Study of Mechanical and Fracture Properties of Concrete with Different Lengths of Polypropylene Fibers

Abstract

This study investigates the effect of polypropylene fibers of different lengths (54 mm, 38 mm, 19 mm) on the mechanical and fracture properties of high-strength concrete. Unlike most existing research focusing on a single fiber length, this work evaluates a fixed hybrid ratio of 4:1:1, thereby addressing the synergistic action of macro- and microfibers. Three dosages were tested and compared to a reference mixture without fibers. Validation was performed by repeated testing of multiple specimens and statistical evaluation of mean values and standard deviations. The results showed that the optimal hybrid mixture (2.0/0.5/0.5 kg/m³) increased compressive strength by 28.7% and splitting tensile strength by 30.1% relative to the reference. Fracture toughness and specific fracture energy also improved significantly, demonstrating enhanced crack resistance and energy absorption. The main contribution of this work is to provide experimental evidence that a hybrid combination of polypropylene fibers at a fixed ratio can improve both mechanical strength and fracture resistance, with direct implications for durability and service life.

1. Introduction

In the search for sustainable building solutions, the construction industry is constantly changing and looking for new advances. As more and more emphasis is being placed on reducing carbon dioxide production and consequently extending the lifetime of buildings, the longest possible service life of concrete structures is becoming one of the priority requirements. Therefore, the aim is to reduce the number and size of cracks, water permeability, and chemical intrusion, but also to increase corrosion protection [1,2]. Steel is susceptible to corrosion, and therefore reinforced concrete elements, if not properly designed and constructed, can deteriorate due to steel corrosion. The production of steel mesh and fiber also creates a significant carbon footprint [3].

One of these advancements is the introduction of fiber-reinforced concrete (FRC), a unique material that has great potential to meet sustainability requirements. FRC is a blend of traditional concrete and durable steel fiber and encompasses five core component materials: cement, water, steel fiber, additives, and aggregate. Engineers can select and combine these materials as needed to meet specific construction demands [4,5,6]. Ultimately, the strength and functional capabilities of the FRC depend on the quality of the fiber materials chosen. Fibers, with a cross-sectional area ranging from 0.1 mm² to 1.5 mm² and with varying lengths of 6 mm to 80 mm [7], are commonly utilized in construction. These essential materials are interlocked by the cement, which binds them together with the other elements, such as aggregate. Antifreeze and accelerator are additives that are used with caution in FRC to avoid compromising quality. Coarse and fine natural aggregates like gravel and sand, respectively, are examples that should be carefully combined for optimal concrete quality. With dispersed reinforcement in the form of fibers such as steel, polymer, glass, and more, FRC is a cementitious composite material [8] Highly coveted for its immense strength, fracture toughness, and ability to control cracks, FRC is an in-demand material. Cementitious substances benefit greatly from the FRC, as it enhances their ductility, flexural strength, and shear strength. FRC is also known to decrease shrinkage, creep, and permeability in concrete while simultaneously bolstering its resistance to fatigue, impacts, and explosions [9].

Good cost-effectiveness is a major benefit of FRC. However, similar to any technology, there are also downsides and faults associated with the increased use of FRC. Research and development focused on FRC, specifically relating to shear failure and fatigue failure, is necessary to improve its performance. Technological instability, mainly due to a lack of appropriate consistency in fresh FRC, can result in failures like crack development. Inappropriate production and application of steel fiber reinforced concrete also contribute to technological issues [10].

Other alternatives include natural fiber, which is a cheaper, environmentally friendly, and sustainable solution for improving the tensile properties of concrete. The sources of natural fiber can be stems, husks, leaves, and pulp of plants. Or polypropylene fiber [11]. Polypropylene fiber (PPF) is polymer fiber. A plastic material that is often used is polypropylene. For example, packaging and straws made of plastic are used every day. Polypropylene is a water-resistant fiber. This fiber is a fiber that has a low density and does not absorb water, so this fiber does not significantly change the physical properties of concrete, but it can change the mechanical properties. PPF differs in length but most importantly in the specific functions they perform. Coarse fibers are also called structural fibers because they replace traditional reinforcement in the form of steel rods and can transmit the loads that act on the structure. The main function of microfibers is to overcome plastic shrinkage in concrete and to limit cracks. Polypropylene fibers also have increased abrasion and fire resistance as a result of their low melting point (about 170 °C), which can prevent cracking and collapse of structural elements [12,13].

