Fiber Reinforced Concrete: Comparative Analysis of Helix Steel Fibers and Plain Steel Fibers in Structural Applications

Prakhar singh

Department of Structural Engineering, SSTC Bhilai Chhattisgarh, India

Rungta College of Engineering and Technology Bhilai, Chhattisgarh, India

prakharsinghk5@gmail.com

Abstract: This is an experimental study focused on studying the effect of adding helix fiber to normal concrete to evaluate compressive strength, tensile strength, stability, elastic modulus, and to improve the desired consistency of the mixture required for workable concrete compared to normal concrete. As concrete is strong in compression and week in tension, steel is placed in concrete where ever tension is anticipated. The process is proven to be cumbersome, time consuming and expensive. Fiber reinforced concrete has emerged as a consequence which offers improved tensile strength in addition to increased compressive strength. Steel Helix fiber with more frictional resistance are added in concrete matrix to improve tensile strength of concrete. Grade M 20 concrete mixture was used and the mix ratio was determined. Stress-strain curves were recorded for different dosages of helix fiber after 28 days to determine the elastic modulus. The results showed that the addition of spiral fiber to normal concrete increases the compressive strength, tensile strength, elastic modulus, and ductility of the concrete compared to normal concrete. The results of this study suggest that the use of helical fibers instead of normal rebars increases the strength of concrete and fly ash is added to concrete to maintain the workability of concrete. This improves the strength parameters and workability of concrete as per the desired requirements.

 Keywords: Structural Engineering, Fiber-Reinforced Concrete, FRC Composite, Future Technology, Concrete Strength, Concrete Cracks.

Introduction

concrete has some undesired properties that require development of concrete mix to produce concrete that has better performance in the different loading conditions and extreme exposure. Brittleness is one of the most critical issues related to concrete efficiency and it needs more attention due its significant importance if any failure could happen. The relatively low tensile strength of concrete is the cause of concrete brittleness which is assumed to be 1/10 of its compressive strength [1]. Reinforced concrete with steel bars has better performance as a composite material that can work properly against the different types of stresses. Increasingly, concrete is reinforced with small, randomly distributed fibers for many application in the field of concrete works [2]. These fibers are added in order to increase the energy absorption capacity and toughness of the material in addition to enhance the tensile strength of concrete [3]. High-modulus fibers can be used to substitute, partially or totally, conventional reinforcement to enhance concrete toughness for structural applications. Early applications of fibers in concrete include horse hair and the addition of asbestos fibers in 1900 to reinforce pottery, straw to mud bricks, to reinforce plaster and asbestos [4] . Control of Cracking and Improvement of Ductility Objective: To reduce crack formation and enhance the ductility of concrete. Rationale: The presence of steel fibers can postpone the initiation and spread of microcracks, thereby improving crack management [5] . Additionally, they contribute to post-cracking ductility, rendering concrete less prone to brittle failure. Steel fibers improve resistance to shear stresses, thereby diminishing the necessity for additional reinforcement [6]. This is especially advantageous in scenarios where concrete is vulnerable to shear failures, such as at beam-column connections or slab foundations [7].

Materials Used

Cement

OPC 43 Grade cement, Figure 1 governed by IS 8112:2013, is a versatile construction material widely used in residential and moderate-strength applications due to its balanced performance characteristics. It achieves a compressive strength of 43 MPa after 28 days of curing, with early strength milestones of ≥23 MPa in 3 days and ≥33 MPa in 7 days, making it suitable for a variety of structural works [5] . Its chemical composition predominantly consists of 60-67% lime (CaO), 17-25% silica (SiO₂), 3-8% alumina (Al₂O₃), and 0.5-6% iron oxide (Fe₂O₃), along with minor components such as magnesia (MgO) and sulfur trioxide (SO₃), ensuring optimal hydration and strength development. The cement has a fineness of at least 225 m²/kg, a standard consistency range of 27-33%, and soundness of ≤10 mm as measured by the Le Chatelier method, with an initial setting time of at least 30 minutes and a final setting time not exceeding 600 minutes. Manufactured by calcining a carefully proportioned mixture of limestone and clay at high temperatures in a rotary kiln to produce clinker, it is then finely ground with gypsum to regulate setting time. OPC 43 Grade cement is used extensively in the construction of walls, columns, beams, plaster, masonry work, and general-purpose concrete for pavements and walkways [2]. Its advantages include high early strength, wide availability, good workability, and economical cost, although its relatively high heat of hydration makes it less ideal for mass concreting projects. The cement is tested for physical and chemical properties under strict quality control measures, with its performance verified through compression tests, soundness tests, and setting time evaluations. To address environmental concerns, the production process is being optimized with the addition of supplementary cementitious materials such as fly ash and slag, reducing its carbon footprint while maintaining the desired quality and strength parameters[7] . This balance of performance, cost-effectiveness, and sustainability makes OPC 43 Grade cement a critical component in modern construction [4] .

