Seminar Report On Fiber Rainforced Concrete

FIBRE REINFORCED CONCRETE Chapter- 1 INTRODUCTION INTRODUCTION

Fibre reinforced concrete (FRC) may be defined as a composite materials made with Portland cement, aggregate, and incorporating discrete discontinuous fibres. Portland cement concrete is considered to be a relatively brittle material, with a low tensile strength and a low strain capacity. When subjected to tensile stresses, non-reinforced concrete will crack and fail. Since mid 1800's steel reinforcing has been used to overcome this problem. As a composite system, the reinforcing steel is assumed to carry all tensile loads. Fibre reinforced concrete (FRC) is Portland cement concrete reinforced with more or less randomly distributed fibers. In FRC, thousands of small fibers are dispersed and distributed randomly in the concrete during mixing, and thus improve concrete properties in all directions. Fibers help to improve the post peak ductility performance, pre-crack tensile strength, fatigue strength, impact strength and eliminate temperature and shrinkage cracks. The role of randomly distributes discontinuous fibres is to bridge across the cracks that develop provides some post- cracking “ductility”. If the fibres are sufficiently strong, sufficiently bonded to material, and permit the FRC to carry significant stresses over a relatively large strain capacity in the post-cracking stage. The real contribution of the fibres is to increase the toughness of the concrete (defined as some function of the area under the load vs. deflection curve), under any type of loading. That is, the fibres tend to increase the strain at peak load, and provide a great deal of energy absorption in post-peak portion of the load vs. deflection curve. When the fibre reinforcement is in the form of short discrete fibres, they act effectively as rigid inclusions in the concrete matrix. Physically, they have thus the same order of magnitude as aggregate inclusions; steel fibre reinforcement cannot therefore be regarded as a direct replacement of longitudinal reinforcement in reinforced and prestressed structural members. However, because of the inherent material properties of fibre concrete, the presence of fibres in the body of the concrete or the provision of a tensile skin of fibre concrete can be expected to improve the resistance of conventionally reinforced structural members to cracking, deflection and other serviceability conditions. 1

FIBRE REINFORCED CONCRETE

Figure1. The load vs. deflection curve

The fibre reinforcement may be used in the form of three dimensionally randomly distributed fibres throughout the structural member when the added advantages of the fibre to shear resistance and crack control can be further utilized. On the other hand, the fibre concrete may also be used as a tensile skin to cover the steel reinforcement when a more efficient two – dimensional orientation of the fibres could be obtained. Fibre reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. Fibres include steel fibres, glass fibres, synthetic fibres and natural fibres. Within these different fibres that character of fibre reinforced concrete changes with varying concretes, fibre materials, geometries, distribution, orientation and densities. Fibre reinforced is mainly used in shotcrete, but can also be used in normal concrete. Fibrereinforced normal concrete are mostly used for on-ground floors and pavements, but can be considered for a wide range of construction parts (beams, pliers, foundations etc) either alone or with hand-tied rebars.

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FIBRE REINFORCED CONCRETE Chapter- 2 HISTORY OF DEVELOPMENT OF FIBRE REINFORCED CONCRETE

HISTORY OF DEVELOPMENT OF FIBRE REINFORCED CONCRETE

The use of fibres to reinforce and enhance the properties of construction materials goes back at least 3500 years, when straw was used to reinforce sun-baked bricks in Mesopotamia. Egyptians also used straw to reinforce mud bricks, but there is evidence that asbestos fiber was used to reinforce clay posts about 5000 years ago. Cement-bound products have been reinforced by various types of fibre at least since the beginning of the last century, and steel and synthetic fibres have been used to improve the properties of concrete for the past 30 or 40 years. Portland cement concrete is considered to be a relatively brittle material. When subjected to tensile stresses, non-reinforced concrete will crack and fail. Since mid 1800's steel reinforcing has been used to overcome this problem. As a composite system, the reinforcing steel is assumed to carry all tensile loads. The problem with employing steel in concrete is that over time steel corrodes due to the ingress of chloride ions. In the northeast, where sodium chloride de-icing salts are commonly used and a large amount of coastal area exists, chlorides are readily available for penetration into concrete to promote corrosion, which favors the formation of rust. Rust has a volume between four to ten times the iron, which dissolves to form it. The volume expansion produces large tensile stresses in the concrete, which initiates cracks and results in concrete spalling from the surface. Although some measures are available to reduce corrosion of steel in concrete such as corrosion inhibitive admixtures and coatings, a better and permanent solution may be replace the steel with a reinforcement that is less environmentally sensitive. Fibre Reinforced Concrete (FRC) was invented by French gardener Joseph Monier in 1849 and patented in 1867. More recently micro fibres, such as those used in traditional composite materials have been introduced into the concrete mixture to increase its toughness, or ability to resist crack growth. Several different types of fibres, both manmade and natural, have been incorporated into concrete. Use of natural fibres in concrete precedes the advent of conventional reinforced concrete in historical context. However, the technical aspects of FRC systems remained 3

FIBRE REINFORCED CONCRETE essentially undeveloped. Since the advent of fibre reinforcing of Concrete in the 1940's, a great deal of testing has been conducted on the various fibrous materials to determine the actual characteristics and advantages for each product. Several different types of fibbers have been used to reinforce the cement-based Matrices. The choice of fibbers varies from synthetic organic materials such as polypropylene or carbon, synthetic inorganic such as steel or glass, natural organic such As cellulose or sisal to natural inorganic asbestos. Currently the commercial products are reinforced with steel, glass, polyester and polypropylene fibres. The selection of the type of fibres is guided by the properties of the fibres such as diameter, specific gravity, young’s modulus, tensile strength etc and the extent these fibres affect the properties of the cement matrix.

