Fiber reinforced concrete - Fibers types and properties, Behavior of FRC in compression, tension including pre-cracking stage and post-cracking stages, behavior in flexure and shear.

1. Fiber Reinforced Concrete
By
Dr. Vinay Kumar B M

2. Background
• Concrete made with portland cement has certain
characteristics: it is relatively strong in compression but
weak in tension, tends to be brittle and little resistance
to cracking.
• Structural cracks develops due to drying shrinkage and
other causes even before loading.
• Brittleness can cause sudden and catastrophic failure,
especially in structures which are subjected to
earthquake, blast or suddenly applied loads .

3. • 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.

4. Definition
“Concrete containing a cement, water, fine and
coarse aggregate,and discontinuous discrete fibers is
called fiber-reinforced concrete”
• It may also contain pozzolans and other admixtures
commonly used in conventional concrete.
• Fibers of various shapes and sizes produced from steel,
plastic, glass, and natural materials are being used.
• however, for most structural and nonstructural
purposes, steel fiber is the most commonly used of all
the fibers

5. The concept of toughness
• Toughness is defined as the area
under a load-deflection (or
stress-strain) curve.
• Adding fibres to concrete greatly
increases the toughness of the
material.
• fibre-reinforced concrete is able
to sustain load at deflections or
strains much greater than those
at which cracking first appears in
the matrix.

6. Typical stress-strain curves for fibre-reinforced
concrete

7. The use of fibres:
• Fibres should be significantly stiffer than the matrix,
i.e. have a higher modulus of elasticity than the
matrix.
• Fibre content by volume must be adequate.
• There must be a good fibre-matrix bond.
• Fibre length must be sufficient.
• Fibres must have a high aspect ratio, i.e. they must be
long relative to their diameter.

8. Aspect Ratio
• The aspect of ratio of the fibre is the ratio of its length
to its diameter
• Typical aspect ratio ranges from 30 to 150
• Maximum usage: 2% by volume.

9. Types of fibres
1. Steel fibre
2. Polypropylene & Nylon fibre
3. Asbestos fibre
4. Glass fibre
5. Carbon fibre

10. Steel fibre
• Most commonly used
• Generally round, diameter vary from 0.25 to 0.75mm
• Significant improvement in flexural, impact and
Fatigue strength.
• Overlays of roads, airfield pavements and Bridge
decks

11. Types of steel fibres

12. Polypropylene & Nylon fibre
• Are suitable to increase impact strength.
• Very high tensile strength, but very low MOE and
higher elongation
• Do not contribute to the flexural strength

13. Asbestos Fibres
• Mineral fibre and has proved to be most successful of all
fibres as it can be mixed with portland cement.
• Tensile strength of asbestos varies between 560 & 980
Mpa

14. Glass Fibres
• Recent introduction in making fibre concrete.
• It has very high tensile strength 1020 to 4080 Mpa.
• Alkali-Resistant glass fibre by trade name “CEM-FIL”
has been developed and used.

15. Carbon Fibre
• Posses very high tensile strength 2110 t0 2815 Mpa
and MOE
• used for structures like cladding, panels and shells.

16. Classification according to volume fraction
1. Low volume fraction (<1%)
2. Moderate volume fraction (between 1 and 2%)
3. High volume fraction (greater than 2)

17. Low volume fraction
• The fibers are used to reduce shrinkage cracking.
• These fibers are used in slabs and pavements that
have large exposed surface leading to high shrinkage
crack.

18. Moderate volume fraction
• The presence of fibers at this volume fraction increase
the modulus of rupture, fracture toughness, and impact
resistance.
• These composite are used in construction methods such
as shotcrete and in structures that require energy
absorption capability, improved capacity against
Delamination, spalling, and fatigue.

19. High volume fraction
• The fibers used at this level lead to strain hardening
of the composites.
• Because of this improved behavior, these composites
are often referred as high-performance fiber-
reinforced composites (HPFRC).
• In the last decade, even better composites were
developed and are referred as ultra-high-
performance fiber reinforced concretes (UHPFRC).

20. Role of Fiber Size
• To bridge the large number of micro cracks in the
composite under load and to avoid large strain
localization it is necessary to have a large number of
short fibers.
• The uniform distribution of short fibers can increase the
strength and ductility of the composite.
• Long fibers are needed to bridge discrete macro cracks
at higher loads; however the volume fraction of long
fibers can be much smaller than the volume fraction of
short fibers.
• The presence of long fibers significantly reduces the
workability of the mix

22. Salient Features of FRC
• Examination of fractured specimens of fiber-reinforced
concrete shows that failure takes place primarily due
to fiber pull-out or debonding.
• Steel fibre in terms of durability is the best.
• The addition of any type of fibres to plain concrete
reduces the workability.
• Concrete mixtures containing fibres posses very low
consistencies; however, the placeability and
compactability of concrete is much better than
reflected by the low consistency

23. Mix Design for SFRC
• Just as different types of fibres have different
characteristics, concrete made with steel fibres will also
have different properties.
• When developing an mix design, the fibre type and the
application of the concrete must be considered.
• sufficient quantity of mortar fraction in the concrete to
adhere to the fibres and allow them to flow without
tangling together(Balling of fibres).

