The Effects of Steel Fiber Types and Volume Fraction on the Physical and Mechanical Properties of Concrete

Abstract

Different types and amounts of steel fibers have varying effects on the improvement of concrete’s mechanical properties. In order to identify the most suitable steel fiber types for the practical production of prefabricated pavements and derive a formula to predict and evaluate the mechanical properties based on steel fiber volume fraction, this study conducted experimental research on the physical mechanical properties of concrete using the method of equal volume substitution of coarse aggregate. The influence of steel fiber type and volume fraction on the microstructure and failure mechanism of steel fiber reinforced concrete (SFRC) was analyzed through electron microscopy scanning. The mechanical properties of plain concrete were used as benchmark. The results showed that when the steel fiber volume fractions were 0.6% and 1.5%, the bending and split tensile strengths of milled steel fibers were 3.1% higher than those of hooked-end steel fibers and the compressive strength of SFRC was significantly increased by 13.5%. The comprehensive mechanical properties of wave-shaped steel fibers were inferior to the first two types. Considering the requirements of SFRC in engineering, milled steel fibers are more suitable to be concrete components. This is because the tangling property of the hooked-end steel fibers makes them difficult to be shaped. In contrast, the smooth surface of milled steel fibers exerts a smaller impact on the forming of specimen and they are more economical. The mechanical properties of SFRC improve as the volume fraction of steel fibers increases from 0% to 2%, but start to decline when the volume fraction exceeds 2%. A volume fraction of 0.6% for steel fibers is sufficient to meet the standard for bending and tensile strength in heavy-duty concrete pavements. Finally, the relationship expressions between the compressive strength, flexural strength, and split tensile strength of SFRC and the steel fiber volume fraction were obtained through fitting the experimental data using Origin software.

1. Introduction

Concrete is a widely used construction material. With the development of modern construction projects and infrastructure, there is a growing demand for its use. The physical and mechanical properties of concrete are important to ensure engineering’s quality and safety. Therefore, more researchers focus on the study of steel fiber reinforced concrete to enhance its resistance to cracks, impacts, and fatigue. SFRC has outstanding mechanical properties, such as tensile strength [1], bending strength [2], crack resistance [3], impact resistance [4], fatigue resistance [5,6], and high toughness [7]. Its high durability makes it a favorite material for engineers [8,9,10].

