength losses under harsh environmental exposure [19]. Similarly, [20] reported that adding macro BFs to recycled aggregate concrete  mixes restored splitting and flexural strengths to levels comparable to those made with natural aggregates. The study [21] further demonstrated that macro BFs addition increased splitting TS by 39.88 % and substantially improved both initial and unstable fracture toughness.

The fiber bridging mechanism plays a pivotal role in post-cracking behavior in fiber-reinforced cementitious composites. Studies have shown that polypropylene fibers develop strong bonds with the cementitious matrix, thereby enhancing tensile capacity and impeding microcrack propagation. Micro polypropylene fibers improve peak load resistance and deformation capacity, while low dosages of macro polypropylene fibers offer greater post-crack energy absorption than higher dosages of micro polypropylene fibers [22]. Scanning electron microscopy observations indicate that steel fibers form thick interfacial transition zones that promote efficient stress transfer, whereas synthetic fibers help reduce micro cracking and improve durability over time. Nevertheless, significant research gaps remain particularly in identifying optimal hybrid fiber combinations, evaluating the long-term performance of FRC, and developing more sustainable fiber alternatives [23]. The study [24] introduced a probabilistic micro-mechanical model to assess crack-bridging stress–displacement behavior in short hooked-end steel fiber composites, recommending fiber volume fractions between 0.75 % and 1 %. Similarly, the study [25] found that fiber length and orientation markedly affect crack opening behavior and the softening characteristics observed after cracking.

This study aims to develop and evaluate HBFC mixtures optimized for the workability and flexural performance requirements of aerodrome pavements, while the objectives are to:

1.    Source and characterize high-performance materials, including micro and macro, Portland Cement M600, and other mix components, in compliance with relevant standards.

2.    Prepare a series of concrete mixtures incorporating the relative proportions of micro and macro BFs varying both the total fiber dosage.

3.    Determine the flexural strength of the mixtures at 28 days under controlled curing conditions, using standardized testing procedures and.

4.    Analyze and compare the flexural performance and workability trends of hybrid fiber mixtures against single and hybrid fiber systems to determine the optimum ratio.

Although BFs have gained attention for their mechanical, thermal, and chemical durability advantages in concrete, most existing studies focus on their application in general structural elements rather than in high-stress pavement systems such as aerodromes. Research has largely examined either micro BFs or macro BFs individually, with limited emphasis on hybridization strategies that balance early-age crack resistance with post-crack load capacity. The majority of available literature seldom analyzes dosage optimization data for maximum performance at constant total volume fractions. There is also an insufficient link between fiber distribution and mechanical performance. Many studies neglect the combined impact of high loading, environmental cycles, and crack control in the long-term performance of fiber-reinforced pavement concrete and its practical implications for aerodrome pavements.

                                                                                    2.     Methods and Materials

2.1.               Material for the Experiment

The mechanical performance, durability, and crack resistance of pavement-grade concrete are closely tied to the physical and chemical characteristics of its constituent materials. For this study, materials were carefully selected to meet the high-performance demands of aerodrome concrete pavements.

Cement: The primary binder was Portland Cement M600, supplied by Akkermann Cement, meeting the requirements of CEM I 52.5 under GOST 31108-2020 [26]. This high-strength cement is noted for its rapid early strength development, making it particularly suitable for rigid pavements subjected to heavy traffic and requiring early reopening. Its quick-setting properties not only reduce construction time but also ensure long-term durability and resistance to cyclic loading.

Water: Mixing water was potable, complying with GOST 23732-79 [27]. Under GOST 2874-82 [28], drinking water is deemed fit for concrete mixing without additional quality verification, ensuring consistent cement hydration and reliable long-term performance.

Superplasticizer: To maintain workability at a low water–cement ratio, Polyplast Target was added at 0.7 % of cement weight. This polycarboxylate-based admixture is available in two variants: Type 1, which contains air-entraining agents to enhance freeze–thaw resistance and Type 2, which includes accelerators and structural modifiers for improved early strength gain and cohesiveness. Both variants conform to GOST 30459 [29], promoting uniform fiber dispersion, better matrix cohesion, and enhanced crack resistance in high-strength FRC.

