Mechanical Properties of Fiber-Reinforced Concrete using Steel Fibers Derived from Discarded Tires

Mechanical Properties of Fiber-Reinforced Concrete using Steel Fibers Derived from Discarded Tires

Dr. Olusegun A Afolabi, Mr. Samuel O Oshadare

Department of Civil & Environmental Engineering, University of Lagos, Nigeria

Abstract – Waste tire disposal poses a significant global challenge, with an estimated 1.5 billion tires discarded annually. Among various recycling approaches, extracting steel fibers from waste tires for use in concrete reinforcement has gained attention. This method not only reduces the volume of tire waste sent to landfills but also improves the mechanical performance of concrete. While concrete is widely valued for its versatility, it suffers from inherent limitations such as low tensile strength and brittle failure. The incorporation of steel fibers addresses these weaknesses by bridging cracks and enhancing post-cracking strength and toughness. Tires, which do not biodegrade naturally, are major contributors to environmental pollution and greenhouse gas emissions. This study explores the use of steel fibers recovered from discarded tires as a sustainable reinforcement material in concrete. The primary objective is to assess the impact of these fibers on concrete's workability, compressive strength, and tensile strength, using slump cone tests for workability evaluation. Concrete mixes were prepared with steel fiber dosages of 1%, 2%, 4%, and 6% by volume, alongside standard materials including cement, water, fine and coarse aggregates, and a superplasticizer. The results indicate that increasing the steel fiber content reduces the slump valuereaching as low as 20 mm at 4% and 6% contentdue to the interlocking effect of fibers, which restricts the movement of the cement paste and aggregates while increasing internal friction. As fiber content increases, the concrete becomes stiffer and more cohesive, making placement and compaction more difficult. Nonetheless, the addition of steel fibers significantly enhances the tensile strength of concrete, with the most notable improvements observed at higher fiber dosages.

Keywords: waste tire recycling, steel fiber-reinforced concrete, sustainable construction, tensile strength, compressive strength.

1. INTRODUCTION

   The improper disposal of used vehicle tires poses a serious environmental threat across the globe. Each year, billions of tires are discarded, occupying landfill space, creating fire hazards, and becoming breeding grounds for disease-carrying pests. Due to their durable composition and resistance to natural degradation, tires do not decompose easily, which makes their accumulation a long- term problem. On the other hand, concrete is one of the most widely used construction materials due to its high compressive strength, durability, and versatility. However, conventional concrete has well-known weaknesses: low tensile strength, limited ductility, and poor resistance to cracking under tension. These shortcomings can compromise structural safety and service life, especially under dynamic loading or extreme environmental conditions.

   Fiber-reinforced concrete (FRC) has emerged as a practical solution to overcome these limitations. By introducing short, discrete fibers into the concrete mix, the material gains improved tensile strength, crack control, and post-cracking behavior. Fiber- reinforced concrete (FRC) incorporates fibrous materials to enhance its structural integrity. This type of concrete contains short, discrete fibers that are uniformly distributed and randomly oriented. These fibers can be made of steel, glass, synthetic materials, or natural substances, each imparting unique property to the concrete. The characteristics of FRC are influenced by the types of concrete, fiber materials, geometries, distribution, orientation, and densities. The use of fibers for reinforcement dates back to ancient times, with horsehair used in mortar and straw in mud bricks [1]. In the early 1900s, asbestos fibers were incorporated into concrete. The mid-20th century saw the advent of composite materials, including FRC. As health risks associated with asbestos were recognized, alternatives like steel, glass (GFRC), and synthetic fibers (such as polypropylene) became prevalent by the 1960s. Research into FRC continues to evolve [2].