Scholars around the world have studied how the volume fraction and type of steel fiber can affect concrete’s mechanical properties. Current studies suggest [11,12] that increasing the volume fraction of steel fibers can considerably improve concrete’s mechanical properties. This is because the mechanical properties of SFRC primarily depend on steel fiber parameters. Upgrading the fiber volume fraction strengthens the bond between steel fibers and the concrete matrix, as well as SFRC’s overall elastic modulus, thereby enhancing its mechanical properties [13]. Zhang Tianyang et al. [14] employed the orthogonal experimental method to evaluate the impact of cement content, fly ash replacement rate, steel fiber volume fraction, and sand rate on the workability and fundamental mechanical characteristics of Steel fiber fine stone concrete (SFRC-FA). According to their study, the steel fiber volume fraction had negligible effects on SFRC-FA’s compressive strength. It played a crucial role in enhancing its splitting tensile strength and four-point bending strength. Yue Jianguang et al. [15] performed 3pb experiments and AE monitoring techniques to analyze how the steel fiber volume fraction affected SFRC’s fracture process. They obtained the relationship between SFRC’s fracture energy, the axial tensile strength, and the steel fiber volume ratio. Wang Huiming et al. [16] studied the effects of different factors on the damage evolution of SFRC. The factors included steel fiber volume fraction, fiber length, and coarse aggregate shape. They concluded that if the steel fibers had higher volume fraction, longer length, and irregular coarse aggregate shape, the damage evolution process of SFRC cubic specimens would be prolonged. E. Garcia-Taenguaa et al. [17] conducted tensile tests on steel bars embedded into SFRC specimens to determine the bond characteristics between them. They derived stress-slip curves for the SFRC-steel bar bond. Mahmood et al. [18] investigated the role of moment redistribution and steel fiber volume fraction on the bending behavior of SFRC beams. Their findings indicated that augmented steel fiber volume fraction could significantly enhance the load-carrying capacity of reinforced concrete beams. The addition of steel fibers to the concrete resulted in more cracks and shorter crack spacing. Babar et al. [19] found that moderate to low volume fractions of steel fibers were advantageous in enhancing the compressive strength of concrete, while high volume fractions of steel fibers had a detrimental effect on compressive strength. Wang et al. [20] investigated the impact of volume fractions (0%, 1%, 2%, and 3%) on the compressive and flexural strength of ultra-high-performance concrete (UHPC). The results indicated that the compressive strength increased with the increase in steel fiber volume fraction. At a steel fiber volume fraction of 1%, the flexural strength slightly improved, and at volume fractions of 2% and 3%, the flexural strength significantly increased. Jang et al. [21] discovered that the fracture modulus of steel fiber-reinforced concrete increased with an increase in the volume fraction of steel fibers. Kang et al. [22] studied high-performance concrete with different steel fiber volume fractions (1%, 2%, 3%, 4%, and 5%), and the results demonstrated that the tensile strength of concrete gradually increased with the increase in the steel fiber volume fraction. Sucharda et al. [23] examined the impact of flat and hooked-end steel fibers at different volume fractions on the fracture properties of self-compacting concrete. The results showed that the best improvement in crack resistance was achieved when hooked-end steel fibers were used at a volume fraction of 1.5%. Xu et al. [24] investigated the influence of flat, hooked-end, and crimped steel fibers on the flexural performance of concrete. The results revealed that compared to the plain concrete matrix, the flexural strength of specimens with flat, hooked-end, and crimped steel fibers increased by 74.29%, 165.07%, and 112.65%, respectively. Qin Honggen et al. [25] investigated the influence of steel fiber volume fraction and steel fiber types on concrete’s mechanical properties. The results showed that an increase in steel fiber volume fraction improves the concrete’s mechanical properties in varying degrees. Notably, different fiber types exhibited distinct reinforcing effects on concrete: the reinforcing effects were stronger for bow-shaped and dumbbell-shaped steel fibers than for straight and wavy fibers. Among them, dumbbell-shaped steel fibers exhibited the highest efficacy [26]. In cement-based composite materials, the stress usually concentrated at the crack tip, and such phenomenon could be effectively mitigated through steel fiber’s bridging effect [27]. During the early formation of the interface transition zone, the boundary effect and the microseepage effect on the phenomenon were strong. Lu Fucong et al. [28] conducted a study on steel fibers’ reinforcing mechanism in concrete as well as SFRC’s micro-damage mechanism. After considering the influence of steel fiber parameters, they proposed three empirical formulas for the compressive strength, splitting strength, and bending strength of SFRC. Using a three-point bending test, Huang Yiqun et al. [29] examined the fracture performance of hooked-end SFRC under different fiber volume fractions. Based on the experimental results and microscopic models, they developed a macroscopic model of SFRC with a better calculation efficiency.

The type and dosage of steel fibers play a crucial role in determining key performance indicators such as strength, toughness, and crack resistance in concrete. Therefore, conducting in-depth research on the mechanisms and performance effects of different types of steel fibers can provide a scientific basis for the design, optimization, and practical application of SFRC, thus promoting advancements in the field. Additionally, to better design and assess the performance of steel fiber-reinforced concrete, it is necessary to derive a calculation formula that can predict and evaluate its mechanical properties based on the volumetric dosage of steel fibers. Furthermore, further research is needed to enhance the understanding of the microstructural aspects and mechanisms of failure in order to provide a comprehensive explanation of the fracture mechanisms. Therefore, this study will conduct experiments on various types of steel fibers and different steel fiber volume fractions in SFRC. By analyzing data from mechanical property tests, it aims to identify a type of steel fiber that is most suitable for prefabricated road surfaces. For reference in practical engineering, the study will propose a fitting formula to calculate the steel fiber volume fraction and concrete’s mechanical properties.

2. Test Overview

2.1. Materials and Preparation

Cement: Ordinary Portland cement with a grade of P.O 42.5R (Easthope Chongqing CEMENT Co., Ltd., Chongqing, China). The test results of each index are shown in

Table 1. Cement properties and test results.

Project		Technical Indicators	Measured Value	Test Method
Standard consistency water consumption (%)		≤28	27.8	GB/T 1346
Stability (mm)		≤5.0	1.7
Condensation time (min)	Initial coagulation	≥45	180
	Final coagulation	≤600	245
Compressive strength (MPa)	3 d	≥4.0	5.0	GB/T 17671
	28 d	≥6.5	8.3
Flexural strength (MPa)	3 d	≥22.0	25.6
	28 d	≥42.5	58.6
Specific surface area (m2/kg)		≥300	340	GB/T 8074

.