Aggregates. Locally sourced fine and coarse aggregates were used. Fine aggregates consisted of natural medium sands with a fineness modulus of 2.0–2.5, meeting GOST 8736-93 [30], and predominantly ranging from 0.63 to 1.25 mm in size. Coarse aggregates comprised 20 mm crushed stone from igneous and metamorphic rocks, compliant with GOST 8267-93 [31] and GOST 26633-91 [32]. Additionally, vein quartz with high purity, rough surface texture, and minimal clay or organic impurities was incorporated to improve packing density and enhance paste–aggregate bonding.

Basalt Fibers. Basalt, a naturally occurring igneous rock formed from cooled volcanic lava, is increasingly recognized as a sustainable and safe alternative to asbestos for reinforcement. The transformation of basalt into fibers yields a high-performance material with exceptional heat resistance and stability under extreme environmental conditions [33]. Within the context of its application to reinforced concrete, there are two major types of BFs. Micro fibers, which are fine filaments with lengths of 15–30 mm and a diameter range of 9–25 μm and are bridged at the micro-crack stage. They are also useful for preventing plastic shrinkage during the early stages of curing because they offer good dispersion and a broad surface area [34]. Macro Fibers, which are coarse BFs (50 mm length, 1 mm diameter) with tensile strengths up to 1650 MPa were incorporated to improve post-cracking load-bearing capacity and flexural strength. Their strong mechanical interlock with the cement matrix enhances ductility and toughness, critical for pavements exposed to heavy dynamic loads. Their corrosion resistance and environmental benefits make them advantageous over synthetic alternatives [35].

2.2.               Methodology

2.2.1. Material preparation

This experiment employed advanced construction materials to produce HBFC specifically designed for the demanding conditions of high-stress pavement applications. The binding material used was Portland Cement M600 (CEM I 52.5), compliant with GOST 31108-2020 [26] and procured from Akkermann Cement. This high-strength cement was chosen for its rapid strength development and excellent adhesion properties, making it well-suited for rigid pavement structures exposed to heavy traffic loads and crack-inducing stresses. All mixing operations utilized potable water that conformed to GOST 23732-79 [27] standards, ensuring consistent cement hydration without the presence of detrimental impurities.

A polycarboxylate-based superplasticizer was incorporated at a dosage of 0.7 % of the cement weight. This admixture kept the concrete workable even with a low water–cement ratio. The main thing we changed across mixes was how we combined the two types of BFs: micro and macro. The micro fibers were there to stop early-age and plastic shrinkage cracks from forming. The macro fibers, which are thicker and harder to pull out, were meant to span larger cracks and help the concrete hold together after cracking. By altering the relative amounts of micro and macro BFs as well as the overall fiber dosage, a number of fiber combinations were tested. In order to assess the impact of fiber dosage and hybridization on concrete performance, the mixtures included single-fiber systems and hybrid combinations ranging from 2:0 (all micro fibers) to 0:2 (all macro fibers), with mixes in between like 1.5:0.5, 1:1, and 0.5:1.5.

Material batching was done to ensure consistency. Mixing was performed in a pan mixer, beginning with the dry components, followed by the gradual addition of water blended with the super-plasticizer. Fibers were introduced slowly to promote uniform distribution and prevent clustering.

2.2.2. The cone draft test

A standard slump cone with dimensions of 300 mm height, 200 mm base diameter, and 100 mm top diameter was used for the slump test. The cone was placed on a flat, non-absorbent surface and filled with fresh concrete in three equal layers by volume. Each layer was compacted with 25 strokes of a tamping rod (16 mm diameter, 600 mm length) to ensure uniform consolidation and removal of entrapped air. After the third layer was rodded, the top surface was leveled, and the cone was carefully lifted vertically in 5–10 seconds without any lateral or twisting motion. The concrete was allowed to subside freely, and the slump value was determined by measuring the vertical difference between the height of the cone (300 mm) and the highest point of the slumped concrete. This value was recorded to the nearest 5 mm. The slump test results were used to evaluate the influence of different fiber dosages on workability.

