Study on the Hybrid Effect of Basalt and Polypropylene Fibers on the Mechanical Properties of Concrete

Abstract

Hybrid fiber-reinforced concrete (HFRC), renowned for its significantly enhanced mechanical properties and structural integrity, is widely used in infrastructure construction and has become a key avenue of modern high-performance concrete development. The hybrid application of basalt fiber (BF) and polypropylene fiber (PPF) at optimized ratios generates synergistic effects, improving both mechanical performance and material service reliability. To explore and evaluate the synergistic mechanism of BF-PPF hybrid fibers on concrete’s mechanical properties and performance, this study employs an orthogonal experimental design and mechanical testing methods, measuring the materials’ static compressive strength (loading rate: 0.6 mm/min), splitting tensile strength (loading rate: 0.12–0.14 MPa/s), dynamic elastic modulus (measured by the ultrasonic method), and dynamic compressive strength (loading rates: 0.6 mm/min, 6 mm/min, and 60 mm/min). For these tests, we prepared 100 mm × 100 mm × 100 mm cubic specimens (for static compressive, dynamic compressive, and splitting tensile tests) and 400 mm × 100 mm × 100 mm prismatic specimens (for dynamic elastic modulus tests), with three parallel specimens in each test group. In addition, the microstructure was characterized by scanning electron microscopy (SEM) to observe the fiber-matrix interaction. The results show that when the BF/PPF volume ratio is 1:2 (BF0.05PPF0.1), the concrete’s compressive strength, splitting tensile strength, and elastic modulus increase by 13.7%, 76.3%, and 116.0%, respectively, with corresponding synergistic effect indices (Q) of 0.057, 0.213, and 0.241, indicating obvious positive synergy. Under dynamic loading, hybrid combinations with higher PPF content (e.g., BF0.05PPF0.1) exhibit strain-rate-dependent enhancements in compressive strength and better impact resistance. SEM analysis reveals that fibers inhibit microcrack propagation through fiber bridging, network distribution, and pull-out resistance, while also improving the interfacial transition zone’s structure. These findings provide theoretical support for the engineering application of composite fiber-reinforced concrete materials.

1. Introduction

Concrete is one of the most widely used building materials in hydraulic engineering. However, due to its inherent weaknesses, such as low tensile strength and limited strain capacity [1,2], concrete structures are prone to cracking during service, which significantly compromises their durability. Fiber-reinforced concrete can effectively enhance concrete’s internal structure. With an appropriate fiber dosage, concrete’s failure mode can be transformed from brittle to ductile [3,4,5]. Compared to single-fiber reinforcement, hybrid fiber systems that incorporate different fiber types demonstrate improved crack resistance, interfacial effects, initial crack strength, and overall strength and toughness [6,7].

Basalt fiber (BF) and polypropylene fiber (PPF), as two distinct fiber additives, are commonly employed to enhance concrete performance. Of the two, basalt fiber is recognized as an excellent “green material,” with all its raw materials derived from natural sources and its production characterized by low energy consumption. It exhibits remarkable properties, including resistance to corrosion, wear, and heat, as well as impressive mechanical performance [8,9], making it a widely used reinforcing agent. Polypropylene (PPF), on the other hand, is a commonly utilized thermoplastic resin, known for its superior chemical resistance, thermal stability, good abrasion resistance, and favorable processing characteristics [10,11], with its applications spanning various sectors, including construction and electronics. However, conventional PPF concrete has fallen short of meeting current mechanical property requirements; nevertheless, research on these fibers’ optimal blending ratios and synergistic effects remains in the experimental phase. Therefore, clarifying the anticracking mechanisms of hybrid fiber-reinforced concrete and optimizing its mix proportions are of significant practical importance for improving crack resistance in hydraulic concrete structures.

In recent years, researchers have conducted a large number of studies on the mixing effect of fiber-reinforced concrete. Fu et al. [12,13,14] demonstrated through mechanical testing (including compressive and flexural strength) that the combination of basalt and polypropylene fibers effectively inhibits crack propagation under loading, significantly enhancing flexural and splitting tensile strengths. However, improvements in elastic modulus and compressive strength were less significant compared to those in tensile properties. Mao et al. [15] suggested that the incorporation of BF and PPF could effectively suppress both early-age shrinkage cracking and later autogenous shrinkage, and Fu Qiang et al. [16] found that moderate dosages of BF and PPF positively enhance concrete’s ductility and toughness. Niu et al. [17] investigated the effects of basalt-polypropylene fibers on the compressive strength and splitting tensile strength of concrete, and found that the influence of the fiber mixture on splitting tensile strength was more significant compared to its impact on compressive strength. Moreover, excessive fiber content may negatively affect concrete’s mechanical properties. Deng [18] experimentally confirmed that when the total fiber content reaches 6 kg/m³, with a BF:PPF ratio of 1:2, the hybrid system achieves optimal compressive and tensile performance. However, although existing studies confirm the mechanical improvements achieved through BF-PPF hybridization, research on the synergistic effect of hybrid fibers is still relatively scarce, especially in exploring the mechanism of synergistic concrete performance enhancement by hybrid fibers, where there remain gaps in understanding.