Coarse Aggregate: Crushed stone with a particle size of 4.75–19 mm (Dazu County Gulong Detritus Factory, Chongqing, China);

Fine Aggregate: Mechanism sand with a fineness modulus of 2.6 (Ningbo Fenghua Xikou Huiyuan Building Materials Co., Ltd., Ningbo, China);

Water reducer: High-performance polycarboxylate water reducer (Sobute New Materials Co., Ltd., Jiangsu Province, China);

Water: Tap water;

Steel fiber: Three representative steel fibers produced by a certain metal products company (Chongqing Ganggang Fiber Co., Ltd., Chongqing, China), as shown in Figure 1, were selected.

The physical property indices of different steel fiber types are presented in

Table 2. The physical property indices of different steel fiber types.

Class	Specifications and Models (mm)	Tensile Strength (MPa)	Bending Properties	Diameter (mm)	Length (mm)	Length-to-Diameter Ratio
Wavy	0.7 × 50	≥1000	3 mm/90°	0.7 ± 0.07	50 ± 2	71.5
End hook shape	0.8 × 58	≥1000	3 mm/90°	0.8 ± 0.08	58 ± 2	72.5
Milling shape	0.6 × 60	≥800	3 mm/90°	0.6 ± 0.15	40~50	60~80

.

The concrete mixing process: after weighing the coarse aggregates, fine aggregates, and cement, the author poured them into the container in sequence and mixed them for 30 s. Once they were thoroughly mixed, water was added and the materials were remixed for approximately 3 min to achieve the desired consistency.

For the SFRC mixing process, the coarse aggregates and fine aggregates were first mixed for 30 s, followed by the addition of cement, which was stirred for another 30 s. Next, the water and a specific amount of water reducer were added and thoroughly mixed. The steel fibers, previously weighed and properly dispersed, were added gradually and evenly to ensure that they did not clump together. The mixture was then thoroughly stirred for approximately 3 min. The mixing process of steel fiber concrete is shown in Figure 2.

Experimental production of concrete compression and tensile strength specimens shall use non-standard cubic test blocks measuring 100 × 100 × 100 mm, while the flexural strength specimens shall utilize non-standard rectangular test blocks measuring 100 × 100 × 400 mm.

Main test equipment: HJW-60 single horizontal shaft concrete mixer (Beijing CCCC Jianyi Technology Development Co., Ltd., Jiangsu Province, China), WAW-300B electro-hydraulic servo universal testing machine (Jinan Chenxin UTM Manufacturing Co., Ltd., Shandong, China), WAW-1000D electro-hydraulic servo universal testing machine (Jinan Chenxin UTM Manufacturing Co., Ltd., Shandong, China) and electron microscope (SEM, KYKY-EM6200, Beijing KYKY Co., Ltd., Beijing, China) scanner.

2.2. Steel Fiber Reinforced Concrete Mix Proportion

The mix proportion of SFRC was designed based on plain concrete mix proportion and equal substitution of coarse aggregate by volume. When analyzing the impacts of steel fiber types on SFRC’s performance, two different steel fiber volume fractions of 0.6% and 0.9% were utilized. Similarly, when examining the effects of volume fraction on SFRC’s performance, steel fiber volume fractions of 0.6%, 1.0%, 1.5%, 2.0%, and 2.5% were employed.

Table 3. Steel fiber reinforced concrete mix ratio.

Configuration Method	Steel Fiber Type	Test Block Number	Amount of Material (kg/m3)
			Cement	Coarse Aggregates	Fine Aggregates	Water	Water Reducer	Steel Fiber
		PC	405	1253	646	142	4.0	0
Equal volume replacement coarse aggregate method	Wave shape	EW0.6	433	1237	637	152	4.3	47
		EW1.5	474	1212	624	166	4.7	117
	End hook shape	EEH0.6	433	1237	637	152	4.3	47
		EEH1.5	474	1212	624	166	4.7	117
	Milling shape	EM0.6	433	1237	637	152	4.3	47
		EM1.0	451	1225	631	158	4.5	78
		EM1.5	474	1212	624	166	4.7	117
		EM2.0	491	1197	617	172	4.9	156
		EM2.5	514	1185	610	180	5.1	195

presents the mix proportion of materials used in the SFRC design.