2.2.3. The flexural strength test

The other batch of fresh concrete was then cast into standard molds and compacted using a vibrating table to remove entrapped air. Concrete beam specimens measuring 100×100×400 mm were prepared for each mix. After casting, the beams were demolded at 24 hours and cured in water at until the testing age of 28 days in a controlled laboratory environment. For testing, each specimen was placed on two supporting rollers spaced 300 mm apart, with the load applied at two points equally distant from each support (third-point loading configuration). The load was applied continuously and without shock using a universal testing machine until failure occurred.

The flexural strength (modulus of rupture, ) was calculated using Equation (1):

                                                                        (1)

where  is the maximum applied load (N);  is the span length between supports (mm);  is the average specimen width (mm);  is the average specimen depth (mm)

The values for the flexural strength were carefully recorded and analyzed in relation to the predetermined hybrid fiber ratios.

                                                                                   3.     Results and Discussion

Two types of BFs were incorporated in this study to provide multiscale crack control and improve the structural performance of airport pavement concrete under severe aircraft loading circumstances:

Micro BFs were 18.2 mm long and 17 μm in diameter. These fibers are useful in preventing the start and spread of microcracks in the cementitious matrix because of their high aspect ratio and small diameter. Microcracks frequently act as precursors to bigger structural cracks in airport pavements, where concrete is subjected to repetitive wheel loads, heat gradients, shrinkage effects, and cyclic environmental exposure. By adding micro BFs, the matrix’s stress distribution is improved, early fracture resistance is increased, and the chance of crack coalescence is decreased. This helps to increase fatigue resistance and durability, both of which are essential for preserving pavement integrity and reducing maintenance needs.

Macro BFs had a length of 50 mm and a diameter of 1 mm, corresponding to an aspect ratio of approximately 50. The fibers were specifically chosen for aerodrome pavement applications because of their strong geometry and great resistance to mechanical damage during mixing and placing, even though this aspect ratio is lower than that of many BFs reported in structural concrete investigations. The comparatively large diameter reduces the chance of fiber rupture under loading by strengthening mechanical anchoring and enhancing fiber stability within the concrete matrix.

Effective crack management is crucial because heavy aircraft loading exposes aerodrome pavements to high wheel loads, impact stresses, cyclic loading, and harsh weather conditions. By integrating the crack-bridging and toughness-enhancing properties of macro fibers with the crack-arresting capabilities of micro fibers, a hybrid system of micro and macro BFs offers multiscale reinforcement. While macro fibers enhance post-cracking behavior, energy absorption, crack-width management, and load transfer across larger cracks, micro fibers restrict the start and spread of micro cracks inside the cementitious matrix. When combined, they improve flexural performance, fatigue resistance, durability, and long-term structural dependability of concrete airport pavements under frequent aircraft stress.

3.1.               Workability

The use of hybrid BFs in aerodrome concrete pavements has been widely explored to enhance mechanical performance and durability under heavy loading conditions. However, the addition of fibers can significantly affect the fresh properties of concrete, particularly its workability. This section shows how different hybrid BF amounts affected the workability of aerodrome pavement concrete, based on slump test results. Fig. 1 gives the slump values. The mix without any fibers-labeled K-had the highest slump at 40 mm (Fig. 2), meaning it flowed easily. Adding fibers brought the slump down, though how much it dropped depended on the fiber type and how much was used.

Figure 1. Slump (mm) against Hybrid Fiber Ratio.