Figure2. Steel, glass, synthetic and natural fibres with different lengths and shapes can be used in concrete

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FIBRE REINFORCED CONCRETE Chapter-3 EFFECT OF FIBRES IN CONCRETE EFFECT OF FIBRES IN CONCRETE Fibres are usually used in concrete to control plastic shrinkage cracking and drying shrinkage cracking. They also lower the permeability of concrete and thus reduce bleeding of water. Some types of fibres produce greater impact, abrasion and shatter resistance in concrete. Generally fibres do not increase the flexural strength of concrete, so it cannot replace moment resisting or structural steel reinforcement. Some fibres reduce the strength of concrete. The amount of fibres added to a concrete mix is measured as a percentage of the total volume of the composite (concrete and fibres) termed volume fraction (Vf). Vf typically ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing fibre length (l) by its diameter (d). Fibres with a non-circular cross section use an equivalent diameter for the calculation of aspect ratio. If the modulus of elasticity of the fibre 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 fibre usually segments the flexural strength and toughness of the matrix. However, fibres which are too long tend to “ball” in the mix and create workability problems.It is important to understand the effect of fibers in concrete mechanism. The composite will carry increasing loads after the first cracking of the matrix if the pull-out resistance of the fibers at the first crack is greater than the load at first cracking. At the cracked section, the matrix does not resist any tension and the fibers carry the entire load taken by the composite. With an increasing load on the composite, the fibers will tend to transfer the additional stress to the matrix through bond stresses. This process of multiple cracking will continue until either fibers fail or the accumulated local debonding will lead to fiber pull-out. Fibres such as graphite and glass have excellent resistance to creep, while the same is not true for most resins. Therefore, the orientation and volume of fibres have a significant influence on the creep performance of rebars/tendons. It has been recognized that the addition of small, closely spaced and uniformly dispersed fibres to concrete would act as crack arrester and would substantially improve its static and dynamic properties. Reinforced concrete itself is a composite material, where the reinforcement acts as the strengthening fibre and the concrete as the matrix. It is therefore imperative that the behavior under thermal stresses for the two materials be similar so that the differential deformations of concrete and the reinforcement are minimized. 5

FIBRE REINFORCED CONCRETE Chapter-4 FACTORS EFFECTING PROPERTIES OF FIBRE REINFORCED CONCRETE

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FACTORS EFFECTING PROPERTIES OF FIBRE REINFORCED

CONCRETE Fibre reinforced concrete is the composite material containing fibres in the cement matrix in an orderly manner or randomly distributed manner. Its properties would obviously, depends upon the efficient transfer of stress between matrix and the fibres. The factors are briefly discussed below:

Relative fibre Matrix Stiffness The modulus of elasticity of matrix must be much lower than that of fibre for efficient stress transfer. Low modulus of fibre such as nylons and polypropylene are, therefore, unlikely to give strength improvement, but the help in the absorbsion of large energy and therefore, impart greater degree of toughness and resistance to impart. High modulus fibres such as steel, glass and carbon impart strength and stiffness to the composite. Interfacial bond between the matrix and the fibre also determine the effectiveness of stress transfer, from the matrix to the fibre. A good bond is essential for improving tensile strength of the composite. The interfacial bond could be improved by larger area of contact, improving the frictional properties and degree of gripping and by treating the steel fibres with sodium hydroxide or acetone.

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FIBRE REINFORCED CONCRETE

Figure3. Reinforcement in a concrete matrix Volume of Fibres The strength of the composite largely depends on the quantity of fibres used in it. Figure 5 and 6 show the effect of volume on the toughness and strength. It can see from Figure 5 that the increase in the volume of fibres, increase approximately linearly, the tensile strength and toughness of the composite. Use of higher percentage of fibre is likely to cause segregation and harshness of concrete and mortar.

Figure4. Stress v/s Strain on fibre volume curve

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FIBRE REINFORCED CONCRETE

Figure5. Effect of volume of fibres Figure6. Effect of volume of fibres In flexure

in tension

Aspect Ratio of the Fibre Another important factor which influences the properties and behavior of the composite is the aspect ratio of the fibre. The Aspect Ratio of a fibre is the ratio of its length to its ‘equivalent’ diameter. As long as a fibre’s basic shape, tensile strength, dosage and anchorage mechanism remain the same, a higher aspect ratio will result in a steel fibre reinforced concrete element having a higher post-crack load carrying capacity. This improved performance is due to the increased fibre count i.e. there are more fibres providing tensile capacity at each cracked section. The Aspect Ratio of the fibres chosen for a particular application is a function of economics and performance. It has been reported that up to aspect ratio of 75, increase on the aspect ratio increases the ultimate concrete linearly. Beyond 75, relative strength and toughness is reduced. Table 1.1 shows the effect of aspect ratio on strength and toughness.