In Table 2, “PC” refers to plain concrete; “EW”, “EEH” and “EW” refer to wavy steel fiber reinforced concrete, end-hooked steel fiber reinforced concrete, and milled steel fiber reinforced concrete, respectively, designed with an equal volume replacement coarse aggregate ratio. “n” represents the volume fraction of steel fiber in “SMn”, “EWn”, “EEHn”, and “EMn”.

2.3. Mechanical Properties Test of Different Mix Proportions

With the same mix proportion as plain concrete, the author produced SFRC test specimens by incorporating equal volumes of steel fiber for mechanical performance testing.

The test results for specimens produced by various mixing methods are presented in

Table 4. Test results of SFRC mechanical properties under different ratio designs.

Configuration Method	Steel Fiber	Number	Compressive Strength/MPa			Flexural Strength/MPa			Split Tensile Strength/MPa
			3 d	7 d	28 d	3 d	7 d	28 d	3 d	7 d	28 d
—	—	PC	38.66	42.78	46.78	5.13	5.89	6.51	3.17	3.39	3.46
Equal volume substitution method	Milling shape	EM1.5	42.70	48.21	56.19	7.38	7.81	8.29	3.66	4.33	4.89

.

3. Tests on the Effect of Steel Fiber Type on Concrete Properties

3.1. Cubic Compressive Test

3.1.1. Experimental Procedure

The specific steps for conducting a cube compression test are as follows:

(1)

After the curing age reaches 3 days, 7 days, and 28 days, respectively, remove the specimens, wipe them clean, and prepare for the test.

(2)

Place the specimens on the compression plate of the testing machine, with the molded surface as the compression surface, aligning the center of the specimen with the center of the compression plate.

(3)

Start the testing machine until the upper pressure plate is close to the specimen, and then apply pressure. The loading rate should be controlled at 0.5 MPa/s until the specimen fails.

3.1.2. Analysis of Test Results

Based on Figure 3, when the volume fraction of steel fibers is 0.6%, the maximum compressive strength of wavy steel fiber reinforced concrete (EW) at 28 days of age is 49.08 MPa, while the maximum compressive strength of end-hooked steel fiber reinforced concrete (EEH) is 48.41 MPa, and the maximum compressive strength of milled steel fiber reinforced concrete (EM) is 51.53 MPa. These values are 4.9%, 3.9%, and 10.2% higher than the maximum compressive strength of PC, which is 46.78 MPa.

When the volume fraction of steel fibers was 1.5%, the maximum compressive strength of EW at 28 days of age was 52.98 MPa, while that of EEH was 49.86 MPa, and that of EM was 56.19 MPa. These values increased by 13.3%, 6.6%, and 20.1%, respectively, compared to the maximum compressive strength of PC, which was 46.78 MPa.

The experimental results show that the increase in the compressive strength of concrete with the addition of steel fibers is only around 10% for different ages of concrete, but milled steel fibers have increased it by approximately two times. The milled steel fiber has a twisted shape with acute angles and a flat surface, which increase its contact area with the concrete and strengthens the bond between the fibers and the matrix. Furthermore, the anchoring effect of the steel fibers in the concrete prevents the propagation of cracks. When micro-cracks appear within the concrete, the tensile and frictional effects of the steel fibers restrict their development, thereby enhancing the concrete’s compressive strength.

3.2. Four-Point Bending Test

3.2.1. Experimental Procedure

The specific steps for conducting a flexural test are as follows:

(1)

After the curing age reaches 14 days, 28 days, and 60 days, respectively, remove the specimens, wipe them clean, and prepare for the test.

(2)

Place the specimen on two cylindrical steel bars with a certain distance between them, as shown in Figure 3. Position the center of the compression surface of the specimen on the cylindrical steel bars and apply a downward load. The compression surface of the specimen should be the molded surface.

(3)

Start the testing machine until the upper pressure plate is close to the cylindrical steel bars placed on the compression surface of the specimen. Begin applying pressure at a loading rate controlled at 0.05 MPa/s until the specimen fails.

3.2.2. Analysis of Test Results

The histogram in Figure 4 indicates that at 28 days, the maximum flexural strengths of 7.42 MPa and 8.22 MPa were achieved with 0.6% and 1.5% substituted wavy steel fibers in the concrete, respectively. These results were 14.0% and 26.3% higher than the maximum flexural strength of the plain concrete, which was 6.51 MPa.

