Untitled I STUDY ON EFFECTS OF FLOWABILITY ON STEEL FIBER DISTRIBUTION PATTERNS AND MECHANICAL PROPERTIES OF SFRC A thesis submitted in fulfilment of the requirements for the degree of Master of Engin[.]
INTRODUCTION
General
Concrete is a vital construction material composed of crushed stone or gravel, sand, and other aggregates bonded by cement hydration, making it essential for building structures, bridges, roads, and hydraulic and harbor engineering projects It offers excellent compressive strength, durability, fire resistance, and ease of shaping, often utilizing local resources for cost-effective construction Despite these advantages, concrete's drawbacks include its high self-weight, lengthy curing time to achieve desired strength, and susceptibility to cracking Over the years, the development of concrete technology has seen significant advancements, enhancing its performance and expanding its applications.
Steel fiber reinforced concrete (SFRC), developed in the early 1970s, is a composite material where short steel fibers are randomly dispersed within the concrete matrix to enhance its mechanical properties Originally designed to address low tensile strength and poor ductility, SFRC utilizes steel fibers to restrain micro-crack development, reduce crack growth, and improve overall durability By increasing tensile strength, SFRC also enhances critical properties such as shear strength, flexural strength, and cracking resistance, leading to better performance under various loading conditions Consequently, SFRC structures demonstrate improved resilience against shear, flexural, punching, impact loads, as well as fatigue and complex recycled actions, making them a versatile choice in modern construction.
Steel fibers in concrete significantly enhance mechanical properties and durability when properly proportioned and produced They improve conventional characteristics such as compressive strength, modulus of elasticity, fracture resistance, shear resistance, tensile strength, and flexural strength Additionally, steel fibers increase energy absorption, toughness, peak strain, and residual tensile strain under compression, especially in high-strength concrete SFRC can achieve multiple cracking and strain hardening before failure under tension and multi-axial loads, involving complex interactions between fibers and the concrete matrix The presence of steel fibers boosts flexural toughness, residual tensile strains post-peak stress, fracture energy, and post-cracking resistance by bridging cracks and restraining crack growth They also reduce moisture diffusion and drying shrinkage within the concrete Furthermore, combining steel fibers with polypropylene fibers can enhance residual mechanical properties and high-temperature resistance, making SFRC a durable and resilient construction material.
Steel fibers are the key reinforcement material in Steel Fiber Reinforced Concrete (SFRC), distributed randomly within the concrete matrix According to ACI A820 [16], four main types of steel fibers are identified based on their source materials: cold drawn wire (Type I), cut sheet (Type II), melt-extracted (Type III), and other fibers (Type IV) These fibers can be straight or deformed, with a tensile strength exceeding 345 MPa, and their dimensions are specified in terms of equivalent diameter and length Standard test methods, such as ASTM C 1517-03, are used to determine fibers' tensile strength and Young’s modulus The most basic steel fibers are straight and made from smooth wire, but they lack sufficient anchorage in concrete, leading to suboptimal utilization of steel strength Advances in production technology have led to over 90% of fibers being deformed, featuring shapes like flattened, spaded, coned, twisted, crimped, hooked, and surface-textured, with various cross-sections including circular, square, rectangular, or irregular These fibers are typically produced as cold-drawn wire with hooked or enlarged ends, or as milling-type with deformed shapes, designed to improve bonding with concrete The shape and surface modifications enhance the fiber's anchorage effect, with different designs offering varying levels of reinforcement efficiency When used in high-strength concrete, it is essential to match the tensile strength of steel fibers with the concrete's strength, leading to the production of fibers with graded strength specifications [9, 10].
Fig 1-1 Steel Fiber: Cold-Drawn Wire with Hooked Ends
Fig 1-2 Steel fiber: Cut Sheet Type with Enlarged Ends (left) or Indentations (right)
Fig 1-3 Steel Fiber: Milling Type with Deformed Shape
The orientation of steel fibers in concrete matrix is influenced by various factors, including the concrete matrix, steel fiber characteristics, volume fraction, workability of fresh concrete, casting methods, and boundary conditions, with the concrete matrix, workability, and fiber properties being the most critical (references [19], [18]) While many studies have examined steel fiber distribution patterns and their impact on the mechanical properties of SFRC, there is limited research on how concrete flowability affects fiber orientation (references [20-23], [1], [22-26], [27], [28]) Due to the low flowability of plastic concrete, the mixing procedure plays a key role in controlling fiber distribution, with proper mixing ensuring fibers are randomly oriented within the fresh concrete (references [27], [29]).
The introduction of superplasticizer into concrete has facilitated the widespread use of long-distance transportation of premixed concrete, promoting sustainable construction practices Due to its self-compacting nature, fresh premixed concrete affects the distribution of steel fibers, which differs from traditional plastic concrete Increased flowability in fresh concrete causes the steel fiber distribution to become more uneven, with higher fiber concentration at the bottom and lower at the top; consequently, higher flowability correlates with reduced uniformity of steel fibers within the SFRC cross-section This variation in fiber distribution can influence the mechanical properties and performance of hardened concrete, making it essential to study how flowability impacts the overall properties of SFRC to optimize its use in construction.
