FIBRE REINFORCED CONCRETE in Construction.pptx

Types of Concrete

1. FIBRE REINFORCED CONCRETE UNIT-6

2. FIBRE REINFORCED CONCRETE FRC is a composite material of conventional concrete or mortar reinforced by random dispersal of short , discontinuous and discrete fine fibres of specific geometry . Fibre is a small piece of reinforcing material either circular or flat possessing certain characteristic properties. It is described by a parameter known as Aspect Ratio . Aspect Ratio = Length of fibre /Diameter of fibre Increase in aspect ratio upto 75 there is an increase in relative strength and toughness. Usually the aspect ratio ranges from 30 to 150.

3. TYPES OF FIBRE 1. Metallic Fibre Steel fibre , Aluminium fibre , Low Carbon Steel Fibre , Galvanized iron fibre . 2. Non metallic Fibre Glass fibre Cellulose fibre Carbon fibre Synthetic fibres : Polypropylene fibre Nylon fibre Natural fibres : Coir Hay

4. STEEL FIBRE

5. STEEL FIBRE Aspect ratio of 30 to 250 . Diameters vary from 0.25mm to 0.75mm . Possess high structural strength . Reduced crack width & control the crack widths tightly , thus improving durability . Improved impact and abrasion resistance . Used in precast and structural applications , highway and airport pavements , refractory and canal linings, industrial flooring, bridge decks.

6. GLASS FIBRE

7. GLASS FIBRE High tensile strength of 1020N/mm² to 4080N/mm². Shape: Round and Straight. Diameter of 0.0005 to 0.015mm. Fibres of length 25mm are used. Improved impact strength Increased flexural strength, ductility and resistance to thermal shock . Used in swimming pools, roofs, sewer lining.

8. Carbon Fibre Carbon fibres are very small in diameter and are generally used in shorter lengths. They are also manufactured as continuous mats and continuous straight fibres . They can be manufactured in strength as high as steel with a density only one-fifth that of steel. Carbon fibre is inert in aggressive environments, abrasion-resistant and stable at high temperatures, with relatively high stiffness.

9. Carbon Fibre Carbon fibres perhaps possess very high tensile strength in the range of 2110 to 2815 N/mm 2 . It has been reported that cement composite made with carbon fibre , as reinforcement will have very high modulus of elasticity and flexural strength. The uniform dispersion of carbon fibre in concrete is more difficult than the other fibre types.

10. Polypropylene fibre Polypropylene fibres are specially engineered for use in concrete and mortar as a micro reinforcement system. They posses very high tensile strength. Low modulus of elasticity and higher elongation do not contribute to the flexural strength.

11. Polypropylene fibre These fibres get uniformly dispersed in the concrete Mortar as millions of fibres in every cubic meter to reduce plastic shrinkage and settlement cracks. Reduce permeability. Increase impact and abrasion resistance to freeze/thaw, Reduce honey combing, segregation, unequal bleeding. Prevent corrosion of reinforcement, Prevent explosive scaling of concrete due to tire and there by vastly improve overall quality and durability.

12. Asbestos fibre 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 have a thermal, mechanical and chemical resistance making it suitable for sheets, pipes, tiles and corrugated roofing elements. Asbestos cement products contain about 8 to 16 percent by volume of asbestos fibres .

13. Asbestos fibre The flexural strength of asbestos cement board is approximately 2 to 4 times that of unreinforced matrix. However due to their relative short length (10mm), they have low impact strength. There is some health hazards associated with the use of asbestos cement. In the near future, it is likely that glass fibre reinforced concrete will replace asbestos completely.

14. SYNTHETIC FIBRE

15. SYNTHETIC FIBRE Man-made fibre from petrochemical and textile industries. It is cheap and abundantly available . It has high chemical resistance and high melting point . The modulus of elasticity of fibre are low. Various types of fibre include acrylic, carbon, nylon, polyester, polyethylene, polypropylene . Field of applications includes in cladding of panels and shotcrete.

16. NATURAL FIBRES

17. NATURAL FIBRES These fibres are obtained at low cost & low level of energy using local manpower & technology. These includes jute, coir and bamboo . The main disadvantage is they may undergo decay . These fibres have low modulus of elasticity, high impact strength. The fibre can also be classified as hard intrusion or soft intrusion depending upon the modulus of elasticity of fibre and modulus of elasticity of concrete matrix . If modulus of elasticity of fibre is more than modulus of elasticity of concrete , it is called Hard Intrusion . (Ex: Steel, glass, carbon)

18. NATURAL FIBRES If modulus of elasticity of fibre is less than modulus of elasticity of concrete , it is called Soft Intrusion. (Ex: Asbestos, Jute) Hard intrusion improves flexural strength & impact strength. Soft intrusion improves only impact strength .

19. Basalt Fibre Reinforecement

20. NECESSITY OF FIBRE REINFORCED CONCRETE Ordinary concrete has high compressive strength, low tensile strength, limited ductility and little resistance to cracking. It shows brittle characteristics to applied load. Reinforcing the concrete structural elements using steel bars improves tensile strength of the structural element but not the inherent tensile strength of concrete . Hence fibres are added to concrete matrix to improve the inherent tensile strength, ductility and resistance to cracking. This composite material is known as FRC .

21. NECESSITY OF FIBRE REINFORCED CONCRETE Addition of fibre in concrete matrix is convenient practical and economical method of improving inherent tensile strength, ductility and resistance to cracking . FRC is most suited for concrete used in hydraulic structures, highway pavements, airport runways, bridge decks, heavy duty floors and tunnel lining.

22. FACTORS AFFECTING THE CHARACTERISTICS OF FRC Water cement ratio (0.4-0.6) Percentage of fibre (Volume fraction) Dia and length of fibre ( Aspect Ratio) Orientation of the fibre Relative Fibre Matrix Workability and compaction of concrete.

