Influence of Hybrid Fibers on Workability, Mechanical and Dynamic Properties of Ultra-High Performance Concrete

Abstract

Ultra-high performance concretes (UHPCs) have been widely used in the construction industry due to their high strength and long-term performance. The purpose of this article is to review the literature on UHPC that contained at least two types of hybrid fiber with different lengths, diameters, and volumetric contents. The results show that the type of fiber, its geometry, including length, diameter, and shape, as well as volumetric content, affect the properties of the concrete, not only in the hardened state, but also in the fresh state. The compressive and flexural strength results increase with higher impact velocity and steel fiber content, with a higher content of shorter fibers contributing to increased strength and energy absorption. Tensile strength increases with the length of the steel fibers and the higher content of polyolefin, polyoxymethylene, and polyester fibers. Investigating new types of fiber, various shape factors, geometries, and anchoring mechanisms of hybrid fibers is essential to improve the workability, adhesion, and strength of the material.

1. Introduction

The growing global population, which was 5 billion in 1987, reached 8 billion in 2024, with a projected 9.7 billion by 2050, is driving an increasing demand for cement, which is essential in the construction process. This contributes to an increase in energy consumption related to cement production. It is estimated that 1 Mg of cement generates as much as 0.73 to 0.99 Mg of CO₂ [1]. Due to increasingly stringent production standards associated with material reuse, reduction of carbon footprints, and limiting temperature growth, optimizing the use of concrete in construction is essential. This can be achieved by reducing the content of clinker in cement, replacing part of the cement with supplementary cementitious materials (SCM), using recycled aggregate, or making effective use of cross-section capacity [2,3,4]. These sustainability-driven approaches align with broader trends in material engineering that aim to incorporate renewable and low-impact raw materials into construction products. Similar concepts have been explored, for example, in the development of epoxy-based modifiers for bitumen derived from plant-based feedstocks [5]. To achieve this, concrete mixes must be designed to achieve high-strength parameters while optimizing the use of cement and SCM. Concrete can be reinforced with recycled fibers, such as steel wires obtained by pyrolysis of used tires [6,7], natural fibers such as coconut fibers [8], or fibers extracted from palm trees [9,10].

High-rise buildings, long-span bridges, impact-resistant shelters, structures located in seismic zones, and reinforcements for damaged or poorly executed structures are examples of applications requiring high-quality cementitious composites [11,12,13,14,15,16,17,18]. Normal-strength concrete (up to 55 MPa compressive strength) has significant limitations for such structures. The concept of ultra-high performance concrete (UHPC) was first proposed by De Larrard and Sedran [19], who achieved a 28-day compressive strength of 165 MPa and a splitting tensile strength of 6.6 MPa for fiber-free concrete with a water-to-binder ratio (W/B) of 0.14. It is generally accepted that the compressive strength of this type of concrete should exceed 120 MPa [20]. A key advantage of UHPC with added fibers is its high tensile strength and high energy absorption capacity [21]. Due to its excellent mechanical properties, ultra-high-performance concrete is one of the most promising construction materials dedicated to civil and military infrastructure systems, including long-span bridges, defensive structures, and high-rise buildings [22]. UHPC is characterized by a low water-to-binder ratio, resulting in low permeability and high resistance to chloride and sulfate penetration, significantly reducing the corrosion of reinforcement bars or steel (ST) fibers [23,24,25,26,27,28]. UHPC exhibits excellent resistance to carbonation due to its high strength parameters, which delay crack formation and control crack width in the development phase, making it difficult for water to penetrate [29]. Furthermore, it has a high particle packing density, low porosity, and excellent impermeability, resulting in high durability and extended service life of structures [30]. Initially, ultra-high performance concretes without fibers were used, in which the improvement of parameters was achieved by designing mixtures with high-quality cement and aggregates, along with an appropriately low W/B ratio, which for UHPC typically ranges between 0.14 and 0.20 compared to 0.4–0.5 for conventional concrete [31,32,33,34,35,36,37]. However, such UHPCs exhibited a high degree of brittleness [38,39], leading to the initial use of a single type of steel fiber, allowing for an improvement in properties sufficient to meet material requirements. The addition of a single type of fiber improved the static properties of UHPC, particularly the compressive and tensile strength. However, the use of a single type of fiber does not always guarantee a sufficient improvement in compressive strength, as observed in the case of UHPC containing 18 mm basalt fibers, 10 mm polypropylene fibers, or 12 mm glass fibers [40]. Chen et al. [41] also reported a decrease in the compressive strength of ultra-high performance concrete with the addition of up to 2.5% basalt, glass, or polypropylene fibers. With the advancement of production technology, two or more types of fiber of different sizes, types, and shapes began to be used, which complemented each other due to their diverse properties and allowed a reduction in final costs [42,43,44,45,46,47]. In the case of hybrid combinations, it was also possible to achieve higher tensile strength, abrasion, and fatigue resistance, as well as energy absorption capacity [48]. In such complex reinforcement systems, short fibers inhibit microcrack formation, acting during the lower load phase. On the other hand, the addition of long fibers improves strength under higher loads by slowing macrocrack propagation and improving ductility through the crack bridge [49,50,51]. Due to the larger contact surface between the long fibers and the concrete matrix, and sometimes their special shape, which enhances anchoring, their extraction is more difficult [52]. This enables a transformation of the behavior of UHPC from brittle to ductile, improving energy absorption capacity, increasing safety, and reducing the risk of structural failure [53].

A properly selected amount of fibers creates a spatial skeletal structure that strengthens and hardens the UHPC, bridges, and reduces cracks [8,22]. The first type of fiber used was steel fibers of various lengths (from 6 to 50 mm) and shapes (straight, twisted, wavy, or hooked ends) [54,55]. They are particularly effective in improving fracture resistance and increasing compressive and tensile strength [56,57,58,59]. However, they are highly dense, are relatively expensive to produce and exhibit poor corrosion resistance. Additionally, the carbon dioxide emissions generated during their production have a negative impact on the natural environment. Another type of fiber that has attracted researchers’ attention is basalt fiber [60,61,62], which, due to its thermal stability, corrosion resistance, lower cost, and lower energy consumption during production, has become an alternative to steel fiber [63,64,65,66,67]. Basalt fibers are available in various dimensions, that is, lengths ranging from 12 to 50 mm and diameters ranging from 0.012 to 0.45 mm. Furthermore, they demonstrate high resistance to corrosion in salt solutions, especially aqueous solutions, and high resistance to frost after freeze–thaw cycles [68,69]. The addition of basalt fibers improves the strength properties and impact resistance of concrete, increases crack resistance, and extends its useful life. A hybrid mix containing steel and basalt fibers allows optimal use of the properties of both types of fibers [70,71]. The compressive and flexural strength of UHPC can increase by more than 50%, while the splitting tensile strength can reach twice the value. Furthermore, the hybrid fiber-reinforced UHPC demonstrates high resistance to dynamic impacts, such as explosions or projectile strikes [72]. Another addition to concrete mixes in the form of hybrid fibers includes micro- and macro-sized polymer fibers with various chemical compositions [73,74,75,76]. These include polyvinyl alcohol (PVA), polyoxymethylene (POM), polyethylene (PE), polypropylene (PP), and polyester (PET) fibers, with lengths ranging from 13 to 40 mm and diameters from 0.012 to 0.9 mm, depending on the performance requirements for the concrete mix [77,78,79,80]. Glass fibers are also used, with lengths of 6–18 mm and diameters of 0.012–0.025 mm. They can be added to mixes as a single type of fiber or combined with steel fibers, resulting in high mechanical performance values [81,82,83]. In the course of ongoing research, steps have also been taken to investigate the effects of other types of fiber combinations, such as carbon and steel fibers [84] or glass and carbon fibers [85]. Furthermore, to improve the properties of UHPC mixes, combinations of basalt and polypropylene fibers can be used [86,87,88,89]. The addition of such types of fiber can improve impact resistance, reduce shrinkage, increase crack resistance, and improve flexural and tensile strength, with a limited increase in compressive strength.

The increase in UHPC dynamic strength has been recognized to have potential for further research, offering prospects for the use of hybrid fibers in structures exposed to impact and explosion loads [90,91,92,93,94]. Hybrid fiber combinations have been found to produce better results in achieving higher dynamic performance parameters compared to single-type fiber [42]. The mix can be designed so that some fibers are pulled out of the concrete, while others absorb energy during cracking. When ultra-high performance concrete reinforced with hybrid fibers is used in protective structures, this material may be subjected to destructive dynamic loads, such as high-speed impacts [95,96]. Another problem is the appearance of explosive spalling, which occurs in ultra-high performance concrete during exposure to fires due to its low permeability and dense structure [97,98]. This phenomenon is caused by high steam pressure and thermal stresses that develop under fire conditions, which can be reduced by adding fibers [99]. However, the addition of steel fibers alone is not effective in preventing explosive spalling of concrete [100,101,102]. An effective solution to avoid explosive spalling is the hybridization of steel fibers with polymer or natural fibers [103,104,105,106,107]. Polymer fibers melt at approximately 200 °C, helping to release trapped steam through a network of newly formed microchannels [108]. After the addition of basalt fibers, it was possible to determine residual strength after exposure to temperatures ranging from 500 to 1000 °C, while the mix containing only steel fibers experienced spalling and did not maintain structural integrity, preventing the assessment of residual strength. Basalt fiber, with its favorable mechanical properties, chemical stability, environmental friendliness, and relatively low production cost, can be used as a single type of fiber additive or in hybrid fiber mixes [109,110,111,112,113]. The skillful application of fibers enables the creation of more slender structural elements while maintaining the same load-bearing capacity, preserving the properties of the mix and reducing the overall amount of reinforcement. This, in turn, results in a reduction in weight, lower overall construction costs, shorter construction times, and an increase in usable space in buildings with the same building area.

The current state of knowledge is primarily based on studies evaluating the effect of single-type fibers on the properties of normal-strength concrete under static loads. On the contrary, research focusing on hybrid fiber systems in high- and ultra-high performance concrete, especially under dynamic conditions, is still limited. Although there are some review articles on UHPC or on concrete with selected hybrid fiber types, a consolidated and wide-ranging review that examines various fiber combinations and their influence on UHPC performance parameters including workability, static strength, and dynamic resistance is still lacking.

This article aims to address this gap by comprehensively presenting the effects of hybrid fiber use on UHPC. It incorporates current knowledge, highlights technological challenges, and outlines future research directions. The study offers a comparative, data-driven synthesis that can support the design of more durable and resilient concrete systems for demanding civil and military infrastructure applications.

2. Description of Type, Shape, and Properties of Fibers

Fibers used as an additive in mixtures to improve the properties of ultra-high performance concrete (UHPC) can be classified according to the material from which they are produced, e.g., steel (ST), synthetic (polypropylene—PP, polyvinyl alcohol—PVA, polyethylene—PE, polyester—PES, polyoxymethylene—POM or polyolefin—POL), inorganic (basalt—B, glass—G) or carbon (C). They are also divided into two categories according to their modulus of elasticity, which can be higher or lower compared to the UHPC matrix [23]. Furthermore, fibers can be classified by their length, that is, ultra-short with L_f ≤ 8 mm, short with 8 mm ≤ L_f ≤ 13 mm, long with 13 mm ≤ L_f ≤ 30 mm, and ultra-long with L_f > 30 mm. They can also be classified according to their shape, such as straight, twisted, hooked, wavy, with round ends, paddle-shaped ends, etched, indented, or thickened ends [114]. Other important parameters include the cross-sectional diameter and the surface area of the fiber that adheres to the matrix. When selecting fibers for use, three main requirements should be considered: firstly, the compatibility of the fiber material properties with concrete; secondly, the optimal shape factor, which allows the composite to behave appropriately after matrix cracking; and the thirdly, the matrix–fiber interaction, enabling stress transfer [115,116,117].

Table 1. Physical, mechanical properties, and geometry of fibers used in HPC and UHPC.

Fiber Type	Density (g/cm3)	Length (mm)	Diameter (mm)	Tensile Strength (MPa)	Elastic Modulus (GPa)	Maximum Elongation (%)
steel	7.5–7.9	6–62	0.1–1.0	345–2900	200–250	0.5–4.0
polypropylene	0.90–0.95	12–50	0.015–0.6	270–760	1.5–11.0	10–80
polyvinyl-alcohol	1.2–1.3	6–12	0.01–0.04	770–2500	16–42	5.5–8.0
polyethylene	0.92–0.97	12–19	0.023–1.0	80–3800	5–116	3–100
polyester	1.22–1.38	3–30	0.01–0.9	580–1200	6–18	7–35
polyoxymethylene	1.4–1.5	8–24	0.2	900–970	8–10	15–18
polyolefin	0.9–1.01	30–60	0.3–0.7	275–600	2.7–9	15–40
basalt	1.9–2.8	10–50	0.009–0.45	1245–4840	40–110	2.2–3.15
glass	2.46–2.74	6–18	0.008–0.025	1400–4650	69–87	2.5–5.4
carbon	1.4–2.2	6–30	0.007–0.02	1500–4000	200–1000	0.3–2.5

presents the physical and mechanical properties, as well as the geometry of selected fibers that have been used as components in UHPC mixtures. Figure 1 shows the fibers used by the authors of this publication in various combinations of fibers during tests of their impact on different UHPC properties.

3. Summary of Experimental Procedures

The detailed experimental procedures adopted in the reviewed studies are summarized in

Table 2. Summary of experimental procedures used in static studies on UHPCs with hybrid fibers.

Reference	Fiber Types/ Length (mm)	Curing Conditions	Loading Rate	Test Age	Test Standard	Notes
[48]	ST/6, 10, 13	50 °C, 1 d, cast 20 °C, 27 d water	0.2 mm/min 0.05 mm/min 0.05 mm/min	28 days	Chinese national standards	Compression test Flexural test Splitting tensile test
[135]	ST/6, 13	20 °C, 28 d, humidity > 95%	2.4 kN/s	28 days	GB/T 17671 -1999	Compression test Flexural test
[136]	ST/6, 13	21 °C, 1 d, cast 21 °C, 27 d, water	0.1 mm/min	28 days	BS EN-196-1 (2005)	Compression test
[137]	ST/6, 10, 15	20 °C, 36 h, cast 80 °C, 48 h, steam 20 °C, 24.5 d, air	1 kN/s 0.3 mm/min	28 days	Not reported ASTMC78	Compression test Flexural test
[138]	ST/13, 19.5, 30	Not reported	0.1 mm/min 0.4 mm/min	Not reported	Not reported ASTMC1609	Compression test Flexural test
[139]	ST/13, 30 ST/13 PVA/8	23 °C, 1 d, cast 23 °C, 27 d, humidity = 100%	0.05 mm/min 0.05 mm/min	28 days	ASTMC109 ASTMC1609	Compression test Flexural test
[140]	ST/19.5 ST h-e/13, 25 PVA/6, 12 PE/12	Not reported	0.6 kN/s 1 mm/min -	Not reported	ASTMC39 Not reported Not reported	Compression test Axial test Elastic modulus
[141]	ST/19.5 ST h-e/30 ST tw/30	20 °C, 1 d, cast 90 °C, 2 d, steam 20 °C, 25 d, air	Not reported 0.4 mm/min	28 days	Not reported ASTMC1609	Compression test Flexural test
[70]	ST/12 B/12	20 °C, 28 d, air	Not reported	28 days	GB/T17671-2021 Not reported GB/T50081-2019	Compression test Flexural test Splitting tensile test
[75]	ST/13 B/12 POL/40 PVA/8, 12 PES/30	Not reported	1 MPa/s 0.15 mm/min -	Not reported	Not reported	Compression test Axial test Elastic modulus
[142]	ST/13 B/12	20 °C, 28 d, air	Not reported	28 days	ASTMC109	Compression test
[76]	ST/13 B/12, 30 PP/16, 40 PVA/12 PES/30 G/6, 12, 18	20 ± 2 °C, 28 d humidity > 95%	1 MPa/s 0.2 mm/min -	28 days	Not reported	Compression test Flexural test Elastic modulus
[143]	ST/16.5, 19.5 B/12 PE/18 PVA/12	23 ± 2 °C, 2 d, cast 23 ± 2 °C, 28 d, water	Not reported 0.1 mm/min	30 days	ASTMC109 Not reported	Compression test Axial test
[144]	ST/13 PE/12	20 °C, 1 d, cast 90 °C, 9 d, water 20–24 °C, 2 d, air	Not reported 0.5 mm/min	12 days	ASTMC109 Not reported	Compression test Axial test
[145]	ST/12 POM/13	20 ± 2 °C, 28 d, air	Not reported	28 days	BS EN-196-1 (2005)	Compression test
[146]	ST/13 PE/19	20 °C, 1 d, cast 20 °C, 27 d, water	100 kN/min 0.2 mm/min	28 days	ASTMC109 ASTMC1609	Compression test Flexural test
[147]	ST/13 PE/6, 12	20 °C, 1 d, cast 20 °C, 27 d, water	Not reported 0.6 mm/min	28 days	BS EN 12390 Not reported	Compression test Axial test
[42]	ST/13 POM/12	20 °C, 28 d, air	0.2 mm/min -	28 days	ASTMC469	Compression test Elastic modulus
[148]	ST/14 PVA/12	20 °C, 28 d, humidity > 95%	2.4 kN/s	28 days	Not reported	Compression test
[149]	ST/18 PE/13	20 °C, 1 d, cast 90 °C, 3 d, steam	Not reported 0.5 mm/min	4 days	Not reported	Compression test Axial test
[150]	ST h-e/30 P/30	20 ± 2 °C, 28 d, humidity > 95%	Not reported	28 days	GB/T17671-2021 Not reported	Compression test Flexural test
[151,152]	ST h-e/50 PP/12	20 °C, 1 d, cast 20 °C, 7 d, water 20 °C, 20 d, air	Not reported	28 days	PN-EN 12390-3 PN-EN 12390-5 PN-EN 12390-6 ASTM C469	Compression test Flexural test Splitting tensile test Elastic modulus
[153]	ST h-e/50 PP/12	23 °C, 1 d, cast 23 °C, 14 d, water 23 °C, 14 d, air	Not reported	29 days	PN-EN 12390-3 PN-EN 12390-5 PN-EN 12390-6 ASTM C469	Compression test Flexural test Splitting tensile test Elastic modulus
[154]	ST h-e/50 PP/12	20 °C, 2 d, cast 20 °C, 26 d, water	Not reported	28 days	PN-EN 12390-3 PN-EN 12390-5 PN-EN 12390-6 ASTMC469	Compression test Flexural test Splitting tensile test Elastic modulus
[85]	ST/15 G/6-12 C/20-30	20 °C, 1 d, cast 90 °C, 4 d, water 20 °C, 2 d, cast	0.005 mm/s 1 MPa/min	7 days	ASTMC39 ASTMC293	Compression test Flexural test
[82]	ST/15 G/6.35	22 °C, 16 h, cast 22 °C, 27 d, 8 h, Water	Not reported 0.02 mm/min Not reported Not reported	28 days	ASTM C469 RILEM50-FMC/198 Not reported ASTMC469	Compression test Flexural test Splitting tensile test Elastic modulus
[84]	ST cr/25 G/12 C/20-30	23 °C, 2 d, cast 23 °C, 26 d, air	Not reported 0.4 mm/s	28 days	ASTMC109 ASTMC78	Compression test Flexural test Elastic modulus
[155]	B/12 PP/12	20 °C, 2 d, cast 20 °C, 26 d, water	0.05 mm/s	28 days	PN-EN 12390-3 PN-EN 12390-5 PN-EN 12390-6	Compression test Flexural test Splitting tensile test

h-e = hooked-end; tw = twisted; cr = crimped.

to improve clarity and reproducibility. Most of the specimens were cured at temperatures ranging from 20 °C to 23 °C for 28 days, often after initial curing in molds and subsequent curing with water or steam at elevated temperatures. Loading rates varied depending on the type of test, typically between 0.05 mm/min and 1 mm/min for compressive and flexural tests, with some studies applying load-controlled rates (e.g., 1 MPa/s or 2.4 kN/s). The test was carried out most frequently at 28 days of age and various international standards were used, including: GB/T 17671-1999 [118], BS EN-196-1 (2005) [119], ASTMC78 [120], ASTMC1609 [121], ASTMC109 [122], ASTMC39 [123], GB/T17671-2021 [124], GB/T50081-2019 [125], BS EN-12390 [126], ASTMC469 [127], PN-EN 12390-3 [128], PN-EN 12390-5 [129], PN-EN 12390-6 [130], ASTMC293 [131], RILEM50-FMC/198 [132], ASTMC78 [133] and ASTME23 [134].

Additional details on dynamic compressive strength and fracture energy tests, including testing apparatus and specimen preparation, are summarized based on reviewed studies (

Table 3. Summary of experimental procedures used in dynamic studies on UHPC with hybrid fibers.