3.1.1. Single fiber dosages

When we added micro BFs (A), the slump dropped further as the fiber content went up. Looking at Fig. 1, slump fell from 35 mm at 0.25 % A down to just 11 mm at 2 % A. This reflects a dramatic drop, just like in the study This huge reduction occurs as a result of the high specific surface area of the micro BFs, which causes them to intertwine, increasing internal friction within the fresh mixture and restricts the movement of cement paste and aggregates. This reflects a remarkable decrease, just like in the study [36]; the slump of the BF-reinforced concrete, the slump has slightly dropped with the increasing dosage of BF up to 0.5 %, which peaked at 43 mm in the plain concrete matrix. However, when adding BF with the volume fraction of 0.125 %, 0.25 %, 0.375 % and 0.5 %, the slumps of the concrete matrix has fell to 40 mm, 33 mm, 26 mm, and 18 mm, respectively. For mixtures containing only macro BFs (B), the reduction in slump was less pronounced. The recorded slump values ranged from 36 mm at 0.5 % B to 31 mm at 2 % B (Fig. 3), suggesting that macro fibers have a comparatively lesser impact on workability due to their lower surface area and interaction intensity.

Figure 2. Control mixture (K) with 40 mm slump.

Figure 3. 2B, BF-reinforced concrete with 31 mm slump.

3.1.2. Hybrid fiber combinations

The incorporation of hybrid micro–macro BFs combinations had a pronounced effect on concrete workability, often intensifying slump loss. High total fiber content mixes, such as 2A1B and 2A0.5B, recorded extremely low slump values of just 10 mm, indicating severe stiffness likely caused by fiber clustering and poor dispersion. Moderate-dosage hybrids, including 1.5A2B, 1.5A1.5B, and 1.5A1B, achieved slumps between 17 mm and 25 mm, reflecting a more balanced trade-off between reinforcement and fresh mix consistency. Lower-dosage combinations, such as 0.5A2B and 0.5A1.5B, maintained better workability, with slump values ranging from 28 mm to 33 mm. For context, [37] reported that the compacting factor of macro-BF-reinforced mixes was up to 14 % lower than that of plain concrete, with slumps dropping from 220 mm (control) to 50–85 mm at macro BF contents of 0.5–1.5 %. Overall, micro BFs (A) caused more severe slump reduction than macro BFs (B), with the greatest impact observed in high-dosage hybrids. Never the less, certain lower-dosage combinations preserved acceptable workability while potentially enhancing mechanical performance. These results underscore the need to optimize fiber type and dosage, and to consider chemical admixtures such as superplasticizers, to counteract workability loss in aerodrome concrete pavement applications.

3.2.               Flexural Strength (MPa)

Flexural strength is a critical property for aerodrome concrete pavements, as they are subjected to heavy aircraft wheel loads and dynamic stresses. The incorporation of hybrid BFs has been explored to enhance the post-crack energy absorption and load-bearing capacity of concrete pavements. This section presents the experimental results on the influence of varying dosages and combinations of micro (A) and macro (B) BFs on the 28-day flexural strength of concrete.

The control mix (K) in Fig. 4, without fibers, recorded a flexural strength of 8.8 MPa, providing a baseline for comparison. Every mix with fibers-whether single type or hybrid-showed higher flexural strength than the plain concrete.

Figure 4. Flexural Strength (MPa) against Hybrid Fiber Combination.

3.2.1. Single fiber dosages

As shown in Fig. 4, the mixes with only micro BFs (A) generally gained flexural strength with increasing fiber dosage. Strength went up from 8.6 MPa at 0.25 % A (Fig. 5) to a high of 9.9 MPa at 1.5 % A. A slight reduction to 9.5 MPa was observed at 2 % fiber dosage, which may be attributed to reduced fiber dispersion efficiency and localized fiber clustering. The macro-only mixes (B) also beat the control mix. Their strengths ranged from 9.2 MPa at 2 % B up to 9.7 MPa at 1 % B. The ability of macro fibers to bridge cracks, which allows for stress redistribution and improves post-cracking load transmission, is responsible for the improvement. It should be noted that the optimal fiber dosage reported in this study is different from that reported by [38], who found that self-compacting concrete with a flexural strength of 7.06 MPa, residual strength of 2.724 MPa, and impact strength of 4457.44 kN·mm performed best at a BF content of 0.4 %. Such differences are expected because the two studies employed different concrete types, mixture compositions, fiber characteristics, and testing procedures. Hybrid BF combinations, especially the 1.5A0.5B mixture, had the best flexural performance in the current study, demonstrating the impact of fiber hybridization on pavement-grade concrete behavior.

