6. fibre reinforced concrete

Fibre Reinforced Concrete
CE 342 Concrete Technology
Prof. Ravindra Gettu
IIT Madras

Fibre-Reinforced Concrete (FRC)
Definition:
Conventional fibre performance concrete
is that which has a homogeneous
distribution of randomly-oriented short
fibres.
The fibres are generally
• much shorter than the dimensions of
the concrete element
• stronger and can elongate more than
the matrix under tension
• introduced in the matrix during the
mixing of the concrete

FRC: General
Matrices
In addition to concrete, FRC matrices can be made up of
• Hardened cement paste without/with admixtures
• Cement mortar
Fibres
• Metallic (steel, stainless steel)
• Polymeric (polypropylene, nylon, acrylic, polyester, etc.)
• Carbon
• Mineral (glass, basalt)
• Naturally-occurring (sisal, cellulose, jute, coconut, etc.)

FRC: Historical Perspective
• Straw and hair have been used
for centuries in mud bricks
• Patent of Alfsen in France for
using fibres to increase tensile
strength of concrete (1918)
• Patent of Martin for the use of
smooth and deformed steel
fibres in concrete (1926)
• Patent of Constantinesco for the
use of fibres in concrete to
increase the toughness (1943);
military applications and
machinery foundations
Barn swallow nest (made of
mud, straw, leaves, feathers)
Wide usage of fibre
reinforced concrete
began in the 1960s.

FRC: Function of the Fibre
Comparison of the tensile
response of different fibre-
reinforced cement-based
composites

FRC: Fibre-Matrix Interaction
Uncracked matrix
Cracked matrix
fibre pullout
with/without
interface crack
fibre rupture
or

In a bond test, fibre rupture occurs
• when bond strength is high
• when embedment length is large
• when fibre tensile strength is
low

In a composite with strong fibres in a brittle matrix, post-
crack load-carrying capacity increases with fibre volume
fraction.
Increasing fibre content

For low volume fractions of fibres (Vf < 1%) , single
(or few) cracks occur at failure.
For high volume fractions of fibres (Vf > 5%),
multiple cracks occur.
Cement mortar with 12%
volume fraction of fibrillated
continuous uniaxial
polypropylene fibres.
At tensile strain of 1%.

Depends on
• Condition of the matrix (cracked or uncracked)
• Matrix composition
• Type of fibre (mechanical characteristics)
• Geometry of the fibre
• Surface characteristics of the fibre
• Distribution of the fibre in the matrix
• Volume fraction of fibres
• Durability and long-terms effects

FRC: Types of Fibres
Fibre Diameter
(microns)
Unit
weight
(gm/cc)
Elastic
modulus
(GPa)
Tensile
strength
(MPa)
Elongation
limit
(%)
Steel
AR-Glass
Polypropylene
Aramid
Carbon
Nylon
Sisal
Hardened
cement paste
5-1000
9-15
20-200
10-12
7-100
20
10-50
7.8
2.6-2.8
0.9
1.4
1.8-2.1
1.2
1.5
1.8-2.4
200
70-85
3.5-5
60-130
230-640
4-5
13-26
10-45
500-2000
1700-4000
300-750
3200-3600
2600-5500
900-1000
280-900
2-4
0.5-3.5
2-3.6
8-15
2.0-4.5
0.4-2.1
13-20
3-5
0.01

FRC: Polypropylene Fibres
fibrillated or
monofilament

FRC: Polypropylene Fibres
Used for
• Controlling plastic shrinkage cracking
• Increasing post-crack load-carrying capacity
• Increasing fire resistance (especially in high-strength
concrete tunnel linings)
Applications include
• Industrial slabs and pavements
• Mine walls and waste disposal covers (shotcrete)
• Repair or plastering mortar
• Thin sheet and extruded products

FRC: Glass Fibres
E- and A-glass fibres lose their strength in a typical cement-based
matrix. Therefore, only alkali-resistant AR-glass is widely used.