Figure 1- Cement, Sand, Aggregate

Aggregate

Aggregate refers to a granular material Figure 1 used in construction, formed from natural rock or synthetic processes, with applications in concrete, asphalt, and foundational works [6]. It is classified based on size (fine aggregates like sand and coarse aggregates such as gravel or crushed stone), shape (rounded, angular, flaky, or elongated), texture (smooth or rough), and origin (natural, recycled, or manufactured). Key parameters influencing its performance include gradation, specific gravity, bulk density, water absorption, moisture content, and surface area. Mechanical properties like compressive strength, abrasion resistance, and impact value are critical for durability and load-bearing capacity [8]. Chemical properties, such as alkali-silica reactivity (ASR), chloride ion penetration, and resistance to weathering agents, also play a significant role in preventing long-term degradation. Environmental considerations include sustainability in sourcing, the potential for recycling, and energy efficiency during production, with an emphasis on reducing carbon emissions [5] . Thermal properties, such as heat capacity and conductivity, are vital in assessing the aggregate’s suitability for specific climates and applications. Aggregates are tested for compatibility with binding materials, durability under freeze-thaw cycles, resistance to chemical attacks, and thermal expansion, ensuring structural integrity and longevity in construction projects [6]. When used in combination with helix or lain steel fibers, Figure 2 the interaction between aggregates and fibers is critical in enhancing tensile strength, crack resistance, and ductility in concrete [7] . This synergy improves load transfer efficiency and mitigates brittle failures under dynamic and static loads, making it suitable for advanced applications in seismic-resistant structures, industrial floors, and high-performance pavements [3]. Additionally, ongoing research explores the optimization of aggregate-fiber bonding to maximize the structural benefits of fiber-reinforced composites [9] .

Water

Water is a vital component in the preparation of helix and plain steel fiber-reinforced concrete, serving multiple functions that influence the overall performance, strength, and durability of the composite material [8]. It facilitates the chemical hydration of cement, which is crucial for the formation of a robust and cohesive matrix. The selection of water should prioritize purity, avoiding contaminants like oils, acids, alkalis, salts, sulfates, chlorides, and organic materials, as these can disrupt the hydration process and potentially corrode steel fibers, thereby weakening the material over time [7] . The water-to-cement ratio (w/c) is a critical parameter that determines not only the workability of the concrete mix but also its mechanical properties, including compressive strength, flexural toughness, and crack resistance [4] . A low w/c ratio enhances the densification of the matrix and improves the bond between the steel fibers and the surrounding cement paste, while excessive water can lead to issues like segregation, bleeding, and reduced fiber-matrix interaction Figure 2. In helix fiber-reinforced concrete, the unique twisted geometry of the fibers demands meticulous control over water content to ensure uniform dispersion and avoid fiber clustering, which could compromise load distribution and overall performance [1][10]. Moreover, sufficient water must be available to produce a mix that is workable enough to accommodate the incorporation of steel fibers without compromising their alignment or structural contribution [11] . Proper curing is another critical aspect, as it prevents premature drying, reduces shrinkage, and ensures the long-term hydration of cement, which in turn enhances durability, impact resistance, and fatigue performance [10] . The interplay between water content, fiber volume fraction, and cementitious components must be carefully balanced to achieve an optimized mix design tailored for specific applications, such as high-strength pavements, industrial floors, and precast elements requiring superior toughness and crack control [12].

Figure 2- Fly ash, Helix fibre, Plain steel fibre

Fly ash

Fly ash, a by product of coal combustion in thermal power plants, is often used as a supplementary cementitious material with Ordinary Portland Cement (OPC) 43 grade Figure 1 to enhance the properties of concrete. Incorporating fly ash reduces the heat of hydration, improves workability, and increases long-term strength and durability by refining the pore structure [3] . When combined with steel fibers, both helix-shaped and plain, the mechanical performance of the concrete is further augmented. Helix fibers, due to their spiral geometry, provide superior anchorage and crack bridging, enhancing tensile strength, ductility, and energy absorption capacity Figure 2. Plain steel fibers contribute to crack resistance and improve flexural strength. Together with fly ash, the synergistic effect optimizes cost efficiency, sustainability, and structural performance, making this composite mix ideal for applications requiring high toughness and durability, such as industrial floors, pavements, and precast elements [9].

Plain steel fibers

Plain steel fibers are commonly utilized in concrete to improve its mechanical properties, particularly in applications using Ordinary Portland Cement (OPC) of 43-grade Figure 1. When combined with helix and plain steel fibers, this composite design leverages the tensile strength and crack resistance of steel while enhancing the ductility and toughness of the concrete matrix. Helix steel fibers, characterized by their spiral shape, provide improved anchorage and energy absorption, resulting in better resistance to dynamic and impact loads. In contrast, plain steel fibers, with their straight or slightly deformed geometry, enhance the first crack strength and flexural toughness. The synergistic use of these fibers Figure 2 ensures a uniform stress distribution and mitigates shrinkage-induced cracking [7] . Typically, these fibers are incorporated at dosages between 0.5% to 2% by volume, ensuring proper mixing to prevent fiber clumping and achieve optimal performance. Such combinations are ideal for high-performance concrete applications like industrial flooring, precast structures, and overlays, where durability and structural integrity are paramount [9].