References	Fiber Types	Curing Conditions	Samples	Test Age	Loading Rate/ Strain Rate	Test Standard	Notes
[135]	ST/6, 13	20 °C, 28 d, humidity > 95%	d = 92 mm h = 46 mm	28 days	i.v. = 8.0–13.9 m/s s.r. = 107.8– 204.8 s−1	Not reported	Dynamic compression test Fracture energy
[136]	ST/6, 13	21 °C, 1 d, cast 21 °C, 27 d, water	50.8 × 25.4 × 25.4 mm	28 days	-	ASTME23	Impact energy absorption
[150]	ST h-e/30 P/30	20 ± 2 °C, 28 d, humidity > 95%	d = 75 mm	28 days	s.r. = 55–214 s−1	Not reported	Dynamic compression test
[42]	ST/13 POM/12	20 °C, 28 d, air	d = 100 mm h = 50 mm	28 days	s.r. = 46– 171 s−1	Not reported	Dynamic compression test Elastic modulus Ultimate toughness
[149]	ST/18 PE/13	20 °C, 1 d, cast 90 °C, 3 d, steam	dumbbell specimen 330 × 50 × 13 mm	4 days	s.r. = 8– 49 s−1	Not reported	Axial tensile test Impact energy

d = diameter; h = height; i.v. = impact velocity; s.r. = strain rate.

).

Dynamic tests commonly utilized the Split Hopkinson Pressure Bar (SHPB) apparatus, with incident and transmission bar diameters of 75 to 100 mm and lengths ranging between 4000 and 6000 mm [135,150]. The samples were typically cylindrical (diameters 75–100 mm, height 46–50 mm), carefully ground to ensure parallel surfaces and minimize friction, often using lubricants such as Vaseline [42,150]. Strain gauges attached to the bars captured incident, reflected, and transmitted waves, which were recorded using dynamic strain acquisition systems at frequencies up to 1 MHz [42]. To ensure consistent dynamic loading, wave shaping materials such as brass or paper were employed to moderate stress wave frequencies and rise time [42,150]. Additionally, high-speed drop weight impact tensile equipment with adjustable drop heights (0–800 mm) was utilized for tensile impact tests, employing dumbbell-shaped specimens (330 × 50 × 13 mm) to ensure uniform axial loading [149]. Comprehensive data acquisition and processing ensured accurate measurement of strain rates and impact velocities during the testing.

Given the diversity of test protocols, material compositions, and mechanical properties reported in the referenced studies, statistical significance tests (e.g., ANOVA or t-tests) were not performed in this review. Instead, data are reported as mean values and percentage changes to highlight general trends and comparative behaviors in fiber combinations. Each data set is analyzed within the context of its own experimental framework.

4. Hybrid Fiber Composition

4.1. Steel—Steel Hybrid Fiber

4.1.1. Physical Characteristics

The effect of straight steel fibers with lengths of 6 mm and 13 mm and a diameter of 0.2 mm is presented in [135]. The fibers differed only in length. For a 2% content of straight fibers in the UHPC mixture, no significant differences in flow were observed depending on the percentage content, unlike in the case of hooked-end fibers. As the content increased to 1.5% of 13 mm fibers, the flow of the mixture decreased by 3.3% compared to a 0.5% addition of 13 mm fibers. These results align with [136], where the influence of straight two-length steel fibers was investigated. The highest fluidity was obtained for a content of 0.5% 13/0.2 mm fibers and 1.5% 6/0.16 mm fibers, with an increase of 4.2% compared to an addition of 2% of 13 mm fibers. The negative effect of fibers is mainly due to three reasons: (1) the fiber size is significantly longer than the aggregate, resulting in a much higher contact surface between the matrix and the fibers than for the aggregate alone, (2) the granular skeleton structure is disrupted in the presence of rigid fibers, (3) the shape of steel fiber, which often includes deformations to improve anchorage in the matrix [156,157]. The fibers were observed to align with the flow of the fresh mixture. However, longer or deformed fibers increase internal friction during mixing, reducing the flowability of the UHPC. Their geometry disrupts the mobility of particles and may require additional admixtures to maintain workable consistency. Short fibers can cause the rotation of long fibers opposite to the flow direction, which explains why the flowability of hybrid mixtures is higher than that of a single type of steel fiber.

The addition of hooked-end steel fibers (length/diameter = 30/0.5 mm) in the amount of 0.5 to 1.5% to UHPC, in combination with 13/0.2 mm straight steel fibers, in a 2% total fiber blend, required increasing the amount of high range water reducer (HRWR) from 25% to 150% [139]. Despite this, the flow time increased by up to 90%. The sample containing up to 1.5% steel fibers (straight, corrugated, hooked end) exhibited the lowest flowability when hooked-end fibers were added, indicating a negative impact of this type of fiber on flow properties [158].

In the case of a UHPC mixture with 0.5% hooked-end steel fibers (13/0.2 mm) and 0.5% straight fibers (19.5/0.2 mm), no reduction in fluidity was observed compared to a mixture with 1% straight steel fibers (19.5/0.2 mm) [140]. However, increasing the fiber content to 0.75% led to a decrease in fluidity. Furthermore, longer hooked-end fibers (25 mm) reduced the flowability of the mix more than shorter fibers (13 mm), suggesting that short fibers created a denser particle skeleton, which decreased the flow parameters. Ensuring proper fluidity is crucial for the usability of the concrete mixture, as low fluidity can affect fiber dispersion and lead to reduced strength. The results were consistent with [159], where a 40% reduction in flowability was observed in a hybrid UHPC mixture with 1% straight steel fibers (6/0.16 mm) and increasing amounts of hooked-end fibers (30/0.38 mm) from 0.5% to 2%.

Table 4. Influence of hybrid steel–steel fibers on flow of UHPC mixes.

Hybrid Fibers/Steel—Steel								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
straight	6	0.2	0.5 1.0 1.5	straight	13	0.2	1.5 1.0 0.5	flowability = 235 mm (−5.1%) flowability = 240 mm (−3.0%) flowability = 243 mm (−1.8%)	[135]
straight	6	0.16	0.5 1 1.0 1 1.5 1	straight	13	0.2	1.5 1 1.0 1 0.5 1	slump = 290 mm (+0.7%) 2 slump = 295 mm (+2.4%) 2 slump = 300 mm (+4.2%) 2	[136]
straight	19.5	0.2	0.5 0.25	hooked-end	13	0.2	0.5 0.75	flowability = 220 mm (0%) 1 flowability = 205 mm (−6.8%) 1	[140]
			0.5 0.25	hooked-end	25	0.2	0.5 0.75	flowability = 190 mm (−13.6%) 1 flowability = 180 mm (−18.2%) 1
straight	13	0.2	1.5 1.0 0.5	hooked-end	30	0.5	0.5 1.0 1.5	flow time = 42 s (+44.8%) flow time = 50 s (+72.4%) flow time = 55 s (+89.7%)	[139]

¹ comparison with a mixture with 1% straight steel fibers; ² comparison with a mixture with 2% straight steel fibers 13 mm long; l_f = length; d_f = diameter; V_f = fiber volume.

summarizes the workability studies of the UHPC mixes containing hybrid steel fibers.

4.1.2. Mechanical Properties

Fibers enhance UHPC by inhibiting microcrack propagation and bridge developing cracks. Short fibers are activated early, while longer fibers delay macrocrack formation and enhance ductility.

Kim et al. [140] noted that the highest compressive strength was achieved in a hybrid mixture that combined straight steel fibers with a length/diameter ratio of 19.5/0.2 mm at a dose of 0.5% and hooked fibers of 13/0.2 mm at a dose of 0.5%. Increasing the length of the hooked fibers to 25 mm resulted in a decrease in compressive strength, which was associated with reduced workability and difficulty evenly dispersing the fibers in this hybrid combination. The results were confirmed by Ma et al. [160], who investigated the effect of straight fibers (13 mm) and hooked fibers (13 mm) with a total content of 2.5% in UHPC. The compressive strength decreased as the proportion of hook-end fibers increased to 1.25% and was the lowest when 2.5% of these fibers were used.

In the case of UHPC mixes containing hybrid straight steel fibers with a total content of 1.5%, the highest compressive strength was achieved with the addition of 0.5% fibers (length/diameter 10/0.12 mm) and 1.0% fibers (13/0.12 mm) [48]. The lowest values were observed in the mixes containing the shortest fibers (6 and 10 mm).

The results presented in [135] confirm the relationships with respect to the influence of steel fibers with a length/diameter ratio of 6 and 13/0.2 mm. The highest compressive strength was obtained for hybrid mixtures with a fiber content of 2% for the highest proportion of 13 mm fibers. The strength increased by 46.2% compared to the fiberless mix. The decrease in compressive strength of concrete mixes with an increasing proportion of short fibers is consistent with [161], where identical-length steel fibers were also used.

Similar conclusions were presented in [136], where the compressive strength was tested for a mixture containing straight steel fibers with a length/diameter ratio of 6/0.16 and 13/0.2 mm, with a total content of 2%. The highest strength was obtained in the mixture with the highest proportion of 13 mm fibers and a total content of 1.5%.

The results of hybrid mixes with a total steel fiber content of 2.5% indicate a greater influence on compressive strength for fibers with a length/diameter ratio of 6/0.2 mm compared to longer fibers of 10/0.2 mm [137]. In combination with straight fibers of 15/0.2 mm at a dose of 1.5%, the addition of fibers of 6 mm at 1% led to a higher compressive strength. The results are consistent with those of Spiesz and Brouwers [136], who evaluated the behavior of hybrid mixtures reinforced with steel fibers of 13 and 6 mm lengths.

In [138] a hybrid mixture was examined with the addition of straight steel fibers with lengths/diameter of 13 and 19.5/0.2 mm and 30/0.3 mm, with a total content of 2%. For the mixture containing 19.5 and 30 mm fibers, an increase in compressive strength was observed with a higher proportion of long fibers. The highest result was achieved with a fiber content of 1.5%. The increase in strength was possible because of the uniform distribution of fibers, which allowed the use of their properties. The longer the fibers used, the greater the friction forces between them and the matrix. Therefore, to overcome these forces, a greater external force was required. In the case of a mixture with 13 mm and 30 mm fibers, the highest compressive strength was observed with the largest proportion of short fibers, which contrasts with the results of the previous analysis. According to research in [162], macro-steel fibers can have a positive effect by inhibiting cracks and increasing maximum strength, but also a negative effect by increasing air content.

A UHPC mixture with 2% steel fibers achieved the highest compressive strength with a combination of 1% straight fibers with a length/diameter of 13/0.2 mm and 1% hooked-end fibers of 30/0.5 mm—an increase of 18.6% compared to the mix without fibers [139]. The effect of increasing the amount of straight steel fibers to 5% was also investigated, and it did not lead to a further increase in strength. The results obtained were lower than those for the hybrid mixture with 2% fibers. This indicates the existence of a certain ultimate volumetric content of fibers, which depends not only on their type but also on the fiber content and the selection of other UHPC ingredients. In each of the above mixes, a water-reducing additive was added, the amount of which increased with fiber content, leading to increased matrix porosity and reduced compressive strength [163]. The largest amount of reducer was added to the mixture containing 2% hooked-end fibers. As the fiber content increased, fiber agglomeration and the need for more energy was observed to evenly distribute them in the mixture.

Yoo et al. [141] reported compressive strength results of UHPC with a total addition of 2% steel fibers, in which straight steel fibers with a length/diameter of 19.5/0.2 mm and hooked-end steel fibers of 30/0.38 mm or twisted fibers of 30/0.3 mm were used. The highest compressive strength was achieved by samples with a 1.5% addition of straight 19.5 mm fibers and 0.5% twisted fibers. Each mixture contained a 2% fiber addition. However, the number of 19.5 mm fibers was approximately 8.3 and 5.2 times higher than the number of hooked end and twisted fibers, respectively. As the percentage of hooked-end or twisted fibers increased, the compressive strength decreased. Based on the described results, straight fibers with a length of 19.5 mm can be concluded to be the most effective in improving compressive strength, as replacing hooked end and twisted fibers with straight fibers resulted in an increase in strength. The results were confirmed in [164], where the effect of two types of steel fibers with a total amount of 2% was investigated in the ultra-high performance concrete mix. The replacement of 1% of hooked-end fibers of 30/0.375 mm with straight fibers of 13, 16.3, or 19.5/0.2 mm resulted in an increase in compressive strength in the range of 1.5–6.1%. Furthermore, the hooked-end and twisted fibers were difficult to evenly distribute in the mix, leading to the formation of weak zones [165].

Shao et al. [48] noted that the highest flexural tensile strength was achieved in the case of a UHPC mix with the addition of 0.5% straight steel fibers with a length/diameter of 10/0.12 mm and 1% straight steel fibers of 13/0.12 mm, which was twice as much as the control sample. Longer fibers (10 and 13 mm) had a greater impact in improving maximum tensile strength compared to shorter fibers (6 mm). It was also observed that 13 mm fibers, despite their lower number, had a greater effect than 10 mm fibers. As the width of the crack in the matrix increased, the stress between the cracks was redistributed, and the load was transferred by the fibers due to anchoring and friction between the matrix and the fibers [166].

The effect of 6 and 13/0.2 mm straight steel fibers on flexural tensile strength for hybrid UHPC mixes with a total fiber content of 2% was confirmed in [135]. A mixture with the addition of 1.5% 13 mm fibers and 0.5% 6 mm fibers achieved a 78% increase in strength compared to the control sample. In the case of the reverse fiber content, the mixture achieved a 25% increase in strength, which was a three-fold lower improvement in flexural strength with the same volume content of fibers. When the UHPC maximum strength was reached, the fibers took over the load bearing, which was noted by distinctly audible fiber tearing sounds.

The test results of UHPC mixes with a total hybrid steel fiber content of 2.5% indicate that in combinations with steel fibers of length/diameter 15/0.2 mm, higher flexural tensile strength with the addition of 6/0.2 mm fibers compared to 10/0.2 mm fibers was obtained [137]. This was because the short fibers allowed the sample to carry the load up to the point of crack formation. After cracking, long fibers played a key role, allowing a further increase in load until the strength was exceeded and the cross section failed [167,168,169].

The analysis of the results of the flexural tensile strength for a mixture with the addition of 2% straight fibers with a length/diameter of 13 or 19.5/0.2 mm, and 30/0.3 mm was presented in [138]. The highest strength was achieved for a combination of 1.5% 19.5 mm fibers and 0.5% 30 mm fibers. Based on the results obtained, it was concluded that longer fibers allow for an increase in flexural strength as a result of a larger contact surface between the matrix and the fiber. The results were confirmed in [167], where the effect of steel fibers 13, 16.3, and 19.5/0.2 mm on the flexural strength of UHPC was determined, showing an improvement with increasing fiber length. Longer fibers offered better slip resistance, increased energy dissipation, and crack bridging, allowing increased flexural strength [170].

The impact of the hybrid combination of straight steel fibers with a length/diameter of 13/0.2 mm and hooked fibers of 30/0.5 mm as an additive to the UHPC mix was presented in [139]. The highest flexural strength for a total fiber content of 2% was achieved by the mixture with 1% of each type of fiber. No agglomeration was observed, allowing the fibers to operate effectively in areas where cracks occurred under increasing load. Hooked-end fibers had better anchorage during pull-out than straight fibers, but increasing their content to 5% by volume did not result in improved strength, indicating the occurrence of agglomeration and interlocking of fibers through their curved ends. The high density and poor workability prevented an even distribution of the ingredients.

Yoo et al. [141] presented the results of the flexural strength test for a hybrid UHPC mix with straight fibers of length/diameter 19.5/0.2 mm and hooked-end fibers 30/0.38 mm or twisted fibers 30/0.3 mm. For a total fiber content of 2%, a smaller effect was observed for hooked-end fibers as their content increased from 0.5% to 1.5%. This was due to the fact that a high concentration of hooked-end fibers caused cracking in the matrix, reducing the ability of neighboring fibers to pull out. The mixture with straight and twisted fibers achieved the highest flexural strength for a fiber content of 1% each. Increasing the content of twisted fibers to 1.5% resulted in the lowest increase in flexural tensile strength, with only a 0.6% improvement compared to the mix with 2% straight steel fibers.

Marković [171] and Lee et al. [172] analyzed the cracking patterns of the bending samples. In the case of longer fibers, damage was observed over larger distances, requiring cracking of a relatively larger concrete surface. The slip resistance of straight fibers is provided solely by interfacial friction, while the hooked-end fibers exhibit both friction and mechanical anchorage generated by the end hooks [173,174]. When a straight fiber reaches its maximum pull-out load, it detaches from the matrix and begins to slide out. In contrast, for hooked-end fibers, a sudden drop in strength is observed because of the need for fiber straightening or matrix cracking, whereas this occurrence is not present for straight fibers. According to [175], the average spalling area was greater for longer fibers and the spalling of the matrix occurred before the maximum load was reached, which reduced the effective length of the anchor compared to the initial length of the fiber.

A UHPC mix with the addition of two types of straight steel fibers with a length/diameter of 6, 10, or 13/0.12 mm and a total fiber content of 1.5% was tested for splitting tensile strength [48]. The addition of fibers transformed sudden cracking into ductile cracking after reaching maximum load. The highest strength was recorded for the mix containing 0.5% fibers with a length of 10 mm and 1% fibers with a length of 13 mm, showing a 75% increase compared to the control mix. The larger matrix-fiber contact surface for longer fibers resulted in more effective crack bridging, inhibiting crack propagation after matrix damage. The load is transferred to the fibers, allowing for a further increase in load until the fibers fail or are pulled out of the matrix.

A mixture containing straight steel fibers with a length/diameter ratio of 19.5/0.2 mm and hooked fibers of 13 or 25/0.2 mm, with a total fiber content of 1%, was tested for axial tensile strength [140]. In each of the mixes, replacing a portion of straight steel fibers with hooked-end fibers resulted in a strength increase ranging from 0.9% to 14.4%. The highest strength improvement was achieved with a 0.5% substitution of straight fibers for 25 mm hooked-end fibers. The hooked ends of the fibers exhibited a beneficial effect by increasing the energy required to remove them from the concrete matrix. However, the matrix surrounding the fibers was mechanically fragmented and crushing could appear, especially with shorter embedding lengths [176]. Tensile strength is related to the strength of the interfacial bond between the fibers and the matrix. The resistance mechanism can be improved by using fibers with rough surfaces, increasing the fiber-matrix contact area, and improving the matrix strength [177]. Yoo et al. [178] conducted tests to determine the pull-out forces of steel fibers with the following length/hook-end configurations: 30 mm/single hook with a 5 mm bend at 30°, 45°, and 60°; 25 mm/single hook with a 2.5 mm bend at 45°; and 30 mm/double hook with a 5 mm bend at 45°, tested through axial pull-out from a UHPC matrix. The shortest 25 mm fiber was pulled from the matrix before reaching the maximum load of the sample, indicating limited use of its load-bearing capacity. For the remaining types of fibers, the fiber rupture occurred without significant matrix damage, which was due to the pull-out resistance of the fiber exceeding the tensile strength of the single fiber.

A UHPC mix containing straight steel fibers (19.5/0.2 mm) in the amount of 0.5% and hooked fibers (13/0.2 mm) with a total content of 0.5% achieved the highest elastic modulus [140]. Neither increasing the fiber content to 1.5% nor increasing the fiber length resulted in a higher modulus. This was attributed to the reduced workability of the mixture and the difficulty in achieving uniform fiber dispersion.

Table 5. Static properties of UHPC reinforced with hybrid steel fibers.