24. • Cement content is, therefore, usually higher for SFRC
than conventional mixes.
• Coarse aggregates of sizes ranging from 10 mm to 20
mm are commonly used with SFRC.
• Larger aggregate sizes usually require less volume of
fibres per cubic meter.
• SFRC with 10 mm maximum size aggregates typically
uses 50 to 75 kg of fibres per cubic meter, while the one
with 20 mm size uses 40 to 60 kg.

25. • Fine aggregates in SFRC mixes typically constitute
about 45 to 55 percent of the total aggregate
content.

26. Typical mix proportions:
• cement 325 to 560 kg
• water-cement ratio 0.4-0.6
• Ratio of fine aggregate to total aggregate 0.5-1.0
• Maximum aggregate size 10mm
• fibre content 0.5-2.5% by volume of concrete
• An appropriate pozzolan may be used as a replacement for
a portion of the Portland cement to improve workability,
reduce heat of hydration and production cost.

27. Workability of FRC
• We know that it is usually wrong to add water to
concrete for workability.
• Main problem with workability of steel fiber reinforced
concrete is in getting proper distribution of the fibers so
that they don't ball up.
• Difficulty is usually overcome by slow, continuous and
uniform feeding of the fibers into the wet or dry mix by
means of vibratory feeders.

28. • Addition of water to improve workability can reduce
the flexural strength significantly.
• suitable admixture should be used to improve the
workability of FRC.

29. Test for workability
1. Slump test- subsidence in mm : when slump value exceed
40 mm .
2. Inverted slump test-time in seconds : required by the
concrete to flow through a slump cone kept in inverted
position(flow of concrete is aided by immersing vibrator)
3. Compacting factor test-degree of compaction : suitable for
mix having medium and low workability(0.91 to 0.81).
4. VB test-time in seconds.

30. Behavior of FRC in Compression
• The increase in strength by steel fibres very rarely
exceeds 28%.
• Fibre quantity is generally limited to 55-65Kg/M3.
• Other factors that are considered in the design are:
MOE, strain at peak load and post cracking behavior.

31. • Addition of fibres increases ductility and hence
results in high energy absorption capacity.
• Increase in ductility provided by the fibres depends
on number of factors:
– fibre content,
– fibre geometry &
– Matrix.

32. • Increase in fibre content results in increase in the
energy absorption capacity. however, the relative
magnitude in energy increases with 0 to 0.7%.
• Regards to the fibre shape, aspect ratio is important
parameter. As the aspect ratio is increased the
ductility also increases.

33. • The composition of the matrix to the strength and
energy absorption in 2 ways:
– The first is its bonding characteristics with fibres.
– Second is the brittleness of the matrix which, itself
plays an important role in behavior of FRC

34. • Normal strength concrete is brittle than high strength
concrete and the incorporation of fibres makes the
composite more ductile.
• Higher fibre volume fraction is required for high
strength concrete to produce ductile failure.
• For high strength concrete a fibre content of 110 to
130 kg/m3 is required to obtain a ductile behavior

35. Modulus of Elasticity
• Modulus of elasticity of FRC increases slightly with an
increase in the fibers content.
• It was found that for each 1 percent increase in fiber
content by volume there is an increase of 3 percent in
the modulus of elasticity.

36. Tensile strength of FRC
• Fibres aligned in the direction of the tensile stress
may bring about very large increases in tensile
strength.
• For more or less randomly distributed fibres, the
increase in strength is much smaller, ranging from as
little as no increase in some instances to perhaps
60%.
• The presence of 3 percent fiber by volume was
reported to increase the splitting tensile strength of
mortar about 2.5 times that of the unreinforced one.

37. Influence of fibre content on tensile strength

38. Behaviour of FRC under Flexure
• The most important contribution of fiber reinforcement
in concrete is not to strength but to the flexural
toughness of the material (total energy absorbed in
breaking a specimen in flexure).
• Toughness of material can be increased (15-30%).
• Tests are usually done by using 100x100x500mm
beams under third point loading.

39. • Increase in flexural strength are normally higher than
increase in either compressive or splitting tensile
strength.
• The increases in flexural strength is particularly
sensitive, to the fibre volume and aspect ratio of the
fibres, with higher aspect ratio leading to larger
strength increases.

40. Influence of fibre content

41. Influence of fibre length
• Longer fibres with higher aspect ratios provide better
performance in both strength increase and energy
absorption as long as they are properly mixed, placed
compacted and finished.

42. Corrosion of Steel Fibers
• A 10-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 fibers
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.

43. Durability of FRC
• Fiber-reinforced concrete is generally made with a
high cement content and low water/cement ratio.
• When well compacted and cured, concretes containing
steel fibers seem to possess excellent durability as long
as fibers remain protected by cement paste.
• Ordinary glass fiber cannot be used in portland
cement mortars and concretes because of chemical
attack by the alkaline cement paste.

44. Applications
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 FRC slab was
used to overlay an existing asphaltic-paved aircraft
parking area.

45. Tunnel Lining and Slope Stabilization:
• Steel fiber reinforced shotcrete (SFRS) are being used
to line underground openings and rock slope
stabilization.
• It eliminates the need for mesh reinforcement and
scaffolding

46. 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 fiber reinforced cement or concrete (GFRC) ,made by
the spray-up process, have been used to construct wall
panels.
• Steel and glass fibers addition in concrete pipes and
manholes improves strength, reduces thickness, and
diminishes handling damages.

47. Dams and 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

48. Other Applications:
• These include machine tool frames, lighting poles, water
and oil tanks and concrete repairs.