Introduction to Steel Fiber Reinforced Concrete (SFRC)

Introduction to Steel Fiber Reinforced Concrete (SFRC)

FRC (Fiber Reinforced Concrete)Fiber Reinforced Concrete is a composite material consisting of cement, aggregate and discontinues, discrete, uniformly dispersed suitable fibers.

Why to use FRCPlain concrete is a brittle material with limited ductility and low tensile strength and strain capacity. Internal micro-cracks are present in that concrete. Thus the concrete leads to heavy brittle fracture. The role of randomly distributed fibers is to bridge across the cracks and to provide some post-cracking ductility. If the fibers are sufficiently strong and bonded to material then FRC will carry significant stress over a relatively large strain capacity in the post cracking stage. Classification of FRC Steel Fiber Reinforced Concrete (SFRC) Glass Fiber Reinforced Concrete (GFRC) Synthetic Fiber Reinforced Concrete (SNFRC) Natural Fiber Reinforced Concrete (NFRC)

Recently some new FRC named High Performance Fiber Reinforced Concrete (HPFRC) and Ultra High Performance Fiber Reinforced Concrete (UHPFRC) have introduced which shows significant changes over plain concrete.

Different Types of Fiber

Steel Fiber Glass Fiber Synthetic Fiber Natural Fiber Steel Fiber Reinforced Concrete (SFRC)SFRC is concrete made of hydraulic cements containing fine and coarse aggregate and discontinuous discrete steel fibers.

Steel Fibers Steel fibers used for reinforcing concrete are defined as short, discrete length of steel having an aspect ratio (ratio of length to diameter) from about 20 to 100 with any of several cross sections and that are sufficiently small to be randomly dispersed in a unhardened concrete mixture using usual mixing procedures.Classification of Steel Fiber

# ASTM A 820 provides a classification of four general types of steel fibers.

Type I - Cold-drawn wireType II - Cut sheet Type III Melt-extracted Type IV Other fibers # The Japanese society of civil engineers (JSCE) has classified steel fibers based on the shape of their cross-section. Type I Square sectionType II Circular SectionType III Crescent Section

CROSS SECTION OF STEEL FIBERSSteel Fiber cutting machine

Composition of Steel FiberThe composition of steel fibers generally includes carbon steel or stainless steel

The length dimension ranges from 6.4mm to 76mm while the diameter ranges from 0.25mm to 0.75mm.

The steel fibers are described by a convenient parameter Aspect Ratio. The aspect ratio is determined by length to diameter ratio. It varies from 20 to 100.

Essential Properties of Steel Fibers Fibers should have a relatively high strength and modulus of elasticity.

Fibers should be protected from Corrosion by the alkaline environment of the cementitious matrix.

Fibers should have the property of surface roughness to enhance bonding with the matrix.

In different environments the compositions of fibers should be varied. I.e. In high temperature stainless steel fibers should be used.

According to ASTM A 820 standards, the minimum yield strength of steel fibers should be 50,000 psi (345 Mpa) while it should be 80,000 psi (552 MPa) according to JSCE standard.

Mix design of SFRC

As per ACI 544.1R, the mixing proportion of materials for SFRC will be as follows.

Property 3/8- in. maximum sized aggregate- in. maximum sized aggregate 1- in. maximum sized aggregate Cement (kg/m3) 350-600 300-550 250-450 w/c ratio 0.35-0.45 0.35-0.5 0.35-0.55Percent of fine to coarse aggregate (%) 45-60 45-55 40-55Entrained Air Content (%) 4-8 4-6 4-5Deformed Fiber (%) 0.4-1.0 0.3-0.8 0.2-0.7Smooth Fiber (%) 0.8-2.0 0.6-1.6 0.4-1.4

Fiber Content, (% of total volume)Properties of Concrete Improved by Steel Fibers:

Compressive strength :

In compression the ultimate strength is only slightly affected by the presence of Steel Fibers. The presence of steel fibers increases compressive strength from 0 to 15% for up to 2% of volume of fibers.

Tensile strength :

In direct tension, the improvement in strength is reported from 30-40% for addition up to 2% by total volume of fibers.It is observed that the split tensile strength increases from 10-45% for addition up to 3% by total volume of fibers.

Flexural Strength: Increase in the flexural strength of SFRC are much greater than in tension or compression because of ductile behavior.

The changed elastic distribution is importantly plastic in the tension zone and elastic in the compression zone, which poses a shift of neutral axis towards the compression zone.

It changes from 20-100% for addition up to 3% by total volume of fibers.

Fatigue:For a given type of fiber there is a significant increase in flexural fatigue strength with increasing percentage of steel fibers. Almost 1 1/2 times increase in fatigue strength due to the increasing use of steel fibers.Impact: Under flexural impact loading, the peak load for SFRC is 40 percent higher than the normal concrete. So, increased use of steel fibers increase the impact resistance of the concrete.

Application of SFRC:

SFRC has been tried on overlays of air-field, road pavements, industrial floorings, bridge decks, canal lining, explosive resistance structures, refractory linings etc.

It can also be used for the fabrication of precast products like pipes, boats, beams, stair case steps, wall panels, roof panels, manhole covers etc.

SFRC is also being tried for the manufacture of prefabricated formwork molds of U shape for casting lintels and small beams.

Limitations Of Steel Fiber Reinforced Concrete :

The use of SFRC requires a more precise configuration compared to normal concrete.Unless steel fibers are added in adequate quantity, the desired improvements cannot be obtained.As the quantity of fibers is increased, the workability of the concrete is decreased. If proper techniques and proportions are not used, the fibers may also cause a finishing problem, with the fibers coming out of the concrete.Another problem is the corrosion of the surface which may influence the appearance of the surface.Steel fibers are not cost effective. Due to the addition of 1% steel fiber of the total volume, there will be a massive change in the total cost of the construction.

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