Hybrid Fibers/Steel—Steel								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
straight	6	0.12	0.5	straight	10	0.12	1.0	compressive strength = 139.9 MPa (+54.1%) flexural strength = 32.8 MPa (+154.3%) splitting tensile strength = 10.4 MPa (+62.5%)	[48]
			1.0				0.5	compressive strength = 133.3 MPa (+46.8%) flexural strength = 30.1 MPa (+133.3%) splitting tensile strength = 9.9 MPa (+54.7%)
			0.5	straight	13	0.12	1.0	compressive strength = 144.9 MPa (+59.6%) flexural strength = 35.2 MPa (+172.9%) splitting tensile strength = 10.6 MPa (+65.6%)
			1.0				0.5	compressive strength = 135.8 MPa (+49.6%) flexural strength = 33.0 MPa (+155.8%) splitting tensile strength = 10.1 MPa (+57.8%)
straight	10	0.12	0.5	straight	13	0.12	1.0	compressive strength = 148.4 MPa (+60.4%) flexural strength = 38.6 MPa (+199.2%) splitting tensile strength = 11.2 MPa (+75.0%)
			1.0				0.5	compressive strength = 145.2 MPa (+59.9%) flexural strength = 35.9 MPa (+178.3%) splitting tensile strength = 11.1 MPa (+73.4%)
straight	6	0.2	0.5	straight	13	0.2	1.5	compressive strength = 142.5 MPa (+46.2%) flexural strength = 32 MPa (+77.8%) compressive strength = 127.5 MPa (+30.8%) flexural strength = 24 MPa (+33.3%) compressive strength = 123 MPa (+26.2%) flexural strength = 22.5 MPa (+25%)	[135]
			1.0				1.0
			1.5				0.5
straight	6	0.16	0.5 1.0 1.5	straight	13	0.2	1.5 1.0 0.5	compressive strength = 141.5 MPa (+42.9%) compressive strength = 130 MPa (+31.1%) compressive strength = 125.5 MPa (+26.8%)	[136]
straight	6	0.2	1.0	straight	15	0.2	1.5	compressive strength = 129.5 MPa (+7.7%) 1 flexural strength = 29.2 MPa (+51.3%) 1 compressive strength = 134.7 MPa (+12.0%) 1 flexural strength = 26.1 MPa (+35.2%) 1	[137]
straight	10	0.2	1.0
straight	13	0.2	0.5   1.0   1.5   0.5   1.0   1.5	straight	30	0.3	1.5   1.0   0.5   1.5   1.0   0.5	compressive strength = 209.6 MPa (+7.4%) flexural strength = 41.7 MPa (+181.8%) compressive strength = 201.3 MPa (+3.1%) flexural strength = 40.5 MPa (+173.7%) compressive strength = 218.4 MPa (+11.9%) flexural strength = 37.1 MPa (+150.7%) compressive strength = 225.1 MPa (+15.3%) flexural strength = 43.1 MPa (+191.2%) compressive strength = 218.5 MPa (+11.9%) flexural strength = 46.2 MPa (+212.2%) compressive strength = 205.1 MPa (+5.1%) flexural strength = 48.0 MPa (+224.3%)	[138]
straight	19.5	0.2
straight	13	0.2	1.5   1.0   0.5	hooked-end	30	0.5	0.5   1.0   1.5	compressive strength = 160 MPa (+14.3%) flexural strength = 23.4 MPa (+143.8%) compressive strength = 166 MPa (+18.6%) flexural strength = 26.5 MPa (+176.0%) compressive strength = 150 MPa (+7.1%) flexural strength = 21.6 MPa (+125%)	[139]
straight	19.5	0.2	0.5     0.25     0.5     0.25	hooked-end         hooked-end	13           25	0.2           0.2	0.5     0.75     0.5     0.75	compressive strength = 127.3 MPa (+3.2%) 2 axial tensile strength = 11.2 MPa (+0.9%) 2 elastic modulus = 47.1 GPa (+29.8%) 2 compressive strength = 124.3 MPa (+0.7%) 2 axial tensile strength = 11.4 MPa (+2.7%) 2 elastic modulus = 41.8GPa (+15.1%) 2 compressive strength = 120.9 MPa (−2.0%) 2 axial tensile strength = 12.7 MPa (+14.4%) 2 elastic modulus = 40.0 GPa (+10.2%) 2 compressive strength = 126.3 MPa (+2.4%) 2 axial tensile strength = 11.8 MPa (+6.3%) 2 elastic modulus = 40.3 GPa (+11.0%) 2	[140]
straight	19.5	0.2	0.5   1.0   1.5   0.5   1.0   1.5	hooked-end         twisted	30           30	0.38           0.30	1.5   1.0   0.5   1.5   1.0   0.5	compressive strength = 186.1 MPa (−15.5%) 3 flexural strength = 36.6 MPa (+0.6%) 3 compressive strength = 187.9 MPa (−14.7%) 3 flexural strength = 39.5 MPa (+8.5%) 3 compressive strength = 199.0 MPa (−9.6%) 3 flexural strength = 45.4 MPa (+24.7%) 3 compressive strength = 185.4 MPa (−15.8%) 3 flexural strength = 38.7 MPa (+6.3%) 3 compressive strength = 198.9 MPa (−9.7%) 3 flexural strength = 44.9 MPa (+23.4%) 2 compressive strength = 202.2 MPa (−8.2%) 3 flexural strength = 39.5 MPa (+8.5%) 3	[141]

¹ comparison with a mixture with 1% straight steel fibers 6 mm long; ² comparison with a mixture with 1% straight steel fibers 19.5 mm long; ³ comparison with a mixture with 2% straight steel fibers 19.5 mm long.

presents the results of static tests on UHPC reinforced with hybrid steel fibers.

In Figure 2, the compressive strength results of UHPC reinforced with hybrid steel fibers are presented. The compressive strength ranges from 120 MPa to more than 220 MPa, depending on the initial mixture and the type and number of fibers in the hybrid combination. It can be observed that it is not necessary to use more than 2% of the fibers, as increasing their proportion did not result in a significant increase in compressive strength. Determining the appropriate combination of fibers is more important. The highest results exceeding 195 MPa were achieved by mixtures combining straight steel fibers of 13 or 19.5 mm and 30 mm in length, as well as straight fibers of 19.5 mm combined with twisted or hooked fibers of 30 mm. The lowest compressive strength values were obtained for straight fiber combinations with lengths of 6, 10, 13, or 15 mm, with a total content of 1.5–2.5%. On the basis of the results, it can be concluded that for hybrid steel combinations, the addition of longer fibers is beneficial.

Figure 3 presents the results of the flexural strength tests. The strengths obtained ranged from 20 to 50 MPa. Increasing the fiber content above 2% did not improve the results, indicating that for flexural strength, planning the appropriate fiber combination is more important than simply increasing its amount, which negatively affects workability. The highest results were achieved with combinations of straight fibers of 13 or 19.5 mm and 30 mm, as well as straight fibers of 19.5 mm combined with twisted or hooked fibers of 30 mm. The addition of straight fibers of 6, 13, and 15 mm, along with a combination of straight fibers of 13 mm and hooked-end fibers of 30 mm, resulted in the lowest strength increases. Fibers that are too short tend to slip out of the matrix without fully using their properties.

4.1.3. Dynamic Properties

In all hybrid combinations tested, the increase in strain rate led to notable gains in dynamic compressive strength and fracture energy, unless otherwise noted.

Under impact, energy is absorbed sequentially through cracking of the matrix, fiber bridging, and eventual fiber pull-out or rupture. This sequence is influenced by fiber geometry, distribution, and anchorage.

Ultra-high performance concrete (UHPC) with the addition of hybrid steel fibers with a length/diameter ratio of 6 and 13/0.2 mm and a total content of 2% showed a higher dynamic compressive strength compared to the control sample [135]. Shorter fibers form a denser network, increasing the surface of the fiber–matrix interaction and improving the dynamic strength. However, overly short fibers may be ineffective in bridging macrocracks. Under static load, weaknesses such as pores and microcracks expand and a slow increase in load allows their development [179]. However, the behavior under dynamic loading is different. The duration of the load application is too short for these weaknesses to develop fully. This leads to the formation of a large number of microcracks, which requires more energy for their initiation compared to the gradual propagation of macrocracks under static loading. This explains the fracture energy results obtained for the hybrid fiber-reinforced UHPC samples tested. The results were higher than those of mixes containing a single type of fiber, indicating a positive hybridization effect and mutual synergy of fibers within the matrix. The energy required to break concrete into two parts involves both matrix breaking and fiber removal. In the context of short-duration impact loading, slow fiber slippage is excluded, leading to the maximum use of friction at the fiber-matrix interface.

Impact resistance tests of a UHPC mix with straight steel fibers of length/diameter 6/0.16 mm and straight steel fibers of 13/0.2 mm showed a decrease in fracture energy with an increasing share of short 6 mm fibers [136]. The maximum value was obtained for the mixture with 13 mm fibers at a content of 2%, while at the same fiber content with 6 mm fibers, a decrease of 52% was recorded. Concrete is damaged according to the stress distribution generated during impact. The results indicate that short fibers, because of their insufficient length, do not allow their bridging properties to be fully utilized. As a result, they are pulled from the concrete with a lower force, absorbing less energy compared to long fibers.

Table 6. Dynamic properties of UHPC reinforced with hybrid steel–steel fibers.

Hybrid Fibers/Steel—Steel								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
straight	6	0.2	0.5	straight	13	0.2	1.5	dynamic c.s. (i.v. 8.9 m/s) = 124.0 MPa (+23.5%) fracture energy (i.v. 8.9 m/s) = 2.41 MJ/m3 (+36.9%) dynamic c.s. (i.v. 11.6 m/s) = 171.8 MPa (+33.8%) fracture energy (i.v. 11.6 m/s) = 3.17 MJ/m3 (+63.4%) dynamic c.s. (i.v. 13.9 m/s) = 190.4 MPa (+29.7%) fracture energy (i.v. 13.9 m/s) = 3.61 MJ/m3 (+16.5%)	[135]
			1.0				1.0	dynamic c.s. (i.v. 8.9 m/s) = 151.5 MPa (+50.9%) fracture energy (i.v. 8.9 m/s) = 2.59 MJ/m3 (+47.2%) dynamic c.s. (i.v. 11.7 m/s) = 177.2 MPa (+38.0%) fracture energy (i.v. 11.7 m/s) = 3.27 MJ/m3 (+68.6%) dynamic c.s. (i.v. 13.9 m/s) = 192.0 MPa (+30.8%) fracture energy (i.v. 13.9 m/s) = 3.88 MJ/m3 (+25.2%)
			1.5				0.5	dynamic c.s. (i.v. 8.9 m/s) = 159.7 MPa (+59.1%) fracture energy (i.v. 8.9 m/s) = 2.72 MJ/m3 (+54.6%) dynamic c.s. (i.v. 11.7 m/s) = 184.3 MPa (+43.5%) fracture energy (i.v. 11.7 m/s) = 3.29 MJ/m3 (+69.6%) dynamic c.s. (i.v. 13.9 m/s) = 204.8 MPa (+39.5%) fracture energy (i.v. 13.9 m/s) = 4.15 MJ/m3 (+33.9%)
straight	6	0.16	0.5 1.0 1.5	straight	13	0.2	1.5 1.0 0.5	impact energy absorption = 59 J impact energy absorption = 47.5 J impact energy absorption = 40.5 J	[136]

c.s. = compressive strength; i.v. = impact velocity.

presents and analyzes the results of the dynamic tests of UHPC reinforced with hybrid steel fibers.

Figure 4 presents the results of the dynamic compressive strength and fracture energy depending on the impact velocity. For the UHPC mixture with the addition of 6 mm and 13 mm straight steel fibers, an increase in impact velocity resulted in higher dynamic compressive strength and fracture energy. For a total fiber content of 2%, when the proportion of 6 mm ST fibers increased, these parameters also increased. This is due to the higher overall content of shorter fibers and their greater number in the matrix, which creates a more complex internal structure.

4.2. Steel—Basalt Hybrid Fiber

4.2.1. Physical Characteristics

Gong et al. [70] performed flowability tests of UHPC mixes with steel fibers of length/diameter 12 mm/0.3 mm and basalt fibers of 12/0.45 mm, with a total fiber content of 2%. They found that as the basalt fiber content increased from 0.5% to 1.5%, the flowability decreased from 30.7 to 43.6% compared to the fiberless mixture. This was due to the blocking properties of the basalt fibers, which tend to clump and curl, causing the matrix to become sticky. Basalt fibers have a rough structure that increases the fiber-matrix contact area, unlike steel fibers, which are typically smooth. This structure increases friction, leading to a reduction in flowability as the basalt fiber content increases. The mixture with 1.5% basalt fibers and 0.5% steel fibers did not achieve the minimum flow of 200 mm, and a decrease in strength was also recorded. The general reduction in flowability was caused by the increased difficulty in the movement of particles within the matrix and the increase in friction between the ingredients. Fibers act as a skeleton that has difficulty shifting position between the components. Furthermore, basalt fibers absorb a small amount of water during mixing, reducing the mobility of the mix. Steel fibers, with a smoother surface, exert less impact on the flow of fresh UHPC.

Reduced flowability was also observed in a hybrid UHPC with 2% steel fibers of a length/diameter of 13/0.2 mm and 0.5% basalt fibers of 12 or 30/0.02 mm [76]. The flowability was 34 to 35% lower than the fiber-free mix and lower for the shorter basalt fibers. The flowability of the hybrid mixes was lower than that of the mix with 2.5% steel fibers of 13 mm length. These results confirmed the behavior of UHPC mixes with basalt fibers. A summary of the results of the workability test is presented in

Table 7. Influence of hybrid fibers on flow of UHPC mixes.

Hybrid Fibers								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
straight steel	12	0.3	0.5 1.0 1.5	basalt	12	0.45	1.5 1.0 0.5	flowability = 175 mm (−43.6%) flowability = 200 mm (−35.5%) flowability = 215 mm (−30.7%)	[70]
straight steel	13	0.2	2.0 2.0	basalt	12 30	0.02 0.02	0.5 0.5	flowability = 410 mm (−34.9%) flowability = 415 mm (−34.1%)	[76]

. The results indicate that basalt fibers have a negative effect on the flowability of hybrid mixes, regardless of the fiber lengths used.

4.2.2. Mechanical Properties

The ultra-high performance concrete (UHPC) with the mix addition of steel fibers (diameter/length of 16.5/0.2 mm at 0.33% and 19.5/0.2 mm at 0.67%) and basalt fibers (12/0.012 mm at 0.5%) exhibited lower compressive strength than the mix with 1.5% steel fibers [143]. The results suggest that basalt fibers negatively affect the compressive strength of concrete, as these fibers tend to agglomerate, creating spaces where there are no fibers. Upon considering the results, it can be inferred that the addition of an appropriate amount of basalt fibers is crucial to improving the compressive strength. Incorrectly chosen fibers, regardless of type, length, or content, can have the opposite effect of what is intended. As fiber content increases, the reinforcement effect decreases due to uneven fiber distribution in concrete and weak bond strength due to inhomogeneity of the mix [170]. Furthermore, exceeding a certain fiber content, which depends on the specific properties of the mix, causes the fibers to overlap, leading to an increase in the pores within the concrete. This can create weak interfaces and gaps [180,181].

A hybrid fiber UHPC with 2% fiber addition was tested to determine its compressive strength as the percentage of fiber content changed [70]. A decrease in compressive strength was observed as the amount of basalt fibers (12/0.45 mm) increased compared to steel fibers (12/0.3 mm). Compared to the fiber-free mixture, the compressive strength increased from 37.6% to 50.5%. However, because of the lower stiffness of the basalt fibers, the compressive strength of the UHPC decreased when the basalt fibers replaced the steel fibers.

Li and Deng [75] presented the results of the compressive strength tests for a hybrid mix with 1.3% steel fibers (13/0.2 mm) and 0.5% basalt fibers (12/0.02 mm). The compressive strength was found to increase by 17.3% compared to the fiber-free mix. However, the compressive strength of the hybrid fiber mix was lower compared to the 1.3% steel fiber mix, indicating a negative impact of the basalt fibers. The reason for this could be the uneven distribution of fibers in the concrete matrix.

Other results of studies on the hybrid combination of steel fibers (13/0.2 mm) and basalt fibers (12/0.02 mm) also indicate that the compressive strength is primarily determined by the percentage of steel fibers [142]. The mixture with 2% steel fibers achieved a compressive strength of 148 MPa, while replacing 0.5% of steel fibers with basalt fibers resulted in a reduction of 8%. Despite the reduced compressive strength, the combination of these two types of fibers showed positive properties. While samples with 2% steel fibers and those without fibers experienced spalling at temperatures between 500 and 1000 °C, samples with basalt fibers maintained integrity, and it was possible to determine the compressive strength test.

A mixture with 2% steel fibers (13/0.2 mm) and 0.5% basalt fibers (30/0.02 mm) achieved a higher compressive strength than an identical mix in which the basalt fibers mentioned above were replaced with basalt fibers of 12/0.02 mm with the same volume content [76]. Longer fibers, due to their lower quantity, were more evenly distributed in the matrix and effectively bridged cracks, leading to a positive effect on increased strength.

The flexural tensile strength of UHPC with a total fiber content of 2%, comprising steel fibers (12/0.3 mm) and basalt fibers (12/0.45 mm), decreased as the basalt fiber content increased from 0.5% to 1.5% [70]. Basalt fibers exhibit a better anchoring effect due to their rough structure, but compared with steel fibers, their mechanical properties do not contribute as much to increasing the concrete strength. Basalt fibers inhibited the development of microcracks, but the maximum strength value was mainly dependent on steel fibers. A positive hybridization effect occurred in the mixes when 0.5% and 1% basalt fibers were mixed with 1.5% and 1% steel fibers, respectively.

The UHPC flexural strength of the mixture with 2% steel fibers (13/0.2 mm) achieved a higher value with the addition of basalt fibers (30/0.02 mm) than with basalt fibers (12/0.02 mm) [76]. Compared to the reference mix, the strength increased by 75.5% for the shorter basalt fibers and by 110% for the longer fibers, indicating a high level of synergy between the fibers and the matrix. These results are consistent with those of Branston et al. [64], who found that longer basalt fibers led to a greater increase in flexural tensile strength.

The axial tensile strength of a UHPC mix with 1.3% steel fibers (13/0.2 mm) and 0.5% basalt fibers (12/0.02 mm) was studied by Li and Deng [75]. Compared to control samples, the mixture achieved a strength that was 30.8% and 4.3% higher than the sample with 1.8% steel fibers, indicating a positive hybridization effect. The increase is attributed to the mechanism that occurs during the interaction of force with the hybrid fiber matrix. As the tensile stresses increase to the level at which the fiberless matrix will fail, the load is transferred by the fibers that bridge the cracks. Furthermore, compared to ordinary concrete, the onset and propagation of cracks is delayed.

The mixture with two types of straight steel fibers (16.5/0.2 mm at 0.33% and 19.5/0.2 mm at 0.67%) along with basalt fibers (12/0.012 mm at 0.5%) showed a 14.7% higher axial tensile strength compared to the control mixture [143]. Furthermore, the strength was higher than in mixes with 1.5% steel fibers (1% fibers of 19.5 mm and 0.5% fibers of 16.5 mm), indicating a positive hybridization effect with basalt fibers. The results suggest that the partial replacement of steel fibers with basalt fibers can increase the strength value at which the first crack occurs, as well as the ultimate strength. Basalt fibers, unlike other types of fiber, are composed of minerals with a composition similar to that of cement, leading to strong chemical bonds between the matrix and the fiber [182]. Due to their density being close to that of concrete, basalt fibers can improve the mechanical properties and durability of concrete without significantly increasing its weight [183]. Furthermore, its high tensile strength delays crack propagation, slows the crack widening rate, and reduces tensile stresses that exceed the strength of the concrete matrix [184]. Nikmehr et al. [185] also confirmed that basalt fibers controlled the advancing cracks and had a positive effect on the bridging.

The modulus of elasticity of the UHPC mix was determined with the addition of 0.5% basalt fibers (12 mm long) and 1.3% straight steel fibers (13 mm long) in [75]. The results showed a 22.2% increase in modulus compared to the control mix without fibers and an 11.4% increase compared to the mix with 1.8% straight steel fibers (13 mm long). This indicates a positive effect of hybridization with basalt fibers.

The modulus of elasticity of concrete with 2% straight steel fibers (13 mm long and 0.2 mm diameter) achieved a higher value when 0.5% of the basalt fibers (30/0.02 mm long/diameter) were replaced by an equal amount of shorter basalt fibers (12/0.02 mm) [76]. Compared to the reference mix, the modulus increased by 20% for the longer basalt fibers and by 15% for the shorter ones, suggesting effective cooperation between the fibers and the matrix, regardless of the length of the fiber.

The results of the static properties test of UHPC reinforced with steel and basalt fibers are analyzed in

Table 8. Static properties of UHPC reinforced with hybrid steel–basalt fibers.

Hybrid Fibers								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
straight steel	12	0.3	0.5	basalt	12	0.45	1.5	compressive strength = 139 MPa (+37.6%) flexural strength = 31.5 MPa (+31.3%) splitting tensile strength = 7.6 MPa (+123.5%)	[70]
			1.0				1.0	compressive strength = 146 MPa (+44.6%) flexural strength = 38.5 MPa (+60.4%) splitting tensile strength = 8.6 MPa (+152.9%)
			1.5				0.5	compressive strength = 152 MPa (+50.5%) flexural strength = 42.5 MPa (+77.1%) splitting tensile strength = 10.2 MPa (+200%)
straight steel	13	0.2	1.3	basalt	12	0.02	0.5	compressive strength = 107.3 MPa (+17.3%) axial tensile strength = 5.44 MPa (+30.8%) elastic modulus = 45.1 GPa (+22.2%)	[75]
straight steel	13	0.2	1.0 1.5	basalt	12	0.02	1.0 0.5	compressive strength = 113.4 MPa (−10.6%) compressive strength = 135.7 MPa (+7.0%)	[142]
straight steel	13	0.2	2.0	basalt	12	0.02	0.5	compressive strength = 126.9 MPa (+10.1%) flexural strength = 6.6 MPa (+75.5%) elastic modulus = 41.0 GPa (+14.9%)	[76]
			2.0	basalt	30	0.02	0.5	compressive strength = 135.2 MPa (+17.3%) flexural strength = 7.9 MPa (+110.1%) elastic modulus = 42.8 GPa (+19.9%)
straight steel	16.5 19.5	0.2 0.2	0.33 0.67	basalt	12	0.012	0.5	compressive strength = 128 MPa (−14.1%) 1 axial tensile strength = 14.7 MPa (+3.1%) 1	[143]

¹ comparison with a mix with 1.5% straight steel fibers.

.

The results of the compressive strength and flexural tensile strength tests of UHPC with the addition of steel and basalt fibers are presented in Figure 5. All programs used steel fibers of 12 or 13 mm in length and basalt fibers of 12 mm (except for a mixture with 2.5% fibers). The compressive strength results ranged from 105 to 152 MPa (Figure 5a). An increase in compressive strength was observed as the steel fiber content increased to 1.5%. For UHPC with 2.5% hybrid fibers, a positive effect was observed when the fiber length increased from 12 to 30 mm. In Figure 5b, it can be seen that the flexural tensile strength increased by 31 to 77% with the increase in the amount of 12 mm straight steel fibers replacing 12 mm basalt fibers for a total fiber amount of 2%. The highest strengths were achieved for short hybrid fibers with 1.5% ST and 0.5% B. Increasing the fiber content to 2.5% resulted in a decrease in strength, but the results were still 76–110% higher than the fiber-free UHPC.

