FIBER REINFORCED CONCRETE

FIBER REINFORCED CONCRETE

ABSTRACT

Concrete is weak in tension and strong in compression .Even though reinforcement is

provided in tension zone micro cracks are developed in the tension and compression zone.

The propagation of these cracks can be arrested by using fiber reinforcement in concrete. The

fiber reinforcement is provided using different materials like steel carbon, glass fibers and

polypropylene fibers. The fibers are very small which are distributed over the whole area of

concrete .because of this we can not only arrest crack formation but also we can increase

flexural ,shear ,torsion, strength, freezing &thawing resistance.

INTRODUCTION

In all countries, the construction industry is rapidly developing based on the invention of

different materials and products in engineering fields. Engineers have attempted various types

of materials in order to make the task more efficient reducing time, cost, improving

durability, quality and performance of structures during their lifetime. Sophisticated analyses

on structural Idealization have made a tremendous impact on the development of construction

materials. This paper describes the general properties and application of fiber-reinforced

concrete used in construction. The promise of thinner and stronger elements reduced weight

and controlled cracking by simply adding a small amount of fibers is an attractive feature of

fiber-reinforced concrete. The quality of good and durable concrete does not depend only on

the quality of raw materials but also on proper mix-design, use of admixtures, placement,

vibration and efficient curing. A number of additives are being used with concrete to enhance

structural properties. Such additives are different types of fiber, namely steel, carbon,

asbestos, jute, glass, polythene, nylon, polypropylene, fly ash, polymer, epoxy,

superplasticiser, etc.

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FIBER REINFORCED CONCRETE

ROLES OF FIBRE

Fibers are usually used in concrete to control cracking due to both plastic shrinkage and

drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of

water. Some types of fibers produce greater impact, abrasion and shatter resistance in

concrete. The amount of fibers added to a concrete mix is expressed as a percentage of the

total volume of the composite (concrete and fibers), termed volume fraction (Vf). Vf typically

ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing fiber length (l) by its

diameter (d). Fibers with a non-circular cross section use an equivalent diameter for the

calculation of aspect ratio. If the modulus of elasticity of the fiber is higher than the matrix

(concrete or mortar binder), they help to carry the load by increasing the tensile strength of

the material. Increase in the aspect ratio of the fiber usually segments the flexural strength

and toughness of the matrix. However, fibers which are too long tend to "ball" in the mix and

create workability problems

WHY FIBER REINFORCED CONCRETE IS USED?

Plain and reinforced concrete stuctures are full of flaws such as pores, air voids, shrinkage

cracks, etc., even before mechanically loaded.These flaws, especially small in size (micro

cracks), grow stably under external loading and unite with existing or newly formed micro

cracks until large fracture is formed which causes the collapse of the structure.Concrete is a

material weak in tension and its tensile strength approximately ranges from 8 to 15 percent of

its compressive strength.The initiation and propogation of these initial cracks and flaws

during loading govern the mechanical behaviour of concrete subjected to different loading

conditions.For a concrete structure subjected to tension, the cracks propogate in a direction

perpendicular to the applied load.On the other hand, for a concrete structure subjected to

purely uniaxial copression, the cracks propagate in the same direction as the applied

compressive load. Since different mechanical responses of concrete structures under different

loading conditions can be explained by fracture process, it is essential to understand when the

cracks initiate and how they propagate with increasing load.The presence of micro cracks at

the mortar aggregate interface is responsible for the weakness of plain concrete.The weakness

can be removed to some extent byinclusion of fibers in the mix.

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FIBER REINFORCED CONCRETE

CRACK ARRESTING MECHANISM OF FIBERS

When the loads imposed on concrete approach that for failure, cracks will propagate,

sometimes rapidly fibers in concrete provide a means of arresting the crack growth. After the

concrete cracks in tension, the fibers continue to take the load, provided the bond is good.