Fibers are often used in the production of industrial flooring as well as roads or runways at airports. This material is also used to create machine foundations and other elements subject to dynamic loads. It is also frequently used as a shotcrete technique, for example, when covering underground structures or in renovation and repair work. Today it is often used as material for structural elements [14,15].

Although several studies have explored the use of polypropylene fibers in concrete, there is a lack of research addressing the combined effect of different fiber lengths in a single mix and their synergistic behavior. Most existing studies focus on a single polypropylene fiber length or type, mainly investigating either macro-fibers for structural performance or micro-fibers for shrinkage crack control. To our knowledge, there is limited research systematically combining different lengths of polypropylene fibers within one mixture to study their synergistic action, even though real structures are subjected to multi-scale cracking. Based on preliminary experiments and the recent literature (e.g., [16]), we therefore adopted a fixed hybrid ratio of 4:1:1 (54, 38, and 19 mm fibers) to represent a balanced configuration of macro- and microfibers. Recent research has further highlighted the benefits of advanced fiber-reinforced concrete (FRCs). For example, a recent study published in [17] demonstrated multifunctional improvements that can be achieved with hybrid fiber systems, particularly in terms of crack resistance and long-term performance. Similarly, [18,19] reported that fiber geometry, dispersion, and hybridization strongly influence mechanical behavior and durability. These contributions underline the need for systematic studies on hybrid fiber combinations, especially for polypropylene fibers, which remain underexplored compared to steel or basalt fibers. There are also studies combining experimental measurements and the latest techniques using machine learning models to analyze the behavior of wire-reinforced concrete [20,21]. The results of these studies demonstrate the need for further experiments of various types in order to supplement the input data for the models.

Our study presents an innovative approach by combining structural (macro) and shrinkage-reducing (micro) fibers in a fixed ratio to determine the most effective combination for enhancing both mechanical and fracture properties of high-strength concrete. Such an approach simulates real-world conditions more closely, where concrete structures are subjected to multi-scale cracking. The findings of this study have direct implications for practical applications such as airport pavements, industrial flooring, and precast structural elements, where resistance to cracking and durability are critical.

The novelty of this work lies in the combination of three fiber lengths in a fixed ratio, tested at varying dosages, and the parallel assessment of both mechanical and fracture parameters. By doing so, the study not only provides new insights into the hybrid effect of polypropylene fibers but also delivers results directly applicable to practical design scenarios such as airport pavements, industrial floors, and precast elements. The purpose of this study is to experimentally test C50/60 grade concrete with fiber of three different lengths to obtain mechanical parameters.

2. Materials and Methods

The aim is to compare the basic material properties of concrete with the same composition but with the addition of polypropylene fibers of different lengths and in different proportions. Polypropylene fibers of 54, 38, and 19 mm length were used as dispersed reinforcement. For the preparation of the samples, Forta Ferro [22,23] fibers were selected. It should be noted that only Forta Ferro fibers were used in this study. While this product is representative of high-performance polypropylene fibers, the results may not fully capture the behavior of other brands or fiber types with different geometry, surface treatment, or mechanical properties. These are high-performance fibers produced by combining polypropylene monofilament non-fibrillated fiber and mesh fibrillated fibers. They are used to reduce shrinkage of concrete in the plastic and hardened state, improve impact resistance, and increase fatigue resistance and toughness of concrete. These fibers exhibit high durability, especially under heavy loads. The preparation and description of the material and the individual mixtures are described in the next subsection. Individual testing methods are also given.