To address these gaps, this study employs an orthogonal experimental design to investigate the effects of different volume ratios (BF:PPF = 1:1, 1:2, 2:1) on the compressive strength, splitting tensile strength, and elastic modulus of hybrid fiber-reinforced concrete. Dynamic loading tests (strain rates ranging from 10⁻⁴ s⁻¹ to 10⁻² s⁻¹) combined with SEM microstructural characterization are conducted to reveal fiber-matrix synergy mechanisms and determine optimal hybrid ratios. The research aims to provide theoretical foundations and technical support for the engineering application of hybrid fiber-reinforced concrete materials.

2. Materials and Methods

2.1. Raw Materials

2.1.1. Concrete Material

The concrete materials used in this test were as follows: Portland cement (P.O 42.5), produced by Tianrui Company in Zhengzhou, China (cement strength indicators shown in

Table 1. Strength index of P.O 42.5.

Instar	3d Flexural Strength (MPa)	28d Flexural Strength (MPa)	3d Compressive Strength (MPa)	28d Compressive Strength (MPa)
Standard value	≥3.5	≥6.5	≥17.0	≥42.5
Measured value	5.6	7.9	26.2	45.7

); JDU-1 high-performance air-entraining agent; RD-N high-efficiency water-reducing agent; tap water from Zhengzhou City, China; grade I fly ash produced by Zhengzhou Thermal Power Plant, China; medium-grade river sand (Zone II) from Tanghe County, Nanyang City, Henan Province, China; and coarse aggregates consisting of continuously graded (5–20 mm), rough-textured, hard angular crushed stones sourced from Jiayu Town, Xingyang City, Zhengzhou, Henan Province, China.

2.1.2. Fibrous Material

The hybrid fibers used were chopped basalt fibers (BFs) and polypropylene fibers (PPFs). The fibers’ monofilament lengths were, respectively, selected as 18 mm and 19 mm, as these lengths are widely used in the literature and can enhance concrete’s mechanical properties at appropriate dosages [14,19]. The performance parameters of these fibers are provided in

Table 2. Fiber physical and mechanical property parameters.

Material Name	Single Length (mm)	Filament Diameter (μm)	Density (kg/m3)	Fracture Ductility (%)	Modulus of Elasticity (GPa)	Tensile Strength (MPa)
Basalt fiber	18	14	2650	2.5	105	3500
Polypropylene fiber	19	23	910	15	7.2	745

, and Figure 1 illustrates the natural accumulation state of BF and PPF in their physical forms.

2.2. Preparation and Method

2.2.1. Hybrid Fiber Orthogonal Design

Table 3. Orthogonal test design of hybrid fiber concrete.

Number	Test Piece	Two Factors Five Levels Orthogonal		Sequence
		BF Dosage/%	PPF Dosage/%
A0	PC	0	0	1
Z1	BF0.05 PPF0.05	0.05	0.05	2
Z2	BF0.075 PPF0.075	0.075	0.075	3
Z3	BF0.05 PPF0.1	0.05	0.1	4
Z4	BF0.1 PPF0.05	0.1	0.05	5
B1	BF0.1	0.1	0	6
B2	BF0.15	0.15	0	7
P1	PPF0.1	0	0.1	8
P2	PPF0.15	0	0.15	9

shows the orthogonal design of mixed basalt and polypropylene fiber dosages. The optimal volume dosage of PPF for mechanical performance is 0.1% to 0.15% [20], and that of BF is 0.1% [21,22]. Therefore, in the experiment, the dosages of PPF and BF were selected as 0, 0.5%, 0.075%, 0.1% and 0.15%, respectively. A two-factor, five-level orthogonal test was designed, with PC as the control group. A total of 9 groups of samples with different fiber types and dosages were prepared, and the specific mix ratio design of the materials is shown in Table 2.