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FIBRE REINFORCED CONCRETE Table1. Aspect ratio of the fibre Aspect Type of concrete Ratio Relative strength Relative toughness

Plain concrete 0 1 1

With 25 1.5 2.0

Randomly 50 1.6 8.0

Dispersed fibres 75 1.7 10.5

100 1.5 8.5

Orientation of Fibres One of the differences between conventional reinforcement and fibre reinforcement is that in conventional reinforcement, bars are oriented in the direction desired while fibres are randomly oriented. To see the effect of randomness, mortar specimens reinforced with 0.5% volume of fibres were tested. In one set specimens, fibres were aligned in the direction of the load, in another in the direction perpendicular to that of the load, and in the third randomly distributed. It was observed that the fibres aligned parallel to the applied load offered more tensile strength and toughness than randomly distributed or perpendicular fibres. Workability and Compaction of Concrete It is well known that the addition of any type of fibers to plain concrete reduces the workability. Incorporation of steel fibre decreases the workability considerably. This situation adversely affects the consolidation of fresh mix. Even prolonged external vibration fails to compact the concrete. The fibre volume at which this situation is reached depends on the length and diameter of the fibre. Since fibres impart considerable stability to a fresh concrete mass, the slump cone test is not a good index of workability. For example, introduction of 1.5 volume percent steel or glass fibres to a concrete with 200 mm of slump is likely to reduce the slum of the mixture to about 25 mm, but the placeability of the concrete and its compactability under vibration may still be satisfactory. Therefore, the Vebe test is considered more appropriate for evaluating the workability of fibre-reinforce concrete mixtures. Another consequence of poor workability is non-uniform 9

FIBRE REINFORCED CONCRETE distribution of the fibres. Generally, the workability and compaction standard of the mix is improved through increased water/ cement ratio or by the use of some kind of water reducing admixtures. Size of Coarse Aggregate Maximum size of the coarse aggregate should be restricted to 10mm, to avoid appreciable reduction in strength of the composite. fibres also in effect, act as aggregate. Although they have a simple geometry, their influence on the properties of fresh concrete is complex. The inter-particle friction between fibres and between fibres and aggregates controls the orientation and distribution of the fibres and consequently the properties of the composite. Friction reducing admixtures and admixtures that improve the cohesiveness of the mix can significantly improve the mix. Mixing Mixing of fibre reinforced concrete needs careful conditions to avoid balling of fibres, segregation and in general the difficulty of mixing the materials uniformly. Increase in the aspect ratio, volume percentage and size and quantity of coarse aggregate intensify the difficulties and balling tendency. Steel fibre content in excess of 2% by volume and aspect ratio of more than 100 are difficult to mix. Mixing of FRC can be accomplished by many methods. The mix should have a uniform dispersion of the fibres in order to prevent segregation or balling of the fibres during mixing. Most balling occurs during the fibre addition process. Increase of aspect ratio, volume percentage of fibre, and size and quantity of coarse aggregate will intensify the balling tendencies and decrease the workability. To coat the large surface area of the fibres with paste, experience indicated that a water cement ratio between 0.4 and 0.6, and minimum cement content of 400 kg/m3 are required. Compared to conventional concrete, fibre reinforced concrete mixes are generally characterized by higher cement factor, higher fine aggregate content, and smaller size coarse aggregate. A fibre mix generally requires more vibration to consolidate the mix. External vibration is preferable to prevent fibre segregation. Metal trowels, tube floats, and rotating power floats can be used to finish the surface.It is important that the fibres are dispersed uniformly throughout the mix; this can be done by the addition of the fibres before the water is added. 10

FIBRE REINFORCED CONCRETE When mixing in a laboratory mixer, introducing the fibres through a wire mesh basket will help even distribution of fibres. For field use, other suitable methods must be adopted.

The typical proportion for fibre reinforced concrete is given below: Cement content W/C ratio

: 325 to 550 kg/cm2

: 0.4 to 0.6

% of sand to total aggregate : 50 to 100% Maximum aggregate size Air-content

: 10 mm

: 6 to 9%

Fibre content : 0.5 to 2.5% by volume of mix

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FIBRE REINFORCED CONCRETE Chapter-5 DIFFERENT TYPE OF FIBRES DIFFERENT TYPE OF FIBRES Following are the different type of fibres generally used in the construction industries. Steel fibre Reinforced Concrete Polypropylene fibre Reinforced (PFR) cement mortar &concrete Glass-fibre Reinforced Concrete Asbestos fibres Carbon fibres Organic fibres