When the fiber strain reaches its breaking strain, the fibers fracture resulting in load transfer

to the fibers of adjacent layers. This process continues which results in shifting of neutral

axis. Failure occurs when the concrete in compression reaches ultimate strain. Reinforcing

steel bars in concrete have: the same beneficial effect because they act as long continuous

fibers. Short discontinuous fibers have the advantage, however, of being uniformly mixed

and dispersed throughout the concrete. Fibers are added to a concrete mix which normally

contains cement, water and fine and coarse aggregate .Among the more common fibers used

are steel, glass, asbestos and polypropylene

TYPES OF FIBER REINFORCED CONCRETE

Natural fiber reinforced concrete

Steel fiber reinforced concrete

Polypropylene fiber reinforced concrete

Nylon fiber reinforced concrete

Asbestos fiber reinforced concrete

Glass fiber reinforced concrete

Carbon fiber reinforced concrete

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FIBER REINFORCED CONCRETE

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FIBER REINFORCED CONCRETE

NATURAL FIBER REINFORCED CONCRETE

Naturally available reinforcing materials can be used effectively as reinforcement in Portland

cement concrete. Natural fiber reinforced concrete is suitable for low-cost construction,

which is very desirable for developing countries. It is important for researchers, design

engineers, and the construction industry to vigorously pursue the use of local materials. For

economical engineering solutions to a variety of problems, natural fiber reinforced concrete

offers a viable alternative that needs to be fully investigated and exploited. Wood fibers

derived from the Kraft process possess highly desirable performance-to-cost ratios, and have

been successfully substituted for asbestos in the production of thin-sheet cement products,

such as flat and corrugated panels and non-pressure pipes. Straw-reinforced, sun-dried mud

bricks for wall construction, and horse hair in mortar, are typical examples of how natural

fibers were used long ago.

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FIBER REINFORCED CONCRETE

APPLICATIONS

In Africa, sisal fiber reinforced concrete has been used extensively for making roof tiles, corrugated

sheets, pipes, silos, and gas and water tanks. Elephant grass fiber reinforced mortar and cement

sheets are being used in Zambia for low-cost house construction, while wood and sisal fibers are

being used for making cement composite panel lining, eaves, soffits, and for sound and fire insulation.

Kraft pulp fiber reinforced cement has found major commercial applications in the manufacture of flat

and corrugated sheet, non-pressure pipes, cable pit, and outdoor fiber reinforced cement paste or

mortar products for gardening. The durability of these products in outdoor exposure has been

demonstrated with nearly 10 years of commercial use of these materials.

Fiber typeFiber length

[mm]

Fiber

diameter

[mm]

specific

gravity

Modulus of

elasticity

[106 MPa]

Ultimate

tensile

strength

[103 MPa]

Elongation

at break [%]

Water

absorption

[%]

Coconut 51-102 0.10-0.41 1.12-1.15 19-26 120-200 10-25 130-180

Sisal N/A N/A N/A 13-26 276-568 3-5 60-70

Sugar cane

BagasseN/A 0.20-0.41 1.2-1.3 15-19 184-290 N/A 70-75

Bamboo N/A 0.05-0.41 1.5 33-40 350-500 N/A 40-45

Jute 178-305 0.10-0.20 1.02-1.04 26-32 250-350 1.5-1.9 N/A

Flax 508 N/A N/A 100 1000 1.8-2.2 N/A

Elephant

grassN/A N/A N/A 4.9 178 3.6 N/A

Water reed N/A N/A N/A 5.2 70 1.2 N/A

Plantain N/A N/A N/A 1.4 92 5.9 N/A

Musamba N/A N/A N/A 0.9 83 9.7 N/A

Wood fiber 3-5 0.03-0.08 1.5 N/A 700 N/A 50-75

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FIBER REINFORCED CONCRETE

STEEL FIBER REINFORCED CONCRETE

During recent years, steel fiber reinforced concrete has gradually advanced from a new,

rather unproven material to one which has now attained acknowledgment in numerous

engineering applications. Lately it has become more frequent to substitute steel reinforcement

with steel fiber reinforced concrete. The applications of steel fiber reinforced concrete have

been varied and widespread, due to which it is difficult to categorize. The most common

applications are tunnel linings, slabs, and airport pavements.

Many types of steel fibers are used for concrete reinforcement. Round fibers are the most

common type and their diameter ranges from 0.25 to 0.75 mm. Rectangular steel fibers are

usually 0.25 mm thick, although 0.3 to 0.5 mm wires have been used in India. Deformed

fibers in the form of a bundle are also used. The main advantage of deformed fibers is their

ability to distribute uniformly within the matrix.

Fibers are comparatively expensive and this has limited their use to some extent.