The experimental design employed a two-factor (BF and PPF volume fraction), five-level (0%, 0.05%, 0.075%, 0.1%, and 0.15%) orthogonal strategy to efficiently explore the individual and interactive effects of the fibers. As established in previous studies [23,24], the mechanical properties of fiber-reinforced concrete exhibit positive correlations with fiber dosage within a specific range, although exceeding this range can lead to diminished workability and increased complexity in the material’s nonlinear behavior, thereby reducing economic viability.

To comprehensively capture these effects, a total of nine mixture proportions were strategically selected as control points, as illustrated in Figure 2. This selection included plain concrete (PC) as the baseline (A0); single-fiber control groups at low, medium, and high dosages (B1, B2 for BF; P1, P2 for PPF) to isolate the contribution of each fiber type; and hybrid-fiber groups along and near the diagonal (Z1–Z4), where the total fiber volume was maintained within the optimal range of 0.1% to 0.15%. This approach allows for the systematic investigation of synergistic effects at different blending ratios (e.g., 1:1, 1:2, 2:1) while controlling for total fiber content, as well as enabling a clear comparative analysis of the factors influencing the mechanical properties, distinguishing between the effects of individual fibers and their hybrid interactions.

2.2.2. Mix Ratio and Matching Process

This experiment’s mix design was based on the “Mandatory National Standard for Common Portland Cement” (GB/T 50081-2002) [25] and formulated through comparison with practical construction mix ratios. Hybrid fiber volume dosages of BF and PPF were selected-BF0.05% + PPF0.05%, BF0.075% + PPF0.075%, BF0.05% + PPF0.1%, and BF0.1% + PPF0.05%-and the control groups included single-doped fiber systems-BF (0.1%, 0.15%) and PPF (0.1%, 0.15%)-along with plain concrete (PC) without fibers. The concrete mix design was based on a constant base proportion with a water-to-cement ratio of 0.45, and the mass contents per cubic meter were as follows: cement, 272 kg; water, 175 kg; fine aggregate, 642 kg; coarse aggregate, 1193 kg; fly ash, 117 kg. The dosage of the high-performance air-entraining agent (JDU-1) and high-efficiency water-reducing agent (RD-N) were fixed at 19.45 g/m³ and 3.89 kg/m³, respectively, ensuring that the only variable was the type and volume fraction of fibers, as detailed in the experimental design (Table 3).

The preparation of test specimens was conducted in accordance with the Standard for Test Methods of Mechanical Properties of Ordinary Concrete (GB 175−2023) [26]. During preparation, coarse and fine aggregates, cement, and fly ash were first dry-mixed in a drum mixer for 1 min to ensure homogeneity, with BF and PPF then added in 2–3 batches; prior to each addition, the fibers were teased apart and dry-mixed at 1 min intervals to ensure uniform dispersion within the dry mixture. Subsequently, the water mixed was added to the drum along with the air-entraining and water-reducing agents in the prescribed amounts, and the mixture was thoroughly stirred. The uniformly blended slurry was then poured into molds and compacted on a vibrating table (see Figure 3 for preparation process), with the entire specimen mixing and preparation process completed within 15 min. After demolding following static curing, the specimens were standard-cured for 28 days to meet normative requirements.

2.3. Test Methods

2.3.1. Uniaxial Compression Test

Compressive strength has always been one of the main indicators used to evaluate the mechanical properties of concrete. Peak strain reflects the maximum degree of deformation that a material can withstand before failure or yield, a key mechanical parameter of concrete. In this study, in accordance with the requirements of TCECS 864-2021 “Standard Test Methods for Ultra-High Performance Concrete” [27], the DY-3008DX electro-hydraulic servo universal testing machine was used for the tests, and the quasi-static loading rate was 0.6 mm/min. Cubic samples of 100 mm × 100 mm × 100 mm were used, and three samples were prepared for each group, taking the average of the test results as the final value. By analyzing the stress-strain diagrams obtained from the tests, the peak strain and failure load of the concrete specimens were derived, and the compressive strength was calculated using Formula (1). The uniaxial compression testing machine is shown in Figure 4. The compressive strength is calculated using Equation (1):

$$f_{c,u}={\displaystyle \frac{F}{A}}$$

(1)

In the formula, f_c,u is the compressive strength of the specimen, MPa; F is the applied load, N; and A is the compression area, that is, the upper surface area of the specimen, mm².