The maximum flexural strengths of 7.93 MPa and 8.33 MPa were attained at 0.6% and 1.5% substitution levels of end-hook-shaped steel fibers in the concrete, respectively. These values were 21.8% and 28.0% higher than the maximum flexural strength of the plain concrete, which was 6.51 MPa.

The maximum flexural strengths of 7.81 MPa and 8.29 MPa were attained at 0.6% and 1.5% substitution levels of milled-shaped steel fibers in the concrete, respectively. These values were 20.0% and 27.3% higher than the maximum flexural strength of the plain concrete, which was 6.51 MPa.

Increasing the volume fraction of steel fibers from 0.6% to 1.5% resulted in a more than 20% increase in flexural strength for the three types of steel fiber reinforced concrete compared to plain concrete. This significant improvement is attributed to the tensile zone cracking and upward movement of the neutral axis of SFRC under bending loads, which force the steel fibers in the tensile zone to bear the tensile force along with the bond force between steel fibers and the matrix, ultimately increasing the toughness and flexural strength of concrete. Despite the relatively smooth surface of hooked-end steel fibers, the bent-hook shape on both ends of the steel fibers allows for better adhesion and bonding with the concrete matrix, resulting in a more compact anchoring effect. This anchoring effect better enhances the tensile performance of the concrete, ultimately improving its flexural strength.

3.3. Splitting Tensile Test

3.3.1. Experimental Procedure

The specific steps for conducting a splitting tensile test are as follows:

(1)

When the curing age reaches 3 days, 7 days, and 28 days, respectively, remove the specimens, wipe them clean, and prepare for the test.

(2)

Mark mutually parallel lines on the top, front, and bottom surfaces of the specimen to accurately identify the location of the splitting plane.

(3)

Place the specimen at the center of the compression plate of the testing machine, and insert cylindrical steel bars between the upper and lower pressure plates and the specimen.

(4)

Start the testing machine until the upper pressure plate is close to the specimen, and begin applying pressure. The loading rate should be controlled at 0.05 MPa/s until the specimen fails.

3.3.2. Analysis of the Test Results

As shown in Figure 5, the maximum split tensile strengths of 4.195 MPa and 4.842 MPa were achieved when using 0.6% and 1.5% substitution of wavy steel fibers in the matrix concrete, respectively. These values were 21.2% and 39.9% higher than the maximum splitting tensile strength of 3.462 MPa observed in the matrix concrete.

The maximum split tensile strengths of 4.316 MPa and 4.990 MPa were achieved at 0.6% and 1.5% of the replacement end-hooked steel fibers in the matrix concrete, respectively, which were 24.7% and 44.1% higher than the maximum splitting tensile strength of 3.462 MPa in the matrix concrete.

The split tensile strengths reached a maximum of 4.211 MPa and 4.890 MPa, respectively, at 0.6% and 1.5% admixture of the substituted milled-shaped steel fibers in the matrix concrete, which were 21.6% and 41.2% higher than the maximum splitting tensile strength of 3.462 MPa in the matrix concrete.

When the steel fiber content was increased from 0.6% to 1.5%, the splitting tensile strength of the three types of steel fibers had reached more than 40.0% that of plain concrete. This is due to the fact that the shear failure of SFRC is completely different from that of ordinary concrete. Ordinary concrete fails in three stages after shear, while the steel fibers in SFRC prevent its shear deformation and crack propagation, enabling it to maintain its integrity even after shear failure. Therefore, the shear strength is improved. Although the wavy steel fibers exhibit stronger bonding with the concrete at the bend, their improvement in flexural and splitting strength is less significant. This could be due to the local tension of the fibers in the tensile zone of the SFRC specimen before initial cracking and the spring effect that occurs when the fibers are stretched after cracking.

At volume fractions of 0.6% and 1.5%, compared to ordinary concrete as a benchmark, the milled-shaped steel fibers showed the highest increase in compressive strength by 13.5%, with a difference in flexural and splitting tensile strength of only 3.1% when compared to the end-hook-shaped steel fibers. However, the wavy steel fibers had poorer overall mechanical properties when compared to the former two.