This study investigates the impact of flowability on various properties of Steel Fiber Reinforced Concrete (SFRC), including fiber distribution rate, distribution coefficient, and fiber orientation coefficient It examines how these factors influence key mechanical properties such as compressive strength, splitting tensile strength, flexural strength, flexural toughness, and fracture energy The research also presents test mechanisms for accurately determining the strengths of high-flow SFRC and provides recommended equations for calculating bending stiffness and flexural stress based on steel fiber distribution.
Research Objectives
This research aims to investigate how the flowability of fresh Steel Fiber Reinforced Concrete (SFRC) influences steel fiber distribution patterns and the resulting mechanical properties It seeks to establish clear relationships between fiber distribution and mechanical performance to enable accurate prediction of the flexural behavior of flowable and self-compacting SFRC The findings will offer valuable technical insights to optimize the design and construction of flowable/self-compacting SFRC, ensuring improved structural performance and durability.
The specific objectives of this research are:
(1) To compare differences of steel fiber distribution patterns of SFRC with different flowability;
(2) To test basic mechanical properties such as compressive strength, splitting tensile strength and flexural strength of SFRC with different flowability;
(3) To assess influence of flowability on the basic mechanical properties of SFRC;
(4) To discuss relationships between steel fiber distribution patterns and mechanical properties of SFRC;
(5) To analyse mechanism causing the differences of mechanical properties of different flowability SFRC;
(6) To propose formulas for calculating the modulus of elasticity, moment of inertia, bending stiffness, moment of elastic resistance and flexural stress of flowable SFRC.
Thesis Arrangement
The thesis is organized into six chapters A brief description of each chapter is given below
This chapter offers a concise overview of prior research on Steel Fiber Reinforced Concrete (SFRC), highlighting the importance and necessity of conducting this study It clearly outlines the primary objectives aimed at advancing understanding in this field Additionally, the chapter introduces the overall structure of the thesis, guiding readers through the subsequent sections.
This chapter offers a comprehensive literature review of previous research on steel fiber distribution patterns and the mechanical properties of Steel Fiber-Reinforced Concrete (SFRC) It highlights key findings related to how steel fiber placement influences the material's strength and durability The chapter also identifies ongoing challenges, particularly the impact of flowability on SFRC performance, emphasizing the need for further investigation Additionally, it outlines specific research questions and assumptions to guide future studies, aiming to enhance understanding and optimize SFRC applications.
This chapter details the comprehensive experimental design and mechanical property testing of steel fiber-reinforced concrete (SFRC) It covers the entire process, including raw material selection and testing, concrete mix design, specimen preparation and curing, and the cutting methods used for steel fiber distribution pattern analysis The chapter also presents key test procedures such as compressive strength testing, splitting tensile tests in three directions, and flexural toughness evaluations of SFRC specimens.
Chapter 4 Evaluation of Steel Fiber Distribution Patterns
This chapter analyzes the impact of flowability on steel fiber distribution patterns in SFRC by calculating three key factors: distribution rate, distribution coefficient, and orientation coefficient The results are systematically plotted, compared, and evaluated to understand how flowability influences fiber dispersion, providing insights into optimizing material performance and ensuring uniform fiber distribution in reinforced concrete composites.
Chapter 5 Mechanical Properties of SFRC Correlating with Steel Fiber Distribution Patterns
This chapter summarizes the mechanical properties test results, including the analysis of load-deflection curves for flexural strength It employs two standard methods—ASTM C1018 and Chinese Standard JG/T472-2015—to assess the flexural toughness of steel fiber-reinforced concrete (SFRC) Fracture energy is calculated to evaluate material performance, and new formulas for calculating the moment of inertia and flexural stress of flowable SFRC are proposed.
This chapter provides conclusions of this research project and recommendations for future research.
LITERATURE REVIEW
General
SFRC (Steel Fiber Reinforced Concrete) is gaining widespread attention due to its superior crack control, enhanced shear and flexural performance, and increased earthquake resistance Its innovative properties make it an ideal choice for structural applications requiring durability and safety As a result, researchers worldwide are increasingly focused on exploring the benefits and potential uses of this advanced composite material.
Previous studies have studied several factors influencing steel fiber distribution
The distribution of steel fibers plays a crucial role in determining the mechanical performance of Steel Fiber Reinforced Concrete (SFRC) Altering the way steel fibers are distributed within the concrete matrix can significantly impact its mechanical properties Understanding how to modify steel fiber distribution is essential for optimizing SFRC performance in engineering applications Controlling fiber distribution during the design process allows engineers to enhance the structural properties and ensure the desired performance of SFRC in various constructions.
Given the widespread use of premixed and flowable concrete in modern construction, it is essential to investigate how flowability influences the distribution patterns of steel fibers and the resulting mechanical properties of Steel Fiber Reinforced Concrete (SFRC) Understanding these effects can improve material performance and optimize construction practices.
This chapter reviews previous research on steel fiber distribution patterns and the mechanical properties of Steel Fiber Reinforced Concrete (SFRC) It highlights unresolved issues related to how flowability influences the performance of SFRC Additionally, the chapter identifies key research questions and hypotheses to address these challenges.