2.3.2. Splitting Tensile Test

In the study of fiber-reinforced concrete’s mechanical properties, splitting tensile strength serves as one of the critical indicators for evaluating performance quality. The splitting tensile strength tests were conducted in accordance with TCECS 864-2021 “Standard Test Methods for Ultra-High Performance Concrete” [27], employing a TYA-2000 digital-display pressure testing machine with matched splitting tensile strength fixtures. The loading rate was maintained within 0.12 MPa/s to 0.14 MPa/s and cubic specimens measuring 100 mm × 100 mm × 100 mm were used, with three specimens prepared per test group and the average of the test results adopted as the final value. The peak strain and failure load of the concrete specimen were derived from the stress-strain diagram obtained from the splitting tensile test, and based on the maximum load recorded in the splitting tensile test, the splitting tensile strength of the concrete diagram specimen was calculated using Equation (2). The splitting tensile test is shown in Figure 5.

$$f_{s,t}={\displaystyle \frac{2F}{\pi A}}=0.637{\displaystyle \frac{F}{A}}$$

(2)

In the formula, f_s,t is the splitting tensile strength, MPa; F is the applied load, N; and A is area of the split surface, mm².

2.3.3. Elastic Modulus Test

In concrete materials, the dynamic elastic modulus is a key parameter used to characterize the material’s brittleness and elastic behavior. The dynamic elastic modulus test is conducted using an ultrasonic measurement method, which calculates the modulus based on the propagation velocity of ultrasonic waves through the specimen in this case, a DT-20 dynamic elastic modulus tester-with the resonance method employed to assess the axial vibration characteristics of the prismatic specimens. The test specimens were prismatic blocks measuring 400 mm × 100 mm × 100 mm, with three samples prepared for each test group and the final result for each group determined as the average of the three measurements. The experimental setup and instrumentation are illustrated in Figure 6. The calculation formula for the dynamic elastic modulus of a specimen is shown in Equation (3):

$$Ed={\displaystyle \frac{13.244\times {10}^{-4}\times W{L}^3{f}^2}{a^4}}$$

(3)

In the formula, E_d is the dynamic elastic modulus, MPa; W is the sample quality, kg; L is the specimen length, mm; f is the transverse vibration frequency, Hz; and a is the side length of the section, mm.

2.4. Fiber Hybrid Synergistic Benefit Theory

The synergistic effect of hybrid fibers in improving concrete performance is primarily evaluated by assessing whether their contribution is more effective compared to that of a single fiber type, typically achieved by calculating the contribution of the fiber mixture to the enhancement of concrete properties [28,29]. Therefore, quantifying the synergistic effect of hybrid fibers is critical for determining the positive or negative contributions of the hybrid effect, and in this study, a synergistic evaluation index (Equations (4)–(6)) was employed to assess the synergistic interaction of hybrid fibers.

$$Q={\displaystyle \frac{S-(S_1{\phi }_1+S_2{\phi }_2+S_3{\phi }_3+\cdots )}{S_1{\phi }_1+S_2{\phi }_2+S_3{\phi }_3+\cdots }}$$

(4)

$${\phi }_1+{\phi }_2+{\phi }_3+\cdots =1$$

(5)

$${\phi }_i={\displaystyle \frac{V_i}{V}},i=1,2,3,\cdots \cdots$$

(6)

In the formula, Q represents the cooperative evaluation index, S represents the performance index attribute of the hybrid-fiber concrete, S_i is the performance index value of the single-doped fiber concrete, V_i is the volume of the single-type fibers, V is the total volume of hybrid fibers, and ${\phi }_i$ is the volume fraction of single-type fibers. The collaborative evaluation index Q > 0 indicates a positive hybrid effect on performance, meaning that the blending method is stronger than the sum of the performance improvement of all parts of the fiber, and an improvement in a certain aspect of performance is realized in the concrete. On the contrary, if Q < 0, this shows a negative hybrid effect on performance, meaning that the blending method is worse than the sum of the performance improvement from all parts of the fiber. If Q = 0, that is, the synergy index is 0, then there is no performance synergy.

3. Results and Discussion

3.1. Compressive Strength

Figure 7 presents the compressive strength values and peak strain results from three groups of test specimens. The experimental results indicate that when the total fiber volume content ranges between 0.1% and 0.15%, the concrete’s compressive strength shows a positive correlation with total fiber content: at 0.1% total fiber content, the specimens’ compressive strengths ranked in descending order as BF0.1 > BF0.05PPF0.05 > PPF0.1; when the total fiber content increased to 0.15%, the ranking became BF0.15 > BF0.1PPF0.05 > PPF0.15. Specimens with a 1:1 blending ratio of BF and PPF exhibited the lowest compressive strength, demonstrating a negative synergy effect; however, when the BF:PPF ratio was adjusted to 1:2, the compressive strength exceeded that of single-fiber-reinforced specimens. This suggests that the optimal total fiber content of BF and PPF should be maintained within 0.1–0.15%.