Helix steel fibers

Helix steel fibers, combined with OPC 43-grade cement Figure 1, significantly enhance the mechanical properties and durability of concrete by introducing a multi-dimensional reinforcement system. These helical fibers, often twisted into a corkscrew shape, provide superior tensile strength, ductility, and crack resistance by bridging micro-cracks and delaying their propagation under stress. When paired with plain steel fibers, the hybrid reinforcement system optimizes performance Figure 2 helix fibers contribute to improved energy dissipation, crack control, and impact resistance, while plain fibers enhance flexural strength, load-bearing capacity, and resistance to fatigue [5] . This combination reduces the risk of brittle failure and improves post-cracking behavior, resulting in a ductile and tough composite material [12] . Additionally, this mix minimizes shrinkage cracks, increases bond strength with the cement matrix, and offers enhanced resistance to freeze-thaw cycles and chemical attacks. It is particularly effective in high-performance applications such as industrial flooring, precast elements, earthquake-resistant structures, and tunnel linings, delivering long-term reliability under dynamic, cyclic, and static loading conditions while reducing reliance on traditional reinforcement bars and lowering maintenance costs [7] .

Methodology

Methodology for Preparing Concrete Mix with OPC 43 Grade Cement, Fly Ash, and Plain/Helix Steel Fibers

1. Material Procurement and Inspection

Cement: Obtain OPC 43-grade cement certified to IS 12269. Inspect for lumps or discoloration, which indicate moisture exposure.

Fly Ash: Use fly ash as per IS 3812 (Part 1). Confirm its fineness using a sieve test (90 µm sieve) and check its specific gravity for mix design adjustments.

Aggregates: Test fine aggregates for silt content (limit ≤3% by weight). For coarse aggregates, ensure gradation complies with IS 383, and remove any debris or clay particles.

Fibers:

Plain Steel Fibers: Measure aspect ratio (length/diameter, typically 40–100) and ensure fibers are free from rust or oil.

Helix Steel Fibers: Verify the pitch, diameter, and length of the fibers. Inspect for uniformity and cleanliness.

Water: Ensure water is potable and free from impurities like chlorides or sulfates that may impact hydration.

2. Mix Design

Design the concrete mix using IS 10262 guidelines. Adjust the proportions to include fly ash (15–25% of total cementitious material) Figure 3.

Choose water-to-cementitious material ratio (w/cm) based on the desired strength and workability, typically 0.4–0.5 for OPC 43-grade cement with fly ash [12] .

Determine fiber content:-

For Plain Steel Fibers, use 0.5–1.5% by volume of concrete.

For Helix Steel Fibers, use the same volume percentage, ensuring their helical shape is considered to maintain workability.

Incorporate admixtures like superplasticizers, if required, to counteract any reduction in workability due to fiber addition.

3. Batching and Preparation

Weigh all materials accurately to match the mix proportions. Use a batching plant or manual weighing for smaller batches [7] . Arrange mixing equipment (concrete mixer or batching plant) and ensure it is clean and free of residual materials Figure 3.

4. Mixing Process

Step 1: Dry Mixing

Add OPC 43-grade cement, fly ash, fine aggregates, and coarse aggregates into the mixer.

Dry mix for 1–2 minutes to ensure uniform distribution of the dry components.

Step 2: Water Addition

Gradually introduce water while the mixer rotates. Monitor the mix consistency closely and avoid over-wetting.

For better control, add water in increments, checking the mix visually or through a slump test.

Step 3: Fiber Introduction

For Plain Steel Fibers:

Sprinkle fibers gradually into the mixer to prevent clumping. Mixing time may need to be extended by 2–3 minutes to ensure even dispersion.

For Helix Steel Fibers:

Introduce helix fibers slowly, as their shape can cause tangling if added too quickly. Mix for an additional 3–5 minutes to ensure uniform distribution throughout the matrix.

Step 4: Final Mixing

Allow the mixer to run for a final 2–3 minutes to homogenize the mix. Inspect visually to confirm there are no fiber clumps or segregation [5].

5. Casting

Prepare moulds Figure 3 (e.g., cubes for compressive strength, cylinders for split tensile strength, or beams for flexural strength). Apply a light coating of release agent to facilitate demoulding.

Place the concrete mix into moulds in layers (50 mm at a time). Compact each layer using a needle vibrator or table vibrator to remove air pockets. Avoid over-vibration, as it can lead to fiber segregation or bleeding [9] .

6. Surface Finishing

Once filled, level the top surface of the moulds using a trowel. Avoid disturbing the fibers during this step. Smooth the surface evenly, ensuring there are no protruding fibers.