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FIBER REINFORCED CONCRETE

PROPERTIES OF CONCRETE IMPROVED BY STEEL FIBERS

Below are some properties that the use of steel fibers can significantly improve:

Flexural Strength: Flexural bending strength can be increased of up to 3 times more

compared to conventional concrete.

Fatigue Resistance: Almost 1 1/2 times increase in fatigue strength.

Impact Resistance: Greater resistance to damage in case of a heavy impact.

Permeability: The material is less porous.

Abrasion Resistance: More effective composition against abrasion and spalling.

Shrinkage: Shrinkage cracks can be eliminated.

Corrosion: Corrosion may affect the material but it will be limited in certain areas.

DISADVANTAGES OF STEEL FIBER REINFORCED CONCRETE

Though steel fiber reinforced concrete has numerous advantages, it has certain concerns that

are yet to be resolved completely.

There are complications involved in attaining uniform dispersal of fibers and consistent

concrete characteristics.

The use of SFRC requires a more precise configuration compared to normal concrete.

Another problem is that unless steel fibers are added in adequate quantity, the desired

improvements cannot be obtained.

However, as the quantity of fibers is increased, the workability of the concrete is affected.

Therefore, special techniques and concrete mixtures are used for steel fibers. If proper

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FIBER REINFORCED CONCRETE

techniques and proportions are not used, the fibers may also cause a finishing problem, with

the fibers coming out of the concrete.

BENEFITS OF STEEL FIBERS

Improve structural strength

Reduce steel reinforcement requirements

Improve ductility

Reduce crack widths and control the crack widths tightly thus improve durability

Improve impact & abrasion resistance

Improve freeze-thaw resistance

USES

SFRC elements are suitable to use in the following areas:

Slabs and Bridge Decks, Airport Pavements,Parking Areas, Fence Posts.

Storage tanks, Precast Concrete Members, Slab-Column connections, hotcreting and

Repair of cavitations.

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FIBER REINFORCED CONCRETE

POLYPROPYLENE FIBER REINFORCED CONCRETE

PP fibers can be produced as monofilaments or as collated fibrillated fiber bundles;their

properties are related to the degree of crystallinity. PP is a linear hydrocarbon, although in

some cases methyl side groups are attached to alternate carbons to improve oxidation

resistance

Polypropylene is one of the cheapest & abundantly available polymers polypropylene fibers

are resistant to most chemical & it would be cementitious matrix which would deteriorate

first under aggressive chemical attack. Its melting point is high (about 165 degrees

centigrade). So that a working temp. As (100 degree centigrade) may be sustained for short

periods without detriment to fiber properties.

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FIBER REINFORCED CONCRETE

Polypropylene fibers being hydrophobic can be easily mixed as they do not need lengthy

contact during mixing and only need to be evenly distressed in the mix.

Polypropylene short fibers in small volume fractions between 0.5 to 15 commercially used in

concrete.

The main features: as a secondary concrete rebar materials, polypropylene fiber HDF-PP can

be greatly enhanced its anti-cracking, permeability, impact resistance, earthquake resistance,

antifreeze, scour, anti-burst, anti-aging properties And easy, pumping and water.

    ■Cracks in concrete block production

    ■Concrete improve the performance of the anti-infiltration

    ■The freeze-thaw resistance of concrete to improve performance

    ■To improve the impact resistance of concrete, bending, anti-fatigue, anti-seismic

performance

    ■To improve the durability of concrete, anti-oxidation

    ■To improve the fire-resistant properties of concrete

BENEFITS OF POLYPROPYLENE AND NYLON FIBERS

Improve mix cohesion, improving pumpability over long distances

Improve freeze-thaw resistance

Improve resistance to explosive spalling in case of a severe fire

Improve impact resistance

Increase resistance to plastic shrinkage during curing

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FIBER REINFORCED CONCRETE

GLASS FIBER REINFORCED CONCRETE

Glass fiber reinforced concrete (GFRC) consists of high strength glass fiber embedded in a

cementitious matrix. In this form, both fibers and matrix retain their physical and chemical

identities, while offering a synergism: a combination of properties that cannot be achieved

with either of the components acting alone. In general, fibers are the principal load-carrying

members, while the surrounding matrix keeps them in the desired locations and orientation,

acting as a load transfer medium between them, and protects them from environmental

damage. In fact, the fibers provide reinforcement for the matrix and other useful functions in

fiber-reinforced composite materials. Glass fibers can be incorporated into a matrix either in

continuous lengths or in discontinuous (chopped) lengths.