   Fibers in concrete are primarily used to control cracking due to plastic and drying shrinkage, reduce permeability, and minimize water bleeding. Some fibers enhance concrete's impact, abrasion, and shatter resistance. In certain cases, larger steel or synthetic fibers can entirely replace rebar [3]. FRC has largely supplanted rebar in the underground construction industry, especially for tunnel segments, due to concerns about the corrosion of steel reinforcements in moist environments. Some fibers, however, may reduce the compressive strength of concrete, and lignocellulosic fibers can degrade in a cement matrix due to hydrolysis. The fiber content in a concrete mix is measured as a percentage of the total composite volume (concrete and fibers), known as "volume fraction" (Vf), typically ranging from 0.1 to 3%. The aspect ratio (l/d) of a fiber is the ratio of its length (l) to its diameter (d) [4]. Fibers with a higher modulus of elasticity than the concrete matrix help increase tensile strength. A higher aspect ratio generally improves the flexural strength and toughness of the matrix. For optimal effectiveness, fibers should be longer than the maximum aggregate size in the concrete mix. Normally, concrete contains aggregate with an equivalent diameter of 19 mm (0.75 in), making

   fibers longer than 20 mm (0.79 in) more effective. However, excessively long fibers can cause workability issues by forming "balls" in the mix [5]. Among the different types of fibers used in FRC such as polypropylene, glass, carbon, and steel fibers are particularly effective for structural applications because of their high tensile strength, durability, and toughness. Different researchers have worked on FRC with different fibers, example is Bheel et al. [6] have used industrial and agricultural waste products as raw materials for the construction industry to create sustainable environments and reduce pollution. This study investigated concrete with 0.25%, 0.50%, 0.75%, and 1% jute fiber (JF) as reinforcement and 10%, 20%, 30%, and 40% wheat straw ash (WSA) as a replacement for fine aggregates. The results showed that compressive, splitting tensile, and flexural strengths improved to 32.88 MPa, 3.80 MPa, and 5.30 MPa at 0.50% jute fiber and 30% WSA. The modulus of elasticity increased, while permeability and workability decreased with higher dosages of jute fiber and WSA. Khan et al. [7] aimed to address this gap by analyzing JF's mechanical and environmental benefits. Experiments added JF at 0%, 0.10%, 0.25%, 0.50%, and 0.75% by weight. Tests included slump tests for fresh concrete, and compressive strength (CS), split tensile strength (STS), and flexural strength (FS) tests for hardened concrete, as well as water absorption (WA) for durability. The study computed embodied carbon (EC) ratios for different mixes. Results showed that 0.10% JF optimized CS, STS, and FS by 6.77%, 6.91%, and 9.63%, respectively, while reducing environmental impact. Using Response Surface Methodology (RSM), definitive equations were developed to evaluate JF's effects, highlighting its potential for future concrete advancements. Reddy [8] examined the durability of concrete with polypropylene fiber, focusing on unit weight, workability,

   compressive strength, tensile strength, and flexural strength. Polypropylene fiber was added in volume fractins of 0%, 0.2%, 0.3%, and 0.5%, with a fiber length of 12 mm. The addition decreased workability but improved mechanical properties, particularly with 0.5% fiber, enhancing split tensile and flexural strength. Silica fume replaced 8% of the cement weight. Polypropylene fibers reduced permeability, shrinkage, and expansion, delaying degradation. Results for compressive, split tensile, and flexural strengths were compared between higher and lower grades of concrete. Alwesabi et al. [9] study found that hybrid polypropylene fiber (PF) and macro steel fiber (MSF) improved concrete's mechanical and ductility characteristics better than single fiber types. They examined the impact of PF, MSF, and PF-MSF hybrid fibers on concrete's density, water absorption, voids, compressive strength, and toughness. Specimens with different PF (0%, 0.1%, 0.175%, 0.25%, 1.0%) and MSF (0%, 0.75%,