Although several studies confirm the positive impact of basalt fibers on the mechanical performance of UHPC, particularly in terms of crack resistance and residual strength after thermal exposure, there are also reports of conflicting findings regarding their influence on compressive strength. For example, Gong et al. [70] observed a noticeable reduction in compressive strength with increased basalt fiber content, attributed this to agglomeration tendencies and reduced workability that compromised fiber dispersion. In contrast, Li and Deng [75] reported strength improvements when basalt fibers were added in moderate amounts, suggesting that the rough surface texture of these fibers enhances the mechanical interlock with the matrix. These contradictory outcomes likely arise from differences in fiber dimensions, fiber-to-matrix ratios, and the homogeneity of fiber distribution. Furthermore, the effects of mixing procedures, superplasticizer dosage, and curing regimes can substantially influence the interaction between basalt fibers and the cementitious matrix. Such variables highlight the importance of contextual interpretation of the test results and suggest that the performance of basalt fiber-reinforced UHPCs is highly sensitive to mixing design and processing conditions.

4.3. Steel—Polymer Hybrid Fiber

4.3.1. Physical Characteristics

Table 9. Influence of hybrid steel–polymer fibers on flow of UHPC mixes.

Hybrid Fibers								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
straight steel	13	0.22	1.0 1.0 2.0 2.0	polyethylene	19	0.023	0.5 1.0 0.5 1.0	flowability = 174 mm (−25.3%) flowability = 145 mm (−37.8%) flowability = 156 mm (−33.1%) flowability = 137 mm (−41.2%)	[146]
straight steel	13	0.2	1.5	polyvinyl- alcohol	8	0.038	0.5	flow time = 48 s (+65.5%)	[139]
straight steel	13	0.2	2.0 2.0 2.0 2.0 2.0	polypropylene	16 40	0.15 0.6	0.5 0.5	flowability = 540 mm (−14.3%) flowability = 543 mm (−13.8%)	[76]
				polyvinyl-alcohol polyester	12 30 30	0.04 0.75 0.9	0.5 0.5 0.5	flowability = 410 mm (−34.9%) flowability = 542 mm (−14.0%) flowability = 533 mm (−15.4%)
straight steel	13	0.2	1.0 2.0	polyoxyme- thylene	12	0.2	2.0 1.0	flowability = 155 mm (−20.5%) flowability = 160 mm (−18.0%)	[42]
straight steel	19.5	0.2	0.25 0.5 0.75	polyvinyl-alcohol	6	0.012	0.75 0.5 0.25	flowability = 140 mm (−36.4%) 1 flowability = 140 mm (−36.4%) 1 flowability = 170 mm (−22.7%) 1	[140]
			0.25 0.5 0.75	polyvinyl-alcohol	12	0.012	0.75 0.5 0.25	flowability = 160 mm (−27.3%) 1 flowability = 160 mm (−27.3%) 1 flowability = 165 mm (−25.0%) 1
			0.25 0.5 0.75	polyethylene	12	0.02	0.75 0.5 0.25	flowability = 160 mm (−27.3%) 1 flowability = 160 mm (−27.3%) 1 flowability = 160 mm (−27.3%) 1
hooked-end steel	30	0.9	0.5 1.0 1.5	plastic synthetic	30	0.9	1.5 1.0 0.5	flowability = 158 mm (−12.2%) flowability = 160 mm (−11.1%) flowability = 161 mm (−10.6%)	[150]
hooked-end steel	50	1.0	0.5 1.0 1.0 1.5	polypropylene	12	0.025	0.025 0.05 0.05 0.1	slump = 111 mm (−7.5%) 2 slump = 102 mm (−15%) 2 slump = 95 mm (−18.1%) 3 slump = 88 mm (−24.1%) 3	[154]

¹ comparison to a mixture with 1% straight steel fibers; ² water/binding material ratio = 0.3; ³ water/binding material ratio = 0.25.

shows the influence of steel–polymer hybrid fibers on the properties of the fresh UHPC mix.

Hybrid mixes with a total fiber content of 1%, consisting of ST fibers with a length/diameter ratio of 19.5/0.2 mm and PVA fibers of 6/0.012 mm or PVA 12/0.012 mm, or PE fibers of 12/0.2 mm, exhibited a reduction in slump flow diameter compared to the control mix with 1% steel fibers [140]. The shorter PVA fibers, because of their lower aspect ratio, caused a greater reduction in workability compared to the longer PVA and PE fibers. A significant reduction in workability was observed with a fiber content of 0.5%.

Increasing the content of straight steel fibers with a length/diameter of 13/0.22 mm, as well as polyethylene fibers with a length/diameter of 19/0.023 mm, negatively affected the flowability of the hybrid UHPC mix [146]. However, the steel fibers reduced the fluidity to a lesser extent. An increase in polyethylene fibers from 0.5% to 1% resulted in a greater decrease in flowability compared to the additional application of 1% steel fibers. This is because steel fibers are heavier, have a larger diameter, and are present in smaller quantities by volume, which makes the reinforcing network less complex. As a result, the components can move more easily because there are fewer bonds that hold the mixture in its initial shape.

The flow time of a hybrid mixture with 1.5% steel fibers (length/diameter 13/0.2 mm) and 0.5% polyvinyl alcohol fibers (8/0.038 mm) indicates the negative impact of PVA fibers on the fluidity of the mixture [139]. A mixture with identical composition but with 3% steel fibers achieved a shorter flow time. The results show that the addition of 0.5% PVA fibers reduces the fluidity by more than 1.5% of the steel fibers, partly due to the high water absorption of the PVA fibers.

Li and Deng [76] observed a decrease in the diameter of the slump flow for mixes containing straight steel fibers with a length/diameter ratio of 13/0.2 mm and five types of non-metallic fibers. The highest flow was achieved by the UHPC mix with 2.5% steel fibers. The replacement of 0.5% steel fibers with non-metallic fibers generally had a minor effect on flow. Only for the PVA fibers with the smallest diameter was a noticeable reduction in flow diameter observed. This was related to the significantly higher volume of PVA fibers compared to those of the other mixes, which strongly influenced the binding of the matrix.

Xiang et al. [42] noted that a mix with a total addition of 3% hybrid fibers (straight steel fibers with a length/diameter ratio of 13/0.2 mm and polyoxymethylene fibers of 12/0.2 mm) achieved a slightly lower slump flow diameter than a sample containing a single type of fiber. Compared to the control sample without fibers, the diameter of the slump flow was less by 18–23%. The addition of polyoxymethylene fibers in the amount of 1% or 2% to, respectively, 2% or 1% steel fibers led to the formation of denser skeletons, creating resistance during movement of the mixed components. Although a single type of fiber affected fluidity in the same way, the combination of both types of fiber indicated a mutual interaction between them and the other components of the mixture.

Zhao et al. [150] presented the results of the slump flow of a hybrid mix containing hooked steel fibers with a length/diameter ratio of 30/0.9 mm and plastic fibers of 30/0.9 mm. As the proportion of synthetic fibers increased, the diameter of the slump flow decreased.

A mixture containing hooked-end steel fibers with a length/diameter ratio of 50/1 mm and polypropylene fibers of 12/0.025 mm showed a decrease in slump with increasing fiber content [154]. When the water-to-binder ratio was lower and the fiber content remained the same, a further reduction in fluidity was observed. The reduction in slump was caused by significant adhesion forces that developed between the fibers and the moving matrix.

The results indicate a substantial decrease in the workability of the hybrid mix when part of the steel fibers were replaced with polymer fibers. Proper mix design is crucial when polymer fibers are incorporated due to the limitations highlighted by the findings. Low fluidity may affect fiber dispersion, lead to uneven distribution, fiber agglomeration, and poor workability, which can ultimately result in reduced mechanical properties of the UHPC. The main factors influencing the flowability of the fresh mix are the specific surface area of the fibers, their hydrophilicity, and the degree of flexibility of the fiber [186]. Plastic fibers have a large number of hydrophilic groups on their surface that absorb water. The flexibility of these fibers allows them to deform and become entangled with steel fibers, restricting their movement [136,187,188]. In the case of fibers with a length of 30 mm, a slight increase in flow was observed with a higher content of steel fibers, which is consistent with the explanation mentioned above.

4.3.2. Mechanical Properties

Table 10. Static properties of UHPC reinforced with hybrid steel–polymer fibers.

Hybrid Fibers								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
straight steel	13	0.2	1.5	polyvinyl- alcohol	8	0.038	0.5	compressive strength = 160 MPa (+14.3%) flexural strength = 23.4 MPa (+143.8%)	[139]
straight steel	13	0.2	1.0	polye- thylene	12	0.024	2.0	compressive strength = 211 MPa (−6.4%) 1 axial tensile strength = 16.1 MPa (+2.6%) 1	[144]
			1.5				1.5	compressive strength = 213 MPa (−9.8%) 1 axial tensile strength = 15.6 MPa (−0.6%) 1
			2.0				1.0	compressive strength = 216 MPa (−8.5%) 1 axial tensile strength = 17.4 MPa (+10.8%) 1
			2.5				0.5	compressive strength = 227 MPa (−3.8) 1 axial tensile strength = 16.2 MPa (+3.2%) 1
straight steel	13	0.2	1.5 1.5 1.5 1.5	polyoxyme- thylene	13	0.2	0.5 1.0 1.5 2.0	compressive strength = 145 MPa (-) compressive strength = 142.5 MPa (-) compressive strength = 142 MPa (-) compressive strength = 146 MPa (-)	[145]
straight steel	13	0.22	1.0   1.0   2.0   2.0	polye- thylene	19	0.023	0.5	compressive strength = 144 MPa (−0.7%) flexural strength = 27.5 MPa (+83.3%)	[146]
							1.0	compressive strength = 142.5 MPa (−1.7%) flexural strength = 28.5 MPa (+90%)
							0.5	compressive strength = 149 MPa (+2.8%) flexural strength = 28.5 MPa (+90%)
							1.0	compressive strength = 147 MPa (+1.4%) flexural strength = 24 MPa (+60%)
straight steel	13	0.2	2.0	polypropylene	16	0.15	0.5	compressive strength = 142.4 MPa (+23.3%) axial tensile strength = 6.2 MPa (+64.9%) elastic modulus = 44.5 GPa (+24.7%)	[76]
				polypropylene	40	0.6	0.5	compressive strength = 146.4 MPa (+27.0%) axial tensile strength = 6.6 MPa (+75.5%) elastic modulus = 40.1 GPa (+12.3%)
				polyvinyl-alcohol	12	0.04	0.5	compressive strength = 135.6 MPa (+17.6%) axial tensile strength = 7.0 MPa (+86.2%) elastic modulus = 46.2 GPa (+29.4%)
				polyester	30	0.75	0.5	compressive strength = 146.2 MPa (+26.8%) axial tensile strength = 6.4 MPa (+70.2%) elastic modulus = 49.8 GPa (+39.5%)
				polyester	30	0.9	0.5	compressive strength = 144.6 MPa (+25.4%) axial tensile strength = 5.9 MPa (+56.9%) elastic modulus = 46.3 GPa (+29.7%)
straight steel	13	0.2	0.5   1.0   1.5	polyethylene	6	0.024	1.5   1.0   0.5	compressive strength = 129.9 MPa (−14.0%) 2 axial tensile strength = 6.15 MPa (−23.1%) 2 compressive strength = 137.5 MPa (−8.9%) 2 axial tensile strength = 6.95 MPa (−13.1%) 2 compressive strength = 142.3 MPa (−5.8%) 2 axial tensile strength = 7.80 MPa (−2.5%) 2	[147]
			0.5   1.0   1.5	polyethylene	12	0.024	1.5   1.0   0.5	compressive strength = 132.7 MPa (−12.1%) 2 axial tensile strength = 6.10 MPa (−23.8%) 2 compressive strength = 139.6 MPa (−7.6%) 2 axial tensile strength = 7.30 MPa (−8.8%) 2 compressive strength = 143.2 MPa (−5.2%) 2 axial tensile strength = 7.60 MPa (−5.0%) 2
			0.5   1.0   1.5	polyethylene	18	0.024	1.5   1.0   0.5	compressive strength = 131.5 MPa (−12.9%) 2 axial tensile strength = 6.15 MPa (−23.1%) 2 compressive strength = 139.7 MPa (−7.5%) 2 axial tensile strength = 6.65 MPa (−16.9%) 2 compressive strength = 140.9 MPa (−6.7%) 2 axial tensile strength = 7.30 MPa (−8.8%) 2
straight steel	13	0.2	1.0   2.0	polyoxyme- thylene	12	0.2	2.0   1.0	compressive strength = 137 MPa (+19.1%) elastic modulus = 43 GPa (+2.4%) compressive strength = 154 MPa (+33.9%) elastic modulus = 46 GPa (+9.5%)	[42]
straight steel	13	0.2	1.3	polyolefin	40	0.6	0.5	compressive strength = 121.4 MPa (+32.7%) axial tensile strength = 4.3 MPa (+3.4%) elastic modulus = 36.9 GPa (+0%)	[75]
				polyvinyl-alcohol	12     8     12	0.04     0.2     0.2	0.5     0.5     0.5	compressive strength = 101.2 MPa (+10.6%) axial tensile strength = 5.1 MPa (+22.6%) elastic modulus = 42.3 GPa (+14.6%) compressive strength = 108.1 MPa (+18.1%) axial tensile strength = 4.7 MPa (+13.0%) elastic modulus = 37.2 GPa (+0.8%) compressive strength = 106.1 MPa (+16.0%) axial tensile strength = 4.95 MPa (+13.0%) elastic modulus = 55.8 GPa (+51.2%)
				polyester	30     30	0.75     0.9	0.5     0.5	compressive strength = 108.3 MPa (+18.4%) axial tensile strength = 5.1 MPa (+22.6%) elastic modulus = 44.5 GPa (+20.6%) compressive strength = 119.8 MPa (+30.9%) axial tensile strength = 4.2 MPa (+1.0%) elastic modulus = 40.2 GPa (+8.9%)
straight steel	14	0.22	0.5 1.0 1.5	polyvinyl-alcohol	12	0.026	1.5 1.0 0.5	compressive strength = 95.8 MPa (+35.7%) compressive strength = 98.4 MPa (+39.4%) compressive strength = 105.4 MPa (+49.3%)	[148]
straight steel	18	0.16	0.5   1.0   1.5	polye- thylene	13	0.02	1.5   1.5   1.5	compressive strength = 175.5 MPa (+24.4%) 3 axial tensile strength = 11.7 MPa (+7.3%) 3 compressive strength = 180.4 MPa (+27.8%) 3 axial tensile strength = 11.2 MPa (+2.8%) 3 compressive strength = 187.9 MPa (+33.1%) 3 axial tensile strength = 12.4 MPa (+13.8%) 3	[149]
straight steel	16.5 19.5 16.5 19.5	0.2 0.2 0.2 0.2	0.33 0.67 0.33 0.67	polye- thylene polyvinyl- alcohol	18   12	0.012   0.012	0.5   0.5	compressive strength = 142 MPa (−4.7%) 4 axial tensile strength = 16.2 MPa (+65.3%) 4 compressive strength = 143 MPa (−4.0%) 4 axial tensile strength = 11.8 MPa (+20.4%) 4	[143]
straight steel	19.5	0.2	0.75   0.5   0.25	polyvinyl- alcohol	6	0.012	0.25   0.5   0.75	compressive strength = 123.2 MPa (−0.2%) 1 axial tensile strength = 7.6 MPa (−31.5%) 1 compressive strength = 119.7 MPa (−3.0%) 1 axial tensile strength = 6.9 MPa (−37.8%) 1 compressive strength = 115.7 MPa (−6.2%) 1 axial tensile strength = 6.8 MPa (−38.7%) 1	[140]
			0.75   0.5   0.25	polyvinyl- alcohol	12	0.012	0.25   0.5   0.75	compressive strength = 121.0 MPa (−2.0%) 1 axial tensile strength = 7.1 MPa (−36.0%) 1 compressive strength = 119.4 MPa (−3.3%) 1 axial tensile strength = 6.7 MPa (−39.6%) 1 compressive strength = 113.4 MPa (−8.1%) 1 axial tensile strength = 5.4 MPa (−51.4%) 1
			0.75   0.5   0.25	polyethylene	12	0.02	0.25   0.5   0.75	compressive strength = 125.0 MPa (+1.3%) 1 axial tensile strength = 7.5 MPa (−32.4%) 1 compressive strength = 119.9 MPa (−2.8%) 1 axial tensile strength = 6.5 MPa (−41.4%) 1 compressive strength = 114.8 MPa (−7.0%) 1 axial tensile strength = 6.0 MPa (−46.0%) 1
hooked-end steel	30	0.9	0.5   1.0   1.5	plastic synthetic	30	0.9	1.5   1.0   0.5	compressive strength = 120 MPa (+9.1%) flexural strength = 23 MPa (+31.4%) compressive strength = 125 MPa (+13.6%) flexural strength = 26.5 MPa (+51.4%) compressive strength = 142 MPa (+29.1%) flexural strength = 24.5 MPa (+40.0%)	[150]
hooked-end steel	50	1.0	0.5       1.0	polypropylene	12	0.025	0.06       0.06	compressive strength = 107.2 MPa (+7.8%) splitting tensile strength = 10.2 MPa (+104%) flexural strength = 9.1 MPa (+11.0%) elastic modulus = 38.9 GPa (+0.5%) compressive strength = 111.0 MPa (+11.7%) splitting tensile strength = 11.0 MPa (+120%) flexural strength = 9.4 MPa (+14.6%) elastic modulus = 39.4 GPa (+1.8%)	[151]
hooked-end steel	50	1.0	1.0       1.5	polypropylene	12	0.025	0.05       0.1	compressive strength = 113.6 MPa (+0.5%) splitting tensile strength = 10.2 MPa (+64.5%) flexural strength = 8.7 MPa (+17.6%) elastic modulus = 39.0 GPa (+1.8%) compressive strength = 112.4 MPa (−0.5%) splitting tensile strength = 10.4 MPa (+67.7%) flexural strength = 9.1 MPa (+23.0%) elastic modulus = 39.1 GPa (+2.1%)	[153]
hooked-end steel	50	1.0	0.25       0.5       0.75	polypropylene	12	0.025	0.75       0.5       0.25	compressive strength = 122.3 MPa (−5.6%) splitting tensile strength = 9.3 MPa (+36.8%) flexural strength = 8.6 MPa (+50.9%) elastic modulus = 29.6 GPa (−8.9%) compressive strength = 133.9 MPa (+3.4%) splitting tensile strength = 10.0 MPa (+47.1%) flexural strength = 9.2 MPa (+61.4%) elastic modulus = 32.5 GPa (+0%) compressive strength = 144.7 MPa (−4.2%) splitting tensile strength = 13.5 MPa (+51.7%) flexural strength = 9.8 MPa (+16.7%) elastic modulus = 34.3 GPa (−10.7%)	[152]
hooked-end steel	50	1.0	0.5       1.0         1.0         1.5	polypropylene	12	0.025	0.025       0.05         0.05         0.1	compressive strength = 106.6 MPa (−6.3%) 5 splitting tensile strength = 9.9 MPa (+98.0%)5 flexural strength = 8.9 MPa (+8.5%) 5 elastic modulus = 32.4 GPa (−16.3%) 5 compressive strength = 111.0 MPa (−2.5%) 5 splitting tensile strength = 11.0 MPa (+120.0%) 5 flexural strength = 9.4 MPa (+14.6%) 5 elastic modulus = 39.4 GPa (+1.8%) 5 compressive strength = 110.6 MPa (−5.5%) 6 splitting tensile strength = 10.5 MPa (+61.5%) 6 flexural strength = 9.1 MPa (+16.7%) 6 elastic modulus = 37.3 GPa (−4.1%) 6 compressive strength = 106.4 MPa (−9.1%) 6 splitting tensile strength = 10.7 MPa (+64.6%) 6 flexural strength = 9.6 MPa (+23.1%) 6 elastic modulus = 39.2 GPa (+0.8%) 6	[154]

¹ comparison with a mix with 1% straight steel fibers; ² comparison with a mix with 1% straight steel fibers; ³ comparison with a mix with 1.5% polyethylene fibers; ⁴ comparison with a mix with 1.5% straight steel fibers; ⁵ water/binding material ratio = 0.3; ⁶ water/binding material ratio = 0.25.

shows a summary of recent tests of the mechanical properties of UHPC with hybrid fiber reinforcement under static load.

The UHPC mix with the addition of 1.5% straight steel fibers with a length/diameter ratio of 13/0.2 mm and 0.5% PVA fibers of 8/0.038 mm achieved a 14.3% higher compressive strength compared to the control sample and the mix containing 2% steel fibers [139]. The results indicate a positive hybridization effect without the formation of fiber agglomeration. Steel fibers were evenly distributed, allowing their properties to contribute to the increase in compressive strength.

The compressive strength of the mixes with a total hybrid fiber content of 3% reached the highest value for the mixture containing 2.5% straight steel fibers with a length/diameter ratio of 13/0.2 mm and 0.5% polyethylene fibers of 12/0.024 mm [144]. Due to the higher modulus of elasticity and tensile strength of steel fibers compared to polypropylene fibers, the compressive strength of concrete improved with an increase in the content of steel fibers [189]. Increasing the proportion of polymer fibers led to an increase in the volume of pores within the matrix and disrupted its density, negatively affecting the strength parameters [190,191].

The compressive strength of the mixture with 1.5% steel fibers with a length/diameter ratio of 13/0.2 mm achieved values ranging from 142.5 to 146 MPa, depending on the content of polyoxymethylene (POM) fibers, which varied from 0.5% to 2% [145].