7. Curing

Cover the filled moulds with a damp cloth or polyethylene sheet to prevent evaporation and allow setting for 24 hours. After demoulding, immerse the specimens in a water curing tank at 27 ± 2°C. Maintain consistent curing conditions for the required duration (7, 14, or 28 days). For site applications, consider spraying water or using curing compounds if immersion curing is impractical.

8. Inspection and Testing

Periodically check the curing specimens for any visible cracks, fiber distribution, or anomalies. Conduct tests at 7, 14, and 28 days to measure compressive strength, split tensile strength, and flexural strength. Compare results between plain steel fibre and helix steel fibre concrete to evaluate their respective performance [12] .

Observations During the Process-

Monitor the mixer for clumping or uneven distribution of fibers, especially with helix fibers.

Ensure proper fiber orientation and bonding with the matrix by observing surface quality after casting.

Adjust admixture dosage as needed to maintain workability, especially for high-fibre-content mixes [11] .

Figure 3- Concrete mix. Cube mould

Testing

Test Results Of Materials

1. Cement

Portland cement Figure 1, specifically Ordinary Portland Cement (OPC) 43 grade, boasting a commendable compressive strength of 43 MPa at the culmination of 28 days, stands as the binding material sourced from ACC Cement. Rigorous assessments were undertaken in accordance with the esteemed Bureau of Indian Standards (BIS) guideline denoted as 8112-1989, with the ensuing outcomes meticulously delineated in Table 1.

2. Fine Aggregate

For the purpose of experimentation, the fine aggregate utilized in the study was sourced from locally available river sand and subjected to testing in accordance with the specifications outlined in BIS: - 383-1970. The testing procedures were conducted as per the guidelines [3] [13] stipulated in IS: 2386-1963. The majority of the fine aggregate particles are able to pass through the IS Sieve 4.75 mm, while being retained on the IS Sieve 75 micron (0.075 mm) within the specified grading zone as detailed in (Table 2) The properties and test outcomes are documented [11].

Table 1- Test Results Of OPC 43 Grade Cement

3. Coarse Aggregate

Crushed angular stone aggregates, boasting a nominal maximum size of 20 mm, are employed in the formulation of concrete mixtures. These aggregates have undergone meticulous testing in accordance with the rigorous standards set forth by the Bureau of Indian Standards, namely BIS: 383-1970 and BIS: 2386-1963. The comprehensive properties and test results are meticulously delineated in the accompanying. These locally sourced aggregates, derived from crushed angular stones, constitute the fundamental coarse aggregate component. Materials that successfully traverse a 75 mm IS Sieve and are retained by a 4.75 mm IS Sieve are classified as coarse aggregates.

Table 2- Specified Grading Zone

Table 3- Test Results Of Fine Aggregate

 4. Workability

Standard: IS 1199: 1959 – Methods of Sampling and Analysis of Concrete

Theory: -

Workability refers to the ease with which concrete can be mixed, transported, placed, and compacted without segregation. The presence of steel helix fibers typically reduces workability due to increased inter-particle friction between the fibers and the concrete matrix [1] . As a result, slump values tend to decrease with increasing fiber content.

However, the fibers improve cohesion and help control bleeding and segregation in the concrete mix. In some cases, workability can be adjusted by using superplasticizers or other admixtures.

A lower slump does not necessarily indicate poor workability for fiber-reinforced concrete, especially if the fibers are improving other properties like toughness and crack resistance.

Test Procedure:

1. Sample Preparation:

Mix the fiber-reinforced concrete thoroughly to ensure an even distribution of fibers.

2. Slump Cone Test:

Fill the slump cone in three layers, compacting each layer with 25 strokes using a tamping rod.

Remove the cone and measure the slump (the reduction in height of the concrete) in millimeters.

3. Observation:

Record the slump value.

4. Alternate Workability Tests:

For stiffer fiber-reinforced concrete mixes where the slump test may not be effective, use other tests like:

Vee bee Test: Measures the time required to achieve full compaction of the concrete.

Compaction Factor Test: Measures the degree of compaction achieved for the same effort.

Analysis Of Result

The slump test results for concrete containing fibers typically show a decrease in workability as the fibre content increases. Plain Steel Fibers: These fibers cause a larger drop in slump because of their higher aspect ratio, larger size, and tendency to interlock. For example, a concrete mix with a 100 mm slump could drop to 30-50 mm after adding 1% by volume of plain steel fibers. Steel Helix Micro fibers. These fibers cause a smaller decrease in slump, often retaining more of the mix’s original workability. A mix with an initial 100 mm slump may reduce to around 60-80 mm after adding 0.5-1% helix microfibers, showing better workability than the mix with plain steel fibers.

5. Compressive Strength Test

Standard: IS 516: 1959 – Method of Tests for Strength of Concrete

Theory: -

Compressive strength is the most critical property of concrete since it primarily resists compressive loads in structures. Concrete typically exhibits high compressive strength but is prone to brittle failure without any post-cracking strength(Figure 5- Compressive strength test, Curing, Flexural strength test).