   0.825%, 0.9%, 1.0%) ratios

   were tested. Results showed that hybrid FRC significantly improved compressive toughness compared to mono fiber inclusion. The 0.1% PF-0.9% MSF hybrid FRC had the highest compressive toughness, 106.5% higher than the control mix, with notable improvements in compressive strength, density, and reduced water absorption and voids. Growing challenge of waste tire management and the limitations of conventional concrete represent two converging issues in modern civil engineering. Waste tires pose serious environmental hazards due to their resistance to degradation, leading to pollution, fire risks, and mosquito breeding when improperly disposed of. Meanwhile, concrete, though widely used, suffers from brittleness and low tensile strength, often requiring costly and corrosion-prone steel reinforcement. To promote sustainable construction, innovative materials are needed to enhance concrete performance while addressing waste management problems. This study therefore aims to assess and understand the influence of steel fibers derived from waste tires on the mechanical properties of fiber-reinforced concrete (FRC), determining whether these recycled fibers can serve as a viable alternative to conventional reinforcement by improving performance, durability, and reliability under compressive and tensile stresses. In doing so, the research supports waste valorization, material efficiency, and sustainable infrastructure development. Specifically, the objectives are: (i) to examine the effect of tire-derived steel fibers on the workability of concrete;

   (ii) to evaluate the compressive strength of FRC at different curing ages; (iii) to assess the split tensile strength and crack-bridging capacity of the fibers; and (iv) to compare performance across different fiber dosages to determine the optimum mix.

2. MATERIALS AND METHODOLOGY

   This section presents the experimental methodology adopted to evaluate the mechanical behavior of concrete reinforced with steel fibers recovered from discarded vehicle tires.

   1. Materials

      Materials used included Ordinary Portland Cement (Grade 42.5R, NIS 444-1:2003), clean river sand (Zone II, BS 882), crushed granite (1020 mm, angular), potable water (BS EN 1008:2002), and a PCE-based superplasticizer to improve workability. Recycled steel fibers were sourced from mechanically shredded end-of-life tires, magnetically separated, cleaned, and trimmed to 2040 mm length with an aspect ratio of 60130 and tensile strength of about 1000 MPa. Their rough surface, often coated with residual rubber, enhanced the bond with the cement matrix

      while providing post-cracking load resistance and toughness, offering a sustainable alternative to industrial fibers.

      (a) (b)

      Figure 1: Steel fibers: (a) Extracted tyre fibers (b) Trimmed fibers

   2. Mix proportion

      A uniform mix design was maintained across all specimens to isolate the effect of fiber addition. The mix proportion by weight was 1:1.875:2.625 (cement:fine aggregate:coarse aggregate) with a constant watercement ratio of 0.50 and superplasticizer dosage of 0.4% by cement weight. Steel fibers were incorporated at 0%, 1%, 2%, 4%, and 6% by cement weight to evaluate performance variation. Concrete mixing was performed using a mechanical mixer to ensure uniform blending. Coarse and fine aggregates were dry-mixed before adding cement, followed by the gradual introduction of water and superplasticizer. Steel fibers were added incrementally to prevent clustering and ensure even dispersion. The fresh mix was placed into cube and cylindrical molds, vibrated for compaction, and allowed to set for 24 hours before demolding. Specimens were cured in water for 7, 14, and 28 days. A total of 30 specimens were prepared and tested for workability, compressive strength, and split tensile strength to determine the optimal fiber dosage for enhanced performance.

      Table 1: Mix proportion

      Materials	(0%	Mix 1 (1%	Mix 2 (2%	Mix 3 (4%	Mix 4 (6%
      	Fibre)	Fibre)	Fibre)	Fibre)	Fibre)
      Cement (g)	3,000	3,000	3,000	3,000	3,000
      Water (g)	1,500	1,500	1,500	1,500	1,500
      Fine Aggregate (FA) (g)	5,625	5,625	5,625	5,625	5,625
      Coarse Aggregate (GA) (g)	7,875	7,875	7,875	7,875	7,875
      Steel Fiber (g)	0	30	60	120	240

      Superplasticizer (SSP) (g) 12 12 12 12 12

      Dangote 3X cement was used, and tests followed BS EN 196-3 (2016) and BS EN 197-1 (2011). Cement fineness was determined using a 90 µm sieve, ensuring residue did not exceed 10%. Standard consistency, initial and final setting times, specific gravity, and moisture content were all measured according to standard procedures to assess cement quality. Fine and coarse aggregates were tested per BS EN 12620 (2013) and BS EN 1097-2 (2010) for sieve analysis, fineness modulus, specific gravity, water absorption, and bulk density. These tests established the gradation, particle size distribution, and density characteristics of the aggregates.