4. Experimental Study on the Effect of Steel Fiber Volume Fraction on Concrete Performance

4.1. Cubic Compression Test

This study employed 100 × 100 × 100 mm cubic test specimens. One group included three specimens after curing for 28 days; the concrete specimens were tested to determine their compressive strengths. The compressive strengths of the 28-day specimens vary with the volume fraction of steel fibers, as depicted in Figure 6. This methodology can be employed to explore the flexural mechanical properties of Steel Fiber Reinforced Concrete (SFRC) materials across a range of steel fiber volume fractions from 0% to 2.5%.

The polynomial fitting function of Origin software was used, and the fitting equation is shown in Equation (1):

$$f_{fcu}=-2.72{{\rho }_f}^2+10.25{\rho }_f+46.53$$

(1)

The shapes of the test specimens after compression failure are shown in Figure 7.

According to the test results:

(1)

The compressive strength of the test specimen initially increased and then decreased as the volume fraction of steel fibers increased. When the volume fraction of the milled steel fiber reached 2.0%, the compressive strength of the specimen reached its maximum to 56.76 MPa.

(2)

In ordinary concrete specimens, significant concrete spalling and severe brittle failure were observed after compression, indicating a lack of integrity. In contrast, the concrete specimens containing steel fibers remained relatively intact, with only some cracks or minor damage. This suggests that the addition of steel fibers impedes the propagation of cracks in the matrix, slowing down the spread and severity of crack damage. Once cracks initiate in the steel fiber-reinforced concrete, they continue to propagate. However, when the cracks encounter the steel fibers crossing the matrix cracks, the high elastic modulus of the steel fibers prevents their rupture, forcing the cracks to bridge or change their direction, resulting in the generation of smaller cracks and suppressing further crack development. Ultimately, this enhances the concrete’s resistance to deformation and overall toughness.

(3)

During the crack propagation process, the steel fibers exhibit high tensile strength and are resistant to fracture. However, their bond strength with the matrix concrete is relatively low. With a lower steel fiber content, as the load increases, most of the concrete specimens detach from the matrix, causing the steel fibers to be pulled out and leading to specimen failure. However, with a higher steel fiber content, the number of fibers increases, including those crossing the cracks, which provide resistance to crack propagation. Consequently, the specimens maintain better integrity due to the connectivity provided by the steel fibers.

4.2. Four-Point Bending Test

This study employed 100 × 100 × 400 mm cuboid specimens, with three specimens per group. The development pattern of the flexural strength at 28 days with an increasing volume fraction of steel fibers can be observed in Figure 8. With this approach, the flexural mechanical properties of Steel Fiber Reinforced Concrete (SFRC) materials within a range of steel fiber volume fractions from 0% to 2.5% can be investigated.

The polynomial fitting function of Origin software was used, and the fitting equation is shown in Equation (2):

$$f_{ff}=-0.66{\rho }^2+2.19{\rho }_f+6.57$$

(2)

According to the analysis in Figure 8 and Figure 9:

After 28 days of curing, the plain concrete test specimen exhibited a bending strength of 6.51 MPa. The reinforced concrete specimen with milled-shaped steel fibers had the highest bending strength of 8.37 MPa, which was 28.6% higher than the former. During the loading test, the plain concrete specimens exhibited brittle failure, meaning that once cracks appeared, they rapidly propagated and resulted in failure. However, when steel fibers were added to the concrete, the ductility of the material improved to some extent, characterized by the presence of numerous and smaller cracks.

In the experiment, the steel fiber-reinforced concrete specimens initially developed small cracks in the region of stress concentration. As the load increased, these cracks gradually propagated and multiplied. At the same time, the steel fibers started bearing the external load and impeding further crack development and propagation, allowing the specimens to continue carrying ductile loads. However, as the cracks continued to expand and the crack widths reached a certain point, the steel fiber-reinforced concrete specimens eventually lost their resistance and experienced failure.

4.3. Splitting Tensile Test

This study employed 100 × 100 × 100 mm cubic test specimens, three specimens for each group. The development pattern of the split tensile strength at 28 days with an increasing volume fraction of steel fibers can be observed in Figure 10. Using this approach, the tensile mechanical properties of Steel Fiber Reinforced Concrete (SFRC) materials within a range of steel fiber volume fractions from 0% to 2.5% can be studied.