Steel helix fibers help improve the concrete's ductility by resisting crack propagation after initial micro-cracks form. These fibers bridge the cracks, thereby preventing the immediate loss of load-bearing capacity and providing residual strength even after cracking occurs.

The addition of fibers typically results in better toughness and a more ductile failure mode, allowing the structure to absorb more energy before collapsing  [11].

Test Procedure:-

Mix Design and Preparation:

Mix concrete according to standard proportions, incorporating steel helix fibers (typically between 0.5% to 2% by volume of the concrete mix).

Properly mix the fibers into the concrete to avoid fiber balling or clumping, ensuring uniform dispersion.

Casting:

Cast cube molds of dimensions 150 mm × 150 mm × 150 mm.

Ensure proper compaction by vibration to eliminate air voids, which can affect the test results [5] .

Curing:-

Submerge the specimens in water at 27 ± 2°C for 7, 14, or 28 days, depending on the testing requirement.

Testing:

Place the cube specimen between the loading plates of a compression testing machine.

Apply a uniform load at a rate of 140 kg/cm² per minute until the specimen fails.

Result:

The compressive strength of SHFRC may slightly improve compared to plain concrete due to crack arresting behavior, but the most significant improvement is observed in the post-cracking performance [7] .

Compressive Strength = Load / Cross-sectional Area

Table 4- No. Of Sampling

Table 5-% Of Crushing Strength at Days

Analysis Of Result

The study examines the effect of varying percentages of Plain steel fibers and Steel Helix Micro Fibers on compressive strength. Plain steel fibers was introduced at rates of 0.5%, 1.0%, 1.5%, and 2.0%, revealing notably heightened compressive strength at the 1.0% mark. Specifically, compressive strength at 1.0% for 7 days registered at 20.2 N/mm²escalating to 28 N/mm²by day 28. Conversely, the investigation also incorporated Steel Helix Micro Fibers in identical percentages, demonstrating analogous trends. Notably, the compressive strength peaked at 1.0%, with values of 21.25 N/mm²after 7 days and 29.5 N/mm²after 28 days. These findings were obtained through compression testing, employing a loading rate of 14 N/mm²per minute.

6. Flexural Strength Test

Standard: IS 516: 1959 – Method of Tests for Strength of Concrete

Theory:-

Flexural strength Figure 5 (also called the modulus of rupture) is crucial for concrete used in structures that undergo bending, such as beams, slabs, and pavements. Regular concrete fails in a brittle manner under tensile stress due to its low tensile strength. Steel helix fibers help improve the tensile capacity of concrete, enabling it to resist bending stresses and delaying the formation and growth of cracks. In the flexural test, the fibers distribute tensile stress across the matrix and bridge cracks that develop during loading [11] . This results in improved ductility and toughness. Post-cracking, the fibers carry a portion of the load, increasing the load-bearing capacity after initial cracking, which is essential for structural integrity under flexural loads  [14].

Test Procedure: -

Mix Design:

Prepare concrete with steel helix fibers incorporated into the mix. The percentage of fibers should be adjusted based on the desired performance, typically between 0.5% to 2% by volume  [14].

Casting:

Cast beam specimens of size 150 mm × 150 mm × 700 mm or 100 mm × 100 mm × 500 mm.

Compact the beam molds to ensure proper consolidation of the mix.

Curing:

Cure the beams by immersing them in water at 27 ± 2°C for 28 days.

Testing:

Place the beam specimen on two supports in a third-point loading configuration, which ensures a uniform distribution of stresses along the span.

Apply load at the one-third points of the beam span until failure occurs [7] .

Observation: -

Record the maximum load PPP at failure and calculate the flexural strength using the formula:-

The Flexural Strength or modulus of rupture (f_b) is given by

f_b = pl/bd² (when a > 20.0cm for 15.0cm specimen or > 13.0cm for 10cm specimen)

or

f_b = 3pa/bd² (when a < 20.0cm but > 17.0 for 15.0cm specimen or < 13.3 cm but > 11.0cm for 10.0cm specimen.)

Where,

a = the distance between the line of fracture and the nearer support, measured on the center line of the tensile side of the specimen

b = width of specimen (cm)

d = failure point depth (cm)

l = supported length (cm)

p = max. Load (kg)

Analysis Of Result

Delineates the correlation between flexural strength and the proportion of Plain steel fibers incorporated at varying rates: 0.5%, 1.0%, 1.5%, and 2.0%. Remarkably, the apex of flexural strength manifests at the 1.0% inclusion level. Specifically, after 7 days, the flexural strength registers at 1.43 N/mm², escalating to 3.1 N/mm² after 28 days.

Similarly, Table 12 elucidates the interplay between flexural strength and the percentage of Steel Helix Micro Fibers introduced across analogous increments: 0.5%, 1.0%, 1.5%, and 2.0%. Once again, the pinnacle of flexural strength emerges at the 1.0% dosage. Notably, after 7 days, the flexural strength stands at 2.1 N/mm², advancing to 3.8 N/mm² after 28 days.