On the basis of the results, it can be concluded that the influence of polymer fibers on the compressive strength of concrete is not straightforward. The compressive strength depends primarily on the proportion of steel fibers in the mixture. The mixture containing only steel fibers achieved a compressive strength that exceeded 160 MPa, with a growing trend of up to 2.5% fiber content. This growing trend suggests that the steel fibers were evenly distributed, improving the bond strength between the concrete matrix and the fiber. The mix containing only POM fibers achieved compressive strength of up to 110 MPa as the fiber content increased. Although POM fibers do not significantly affect compressive strength, they do not interfere with the uniform distribution of steel fibers in the matrix, thus helping to form an effective internal fiber network within the concrete.

Nikmehr et al. [146] observed slight decreases in compressive strength for a mixture containing 1% straight steel fibers with a length/diameter ratio of 13/0.22 mm and 0.5% or 1% polyethylene fibers of 19/0.023 mm (a reduction of 0.7% and 1.7%, respectively, compared to the mix without fibers). When the steel fiber content was increased to 2%, the addition of 0.5% or 1% polyethylene fibers led to strength increases of 2.8% and 1.4%, respectively. The inclusion of PE fibers trapped more air voids during mixing and could have weakened the fiber-matrix bond. Strength improvement was mainly driven by steel fibers, as they contribute to the delay in crack formation and propagation. Additionally, the high aspect ratio of PE fibers, although potentially helpful for anchoring and twisting, caused fiber grouping and uneven distribution in this case [192].

In the case of mixes with two types of fibers: 2% straight steel fibers with a length/diameter ratio of 13/0.2 mm and 0.5% of one of five different types of non-metallic fibers (PP 16/0.15 mm or 40/0.6 mm, PVA 12/0.04 mm, and polyester 30/0.75 mm or 30/0.9 mm), compressive strength increases ranging from 17.6% to 27% were noted compared to the mix without fibers [76]. The mixture with PVA fibers showed the lowest strength gain compared to the control mixture and an 8.6% reduction compared to the mix with 2.5% steel fibers. The same volume content of non-metallic fibers contained many more fibers than steel fibers, resulting in uneven fiber dispersion, mixing difficulties, and fiber agglomeration.

Chu [147] investigated the effect of polyethylene fibers with lengths/diameter ratios of 6, 12, and 18/0.024 mm and straight steel fibers of 12/0.2 mm, on the compressive strength of UHPC with a total fiber content of 2%. As the percentage of non-metallic fibers increased, the compressive strength decreased, with no significant differences observed between the different lengths of the fiber. However, according to Kwan et al. [193], excessively short fibers can lead to complete slippage within the mixture. Even if the bond strength is sufficient at the initial crack, increasing the applied force could cause the short fibers to fail without providing adequate anchorage. The best combination was achieved with steel fibers and 12 mm polyethylene fibers, resulting in the smallest strength reductions, suggesting that the fibers were evenly distributed and effectively prevented microcracking without negatively affecting peak strength.

A reduction in the compressive strength of mixtures with a total fiber content of 3% (steel and polyoxymethylene) was confirmed in [42]. A linear decrease in strength was observed, from 163 MPa for 3% steel fibers to 125 MPa for 3% POM fibers, with intermediate strength values for hybrid mixes. It was observed that the POM fibers were either pulled out or fractured, whereas the steel fibers were predominantly pulled out because of their high strength.

Mixtures with a total addition of 1.8% hybrid fibers, including 1.3% straight steel fibers with a length/diameter of 13/0.2 mm and 0.5% polymer fibers, showed an increase in compressive strength compared to the control sample [75]. Among the polymer fibers used were polyolefin fibers with a length/diameter of 40/0.6 mm, PVA fibers of 12/0.04 mm, 8/0.2 mm or 12/0.2 mm, and polyester fibers of 30/0.75 mm or 30/0.9 mm. An increase in compressive strength was obtained compared to the 1.3% steel fiber mixture only for two mixtures: steel-polyolefin and steel-polyester (30/0.9 mm) fiber combinations. It can be seen that these fibers had the largest diameters, which effectively inhibited macrocracks. The lowest compressive strength was achieved with the addition of fibers with the smallest diameters, which was also confirmed in [76].

Feng et al. [148] observed an increase in the compressive strength of a UHPC mixture with a total fiber content of 2% as the proportion of straight steel fibers with a length/diameter of 14/0.22 mm increased compared to PVA fibers with a length/diameter of 12/0.026 mm. Compressive strength increased from 35.7% to 49.3% as the proportion of steel fibers increased from 0.5% to 1.5%.

Huo et al. [149] determined the compressive strength of mixtures containing 1.5% polyethylene fibers with a length/diameter of 13/0.02 mm and steel fibers with a length/diameter of 18/0.16 mm in amounts of 0.5%, 1%, and 1.5%. The lowest increase in compressive strength was observed with a 0.5% addition of steel fibers (24.4% compared to the fiber-free mixture). Further increases in the steel fiber content to 1% and 1.5% resulted in increases in strength of 27.8% and 33.1%, respectively.

Mixtures containing straight steel fibers with a length/diameter of 16.5/0.2 mm in the amount of 0.33% and 19.5/0.2 mm in the amount of 0.67%, combined with polyethylene fibers (18/0.012 mm) or PVA fibers (12/0.012 mm) in the amount of 0.5%, achieved lower compressive strength than the mixture containing 1.5% steel fibers [143]. The results confirm that steel fibers are responsible for increasing the compressive strength of UHPC, provided other parameters are met, such as proper workability, aggregate classification, water-to-binder ratio, and curing process [194].

An analysis of the results for mixtures with a total fiber content of 1% indicates a linear decrease in compressive strength as the proportion of polymer fibers (PVA 6/0.012 mm, PVA 12/0.012 mm, or PE 12/0.02 mm) increased relative to straight steel fibers (19.5/0.2 mm) [140]. No significant differences in compressive strength were observed depending on the type of polymer fiber used, and the strengths were similar for the three types of fibers. This was attributed to the significantly lower stiffness of the polymer fibers, which, although binding to the concrete matrix, were unable to hold it as effectively as the steel fibers. The high aspect ratio of polymer fibers also hindered their uniform dispersion, causing fiber concentration, which led to the formation of a nonhomogeneous structure.

The combination of hooked steel fibers with a length/diameter of 30/0.9 mm and polymer fibers with the same shape ratio (30/0.9 mm), with a total content of 2%, resulted in an increase in compressive strength [150]. The highest strength was obtained in the mixture with 1.5% steel fibers, showing a 29% increase in strength compared to the fiber-free mixture. A linear decrease in strength was also observed, reaching the lowest value for concrete with the lowest steel fiber content (0.5%), although it still showed a 9% increase in strength compared to concrete without fibers.

Concrete mixtures with a hybrid addition of hooked steel fibers with a length/diameter ratio of 50/1 mm in the amount of 0.5% or 1%, and polypropylene fibers with a length/diameter ratio of 12/0.025 mm in the amount of 0.06%, achieved compressive strength increases of 7.8–11.7% compared to the reference mixture without fibers [151]. The effects of increasing the amount of hooked steel fibers from 1% to 1.5% and polypropylene fibers from 0.05% to 0.1% were described in [153]. The increase in the amount of steel and polypropylene fibers was found to cause a slight decrease in compressive strength as a result of their nonuniform distribution in the mixture. The compressive strength of concrete with the same types of fibers in a total amount of 1% was determined in [152]. With the addition of 0.25% and 0.75% steel fibers, slight decreases in strength of 5.6% and 4.2% were obtained, respectively. A uniform volumetric share of 0.5% ST fibers and 0.5% PP fibers resulted in a slight increase of 3.4% compared to the fiber-free mixture. The effect of the hybrid fibers described above on the compressive strength of UHPC mixtures was confirmed in [154]. Despite the addition of fibers in a total amount of 0.525 to 1.60%, a decrease in compressive strength was obtained compared to the control mixtures. The properties of the fibers were not utilized, which may indicate their non-uniform distribution and the formation of weaker zones without fiber reinforcement through which cracks were initiated. Additionally, the high aspect ratio and the large amount of polypropylene fibers led to their agglomeration and the formation of a more porous matrix [195].

An ultra-high performance concrete mixture with the addition of 1.5% straight steel fibers with a length/diameter ratio of 13/0.2 mm and 0.5% PVA fibers 8/0.038 mm achieved a flexural tensile strength 144% higher than the control UHPC and 10% higher than concrete with a 2% ST addition [139]. After the initial formation of the crack, the load continued to increase until the maximum strength was reached. The load bearing capacity increased due to the steel fibers that feed into the crack interfaces. The tensile strength of the fibers and their behavior during removal from the matrix was used.

The flexural tensile strength of UHPC mixtures with the addition of 1% or 2% straight steel fibers with a length/diameter ratio of 13/0.22 mm and 0.5% or 1% polyethylene fibers 19/0.023 mm was determined in [146]. In the case of the mixture with 1% steel fibers, the addition of polyethylene fibers in the amount of 0.5% or 1% resulted in increases in strength of 83.3% and 90%, respectively. Increasing the amount of steel fibers to 2% and adding polypropylene fibers in the amount of 0.5% or 1% led to strength increases of 90% and 60%, respectively, indicating that with a total fiber content of 3%, they were not evenly distributed and the hydration process may have been hindered. The results demonstrated the positive effect of polymer fibers, which contributed to the increase in flexural strength. These fibers prevented the formation of microcracks and allowed the steel fibers to carry the ultimate load.

The mixture containing a total of 2% fibers, composed of hooked steel fibers with a length/diameter of 30/0.9 mm and polymer fibers of 30/0.9 mm, achieved an increase in the flexural tensile strength compared to the reference mixture [150]. The mixture with an equal volume content of steel and polymer fibers (1% each) reached the highest increase in strength, which was 51.4% compared to the control mixture. The concrete strength for this fiber combination was also higher than the strength of the mixture with a 2% addition of steel fibers, indicating the positive effect of fiber hybridization.

The addition of hooked steel fibers with a length/diameter ratio of 50/1 mm in the amount of 0.5% or 1% and polypropylene fibers of 12/0.025 mm in the amount of 0.06% resulted in an increase in flexural tensile strength as the content of steel fibers increased [151]. The mixture achieved an increase in strength of 11% and 14.6%, respectively, for the increase in the content of steel fibers. The total addition of 1.05 to 1.6% of the fibers mentioned above was presented in [153]. The increase in strength was 17.6% for 1% steel fibers and 0.05% PP fibers compared to the control mixture without fibers. Increasing the share of ST fibers to 1.5% and PP to 0.1% resulted in a further increase in flexural strength of 5.4%. Smarzewski [152] examined the effect of 1% of the aforementioned ST and PP fiber mixtures on the properties of UHPC. For each fiber combination, an increase in flexural tensile strength was achieved from 16.7 to 61.4%, with the best results obtained from an equal distribution of fibers at 0.5% each. The positive effect of adding hooked steel and polypropylene fibers was confirmed in [154]. An increase in strength was observed from 8.5% to 23.1% as the fiber content increased. Furthermore, reducing the water/binder ratio from 0.3 to 0.25 resulted in improved flexural tensile strength.

The axial tensile strength of UHPC mixtures with straight steel fibers (length/diameter = 13/0.2 mm) and polyethylene fibers (12/0.024 mm) with a total fiber content of 3% was determined by Huang et al. [144]. The study considered the following fiber combinations in hybrid mixtures: 1/2%, 1.5/1.5%, and 2/1%. Strength increases were achieved compared to UHPC with 3% steel fibers, except for the 1.5/1.5% combination. Increasing the content of PP fibers to 2% had a positive effect, suggesting that with equal fiber shares, fiber clusters could have formed, creating spaces through which cracks spread.

Hybrid concretes with a total fiber content of 2.5% were tested for axial tensile strength [76]. The mixture included 2% straight steel fibers (length/diameter = 13/0.2 mm) and 0.5% polymer fibers. The effects of several types of synthetic fibers were compared, including PP (16/0.15 mm), PP (40/0.6 mm), PVA (12/0.04 mm), polyester (30/0.75 mm), and polyester (30/0.9 mm). The axial tensile strength of concrete was found to increase for each hybrid mixture by 57–86% compared to the control sample.

The influence of combining PE fibers (length/diameter = 6, 12, and 18/0.024 mm) with straight steel fibers (13/0.2 mm) on UHPC axial tensile strength of was presented in [147]. For each mixture, there was a 2.5–24% decrease in strength compared to the control sample with 2% ST, and this increased with a higher PE fiber content. The lowest strength results were observed for the longest PE fibers, suggesting that the lower amount of these fibers led to an uneven distribution across the sample cross section.

All hybrid concretes with a total fiber addition of 1.8%, including 1.3% steel fibers (length/diameter = 13/0.2 mm) and 0.5% polymer fibers, showed an increase in axial tensile strength compared to the reference mixture without fibers [75]. The polymer fibers used included polyolefin (40/0.6 mm), PVA (12/0.04 mm), PVA (12/0.2 mm), polyester (30/0.75 mm) and polyester (30/0.9 mm).

The axial tensile strength of hybrid mixtures with the addition of 1.5% PE fibers (length/diameter = 13/0.02 mm) and 0.5–1.5% steel fibers (18/0.16 mm) increased with the proportion of ST, ranging from 7.3% to 12.4%, due to the high tensile strength of the steel fibers [149].

Concretes with the addition of straight steel fibers (length/diameter = 16.5/0.2 mm at 0.33% and 19.5/0.3 mm at 0.67%) and polymer fibers at 0.5% achieved higher axial tensile strength compared to a mixture with 1.5% steel fibers [143]. With 0.5% polyethylene fibers (18/0.012 mm), the increase was 65.3%, and with PVA fibers (12/0.012 mm) it was 20.4%. A positive hybrid effect was observed when 0.5% of steel fibers were replaced with polymer fibers, especially PE fibers.

The axial tensile strength was determined for mixtures with a total fiber content of 1% [140]. These included combinations of steel fibers (length/diameter = 19.5/0.2 mm) and polymer fibers, such as PVA (6/0.012 mm), PVA (12/0.012 mm), or PE (12/0.02 mm). Each mixture showed a linear decrease in tensile strength with a reduction in ST fiber content. The flowability results were significantly lower than those for steel fibers, due to the high shape factor of the polymer fibers because of further hindered proper fiber dispersion. The addition of shorter polymer fibers resulted in greater reductions in strength. The incorporation of microfibers has been observed to improve tensile strength, as they are more active during the cracking process, causing macrocrack formation to delay and improve ductility [196,197]. A loud sound heard after the first crack indicated fiber bridging. In UHPC without fiber addition, a sudden failure occurred, as once the concrete matrix reached its maximum strength, there was no component in the mixture capable of carrying the load [76]. Microcracks form in weakened areas, such as pores or interfaces. These cracks connect, but when microfibers are present, their expansion is hindered [198]. The effectiveness of microfibers is related to the smaller spacing between them as a result of their higher density per unit volume.

The effect of hooked-end steel fibers (length/diameter = 50/1.0 mm) at 0.5 or 1% and polypropylene fibers (12/0.025 mm) at 0.06% on tensile splitting strength was reported in [151]. The UHPC mixture achieved a tensile strength increase of 104–120% with an increase in the steel fiber content from 0.5 to 1%. The addition of the aforementioned fibers in a total amount of 1.05–1.6% was presented in [153]. Significant strength gains were observed in the range of 64.5 to 67.7% with increasing total fiber content. The results indicate that for the mixture with 1.06% fibers, the packing density was already high enough that increasing the fiber content further did not result in a clear increase in strength. Smarzewski [152] observed that the splitting tensile strength, after adding the same fibers in a total amount of 1%, increased 36.8 to 51.7%. The highest value was obtained by UHPC with a maximum steel fiber volume fraction of 0.75%, indicating optimal utilization of the parameters of the hooked-end fiber, which contributed to the bridge macrocracks. The positive effect of adding hooked-end ST and PP fibers was also confirmed in [154]. The splitting tensile strength was observed to increase by 98–120% and 61.5–64.6% for water/binder ratios of 0.3 and 0.25, respectively. The increase in UHPC strength with 1.05% fiber addition and w/b ratio = 0.3 was higher than that of 1.6% fiber addition at a lower w/b = 0.25. This can be explained by the uneven distribution of fibers at a higher fiber content, linked to reduced workability.

Li and Deng [76] determined the elastic modulus of concrete with hybrid fiber mixtures, including 2% steel fibers (length/diameter = 13/0.2 mm) and 0.5% polymer fibers. The following types of polymer fibers were used: PP (16/0.15 mm), PE (40/0.6 mm), PVA (12/0.04 mm), polyester (30/0.75 mm), and polyester (30/0.9 mm). For each mixture, the elastic modulus increased by 12.3 to 39.5% compared to the control concrete without fibers. Compared to the mixture with 2.5% steel fibers, only the concrete with steel and polyester fibers (30/0.75 mm) achieved an increase in elastic modulus, suggesting a positive hybridization effect for this combination.

The elastic modulus of UHPC with a total fiber content of 3% was tested in [42]. Steel fibers (length/diameter = 30/0.2 mm) and POM fibers (12/0.2 mm) were used. The highest elastic modulus was obtained by mixing with 3% steel fibers, while increasing the content of POM fiber to 1% and 2% caused a slight linear decrease in the elastic modulus.

Mixtures with a total addition of hybrid fibers of 1.8%, including 1.3% steel fibers (length/diameter = 13/0.2 mm) and 0.5% polymer fibers, achieved an increase in elastic modulus [75]. Among the polymer fibers used were POL (40/0.6 mm), PVA (12/0.04 mm), PVA (8/0.2 mm), PVA (12/0.2 mm), polyester (30/0.75 mm), and polyester (30/0.9 mm). The highest elastic modulus was recorded for the ST and PVA (12 mm) mixture, which was 51% higher compared to the reference mixture. The elastic modulus of UHPC with steel and polyolefin fibers was observed to decrease by 9% compared to concrete with 1.8% ST. This was related to the lower elastic modulus of polyolefin fiber compared to that of ST fiber.

The elastic modulus of concrete with the addition of 0.5 or 1% hooked-end ST fibers (length/diameter = 50/1 mm) and 0.06% PP fibers (12/0.025 mm) increased from 0.5 to 1.8% as the ST fiber content increased [151]. The addition of the fibers mentioned above in amounts of 1.05% and 1.6% was presented in [153]. The UHPC mixtures achieved elastic modulus gains of 1.8% and 2.1%, respectively, with an increasing total fiber content. The impact of hybrid mixtures of identical fibers with a total content of 1% was assessed in [152]. A decrease in elastic modulus of 8.9% and 10.7% was observed for two combinations: 0.25% ST/0.75% PP and 0.75% ST/0.25% PP, but no effect was observed for the 0.5% ST/0.5% PP mixture. The addition of the aforementioned fiber types in total amounts of 0.525% and 1.6% resulted in a decrease in elastic modulus by 4.1% and 16.3%, or a slight increase of 0.8% and 1.8% [154]. In the case of concrete with a total fiber content of 1.05%, reducing the w/b ratio resulted in a decrease in elastic modulus, which can be attributed to insufficient water content for proper mixing and hydration. These results suggest that hooked-end ST has a minimal effect on the elastic modulus value.

Figure 6a presents the results of the compressive strength tests of UHPC with the addition of steel and polyethylene fibers. An increase in compressive strength was observed from 115 to 225 MPa with the growing fiber content. The lowest results were obtained for mixtures with a total fiber content of 1%. For a 2% fiber addition, small variations in strength results were recorded, ranging from 135 to 142 MPa. A positive effect of increasing the length of steel fibers from 13 to 18 mm and reducing the length of PE fibers from 19 to 13 mm was observed with a total fiber content of 2.5%. This suggests better use of longer steel fibers due to higher anchoring forces within the matrix and more effective crack-bridging by shorter polyethylene fibers. For the 3% fiber content, the strength increased with an increasing proportion of steel fibers. The most favorable combination was found to be 2.5% ST13 and 0.5% PE12. The lowest compressive strength was obtained with a mixture of short steel fibers of 13 mm at 2% and PE fibers of 19 mm at 1%, confirming the trends observed with a lower fiber content. Figure 6b presents the results of the compressive strength tests for concrete with the addition of steel and PVA fibers, which ranged from 95 to 160 MPa, depending on the type and amount of fibers, as well as the recipe for the mixture without fibers. For the 1% fiber content, a high concentration of results was observed at 118 MPa. Increasing fiber content to 1.8% and 2% resulted in a decrease in strength to 102 MPa, with a high concentration of results. This may be due to a significant reduction in the workability, leading to agglomeration of components and an impaired hydration process, creating areas of the heterogeneous matrix. The compressive strength of UHPC with the addition of steel and polypropylene fibers, with a total content ranging from 0.5% to 1.5%, was between 105 and 145 MPa (Figure 6c). The combination used hooked steel fibers of 50 mm in length and polypropylene fibers of 12 mm. Increasing the fiber content to 1% showed a positive effect on the compressive strength. Depending on the type of fiber, values between 110 and 145 MPa were achieved, and it was observed that the strength of the UHPC increased with the growing proportion of steel fibers. For a mix with 2% straight steel fibers of 13 mm and 0.5% PP fibers of 16 or 40 mm, a maximum value was obtained, with a slightly higher result observed for the longer PP fibers.