2.1. Concrete Mixtures

In the production of fiber concrete, it is extremely important to follow the prescribed production technology for a given type of fiber. The test samples were produced in four alternatives—samples made from a reference concrete mix (C_reference) and samples with added fiber content in a combination of lengths of 54 mm, 38 mm, and 19 mm in quantities of 2.0/0.5/0.5 kg (C_2.0/0.5/0.5), 4.0/1.0/1.0 kg (C_4.0/1.0/1.0), and 6.0/1.5/1.5 kg (C_6.0/1.5/1.5) per 1 m³ of concrete. The first number indicates the amount of fiber length of 54 mm added, the second number indicates the amount of fiber length of 38 mm, and the last number is the amount of fiber length of 19 mm. The ratio between the fibers was always 4:1:1, but compared to the first set, set two had double and set three had triple the amount of fibers per cubic meter. The constant ratio of 4:1:1 was chosen based on preliminary experiments and findings in the literature [24], since this proportion was found to provide a synergistic effect: longer fibers (54 mm) act mainly as structural reinforcement bridging larger cracks, while the shorter fibers (38 mm and 19 mm) control microcrack propagation and early-age shrinkage. The present work therefore focuses on verifying this hybrid effect at different total fiber dosages. Nevertheless, other ratios may also be beneficial, which will be the subject of future research. The ingredients were dosed according to the technological prescription and in the quantities given in

Table 1. Composition of concrete mixtures.

	C_reference	C_2.0/0.5/0.5	C_4.0/1.0/1.0	C_6.0/1.5/1.5
Portland Cement CEM I 42.5R [kg]	340	340	340	340
Fly ash [kg]	80	80	80	80
Fine aggregate 0/4 [kg]	784	784	784	784
Coarse aggregate 4/8 [kg]	920	920	920	920
Superplastificator [kg]	5	6	6	6
Water [L]	180	162	166	165

with the aim of creating a reference mixture at least at the level of C50/60 concrete. The experimental mixtures were then based on the same composition. The concrete mix was prepared in a forced circulation mixer. The concrete mix was prepared in a forced circulation mixer. To improve fiber separation and minimize clumping, the polypropylene fibers were first blended together with the coarse aggregate for approximately 1 min. This pre-mixing step reduced the risk of fiber balling. Subsequently, fine aggregate, cement, and fly ash were added, followed by 80% of the mixing water. The superplasticizer (Sika ViscoCrete 1035), premixed with the remaining water, was then gradually introduced to enhance workability and further aid in uniform fiber dispersion. After all components were added, the mixture was blended for an extended period of 5 min to ensure homogeneity.

The cube (150/150/150 mm) and prismatic (100/100/400 mm) blocks were manufactured in accordance with the technical standard EN 12390-2 [25]. Steel molds were used for the production of the beams and plastic molds for the production of cubes. To facilitate molding, the molds were pre-treated with a thin layer of separating agent. The filling was carried out in two layers; the concrete was sufficiently compacted on a vibrating table, then the excess concrete was removed, and the surface was leveled with a trowel. The filled forms were placed under the sheeting. After 24 h, the test bodies were unmolded and stored in a metal container at 20 °C and 95% relative humidity until just before testing.

2.2. Experimental Program

The experimental program was prepared based on standard tests and tests that are suitable for fibrous concrete. The individual tests are described below.

2.2.1. Workability

The test determines the consistency of fresh concrete mix by measuring the spread of concrete on a flat plate that is subjected to vibrations. Before the test begins, both the plate and the mold must be moistened. After filling the mold and demolding, the mixture on the plate is subjected to a free fall from a height of 40 mm, which is repeated fifteen times. The test results were as follows: mixture C_reference spread 640 × 650 mm, mixture C_2.0/0.5/0.5 spread 600 × 570 mm, mixture C_4.0/1.0/1.0 spread to 400 × 540 mm, and mixture C_6.0/1.5/1.5 spread to 285 × 290 mm.

2.2.2. Volumetric Mass

After demolding, all samples were measured and weighed, and the values were recorded. The determination of the bulk density was carried out in accordance with EN 12390-7 [26]. Three samples for each mixture were tested.

2.2.3. Compressive Strength

The compressive strength of the test samples was determined according to the technical standard EN 12390-3 [27]. The measurements were carried out on cube-shaped test samples with an edge length of 150 mm. The samples were continuously loaded at a rate of 0.5 MPa/s, without impact, until the moment when the compressive forces in the body were such that the test body broke. The compressive strength of the concrete was determined by the equation of the force achieved and the area measured. Three samples for each mixture were tested. Testing was carried out using a calibrated hydraulic press (Form + Test, capacity 3000 kN). For each mixture, three specimens were tested to obtain average values and standard deviations. The applied loading rate was verified with the data acquisition system.

