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fibre reinforced concrete

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It gives a brief introduction on fibre reinforced concrete

Veröffentlicht in: Ingenieurwesen
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fibre reinforced concrete

  1. 1. “FIBRE REINFORCED CONCRETE” by MAYAKUNTLA PRASANNAKUMAR VENKATESHA A
  2. 2. What is Fibre? • Fibre is a small piece of reinforcing material which increases structural integrity. Why Fibre ? Concrete: • Weak in tension • Brittle
  3. 3. What is fibre reinforced concrete • FRC is a Portland cement reinforced with more or less randomly distributed fibres .
  4. 4. Types of fibres 1. Steel fibre:
  5. 5. 2. Glass fibre:
  6. 6. • Asbestos fibre:
  7. 7. • Polypropylene fibre:
  8. 8. • Carbon fibre:
  9. 9. • Aramid fibre:
  10. 10. Source: Santa Kumar
  11. 11. Factors effecting the properties of FRC 1. Volume of fibres: • low volume fraction (less then 1%): Used in slabs and pavement that have large exposed surface leading to shrinkage cracking • Moderate volume fraction(between 1 and 2%): Used in construction method such as shotcrete • High volume fraction(greater then 2%): Used in making high performance FRC
  12. 12. 2. Aspect ratio of fibre: = fibre length/fibre diameter Source: M.S Shetty
  13. 13. 3. Orientation of fibres: • Aligned in the direction of load • Aligned in the direction perpendicular to load • Randomly distribution of fibers
  14. 14. 4. Relative fibre matrix: • Fibre should be significantly stiffer than matrix • Low modulus of fibres imparts more energy absorption while high modulus of fibres imparts strength and stiffness. • Low modulus fibres e.g. nylon, polypropylene • High modulus of elasticity e.g. steel, glass and carbon fibres.
  15. 15. 5. 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
  16. 16. 6.Size of coarse aggregate: • Restricted to 10mm • Friction between fibres and between fibres and aggregates controls orientation and distribution. 7. 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.
  17. 17. Developments in FRC 1.High fibre volume micro fibre system: • length – 3mm • Diameter – 25 microns • Specific surface > 200 cm2/gram • Mixing of FRC needs careful conditions to avoid balling effect • Sand particles of size not exceeding 1mm • Low sand to cement ratio. • Requires large dosage of super plasticizers • Omni mixer is used for mixing
  18. 18. Omni mixer used in high volume FRC
  19. 19. 2. Slurry infiltrated fibre concrete(SIFCON): • Invented by lankard in 1979 • Pre-placing the dry fibres and cement slurry is infiltrated. • Volume of fibres can be increased to 20% • increase in flexural capacity and toughness. • used in blast resistant structures • better suited for three dimensional application such as zones of reinforcing bars anchorages
  20. 20. 3. Slurry infiltrated mat concrete (SIMCON): • Infiltrating continuous steel fibre mats with a specially designed cement based slurry. • Mats are made up of stainless steel. • Fibre volume is less than that required for SIFCON, but same flexural strength and energy absorption. • Aspect ratio exceeding 500 can be used. • Since mat is predefined configuration, handling is minimized and balling effect is reduced
  21. 21. • Cracks are small and discontinuous and possibility of water seepage is low . • Concrete slurry uses very little water to pack the mat very tight some of the cement remains unhydrated.
  22. 22. Applications of SFRC • Highway and airfield pavements
  23. 23. • Hydraulic structures
  24. 24. • Fibre shotcrete
  25. 25. • Precast applications
  26. 26. • Structural applications
  27. 27. Behaviour of SFRC in Tension • Effect of incorporating fibres – delay and control tensile cracking • Fibres (ductile) + matrix (brittle) composite (ductile) • Sharing of tensile load (most predominant feature of FRC)  until the matrix cracks ( fibre & matrix)  once matrix cracks (fibres)  this mechanism gives rise to favourable dynamic properties 1. Energy absorption 2. Fracture toughness
  28. 28. • Mangat reference (1976) “ The effect of fibres in a cementitious material is principally to cause relief of tensile stress at the crack tip and prevent unstable crack propagation”
  29. 29.  Kelly (1970) • Investigated the mechanism of pull-out. • Load-elongation curve of fibres in tension depends on volume fraction of fibres. • Response in tension (based on FRC or SIFCON) stage1: before cracking the composite elastic- (elastic stage) stage2: after cracking –fibres tend to pull out – sudden change in load elongation curve. - if maximum post cracking stress › cracking stress – (multiple cracking stage) stage 3: beyond the peak point - failure and/or pull out of fibres across single critical crack. • Note : the post cracking strength increases with increase in bond strength, aspect ratio and volume fraction of fibres.
  30. 30. • In the curve OA – debonding of fibre • In case of short fibres – debonding occurs at max load • Debondind energy per unit area = (area of OAB under the stress-strain curve)/(surface area of fibre) • The additional energy dissipation of fibre concrete results from debonding energy as well. Source: santakumar
  31. 31. Source: M.S Shetty