As an inorganic high-elastic-modulus fiber, BF demonstrates excellent bonding with the cement matrix, effectively inhibiting microcrack initiation during the elastic stage and enhancing early-stage performance. However, its crack-arresting capability diminishes as cracks propagate, eventually showing similar tensile properties to plain concrete (PC). In contrast, PPF (a hydrophobic synthetic organic fiber with low elastic modulus) exerts frictional resistance at the fiber-matrix interface, effectively restricting microcrack propagation and significantly improving splitting tensile strength. Experimental evidence shows that both fibers reduce microcrack formation and delay crack development, thereby enhancing concrete performance [30]. Notably, BF-reinforced specimens exhibited lower peak strain compared to PC and PPF specimens, with the latter demonstrating effective crack impedance during later crack propagation stages, maintaining higher compressive strength by preventing crack expansion and spalling [31,32].

As shown in

Table 4. Compressive strength of hybrid fiber-reinforced concrete.

Sample	Compressive Strength (MPa)				Standard Deviation σv	Coefficient of Variation Cv/%	Collaborative Evaluation Index
	1	2	3	Standard Value
PC	45.2	38.8	44.5	42.8	2.87	6.69	—
Z1	45.9	44.3	−	44.3	0.80	1.77	−0.006
Z2	43.5	45.8	44.7	44.7	0.94	2.10	−0.023
Z3	51.8	48.7	−	48.7	1.55	3.08	0.057
Z4	44.9	43.6	46.4	45.0	1.14	2.54	−0.009
B1	44.0	44.2	43.8	44.0	0.16	0.37	—
B2	44.1	43.9	46.3	44.8	1.09	2.43	—
P1	43.9	44.8	46.8	45.2	1.21	2.68	—
P2	40.5	46.7	50.4	46.7	4.08	8.90	—

and

Table 5. Peak strain of compressive strength of hybrid fiber-reinforced concrete.

Sample	Peak Strain (10−3)				Standard Deviation σv	Coefficient of Variation Cv/%
	1	2	3	Standard Value
PC	1.68	1.33	1.66	1.56	0.22	14.31
Z1	1.60	1.24	1.37	1.40	0.22	15.51
Z2	1.07	1.16	1.22	1.15	0.10	8.66
Z3	1.16	0.94	1.01	1.04	0.16	15.06
Z4	1.19	1.24	1.39	1.27	0.13	10.24
B1	1.15	1.21	1.16	1.17	0.04	3.58
B2	0.89	1.07	0.92	0.96	0.14	14.50
P1	1.27	1.30	1.23	1.27	0.04	3.48
P2	1.26	1.57	1.33	1.39	0.20	14.08

, excessive BF proportions in hybrid mixtures caused negative synergy effects; however, when PPF dominated the mixture, a small BF content effectively suppressed early-stage crack propagation while PPF enhanced load transfer through matrix integration. This combination demonstrates a positive synergistic benefit superior to single-fiber mixes, with a synergistic evaluation index of 0.057.

3.2. Splitting Tensile Strength

Figure 8 presents the experimental results for concrete splitting tensile strength and peak strain. The findings demonstrate that when the total fiber volume content ranges between 0.1% and 0.15%, the single addition of BF provides inferior tensile performance enhancement compared to PPF. The hybrid fiber combination shows overall better tensile strength improvement than single-fiber reinforcement: specifically, hybrid fiber specimens BF0.05PPF0.1 and BF0.1PPF0.05 exhibit 76.34% and 69.19% strength increases, respectively, compared to the PC control group. However, when using a 1:1 fiber ratio (groups Z1 and Z2), the tensile performance improvements decrease significantly to only 12.59% and 7.03%, with strength values even lower than single-fiber specimens. Regarding peak strain characteristics, BF-only concrete shows a notably smaller peak strain than both the PC control group and PPF-only concrete, and while PPF addition alone increases specimen peak strain, the hybrid BF-PPF combination demonstrates limited improvement (≤15% increase) compared to the control group, indicating insignificant influence on the concrete’s peak strain [33,34]. In addition, to evaluate the influence of fibers on the toughness of concrete, the tensile strength ratio (splitting tensile strength divided by compressive strength) of each mixture was calculated. The higher the ratio, the better the tensile performance and toughness compared to the compressive strength. The results show that the tensile and compressive ratio of the mixed fiber concrete with a dosage of BF0.05 and PPF0.1 is the highest at 0.069. This indicates that the optimal mix ratio not only enhances the tensile strength of concrete but also improves its toughness, further confirming the positive synergistic effect of BF and PPF at a volume ratio of 1:2.