FRC: Glass Fibres
Used for
• Controlling crack widths
• Increasing post-crack load-carrying capacity
• Panels for architectural cladding and other thin-sheet
products
• Extruded products
• Repair or plastering mortar

FRC: Steel Fibres
Steel fibres can be
• of low- or high-carbon steel, or
stainless steel.
• produced by cold-drawing, shaving
or melt-extraction.
• smooth or deformed (crimped)
• of round or irregular cross-section

FRC: Steel Fibres - Collated
Zerbino

FRC: Steel fibres
Used for
• Limiting crack propagation
• Increasing ductility and energy dissipation during
failure
• Distributing cracking (i.e., finer micro- instead of
localized macro-cracking)
• Increasing abrasion, impact and fatigue resistance
• Decreasing shrinkage cracking

FRC: Steel fibres
• Shotcrete for tunnel and other linings, and repair
of structural elements
• Pavements and industrial floors (as primary
reinforcement and for controlling shrinkage
cracks)
• Pipes
• Thin-walled elements
• Some structural applications

Steel fibre reinforced concrete (SFRC)
Main Applications Today
For providing resistance against
shrinkage cracking, and impact
and local compressive loads in
floors and pavements.
For improving the post-crack
load-carrying capacity of
shotcrete.
Why are not there more structural applications?
•Lack of standards regarding SFRC structural design
•Incomplete material characterization
•Insufficient standard test methods

Steel fibre reinforced concrete (SFRC)
Recent developments:
• Proposal for European test standards
• European structural design code being checked
• Thin-walled elements are being used more
commonly in Japan and Europe
Due to these and ongoing research, promising
applications can be expected in:
• Slender structural elements
• Elements where crack widths have to be limited
• Structures where brittle failure has to be avoided
and/or more energy dissipation capacity is required

Fabrication
SFRC PANELS FOR BUILDING WALLS (Germany)
Two thin SFRC panels separated by lattice girders.
The space in between is filled with plain concrete on site.

Installation on site
SFRC PANELS FOR BUILDING WALLS (Germany)
With SFRC, the lattice girder separation could be increased,
conventional reinforcement was eliminated in the panels
and a higher casting rate could be implemented for the plain
concrete on site.

PRECAST PIPES FOR WATER SUPPLY (Spain)
Concrete pipes with a 3-layer wall
External reinforced concrete layer, steel layer in the middle
& SFRC interior layer
With SFRC, shrinkage cracking in the
interior layer was practically
eliminated.

Berlin-type retaining walls
Made up of SFRC arches with U-shaped or rectangular
cross-sections placed between king posts
SFRC STRUCTURAL ARCHES (United Kingdom)

FRC Shell Structures
City of Arts and Sciences, Valencia (Spain).
White fiber-reinforced shotcrete
Requisites:
• Mouldability
• Shell thickness of 5 cm
• Flexural strength and toughness

FRC Tunnel Lining Segments
• Subway train
line under
construction
in Barcelona
(Spain).
• Elements with
conventional
and fiber
reinforcement.

Why Use Steel Fibre-Reinforced Concrete in
Tunnel Linings ?
• Higher crack resistance at early ages (during
demolding, handling and storage).
• Fewer problems of chipping and local crushing due
to impact (during transportation and placing of the
segments).
• Improved resistance against cracking induced by
concentrated compressive stresses (due to uneven
contact between segments, between rings or
between actuator plates and segment).

Details of Section 4
Excavation: 12 m
diameter.
Lining: 7 identical
segments + half-size key.
Internal diameter: 10.9 m.
Lining thickness: 350 mm.
Ring width: 1.80 m.
Designed with SFRC
reinforced with
conventional rebars (60
kg/m3 of rebars, 30 kg/m3
of fibres, 50 MPa
concrete)
FRC Tunnel Lining Segments
350
11600
10900 350

Casting and Stacking of the Segments

Selection of the Concrete Composition
• Requirements
• 28-day characteristic compressive strength of at least 40
MPa.
• Early-age (4-6 hours) mean compressive strength of at
least 25 MPa.
• 28-day mean equivalent flexural strength of at least 2.9
MPa.
• Practical Considerations
• Adequate “placeability” with 30 kg/m3 of steel fibers.
• Maximum cement content of 400 kg/m3.
• Cement and aggregates should be those normally used
in the prefabrication plant.

