Fibre Reinforced Concrete

Kalyani Quantum 2008 A TECHNICAL PAPER ON

Submitted By

Amar J.Thorat S.E(civil) Shivaratan Desai S.E(civil)

Ganesh M.Gholap F.E(civil)

Rajarambapu Institute of Technology,Rajaramnagar.

.

Contents Abstract Introduction Role of Fibre Behavior of FRC Factor affecting properties of Reinforced Concrete What do fibermesh Types of Fiber & Their application Requirement of good Fibre Advantages of Fibre Conclusion Reference

ABSTRACT Concrete is an important versatile construction material, used in wide variety of situation such as sky scrapper building, bridges, tunnels etc. When we talk of a building material, it is very important to consider its durability as it has indirect effect on economy, serviceability and maintenance. The lack of durability may be caused by external environmental reasons of internal causes within concrete itself. Normally concrete fails due to combined actions of various detrimental agencies.

To increase durability of concrete .We are use so many chemicals, one newly invented material i.e. not chemicals but affect on durability of concrete name of that material is fibermesh. Fibermesh is a 100% virgin polypropylene fibers use in concrete. Also we are using Glass, Steel, Polypropylene ,Nylon.They are specially used in concrete as micro reinforcement system, which reduce cracks up to 95

1. INTRODUCTION: Plain concrete possesses a very low tensile strength, limited ductility and little resistance to cracking. Internal micro cracks are inherently present in the concrete and its poor tensile strength is due to propagation of such micro cracks, eventually leading to brittle fracture of the concrete. In the past, attempts have been made to impart improvement in tensile properties of concrete by way of using conventional reinforced steel bars and also by applying restraining techniques. Although both these methods provide tensile strength to the concrete members, they however do not increase the inherent tensile strength of concrete itself. Recently, however the development of fibre-reinforced composites in the plastics and aerospace fields has provided a technical basis for improving these deficiencies. Fiber-Reinforced Concrete (FRC) results from the addition of either short discrete fibers or continuous long fibers to the cement based matrix.

1.1 ROLE OF FIBRES: When the loads are imposed on concrete failure cracks may propagate sometimes rapidly; fibres in concrete provide a means of arresting the crack growth. Reinforcing steel bars in concrete have the same beneficial effect because they act as long continuous fibres. Short discontinuous fibres have the advantage, however of being uniformly mixed and dispersed throughout the concrete. Fibres are added to a concrete mix which normally contains cement, water and fine and coarse aggregate. As a rule, fibres are generally randomly distributed in the concrete; however, processing the concrete so that the fibres become aligned in the direction of applied stress will result in even greater tensile or flexural strengths. Concrete made with Portland cement has certain characteristics: it is relatively strong in compression but weak in tension and tends to be brittle. The weakness in tension can be overcome by the use of conventional rod reinforcement and to some extent by the inclusion of a sufficient volume of certain fibres.

2. BEHAVIOUR OF FIBER-REINFORCED CONCRETE: The strength of the fiber reinforced concrete can be measured in terms of its maximum resistance when subjected to compressive, tensile, flexural and shear loads. In field conditions, usually some combination of these loads is imposed; however for evaluation purposes, the behavior is characterized under one type of loading without the interaction of other loads. The strength under each individual type of loading is a useful indicator of the FRC material's performance characteristic for design consideration.

2.1 Toughness: Toughness is defined as the area under a load-deflection (or stress-strain) curve. As can be seen from Figure 1, adding fibres to concrete greatly increases the toughness of the material. That is, fibre-reinforced concrete is able to sustain load at deflections or strains much greater than those at which cracking first appears in the matrix.

Fig no.1.

Stress-Strain curve

2.2 Compression: The compressive properties of fiber-reinforced concrete (FRC) are relatively less affected by the presence of fibers as compared to the properties under tension and bending. Important factor is that with the addition of fibers there is an almost negligible increase in strength for mortar mixes; however for concrete mixes, strength increases by as much as 23%.The complete stress-strain curves of steel fiber-reinforced concrete with compressive strengths ranges from 35 MPa to 84 MPa. 2.3 Flexure: There are a number of factors that influence the behavior and strength of FRC in flexure. These include: type of fibres, fibre length (L), aspect ratio (L/df) where df is the diameter of the fibre, the volume fraction of the fibre (Vf), fiber orientation and fibre shape, fibre bond characteristics (fiber deformation). Also, factors that influence the workability of FRC such as W/C ratio, density, air content and the like could also influence its strength. The ultimate strength in flexure could vary considerably depending upon the volume fraction of fibers, length and bond characteristics of the fibers and the ultimate strength of the fibers.