The design of GFRC panels proceeds from a knowledge of its basic properties under tensile,

compressive, bending and shear forces, coupled with estimates of behavior under secondary

loading effects such as creep, thermal and moisture movement.

Glass fiber reinforced concrete architectural panels have general appearance of pre-cast

concrete panels, but are different in several significant ways. For example, GFRC panels will,

on the average, weigh substantially less than pre-cast concrete panels due to their reduced

thickness. The low weight of GFRC panels decrease superimposed loads on the building’s

structural components. The building frame becomes more economical. There are number

differences between structural metal and fiber-reinforced composites. For example, metals in

general exhibit yielding and plastic deformation whereas most fiber-reinforced composites

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FIBER REINFORCED CONCRETE

are elastic in their tensile stress-strain characteristics. Other important characteristics of many

fiber-reinforced composites are their non-corroding behavior, high damping capacity and low

coefficients of thermal expansion.

HEALTH HAZARDS CAUSED DUE TO GLASS FIBER

The National Toxicology Program classifies inhalable glass wool fibers as "Reasonably

anticipated to be a human carcinogen”. Some fiberglass products warn of "possible cancer

hazard by inhalation". The European Union and Germany classify synthetic vitreous fibers as

possibly or probably carcinogenic, but fibers can be exempt from this classification if they

pass specific tests. Evidence for these classifications is primarily from studies on

experimental animals and mechanisms of carcinogenesis. Studies of fiberglass factory

workers show significant increases in lung cancer but do not show clear exposure-response

relationships and maybe confounded by the effects of smoking The Environmental Research

Foundation has documented significant efforts by the fiberglass industry to prevent or remove

cancer causing classifications.

Fiberglass will irritate the eyes, skin and the respiratory system. Potential symptoms include

irritation of eyes, skin, nose, throat; dyspnea (breathing difficulty); sore throat, hoarseness

and cough.

Fiberglass is resistant to mold but growth can occur if fiberglass becomes wet and

contaminated with organic material. Fiberglass insulation that has become wet should be

inspected for evidence of residual moisture and contamination. Contaminated fiberglass

insulation should be promptly removed.

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FIBER REINFORCED CONCRETE

APPLICATIONS

1. The use of alkali-resistant glass fibers for reinforcing cement has received appreciable

attention because of their excellent engineering properties.

2. Glass-fiber reinforced cement products that decrease with time in tensile and impact

strength should not be used for primary structural applications.

3. Glass fibers have been used successfully to avoid cracking problems due to shrinkage

stresses in the production of thin sheet.

4. Combining fiber types in cement composites is a new approach with high potential

for improving the long-term performance of glass-fiber-reinforced cement products.

5. Mixtures of polypropylene and glass fibers or, alternatively, mica flakes used as fibers

may help prevent long-term decreases in tensile and impact strength.

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FIBER REINFORCED CONCRETE

ASBESTOS REINFORCED CONCRETE

Asbestos is a naturally occurring silicate minerals used commercially for their desirable

physical properties. They all have in common their eponymous, asbesti form habit: long,

(1:20) thin fibrous crystals. Asbestos became increasingly popular among manufacturers and

builders in the late 19th century because of its sound absorption, average tensile strength, its

resistance to fire, heat, electrical and chemical damage, and affordability. It was used in such

applications as electrical insulation for hotplate wiring and in building insulation. When

asbestos is used for its resistance to fire or heat, the fibers are often mixed with cement

(resulting in fiber cement) or woven into fabric or mats. Commercial asbestos mining began

in the Eastern Townships of Quebec, Canada and the world's largest asbestos mine is located

in the town of Asbestos, Quebec.

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FIBER REINFORCED CONCRETE

HEALTH HAZARD OF ASBESTOS

The inhalation of asbestos fibers can cause serious illnesses, including malignant lung cancer,

mesothelioma (a formerly rare cancer strongly associated with exposure to amphibole

asbestos), and asbestosis (a type of pneumoconiosis). Long exposure to high concentrations

of asbestos fibers is more likely to cause health problems. This is most common among the

miners of asbestos, since they have the longest exposure to it. The European Union has

banned all use of asbestos and extraction, manufacture and processing of asbestos products.