2.2.4. Tensile Splitting Strength

Testing of the transverse tensile strength of the concrete was carried out in accordance with EN 12390-6 [28]. The cube samples with an edge length of 150 mm were tested. Three samples for each mixture were tested. Loading was carried out continuously at a rate of 0.05 MPa/s through the press until failure of the specimen. The transverse tensile strength is calculated by the maximum force, contact area, and transverse dimension of the specimen.

2.2.5. Elastic Modulus

The measurement was carried out according to the technical standard EN 12390-13 [29] on a test specimen of prismatic shape 100/100/400 mm. The test specimen was placed in a press fitted with a deflection meter. The loading and unloading cycles were recorded using a digital extensometer with 1/1000 mm resolution, ensuring accurate deflection measurements. Three prisms per mixture were tested, and the average modulus was calculated. The loading was carried out gradually by partial forces, and the deflections were gradually read. After reducing the load to the previous level, the elastic deflection was determined. From this, the elastic modulus was determined using the basic equation.

2.2.6. Fracture Mechanics

Fracture mechanics is essential for describing the behavior of structures with cracks. Its goal is to track the processes that lead to damage and fracture of a structural element, as well as to find governing physical laws and mathematical models of material degradation. The determination of fracture parameters was carried out by a three-point bending test on prismatic specimens with dimensions of 100/100/400 mm (see Figure 1).

To obtain reliable fracture parameters, the initial part of the load–deflection curve was adjusted to eliminate artifacts from seating effects and early microcracking. This linearization procedure followed methodologies similar to those described in recent fracture mechanics studies [14]. In practice, the curve was processed step by step:

(i)

interpolation of a straight line through the initial ascending branch,

(ii)

iterative removal of outlier points until the correlation coefficient (R²) approached unity, and

(iii)

recalculation of fracture parameters using the adjusted curve.

This approach ensured reproducibility and comparability of results across specimens.

After preparation, the specimens were notched by half-cutting with a diamond saw to a depth of 1/3 of the cross-section height. The beams were then tested in a Hecker FPZ 100/10 press with a support span of 300 mm. Loading was applied at a deformation rate of 0.05 mm/min while force and deflection were continuously recorded.

A total of five beams were tested for each mixture and for the reference. From each set, three representative samples were selected for adjustment and evaluation in the StiCrack ESC program. The following fracture parameters were determined: static modulus of elasticity, effective fracture toughness (KIce), specific fracture work (WF), and specific fracture energy (GF).

3. Results

This chapter presents clear results according to the experimental program described above. In the measurements, the mean values of each value and the standard deviation were determined. In addition, each set of results contains the percentage difference of the values of the newly designed mixtures against the reference mixture.

3.1. Mechanical Test

Figure 2, Figure 3, Figure 4 and Figure 5 show the results obtained in the laboratory. In Figure 2 it can be seen that the volumetric mass of the individual mixtures is almost similar. The percentage difference against the reference concrete mix in

Table 2. Relative difference of volumetric mass to reference mixture.

Mixture	C_2.0/0.5/0.5	C_4.0/1.0/1.0	C_6.0/1.5/1.5
Relative difference	0.9%	−0.9%	−2.2%

shows very little variation. Only the C_6.0/1.5/1.5 mix shows a difference of more than 2%. Similarly, the standard deviations determined by measuring each mix in several tests are very low, and a comparison of the differences between the single-cast concretes and the standard deviation value shows that the mixes have almost the same volumetric mass.

Based on the results shown in Figure 3, it can be concluded that the concrete mix with 2.0/0.5/0.5 kg of fiber showed the greatest increase in compressive strength. The percentage difference against the reference concrete mix in

Table 3. Relative difference of compressive strength to reference mixture.

Mixture	C_2.0/0.5/0.5	C_4.0/1.0/1.0	C_6.0/1.5/1.5
Relative difference	28.7%	9.3%	14.7%

is shown. Concrete 2.0/0.5/0.5 exhibited the highest compressive strength, while concrete 6.0/1.5/1.5 had the lowest strength, which was 12.2% lower. Concrete 4.0/1.0/1.0 had a strength 17.7% lower than the highest value. Important is the comparison with the reference mixture, where significant increases are evident and yet the standard deviations are small, so that the measurement error does not play a role.