Factors Influencing Steel Fiber Distribution Patterns
Cement paste is essential for coating steel fibers to ensure a strong bond between the fibers and the concrete matrix The distribution of steel fibers within the concrete is significantly influenced by the coarse aggregate, which acts as an internal framework, promoting uniform fiber dispersion and enhancing the overall structural integrity However, the particle grading of coarse aggregate impacts the uniformity of fiber distribution, as random placement occurs in the voids between aggregates If the coarse aggregate particles are too large, steel fibers struggle to move and rotate freely during mixing, which can compromise the reinforcement quality To optimize fiber distribution, the maximum size of coarse aggregate should be appropriately matched to the length of the steel fibers, ensuring effective integration within the concrete.
Steel fiber characteristics such as length, sectional dimension, shape, and surface configuration are crucial for performance The equivalent diameter is used to represent non-circular or irregular cross sections of steel fibers The fiber length influences its bridging capacity among coarse aggregates and impacts the cohesiveness of fresh concrete, while the sectional dimension determines stiffness and prevents bending or clumping, affecting fiber flow in the mix For optimal orientation and minimal bending in concrete, a fiber length of 20-60 mm with an aspect ratio of 30-100 is recommended for SFRC design Additionally, in plastic fresh concrete, steel fiber length should be at least 4/3 of the maximum aggregate size to ensure effective post-cracking performance.
Steel fibers are manufactured in various shapes, including hooked ends, enlarged ends, and crimped designs, to improve their anchorage within concrete Surface configurations such as impressions, roughing, and warping are also produced to enhance bonding with cement paste These shape and surface modifications increase cohesiveness with cement, but they also affect the orientation of steel fibers within the concrete matrix Additionally, when steel fiber density exceeds that of fine aggregate and cement paste, the fibers tend to sediment toward the lower layers of the concrete section, impacting overall performance and durability.
2.2.3 Volume Fraction of Steel Fiber
The strength of Steel Fiber Reinforced Concrete (SFRC) is influenced by the volume fraction of steel fibers, with lower concentrations providing limited reinforcement due to insufficient fiber distribution Conversely, excessively high fiber volumes promote fiber clustering, which negatively impacts the material's properties An optimal range exists, typically between 0.75% and 1.5%, where steel fibers distribute randomly, maximizing reinforcement effectiveness; this range may vary depending on the concrete matrix and fiber type, with a broader suggested range from 0.5% to 2.0% [24, 27].
Workability of fresh concrete depends on three key parameters: flowability, cohesiveness, and water retention When cohesiveness and water retention are maintained, flowability significantly influences the distribution pattern of steel fibers within the mix Proper mixing procedures enable steel fibers to distribute randomly and orient along the flow direction in fresh concrete Increased flowability causes steel fibers to align horizontally along the flow, resulting in a higher concentration of fibers from the top to the bottom layer of steel fiber reinforced concrete (SFRC) However, higher flowability can reduce uniformity across the section compared to lower flowability mixes Additionally, sedimentation of coarse aggregates can drive steel fibers to settle toward the lower layer of the section, affecting overall fiber distribution within SFRC.
Compaction methods such as table vibration, hand tamping and internal vibration have considerable influences on the distribution of the fibers in common concrete [28]
Table vibration increases the tendency of fibers to orient horizontally, with hand tamping causing the least non-uniformity in fiber distribution, while internal vibration can lead to uneven fiber orientation as steel fibers tend to align along the vibrator's axis During concrete vibration, coarse aggregate sediments towards the bottom layer while sand and cement paste rise to the top; this sedimentation process influences the distribution of steel fibers, causing them to settle with the coarse aggregate in the mid-section or on the top layer To achieve uniform Steel Fiber Reinforced Concrete (SFRC), it is essential to optimize vibration time—reducing it as flowability increases to prevent segregation and fiber layering.
The steel fibers in self-compacting SFRC tend to orientate in a horizontal plane, the casting and compaction processes can significantly affect the distribution and orientation of steel fibers [32]
When SFRC mix is cast into a formwork, the boundary constraints significantly influence its formation and mechanical properties In small-scale elements with plastic fresh concrete, the movement and orientation of steel fibers are restricted by coarse aggregates and the formwork interface, causing fibers to preferentially distribute near the formwork surface This boundary condition thus affects both the fundamental mechanical properties and the overall quality assessment of SFRC.
Self-compacting fiber-reinforced concrete (SFRC) experiences disturbed steel fiber orientation and dispersion near rough formwork surfaces due to reduced flow rate, leading to increased randomness within a zone approximately half the fiber length Nonetheless, this localized phenomenon has minimal impact on the overall fiber distribution in larger elements Additionally, rigid formwork surfaces can cause non-random disturbances in fiber dispersion, potentially affecting the mechanical properties of large SFRC elements.
Description of Steel Fiber Distribution in SFRC
Three concepts are commonly used to describe the distribution of steel fibers in the aim section [28, 29, 37]
2.3.1 Distribution Rate/concentration of Steel Fiber
This factor reflects the number of steel fibers per unit sectional area, and is defined as where, n i = the number of steel fibers in ith region of aim section,
A i = the area of ith region,
A f1 = the sectional area of single steel fiber across the aim section
2.3.2 Distribution Coefficient/uniformly Distributed Variable of Steel Fiber
This factor reflects the uniformity of steel fiber distributed in the aim section, which is expressed as
−ϕ(x) = √ ∑(𝑛 𝑚 i −𝜇) 2 /𝜇 (3) where m = the number of regions of aim section,
= the average of number of steel fibers in m regions
The value of ranges from 0 to 1, the larger the distribution coefficient, the better the uniformity is
2.3.3 Orientation Coefficient of Steel Fiber
This factor reflects the ratio of steel fiber in different aim sections, which is expressed as
x = the distribution rate of steel fibers across aim section x,
y = the distribution rate of steel fibers across aim section y
Only when steel fibers are distributed along the direction of tensile stress in the concrete matrix, can they provide crack-bridging effects in hardened concrete.