Steel fibre Reinforced Concrete A number of steel fibre types are available as reinforcement. Round steel fibre the commonly used type are produced by cutting round wire in to short length. The typical diameter lies in the range of 0.25 to 0.75mm. Steel fibres having a rectangular c/s are produced by silting the sheets about 0.25mm thick. Fibre made from mild steel drawn wire. Conforming to IS: 2801976 with the diameter of wire varying from 0.3 to 0.5mm have been practically used in India. Round steel fibres are produced by cutting or chopping the wire, flat sheet fibres having a typical c/s ranging from 0.15 to 0.41mm in thickness and 0.25 to 0.90mm in width are produced by silting flat sheets. Deformed fibre, which are loosely bounded with watersoluble glue in the form of a bundle are also available. Since individual fibres tend to cluster together, their uniform distribution in the matrix is often difficult. This may be avoided by adding fibres bundles, which separate during the mixing process. The steel fibre reinforcement not only improves the toughness of the material, the impact and the fatigue resistance of concrete, but it also increases the material resistance to cracking and, hence to water and chloride ingress with significant improvement in durability of concrete structures. Therefore, the use of SFRC in tunnel structures represents an attractive technical solution with respect to the conventional steel reinforcement, because it reduces both the labor costs (e.g. due to the placement of the conventional steel bars) and the construction costs (e.g. 12

FIBRE REINFORCED CONCRETE forming and storage of classical reinforcement frames, risks of spalling during transportation and laying).

Figure7. Different type of steel fibre

Structural use of SFRC As recommended by ACI Committee 544, ‘when used in structural applications, steel fibre reinforced concrete should only be used in a supplementary role to inhibit cracking, to improve resistance to impact or dynamic loading, and to resist material disintegration. In structural members where flexural or tensile loads will occur the reinforcing steel must be capable of supporting the total tensile load’. Thus, while there are a number of techniques for predicting the strength of beams reinforced only with steel fibres, there are no predictive equations for large SFRC beams, since these would be expected to contain conventional reinforcing bars as well. For beams containing both fibres and continuous reinforcing bars, the situation is complex, since the fibres act in two ways: They permit the tensile strength of the SFRC to be used in design, because the matrix will no longer lose its load-carrying capacity at first crack. They improve the bond between the matrix and the reinforcing bars by inhibiting the growth

of cracks emanating form the deformations (lugs) on the bars. 13

FIBRE REINFORCED CONCRETE However, it is the improved tensile strength of SFRC that is mostly considered in the beam analysis, since the improvements in bond strength are much more difficult to quantify. Steel fibres have been shown to increase the ultimate moment and ultimate deflection of conventionally reinforced beams; the higher the tensile stress due to the fibres, the higher the ultimate moment. Polypropylene Fibre Reinforced (PFR) cement Polypropylene fibre reinforced concrete or mortar is an additive to concrete and mixes which considerably reduces the risk of drying cracks. The plastic fibres consist of polypropylene fibres which are resistant in alkaline environments and the length and dosage of which can vary depending on what properties are required. Polypropylene fibre was first used to reinforce concrete in the 1960s. Polypropylene is a synthetic hydrocarbon polymer, the fibre of which is made using extrusion processes by hotdrawing the material through a die. Polypropylene fibres are produced as continuous monofilaments, with circular cross section that can be chopped to required lengths, or fibrillated films or tapes of rectangular cross section. Polypropylene fibres are hydrophobic and therefore have the disadvantages of poor bond characteristics with cement matrix, a low melting point, high combustibility and a relatively low modulus of elasticity. Long polypropylene fibres can prove difficult to mix due to their flexibility and tendency to wrap around the leading edges of mixer blades. Polypropylene fibres are tough but have low tensile strength and modulus of elasticity; they have a plastic stress-strain characteristic. Polypropylene is one of the cheapest & abundantly available polymers polypropylene fibres 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 fibre properties. Polypropylene fibres 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 fibres in small volume fractions between 0.5 to 15 commercially used in concrete. 14

FIBRE REINFORCED CONCRETE Polypropylene fibre reinforced concrete or mortars are used for: Considerably reducing the risk that drying cracks will arise during dry, windy and hot weather. Improving the cohesion between fresh concrete batches. Improving the ability to pump through pipes and hoses of smaller dimensions and through greater pumping distances. Improving workability while pouring. Areas of application Used in concrete and mortar where there is a risk of dry cracking. Drying cracks are most often formed in the case of concrete pouring that is carried out without protection during weather that is dry, windy or hot. Casting of slabs, beams and cast on structures that are particularly vulnerable. It can be difficult to protect concrete or mortar against dry cracking immediately after pouring has been completed. In such situations, the plastic fibre reinforcement is a simple and effective aid, until the surface can be protected by using conventional methods such as water, well-functioning curing membranes or coverings. Mixing in plastic fibre reinforcement does not replace normal curing methods