The polynomial fitting function of Origin software was used, and the fitting equation is shown in Equation (3):

$$f_{fts}=-0.55{{\rho }_f}^2+1.83{\rho }_f+3.42$$

(3)

According to the analysis in Figure 10 and Figure 11:

(1)

During the split tensile test, the plain concrete specimen was split into two halves. However, SFRC specimen kept its integrity. According to the experimental images, despite the cracks distributed across the middle of the specimen, the steel fibers inside the cracks still connect the specimen, stabilizing the overall structure. The addition of steel fibers to the concrete specimens, through the combined action of the fibers and coarse aggregates, prevents the extension and further inward development of cracks and inhibits the formation of through cracks. Under loading, although the specimens lose their tensile capacity, they still maintain the overall integrity of the structure. The steel fibers constrain the split surface by bearing the compression-shear stress at the loading end. When the split tensile strength deviates from the axial tensile strength, a wall effect occurs in the SFRC split tensile test. A higher volume fraction of the steel fiber would increase the constraint and the degree of deviation.

(2)

The splitting tensile strength of SFRC increased to 4.21 MPa and 4.94 MPa at 0.6% and 2.0% admixture of milled-shaped steel fibers, respectively. This represented a 21.63% and 42.72% improvement compared to the splitting strength of the matrix concrete, which was only 3.46 MPa.

The results suggest that volume fraction of 0.6% is adequate to meet the required bending tensile strength standard for heavy-duty concrete pavements.

4.4. Electron Microscopic Analysis of SFRC

4.4.1. The Microstructure of Different Types of Steel Fibers under Electron Microscopy

Figure 12 shows the microstructures of wavy, hooked-end, and milled steel fibers, when the volume fraction of steel fibers is 0.6% in the SFRC. Firstly, it can be observed that the wavy steel fibers exhibit strong bonding with the concrete at bending points, indicating that the wavy shape allows for a tight bond between the steel fibers and the concrete, enhancing the overall performance of the composite material. Secondly, the surface of hooked-end steel fibers is relatively smooth, but the bent hooks on both ends contribute to strengthening the bond with the matrix, thereby improving the tensile strength of the composite material. Lastly, the milled steel fibers have a flattened shape with protrusions on the cross-section. This design increases the contact area between the steel fibers and the concrete, thus enhancing the bond strength between the two. The protrusions of the milled steel fibers anchor more effectively in the concrete matrix, further enhancing the overall performance of the composite material.

4.4.2. Crack Propagation

Concrete contains numerous microscopic cracks. According to Figure 13a, the hydration reaction of cement is sufficient. The surface of the concrete samples is covered by numerous crossed hydration products, which enhance the connection within the concrete. The hydration products form a network structure within the concrete, filling tiny cracks and forming strong bonds with the surrounding concrete. The formation of this coating and interlocking structure contributes to the improvement of concrete strength and durability. Additionally, some internal voids are filled with hydration products, thereby improving the compactness of the concrete. These filled voids reduce the porosity of the concrete and increase its overall density. The increased compactness aids in enhancing the strength and durability of the concrete [27].

Hydration products (calcium silicate hydrate gel, or C-S-H gel) play a vital role in enhancing the concrete’s strength. Figure 13b depicts the interface of the transition zone between the aggregate and cement paste. In this zone, C-S-H gels are less dense than in other areas of the concrete and are distributed unevenly. Even at a magnification of 2000 times, only a few voids and cracks are observed in areas with less gel distribution. The dense internal structure and lower porosity of the interface transition zone effectively delay the formation of cracks. Due to the compactness and lower porosity of the interface transition zone, the internal structure of the concrete becomes tighter, and the distribution of the gel becomes more uniform. Such a structure is advantageous for enhancing the overall strength and durability of the concrete, thereby improving its strength [30,31,32].

As shown in Figure 14a, while SFRC produces less C-S-H gels compared to plain concrete, it is evident from Figure 14c–e that more C-S-H gels accumulate on the surface of steel fibers, thereby strengthening the bond between steel fibers and the concrete matrix. Figure 14b visualizes the cracks extending linearly along the edge of the aggregate, indicating a debonding phenomenon between the mortar and aggregates, forming distinct boundaries. The propagation path of these cracks demonstrates the failure mode of concrete, while also highlighting the role of steel fibers.

Additionally, it is observed from Figure 14c–e that there are more cracks at the transition zone of the steel fiber–cement slurry interface. This can be attributed to the cracks generated by the concrete spreading to the steel fibers, which are hindered by the steel fibers. As the load increases, the steel fibers are gradually pulled out while obstructing the expansion of cracks; during the loading process, steel fibers play a role in preventing crack propagation. However, as the load increases, the steel fibers are gradually pulled out, resulting in an increase in edge cracks around the steel fibers. This process continues until the steel fibers are completely pulled out, losing their ability to prevent crack propagation. These observations elucidate the mechanism of steel fibers in concrete. They can inhibit crack propagation and improve the toughness and tensile strength of concrete. However, when subjected to loads exceeding their bearing capacity, steel fibers gradually get pulled out, leading to an increase in cracks around the steel fibers.