Subsequently, the specimens undergo compression testing under a loading rate of 14 N/mm² per minute. Flexural strength increases due to the bridging effect of the fibers, which leads to delayed crack propagation and greater energy absorption capacity [7] .

Figure 5- Compressive strength test, Curing, Flexural strength test

7. Splitting Tensile Strength Test

Standard: IS 5816: 1999 – Method of Test for Splitting Tensile Strength of Concrete

Theory:

Concrete has low tensile strength compared to its compressive strength, which makes it vulnerable to cracking under tension. The splitting tensile test is an indirect way to determine tensile strength. Steel helix fibers improve the tensile behaviour of concrete by enhancing its crack resistance. Fibers act as a bridging mechanism that prevents the propagation of cracks, thereby improving the fracture toughness and tensile capacity of concrete. The fibers resist separation of the concrete matrix by transferring loads across cracks, leading to a more gradual failure instead of the brittle failure seen in plain concrete [1] . By improving the tensile strength, fibers help concrete better withstand dynamic loads, thermal stresses, and shrinkage cracking  [8].

Test Procedure:

Sample Preparation:

Cast cylindrical specimens of size 150 mm diameter × 300 mm height.

Ensure proper fiber distribution during the mixing process to prevent clumping.

Curing: Cure the cylinders in water at 27 ± 2°C for 28 days.

Testing: Place the cylindrical specimen horizontally in a splitting tensile testing machine.

Apply the load along the length of the cylinder at a uniform rate until failure occurs along the diameter.

Observation:

Record the failure load PPP.

Calculation:

Calculate the splitting tensile strength using the formula:-

Calculate the splitting tensile strength of the specimen as follows: T= 2P/ pi LD Where: T = Splitting tensile strength, MPa P: Maximum applied load indicated by the testing machine, N D: Diameter of the specimen, mm L: Length of the specimen, mm

Analysis Of Result

Illustrates the correlation between the percentage of flexural strength and the proportion of Plain steel fibers incorporated at varying rates: 0.5%, 1.0%, 1.5%, and 2.0%. Notably, the apex of flexural strength manifests at 1.0%, boasting a commendable 2.8 N/mm² after 7 days, escalating to 3.9 N/mm² after 28 days. Conversely, Table 14 delineates akin findings pertaining to the percentage of flexural strength vis-à-vis the inclusion of Steel Helix Micro Fibers at identical rates. Evidently, the zenith of flexural strength resides at 1.0%, registering a noteworthy 3.15 N/mm² after 7 days, progressing to 4.2 N/mm² after 28 days. Following the experimentation phase, the specimens are subjected to rigorous compression testing, performed under controlled conditions employing a loading rate of 14 N/mm² per minute. Following this, the specimen undergoes compression testing at a loading rate of 14 N/mm² per minute. Steel helix fibers contribute to improved tensile strength due to their crack-bridging ability. This helps in delaying crack initiation and ensuring more gradual crack growth.

8. Rebound Hammer Test

The Rebound Hammer Test is a non-destructive testing (NDT) method used to assess the surface hardness of concrete, which can be correlated to its compressive strength [10] . The test is carried out in accordance with IS 13311 (Part 2): 1992 – Non-destructive Testing of Concrete – Methods of Test – Part 2: Rebound Hammer Table 6- Comparison of Rebound Test Results [8].

Here is an outline of the test procedure and typical results for the rebound number, which reflects the surface hardness and strength, for both Plain Steel Fiber Concrete and Steel Helix Microfiber Concrete of M20 Grade [7].

M20 Grade Concrete Mix:

Mix Proportion: 1:1.5:3 (Cement: Fine aggregate: Coarse aggregate) with a water-cement ratio of approximately 0.5, as per IS 10262:2019.

Target Compressive Strength (fck): 20 MPa (strength at 28 days).

Fiber Addition:

Plain Steel Fibers: Typically 1-2% by volume of concrete.

Steel Helix Microfibers: Typically 0.5-1% by volume of concrete.

The type and content of fibers influence the surface hardness by improving crack resistance and bonding within the concrete matrix.

Rebound Hammer Test Setup (As per IS 13311)

a) Test Equipment:

Rebound Hammer: A spring-loaded device that measures the surface hardness by recording the hammer’s rebound after striking the concrete surface.

b) Test Procedure:

Surface Preparation: Ensure the surface is clean, dry, and free of any loose debris.

Test Points: Choose at least 10 evenly spaced test points on each concrete sample.

Positioning: Hold the hammer perpendicular to the surface.

Rebound Reading: Record the rebound number at each point.

Average Rebound Number: Calculate the average rebound number, which can then be linked to compressive strength using a correlation curve from the manufacturer or IS 13311.

c) Interpretation of Rebound Number:

Higher rebound numbers suggest greater surface hardness and higher compressive strength.