Figure8. Polypropylene fibre reinforced cement-mortar & concrete

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FIBRE REINFORCED CONCRETE Glass-fibre Reinforced Concrete Glass fibre is made up from 200-400 individual filaments which are lightly bonded to make up a stand. These stands can be chopped into various lengths, or combined to make cloth mat or tape. Using the conventional mixing techniques for normal concrete it is not possible to mix more than about 2% (by volume) of fibres of a length of 25mm. The first research on glass fibres in the early 1960s used conventional borosilicate glass (Eglass) and soda-lime-silica glass fibres (A-glass). The test results showed that alkali reactivity between the E-glass fibres and the cement-paste reduced the strength of the concrete. Continued research resulted in alkali-resistant glass fibres (AR-glass), that improved longterm durability, but sources of other strength-loss trends were observed. One acknowledged source was fibre embrittlement stemming from infiltration of calcium hydroxide particles, byproducts of cement hydration, into fibre bundles. The major appliance of glass fibre has been in reinforcing the cement or mortar matrices used in the production of thin-sheet products. The commonly used verities of glass fibres are e-glass used. In the reinforced of plastics & AR glass E-glass has inadequate resistance to alkalis present in Portland cement where ARglass has improved alkali resistant characteristics. Sometimes polymers are also added in the mixes to improve some physical properties such as moisture movement.

Figure9. Glass-fibre reinforced concrete

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FIBRE REINFORCED CONCRETE Asbestos fibres The naturally available inexpensive mineral fibre, asbestos, has been successfully combined with Portland cement paste to form a widely used product called asbestos cement. Asbestos fibres here thermal mechanical & chemical resistance making them suitable for sheet product pipes, tiles and corrugated roofing elements Asbestos cement board is approximately two or four times that of unreinforced matrix. However, due to relatively short length (10mm) the fibre have low impact strength.

Figure10. Asbestos fibres

Carbon fibres Carbon fibres are manufactured by carbonizing suitable organic materials in fibrous forms at high temperatures and then aligning the resultant graphite crystallites by hot-stretching. The fibres are manufactured as either Type I (high modulus) or Type II (high strength) and are dependent upon material source and extent of hot stretching for their physical properties. Carbon fibres are available in a variety of forms and have a febrile structure similar to that of asbestos. Carbon fibre made from petroleum and coal pitch is less expensive than the conventional carbon fibre made from fibrous materials. The Type I and II carbon fibres produced by carbonizing suitable organic materials other than petroleum-based materials are 20 to 40 times stronger and have a modulus of elasticity up to 100 times greater than the pitch-based carbon fibre. Carbon fibres from the most recent & probability the most spectacular addition to the range of fibre available for commercial use. Carbon fibre comes under the very high modulus of 17

FIBRE REINFORCED CONCRETE 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 fibre, and hence are generally treated with resign coating. Tensile creep is reduced slightly, but flexural creep can be substantially reduced when very stiff carbon fibres are used

Figure11. Carbon fibres

Carbon fibre cement-matrix composites are structural materials that are gaining in importance quite rapidly due to the decrease in carbon fibre cost and the increasing demand of superior structural and functional properties. These composites contain short carbon fibres, typically 5 mm in length, as the short fibres can be used as an admixture in concrete (whereas continuous fibres cannot be simply added to the concrete mix) and short fibres are less expensive than continuous fibres. However, due to the weak bond between carbon fibre and the cement matrix, continuous fibres [2±4] are much more effective than short fibres in reinforcing concrete. Surface treatment of carbon fibre (e.g. by heating or by using ozone , silane , SiO2 particles or hot NaOH solution ) is useful for improving the bond between fibre and matrix, thereby improving the properties of the composite. In the case of surface treatment by ozone or silane, the improved bond is due to the enhanced wettability by water. Admixtures such as latex methylcellulose and silica fume also help the bond The effect of carbon fibre addition on the properties of concrete increases with fibre volume fraction, unless the fibre volume fraction is so high that the air void content becomes excessively high .(The air void content increases with fibre content and air voids tend to have

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FIBRE REINFORCED CONCRETE a negative effect on many properties, such as the compressive strength.) In addition, the workability of the mix decreases with fibre content. Moreover, the cost increases with fibre content. Therefore, a rather low volume fraction of fibres is desirable. A fibre content as low as 0.2 vol. % is effective, although fibre contents exceeding 1 vol. % are more common. The required fibre content increases with the particle size of the aggregate, as the Flexural strength decreases with increasing particle size. Effective use of the carbon fibres in concrete requires dispersion of the fibres in the mix. The dispersion is enhanced by using silica fume (a fine particulate) as an admixture [22±24]. A typical silica fume content is 15% by weight of cement. The silica fume is typically used along with a small amount (0.4% by weight of cement) of methylcellulose for helping the dispersion of the fibres and the workability of the mix. Latex (typically 15± 20% by weight of cement) is much less effective than silica fume for helping the fibre dispersion, but it enhances the workability, Flexural strength, Flexural toughness, impact resistance, frost resistance and acid resistance. The ease of dispersion increases with decreasing fibre length The improved structural properties rendered by carbon fibre addition pertain to the increased tensile and flexible strengths, the increased tensile ductility and flexural toughness, the enhanced impact resistance, the reduced drying shrinkage and the improved freeze thaw durability.

Figure12. Figure of Carbon Fibre

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FIBRE REINFORCED CONCRETE Organic fibres Organic fibre such as polypropylene or natural fibre may be chemically more inert than either steel or glass fibres. They are also cheaper, especially if natural. A large volume of vegetable fibre may be used to obtain a multiple cracking composite. The problem of mixing and uniform dispersion may be solved by adding a super plasticizer.