  32. 32. Behaviour of FRC in Compression • Increase in compressive strength of FRC is marginal and ranges from 0% to 20%. • However, post cracking compressive stress-strain response changes substantially. • Change is due to Increase in strain at peak load & ductility beyond ultimate load – higher toughness • Higher toughness – prevents sudden & catastrophic failures (especially in case of EQ & blast type of loads)
  33. 33. •Toughness = total area (A1+A2) / area A1 Source: M.S Shetty
  34. 34. Behaviour of FRC in Flexure • There are 3 stages of response in flexure stage1: process zone - more or less linear response up to elastic limit. - transfer stress from matrix to the fibres by interfacial shear - imposed stress is shared between matrix and fibre until first crack. stage2: pseudo-plastic zone - it is the non-linear portion between the elastic point and max load capacity point. - stress in matrix is progressively transferred to the fibres. - fibres pull-out from the matrix (non-linear load-deflection) - results in multiple cracking
  35. 35. stage3: stress free zone - descending portion following peak strength until strain limit. - load-deflection curve represents ability of the fibre composite to absorb large amounts of energy before failure. - fibres are completely pulled-out • Flexural strength of fibre composite is fc = ultimate strength of fibre composite fm = max strength of plain matrix (concrete) C and D are constants determined experimentally for plain concrete C=1 and D=0 for FRC ultimate strength C=0.95,D=4.95 for FRC first cracking strength C= 0.85, D= 4.95
  36. 36. Source: M.S Shetty
  37. 37. Crack Arresting • Crack resistance is lower than the ultimate stress • Once cracking is subjected to coupling impact of increased loads, material ageing, structure fatigue – increase microcracks • Microcracks – upward shifting of N-A, tension area of concrete is lost – decrease of structural rigidity – deterioration of structural durability. • Propogation of micropcracks – emergency situation • Fracture mechanics – stress singularities at crack tips • Stress intensity factor › critical stress intensity factor of FRC – Propagation of cracks - functional obsoleteness & structural failure
  38. 38. Bridging action of steel fibres
  39. 39. CASE STUDY • Research program is funded by National Basic Research Program of china • Published in 15 may 2013 in JESTR
  40. 40. Case study to arrest cracks Crack arresting and strengthening
  41. 41. • Supposing unilateral crack under pure bending • Stress concentration factor of edge crack is more than central penetrated crack under same loading – unstable propagation • HFRP is bonded to the surface – resists stress concentration of crack at crack tip– edge crack in to internal eccentric crack • From the super position principle where, are stress intensity factors at crack tip A, the rebar and the HFRP sheet. • HFRP – one layer of unidirectional CFRP sheet (300 g/m^2) & one layer of unidirectional GFRP sheet (600 g/m^2) adhered to the bottom by epoxy
  42. 42. Tensile strength is increased by 171% & fracture elongation is increased by 70%
  43. 43. 2 no. of specimens, 8mm dia bars, 3% of nylon (tensile- 6 Mpa)
  44. 44. FEM analysis results STRESS INTENSITY FACTOR VS CRACK HEIGHT WITHOUT HFRP
  45. 45. STRESS INTENSITY FACTOR VS CRACK HEIGHT WITH HFRP •HFRP increases ductility separately by 36% and 106%
  46. 46. Mix Design Procedure • Corresponding to required 28-day field flexural strength of SFRC – design strength of laboratory mix is determined. • For known geometry, stipulated volume fraction- w/c ratio is selected between 0.45 and 0.60 • Depending on max size of agg. & fibre concentration – cement paste content is determined by mass • Ratio of FA to CA varies from 1:1 to 1:3, ratio of 1:1.5 is a good start for volume % of fibre up to 1.5 and length of fibre up to 40mm.
  47. 47. • From w/c ratio & paste content – cement & water content • Fibre content is obtained by taking density of fibres as 7850 kg/m^3 • Total quantity of agg. Is determined as wt. of agg. = wt. of FRC – ( wt. of water, cement & fibre) • Quantities of FA &CA are determined by ratio of FA:CA= 1:1.5 • Trial mix is checked for workability by appropriate test.
  48. 48. Applications in India and abroad • More than 400 tones of Shakti man Steel Fibers have been used recently in the construction of a road overlay for a project at Mathura (UP).
  49. 49. • A 3.9 km long district heating tunnel, carrying heating pipelines from a power plant on the island Amager into the center of Copenhagen, is lined with SFC segments without any conventional steel bar reinforcement.
  50. 50. • Steel fibers are used without rebars to carry flexural loads is a parking garage at Heathrow Airport. It is a structure with 10 cm thick slab.
  51. 51. Conclusions • The total energy absorbed in fiber i.e., area under the load-deflection curve is at least 10 to 40 times higher for fiber-reinforced concrete than that of plain concrete. • Addition of fiber to conventionally reinforced beams increased the fatigue life and decreased the crack width under fatigue loading. • At elevated temperature SFRC have more strength both in compression and tension. • Cost savings of 10% - 30% over conventional concrete flooring systems.

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