Composition and Properties
Component kg/m3
Cement CEM I 52.5R 400
Sand 0/5 mm 745
Gravel 5/14 mm 558
Grava 12/22 mm 559
Water 132.2
Superplasticizer 4.8
Property Test result
Slump after 20 minutes
from casting
3 cm
Density of fresh concrete 2430 kg/m3
28-day cylinder strength
62,8 MPa
(±2,4%)
at 4+0,5
hours
18,7 MPa
(±3,7%)
at 5+0,5
hours
25,0 MPa
(±3,1%)
Compressive
strength with
accelerated
curing
at 6+0,5
hours
28,2 MPa
(±2,4%)

Selection of Fiber Type
Toughness
Characterization
• Belgian standard was chosen
for determining the equivalent
flexural strength (deflection
limit of 1.5 mm).
• Toughness evaluated with
different fibers.
• Fibers had lengths of 50-60
mm and diameters of 0.75-1.0
mm.
0 1 2 3 4
Flecha (mm)
0
10
20
30
40
50
Carga(kN)
Dramix 80/60
Dramix 65/60
Wirand 1.0/50
Novocon 1060
Duoloc 47×1.0
Tests with
45 kg/m3 of fibres
Midspan deflection (mm)
Load(kN)

FRC for Metro Line 9 in Barcelona
From Badalona through the Collserola hills to the Airport.
Total length of 43 km with 46 stations. Tunnel depth = 0-90 m
Project duration: 2002-14
Estimated cost: 2.2 million euros

Shotcrete - Rohtang Road Tunnel
Fiber type : Dramix RC 65/35 BN
Concrete grade : M 35
Length : 14 km
Lining thickness : 100 mm
Year : 2011, Ongoing
Location : Rohtang, Himachal Pradesh
The tunnel creates an all-weather route to
Leh, Lahaul and Spiti valleys in Himachal Pradesh.
Courtesy: BEKAERT

Precast Boundary Wall - Navi Mumbai SEZ
Pvt. Ltd
Courtesy: BEKAERT
Fibre type : Dramix RC-80/60-CN
Project : Navi Mumbai SEZ
Length : 68 km
Location : Navi Mumbai,Maharashtra
The NMSEZ project involved developing
onsite production through concrete batching plant,
which resulted in undisrupted supply of concrete
to site helping completion in 7 months

Precast Elements – Storm Water Drains
Courtesy: PRECISION WIRE INDUSTRIES
NHAI 4-LANE
HIGHWAY
PROJECT.
JHAMTHA
TO KANHAN
RING ROAD;
NEAR
NAGPUR

Container Terminal – Toyota
Courtesy: BEKAERT
Fibre type : Dramix RC 80/ 60 BN
Thickness : 300 mm
Project : Toyota Container Terminal
Area : 17,200 sqm
Location : Bengaluru, Karnatka
With 110 Tons Axle load. Toyota stacks
4 Containers of 26 Tons each
with a Kalmar Reach Stacker.

Slab on grade – BMW India Pvt. Ltd.
Courtesy: BEKAERT
Fibre type : Dramix RC 80/60 BN
Area : 12,500 sqm
Project : BMW India Pvt Ltd
Thickness of slab : 150 mm
Location : Chennai, Tamil Nadu
BMW, a German automobile manufacturing company is
constructing an assembly building. Dramix® steel fibre
dosage 10kg/m3 was chosen as reinforcement.

Fibre-Reinforced Concrete Pavements
Reduced cracking, settlement and joint spacing.
Road at Guest House Dining Hall at IITM; M35
concrete (with 30% fly ash), 25 kg/m3 of 50
mm × 1 mm steel fibres
Road at JEE/GATE Offices at IITM; M30 concrete
(with 15% fly ash), 15 kg/m3 of 60 mm × 0.75 mm
collated steel fibres

Pavement designed and laid
at IIT Madras
• Existing asphalt pavement was distressed
• White topping of average thickness 120 mm designed

Pavement designed and laid
at IIT Madras
• Existing asphalt pavement was distressed
• White topping of average thickness 120
mm designed
• Project details
– Total length – 218 m
– Casting done in January 2011
– Average temperature - 33C
• Initial stretch – 124 m
• Second stretch laid after two days – 94 m

Design Details
• Design axle load – 10 t
• Grade of concrete M35
• Design thickness – minimum 120 mm
• Required design flexural strength of SFRC = 0.7 MPa
• Chose fibre details
– Fibre type – Hooked end steel fibres, 60 mm length, l/d= 80
– Fibre dosage – 15 kg/m3
– Specification for the fibre concrete
• Equivalent flexural strength – 1.68 MPa
• Re,3 = 42 %