The following Figure 4 shows the results of the tensile strength. The percentage difference against the reference concrete mix in

Table 4. Relative difference of tensile strength to reference mixture.

Mixture	C_2.0/0.5/0.5	C_4.0/1.0/1.0	C_6.0/1.5/1.5
Relative difference	30.1%	27.4%	23.3%

is shown. The samples exhibited overall balanced results, with a slight decrease in tensile strength observed with increasing amounts of fiber. More specifically, there was a decrease in strength of 2.2% for the concrete mix containing 4.0/1.0/1.0 kg of fiber and a 5.6% decrease for the concrete mix containing 6.0/1.5/1.5 kg of fiber compared to the concrete mix with 2.0/0.5/0.5 kg of fiber. The Figure 5 shows the results of the elastic modulus. The percentage difference against the reference concrete mix in

Table 5. Relative difference of elastic modulus to reference mixture.

Mixture	C_2.0/0.5/0.5	C_4.0/1.0/1.0	C_6.0/1.5/1.5
Relative difference	4.7%	7.8%	0.0%

is shown.

3.2. Fracture Properties

The results obtained on the samples tested in the test press using an inductance sensor could not be used immediately to obtain the fracture parameters but had to be numerically adjusted. For the correct evaluation of fracture parameters, it is necessary for the load to be applied linearly at the beginning. It was therefore necessary to adjust the initial part of the graph so that the values were linear. This was achieved by interpolating a straight line through the ascending part of the graph and gradually deleting the points that were not linear until the reliability value approached 1. This made it possible to obtain the results of effective fracture toughness and specific fracture energy. From the results shown in Figure 6, it is clear that the highest effective fracture toughness values were achieved by adding fibers in a ratio of 2.0/0.5/0.5 kg and 6.0/1.5/1.5 kg. For test specimens with fibers in a ratio of 4.0/1.0/1.0 kg, this value was 16.5% lower.

The highest values of specific fracture energy (see Figure 7) were achieved by specimens with added fibers in the ratio of 4.0/1.0/1.0 kg. This was followed by specimens with fiber content in the ratio of 6.0/1.5/1.5 kg, which achieved 11.4% lower values, and specimens with fiber content in the ratio of 2.0/0.5/0.5 kg, which achieved 25.2% lower values.

4. Summary and Discussion

This study evaluated the mechanical and fracture behavior of high-strength concrete reinforced with polypropylene fibers (PPF) of three different lengths and at varying dosages. The results show that the addition of PPF significantly influences both compressive and tensile strength, as well as fracture energy and toughness. Among the tested combinations, the mixture labeled C_2.0/0.5/0.5—containing a total of 3.0 kg/m³ of fibers in 54 mm, 38 mm, and 19 mm—demonstrated the most favorable performance across both mechanical and fracture-related parameters. The chosen mixing sequence, together with the use of a superplasticizer, successfully minimized fiber clustering and balling, which are common challenges in polypropylene fiber concretes. This contributed to the reliability of the mechanical and fracture test results.

The observed improvement can be attributed to a multiscale reinforcement mechanism, wherein the longer (54 mm) macrofibers effectively bridge larger cracks and resist post-peak deformation, while the shorter microfibers (38 mm and 19 mm) assist in controlling early-age shrinkage cracks and microcrack propagation. This hybrid action enhances the concrete’s toughness, crack resistance, and ability to dissipate energy during loading. The balanced dosage in C_2.0/0.5/0.5 appears to offer an optimal compromise: sufficient reinforcement to improve performance without the negative side effects of excessive fiber content. In contrast, mixtures with higher fiber dosages (C_4.0/1.0/1.0 and C_6.0/1.5/1.5) showed a decrease in compressive and tensile strength. This may be attributed to potential issues such as fiber clustering, reduced workability, or incomplete compaction, which could hinder matrix continuity or fiber alignment.

The results also confirm observations from recent fracture mechanics studies (e.g., [14]), which emphasize the importance of fiber input features such as length, dispersion, and aspect ratio on fracture energy and toughness. In our case, fiber clustering at higher dosages may have hindered effective dispersion, explaining the reduced compressive and tensile strengths despite higher fracture energy.