Use the rebound number correlation with compressive strength through a calibration curve.

Analysis Of Result

Test Results for Plain Steel Fiber and Steel Helix Microfiber Concrete

a) Plain Steel Fiber Concrete: Plain steel fibers improve the concrete's resistance to cracking, leading to a higher surface hardness. The average rebound number of 26 indicates a surface hardness that correlates well with the target compressive strength of M20 concrete. The presence of steel fibers has moderately enhanced the surface hardness due to the crack-bridging effect, improving the concrete's post-crack performance [9].

Observed Rebound Numbers (Hypothetical for M20 Concrete): Range: 24 to 28 (indicating compressive strength between 20 to 24 MPa).

Average Rebound Number: 26.

Compressive Strength Estimate: An average rebound number of 26 correlates to an estimated compressive strength of about 22 MPa using the calibration curve.

b) Steel Helix Microfiber Concrete: Steel helix microfibers provide enhanced crack resistance and energy absorption, slightly improving the rebound number. The average rebound number of 28 indicates a slightly higher surface hardness compared to plain steel fiber concrete [7] . The helical shape of the fibers provides better bonding with the concrete matrix, leading to enhanced surface hardness and an estimated compressive strength of 24 MPa. The improved load distribution and crack resistance due to the helical fibers increase the overall strength and performance under loading [3].

Observed Rebound Numbers (Hypothetical for M20 Concrete): Range: 26 to 30 (indicating compressive strength between 22 to 26 MPa).

Average Rebound Number: 28.

Compressive Strength Estimate: An average rebound number of 28 corresponds to an estimated compressive strength of approximately 24 MPa

Table 6- Comparison of Rebound Test Results

Steel Helix Microfiber Concrete shows superior surface hardness and slightly higher compressive strength compared to Plain Steel Fiber Concrete, as indicated by the rebound hammer test results.Both fiber types improve the concrete's performance beyond traditional M20 concrete, but the helical shape of the steel microfibers allows for better anchoring, resulting in a greater increase in the surface hardness and overall strength.

Limitations of the Rebound Hammer Test:

• It provides an estimate of surface hardness and not the exact compressive strength.
• It is influenced by surface conditions, aggregate type, and moisture levels.
• Should be used in conjunction with other tests (e.g., core testing) for a more accurate assessment of concrete strength.

The Rebound Hammer Test is a quick and convenient method for on-site evaluation of concrete hardness and can be an effective tool in assessing the uniformity and quality of concrete in existing structures [9] .

Comparing Plain Steel Fibers With Helix Micro Rebars

Comparing plain steel fibers with helix micro rebars involves examining their respective characteristics, applications, and the advantages they offer in reinforcing concrete structures. The below table shows us the various differences amongst helix steel fibers and plain steel fibers [10] .Top of Form

Table 7- Comparing plain steel fibers with helix micro rebars

Conclusion

The integration of fibers into concrete, especially Steel Helix Microfibers (SHMF) and Plain Steel Fibers (PSF), showcases remarkable enhancements in mechanical properties such as tensile strength, flexural strength, and splitting tensile strength, as demonstrated by extensive experimental analysis. Detailed testing, including assessments of compressive strength, flexural strength, and splitting tensile strength, clearly indicates that the inclusion of fibers significantly boosts the durability, toughness, and crack resistance of concrete.

Performance Optimization

Empirical data reveals that the optimal fiber content ranges between 0.5% and 1.0%, with peak performance observed at a 1.0% fiber addition. Specifically, SHMF outperformed PSF in strength enhancement, exhibiting superior compressive strength (29.5 N/mm² after 28 days) and flexural strength (3.8 N/mm² after 28 days) at the 1.0% fiber dosage.

Crack Control and Durability

Addressing the inherent weaknesses of conventional concrete in tensile strength and crack propagation, FRC mitigates cracking issues caused by shrinkage, temperature variations, and mechanical loads. SHMF, with its unique helical structure, provides superior bonding within the concrete matrix, further controlling crack growth and improving post-crack load-bearing capacity.

Workability and Mix Design

While fibers enhance mechanical performance, they also reduce workability due to increased friction between particles. This issue can be managed with the use of superplasticizers, which help maintain a balance between workability and performance.

Environmental and Economic Implications

From an environmental perspective, the use of fibers, particularly SHMF, not only extends the lifespan of concrete structures but also reduces the need for frequent repairs and reconstruction, thereby minimizing resource consumption and carbon emissions associated with frequent concrete production.

Long-Term Durability and Lifecycle Performance

The addition of fibers to concrete significantly enhances its long-term durability, particularly in transportation infrastructure where structures are exposed to environmental stressors such as freeze-thaw cycles, de-icing salts, and chemical attacks.

Energy Absorption and Seismic Resilience

In transportation engineering, structures must be designed to absorb and dissipate energy, particularly in regions prone to seismic activity or high-impact events. FRC, especially with SHMF, improves the energy absorption capability of concrete by providing greater ductility and toughness.