Relationship between Steel Fiber Distribution Patterns and Mechanical
Proper fiber distribution patterns within the concrete matrix are essential for achieving the desired mechanical properties of Steel Fiber Reinforced Concrete (SFRC) An ideal steel fiber distribution should align with the stress characteristics of structural elements like beams, slabs, columns, and shear walls This ensures that steel fibers effectively enhance load resistance, providing optimal structural performance and durability.
In designing SFRC beams, it is essential to strategically distribute steel fibers to optimize both flexural and shear resistance Steel fibers should be placed primarily in the lower section of the beam, oriented parallel to the direction of flexural stress, to enhance flexural capacity Conversely, for shear resistance, steel fibers must be uniformly distributed throughout the beam with an orientation perpendicular to the shear stress direction Proper fiber placement and orientation are crucial for maximizing the structural performance of SFRC beams.
When designing an SFRC floor slab, it is essential to focus on its flexural resistance Steel fibers should be evenly distributed in the lower section of the slab and oriented parallel to the direction of flexural stress to optimize structural performance.
In SFRC column design, steel fibers are not required when flexural resistance is not a concern However, in earthquake-resistant structures, vertically oriented steel fibers play a crucial role in enhancing shear resistance Incorporating steel fibers can significantly improve the seismic performance of SFRC columns, ensuring stability during seismic events Proper selection and orientation of steel fibers are essential for optimizing the structural integrity and safety of earthquake-prone buildings.
In the design of a SFRC shear wall, steel fibers with horizontal orientation could provide resistance of crack expansion
Understanding the distribution patterns of steel fibers within concrete is essential for designing effective Steel Fiber Reinforced Concrete (SFRC) structural members Key factors influencing the mechanical properties of SFRC include fiber orientation, spacing, and dispersion, which directly affect tensile strength, durability, and fracture toughness Investigating the relationship between steel fiber distribution patterns and SFRC mechanical performance is crucial for optimizing material design and ensuring structural reliability.
2.4.1 Distribution Rate/Concentration of Steel Fiber
Stroeven et al (2024) conducted experiments on concrete specimens with a water/cement ratio of 0.5, cement content of 375 kg/m³, and steel fiber volume fractions ranging from 0% to 3.0% Their findings indicate a positive linear relationship between steel fiber concentration and SFRC mechanical properties, such as bending strength and splitting tensile strength The study revealed that steel fibers aligned parallel to the vibration direction are usually more concentrated, but increased flowability of SFRC tends to enhance fiber concentration perpendicular to the vibration direction When the fiber volume fraction is fixed, SFRC with higher flowability shows greater steel fiber concentration perpendicular to the vibration direction, which influences mechanical performance Since fibers aligned parallel to the load direction significantly contribute to the bending and tensile strength of SFRC, flowability should be considered in the mechanical property assessment of SFRC.
2.4.2 Distribution Coefficient /Uniformly Distributed Variable of Steel Fiber
S T Kang et al [23] carried out a series of experiments on concrete specimens with a water/cement ratio of 0.25 and fiber volume fractions of 2.0% Specimens were prepared by placing material parallel to the longitudinal direction of the specimens (PL) and placing material transversely to the longitudinal direction of the specimens (TL) Specimens were cut along transversely (TC), horizontally (HC) and vertical (VC) directions The number of steel fibers in each cutting section were counted and distribution coefficients were calculated In the case of concrete placed parallel to the longitudinal direction of the specimen, the fibers are more uniformly dispersed in the cross section cut transversely compared to the others Furthermore, most of the fibers in the cross section cut transversely specimens are aligned more parallel to the normal direction of the cutting plane, relative to the other specimens On the other hand, in the case of placing concrete transversely to the longitudinal direction of the specimen, the fiber dispersion coefficient is approximately 10% higher in the vertical cutting direction than in the transversal and horizontal cutting directions
The higher distribution coefficients of PL-TC compared to TL-TC indicate that PL beam specimens exhibit superior mechanical properties, such as increased flexural strength This enhancement is primarily due to the greater fiber content in the PL specimens, which effectively bridges transverse cracks aligned with the beam's longitudinal axis, improving structural durability and performance.
Assuming a uniform fiber distribution can result in significant discrepancies between measured and predicted flexural strength values This highlights that relying solely on the simple assumption of uniformity may lead to considerable prediction errors Therefore, the fiber placement direction in fiber reinforced concrete must be carefully considered to ensure accurate strength predictions and optimal structural performance.
2.4.3 Orientation Coefficient of Steel Fiber
The mechanical properties of Steel Fiber Reinforced Concrete (SFRC) exhibit significant variability, largely influenced by steel fiber orientation Research by Laranjeira et al demonstrated that a higher orientation coefficient leads to more uniform fiber alignment, which can enhance the mechanical performance of SFRC in specific directions Notably, flowable concrete with large fiber orientation coefficients may outperform traditional SFRC in those orientations, highlighting the importance of fiber orientation in optimizing mechanical properties.