Figure 5. 0.25A micro basalt fiber concrete under bending stresses.

Figure 6. 1.5A0.5B hybrid basalt fiber concrete specimen for flexural strength testing.

3.2.2. Hybrid fiber combinations

The highest flexural strength values were achieved with hybrid fiber mixtures, demonstrating the synergistic benefits of combining micro and macro BFs. Mixtures, such as 1.5A1.5B (10.2 MPa), 1.5A0.5B (Fig. 6) (10.4 MPa) , 1A1.5B (10.1 MPa), and 1A1B (10.3 MPa) in Fig. 4, all surpassed the performance of single-fiber systems. This enhancement can be attributed to the complementary roles of micro fibers in controlling microcracks and macro fibers in providing post-crack load transfer. Lower-dosage hybrids, including 0.5A1.5B, 0.5A1B, and 0.5A0.5B, also outperformed the control, with flexural strengths ranging from 9.4 to 9.9 MPa. These results align partially with [39], who reported that optimal improvements in mechanical properties occurred at BF contents between 0.1 % and 0.3 %. Similarly, research [40] observed peak performance in SCC70-85/0.10 specimens, which exhibited a 69.90 % increase in flexural strength and a 23.47 % rise in splitting TS compared to the control.

In the present study, optimal performance was achieved at balanced moderate hybrid dosages. The highest flexural strength (10.4 MPa) was recorded for the 1.5A0.5B mix, underscoring the efficiency of micro–macro fiber synergy in enhancing load-carrying capacity. Overall, while both single and hybrid BF systems improved flexural performance, hybrid configurations consistently provided superior results, making them a promising reinforcement strategy for high-performance aerodrome concrete pavements.

Enhanced flexural strength directly improves resistance to cracking under repeated aircraft loading, reducing the risk of structural fatigue and extending pavement service life. The combined crack-control and post-crack reinforcement provided by hybrid BFs can minimize foreign object damage by limiting surface spalling and fragment release, which is critical in airfield safety. Furthermore, hybrid fiber systems improve the distribution of loads throughout the concrete matrix. This means slab thickness could potentially be cut down without affecting the durability, which in turn, cuts down on materials. This may contribute to reduced expenses, material consumption and improved sustainability in pavement construction.

                                                                                                    4.     Conclusion

This study makes it clear: adding BFs – especially when you blend micro and macro types – can really boost the flexural strength of concrete pavements for airports. While single fiber systems (micro or macro) offered measurable improvements over the control mix, hybrid combinations consistently outperformed them due to the complementary functions of the two fiber types: micro BFs effectively controlled microcracking, while macro BFs improved crack bridging and post-crack load transfer. The most notable gain was achieved with the 1.5A0.5B mix, which reached a flexural strength of 10.4 MPa, representing a substantial enhancement over the baseline value of 8.8 MPa. However, excessively high fiber dosages, especially in single-type mixes, showed signs of diminished performance, likely due to fiber balling and reduced workability.

                                                                                         5.     Recommendations

1.    For aerodrome pavement applications, hybrid BF combinations with balanced proportions particularly 1.5 % micro BF and 0.5 % macro BF are recommended to maximize flexural performance without compromising workability.

2.    Where mechanical performance is a priority, hybrid fiber systems should be preferred over single fiber additions, as they harness the synergistic benefits of both micro and macro fibers.

3.    Fiber contents above optimal ranges should be avoided to prevent issues such as poor dispersion, fiber clustering, and reduced workability, which can offset strength gains.

4.    Further field-scale studies on full-scale aerodrome pavement slabs are advised to validate laboratory results under actual aircraft loading conditions and environmental exposure.

                                                                                               6.      Additional information

1.    The authors state that generative artificial intelligence and artificial intelligence-based technologies were not used; however, ProWritingAid and Grammarly were used only for language editing and proofreading.

2.    The authors received no financial support for the research, authorship, and publication of this article.

3.    The authors declared no potential conflicts of interest concerning the research, authorship, and publication of this article.