23. PERCENTAGE OF FIBRE (VOLUME FRACTION) A) Low volume fraction (less than 1%) :Used in slab and pavement that have large exposed surface leading to high shrinkage cracking. B) Moderate volume fraction (between 1 & 2%) : Used in construction method such as shotcrete and in structures which requires improved capacity against spalling and fatigue. C) High volume fraction (greater than 2 %): Used in making high performance fibre reinforced composites .

24. DIA AND LENGTH OF FIBRE (ASPECT RATIO) Increase in the aspect ratio upto 75 , there is increase in relative strength and toughness. TYPE OF CONCRETE ASPECT RATIO RELATIVE STRENGTH RELATIVE TOUGHNESS PLAIN CONCRETE WITH RANDOMLY DISPERSED FIBRE 1.0 1.0 25 1.50 2.0 50 1.60 8.0 75 1.70 10.50 100 1.50 8.50

25. ORIENTATION OF FIBRE A)Aligned in the direction of load B) Aligned in the direction perpendicular to load. C) Randomly distribution of load It is observed that fibre aligned parallel to applied load offered more tensile strength and toughness than randomly or perpendicular distributed fibre .

26. RELATIVE FIBRE MATRIX Fibre should be significantly stiffer than matrix. Modulus of elasticity of matrix must be less than of fibre for efficient stress transfer . Low modulus of fibre imparts more energy absorption while high modulus fibre imparts strength and stiffness . Low modulus fibre Ex: Nylon, polypropylene fibre High modulus fibre Ex: Steel, Glass and carbon fibre .

27. Workability and compaction of concrete: Usage of steel fibres , higher aspect ratio and non-uniform distribution of fibres will reduce workability Prolonged external vibration fails to compact the concrete. These properties can be improved by increasing water/cement ratio or by using water reducing admixtures

28. Size of coarse aggregate: Restricted to 10mm • Friction between fibres and between fibres and aggregates controls orientation and distribution.

29. Mixing: Mixing of FRC needs careful precautions to avoid balling effect and segregation. Increase in aspect ratio, volume percentage and size of coarse aggregate will increase the difficulties .

30. ADVANTAGES OF FRC High modulus of elasticity for effective long-term reinforcement , even in the hardened concrete . Does not rust nor corrode and requires no minimum cover. Ideal aspect ratio which makes them excellent for early age performance. Easily placed, Cast, Sprayed and less labour intensive than placing rebar. Greater retained toughness in conventional concrete mixes.

31. ADVANTAGES OF FRC Higher flexural strength , depending on addition rate . Can be made into thin sheets or irregular shapes . FRC possesses enough plasticity to go under large deformation once the peak load has been reached.

32. DISADVANTAGES OF FRC Greater reduction of workability High cost of materials Generally fibres do not increase the flexural strength of concrete and so cannot replace moment resisting or structural steel reinforcement .

33. FRC MIX PROPORTIONING The mix proportion generally depend on the intended application of the FRC. The important consideration are uniform dispersion of the fibre , adequate workability for placing and compaction with available equipment. The workability of FRC that is influenced by maximum size of aggregate , volume fraction, geometry and aspect ratio of fibre . As the size of aggregate increases , uniform fibre dispersion become difficult.

34. FRC MIX PROPORTIONING Hence coarse aggregate of size upto 10mm are used. Water cement ratio between 0.4-0.6 , cement content of 250kg/m³ are recommended for providing adequate paste content to coat large surface of fibres .

35. BEHAVIOUR OF FRC IN TENSION When a fibre reinforced composite is subjected to tension , the matrix will crack long before the fibre can be fractured. Once the matrix is cracked the composite continues to carry the increasing tensile stress . The peak stress and the peak strain of composite is greater than those of the matrix alone . During the inelastic range between first cracking and the peak, multiple cracking of matrix occurs . Until the initial cracking of the matrix , it is reasonable to assume that both the fibres and matrix behave elastically and there is no spillage between the fibre and the matrix .

36. BEHAVIOUR OF FRC IN TENSION After initial cracking has occurred the composite will carry increasing load only if the pull out resistance of fibre is greater than the load at the initial cracking. In the post cracking stage the failure of composite is generally due to fibre pull out rather than fibre yielding or fracture. Improvement of structural performance of FRC depends on : 1. Strength characteristics of fibre 2. Volume of fibres 3. Dispersion and orientation of fibre 4. Shape and aspect ratio of fibre .

37. BEHAVIOUR OF FRC IN COMPRESSION The increase in compressive strength of FRC is marginal and ranges from 0% to 20%. The post cracking compressive stress and strain shows noticeable increase in strain at peak load and significant increase in ductility beyond ultimate load. Increase in toughness of FRC prevents sudden failure especially under earthquake and blast type of loading. Increase in fibre volume fraction improves ductility and energy absorption capacity of FRC.

38. BEHAVIOUR OF FRC IN FLEXURE In a FRC member subjected to flexure, the load at the first crack will increases due to the crack arresting mechanism of closely spaced fibre . After the concrete cracks in tension the fibre continue to take the load, provided the bond is good when the fibre strain reaches its breaking strain, the fibre fracture resulting in load transfer to the fibres of adjacent layer which on reaching their breaking strain fracture and result in the shifting of the neutral axis. Failure occurs when the concrete in compression reaches the ultimate strain . The most important factors affecting the ultimate load are the volume of fibre and their aspect ratio.

39. BEHAVIOUR OF FRC IN SHEAR The shear strength and toughness index of compact cube specimens shows that the shear strength was not affected by fibre volume. The post cracking toughness increases uniformly with increase in fibre content. This property of post-cracking toughness in FRC makes it very useful in resisting earthquake and blast related loads .