   3. Tests on Fresh Properties

      The slump cone test (in Figure 2) measures the workability and consistency of concrete, including mixes with steel fiber and aggregates. The concrete mix is prepared to ensure homogeneity before testing. The required materials include a slump cone, base plate, tamping rod (16 mm diameter, 600 mm long), measuring scale, and scoop. For the procedure, the cone is placed on a clean, level base plate and filled in three equal layers. Each layer is compacted with 25 uniform strokes of the tamping rod, ensuring penetration into the previous layer. After filling, the surface is leveled, and the cone is lifted vertically within 5 to 10 seconds to let the concrete settle. The slump is measured as the difference between the cone height and the highest point of the settled concrete, recorded in millimeters.

      Figure 2: slump cone test

   4. Tests on Hardened Concrete Properties

      1. Compressive Strength Test

         This test evaluates a materials ability to withstand external loads without cracking or deformation (as shown in Figure 3.3). Concrete was poured into molds, compacted to prevent voids, and cured for 28 days before testing. A 1560 kN capacity compression machine applied a gradual load of 140 kg/cm² per minute until failure. Compressive strength was calculated using Equation 3.8 by dividing the maximum oad by the specimen's cross-sectional area. The test followed BS EN 12390-3 (2019). The test is illustrated in Figure 3.

         (1)

         Figure 3: Compressive strength Test

      2. Split Tensile Strength Test

         Also known as the Brazilian test, this method indirectly measures concrete's tensile strength. Cylindrical specimens (100 mm diameter, 200 mm height) were dried after curing, placed in a testing machine, and covered with a plywood strip. A gradual load between 0.7 to 1.4 MPa/min was applied until failure, following BS EN 12390-6 (2011). The failure load was recorded. The test is shown in Figure 4.

         Figure 4: Split tensile strength test

      3. Flexural Test

         In the flexural strength test, beam specimens were cast, cured, and placed on two support rollers of the testing machine at a

         specified span. The load was applied either at the mid-span in a three- point bending setup or at two points in a four-point bending setup at a constant rate until failure occurred. The maximum load carried by each specimen was recorded, and the flexural strength was calculated using the applied load, span length, and specimen dimensions.

3. RESULTS AND DISCUSSION

   This section presents the results of laboratory tests conducted on fresh and hardened concrete reinforced with steel fibers extracted from discarded tires. The key tests performed include the slump test (for workability), compressive strength test (on cubes), split tensile strength test (on cylinders), and flexural test (on beams). Results are systematically analyzed to assess the influence of varying steel fiber content on concrete properties, with comparisons made against the control mix (0% fiber) to highlight performance improvements.

   1. Properties of Cement

      The physical properties of the Ordinary Portland Cement used are summarized in Table 2. All values conform to BS EN 196-3 (2016) specifications.

      Table 2: Properties of Cement

      S/N	Property	Value Observed
      1	Specific Gravity	2.50
      2	Initial Setting Time	52 min
      3	Final Setting Time	489 min
      4	Fineness	7.5%
      5	Standard Consistency	32%

   2. Properties of Fine Aggregates

      The particle size distribution curve (Figure 5) shows D = 0.25 mm, D = 0.5 mm, and

      D = 0.7 mm, giving a coefficient of uniformity (Cu) of 2.8 and a coefficient of curvature (Cc) of 1.43. Based on these values, the fine aggregate is classified as well-graded sand under the

      Unified Soil Classification System (USCS). All properties comply with BS EN 12620 (2013).

      Table 3: Properties of Fine Aggregates

      S/N	Property	Result
      1	Fineness Modulus	3.3
      2	Specific Gravity	2.58
      3	Water Absorption	2.5%
      4	Bulk Density	1.50 g/cm³

      120

      98

      100

      81 33 .

      80

      60

      40

      32 53 .