The experimental results reveal that hybrid BF-PPF incorporation synergistically enhances the pre-peak elastic mechanical properties of concrete. This combination effectively inhibits microcrack initiation and enhances frictional resistance between matrix cracks. Particularly, a 1:2 ratio of BF-PPF volume demonstrates the strongest synergistic effect in tensile performance improvement. As shown in

Table 6. Splitting tensile strength of hybrid fiber-reinforced concrete.

Sample	Splitting Tensile Strength (MPa)				Standard Deviation σv	Coefficient of Variation Cv/%	Increase Rate (%)	Collaborative Evaluation Index
	1	2	3	Standard Value
PC	1.82	1.9	1.92	1.90	0.06	3.42	0	—
Z1	2.11	2.14	2.33	2.19	0.10	4.56	12.59	−0.138
Z2	1.90	2.04	2.15	2.03	0.10	5.02	7.03	−0.249
Z3	3.19	3.42	3.50	3.37	0.13	3.82	76.34	0.213
Z4	2.98	3.07	3.10	3.05	0.05	1.67	69.19	0.204
B1	1.98	2.06	2.11	2.05	0.05	2.63	8.16	—
B2	2.22	2.31	2.34	2.29	0.05	2.23	20.00	—
P1	2.72	2.88	2.92	2.84	0.08	3.03	54.07	—
P2	2.88	3.05	3.13	3.02	0.10	3.44	65.14	—

and

Table 7. Peak strain of splitting tensile strength of hybrid fiber-reinforced concrete.

Sample	Peak Strain (10−3)				Standard Deviation σv	Coefficient of Variation Cv/%
	1	2	3	Standard Value
PC	0.58	0.60	0.71	0.63	0.06	10.20
Z1	0.62	0.65	0.71	0.66	0.04	5.43
Z2	0.62	0.63	0.64	0.63	0.01	2.13
Z3	1.16	0.94	1.01	0.65	0.08	11.28
Z4	0.68	0.71	0.76	0.72	0.03	4.60
B1	0.42	0.49	0.56	0.49	0.06	12.47
B2	0.36	0.42	0.48	0.42	0.05	13.84
P1	0.70	0.78	0.84	0.77	0.06	7.25
P2	0.58	0.70	0.80	0.69	0.09	12.99

, depicting the synergistic effects in basalt-polypropylene hybrid-fiber concrete splitting tensile strength, this high synergy arises from increased ultimate load capacity through combined fiber action, observed concrete densification, and effective fiber bridging within cracks.

3.3. Modulus of Elasticity

Figure 9 shows the elastic moduli of fiber-reinforced concretes with different admixture ratios obtained from experiments. The test results indicate that at total volume admixture levels of 0.1% to 0.15%, the elastic modulus of single-fiber specimens BF0.1 and BF0.15 increased by 39.75% and 80.39%, respectively, compared to the control PC. The corresponding improvement rates for PPF0.1 and PPF0.15 were 25.75% and 66.81%, demonstrating that the single admixture of basalt fiber (BF) provides better elastic modulus enhancement than polypropylene fiber (PPF) at equivalent volume ratios in concrete. Under hybrid-fiber conditions, the BF0.05 PPF0.1 specimen exhibited the highest elastic modulus of 64.32 GPa, representing a 116.03% improvement over the control PC, exceeding both single-fiber concrete types and demonstrating the positive synergistic effects of hybrid-fiber reinforcement. However, when the fiber ratio was 1:1, the elastic moduli of the specimens became lower than for both single-fiber concretes, showing negative synergistic effects. Notably, the BF0.1PPF0.05 specimen’s elastic modulus reached only 42.26 GPa, inferior to both single-fiber concretes at equivalent volume ratios. This comparative analysis reveals that basalt fiber (BF) effectively enhances the elastic mechanical properties of concrete, whereas polypropylene fiber (PPF) shows relatively weaker elastic-phase improvements. The synergistic effects of basalt-polypropylene hybrid fibers in elastic modulus enhancement, as illustrated in

Table 8. Elastic modulus of hybrid fiber-reinforced concrete.

Sample	Specimen Number	Fiber Content (%)		Modulus of Elasticity (GPa)	Increase Rate (%)	Collaborative Evaluation Index
		BF	PPF
A0	PC	—	—	30.24	0.00	—
Z1	BF0.05PPF0.05	0.05	0.05	32.52	7.53	−0.19
Z2	BF0.075PPF0.075	0.075	0.075	36.42	20.43	−0.31
Z3	BF0.05PPF0.1	0.05	0.1	64.32	116.03	0.24
Z4	BF0.1PPF0.05	0.1	0.05	46.60	54.11	−0.12
B1	BF0.1	0.1	—	42.26	39.75	—
B2	BF0.15	0.15	—	54.55	80.39	—
P1	PFF0.1	—	0.1	38.03	25.75	—
P2	PFF0.15	—	0.15	50.44	66.81	—

, further corroborate these observations on composite fiber interaction mechanisms.