Figure 6d shows the compressive strength results of mixtures with ST fibers and other types of polymer fibers (POM, PES, POL, P). Similar results were obtained for fiber contents ranging from 2% to 3.5%. A slight increase in strength was observed when the fiber content increased from 2% to 2.5%. Further increasing the fiber content to 3.5% did not provide significant benefits.

Similar results for the flexural tensile strength of the mixtures were obtained with a fiber content of 0.5 to 1.6%, ranging from 9 to 10 MPa (Figure 7). These mixtures contained hooked steel fibers 50 mm long and polypropylene fibers 12 mm long. Increasing the total fiber content and changing the proportion between them did not lead to a rise in strength. The long hooked fibers, with their high tensile strength and ability to resist pulling forces from the matrix, were not fully used in load transfer. This was likely due to fiber clustering, which prevented them from effectively stopping crack formation. The addition of 2% fibers significantly increased the flexural tensile strength to 23–28.5 MPa. The highest results were achieved for UHPC with hooked steel fibers of 30 mm and plastic fibers of 30 mm, with an equal volume share of ST and P fibers. The highest UHPC strength was observed with 12 mm ST fibers and 19 mm PP fibers with an equal fiber content. Increasing the fiber amount to 2.5% and 3% resulted in a decrease in strength, linked to reduced workability and the creation of weakened areas.

4.3.3. Dynamic Properties

A UHPC mix with a total fiber content of 2%, consisting of hooked-end steel fibers (length/diameter = 30/0.9 mm) and polymer fibers (30/0.9 mm), achieved an increase in dynamic compressive strength ranging from 0.2% to 17.2%, depending on the mix and strain rate, compared to the control UHPC [150]. As the strain rate increased, the compressive strength also increased, reaching the highest gains for concrete with the lowest number of synthetic fibers. The results indicate that the hooked-end steel fibers were responsible for the greater reinforcing effect. This phenomenon can be explained by the homogeneous dispersion of fibers within the matrix, which effectively compensates for internal defects. Synthetic fibers, which have a lower elastic modulus than steel fibers, were less effective at inhibiting crack propagation under dynamic load, as their greater flexibility results in lower stiffness and reduced capacity to transfer tensile stresses across developing cracks. However, the mixture containing 1.5% synthetic fibers achieved slightly lower strength, suggesting that these fibers help disperse stress concentrations and slow the formation and growth of microcracks. At lower strain rates (45–55 s⁻¹), the fiber-free mixture fractured into large fragments, while the samples with fiber addition maintained their integrity. This is due to the fibers preventing the coalescence of emerging cracks, acting as effective bridges between cracked surfaces, and distributing tensile stresses more evenly across the damaged areas. Increasing the strain rate (93–105 s⁻¹) caused greater damage, with some fibers detached from the matrix near the edges of the specimen. Additionally, hybrid fiber mixtures exhibited less severe cracking under impact loading compared to samples reinforced with a single type of fiber. Strain rates greater than 160 s⁻¹ caused all samples to lose integrity and show severe damage. As a result of the expansion of stresses around steel fibers, matrix spalling was observed. Polymer fibers, characterized by high deformability, effectively controlled crack propagation within the matrix through stretching and eventual rupture upon reaching their tensile limits. Consequently, cracks in the hybrid mixtures were less abrupt than those in the mixtures containing only steel fibers.

UHPC with straight steel fibers (length/diameter = 13/0.2 mm) and POM fibers (12/0.2 mm), with a total fiber content of 3%, underwent dynamic tests [42]. The increase in strain rate resulted in a higher strength for each mix due to the stronger impact on the structure of the material [199,200]. At lower strain rates (40–80 s⁻¹), only a few cracks were observed, bridged by steel fibers, along with minor edge spalling. Increasing the strain rate (80–100 s⁻¹) led to more cracks, fiber pull-out, and fiber breakage. The further increase in the strain rate caused fragmentation to become progressively smaller pieces.

Dynamic values of compressive strength, elastic modulus, and fracture resistance were higher for the mixture with 2% ST. This is due to the lower strength and elastic modulus of POM fibers compared to steel fibers. ST fibers were pulled out, while POM fibers were pulled out or broken under dynamic loads, due to their lower tensile strength and weaker interfacial bond with the concrete matrix, which results in lower resistance during extraction. Importantly, short steel fibers do not create localized energy concentration points, unlike long hooked-end fibers, which exhibit particularly abrupt cracking, because of their geometry that generates stress concentrations at their ends, leading to localized failure in the surrounding matrix. Fiber failure due to exceeding its tensile strength is unfavorable, as it prevents full use of the mechanical properties of fibers [201,202]. Energy is suddenly released, which can cause a sharp decrease in the strength of the material.

A mixture with 1.5% PE fibers (length/diameter = 13/0.02 mm) was enriched with 0.5%, 1%, or 1.5% ST fibers (18/0.16 mm) [149]. For each concrete, an increase in strain rate led to higher axial tensile strength and impact energy. As the ST content increased, the dynamic behavior of the UHPC improved. The axial tensile strength of the mixture with 0.5% ST fibers and a strain rate of 49 s⁻¹ was 29.4 MPa, gradually increasing with the ST fiber content to a maximum of 43.2 MPa at a strain rate of 45 s⁻¹ with 1.5% ST. The impact energy increased 1.7 times with increasing steel fiber content from 0.5% to 1.5%. This indicates a significant improvement in energy dissipation efficiency by steel fibers. Additionally, they activate the potential of PE fibers for pull-out under dynamic impact. Due to their low deformability, steel fibers bear higher external loads from the beginning of crack development. The increase in the dynamic strength of the matrix occurs mainly before the first crack forms, and after its formation, multiple cracks are possible due to fiber bridging [203].

Increased short-fiber density contributes significantly to the energy dissipation capacity of the UHPC under dynamic loading. The higher number of fibers per unit volume leads to more frequent fiber–matrix interactions, which in turn enhances frictional pull-out resistance and crack bridging. The described relationships were confirmed based on the influence of steel microfibers (length/diameter = 6/0.12 mm) in amounts ranging from 1 to 2.5% in concrete mixtures [204]. Three-dimensional tomography-based reconstructions illustrated the spatial distribution of pores and fibers, allowing a precise visualization of fiber alignment and clustering, providing insight into crack formation and propagation pathways within the concrete. For plain concrete, the cracks were concentrated and exhibited pronounced brittle behavior, while fiber-reinforced concrete showed evenly distributed cracks. In regions with high fiber density (reinforcement zones), macrocracks were absent, and only minor microcracks were observed. Fiber inclusion altered the original crack propagation directions, allowing stress redistribution throughout the matrix. This change mitigates concrete brittleness, improving ductility and impact resistance. Peak stress, maximum strain, and ultimate stress were highest for 2% fiber content. Increasing the fiber content to 2.5% resulted in a reduction, indicating an uneven distribution of fibers within the matrix.

For a mixture with ST fibers (12/0.175–0.35 mm) and PE fibers (12/0.039 mm) totaling 2%, high-rate dynamic compression tests (9.5–11.5 m/s) showed some integrity of the sample [205]. Samples with 2% ST fibers exhibited brittle failure with concrete fragmentation, while 2% PE fibers resulted in wide cracks with fewer microcracks. This difference arises due to the significantly higher number of PE fibers compared to ST fibers at the same volumetric fraction (due to the lower density and diameter).

Several studies using high-speed imaging and post-fracture SEM analysis have shown that short fibers are activated earlier during impact, forming a dense bridging network that resists crack propagation. The energy absorbed during debonding and pulling out is largely governed by the extent of interfacial bonding and the frictional resistance during fiber extraction. These mechanisms are particularly efficient in short fibers as a result of their higher probability of activation and shorter embedding lengths.

The dynamic properties of UHPC reinforced with steel and polymer fiber are summarized and analyzed in

Table 11. Dynamic properties of UHPC reinforced with hybrid steel–polymer fibers.

Hybrid Fibers								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
hooked-end steel	30	0.9	0.5	plastic synthetic	30	0.9	1.5	dynamic c.s. (s. r. ε = 56 s−1) = 144.8 MPa (+10.5%) 1.1 dynamic c.s. (s. r. ε = 103 s−1) = 180.0 MPa (+2.0%) 1.2 dynamic c.s. (s. r. ε = 159 s−1) = 213.4 MPa (+3.2%) 1.3 dynamic c.s. (s. r. ε = 207 s−1) = 235.1 MPa (+3.3%) 1.4	[150]
			1.0				1.0	dynamic c.s. (s. r. ε = 49 s−1) = 138.2 MPa (+5.4%) 1.1 dynamic c.s. (s. r. ε = 93 s−1) = 169.2 MPa (4.1%) 1.2 dynamic c.s. (s. r. ε = 166 s−1) = 210.5 MPa (+1.8%) 1.3 dynamic c.s. (s. r. ε = 209 s−1) = 228.2 MPa (+0.2%) 1.4
			1.5				0.5	dynamic c.s. (s. r. ε = 55 s−1) = 153.7 MPa (+ 17.2%) 1.1 dynamic c.s. (s. r. ε = 105 s−1) = 198.8 MPa (+12.7%) 1.2 dynamic c.s. (s. r. ε = 166 s−1) = 235.7 MPa (+14.0%) 1.3 dynamic c.s. (s. r. ε = 206 s−1) = 242.8 MPa (+6.6%) 1.4
straight steel	13	0.2	1.0	polyoxyme- thylene	12	0.2	2.0	dynamic c.s. (s. r. ε = 68 s−1) = 178 MPa (+15.6%) 2.1 dynamic c.s. (s. r. ε = 123 s−1) = 203 MPa (+11.5%) 2.2 elastic modulus (s. r. ε = 68 s−1) = 41 GPa (+7.9%) 2.1 elastic modulus (s. r. ε = 123 s−1) = 45 GPa (+15.4%) 2.2 ultimate toughness (s. r. ε = 68 s−1) = 2.17 J/m3 (+11.9%) 2.1 ultimate toughness (s. r. ε = 123 s−1) = 2.62 J/m3 (+35.1%) 2.2	[42]
			2.0				1.0	dynamic c.s. (s. r. ε = 89 s−1) = 203 MPa (+31.8%) 2.1 dynamic c.s. (s. r. ε = 118 s−1) = 219 MPa (+20.3%) 2.2 elastic modulus (s. r. ε = 89 s−1) = 57 GPa (+50.0%) 2.1 elastic modulus (s. r. ε = 118 s−1) = 73 GPa (+87.2%) 2.2 ultimate toughness (s. r. ε = 89 s−1) = 2.59 J/m3 (+33.5%) 2.1 ultimate toughness (s. r. ε = 118 s−1) = 2.70 J/m3 (+39.2%) 2.2
straight steel	18	0.16	0.5	polye- thylene	13	0.02	1.5	axial tensile strength (s. r. ε = 8 s−1) = 17.3 MPa (+8.8%) 3.1 axial tensile strength (s. r. ε = 22 s−1) = 22.6 MPa (+18.3%) 3.2 axial tensile strength (s. r. ε = 49 s−1) = 29.4 MPa (+23.0%) 3.3 impact energy (s. r. ε = 8 s−1) = 18.5 N/mm2 (+23.3%) 3.1 impact energy (s. r. ε = 22 s−1) = 19.9 N/mm2 (+47.4%) 3.2 impact energy (s. r. ε = 49 s−1) = 16.1 N/mm2 (+33.1%) 3.3	[149]
			1.0				1.5	axial tensile strength (s. r. ε = 6 s−1) = 17.3 MPa (+8.8%) 3.1 axial tensile strength (s. r. ε = 22 s−1) = 26.9 MPa (+40.8%) 3.2 axial tensile strength (s. r. ε = 46 s−1) = 35.3 MPa (+47.7%) 3.3 impact energy (s. r. ε = 6 s−1) = 7.4 N/mm2 (−50.7%) 3.1 impact energy (s. r. ε = 22 s−1) = 15.6 N/mm2 (+15.6%) 3.2 impact energy (s. r. ε = 46 s−1) = 26.8 N/mm2 (+121.5%) 3.3
			1.5				1.5	axial tensile strength (s. r. ε = 6 s−1) = 19.7 MPa (+23.9%) 3.1 axial tensile strength (s. r. ε = 19 s−1) = 31.6 MPa (+65.5%) 3.2 axial tensile strength (s. r. ε = 45 s−1) = 43.2 MPa (+80.7%) 3.3 impact energy (s. r. ε = 6 s−1) = 9.1 N/mm2 (−39.3%) 3.1 impact energy (s. r. ε = 19 s−1) = 17.3 N/mm2 (+28.2%) 3.2 impact energy (s. r. ε = 45 s−1) = 27.3 N/mm2 (+125.6%) 3.3

c.s. = compressive strength; s. r. = strain rates; ^1.x comparison with a mixture without fiber ^1.1 ε = 53 s⁻¹, ^1.2 ε = 99 s⁻¹, ^1.3 ε = 158 s⁻¹, ^1.4 ε = 227.7 s⁻¹; ² comparison with a mixture with 3% polyoxymethylene fiber ^2.1 ε = 78 s⁻¹, ^2.2 ε = 123 s⁻¹; ³ comparison with a mixture with 1.5% polyoxymethylene fiber ^3.1 ε = 9 s⁻¹, ^3.2 ε = 24 s⁻¹, ^3.3 ε = 53 s⁻¹.

.

The dynamic compressive strength of UHPC with the addition of steel and polymer fibers, depending on the strain rates, is presented in Figure 8a. As the strain rate increased, the strength of each mixture also increased. Increasing the proportion of steel fibers while maintaining the same total fiber content in UHPC with hooked ST fibers of 30 mm and P fibers of 30 mm resulted in higher strength. The positive effect of increasing the steel fiber content from 1% to 2% was also confirmed for the combination of 12 mm ST fibers and 12 mm POL fibers. Figure 8b shows the results of the dynamic axial tensile strength test. UHPC with the addition of 18 mm ST fibers and 13 mm PE fibers achieved higher results with increasing strain rates. With a constant PE fiber content of 1.5%, increasing the steel fiber content from 0.5% to 1.5% led to a significant increase in tensile strength. The impact energy of the mixtures containing 18 mm ST fibers and 13 mm PE fibers at 1.5% is illustrated in Figure 8c. The highest energy at the lowest strain rate was obtained with the lowest ST content of 0.5%. However, as the strain rate still increased, the energy decreased. An effective energy improvement with increasing strain rates occurred with 1% and 1.5% steel fiber content, further increasing with a higher ST content. With a total fiber content of 3%, increasing the content of steel fibers from 1% to 2% resulted in an increase in ultimate toughness (Figure 8d).

4.4. Steel—Glass Hybrid Fiber

4.4.1. Physical Characteristics

The effect of glass fibers at a content of 0.5% as an addition to a UHPC mixture with steel fibers (length/diameter = 13/0.2 mm) at a content of 2% was analyzed in [76]. The study used three lengths of glass fibers (6, 12, and 18 mm) with the same diameters and mechanical properties. Compared to the fiber-free mix, a decrease in flowability of 30.2% to 33.2% was observed. Compared to the flowability of the mixture with only 2.5% ST, the negative impact of G fibers on the flowability of fresh UHPC was evident, with a slight increase in flowability as the fiber length increased. The longer the fibers, the greater the friction between the components of the UHPC matrix, which influenced the increase in interaction forces during flow, generating stresses that tended to maintain the initial form of the mixture.

4.4.2. Mechanical Properties

The compressive strength of ultra-high performance concrete with straight steel fibers (length/diameter = 13/0.2 mm) and glass fibers (6, 12, or 18/0.014 mm) was presented in [76]. The best combination effect was achieved with the 12 mm G fibers. However, compared to UHPC with a 2.5% addition of ST fibers, the compressive strength of the hybrid fiber mix decreased, indicating that the compressive strength of UHPC is primarily determined by the more favorable properties of steel fibers. This may also be due to the reduced workability of the mix and the higher probability of particle agglomeration in hybrid fiber reinforced concrete. In the case of short glass fibers, there may also be a slip phenomenon without fiber rupture under full stress, preventing the full use of G fibers in strengthening the mix.

In [85], a positive hybridization effect on the compressive strength of UHPC was reported with glass fibers (length/diameter = 6–12/0.025 mm) and steel fibers (15/0.6 mm) at 1%. In addition to the strength increase compared to the control series without fibers, the UHPC also achieved a compressive strength higher than that of the UHCP with 2% ST fibers. Glass fibers limited the spread of microcracks due to their high adhesion to the UHPC matrix and did not interfere with ST fibers in carrying the load after cracking [206,207]. The combination of G and ST fibers complemented the UHPC matrix due to its different properties, allowing for an increase in the maximum load capacity.

A mixture with glass fibers (length/diameter = 6.35/0.025 mm) was supplemented with straight steel fibers (13/0.2 mm) or hooked steel fibers (60/0.75 mm) [82]. Higher compressive strength was achieved with G and straight ST fibers up to 1.9%. Beyond this level, UHPC reinforced with G and hooked ST fibers showed even greater strength. This suggests that the collaboration between ST and G fibers was regulated through synergy.

Pourjahanshahi i Madani [84] examined the effect of a hybrid mixture of twisted steel fibers (length/diameter = 25/0.7 mm) and glass fibers (12/0.017 mm) on UHPC strength. They observed a slight decrease in compressive strength of 6.1% for UHPC with 1.5% ST fibers and 1% G fibers compared to the reference mix without fibers. The trend of variation in strength indicates partial fiber agglomeration and increased air content. Glass fibers may cause micropore formation during the mixing process, leading to a reduction in compressive strength, as also noted in [208].

The elastic modulus of UHPC with the addition of straight steel fibers (length/diameter = 12/0.2 mm) at 1% and glass fibers (6, 12, or 18/0.014 mm) increased for every fiber combination tested [76]. As the length of the G fibers increased, the modulus increased by 4% to 37%. Only samples with ST and the longest G fibers achieved a higher modulus than UHPC with 2.5% ST. The shortest G fibers gave the weakest stiffening effect due to the lowest workability of the mixture, causing defects in the UHPC matrix that acted as weak structural areas.

The elastic modulus for the hybrid mixture with 1% straight ST fibers (length/diameter = 15/0.2 mm) and 1% G fibers (6–12/0.012 mm) increased by 4.4% compared to the fiber-free mix [85]. However, compared to 2% ST fibers, there was a slight 0.8% decrease in modulus, indicating that despite the significantly higher stiffness of ST compared to G, a remarkably positive hybridization effect of fibers was achieved in UHPC.

Muhyaddin [82] tested ultra-high performance concrete with G fibers of length/diameter = 6.35/0.025 mm combined with straight ST (13/0.2 mm) or hooked-end ST (60/0.75 mm). The same amount of G/ST fibers = 0.5/0.5% or 1/1% was added to the mixtures. The elastic modulus of UHPC with a total fiber content of 2% was slightly higher than that of UHPC with 1% hybrid fibers. Compared to fiber-free UHPC, there was a 12% increase for the mix with straight ST fibers and a 13% increase for the mix with hooked-end ST fibers.

The elastic modulus of UHPC with 1.5% crimped ST fibers (25/0.7 mm) and 1% G fibers (12/0.017 mm) was 2% higher compared to the fiber-free mixture, but 19% lower than UHPC with 1.5% ST [84]. The results confirmed the observed trend of decreasing the modulus of UHPC with combinations of G and ST fibers as the fraction of the lower-stiffness glass fibers increased.

The flexural strength of UHPC with 1% straight ST (15/0.2 mm) and 1% G fibers (6–12/0.012 mm) increased by 39% compared to fiber-free UHPC [85]. The strength also exceeded that of UHPC with 2% ST fibers, indicating a positive hybridization effect when replacing 1% ST with 1% G fibers.

A hybrid mix with G fibers of length/diameter = 6.35/0.025 mm combined with straight ST (13/0.2 mm) or hooked-end ST (60/0.75 mm) was tested for flexural strength [82]. The combination of hooked-end ST and G fibers achieved the highest flexural strength at a total fiber content of 2%, increasing the strength of UHPC by 163% compared to fiber-free concrete. For the mixture with straight ST fibers, the strength increased by 54.3%, indicating that long hook-end ST is more effective in improving flexural strength. Long ST better bridged cracks and delayed crack propagation. Furthermore, the hooked ends ensured good anchorage in the matrix, preventing fiber slippage, which was not observed for the straight and short ST fibers. However, no brittle fractures were observed in any of the fiber combinations tested.

The tensile strength of the mixture with 1.5% crimped ST fibers (25/0.7 mm) and 1% G fibers (12/0.017 mm) increased by 6% compared to the fiber-free mixture, as reported in [84]. However, compared to UHPC with 1.5% ST, the tensile strength was 46% lower, suggesting a lower impact of fiber hybridization. Before cracking occurred, the interaction between the fibers and the UHPC matrix did not affect the flexural behavior. The appearance of cracks depends on the strength of the concrete matrix, while the type of fiber influences the subsequent development of strength [209]. Furthermore, the fiber bridging ability is dependent on chemical adhesion, static friction, and its mechanical anchorage [117].

Axial tensile strength tests showed a positive hybridization effect for each UHPC mixture containing 2% straight ST (12/0.2 mm) and 0.5% G fibers with dimensions of 6, 12, or 18/0.014 mm [76]. UHPC with hybrid fibers achieved higher tensile strengths than the mixture with 2.5% ST, and the strength increased as the length of the G fibers increased. These results can be related to the crack-bridging effect, which is worse for short fibers, and to the reduced workability of UHPC with a higher fiber content.