Potential for Reducing Reinforcement Requirements

Fiber Reinforced Concrete has shown potential in reducing the reliance on conventional steel reinforcement in certain applications. While FRC may not entirely replace rebar in high-flexural-load structures like large-span bridges, the enhanced tensile and flexural properties provided by fibers can reduce the overall amount of steel required, thus lowering material costs and simplifying construction logistics.

Sustainability and Material Efficiency

The use of FRC contributes to sustainable construction practices by improving material efficiency. By enhancing the mechanical properties of concrete, fibers allow for thinner sections and reduced material usage without compromising structural performance. Additionally, innovations in fiber materials, such as the use of recycled fibers or bio-based alternatives, further align FRC with sustainable development goals​

Final Remarks

The findings strongly suggest that fiber reinforcement, particularly with SHMF, can potentially replace traditional concrete in transportation applications due to its superior strength and durability characteristics. However, careful consideration must be given to the precise mix design and fiber dosage, as excessive fiber content can compromise workability and may increase the risk of permeability. The overall sustainability and economic benefits, coupled with enhanced performance in dynamic and static load resistance, make FRC an ideal candidate for modern transportation engineering projects

References 

[1]      F. Althoey, O. Zaid, A. Majdi, F. Alsharari, S. Alsulamy, and M. M. Arbili, “Effect of fly ash and waste glass powder as a fractional substitute on the performance of natural fibers reinforced concrete,” Ain Shams Eng. J., vol. 14, no. 12, p. 102247, 2023, doi: 10.1016/j.asej.2023.102247.

[2]      R. Chakraborty, T. Paul, and A. B. S. Ornob, “A Review on Tensile Behavior of Different Kinds of Fiber Reinforced Concrete,” Int. Exch. Innov. Conf. Eng. Sci., pp. 231–237, 2020, doi: 10.5109/4102495.

[3]      V. Suhasini, N. Sahu, and S. Vishwakarma, “Innovative Approaches to Sustainable Concrete : Boosting Structural Integrity with Crushed Glass Fibbers and PPC,” vol. 11, no. 11, 2024.

[4]      K. Vardhan, S. Goyal, R. Siddique, and M. Singh, “Mechanical properties and microstructural analysis of cement mortar incorporating marble powder as partial replacement of cement,” Constr. Build. Mater., vol. 96, pp. 615–621, 2015, doi: 10.1016/j.conbuildmat.2015.08.071.

[5]      S. Dutta, “Experimental Investigation on Fibre Reinforced Concrete using Binding Wires,” vol. 8, no. 04, pp. 452–457, 2019, [Online]. Available: www.ijert.org

[6]      M. Chandra, P. Modassar, and S. Vi, “A Critical Study of Composite Fibre Reinforced Concrete ( CFRC ) with the Partial Replacement of Cement by FlyAsh,” vol. 3, no. 09, pp. 112–115, 2015.

[7]      E. Arunakanthi and J. D. C. Kumar, “Experimental studies on fiber reinforced Concrete (FRC),” Int. J. Civ. Eng. Technol., vol. 7, no. 5, pp. 329–336, 2016, doi: 10.9790/9622-0705024044.

[8]      “Workable and Robust Concrete Using High Volume,” vol. C, pp. 537–548, 2018.

[9]      R. Siddique, G. Kaur, and Kunal, “Strength and permeation properties of self-compacting concrete containing fly ash and hooked steel fibres,” Constr. Build. Mater., vol. 103, pp. 15–22, 2016, doi: 10.1016/j.conbuildmat.2015.11.044.

[10]    “Journal of technology,” Nature, vol. 178, no. 4539, p. 894, 1956, doi: 10.1038/178894d0.

[11]    K. Vijay, M. Murmu, and S. V. Deo, “Bacteria based self healing concrete – A review,” Constr. Build. Mater., vol. 152, pp. 1008–1014, 2017, doi: 10.1016/j.conbuildmat.2017.07.040.

[12]    G. Singh and R. Siddique, “Effect of iron slag as partial replacement of fine aggregates on the durability characteristics of self-compacting concrete,” Constr. Build. Mater., vol. 128, pp. 88–95, 2016, doi: 10.1016/j.conbuildmat.2016.10.074.

[13]    M. Lin et al., “Combined effects of expansive agents and glass fibres on the fracture performance of seawater and sea-sand concrete,” J. Mater. Res. Technol., vol. 20, pp. 1839–1859, 2022, doi: 10.1016/j.jmrt.2022.08.019.

[14]    R. H. Faraj, H. F. Hama Ali, A. F. H. Sherwani, B. R. Hassan, and H. Karim, “Use of recycled plastic in self-compacting concrete: A comprehensive review on fresh and mechanical properties,” J. Build. Eng., vol. 30, no. December 2019, p. 101283, 2020, doi: 10.1016/j.jobe.2020.101283.