3.4. Strain Rate and Dynamic Compressive Strength

When concrete is subjected to strain loading at rates ranging from 10⁻⁵ to 10⁻⁴ s⁻¹, its uniaxial compressive performance is minimally affected by variations in strain rate, and the results can approximate the static compressive stress strength [35]. In this experiment, the DY-3008DX electro-hydraulic servo universal testing machine was employed, a strain rate of 10⁻⁴ s⁻¹ was used as the quasi-static loading condition, and 10⁻³ s⁻¹ and 10⁻² s⁻¹ were used as the dynamic loading conditions, corresponding to displacement loading rates of 0.6 mm/min, 6 mm/min, and 60 mm/min, respectively.

Table 9. Hybrid fiber-reinforced concrete property rules under different strain rates.

Sample	Different Strain Rate (s−1)
	10−4 s−1		10−3 s−1		10−2 s−1
	Static Compressive Strength (MPa)	Strength Increase Rate (%)	Dynamic Compressive Strength (MPa)	Strength Increase Rate (%)	Dynamic Compressive Strength (MPa)	Strength Increase Rate (%)
PC	42.8	−	46.5	−	48.3	−
Z1	44.3	3.42	46.6	0.22	48.4	0.21
Z2	44.7	4.28	47.6	2.37	48.8	1.04
Z3	48.7	13.70	54.3	16.77	56.8	17.60
Z4	45.0	4.98	48.2	3.66	49.6	2.69
B1	44.0	2.72	47.1	1.29	48.7	0.83
B2	44.8	4.59	47.8	2.80	48.9	1.24
P1	45.2	5.49	48.5	4.30	50.1	3.73
P2	46.7	9.03	52.1	12.04	54.2	12.22

presents the performance data of hybrid fiber-reinforced concrete under different strain rates. Figure 7 illustrates the compressive strength of fiber-reinforced concrete across varying strain rates. As shown in Figure 10, the BF0.05PPF0.1 specimen exhibits uniaxial compressive strengths of 48.7 MPa, 54.3 MPa, and 56.8 MPa at strain rates of 10⁻⁴ s⁻¹, 10⁻³ s⁻¹, and 10⁻² s⁻¹, respectively, demonstrating a positive correlation between compressive performance and strain rate. The PPF0.15 specimen also shows an increasing trend in compressive strength under dynamic loading. However, concrete specimens containing solely basalt fiber (BF) or hybrid fibers with a BF content exceeding 50% show limited improvement in dynamic load resistance.

As an organic synthetic fiber, PPF exhibits poor adhesion to the concrete matrix. Under quasi-static loading conditions, PPF incorporation fails to demonstrate advantages, whereas BF effectively inhibits early-stage damage. However, as the loading rate increases, PPF displays superior impact resistance, indicating that hybridizing PPF and BF at appropriate ratios significantly enhances concrete’s dynamic load resistance.

3.5. SEM Microstructure Analysis

To reveal the morphological characteristics of external fibers within the concrete and the interaction features between the hybrid fibers and matrix, this study prepared SEM specimens (2 cm × 2 cm) of the fiber-reinforced concrete (FRC) mixtures in the laboratory. The microstructures of the hybrid fiber concretes were observed using a Hitachi Regulus 8100 cold-field emission scanning electron microscope.

Figure 11 displays SEM micrographs showing uniaxial compression damage in concrete, revealing that under a load, microdamage and microcracks first initiate at the interface between the aggregates and cement paste. These microcracks gradually propagate into the cement mortar as compression progresses, ultimately connecting with adjacent cracks to form larger penetrating fractures, meaning that a “mechanically vulnerable region” exists between the coarse/fine aggregates and cement paste. As demonstrated by Jia [36], an interfacial transition zone (ITZ) exists between the different concrete phases, constituting a thin layer of highly porous mortar surrounding coarse aggregates or fine sand. The mechanical and apparent properties of the ITZ show no fundamental difference from ordinary mortar, but its high-porosity structure results in comparatively inferior performance; thus, the ITZ can be considered as weakened mortar.