Issues Remaining of Flowable SFRC
Numerous studies have examined the distribution patterns of steel fibers [20-23] and their impact on the mechanical properties of Steel Fiber Reinforced Concrete (SFRC) [1, 22-26], while limited research has addressed the flowability of concrete mixes [27, 28] Due to the inherently low flowability of plastic concrete, the uniform distribution of steel fibers largely depends on the mixing procedure When proper mixing techniques are employed, steel fibers can be randomly oriented within the fresh concrete, ensuring optimal performance [10, 12].
The addition of superplasticizer facilitates the transportation of premixed concrete over long distances, supporting sustainable construction practices Due to the self-compacting nature of fresh premixed concrete, the distribution of steel fibers differs from that in traditional plastic concrete, with higher flowability causing the fibers to accumulate more toward the bottom layers Increased flowability leads to a less uniform distribution of steel fibers within the SFRC cross-section compared to lower flowability mixes This variation in fiber distribution significantly impacts the mechanical properties and overall performance of hardened SFRC, highlighting the importance of understanding how flowability influences SFRC characteristics.
Research Questions and Assumptions
The following research questions are pointed out regarding the unsolved issues of flowable SFRC:
(1) How does the flowability influence steel fiber distribution patterns?
(2) What are the effects of steel fiber distribution patterns on the SFRC mechanical properties?
(3) What is the relationship between steel fiber distribution patterns and SFRC mechanical properties?
With the increases of the flowability of SFRC, there are two possible scenarios of steel fiber distributions
Steel fibers tend to sink toward the lower section of the specimens due to their higher density compared to fresh concrete, as illustrated in Fig 2-1 The cohesive forces of the cement paste and obstruction from coarse aggregates help prevent sedimentation of steel fibers However, increasing the flowability of the concrete reduces its stickiness, resulting in less resistance to fiber sedimentation Consequently, in high-flowability SFRC, steel fibers are more likely to be distributed in the lower region of the mixture. -**Sponsor**Looking to optimize your article and ensure it resonates with readers and search engines alike? [Soku AI](https://pollinations.ai/redirect-nexad/w1A7H8fa?user_id=983577) can help you refine your content with expert-level automation Soku AI, trained by Facebook advertising and marketing specialists, is the world's first fully autonomous Facebook/Meta advertising AI agent With Soku AI, you can turn every business owner into a Facebook ads expert overnight.
Fig 2-1 Simulation of Steel Fiber Distribution Patterns of Different Flowability SFRC
In scenarios where steel fibers align predominantly along the flow direction of the fresh concrete mixture, their orientation is influenced by the flowability of the concrete When the concrete exhibits sufficient flowability to move smoothly within the formwork, the flow can induce steel fibers to rotate and align in the flow direction This orientation enhances the structural properties of the concrete, making it crucial to understand and control flow characteristics during placement Properly managing concrete flowability ensures optimal steel fiber distribution and improved overall concrete performance.
High flowability SFRC From uniform distribution
To distribute more among the bottom of the specimen
Fig 2-2 Simulation of Steel Fiber Distribution Patterns of Different Flowability SFRC
Steel fibers in high-flowability SFRC tend to distribute more in the tensile section or align along the flow direction of the fresh concrete, enhancing flexural performance These distribution and orientation patterns are expected to improve the mechanical properties of SFRC, which will be verified through testing.
When the flowability of SFRC Increases:
- Distribute more in the lower section of the specimens,
- Orientate more along the longer direction of the specimens
- Splitting tensile strength perpendicular to the orientation direction of steel fibers will increase
- Flexural performance of beam will be improved
- Load keeping capability in special conditions will increase
High flowability SFRC From random orientation
To orientate more along the flow direction
- Fracture energy of beam will increase
EXPERIMENTAL DESIGN
General
This chapter outlines a comprehensive experimental design, detailing each step from raw material selection and testing to concrete mix design, specimen preparation, and curing processes It also describes the specimen cutting methods used to analyze steel fiber distribution patterns and the procedures for testing the mechanical properties of steel fiber reinforced concrete (SFRC).
Raw Material Tests
Physical and mechanical properties of raw materials are given in Tables 3-1 to 3-3 Raw materials used for this research are listed below:
Cement: Grade 42.5 Portland cement [35], equivalent to Type GP cement in
Coarse aggregate: continuous grading limestone coarse aggregate (size of 5~20 mm)
Fine aggregate: natural river sand (fineness modulus 2.77)
Steel fiber: cut sheet type with indentations Diameter was 0.8 mm and length was 30 mm, with an aspect ratio of 37.5
Water-reducer: high range water-reducing admixture (HRWRA) with a water-reducing rate of 19%
Table 3-1 Physical and Mechanical Properties of Cement
Water content of standard density (%)
Flexural strength (MPa) Initial Final 3 d 28 d 3 d 28 d
Table 3-2 Physical Properties of Sand
Table 3-3 Physical Properties of Coarse Aggregate
Content of needle-slice particle (%)
Fig 3-1 Sample of Fiber Used
The combination of test parameters is summerised in Table 3-4, in which the volume fraction is used to represent the content of steel fibers
Table 3-4 Combination of Test Parameters
Slump (mm) m co (kg.m -3 ) W/C β so (%) HRWRA (%) ρ f (%)
Mix Design
The study employed a direct mix design method for steel fiber-reinforced concrete, replacing an equal volume of coarse aggregates with steel fibers to enhance structural properties The steel fibers' mass was carefully incorporated into the sand ratio calculations to ensure mix consistency A water to binder (w/c) ratio of 0.4 was maintained to optimize the concrete's strength and durability, aligning with best practices for high-performance concrete mixes.