2.4 Tensile and Splitting Tensile: The failure in tension of cement based matrices is rather brittle and the associated strains are relatively small in magnitude. The addition of fibres to such matrices, whether in continuous or discontinuous form, leads to a substantial improvement in the tensile properties of the FRC in comparison with the properties of the unreinforced matrix. The stress–strain or load–elongation response of fibre composites in tension depends mainly on the volume fraction of fibres. Many of the current applications of fibre reinforced concrete involve the use of fibres ranging around 1.0 percent by volume of concrete. Recently, it has been possible to incorporate relatively large volumes (ranging up to 15 percent) of steel, glass, and synthetic fibers in concrete. 2.5 Shear strength: Shear failure can be sudden and catastrophic.This is true for critical sections where, due to construction constraints, little or no reinforcing steel may be placed. Tests performed to study the shear strength are classified into two general groups: a)The direct shear tests are required to understand the basic transfer behaviour of concrete. b)The tests on beams and corbels are necessary to understand the behaviour of structural members reinforced with fibres. It is proven that the addition of fibers generally improves the shear strength and ductility of concrete. 2.6 Modulus of Elasticity: The modulus of elasticity of a material, whether in tension, compression, or shear, is a fundamental property that is needed for modeling mechanical behavior in various structural applications. If the modulus of elasticity of the fibre is high with respect to the modulus of elasticity of the concrete or mortar binder, the fibres help to carry the load, thereby increasing the tensile strength of the material. 2.7 Creep Shrinkage: From the studies it was observed that the steel fibres were less effective in restraining the creep at high stress to strength ratio (equal to 0.55) in comparison with low stress to strength ratio (equal to 0.33). Large stress to strength ratios increase the lateral strains and hence decrease the interfacial pressure between the fibers and the surrounding concrete. This in effect reduces the restraint to sliding action between the fibers and the concrete matrix and results in larger creep strains.

2.8 Shrinkage: The primary advantage of fibers in relation to shrinkage is their effect in reducing the adverse width of shrinkage cracks. Shrinkage cracks arise when the concrete is restrained from shrinkage movements. The presence of steel fibers delays the formation of first crack, enables the concrete to accommodate more than one crack and reduces the crack width substantially. The addition of small amounts of steel fibers (0.25% by volume) reduced the average crack widths by about 20% and the maximum crack width by about 50% in comparison with unreinforced plain concrete. High strength concretes with silica fume undergo early cracking when deformation is restrained. This phenomenon, which occurs even when concrete is protected against any evaporation, is attributed to shrinkage, because of the exceptionally low water-cement ratio (about 0.26). This phenomenon can be corrected by the use of fibers. 2.9 Strain Capacity: The ability to withstand relatively large strains before failure, the superior resistance to crack propagation and the ability to withstand large deformations and ductility are characteristics that distinguish fibre-reinforced concrete from plain concrete. These characteristics are generally described by "toughness" which is the main reason for using fiber-reinforced concrete in most of its applications. Unlike plain concrete specimens, the presence of fibers imparts considerable energy to stretch and rebound the fibers before complete fracture of the material occurs. Toughness is a measure of the ability of the material to mobilize large amounts of postelastic strains or deformations prior to failure. 2.10 Impact Resistance: Impact resistance is essential for applications such as the bridge piers. It is well recognized that the addition of fibers to concrete enhances the impact resistance. Compared with plain concrete, the increase in impact strengths at full failure were 640%, 847%, 1,824% and 2,806% respectively for concretes with 0.5, 1.0, 1.5, and 2.0% (volume) fiber content. 2.11 Abrasion Resistance When erosion is due to abrasion resulting from high velocity flow and impact of large debris, steel fiber concretes have provided significant erosion resistance.

2.12 Wet-Dry Exposure: The effect of addition of polypropylene fiber to concrete mix and adequate curing in enhancing the degradation resistance of concrete surface skin subjected to cyclic wet/dry

seawater exposure. It indicates that addition of polypropylene fibers effectively retard the deterioration process of the surface skin of the concrete specimens cured in hot weather environment.