As secondary reinforcement, fibers substitute traditional steel materials in concrete through three primary interaction mechanisms, leveraging their distinct material properties. Basalt fiber (BF), an inorganic fiber with a high elastic modulus and strong adhesion to the cement matrix [37], is dispersed as individual filaments with hydration products adhered to their surface, indicating robust bonding. When concrete is under compressive or impact loads, microcracks tend to initiate, but the presence of BF effectively suppresses their formation during the elastic stage, enhancing early mechanical performance. Polypropylene fiber (PPF), on the other hand, exhibits superior chemical stability that promotes internal hydration processes. Both fibers contribute positively to the matrix’s tensile behavior, as corroborated by mechanical tests including compressive strength, splitting tensile strength, and peak strain measurements, confirming that, overall, the crack resistance and concrete performance are improved. The three reinforcement mechanisms are as follows: Firstly, the fiber bridging effect (Figure 12a,b involves single filaments spanning across cracks to connect two paste segments or multiple parallel fibers linking separate paste blocks, effectively transferring stress and delaying crack opening. Secondly, the network distribution pattern (Figure 12c) features intersecting fibers forming spatial meshes that enhance integration with cement paste and aggregates. This network redistributes localized stresses through the matrix and inhibits crack propagation, significantly improving structural integrity and reducing post-failure spalling in PPF-enhanced concrete. Thirdly, the fiber pull-out resistance mechanism (Figure 12d) arises from interfacial friction between fibers and paste, which restricts crack expansion. This results in microdamage patterns resembling steel bar pull-out behavior, with umbrella-shaped cracks emanating from the interface transition zone [28]. Under tensile stress, the paste initially resists deformation until microcracks nucleate near aggregates or pores. Further stress increases provoke radial cracking, until interfacial friction is overcome, leading to either fiber-matrix debonding (Figure 12d) or fiber fracture (Figure 12b).

4. Conclusions

• This study systematically investigated the hybrid effect of basalt fiber (BF) and polypropylene fiber (PPF) on the mechanical properties of concrete under both quasi-static and dynamic loading conditions, and the results demonstrate that concrete’s mechanical performance is highly sensitive to the fiber blending ratio. An optimal volume ratio of BF to PPF at 1:2 (BF0.05PPF0.1) was identified, under which the compressive strength, splitting tensile strength, and elastic modulus increased by 13.7%, 76.3%, and 116.0%, respectively, compared to plain concrete. The calculated synergistic evaluation indices (Q) for these properties were 0.057, 0.213, and 0.241, respectively, confirming a significant positive synergistic effect, with this optimal ratio leveraging the complementary functionalities of the high-modulus BF (effective in crack initiation inhibition) and the low-modulus PPF (effective in crack propagation restraint).
• The compressive strength of concrete shows a positive correlation with strain rate (10⁻⁴~10⁻² s⁻¹). Under dynamic-loading conditions, with the total fiber content ranging 0.1~0.15%, effective compressive strength improvements (13.70%, 16.77%, and 17.60% enhancement rates) were achieved when the BF-PPF ratio reached 1:2. This indicates that hybrid basalt-polypropylene fibers can induce “delayed deformation” in concrete under compression, endowing the material with superior impact resistance under dynamic loads.
• Scanning Electron Microscopy (SEM) analysis revealed the fiber distribution and interaction mechanisms within the concrete specimens studied: the fibers were observed to enhance matrix strength primarily through bridging and tensile actions, which explains the improved tensile resistance and pull-out performance when measured mechanically. Observations confirmed that microcracks typically initiated in the interfacial transition zone (ITZ) before propagating into the mortar matrix, and the hybrid fiber system demonstrated a synergistic effect in reinforcing this critical ITZ region. These microstructural observations offer a plausible mechanistic explanation for the macroscopic mechanical enhancements recorded, and the findings contribute fundamental insights that could inform the design of hybrid fiber-reinforced concretes.
• This study has several limitations that also define avenues for future research. The findings are based on laboratory-scale specimens under standard curing and short-term loading conditions. The optimal fiber ratio (BF0.05PPF0.1) was identified within a total fiber volume fraction range of 0.1~0.15%; different ratios may be optimal for other dosage ranges or specific applications such as seismic resistance or impact protection. Furthermore, while SEM analysis effectively reveals morphological interactions, it cannot provide quantitative data on the chemical bonding at the fiber-matrix interface. Techniques such as Fourier Transform Infrared Spectroscopy (FTIR) could be employed in future work to provide deeper insights into these interface characteristics, and future research should also include (1) long-term investigations into durability aspects such as creep, fatigue, and performance in aggressive environments, and (2) a cost-benefit analysis to assess the economic viability of using this hybrid fiber combination in practical engineering projects.