Figure13. Organic fibre

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FIBRE REINFORCED CONCRETE Chapter-6 CURRENT DEVELOPMENT IN FIBRE REINFORCED CONCRETE

CURRENT DEVELOPMENT IN FIBRE REINFORCED CONCRETE There is a new generation of high performance fiber-reinforced composites. In many of these materials the strength, toughness, and durability are significantly improved which were not easily achieved by normal fibre reinforced concrete. Here some new type of fibre reinforced concrete discussed Compact Reinforced Composites Reactive Powder Concrete Slurry-Infiltrated-fibred concrete Engineered Cementitious Composite (EEC) 6.1.

Compact Reinforced Composites (CRC)

Compact Reinforced Composites is a material consisting of an extremely strong, dense cement matrix, 20-30% silica fumes by weight of cement, 10-20% by volume off conventional reinforcement and 5-10% of fine fibres of 6 mm long and 0.15 mm diameter. While such a material is extremely expensive, it exhibits a flexural strength up to 260 MPa and compressive strength about 200 MPa. Advantage is that it can be moulded and fabricated at site. Researchers in Denmark created Compact Reinforced Composites using metal fibres, 6 mm long and 0.15 mm in diameter, and volume fractions in the range of 5 to 10 %. High frequency vibration is needed to obtain adequate compaction. These short fibres increase the tensile strength and toughness of the material. The increase of strength is greater than the increase in ductility; therefore the structural design of large beams and slabs requires that a higher amount of reinforcing bars be used to take advantage of the composite. The short fibres are an efficient mechanism of crack control around the reinforcing bars. The final cost of the structure will be much higher than if the structure would be made by traditional methods, therefore the use of compact reinforced composites is mainly justified when the structure requires special behavior, such as high impact resistance or very high mechanical properties. 21

FIBRE REINFORCED CONCRETE

Reactive Powder Concrete (RPC) Investigators in France by adding metal fibres, 13 mm long and 0.15 mm in diameter, with a maximum volume fraction of 2.5%. This composite uses fibres that are twice as long as the compact reinforced composites therefore, because of workability limitations, cannot incorporate the same volume fraction of fibres. The smaller volume fraction results in a smaller increase in the tensile strength of the concrete. Commercial versions of this product have further improved the strength of the matrix, chemically treated the surface of the fibre, and added microfibers. Slurry-Infiltrated-fibred concrete (SIFCON) The processing of this composite consists in placing the fibres in a formwork and then infiltrating a high w/c ratio mortar slurry to coat the fibres. Compressive and tensile strengths up to 120 MPa and 40 MPa, respectively have been obtained. Modulus of rupture up 90 MPa and shear strength up to 28 MPa has been also reported. In direct tension along the direction of the fibres, the material shows a very ductile response. This composite has been used in pavements slabs, and repair. SIFCON can be used blast resistant structures and burglar proof safe vaults in bank and residential buildings. Engineered Cementitious Composite (ECC) The ultra high-ductility of this composite, 3-7% was obtained by optimizing the interactions between fiber, matrix and its interface. Mathematical models were developed so that a small volume fraction of 2% was able to provide the large ductility. The material has a very high stain capacity and toughness and controlled crack propagation. The manufacturing of ECC can be done by normal casting or by extrusion. By using an optimum amount of super plasticizer and non-ionic polymer with steric action, it was possible to obtain self-compacting casting. Experimental results with extruded pipes indicate that the system has a plastic yielding behaviour instead of the typical brittle fracture exhibited when plain concrete is used.

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FIBRE REINFORCED CONCRETE Chapter-7 STRUCTURAL BEHAVIOR OF FIBRE REINFORCED CONCRETE STRUCTURAL BEHAVIOR OF FIBRE REINFORCED CONCRETE Fibres combined with reinforcing bars in structural members will be widely used in the future. The following are some of the structural behavior. Flexure The use of fibres in reinforced concrete flexure members increases ductility, tensile strength, moment capacity, and stiffness. The fibres improve crack control and preserve post cracking structural integrity of members. Torsion The use of fibres eliminates the sudden failure characteristic of plain concrete beams. It increases stiffness, torsional strength, ductility, rotational capacity, and the number of cracks with less crack width. Shear Addition of fibres increases shear capacity of reinforced concrete beams up to 100 percent. Addition of randomly distributed fibres increases shear-friction strength, the first crack strength, and ultimate strength. Column The increase of fibre content slightly increases the ductility of axially loaded speci- men. The use of fibres helps in reducing the explosive type failure for columns. High Strength Concrete Fibres increase the ductility of high strength concrete. The use of high strength concrete and steel produces slender members. Fibre addition will help in controlling cracks and deflections. Cracking and Deflection Tests have shown that fibre reinforcement effectively controls cracking and de- flection, in addition to strength improvement. In conventionally reinforced concrete beams, fibre addition increases stiffness, and reduces deflection.