3.FACTORS EFFECTING PROPERTIES OF REINFORCED CONCRETE: Its properties would obviously, depend upon the efficient transfer of stress between matrix and the fibres, which is largely dependent on the type of fiber, fiber geometry, fiber content, orientation and distribution of the fibres, mixing and compaction techniques of concrete, and size and shape of the aggregate. 3.1 Relative Fiber Matrix Stiffness: The modulus of elasticity of matrix must be much lower than that of fiber for efficient stress transfer. High modulus fibres such as steel, glass and carbon impart strength and stiffness to the composite. Interfacial bond between the matrix and the fibres also determine the effectiveness of stress transfer, from the matrix of the fiber. 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. 3.2 Volume of Fibres: The increase in the volume of fibres, increase approximately linearly, the tensile strength and toughness of composite. Use of higher percentage of fiber is likely to cause segregation and harshness of concrete and mortar. 3.3 Aspect Ratio of the Fibre: It has been reported that up to aspect ratio of 75, increase in the aspect increases the ultimate strength of the concrete linearly. Beyond 75, relative strength and toughness is reduced. 3.4 Orientation of Fibres: One of the differences between conventional reinforcement and fiber reinforcement is that in conventional reinforcement, bars are oriented in the direction desired while are randomly oriented. The fibres aligned parallel to the applied load offered more tensile strength and toughness than randomly distributed or perpendicular fibres. 3.5 Workability and Compaction of Concrete Incorporation of steel fiber decreases the workability considerably. Even prolonged external vibration fails to compact the concrete. 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. 3.6 Size of Coarse Aggregate Several investigators recommended that the 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. The inter-partical friction between fibres, and between fibres and aggregates controls the orientation and distribution of the fibres and consequently the properties of the composite. 3.7 Mixing Mixing of the fibre reinforced concrete needs careful conditions to avoid balling of the fibres, segregation and in general the difficulty of mixing the materials uniformly. Increasing the aspect ratio, volume percentage and size and quantity of coarse aggregate intensify the difficulties and balling tendencies. A steel fibre contain in excess of 2% by volume and an aspect ratio of more than 100 are difficult to mix. It is important that the fibres are dispersed uniformly throughout the mix. This can be done by the addition of fibres before the water is added. When mixing in a laboratory mixer, introducing the fibres through a wire mesh basket, will help even distribution of fibres.

WHAT DO FIBERMESH? Fibermesh fibers get uniformly dispersed in the concrete of mortar as millions of fibers in every cubic meter to reduce-

Plastic shrinkage, Settlement cracks, Reduce permeability, Increase impact and shatter resistance, Increase abrasion resistance, Increase resistance to freeze/thaw, Intrinsic cracking. There by vastly improve overall quality and durability.

4. TYPES OF FIBRE AND THEIR APPLICATIONS: Fibre Type

Application

Glass

Precast panels, curtain wall facings, sewer pipe, thin concrete shell roofs, wall plaster for concrete block.

Steel

Cellular concrete roofing units, pavement overlays, bridge decks, refractory, concrete pipe, airport runways, pressure vessels, blastresistant structures, tunnel linings, ship-hull construction.

Polypropylene, Foundation piles, prestressed piles, facing panels, flotation units for nylon walkways and moorings in marinas, road-patching material, heavyweight coatings for underwater pipe. Asbestos

Sheet, pipe, boards, fireproofing and insulating materials, sewer pipes, corrugated and flat roofing sheets, wall lining.

Carbon

Corrugated units for floor construction, single and double curvature membrane structures, boat hulls, scaffold boards.

Mica Flakes

Partially replace asbestos in cement boards, concrete pipe, repair materials.

* Combinations of more than one fibre type can be used for special purposes.

5. REQUIREMENT FOR GOOD FIBRES: For the effective use in hardened concrete Fibre length must be sufficient. Fibres should be significantly stiffer than the matrix, i.e. a higher modulus of elasticity. There must be a good fibre-matrix bond. Fibre content by volume must be adequate. Fibres must have a high aspect ratio, ie they must be long relative to their diameter. It deals with high volume concentrations of fibre. However, for economic reasons, the current trend in practice is to minimize fibre volume, in which case improvements in properties can be marginal. For the quantities of fibres typically used (less than 1% by volume for steel and about 0.1% by volume for polypropylene) the fibres will not have significant effect on the strength or modulus of elasticity of the composite. It must be noted that high volume concentrations of certain fibres may make the plastic concrete unworkable.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.

6.ADVANTAGES: 1. Fibres introduced in the concrete acts as an crack arrester. 2. Fibre-reinforced concrete is able to sustain load at deflections or strains much greater than those at which cracking first appears in the matrix. 3.The addition of fibers generally improves the shear strength and ductility of concrete. 4.The fibres help to carry the load, thereby increasing the tensile strength of the material. 5.The presence of steel fibers delays the formation of first crack, enables the concrete to accommodate more than one crack and reduces the crack width substantially. 6.It has got the ability to withstand relatively large strains before failure. 7.It has superior resistance to crack propagation. 8.It has got the ability to withstand large deformations and ductility. 9.The addition of fibers to concrete enhances the impact resistance.

7. CONCLUSION: 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 every type of fiber has been tried out in cement and concrete, not all of them can be effectively and economically used. Each type of fiber has its characteristic properties and limitations. Some of the fibres that can be used are steel fibres, polypropylene, nylons, asbestos, coir, glass and carbon. At this time, no general IS 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.

8. REFERENCES www.irc.nrc. Concrete technology by M. S. Shetty. www.aci.us.co www.icjonline.com www.ask.com www.google.com

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