SFRC: Important Considerations
Fibre parameters:
• Length, l
• Diameter, d, or aspect ratio, l/d
• Volume fraction, Vf
For a concrete where cracking occurs
along the aggregate-mortar interfaces
(i.e., in usual concretes), an effective
fibre should be much longer than the
maximum aggregate size (dmax).
 l ≥ 2.5 dmax
For a concrete where cracking occurs through the aggregates (e.g., HSC), fibre
length is not important as Vf (which is normally higher than in usual concrete).

SFRC: Important Considerations
Mixing and placing:
• Mixing time should be longer (i.e., 2-3 more minutes)
than usual to get a homogeneous fibre distribution
• High slump can lead to segregation of the fibres
• Excessive vibration can lead to segregation and
preferential orientation of the fibres
• In flowing concrete, fibres are usually oriented along
the direction of flow

FRC: Fresh state
Balling of fibres can occur during mixing due to an
inappropriate combination of fibre dosage, aspect
ratio and/or length
Collated fibres (i.e., fibres held together by water-
soluble glue) can reduce the problem of balling

SFRC: Fresh State
Implications
 Slightly higher paste/mortar content (about 10% more)
 Need for superplasticizer
 Slump of the matrix (without the fibers) should be 3-5 cm more
than that expected in SFRC
Incorporation of fibres can lead to lower workability
Fibras l/d = 100 en
hormigones con diferente
tamaño máximo de agregados
19 mm
Conoinvertido(s)
Contenido de fibras (kg/ m3)
20 40 80 10060
10 mm
Hormigones con agregados de
19 mm de tamaño máximo
l/d = 100
Conoinvertido(s)
20 40 80 10060
l/d = 75
Invertedcone(sec)
Invertedcone(sec)
Fiber content (kg/m3) Fiber content (kg/m3)
Fibras l/d = 100 en
19 mm
Conoinvertido(s)
20 40 80 10060
10 mm
l/d = 100
Conoinvertido(s)
20 40 80 10060
l/d = 75
Invertedcone(sec)
Invertedcone(sec)
l/d=100
Different fibre length
Fibras l/d = 100 en
19 mm
Conoinvertido(s)
20 40 80 10060
10 mm
l/d = 100
Conoinvertido(s)
20 40 80 10060
l/d = 75
Invertedcone(sec)
Invertedcone(sec)
Fibras l/d = 100 en
19 mm
Conoinvertido(s)
20 40 80 10060
10 mm
l/d = 100
Conoinvertido(s)
20 40 80 10060
l/d = 75
Invertedcone(sec)
Invertedcone(sec)
l =19 mm
Different aspect ratio

SFRC: Mechanical Behaviour
Stress (load)-displacement response
• Strength is not affected for low volume fractions of fibers
• Sharp post-peak drop is avoided
• Post-peak ductility increases with an increase in the
dosage and effectiveness of the fibers
stress
(load)
displacement
Usual
concrete
FRC
Toughening
effect of the
fibers in
concrete

Response under compression
Stress-(nominal) strain curve
0 3000 6000 9000 12000
Deformación axial (microdeformaciones)
0
20
40
60
80
100
Tensión(MPa)
HAR con 80 kg/m.cu.
de fibras metálicas
Hormigón
Convencional
HAR
HSC with 80 kg/m3
of steel fibers
HSC
NSC
Stress
Axial strain (microstrains)
Circumferential
extensometer
Specimen
Loading platten
Displacement
extensometer
Strain gauges

Response under uniaxial tension
Stress-crack opening response
Tensión(MPa)
0
1
2
3
4
0 0.5 1 1.5 2
CMOD (mm)
Stress(MPa)
C 70/40
C 70/20
C 70/00
Stress,s(MPa)
Crack opening, w (mm)

Evaluation of concrete in existing structures
Tests on cores
0 500 1000 1500 2000
w (m)
0
1
2
3
s(MPa)
Horizontal
Reference cylinder
Vertical
Vertical core
Horizontal core
Filling
direction