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FIBRE REINFORCED CONCRETE Chapter-8 MECHANICAL PROPERTIES OF FIBRE REINFORCED CONCRETE MECHANICAL PROPERTIES OF FIBRE REINFORCED CONCRETE Addition of fibres to concrete influences its mechanical properties which significantly depend on the type and percentage of fibre. Fibres with end anchorage and high aspect ratio were found to have improved effectiveness. It was shown that for the same length and diameter, crimped-end fibres can achieve the same properties as straight fibres using 40 percent less fibres. In determining the mechanical properties of FRC, the same equipment and procedure as used for conventional concrete can also be used. Below are cited some properties of FRC determined by different researchers. Compressive Strength The presence of fibres may alter the failure mode of cylinders, but the fibre effect will be minor on the improvement of compressive strength values (0 to 15 percent). Fibres do little to enhance the static compressive strength of concrete, with increases in strength ranging from essentially nil to perhaps 15%. Even in members who contain conventional reinforcement in addition to the steel fibres, the fibres have little effect on compressive strength. However, the fibres do substantially increase the post-cracking ductility, or energy absorption of the material. Modulus of Elasticity Modulus of elasticity of FRC increases slightly with an increase in the fibres content. It was found that for each 1 percent increase in fibre content by volume there is an increase of 3 percent in the modulus of elasticity. Flexure The flexural strength was reported to be increased by 2.5 times using 4 percent fibres. Steel fibres are generally found to have aggregate much greater effect on the flexural strength of SFRC than on either the compressive or tensile strength, with increases of more than 100% having been reported. The increase in flexural strength is particularly sensitive, not only to the fibre volume, but also to the aspect ratio of the fibres, with higher aspect ratio leading to larger strength increases. 24

FIBRE REINFORCED CONCRETE

Toughness For FRC, toughness is about 10 to 40 times that of plain concrete. The toughness index of FRC is increased up to 20 folds (for 1.5 percent hooked fibre content) indicating excellent energy absorbing capacity. Splitting Tensile Strength The presence of 3 percent fibre by volume was reported to increase the splitting tensile strength of mortar about 2.5 times that of the unreinforced one. Fatigue Strength The addition of fibres increases fatigue strength of about 90 percent and 70 percent of the static strength at 2 x 106 cycles for non-reverse and full reversal of loading, respectively. Impact Resistance The impact strength for fibrous concrete is generally 5 to 10 times that of plain concrete depending on the volume of fibre used. Corrosion of Steel Fibres When well compacted and cured concretes containing steel fibers seem to possess excellent durability as long as fibers remain protected by the cement paste. In most environments, especially those containing chloride, surface rusting is inevitable but the fibers in the interior usually remain uncorroded. A l0-year exposure of steel fibrous mortar to outdoor weathering in an industrial atmosphere showed no adverse effect on the strength properties. Corrosion was found to be confined only to fibres actually exposed on the surface. Steel fibrous mortar continuously immerse in seawater for 10 years exhibited a 15 percent loss compared to 40 percent strength decrease of plain mortar.

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FIBRE REINFORCED CONCRETE Chapter-9 APPLICATIONS APPLICATIONS

The uniform dispersion of fibres throughout the concrete mix provides isotropic properties not common to conventionally reinforced concrete. The applications of fibres in concrete industries depend on the designer and builder in taking advantage of the static and dynamic characteristics of this new material. The main area of FRC applications are Runway, Aircraft Parking, and Pavements For the same wheel load FRC slabs could be about one half the thickness of plain concrete slab. Compared to a 375mm thickness' of conventionally reinforced concrete slab, a 150mm thick crimped-end FRC slab was used to overlay an existing asphaltic paved aircraft parking area. FRC pavements are now in service in severe and mild environments. Tunnel Lining and Slope Stabilization Steel fibre reinforced shotcrete (SFRS) are being used to line underground openings and rock slope stabilization. It eliminates the need for mesh reinforcement and scaffolding. Blast Resistant Structures When plain concrete slabs are reinforced conventionally, tests showed that there is no reduction of fragment velocities or number of fragments under blast and shock waves. Similarly, reinforced slabs of fibrous concrete, however, showed 20 percent reduction in velocities, and over 80 percent in fragmentations. Thin Shell, Walls, Pipes, and Manholes Fibrous concrete permits the use of thinner flat and curved structural elements. Steel fibrous shotcrete is used in the construction of hemispherical domes using the inflated membrane process. Glass fibre reinforced cement or concrete (GFRC), made by the spray-up process, have been used to construct wall panels. Steel and glass fibres addition in concrete pipes and manholes improves strength, reduces thickness, and diminishes handling damages.

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FIBRE REINFORCED CONCRETE

Figure14. Segments Figure15. Beam

Dam sand Hydraulic Structure FRC is being used for the construction and repair of dams and other hydraulic structures to provide resistance to cavitation and severe erosion caused by the impact of large water born debris. Other Applications These include machine tool frames, lighting poles, water and oil tanks and concrete repairs.