The phenomenon of steel fiber debonding from the cementitious matrix indicates that the steel fibers act as bridges and undergo relative sliding with the cementitious matrix, leading to the detachment of the weak matrix adhered to the fiber surface. Simultaneously, a large number of cracks form around the steel fibers. The phenomenon of steel fiber pullout from the matrix indicates that with the increase in bridging stress, the steel fibers undergo significant elongation and dissipate a considerable amount of energy, thereby significantly enhancing the fracture toughness of the concrete beam. Additionally, the transverse shrinkage of the steel fibers caused by the Poisson effect further promotes their detachment from the matrix until complete separation. At this stage, the steel fibers only resist external loads through sliding friction with the matrix, with the loss of interfacial cohesion [33].

4.4.3. Analysis of Microscopic Reinforcement Mechanism

When PC is under loads, stress will concentrate in the interface transition zone between the aggregate and cement paste. This eventually leads to more cracks and failure. Previous studies show that the interface transition zone is the weakest point of concrete. The interface transition zone, which refers to the thin water layer between the aggregate and cement paste, was first identified by Otsuki [34]. This layer contains numerous pores and microcracks, which substantially reduce the adhesion between the aggregate and cement paste. SFRC consists of two interface transition zones: steel fiber-cementitious matrix and aggregate-cementitious matrix, which contribute to the complex microstructure of fiber-reinforced concrete compared to PC. The formation of a three-dimensional network of fibers introduces significant differences in mechanical properties between fiber-reinforced concrete and ordinary concrete. Therefore, studying the microstructure of SFRC is crucial for understanding its macroscopic performance. In the interface transition zone of SFRC, steel fibers connect the two different substances and effectively obstruct the initiation and development of microcracks. At the same time, the support system formed by the aggregate and the steel fibers of the disordered mesh structure is activated, which results in a more efficient crack arresting effect, which is macroscopically manifested in the increased ductility, splitting tensile strength and flexural strength of the concrete. That is why SFRC has better mechanical properties. However, the excess addition of steel fibers leads to fiber clustering, which makes the concrete less compact and significantly reduces the compressive strength.

5. Conclusions

(1)

When the steel fiber volume fractions were 0.6% and 1.5%, the bending and split tensile strengths of milled steel fibers were 3.1% higher than those of hooked-end steel fibers and the compressive strength of SFRC was significantly increased by 13.5%. The comprehensive mechanical properties of wave-shaped steel fibers were inferior to the first two types.

(2)

The mechanical properties of the three types of steel fiber reinforced concrete in this study all meet the design requirements for SFRC in engineering. Among them, milled steel fibers are more suitable as concrete components due to their smooth surface, which has a smaller impact on specimen forming and is more economical compared to the hooked-end steel fibers, which are difficult to shape due to their tangling property. Therefore, milled steel fiber reinforced concrete is more suitable for practical applications in prefabricated road surfaces.

(3)

Increasing the volume fraction of steel fibers in SFRC from 0% to 2% leads to continuous improvement in all its mechanical properties. However, after the volume fraction exceeds 2%, the mechanical properties of SFRC begin to deteriorate. Thus, the volume fraction of 0.6% is adequate to satisfy the bending tensile strength standard required for heavily loaded concrete pavements. Based on the test results and analysis of SFRC mechanical performance, this paper has developed an expression that relates the increase in compressive strength, bending strength, and splitting tensile strength of concrete to the volume fraction of steel fibers. This expression provides a valuable reference for the application of SFRC in engineering.

(4)

When SFRC is under a load, steel fibers act as a connection repair between the transition zone of the aggregate and slurry interface, effectively hindering the generation of microcracks. At the same time, the support system formed by the aggregate and the steel fibers of the disordered mesh structure is activated, which results in a more efficient crack arresting effect, which is macroscopically manifested in the increased ductility, splitting tensile strength and flexural strength of the concrete. However, excessive mixing of steel fibers can lead to agglomeration, reducing the compactness of concrete, and resulting in a reduction of compressive strength at the macroscopic level.