𝑚 sf0 = the mass of steel fibers used per cubic meter (kg/m 3 ),
𝑚 c0 = the mass of cement used per cubic meter (kg/m 3 ),
𝑚 f0 = the mass of fly ash used per cubic meter (kg/m 3 ),
𝑚 w0 = the mass of water used per cubic meter (kg/m 3 ),
𝑚s0 = the mass of sand used per cubic meter (kg/m 3 ),
𝑚 g0 = the mass of coarse aggregate used per cubic meter (kg/m 3 ),
𝜌 c = the density of cement (kg/m 3 ),
𝜌f = the density of fly ash (kg/m 3 ),
𝜌 = the density of water (kg/m 3 ),
𝜌s = the density of sand (kg/m 3 ),
𝜌g = the density of coarse aggregate (kg.m -3 ),
𝜌sf = the volume fraction of the steel fiber,
𝛼 = the percentage of air within the concrete
The study employs specific mix proportions, with a binder-to-water ratio of 0.4 and 215 kg/m³ of water Class F fly ash replaces 20% of cement by weight to enhance durability and sustainability Steel fiber volume fraction is maintained at 1% to improve tensile strength and crack resistance Sand ratios are adjusted to 35%, 37%, 39%, and 41% for mixes with 80 mm, 120 mm, 160 mm, and 200 mm aggregate sizes, respectively, to optimize flowability and ensure sufficient mortar for proper concrete flow.
Curing of Specimens
3-6 Simulation of Cutting Orientation of The Specimens 30
3-7 Gridding of Section Using AutoCAD 30
3-10 Loading on Specimens for Splitting Tensile Strength 32
4-1 Distribution Rate of Steel Fibers Versus Layers of Specimens of Different
4-2 Transverse Section of Different Flowability SFRC 36
4-3 Vertical Section of Different Flowability SFRC 38
4-4 Horizontal Section of Different Flowability SFRC 40
5-2 Definition of Toughness Indexes According To ASTM C 1018 Method 48 5-3 Definition of Toughness Indexes According to JG/T 472-2015 Method 51 5-4 Load-deflection Curve of Different flowability SFRC 52
5-6 Crack Elongation and Expansion Mechanism of SFRC 60
3-1 Physical and Mechanical Properties Of Cement 24
3-3 Physical Properties of Coarse Aggregate 24
5-2 Flexural Toughness Calculated by ASTM C 1018 48
5-5 Data of Flexural Resistance of SFRC 58
𝑚sf0 the mass of steel fibers used per cubic meter (kg/m 3 ),
𝑚c0 the mass of cement used per cubic meter (kg/m 3 ),
𝑚f0 the mass of fly ash used per cubic meter (kg/m 3 ),
𝑚 w0 the mass of water used per cubic meter (kg/m 3 ),
𝑚 s0 the mass of sand used per cubic meter (kg/m 3 ),
𝑚g0 the mass of coarse aggregate used per cubic meter (kg/m 3 ),
𝜌 c the density of cement (kg/m 3 ),
𝜌 f the density of fly ash (kg/m 3 ),
𝜌 w the density of water (kg/m 3 ),
𝜌 s the density of sand (kg/m 3 ),
𝜌 g the density of coarse aggregate (kg/m 3 ),
𝜌 sf the volume fraction of the steel fiber,
𝛼 the percentage of air within the concrete ρ f the volume fraction of steel fiber n i the number of steel fibers in ith region of the section
A i the area of ith region
A f1 the sectional area of single steel fiber across the section m the number of regions of the section
the average of number of steel fibers in m regions
x he distribution rate of steel fibers across section x
y the distribution rate of steel fibers across section y
𝑓 e,p the equivalent initial flexural strength (MPa)
𝑏 the cross section width of the beam (mm)
ℎ the cross section height of the beam (mm)
𝐿 the span of the beam (mm)
𝛿p the mid-span deflection of the beam under peak-load (mm)
𝛺p the area under the load-deflection curve up to 𝛿p (Nmm)
𝑓ftm the flexural strength of SFRC (MPa)
𝑃 the maximum flexural load (kN)
𝛺p the area under the load-deflection curve from 𝛿p up to 𝛿k (Nmm)
𝛿 p,k the increased mid-span deflection from 𝛿 p to 𝛿 k (mm)
𝛿k the calculated mid-span deflection 𝐿/𝑘 (mm) when k equals to 500, 300,
𝐸𝑖 refer the modulus of elasticity of the mixture constituents
𝑉 𝑖 the volume fraction of the mixture constituents
𝐸j the modulus of elasticity of layer j
𝐸 C the modulus of elasticity of concrete
𝐸 S the modulus of elasticity of steel fiber
𝜌j the distribution rate of layer j‟s steel fibers
𝛼j the ratio of the modulus of elasticity of layer j to the modulus of elasticity of concrete
𝐴j the additional sectional area of layer j
ℎ the height of the section
𝐴 1 the additional sectional area by modulus of elasticity of layer 1
𝐴 2 the additional sectional area by modulus of elasticity of layer 2
𝐴4 the additional sectional area by modulus of elasticity of layer 4
𝐼 0 the altered moment of inertia before crack-elongation
𝑊 0 the moment of elastic resistance of aspect section area 𝐴 0 to the edge of tensile section
𝛼 e the ratio of the modulus of elasticity of steel fiber to the modulus of elasticity of concrete.