      20

      4. 7.63

      22

      0

      0.01

      0.1

      1

      10

      PARTICLE SIZE (mm)

      % PERCENTAGE PASSING

      Figure 5: Particle distribution of Fine aggregates

   3. Properties of Coarse Aggregates

      The particle size distribution curve (Figure 6) indicates D = 6 mm, D = 14 mm, and D

      = 24.7 mm, yielding Cu = 4.11 and Cc = 1.32, confirming the granite as well-graded per

      USCS classification. The fineness modulus of 6.7 satisfies BS EN 933-1 (2012) and BS EN 12620 (2013).

      Table 4: Properties of Coarse Aggregates

      S/N	Property	Result
      1	Fineness Modulus	6.7
      2	Specific Gravity	2.61
      3	Water Absorption	1.4%

      100

      92.12

      90

      80

      71.68

      70

      60

      50

      40

      30

      18.28

      20

      10

      1.84

      0

      0.01

      0.1

      1

      PARTICLE SIZE (mm)

      10

      % PERCENTAGE PASSING

      Figure 6: Particle distribution of Fine aggregates

   4. Fresh properties of concrete

      The slump test results in Figure 7 indicate that increasing the steel fiber content reduces the workability of the concrete mix. The control mix, without fibers, has the highest slump (35 mm), showing better flowability. As steel fiber content increases, the slump decreases progressively, reaching 20 mm at 4% and 6% fiber content. This reduction is due to the interlocking effect of steel fibers, which restricts the movement of cement paste and aggregates, leading to higher internal friction. At higher fiber dosages, the concrete becomes more cohesive and stiffer, making placement and compaction more challenging. The stiffness outcome is similar to the research by (Kytinou, Chalioris and Karayannis [10]. The stabilization of slump at 20 mm for 4% and 6% fiber suggests that beyond a certain threshold, additional fibers have minimal further impact on workability.

      40

      35

      35

      30

      30

      25

      25

      20

      20

      20

      15

      10

      5

      0

      Control

      1% Fiber

      2% Fiber

      Concrete mixture

      4% Fiber

      6% Fibre

      Slump (mm)

      Figure 7:Slump result

      1. Compressive strength result

         The inclusion of steel fibers led to a noticeable and progressive increase in compressive strength across all curing periods, as shown in Figure 8. At 7 days, the control mix (without fiber) achieved a compressive strength of 10.18 MPa, while the strength rose steadily with increasing fiber content, reaching 12.44 MPa at 6% fiber inclusion. This trend continued at 14 and 28 days, with the control mix recording 13.57 MPa and 15.51 MPa respectively, and the 6% fiber mix achieving 16.59 MPa and 18.96 MPa.

         Compressive strength result

         25

         20

         15

         10

         5

         0

         Control

         1%

         2%

         Steel Fiber (%)

         4%

         6%

         7 Days (MPa) 14 Days (MPa) 28 Days (MPa)

         Compressive strength (MPa)

         Overall, the addition of steel fibers enhanced the compressive strength at all ages, indicating improved load-bearing capacity and crack resistance, which align with study by Pourbaba et al. [11]. The most substantial improvements were observed at 6% fiber content, suggesting that steel fibers effectively contribute to the densification and bridging of micro-cracks within the concrete matrix.

         Figure 8: Compressive strength result

         1. Regression Analysis of 28-Day Compressive Strength

            To examine the influence of steel fibre content on 28- day compressive strength, the experimental results were analysed

            using regression modelling. Steel fibre dosage levels of 0%, 1%, 2%, 4%, and 6% were taken as the independent variable, while the corresponding 28- day

            compressive strengths (15.51, 17.00, 18.09, 18.96, and 18.96 MPa) served as the dependent variable. The best- fit quadratic equation obtained was:

            Fc,28 = 15.5789 + 1.5282SF 0.1621SF² Equation 1

            Where SF is steel Fiber conent (%)

            The regression model reveals a predominantly linear increase in compressive strength with fibre incorporation, averaging about 0.51 MPa per 1% steel fibre addition. The presence of the negative quadratic coefficient indicates a second-order effect, producing a concave-down trend and signalling a progressive reduction in marginal strength gains at higher fibre dosages. Statistically, the model exhibits an excellent fit, with a coefficient of determination of R² 0.995, confirming its robustness for predictive applications.