Response under direct shear (push-off test)
Horizontal
LVDT
Vertical
LVDT
Loading
bar
0 0.25 0.5 0.75 1
Vertical displacement (mm)
0
4
8
12
Shearstress,(MPa)
C70/40
C70/20
C70/00
0 0.250 0.25 0.5 0.75 1
Vertical displacement (mm)
0
4
8
12
Shearstress,(MPa)
C70/40
C70/20
C70/00
0 0.25
Maximum stress and post-peak strength increase with fibre content
75 mm
deep
notch
150 mm
260 mm

Flexural Toughness: Conventional Approaches
350 mm
150 mm
75 mm
150 mm
Third-point loaded beam
0 1000 2000 3000 4000
Deflection (m)
0
5
10
15
20
25
30
35
40
45
Load(kN)
20 kg/m3
30 kg/m3
40 kg/m3
0 50 100
Deflection (microns)
0
10
20
30
40
Load(kN)
INITIAL PART
Tests of 30 MPa concrete

Non-dimensional indices: ASTM, Spanish standards
In = area up to a certain deflection divided by the area up to first-
crack such that the index is equal to n for elastic-plastic response

Absolute toughness & Equivalent flexural strength: Belgian,
Japanese, Spanish, Dutch and German standards
Bn = area under load-deflection
curve up to dn (i.e., deflection
equal to the span/n)
Equivalent flexural strength
sn = {Bn/dn} {1.5 span/bd 2}
where b & d = beam width &
depth

Measurements:
• Load-deflection curve
• Load-crack mouth
opening displacement
(P-CMOD) curve
Notched beam
Clip gage
LVDT
0 0.4 0.8 1.2 1.6 2
CMOD (mm)
0
10
20
30
40
load(kN)0 0.2 0.4 0.6 0.8 1
CMOD (mm)
0
0.2
0.4
0.6
0.8
1
deflection(mm)
0 0.4 0.8 1.2 1.6 2
deflection (mm)
0
10
20
30
40
load(kN)
Plain
20 kg/m3
40 kg/m3
Plain
20 kg/m3
40 kg/m3
Flexural toughness based on notched beam response
Toughness measures: equivalent flexural strength(s), residual strength(s)
Proposed by RILEM, adopted by CEN (as European standard)

Time-dependent response
• Restrained plastic shrinkage cracking can be
decreased by the incorporation of polymeric and steel
fibres
• Restrained drying shrinkage crack widths are reduced
with steel fibres
• Impact and fatigue resistance of SFRC is higher than
in plain concrete

SIFCON: Slurry-Infiltrated Fibre Concrete
Properties
• Contains 5-20% volume fraction of steel fibres.
• Cast by preplacing fibres in mould and then filling the
voids with a cement-based slurry. (The fibres tend to be
oriented perpendicular to the casting direction.)
• Unit weight = 1900-3200 kg/m3
• Compressive strength = 60-210 MPa
• Tensile strength = 4-14 MPa
• Ductility can reach values of 1000 times that of the plain
matrix
Applications
• Safety vaults
• Explosion resistant containers
• Repair and rehabilitation

UHPC: Ultra-High Strength Concrete
RPC: Reactive Powder Concrete
Properties
• w/c ≤ 0.2
• High binder content (e.g., a cement dosage of
1000 kg/m3; silica fume dosage of 30%)
• Aggregate grain size is limited to 0.5 mm.
• Short steel fibres of 5-13 mm length at high
dosages (e.g., 2-6% volume fractions) are used.
• Compressive strength can be as high as 230 MPa
• Flexural strength can be as high as 50 MPa
• Elastic modulus = 50-60 MPa
Applications
• Thin and slender precast structural elements
• Strong and compact products

References
• Fiber-Reinforced Cement Composites, P.N.Balaguru &
S.P.Shah, McGraw Hill, New York, 1992
• Les bétons de fibres métalliques, P.Rossi, Presses Ponts
et Chaussées, Paris, 1998
• The Science and Technology of Civil Engineering
Materials, J.F. Young, S. Mindess, R.J. Gray & A. Bentur,
Prentice Hall, 1998
• http://www.ductal-lafarge.com/
• ACI Materials Journal
• Intnl. Journal of Cement Composites
• Materials and Structures Journal
• Concrete International Journal, ACI

6. fibre reinforced concrete

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