Concrete is a vital construction material composed of crushed stone or gravel, sand, and other aggregates bonded by the hydration of cement, making it essential in building structures, bridges, roads, and hydraulic engineering It offers excellent properties such as high compressive strength, durability, fire resistance, and cost-effectiveness due to its easy local resource availability and good forming ability Despite its advantages, concrete has drawbacks including heavy weight, long curing times to achieve desired strength, and susceptibility to cracking Over the years, concrete has undergone significant advancements, leading to improved performance and broader applications in modern civil engineering.
Steel fiber reinforced concrete (SFRC), developed in the early 1970s, is a composite material where short steel fibers are randomly embedded within the concrete matrix to enhance structural performance Its primary goal is to address the limited tensile strength and poor compressive ductility of traditional concrete by using steel fibers to hinder micro-crack development and reduce macro-crack propagation By increasing tensile strength, SFRC improves properties such as shear strength, flexural strength, and crack resistance, leading to enhanced durability under shear, flexural, punching, impact, fatigue, and complex loading conditions, including recycled materials.
Steel fibers in concrete significantly enhance its mechanical properties and durability when incorporated with proper mix proportions and production techniques They improve traditional strength metrics such as compressive strength, modulus of elasticity, fracture resistance, shear resistance, tensile strength, and flexural strength, while also boosting properties like energy absorption, toughness, peak strain, and residual tensile strain under compression, particularly in high-strength concrete Additionally, SFRC exhibits multiple cracking and strain hardening behavior prior to failure under uniaxial and multi-axial loads, driven by nonlinear fracture processes, bond-slip interactions, and elastic responses of fibers and the matrix The inclusion of steel fibers increases flexural toughness, post-peak residual tensile strain, fracture energy, and crack resistance by bridging cracks and preventing crack propagation Furthermore, steel fibers reduce internal moisture diffusion and drying shrinkage of concrete, and when combined with polypropylene fibers, SFRC’s residual mechanical properties and high-temperature resistance are notably improved, ensuring enhanced durability and structural performance.
Steel fibers are the key reinforcement material in Steel Fiber Reinforced Concrete (SFRC), randomly distributed within the concrete matrix According to ACI A820 standards, four main types of steel fibers are identified based on their production source: cold drawn wire (Type I), cut sheet (Type II), melt-extracted (Type III), and other fibers (Type IV), which can be straight or deformed The tensile strength of steel fibers typically exceeds 345 MPa, with their dimensions and permissible variations expressed through equivalent diameter and length, adhering to ASTM C 1517-03 testing standards for tensile strength and Young's modulus While basic straight fibers cut from smooth wire are common, they lack optimal anchorage in concrete, leading to reduced strength utilization Advances in research and manufacturing have resulted in over 90% of fibers being deformed shapes—such as flattened, spaded, coned, twisted, crimped, hooked, and surface-textured fibers—which enhance bonding in concrete These fibers can have circular, square, rectangular, or irregular cross-sections, with variations in diameter and length, as shown in standard engineering diagrams Different fiber shapes—including hooked ends, enlarged ends, and deformed forms—are designed to improve the bond effect, with their effectiveness depending on shape To optimize performance in high-strength concrete, steel fibers must have tensile strengths compatible with the concrete’s strength, necessitating production of fibers with varying strength grades.
Fig 1-1 Steel Fiber: Cold-Drawn Wire with Hooked Ends
Fig 1-2 Steel fiber: Cut Sheet Type with Enlarged Ends (left) or Indentations (right)
Fig 1-3 Steel Fiber: Milling Type with Deformed Shape
The orientation of steel fibers in concrete matrix is influenced by multiple factors, including the concrete matrix, steel fiber characteristics, volume fraction, workability of fresh concrete, casting method, and boundary conditions, with the matrix, workability, and fiber properties being most critical [19, 18] While numerous studies have examined steel fiber distribution patterns and their impact on mechanical properties of SFRC [20-26], limited research has focused on the flowability of the concrete mix [27, 28] Due to the low flowability of plastic concrete, the mixing procedure plays a key role in controlling fiber distribution, ensuring fibers are randomly oriented within the fresh mix when properly mixed [27, 29].
Introducing superplasticizer into concrete enables the long-distance transportation of premixed concrete, supporting sustainable construction practices The self-compacting nature of fresh premixed concrete leads to a different distribution of steel fibers compared to plastic concrete, with flowability influencing fiber dispersion As concrete's flowability increases, the ratio of steel fibers tends to rise from the top to the bottom layer of SFRC, resulting in less uniform fiber distribution in highly flowable SFRC This variation in fiber distribution can impact the performance of the hardened concrete, making it essential to study the effects of flowability on SFRC properties.