            Figure 9: Comparison of actual vs predicted compressive strength result

      2. Split tensile strength result

         Figure 10 indicate that the inclusion of steel fiber significantly improved the split tensile strength of concrete at all curing periods. For the control mix (0% fiber), the split tensile strength values were 1.92 MPa, 2.56 MPa, and 2.93 MPa at 7, 14, and 28 days, respectively. With the gradual increase in fiber content, the tensile strength showed consistent improvement.

         At 6% fiber content, the split tensile strength rose to 2.78 MPa at 7 days, 3.71 MPa at 14 days, and

         4.24 MPa at 28 days. This progressive increase demonstrates that steel fibers contribute to better tensile performance by enhancing crack resistance and improving the post-cracking behavior of the concrete matrix. The strength result align with outcome of study by Abbas et al. [12].

         Overall, the results affirm that steel fiber reinforcement plays a crucial role in enhancing the tensile capacity of concrete, with the most substantial improvements observed at higher fiber dosages.

         Split tensile strength result

         5

         4.5

         4

         3.5

         3

         2.5

         2

         1.5

         1

         0.5

         0

         Control

         1

         2

         Steel Fiber content (%)

         4

         6

         7 Days (MPa) 14 Days (MPa) 28 Days (MPa) Series4

         Split tensile strength (Mpa)

         Figure 10: Split tensile strength result

         1. Regression Analysis of 28-Day Split Tensile Strength

            The relationship between steel fibre content and 28-day split tensile strength was examined using a simple linear regression model. Steel fibre dosage (0 %, 1 %, 2 %, 4 %, and 6 %) was taken as the independent variable, while the corresponding 28-day split tensile strengths (2.93, 3.22, 3.57, 3.83, and 4.24 MPa) were used as the dependent variable.

            The resulting regression equation is:

            Ft,28 = 0.2097SF+ 3.0127 Equation 2

            This shows that each 1 % increase in steel fibre content produced an average increase of about

            0.21 MPa in 28-day split tensile strength. The model achieved a high coefficient of determination (R2=0.974), indicating an excellent fit between steel fibre content and the measured tensile strengths.

            Figure 11: Comparison of actual vs predicted tensile strength result

      3. Flexural Strength Result

         The 28-day flexural strength results show a clear enhancement with increasing steel fiber content. The control mix without fibers recorded a strength of about 3.8 MPa. With the inclusion of 1% fibers, the value increased modestly to around 4.2 MPa, indicating that even a low dosage contributed to crack bridging and improved post-cracking resistance. At 2% fiber content, the flexural strength rose further to approximately 4.6 MPa, while 4% fiber achieved about 5.0 MPa. The maximum strength, roughly 5.6 MPa, was attained at 6% fiber content, representing an overall increase of nearly 47% compared to the plain mix.

         6

         5.51

         5.04

         5

         4.62

         4.17

         4

         3.81

         3

         2

         1

         0

         0%

         1%

         2%

         Fiber content

         4%

         6%

         Flexural Strength (MPa)

         This trend demonstrates the effectiveness of steel fibers in enhancing flexural performance by improving crack arresting and load redistribution across the matrix. The gains are consistent with findings in the literature, where increases of 2060% in flexural strength are typically reported depending on fiber dosage, aspect ratio, and mix design (Afroughsabet & Ozbakkaloglu, 2015; Yoo & Banthia, 2016; Yin et al., 2021). The progressive improvement suggests that the fibers were well dispersed and contributed positively up to the highest tested content. However, it should be noted that very high fiber contents may sometimes reduce workability, potentially leading to fiber clustering if not carefully mixed.

         Figure 12: Flexural Strength Result

         1. Relationship Regression Analysis of 28-Day Split Flexural Strength

            The effect of steel fibre content on flexural strength was evaluated using regression analysis. Experimental results for fibre dosages of 0%, 1%, 2%, 4%, and 6% corresponding to flexural

            strengths of 3.81, 4.17, 4.62, 5.04, and 5.51 MPa respectively were modelled with a second- degree polynomial to capture potential non- linear trends.