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FIBRE REINFORCED CONCRETE Chapter-10 FIBRE REINFORCED CONCRETE VERSUS CONVENTIONALLY REINFORCED CONCRETE

10. FIBRE REINFORCED CONCRETE VERSUS CONVENTIONALLY REINFORCED CONCRETE Unreinforced concrete has a low tensile strength and a low strain capacity at fracture. These shortcomings are traditionally overcome by adding reinforcing bars or prestressing steel. Reinforcing steel is continuous and is specifically located in the structure to optimize performance. Reinforced concrete itself is a composite material, where the reinforcement acts as the strengthening fibre and the concrete as the matrix. It is therefore imperative that the behavior under thermal stresses for the two materials be similar so that the differential deformations of concrete and the reinforcement are minimized. Fibres are discontinuous and are generally distributed randomly throughout the concrete matrix. Although not currently addressed by ACI Committee 318, fibres are being used in structural applications with conventional reinforcement. Because of the flexibility in methods of fabrication, fibre reinforced concrete can be an economic and useful construction material. For example, thin (1/2 to 3/4 in. [13 to 20 mm] thick), precast glass fibre reinforced concrete architectural cladding panels are economically viable in the U.S. and Europe. In slabs on grade, mining, tunneling, and excavation support applications, steel and synthetic fibre reinforced concrete and shotcrete have been used in lieu of welded wire fabric reinforcement. According to the report by ACI Committee 554 the total energy absorbed in fibre debonding as measured by the area under the load-deflection curve before complete separation of a beam is at least 10 to 40 times higher for fibre-reinforced concrete than for plain concrete.

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FIBRE REINFORCED CONCRETE

Comparison of Performance of Standard Concrete and Fibre Reinforced Standard Concrete Exposed To Elevated Temperatures Experimental Programme Table 2- Material used during experiment

1). An increase in compressive strength and tensile strength has been observed for both standard concrete and fibre reinforced standard concrete when exposed to a temperature of 500C. 2). In the range of 50 to 800C the split tensile strength of both standard concrete and fibre reinforced standard concrete is same. 3). Flexural strength of standard concrete is equal to that of the fibre reinforced standard concrete in range of 500C-800C. 4). Beyond 500C, both standard concrete and fibre reinforced standard concrete are found to loose compressive strength gradually. 5). Fibre reinforced standard concrete is found to exhibit more compressive strength split tensile strength and flexural strength than standard concrete at all temperatures. 6). The difference between compressive strength of fibre reinforced standard concrete and standard concrete varies in the range of 6-10percentage. 7). The difference between split tensile strength of fibre reinforced standard concrete and standard concrete varies in the range of 0-12 percentage. 8). The difference between flexural strength of fibre reinforced standard concrete and standard concrete varies in the range of 0-20 percentage.

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FIBRE REINFORCED CONCRETE ADVANTAGES AND DISADVANTAGES OF FIBRE REINFORCED CONCRETE Fibre reinforced concrete has started to find its place in many areas of civil infrastructure applications where the need for repairing, increased durability arises. Also FRCs are used in civil structures where corrosion can be avoided at the maximum. Fibre reinforced concrete is better suited to minimize cavitations/erosion damage in structures such as sluice-ways, navigational locks and bridge piers where high velocity flows are encountered. A substantial weight saving can be realized using relatively thin FRC sections having the equivalent strength of thicker plain concrete sections. FRC controls cracking and deformation under impact load much better than plain concrete and increased the impact strength 25 times. When used in bridges it helps to avoid catastrophic failures. Also in the quake prone areas the use of fibre reinforced concrete would certainly minimize the human casualties. In addition, polypropylene fibres reduce or relieve internal forces by blocking microscopic cracks from forming within the concrete. The main disadvantage associated with the fibre reinforced concrete is fabrication.

The process of incorporating fibres into the cement matrix is labor intensive and costlier than the production of the plain concrete. The real advantages gained by the use of Fibre Reinforced Concrete overrides this disadvantage.

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FIBRE REINFORCED CONCRETE Chapter-11 SUMMERY AND DISUCSSION SUMMERY AND DISUCSSION

A brief state-of-the-art report on fiber reinforced concrete is presented. Our understanding of fiber-matrix interaction, reinforcement mechanisms and performance characteristics is fairly advanced. The theory, properties, and typical applications of concrete reinforced with the various fibres have been described. Knowledge of the theory of the different fibre types and categories is necessary to understand the appropriate testing and degree of quality control/assurance necessary to ensure that design requirements are satisfied. The usefulness of fiber reinforced concrete (FRC) in various civil engineering applications is indisputable. Fiber reinforced concrete has started to find its place in many areas of civil infrastructure applications where the need for repairing, increased durability arises. Also FRC are used in civil structures where corrosion can be avoided at the maximum.Fiber reinforced concrete has so far been successfully used in slabs on grade, shotcrete, architectural panels, precast products, offshore structures, structures in seismic regions, thin and thick repairs, crash barriers, footings, hydraulic structures and many other applications.FRC is slowly becoming a well accepted mainstream construction material. There are currently 200,000 metric tons of fibers used for concrete reinforcement. Steel fiber remains the most used fiber of all (50% of total tonnage used) followed by polypropylene (20%), glass (5%) and other fibers (25%).

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