The tensile splitting strength of UHPC with 1% straight ST of length/diameter 15/0.2 mm and 1% G fibers 6–12/0.012 mm increased by 24% compared to the concrete without fibers [85]. Compared to the mixture with 2% steel fibers, there was a slight 2% decrease.

UHPC with G fibers of length/diameter 6.35/0.025 mm and straight ST 13/0.2 mm or hooked ST 60/0.75 mm was subjected to tensile splitting tests [82]. The strength of UHPC increased for the straight ST and G fiber mixture by 10.7% and 15.5%, respectively, with a total fiber content of 1% and 2%. For the hooked-end fibers, the strength increases were 9.5% and 14.3%. The static properties of UHPC reinforced with steel and glass fiber mixtures are presented in

Table 12. Static properties of UHPC reinforced with hybrid steel–glass fibers.

Hybrid Fibers								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
straight steel	13	0.2	2.0	glass	6	0.014	0.5	compressive strength = 123.0 MPa (+6.7%) axial tensile strength = 6.4 MPa (+70.2%) elastic modulus = 37.2 GPa (+4.2%)	[76]
			2.0	glass	12	0.014	0.5	compressive strength = 132.9 MPa (+15.3%) axial tensile strength = 7.3 MPa (+94.2%) elastic modulus = 46.1 GPa (+29.1%)
			2.0	glass	18	0.014	0.5	compressive strength = 121.1 MPa (+5.0%) axial tensile strength = 8.3 MPa (+120.8%) elastic modulus = 48.9 GPa (+37.0%)
straight steel	15	0.6	1.0	glass	6-12	0.012	1.0	compressive strength = 174.0 MPa (+13.1%) flexural strength = 20.0 MPa (+38.9%) splitting tensile strength = 15.4 MPa (+24.2%) elastic modulus = 47.7 GPa (+4.4%)	[85]
straight steel	13	0.2	0.5       1.0       0.5       1.0	glass	6.35	0.025	0.5       1.0       0.5       1.0	compressive strength = 148 MPa (+8.8%) flexural strength = 10 MPa (+42.9%) splitting tensile strength = 9.3 MPa (+10.7%) elastic modulus = 42.1 GPa (+7.1%) compressive strength = 161.0 MPa (+14.9%) flexural strength = 10.8 MPa (+54.3%) splitting tensile strength = 9.7 MPa (+15.5%) elastic modulus = 44 GPa (+12.0%) compressive strength = 145.5 MPa (+7.0%) flexural strength = 14.7 MPa (+110.0%) splitting tensile strength = 9.2 MPa (+9.5%) elastic modulus = 42.3 GPa (+7.6%) compressive strength = 161.5 MPa (+15.2%) flexural strength = 18.4 MPa (+162.9%) splitting tensile strength = 9.6 MPa (+14.3%) elastic modulus = 44.4 GPa (+13.0%)	[82]
hooked-end steel	60	0.75
crimped steel	25	0.7	1.5	glass	12	0.017	1.0	compressive strength = 139 MPa (−6.1%) elastic modulus = 23.2 GPa (+1.8%) flexural strength = 18.2 MPa (+5.8%)	[84]

.

Figure 9a illustrates the results of the compressive strength of UHPC with the addition of steel and glass fibers. The highest strengths were achieved with a hybrid fiber content of 2%. These concretes contained straight steel fibers of 13 mm in length or hooked steel fibers of 60 mm in length, as well as glass fibers of 6.35 mm or 6–12 mm. Increasing the fiber content to 2.5% led to a decrease in strength. The results of the tensile strength tests are presented in Figure 9b. For the addition of 1% fibers, the mixture with 6.35 mm glass fibers and 60 mm hooked steel fibers achieved results more than 40% higher than UHPC with straight 13 mm fibers. Increasing the fiber content from 1% to 2% resulted in a higher tensile strength, except for the mixture with 1% straight steel fibers of 13 mm and 1% glass fibers of 6.35 mm. This is due to the short length of the steel fibers, which are pulled out of the matrix under tensile forces. Analysis of the results indicates that it is primarily the ST fibers that contribute to the increase in the tensile strength of UHPC. The replacement of 13 mm straight steel fibers with 60 mm hooked steel fibers, while maintaining the same 1% content, increased the tensile strength from 11 MPa to 18.5 MPa. The analysis confirms that long steel fibers have a decisive impact on the increase in flexural strength. However, the use of 25 mm long twisted steel fibers, combined with 12 mm glass fibers, resulted in only a strength increase of 5.8% compared to the fiber-free mixture.

4.5. Hybrid Combinations of Other Fiber Types

The addition of twisted ST fibers with a length/diameter of 25/0.7 mm at 1.5% combined with carbon fibers of 7/0.01 mm at 1% resulted in a decrease of 8.6% in the compressive strength of UHPC compared to fiber-free concrete [84]. This drop suggests that the total fiber content of 2.5% was not evenly distributed and may have a negative impact on hydration processes, preventing a positive effect of hybrid fiber on the compressive strength of the UHPC. The highest strength was achieved with concrete with 1.5% ST, while increasing the ST content to 2.5% led to a reduction in strength.

Straight steel fibers (15/0.6 mm) at 1% were combined with carbon fibers (20–30/0.018 mm) at 1% to assess their impact on the compressive strength of UHPC [85]. A 13.6% increase in strength was observed compared to UHPC without fibers and a 4.6% increase relative to UHPC with 2% ST fibers. These results indicate a positive hybridization effect of ST and C fibers, with an even fiber distribution within the UHPC matrix and proper hydration of the components. Compared to previous results [84], it can be seen that the combination of longer C fibers with shorter straight ST fibers is beneficial in improving the compressive strength of concrete.

Raza et al. [85] proposed another type of hybrid combination in UHPC, consisting of 1% C fibers (20–30/0.018 mm) and 1% G fibers (6–12/0.012 mm). This concrete achieved a compressive strength of 13.2% higher than fiber-free UHPC and 3.4% higher than UHPC with 2% ST fibers. A positive hybridization effect of C and G fibers was recorded. This may be due to the higher tensile strength and elastic modulus of C fibers compared to ST fibers, as well as the strong adhesion of G fibers to the UHPC matrix [210].

UHPC with the addition of basalt fibers measuring 12/0.013 mm and polypropylene fibers of the same size (12/0.013 mm) was investigated in [155]. The samples were prepared with a total fiber content of 1% or 2%. For all hybrid mixtures, a decrease in compressive strength was observed, ranging from 10.8% to 18.1%, increasing with the content of PP fibers. The results showed that increasing the combined amount of PP fibers and B fibers to 2% caused a greater reduction in strength compared to the mixture with a fiber content of 1%. The B and PP fibers tended to reduce the workability of the concrete by absorbing small amounts of water, leading to the formation of pores.

Twisted steel fibers with a length/diameter of 25/0.7 mm at 1.5%, combined with carbon fibers (7/0.01 mm) measuring at 1%, resulted in a 22.7% increase in flexural strength compared to UHPC without fibers [84]. This result was identical to that of UHPC with 1.5% ST fibers, suggesting that the addition of short C fibers does not significantly improve flexural strength. An additional increase in the ST fiber content to 2.5% led to an additional 50% increase in flexural strength.

Straight steel fibers (15/0.6 mm) at 1% were added to UHPC with C fibers (20–30/0.018 mm) at 1%, resulting in an increase of 34.9% in flexural strength compared to UHPC without fibers and causing a slight 0.9% decrease compared to UHPC with 2% ST fibers [85]. A positive effect of the hybridization of C and ST fibers on flexural strength was observed, contrasting with the findings in [84]. This can be attributed to the use of longer C fibers, which played a crucial role in improving the flexural strength of the UHPC compared to the fiber-free UHPC.

Another type of hybrid combination involved C fibers (20–30/0.018 mm) at 1% and G fibers (6–12/0.012 mm) at 1%, added to the UHPC mix [85]. This resulted in a 23.4% increase in flexural strength and is the lowest among hybrid fiber combinations consisting of two of the three fiber types (ST, C and G). The results suggest that the glass fibers had the smallest effect on the improvement in strength, probably because of the lower modulus of elasticity of the G fibers.

The effect of combining B fibers (12/0.013 mm) and PP fibers (12/0.013 mm) on the flexural strength of concrete was evaluated in [155]. Compared to fiber-free concrete, strength increases of up to 7.5% with a total fiber content of 1% and up to 23.7% with fiber addition of 2% were achieved. The strength of the concrete improved with increasing PP fiber content, regardless of the total amount of fiber.

Straight steel fibers with a length/diameter of 15/0.6 mm at 1%, combined with carbon fibers of 20–30/0.018 mm at 1%, influenced the tensile splitting strength of UHPC [85]. This combination of fibers achieved the highest increase in UHPC strength of 32.3% compared to fiber-free concrete. The strength was also 3.8% higher than that of UHPC with 2% ST fibers, demonstrating a positive hybridization effect of C and ST fibers. Increasing the C fiber content to 2% did not result in further improvement in the flexural strength of the UHPC, despite the higher tensile strength of C compared to the ST fibers. This may be due to the smoother surface of the C fibers, which allowed them to be pulled out of the matrix without fully using their tensile capacity.

Another type of hybrid combination involved C fibers (20–30/0.018 mm) at 1% and G fibers (6–12/0.012 mm) at 1% [85]. This fiber mixture led to a 15.3% increase in the tensile splitting strength of UHPC compared to fiber-free concrete. However, this strength gain was the lowest among the fiber combinations tested. The result can be explained by the smooth surface of the G fibers and its short length, which made it easier to remove them from the matrix. Carbon fibers, with their higher tensile strength and longer length compared to G fibers, improved their adhesion to the matrix during tensile stress. UHPC with 2% G fibers achieved the smallest strength increase, indicating that G fibers are less effective in increasing the tensile strength of splitting than ST and C fibers.

UHPC with the addition of basalt fibers with a length/diameter of 12/0.013 mm and polypropylene fibers of 12/0.013 mm was examined in [155]. Compared to the fiber-free mixture, the tensile splitting strength increased by 19.3% to 43.9% for a total fiber content of 1% and by 36.8% to 52.6% for a fiber content of 2%. Strength increased with increasing B fiber content, both for total fiber content of 1% and 2%. This can be explained by the significantly lower tensile strength of PP compared to B fibers, causing the polypropylene fibers to not be pulled out of the matrix and to break when reaching their maximum strength [211].

Straight steel fibers with a length/diameter of 15/0.6 mm at 1% and carbon fibers of 20–30/0.018 mm at 1% were added to the UHPC mixture [85]. This combination resulted in the highest increase in the elastic modulus of the concrete compared to other hybrid combinations of ST, C, and G fibers. The modulus was 5.7% higher than that of fiber-free UHPC. This can be attributed to the higher stiffness of the ST and G fibers used in the hybrid mixture and their uniform distribution within the matrix.

Another type of hybrid combination is the addition of C fibers (20–30/0.018 mm) and G fibers (6–12/0.012 mm) at 1% each to UHPC [85]. The elastic modulus of UHPC increased by 4.6% compared to the plain mixture. This confirms a positive hybridization effect, as the modulus was higher than that of the UHPC with a 2% addition of a single fiber type. It can be concluded that the presence of fibers of varying lengths helps effectively control cracking, especially before the maximum load is reached, indicating their beneficial distribution within the mixture.

Table 13. Static properties of UHPC reinforced with different combinations of hybrid fibers.

Hybrid Fibers								Properties (Effectiveness %)	Ref.
Part A				Part B
Type	lf (mm)	df (mm)	Vf (%)	Type	lf (mm)	df (mm)	Vf (%)
crimpedsteel	25	0.7	1.5	carbon	7	0.010	1.0	compressive strength = 133.5 MPa (−8.6%)flexural strength = 21.1 MPa (+22.7%)	[84]
straight steel	15	0.6	1.0	carbon	20-30	0.018	1.0	compressive strength = 175.0 MPa (+13.6%)flexural strength = 19.3 MPa (+34.9%)splitting tensile strength = 16.4 MPa (+32.3%)elastic modulus = 48.3 GPa (+5.7%)	[85]
carbon	20-30	0.018	1.0	glass	6-12	0.012	1.0	compressive strength = 173.0 MPa (+13.2%)flexural strength = 17.5 MPa (+23.4%)splitting tensile strength = 14.3 MPa (+15.3%)elastic modulus = 47.8 GPa (+4.6%)
basalt	12	0.013	0.25     0.5     0.75     0.5     1.0     1.5	polypropylene	12	0.013	0.75     0.5     0.25     1.5     1.0     0.5	compressive strength = 114.0 MPa (−13.1%)flexural strength = 10.0 MPa (+7.5%)splitting tensile strength = 6.8 MPa (+19.3%)compressive strength = 115.7 MPa (−11.8%)flexural strength = 9.4 MPa (+1.1%)splitting tensile strength = 8.1 MPa (+42.1%)compressive strength = 116.9 MPa (−10.9%)flexural strength = 9.3 MPa (+0%)splitting tensile strength = 8.2 MPa (+43.9%)compressive strength = 107.5 MPa (−18.1%)flexural strength = 11.5 MPa (+23.7%)splitting tensile strength = 7.8 MPa (+36.8%)compressive strength = 111.6 MPa (−14.9%)flexural strength = 9.7 MPa (+4.3%)splitting tensile strength = 8.3 MPa (+45.6%)compressive strength = 117.0 MPa (−10.8%)flexural strength = 9.4 MPa (+1.1%)splitting tensile strength = 8.7 MPa (+52.6%)	[155]

presents the analysis of the static properties of UHPC reinforced with various combinations of fibers.

Figure 10a presents the compressive strength results of UHPC with different combinations of fibers. The addition of 12 mm-length basalt fibers and 12 mm-length polypropylene fibers negatively affected the strength, with an average decrease of 12% compared to the mixture without fibers. The mixtures achieved higher strengths with a higher proportion of B fibers. Despite the high tensile strength of the B fibers and the high ductility of the PP fibers, the final strength of the matrix decreased. When the content of these fibers increased from 1% to 2%, a slight decrease in strength was observed, although higher results were obtained with a higher proportion of basalt fibers. Other fiber combinations used in UHPC included half a dose of 2% of 15 mm ST fibers and 20–30 mm C fibers, or identical C fibers and 6-12 mm G fibers. The resulting UHPC strength was more than 13% higher than the strength of the control UHPC. Changing the shape of the steel fibers to crimped in UHPC with carbon fibers caused a decrease of approximately 9% in strength compared to the fiber-free mixture, due to difficulties with the uniform distribution of the crimped fibers. The flexural tensile strength is presented in Figure 10b. UHPC with 12 mm 1% B fibers and 12 mm PP fibers showed increases in strength of up to 7.5%, which increased with a larger proportion of PP fibers. When the fiber content increased from 1% to 2%, similar trends were observed. The mixtures with 2% steel–carbon or carbon–glass fibers had flexural strengths of 19.3 MPa and 17.5 MPa, respectively. The higher values were due to the longer lengths and higher tensile strength of the steel and carbon fibers, which resulted in higher resistance forces that kept the fibers in the matrix and made the fibers more difficult to remove. UHPC with 1.5% crimped 25 mm ST fibers and 1% 7 mm C fibers achieved the highest strength, with an increase of almost 23% compared to the control mixture. This improvement was due to the shape of the steel fibers, which improved their anchorage in the matrix.

5. Static Load Failure Modes

Figure 11 shows UHPC samples reinforced with various combinations of fibers compared to plain UHPC samples, damaged under compression. The failure of fiber reinforced cubes is more gradual and continuous. On the contrary, UHPC without fiber addition (Figure 11a) experiences sudden failure, with large concrete fragments detaching once the maximum compressive stress is exceeded. In the case of a large number of short polymer fibers, a chaotic network of cracks was observed (Figure 11b). The combination of short PP fibers and short B fibers (Figure 11c–e) reduced both the total number and width of cracks due to the higher strength of the basalt fibers. An effect was particularly evident as their proportion increased. Damage resulting from compressive forces for samples containing 1.5% nonmetallic fibers: long fibers (38 or 54 mm) and short fibers (12 mm) are shown in Figure 11f–m. The crack pattern was more regular and, in some cases, a network of fine cracks was formed (Figure 11f,l). Figure 11j illustrates the concrete failure model with 0.75% PP (38 mm) + 0.75% PVA (12 mm), which shows a different pattern compared to other cases. Due to the highest aspect ratio of the PVA fibers, they were the most numerous in the mixture. The complex network formed by these fibers effectively counteracted the formation of microcracks. In addition to a wide crack and vertical spalling in the corner of the cube, no clear vertical crack was observed. Failure patterns for the polymer and basalt fiber mixture show less complex crack networks (Figure 11g,k,m), attributed to the higher strength of basalt fibers, as particularly visible in Figure 11g. The spalling of concrete at the corner of the sample (Figure 11k) and the detachment of the surface fragment (Figure 11m) suggest a lack of fiber presence at these places, where the matrix strength was exceeded. Characteristic damage patterns are shown in UHPC images with long hooked-end steel fibers (Figure 11n–p). The cube in Figure 11n, which contains 1.5% ST fibers (50 mm) and 0.05% PP fibers (12 mm), failed due to wide horizontal cracks held together by long hooked steel fibers. The absence of microcracks indicates an insufficient amount of polypropylene microfibers, which could not withstand the development stresses. The failure pattern of the mixture with 1.5% ST fibers (50 mm) and 1.5% B fibers (12 mm) (Figure 11o) shows two main horizontal cracks in the upper and lower parts of the sample. We can see a similarity to Figure 11n. However, the higher content and strength of the basalt microfibers in this case resulted in narrower microcracks, and the concrete did not spall. The UHPC containing three types of fiber—0.25% each of ST hooked end (50 mm), PP (40 mm), and G (18 mm), after compression stress, is presented in Figure 11p. A horizontal crack can be related to the presence of long ST and PP fibers, along with a complex crack network facilitated by glass fibers that effectively bridged the expanding cracks.

Images of UHPC damage with hybrid fiber reinforcement confirm the beneficial effect of fiber reinforcement on the crack resistance of concrete and its ability to transfer residual stresses, reducing the risk of sudden failure. In the samples analyzed, the increase in compressive load (up to the point of ultimate strength) was more controlled, and material disintegration occurred gradually. Fibers altered the direction of microcrack development, resulting in increased energy absorption before failure. The width of the crack was limited, as the fibers maintained structural continuity and absorbed energy. Failure patterns depend on the type of fiber, its aspect ratios, the total fiber content, and the percentage contribution of each fiber type within the concrete matrix.

Figure 12 presents various failure models of UHPC samples after splitting tests. The UHPC cube without fiber addition (Figure 12a) fractured along the center of the cross section after reaching the ultimate tensile stress, with a clear separation into two main parts, completely losing its load-bearing capacity. In the case of the sample containing two types of polymer fibers at 0.75% each (POL 12 mm and PP 12 mm) (see Figure 12b), a single main crack was observed, indicating the loss of load bearing capacity by the matrix and the partial transfer of residual stresses by the fibers, which, due to their short length, were pulled out. A network of vertical microcracks was also visible, showing that some of the energy from the fracture was absorbed. The fibers prevented the complete separation of the sample by bridging the cracks, delaying the propagation of the crack, and reducing the width of the crack. The addition of a mixture of PP and B fibers (Figure 12c–e) demonstrates the higher strength of basalt fibers, as the main crack was narrower and only a few vertical microcracks appeared. This indicates that basalt fibers absorbed significantly more fracture energy compared to PP fibers. Damage patterns for samples with 1.5% non-metallic fibers: longer (38 or 54 mm) and shorter (12 mm) are shown in Figure 12f–l. The longer fibers were partially broken, while the shorter ones were pulled out of the separated concrete parts, but both types together were effective in stopping sudden cracking. The network of micro- and macro-cracks suggests energy dissipation by the fibers and local stress redistribution, providing the UHPC with a higher capacity to absorb energy. The increased length of the POL fibers (54 mm compared to 38 mm) in combination with the same B fibers (12 mm) reduced the width of the macro-cracks and generated more micro-cracks, indicating a more efficient use of the fibers (Figure 12m,j). The effect of fiber length was also observed in Figure 12l,f, where in the mixture with 0.75% POL and 0.75% PP (12 mm), increasing the length of the POL fibers from 38 to 54 mm resulted in three vertical cracks that run through the entire cross section and a reduction in the width of the crack. This reflects the participation of an additional part of the matrix and a portion of the fibers in the load-carrying with the improved energy absorption capacity. The failure pattern for the mixture with 0.75% PP fibers (38 mm) and 0.75% PVA fibers (12 mm) (Figure 12i) provides insight into the role of PVA fibers in the transfer of tensile stresses. The network formed by these fibers is complex enough to effectively counteract the formation of microcracks. The resulting cracks are wide because of the full use of the properties of the PP fibers, some of which were partially broken. However, the PVA fibers, due to their shorter length and lower bond strength, were unable to prevent the further development of microcracks, leading to weakened locations through which the cracks propagated. The failure pattern for the mixture with 0.75% PP fibers (38 mm) and 0.75% PVA fibers (12 mm) (Figure 12i) provides information on the behavior of PVA fibers in transferring tensile stresses. The network formed by these fibers is complex enough to effectively counteract the formation of microcracks. The resulting cracks are wide because of the full use of the properties of the PP fibers, some of which were partially broken. However, the PVA fibers, due to their shorter length and lower bond strength, were unable to prevent the further development of microcracks, leading to weakened areas through which the cracks propagated. The sample containing 1.5% PP fibers (40 mm) and 1.5% B fibers (12 mm) effectively transferred tensile stresses without significant cross-sectional damage. The large number of long polymer fibers bridged the cracks, aided by the short basalt fibers, which have higher tensile strength. The failure modes of the samples with hooked-end ST fibers (50 mm) combined with PP fibers (40 mm) are shown in Figure 12o,p. In the mixture with 1.5% ST and 1.5% PP, after exceeding the tensile stresses, three main vertical cracks appeared near the center, indicating the effective use of long fibers, where a greater part of the cross section was engaged in load transfer. The beneficial effect of adding 2.5% ST fibers at the hook end and 0.5% PP fibers is confirmed by the failure pattern in Figure 12p, which shows only one narrow main crack with a small crushed concrete zone under the upper loading fiberboard.