This study evaluates how different flowability levels of SFRC mixes and specimens influence key properties, including fiber distribution rate, distribution coefficient, and fiber orientation coefficient It examines the impact of flowability on the mechanical performance of SFRC, such as compressive strength, splitting tensile strength, flexural strength, flexural toughness, and fracture energy The research also presents test mechanisms used to determine the strengths of high flowability SFRC, along with recommended equations for calculating bending stiffness and flexural stress based on steel fiber distribution Overall, the findings highlight the critical relationship between flowability and the structural properties of SFRC, providing valuable insights for optimizing mix design and durability.
This research aims to investigate how the flowability of fresh Steel Fiber Reinforced Concrete (SFRC) influences steel fiber distribution patterns and the resulting mechanical properties By analyzing these relationships, the study seeks to enable accurate prediction of the flexural performance of flowable and self-compacting SFRC The findings will provide essential technical guidance for the design and construction of flowable SFRC, ensuring improved structural performance and durability.
The specific objectives of this research are:
(1) To compare differences of steel fiber distribution patterns of SFRC with different flowability;
(2) To test basic mechanical properties such as compressive strength, splitting tensile strength and flexural strength of SFRC with different flowability;
(3) To assess influence of flowability on the basic mechanical properties of SFRC;
(4) To discuss relationships between steel fiber distribution patterns and mechanical properties of SFRC;
(5) To analyse mechanism causing the differences of mechanical properties of different flowability SFRC;
(6) To propose formulas for calculating the modulus of elasticity, moment of inertia, bending stiffness, moment of elastic resistance and flexural stress of flowable SFRC
The thesis is organized into six chapters A brief description of each chapter is given below
This chapter offers a comprehensive overview of previous research on Steel Fiber Reinforced Concrete (SFRC), highlighting the importance and necessity of conducting this study to fill existing knowledge gaps The main objectives of the research are clearly outlined, emphasizing its contribution to advancing understanding in the field Additionally, the chapter presents the overall structure of the thesis, guiding readers through the subsequent sections for a cohesive understanding of the study.
This chapter presents a comprehensive literature review of previous studies on steel fibers' distribution patterns and the mechanical properties of Steel Fiber Reinforced Concrete (SFRC) It highlights key findings regarding how fiber distribution affects the material's performance and durability The review also identifies unresolved issues related to the impact of flowability on SFRC's mechanical behavior, emphasizing the need for further research Additionally, the chapter outlines specific research questions and assumptions to guide future investigations into optimizing SFRC performance through improved flowability and fiber dispersion.
This chapter presents a comprehensive experimental design and mechanical property testing process for Steel Fiber Reinforced Concrete (SFRC) It covers critical steps such as raw material selection and testing, concrete mix design, specimen preparation and curing, and cutting methods for steel fiber distribution analysis The chapter also details key tests including compressive strength, splitting tensile strength in three directions, and flexural toughness, providing a thorough evaluation of SFRC's mechanical performance.
Chapter 4 Evaluation of Steel Fiber Distribution Patterns
This chapter presents the calculation of three key factors—distribution rate, distribution coefficient, and orientation coefficient—for each flowability SFRC These results are systematically plotted, compared, and analyzed to assess how flowability impacts steel fiber distribution patterns, providing valuable insights into their relationship and influence on material performance.
Chapter 5 Mechanical Properties of SFRC Correlating with Steel Fiber Distribution Patterns
This chapter summarizes the mechanical properties test results, including the analysis of the load-deflection curve for flexural strength Flexural toughness of fiber-reinforced concrete (SFRC) is assessed using both the ASTM C1018 standard and the Chinese JG/T472-2015 standard Fracture energy is calculated to evaluate material performance, and new formulas for determining the moment of inertia and flexural stress of flowable SFRC are proposed for more accurate analysis.
This chapter provides conclusions of this research project and recommendations for future research
SFRC offers significant benefits including improved crack control, superior shear and flexural strength, and enhanced earthquake resistance These advantages have garnered increasing interest from researchers worldwide, highlighting SFRC's potential as a groundbreaking composite material in structural engineering.
Previous studies have studied several factors influencing steel fiber distribution
Steel fiber distribution is a key factor influencing the mechanical performance of Steel Fiber Reinforced Concrete (SFRC) Altering the distribution of steel fibers within the concrete matrix can significantly impact the material's strength and durability Understanding how to modify fiber distribution is essential for controlling SFRC's mechanical properties and optimizing its performance in engineering applications Effective management of fiber dispersion during the design phase enables engineers to enhance the structural integrity and reliability of SFRC structures.
Cutting Specimens for Steel Fiber Distribution Patterns Analysis
By analyzing specimens cut along transverse, horizontal, and vertical sections and counting the steel fibers in each, we assess the influence of flowability on fiber distribution patterns Four equal cubes were cut along the transverse direction, revealing steel fibers aligned longitudinally across three cross-sections Additionally, two cubes were sliced horizontally into six parts to observe vertically oriented fibers, while the other two were sliced vertically to examine transversely aligned fibers This comprehensive sectioning method provides insights into how flowability affects steel fiber distribution within concrete composites.
Images of the cross sections were acquired and inputted into AutoCAD for gridding as shown in Fig 3-7 Each section was divided into a 4 4 grid for counting steel fibers
Fig 3-6 Simulation of Cutting Orientation of The Specimens
Fig 3-7 Gridding of Section Using AutoCAD
Fig 3-8 Photos of Cut Specimens