            The resulting best- fit equation is:

            Ff,28 = 3.8276 + 0.3479 SF 0.0089SF² Equation 3

            where Ff is the 28 days flexural strength in MPa and SF is the steel fibre content expressed as a percentage by volume.

            The model achieved an R² value of 0.999, indicating an almost perfect correlation between predicted and measured values. The positive linear term reflects the initial increase in flexural strength with fibre addition, while the small negative quadratic term suggests a gradual reduction in the rate of strength gain at higher fibre dosages. This behaviour is consistent with the observed plateau effect in the experimental data.

            Figure 13: Comparison of actual vs predicted flexural strength result

4. CONCLUSION

The research investigated the mechanical behavior of concrete reinforced with recycled steel fibers from waste tires, focusing on workability, compressive strength, and split tensile strength. The key findings are as follows:

1. The study revealed that workability decreased consistently as the volume of steel fibers increased from 0% to 6%. This reduction was attributed to increased internal friction among fibers and aggregates, the greater surface area requiring more paste for lubrication, and the tendency for fiber clustering at higher dosages. Nevertheless, the mixes remained workable and compactable, especially when aided by a superplasticizer, and no severe balling or segregation was observed.

2. In terms of compressive strength, the inclusion of steel fibers produced moderate improvements across all curing ages of 7, 14, and 28 days. At 28 days, the mix containing 6% fibers achieved up to a 22.2% increase compared to the control. The improvement was linked to the fibers ability to restrain crack initiation, slow down crack propagation, and enhance load distribution across the matrix. However, the effect on compressive strength was less pronounced than on tensile properties, since fibers primarily influence tensile behavior rather than compressive resistance.

3. The influence on split tensile strength was far more significant. At 28 days, the addition of steel fibers improved tensile strength by as much as 44.7% at 6% fiber content. This enhancement was attributed to the fibers crack- bridging action, their role in delaying crack growth, and the increased energy absorption and toughness of the concrete. These results confirmed the effectiveness of steel fibers in overcoming concretes inherent weakness in tension.

4. The findings further indicated that fiber dosages in the range of 4% to 6% by weight of cement offered the best balance between mechanical performance and workability. Lower dosages, particularly below 2%, produced limited improvements, while higher dosage, although effective in enhancing strength, could potentially reduce workability and cause placement challenges if not carefully managed.

RECOMMENDATIONS

The study recommends a fiber content of 4% to 6% by weight of cement for most practical applications where tensile strength, toughness, and crack resistance are of primary concern. For precast elements such as panels and blocks, where production conditions are more controlled and uniform dispersion is easier to achieve, even higher fiber dosages may be considered. The results further show that recycled steel fiber-reinforced concrete is well-suited for structural and pavement-related applications, including slabs-on-grade, driveways, pavements, tunnel linings with shotcrete, and non-load-bearing wall panels. However, for load-bearing members, the research emphasizes that fiber reinforcement should complement conventional steel reinforcement rather than replace it entirely, ensuring both safety and structural reliability. With regard to mix design and quality control, the

use of superplasticizers was found to be essential at fiber dosages above 2% in order to maintain adequate workability. Proper mixing procedures were also critical, with fibers added gradually to achieve uniform distribution throughout the matrix. Additionally, trimming and cleaning of recycled fibers were highlighted as necessary steps to reduce surface contamination and promote effective bonding with the cementitious paste. From a cost perspective, the study establishes that recycled tire steel fibers can provide a practical alternative to industrially manufactured fibers, particularly in projects where cost- efficiency and sustainability are priorities. Local sourcing and processing of waste tires not only lower material costs but also reduce the environmental burden associated with tire disposal.

Overall, this research successfully demonstrated that steel fibers recovered from waste tires enhance the mechanical properties of concrete, especially in terms of split tensile strength, crack resistance, and toughness. The findings support the use of recycled fibers as a sustainable, cost- effective, and technically viable material for improving concrete performance in construction. Beyond technical improvements, the study contributes to the expanding body of knowledge on sustainable concrete technologies and highlights innovative pathways for utilizing tire waste in civil engineering, aligning performance objectives with environmental responsibility.

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