The addition of fibers to the UHPC transformed the sudden and brittle failure observed in the UHPC without fibers into a controlled damage process in all hybrid fiber combinations, allowing the transfer of residual stresses. Fibers visible in the crack zone, some anchored in the concrete matrix, and others broken after exceeding the tensile strength, highlight their active role in limiting the crack width. In the case of fibers with high tensile strength, they are observed to gradually pull out of the matrix, creating additional resistance and improving crack propagation resistance. On the other hand, fibers with lower tensile strength tend to rupture. Fibers reduce crack openings, improving structural integrity after the UHPC matrix exceeds its tensile strength, slowing the failure process. In all failure modes presented, various combinations of fibers improved the plastic behavior of the material, which is crucial for fatigue resistance, long-term operational safety, and resistance to dynamic loads.

Figure 13 presents photographs of UHPC samples reinforced with various combinations of fibers and a plain sample in the bending test. The UHPC beam without fibers (Figure 13a) suddenly broke mid-span after reaching ultimate stress, completely losing its load-bearing capacity. The remaining images show the damage patterns of the UHPC samples with hybrid fiber reinforcement. In Figure 13b–l, notched samples with hybrid fiber additives are illustrated. The main crack propagated from the tip of the notch toward the upper edge of the sample, undergoing multiple deflections. In the beam with 0.25% ST fibers (50 mm) and 0.75% PP fibers (12 mm) (Figure 13b), hooked steel fibers and straight PP fibers are visible within the fractured cross section, maintaining the concrete structure and preventing complete separation. The absence of microcracks is attributed to the high concentration of short PP fibers, which, because of their low density and small diameter, are more numerous than steel fibers. These PP fibers initially bear developing stresses until they reach their load capacity, after which the steel fibers take on the additional load. Increasing the steel fiber content to 0.5% (without changing the total fiber volume) reduced the width of the main crack, although additional microcracks appeared due to the lower fiber content of PP (Figure 13c). Figure 13d shows the damage pattern of a beam with 1.5% B fibers (12 mm) and 1.5% PP fibers (40 mm), in which, due to the large number of basalt fibers of high tensile strength, crack bridging is possible. Longer and wavy PP fibers have lower tensile strength but better adhesion, effectively providing crack inhibition. The failure pattern of the sample containing 1% ST fibers (50 mm) and 2% G fibers (18 mm) is shown in Figure 13e, in which a major crack is visible that extends from the notch upward through the cross section. Long ST fibers, visible in the center of the crack, prevent it from widening due to their high tensile strength and hooked ends, which anchor them within the concrete matrix. Increasing the ST fiber content to 1.5% reduced the width of the crack, while 1.5% short G fibers prevented the development of microcracks, leading to more controlled propagation of the main crack (Figure 13f). The damage pattern of the sample with 1.5% ST fibers (50 mm) and 1.5% B fibers (12 mm) is shown in Figure 13g. The crack originating at the tip of the notch and extending into a fracture does not propagate vertically to the top, but it shifts horizontally. The high tensile strength of the fibers and the forces that anchor them in the matrix prevent further vertical cracking. The resulting stresses followed the weakened zone of the beam, indicated by the horizontal crack. The presence of microcracks highlights the lower effectiveness of basalt fibers in preventing their formation compared to polymer fibers, which have a greater elongation capacity and therefore do not fracture suddenly. The impact of three types of fibers (ST 50 mm + PP 40 mm + G 18 mm) with different volume contents during the bending test is illustrated in Figure 13h–l. ST fibers remain embedded in the matrix due to their length and hooked ends, preventing crack widening, but we see surface spalling of concrete fragments as a result of straightening the hooks, as presented in Figure 13i,j. Long polypropylene fibers prevent microcrack formation, which can be observed in failure modes in Figure 13h–k. Increasing their quantity from 0.25% to 0.83% resulted in a reduction in the number of microcracks until they did not occur in the case of Figure 13k. Reducing the content of PP fiber to 0.42% (see Figure 13l) resulted in the formation of additional cracks in the upper part of the cross section. Glass fibers, as a high tensile strength material, help to restrict crack widening. Additionally, because of their low density and small diameter, they are much more numerous in the matrix compared to ST fibers. These fibers, along with the polypropylene fibers, effectively create a complex structural network that prevents the formation of microcracks. However, because of the low ultimate strain of the glass fibers, there are certain limitations. Figure 13m–o show images of the damage patterns of the beams without notches, tested for flexural tensile strength. The UHPC with the addition of PP fibers (12 mm) and B fibers (12 mm) fractured after reaching the maximum load, with a visible crack spreading upward. The absence of visible microcracks is attributed to the presence of polypropylene fibers, while basalt fibers mainly contribute to slowing down crack propagation because of their high tensile strength. The influence of 1% ST fibers (50 mm) combined with 0.05% PP fibers (12 mm) on the flexural strength of the beam is evident in Figure 13o. The ST fibers hold the cross section together, preventing a complete separation of the beam into two parts, even at an advanced stage of loading. The fibers are pulled from the matrix and the straightening of their hooked ends creates local stress concentrations, leading to the spalling of the concrete fragments. Insufficient amounts of PP fibers do not prevent microcrack formation. The addition of fibers to the UHPC prevented sudden failure of the samples after reaching the ultimate strength, unlike the UHPC beam without fibers. Fiber reinforcement increases the tensile strength of the concrete and increases its resistance to local stress concentrations. Fibers absorb part of the energy, which delays the appearance of the first cracks compared to plain UHPC. After the maximum load is reached, the sample retains a certain level of structural integrity, indicating an effective transfer of residual loads through the fibers. Furthermore, the fibers contributed to improving the ductility of the concrete sample, making its behavior under load more predictable.

6. Microstructural Analysis and SEM Observations

The behavior of long hooked-end steel fibers (65 mm) indicated crack formation at the fiber-matrix interface [172]. This was due to internal pressures generated by the fiber’s hooked ends attempting to straighten during pull-out. This phenomenon was not observed with straight steel fibers (even up to 97.5 mm in length), due to their smooth surface that facilitates pulling out of the matrix without causing significant damage. An increase in tensile strength was also observed with increasing straight fiber length (from 65 to 97.5 mm). The higher aspect ratio increased the effective bonding area.

Basalt fibers with lengths of 6 and 12 mm and a diameter of 0.017 mm at concentrations up to 3% improved tensile strength by transferring loads after matrix cracking [113]. At 1% fiber content, a uniform distribution allowed effective load transfer from various directions. Due to their small diameter, these fibers were particularly effective in preventing microcrack formation. High tensile strength prevented their rupture and strong anchoring in the matrix reinforced their effectiveness. However, increasing the fiber content from 2.5% to 3% caused fiber agglomeration and weakened fiber-matrix bonding at these sites, potentially reducing mechanical properties.

SEM images of concrete mixtures containing 0.35% steel fibers and basalt fibers of lengths of 12, 25, 37, and 50 mm (up to 0.3%) confirm a strong bond between the fibers and the matrix [62]. The basalt fibers are visibly attached to fragments of the concrete matrix on fractured surfaces, indicating excellent adhesion. However, longer basalt fibers tended to agglomerate, potentially creating weak zones. Macrocracks propagated through steel fibers, confirming their role in bridging cracks under high-load conditions. The localized agglomeration observed with longer fibers highlights the importance of optimal fiber length selection to achieve uniform dispersion and prevent crack initiation sites.

Microstructural images of UHPC mixtures with basalt (18/0.013 mm), glass (12/0.01 mm), or polypropylene fibers (10/0.023 mm), added in amounts ranging from 0.5 to 2.5%, are presented in [40]. Basalt fibers showed attached matrix fragments, confirming their strong matrix adhesion. Higher fiber content resulted in fiber agglomeration zones, potentially creating areas of weakness. The glass fibers demonstrated strong anchorage, indicated by fractures propagating along the fibers rather than pulling them out. Polypropylene fibers bridged microcracks effectively, but ruptured once their limited tensile strength was exceeded, effectively limiting crack propagation. Quantitative SEM analysis indicated typical microcrack widths ranging from 5 to 20 µm, with fiber pull-out lengths observed in the range of 50–200 µm, clearly demonstrating effective crack bridging and stress dissipation. The strong bond of basalt fibers to the matrix was also confirmed in [83], as evidenced by the partial coverage of the fiber surfaces with cement paste. The additions of glass fibers (24/0.01–0.017 mm) exhibited agglomeration, with cracks propagating along the fiber-matrix interfaces, indicating weak areas.

To support the described mechanisms of crack inhibition and fiber–matrix interaction in UHPC, selected scanning electron microscopy (SEM) images from the previous research of the authors are presented in Figure 14.

These micrographs illustrate key microstructural phenomena observed in hybrid fiber-reinforced UHPCs, including fiber anchorage, pull-out behavior, crack bridging, and fiber rupture. In Figure 14a, a hooked-end steel fiber (50 mm in length, 1 mm in diameter) is shown embedded within the concrete matrix that contains 0.5% steel and 0.06% polypropylene fibers (12 mm). The image clearly reveals a strong mechanical anchorage, suggesting effective stress transfer and interfacial bonding. Figure 14b provides a close-up of the transition zone adjacent to the steel fiber, where fine cracks are visible around the matrix interface, indicating stress concentration and potential crack deflection paths. In Figure 14c, from a sample with 1.0% steel and 0.06% polypropylene fibers, a gap left after the pulling out of the steel fiber is visible, demonstrating one of the main energy dissipation mechanisms in UHPC. Figure 14d captures a polypropylene fiber (12 mm long) that bridges a microcrack within the same sample, confirming the role of synthetic fibers in limiting crack propagation. Figure 14e, from a specimen containing 1.0% steel and 0.25% polypropylene fibers, reveals ruptured polypropylene fibers embedded in the matrix, suggesting that these fibers reached their tensile limits under load. Finally, Figure 14f shows a close-up of the fractured fibers, further supporting the conclusion that polypropylene fibers contribute to UHPC toughness not only by bridging, but also by rupturing and dissipating stress.

These findings are closely aligned with previous research, strengthening the credibility of observed fiber–matrix interactions and highlighting the critical role of fiber geometry and distribution on the mechanical performance of UHPC under various loading conditions.

7. Conclusions

Hybrid fiber reinforcement in UHPC leads to notable improvements in both fresh and hardened properties, with clear trends emerging for workability, mechanical strength, and dynamic performance.

• The fluidity of UHPC continuously decreases as the total fiber content increases. In particular, adding more steel fibers (especially longer or hooked-end fibers) significantly reduces flowability. For example, the flow time of a UHPC mix can increase by over 70% (indicating a loss of slump flow) when shorter steel fibers are used, compared to mixes with longer fibers. Among the different types of fiber, basalt fibers impair workability more than steel fibers because of their rough surface and tendency to clump. Likewise, a higher proportion of polymer micro-fibers (e.g., polypropylene) in a steel-polymer hybrid mix causes a pronounced drop in fluidity as these small-diameter fibers absorb water and coil together. These findings underscore the need to balance the fiber content with the mix design to maintain self-compacting properties.
• An optimal total fiber content of around 2% by volume was identified to maximize strength gains. Exceeding ~2% fiber dose showed diminishing returns or even slight reductions in compressive strength due to fiber clustering and loss of workability. For example, steel + glass fiber hybrids at 2% total fiber achieved up to 15% higher compressive strength than plain UHPC, while increasing to 2.5% fiber caused a minor strength decline. In general, hybrid fibers moderately improved compressive strength (often of the order of 10–15%). The fiber length was crucial: mixtures with fibers ~20–30 mm long achieved the highest compressive strengths, while the use of fibers shorter than 15 mm led to lower strength. This is attributed to the fact that longer fibers provide better crack bridging, whereas very short fibers may not anchor as effectively. Additionally, the base matrix quality (e.g., a low water-to-binder ratio and proper curing) remains important, as fiber effects on compressive strength can be overshadowed by a weak matrix. Among polymer fibers, the type of fiber influenced compressive results: polyolefin, polyoxymethylene (POM) and polyester fibers yielded the highest compressive strengths, whereas the use of polyvinyl alcohol (PVA) fibers led to comparatively lower strength. This suggests that stiffer polymer fibers contribute more to load-bearing capacity than more ductile or hydrophilic ones in compression. In general, the highest compressive strength gains observed with hybrid fibers were in the order of 10 to 15% in most studies, confirming that fibers primarily enhance tensile-related properties while providing a moderate boost in compressive strength.
• Hybrid fiber reinforcement had a much more pronounced effect on the tensile capacity (including splitting tensile and flexural strength) of the UHPC. Using longer steel fibers (≥30 mm) or fibers with improved anchorage (e.g., hooked ends) was particularly beneficial for tensile strength, as these fibers engage more fully across cracks. In contrast, increasing the proportion of shorter or more flexible fibers (such as certain polymers) tended to decrease the direct tensile strength if those fibers lack the high tensile capacity of steel. The best improvements in tensile performance were achieved by synergistic fiber combinations that utilize fibers of different types and sizes to bridge cracks at multiple scales. For example, a hybrid mix with 1.5% steel + 0.5% basalt fibers (both relatively short fibers) produced the highest tensile splitting strength in its category. Another effective mix was 2.0% steel + 0.5% basalt (using short steel and long basalt fibers), which maximized the combined benefits of both fibers. In steel–polymer hybrids, extending the steel fiber length while adding a small fraction of the micro-polymer fibers improved the overall tensile performance, as long steel fibers carry loads and polymer fibers help control microcracks. In all the studies surveyed, increases in tensile strength of +23% to +50% with hybrid fibers (depending on fiber types and proportions) were recorded compared to an unreinforced UHPC. In particular, the combination of steel with carbon fibers (total 2%) yielded a roughly 35% increase in tensile strength, while a mixture of carbon + glass fiber showed up to 23% tensile improvement. The most dramatic tensile gains were seen with certain polymer–fiber mixes: for example, adding 0.5% basalt + 1.5% polypropylene fibers (short fibers) was identified as an optimal blend, boosting the tensile strength by more than 50%. The flexural strength also improved substantially. Many hybrid combinations achieved double-digit percentage increases in flexural strength; in one case, a balanced hybrid of 1% steel + 1% polymer fibers achieved a 51.4% higher flexural strength than the plain UHPC, outperforming even a mix with 2% steel fibers alone. In extreme cases with very high fiber content, the flexural strength more than doubled, e.g., a hybrid mix attained flexural strength of 32.8 MPa, which was 154% higher than the fiber-free reference. These results highlight that hybrid fiber reinforcement is most impactful in tension-driven properties, with carefully chosen fiber combinations (such as steel + synthetic fibers or steel + basalt) providing complementary benefits: stiff fibers (steel, carbon, basalt) carry load and bridge larger cracks, while ductile or finer fibers (polypropylene, POM, glass) fill micro-cracks and enhance post-crack ductility.
• Under high strain-rate loading tests, impact or dynamic, UHPC with hybrid fibers demonstrated improved strength and energy absorption relative to static conditions. Dynamic compressive and tensile strengths increased with increasing strain rate for all fiber-reinforced mixes, reflecting sensitive behavior of the material strain-rate. Crucially, greater fiber content led to better performance under impact: substituting polymer fibers with more steel fibers consistently improved dynamic strength. For example, in one study, increasing the content of steel fibers from 0.5% to 1.5% (while decreasing polymer fibers) increased the axial tensile strength under impact from ~29.4 MPa to 43.2 MPa, and the absorption of impact energy increased by 1.7 times. This shows a nearly 70% improvement in dynamic tensile capacity due to a higher fiber dose. Fiber size also played a role: mixtures with a higher proportion of short fibers exhibited better impact resistance, since a larger number of short fibers can intercept and blunt cracks more effectively under high strain rates. Short steel fibers, in particular, contributed significantly to energy dissipation, as they prevent crack coalescence and rupture sequentially, absorbing impact energy. Overall, UHPC with hybrid fibers showed improved toughness and crack resistance under dynamic loading, with performance gains most pronounced when steel fibers (especially short, well-distributed ones) were used to complement other fibers. These findings suggest that optimized hybrid fiber networks can make UHPC more resilient to impact, providing both higher dynamic strength and improved post-crack energy absorption compared to unreinforced matrices.

8. Practical Recommendations

Based on the comprehensive review of hybrid fiber UHPC, the following practical recommendations are proposed to guide both research and application.

• Optimize the mixture and fiber distribution. Optimize the granular composition of the UHPC mixes to ensure a uniform distribution of the mixed fibers while maintaining self-compacting workability. It is recommended that various cements, mineral fillers, fine powders, and even nanomaterials be explored to improve the flow of the mix and the bond between the fibers and the matrix. A key goal is to reduce the gap between the tensile and compressive strength of UHPC. A balanced matrix can better utilize fiber reinforcement, maximizing the structural performance of the composite.
• Enhance fiber-matrix bonding. Focus on improving the bond properties and efficacy of each fiber type in hybrid combinations. This includes investigating new fiber materials and shapes (different polymers, carbon, glass, etc.), as well as specialized fiber geometries or surface treatments (e.g., hooked ends, crimping) that anchor fibers more effectively. Optimizing fiber–matrix adhesion will lead to higher tensile strength and toughness. When designing hybrids, use multiple fiber types strategically: longer or thicker fibers (especially steel fibers) should primarily boost flexural and post-cracking load capacity, whereas shorter or thinner fibers (e.g., micro polypropylene or PE fibers) should be included to increase tensile strength, crack resistance, impact resistance, and fatigue performance. It is important to determine the optimal fiber content and the best long-to-short fiber ratio for each application. This is especially true for large structural elements under cyclic or impact loads, as fiber efficiency observed in small lab samples may vary at the structural scale.
• Ensure long-term durability. Conduct long-term durability studies for UHPC with hybrid fibers to fully assess its performance in harsh environments. Evaluate resistance of the material to deterioration mechanisms such as freeze–thaw cycles, chemical attack, penetration of chloride (from deicing salts or seawater), and exposure to sulfate. Ensuring that hybrid fiber UHPC can maintain its integrity under these conditions is critical for real-world use. Different fiber types have distinct effects on durability–for example, incorporating synthetic fibers or basalt fibers can enhance resistance to chloride ingress, sulfate attack, and even fire, more effectively than steel fibers alone. Therefore, further research and field trials should identify optimal fiber combinations for structures exposed to aggressive environments to leverage fibers that improve durability while still providing mechanical strength.
• Develop standardized design models. Update and refine the empirical formulas and constitutive models used for UHPC to account for hybrid fiber reinforcement. Existing predictive models are often based on specific test results and do not generalize well due to variations in test methods and equipment. Extensive testing and data collection should be performed to develop standardized models and design guidelines that reliably capture the behavior of hybrid fiber UHPC. These models should be verified across different mix compositions and loading conditions, ensuring engineers can predict performance without needing case-by-case experimentation.
• Study dynamic and high-temperature behavior. Address the current lack of data on the behavior of hybrid fiber UHPC under dynamic loading (impact, blast) and high temperature conditions. Specifically targeted studies are needed to observe crack propagation and failure mechanisms at high strain rates and elevated temperatures. High-speed imaging, detailed fracture analysis (e.g., via scanning electron microscopy or X-ray CT), and controlled fire tests can shed light on how different fiber types contribute to toughness in extreme conditions. While it is evident that adding hybrid fibers improves impact and thermal performance, research should quantify how much each fiber type and proportion contributes to these improvements, thus guiding the design of UHPC for earthquake, blast, or fire-prone structures.
• Improve sustainability and reduce cost. Tackle the high production costs and environmental impact of UHPC by developing more sustainable manufacturing methods. UHPC are currently relying on expensive raw materials and energy-intensive processes, and the lack of standard design codes can further increase costs. Research should focus on reducing the carbon footprint and cost of UHPC through measures such as the use of locally available or recycled aggregates, the incorporation of cheaper fibers (or recycled fibers) with lower energy embodied, and the addition of supplementary cementitious materials (such as fly ash, slag, or silica fume) to reduce cement usage. Innovations in mix design that maintain performance while lowering CO₂ emissions and production energy demand will make UHPC with hybrid fibers more economically viable for widespread use.
• Adopt advanced production techniques. Embrace automation and digital fabrication technologies to streamline UHPC production. Automated batching and fiber dispersion systems, along with rigorous quality control, can ensure consistency and reduce labor costs. Support for technologies such as 3D concrete printing is particularly promising–these allow the creation of complex or non-standard structural elements with UHPC, potentially unlocking new applications. By automating production and using digital design-to-fabrication methods, manufacturers can reduce waste and variability, thus lowering costs and accelerating the adoption of hybrid fiber UHPC in construction.