“FIBRE REINFORCED CONCRETE”

byMAYAKUNTLA PRASANNAKUMAR

VENKATESHA A

What is Fibre?

• Fibre is a small piece of reinforcing material which increases structural integrity.

Why Fibre ?Concrete:

• Weak in tension

• Brittle

What is fibre reinforced concrete

• FRC is a Portland cement reinforced with

more or less randomly distributed fibres .

Types of fibres

1. Steel fibre:

2. Glass fibre:

• Asbestos fibre:

• Polypropylene fibre:

• Carbon fibre:

• Aramid fibre:

Source: Santa Kumar

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

2. Aspect ratio of fibre:

= fibre length/fibre diameter

Source: M.S Shetty

3. Orientation of fibres:

• Aligned in the direction of load

• Aligned in the direction perpendicular to load

• Randomly distribution of fibers

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.

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

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.

Developments in FRC1.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

Omni mixer used in high volume FRC

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

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

• 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.

Applications of SFRC

• Highway and airfield pavements

• Hydraulic structures

• Fibre shotcrete

• Precast applications

• Structural applications

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

• 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”

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.

• 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

Source: M.S Shetty

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)

•Toughness = total area (A1+A2) / area A1

Source: M.S Shetty

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

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

Source: M.S Shetty

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

Bridging action of steel fibres

CASE STUDY

• Research program is funded by National Basic Research

Program of china

• Published in 15 may 2013 in JESTR

Crack arresting and strengthening

Case study to arrest cracks

• 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

Tensile strength is increased by 171% &

fracture elongation is increased by 70%

2 no. of specimens, 8mm dia bars, 3% of nylon (tensile- 6 Mpa)

FEM analysis results

STRESS INTENSITY FACTOR VS CRACK HEIGHT WITHOUT HFRP

STRESS INTENSITY FACTOR VS CRACK HEIGHT WITH HFRP

•HFRP increases ductility separately by 36% and 106%

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.

• 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.

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).

• 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.

• 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.

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.