CARBON FIBER REINFORCED CONCRETE

Carbon fibers are the most recent & probably the most spectacular addition to the range of

fiber available for commercial use. Carbon fiber comes under the very high modulus of

elasticity and flexural strength. These are expansive. Their strength & stiffness characteristics

have been found to be superior even to those of steel. But they are more vulnerable to damage

than even glass fiber, and hence are generally treated with resign coating.

BENEFITS OF CFRC

· High strength lightweight concrete cabe achieved.

· More durable in hot weather & less shrinkage value

· Increased freezing - thawing resistances

Uses

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FIBER REINFORCED CONCRETE

Where the lightweight concreting is required.

Precast thin sections with lightweight concreting (up to Specific Gravity 1.0)

Suitable for high temperature and low humidity areas

COMPARISION OF DIFFERENT PROPERTIES AMONG DIFFERENT TYPES OF

CONCRETE

SOME DEVELOPMENTS IN FIBER-REINFORCED CONCRETE

An FRC sub-category named Engineered Cementitious Composite (ECC) claims 500 times

more resistance to cracking and 40 percent lighter than traditional concrete.ECC claims it can

sustain strain-hardening up to several percent strain, resulting in a material ductility of at least

two orders of magnitude higher when compared to normal concrete or standard fiber-

reinforced concrete. ECC also claims a unique cracking behavior. When loaded to beyond the

elastic range, ECC maintains crack width to below 100 µm, even when deformed to several

percent tensile strains. Field results with ECC and The Michigan Department of

Transportation resulted in early-age cracking

Recent studies performed on a high-performance fiber-reinforced concrete in a bridge deck

found that adding fibers provided residual strength and controlled cracking. A new kind of

natural fiber-reinforced concrete (NFRC) made of cellulose fibers processed from genetically

modified slash pine trees is giving good results. The cellulose fibers are longer and greater in

diameter than other timber sources. Some studies were performed using waste carpet fibers in

concrete as an environmentally friendly use of recycled carpet waste. A carpet typically

consists of two layers of backing (usually fabric from polypropylene tape yarns), joined by

CaCO3 filled styrene-butadiene latex rubber (SBR), and face fibers (majority being nylon 6

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2-3300-4000.5-1.01.7-2.07.5Carbon

32002.32.5-3.40.02-20Asbestos

0.25170--2.90.01-200Mica Fl

0.5520.00.97.5Polypropylene

2-3802-3.52.69-15Glass

1-32003-47.85-500Steel

Tensile Strength, GPaModulus of Elasticity, GPa

Failure Srain, %

Specific Gravity

Diameter µm

Fiber

FIBER REINFORCED CONCRETE

and nylon 66 textured yarns). Such nylon and polypropylene fibers can be used for concrete

reinforcement. Other ideas are emerging to use recycled materials as fibers.

CONCLUDING REMARKS

Innovations in engineering design, which often establish the need for new building materials,

have made fibre-reinforced cements very popular. The possibility of increased tensile

strength and impact resistance offers potential reductions in the weight and thickness of

building components and should also cut down on damage resulting from shipping and

handling. Although ASTM C440-74a describes the use of asbestos-cement and related

products, there are, at this time, no general ASTM standards for fibre-reinforced cement,

mortar and concrete. Until these standards become available, it will be necessary to rely on

the experience and judgment of both the designer and the fibre manufacturer. The onus is

thus on the designer to be aware of the limitations presently inherent in some of these

composites, particularly the durability of glass-fibre-reinforced products

REFERENCES

Building Research Station (1976), A Study of the Properties of Cem-Fil/OPC Composites, Building Research Establishment Current Paper, CP38/76, Garston, England.

Cheetham, C.J. and P. Maguire, (1979), Coating of Glass Fibres, U.S. Patent 4,173,486.

Majumdar. A.J. and J.F. Ryder, (1968), Glass Fibre Reinforcement of Cement Products, Glass Technology, Vol. 9 (3), pp. 78-84.

Majumdar. A.J. and R.W. Nurse, (1974), Glass Fibre Reinforced Cement, Materials

Ramachandran, V.S.,(1979), Superplasticizers in concrete, National Research Council of Canada, Division of Building Research, Canadian Building Digest pp. 203

Science and Engineering, Vol. 15, pp. 107-127.

Wikipedia

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FIBER REINFORCED CONCRETE

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