Although the present work focused on short-term mechanical and fracture behavior, the results also have implications for long-term durability. Improved crack resistance and higher fracture energy are directly linked to reduced water ingress and chloride penetration, both of which are critical factors for the service life of reinforced concrete structures. By limiting crack width and enhancing energy absorption, hybrid polypropylene fibers may contribute to lower permeability, increased resistance to freeze–thaw cycles, and improved performance in aggressive environments. This suggests that the use of hybrid PPF can support sustainability objectives by extending the lifetime of concrete structures.

One limitation of this study is that the fiber length ratio was fixed at 4:1:1. Although this ratio was selected to represent a balanced hybrid effect of macro- and microfibers, it does not exclude the possibility that other combinations could provide equal or superior performance. Similar hybrid effects were reported by [xp], who showed that multi-scale polypropylene fibers enhance both early-age strength (due to microfibers) and later-stage toughness (due to macrofibers). Future investigations should therefore systematically vary the fiber ratios to optimize performance.

Although these higher-dosage mixtures demonstrated improved specific fracture energy—indicating better post-crack energy absorption—the mechanical strength metrics suggest diminishing returns beyond a certain fiber content threshold. From the fracture mechanics perspective, the results confirm that PPF addition increases the effective fracture toughness and fracture energy of concrete. The specific combination of lengths and dosage in the C_2.0/0.5/0.5 mixture provides a synergistic effect: longer fibers enhance post-peak behavior, while shorter fibers distribute stress more evenly and suppress crack initiation. While this study provides clear evidence of the mechanical benefits of fiber hybridization, it is limited to a fixed fiber length ratio and a specific type of PPF.

Future research should investigate a broader spectrum of fiber ratios, types (e.g., monofilament vs. fibrillated), and surface treatments. Additionally, microstructural analysis techniques such as scanning electron microscopy (SEM), X-ray computed tomography (XCT), or digital image correlation (DIC) could offer deeper insight into the fiber-matrix interaction, fiber dispersion, and pull-out behavior. Such advanced analysis would help to better understand the micromechanical mechanisms that govern fiber effectiveness and could lead to the design of even more efficient fiber-reinforced concrete systems tailored for demanding structural applications. Another limitation of this study is that only one commercial polypropylene fiber type (Forta Ferro) was tested. While the observed synergistic effect of combining different fiber lengths is expected to be transferable to other polypropylene fibers, variations in fiber geometry and surface treatment may affect the magnitude of the improvement. Future research should therefore evaluate whether these findings can be generalized to a wider range of polypropylene fiber products.

5. Conclusions

This study investigated the effect of polypropylene fibers of three different lengths (54 mm, 38 mm, and 19 mm) combined in a fixed 4:1:1 ratio on the mechanical and fracture properties of high-strength concrete (C50/60). Based on the experimental results, the following conclusions can be drawn:

• The addition of hybrid polypropylene fibers significantly improved the strength properties. The optimal mixture (2.0/0.5/0.5 kg/m³) increased compressive strength by 28.7% and splitting tensile strength by 30.1% compared with the reference concrete without fibers.
• The elastic modulus was only slightly affected, with changes within ±8% depending on fiber dosage.
• Fracture parameters were strongly influenced: fracture toughness and specific fracture energy were notably higher in fiber-reinforced mixtures. The hybridization provided a synergistic effect, where longer fibers bridged macrocracks and shorter fibers reduced microcrack propagation.
• Higher fiber contents (≥4.0/1.0/1.0 kg/m³) led to reduced workability and strength, likely due to fiber clustering and compaction difficulties, although fracture energy was still improved.
• The novelty of this work lies in the systematic evaluation of a fixed hybrid fiber ratio (4:1:1), combining macro- and microfibers in one mixture and simultaneously assessing mechanical and fracture properties.
• Limitations of this study include the use of only one commercial fiber type (Forta Ferro), the absence of microstructural validation (e.g., SEM or XCT), and the lack of long-term durability tests. Future research should therefore focus on different fiber brands, detailed microstructural analyses, and durability studies (e.g., freeze–thaw, chloride penetration, and carbonation).

Overall, the findings confirm that a carefully designed hybridization of polypropylene fibers can enhance both strength and fracture performance of high-strength concrete, offering a practical path towards more durable and sustainable concrete structures.