How do Beam-Column Joints in RC Buildings Resist Earthquakes?
Behaviour of Cocktail Fibre
1. 1
CHAPTER 1
INTRODUCTION
1.1 GENERAL
Earthquakes present a threat to public safety and welfare in a significant portion
everywhere. We cannot stop earthquakes but we can protect ourselves from them, as
“earthquakes don’t kill human beings, but the structures do”.
The behaviour of reinforced concrete moment resisting frame structures in recent
earthquakes all over the world has highlighted the consequences of poor performance of
beam column joints. Beam column joints in a reinforced concrete moment resisting frame
are crucial zones for transfer of loads effectively between the connecting elements (i.e.
beams and columns) in the structure. In the analysis of reinforced concrete moment
resisting frames, the joints are generally assumed as rigid.
In Indian practice, the joint is usually neglected for specific design with attention
being restricted to provision of sufficient anchorage for beam longitudinal reinforcement.
This may be acceptable when the frame is not subjected to earthquake loads. The poor
design practice of beam column joints is compounded by the high demand imposed by the
adjoining flexural members (beams and columns) in the event of mobilizing their inelastic
capacities to dissipate seismic energy.
Since past three decades, extensive research has been carried out on studying the
behaviour of joints under seismic conditions through experimental and analytical studies.
Various international codes of practices have been undergoing periodic revisions to
incorporate the research findings into practice.
Fibre Reinforced Concrete (FRC) has potential application in building frames due
to its high seismic energy absorption capability and relatively simple construction
technique. To tap such potential, the existing body of knowledge on FRC must be
expanded to provide a proper basis for officials to add this method of construction to the
provisions of the building code.
This thesis aims to add to that body of knowledge through experimental
investigation on the behaviour of beam column joint under seismic loading.
2. 2
1.2 BACKGROUND
Plain concrete is weak in tension and has limited ductility and little resistance to
cracking. Microcracks are present in concrete and because of its poor tensile strength; the
cracks propagate with the application of load, leading to brittle fracture of concrete.
Microcracks in concrete are formed during its hardening stage. A discontinuous
heterogeneous system exits even before the application of any external load. When the
load is applied, microcracks start developing along the planes, which may experience
relatively low tensile strains, at about 25-35% of the ultimate strength in compression.
Further application of the load leads to uncontrolled growth of the microcracks.
The low resistance to tensile crack propagation in turn results in a low fracture
toughness, and limited resistance to impact and explosive loading. The low tensile
strength of concrete is being compensated for in several ways, and this has been achieved
by the use of reinforcing bars and also by applying prestressing method. Though these
methods provide tensile strength to concrete, they do not increase the inherent tensile
strength of concrete itself.
Further, conventionally reinforced concrete is not a two phase material in true
sense. Conventionally reinforced concrete a true two phase material only after cracking
when cracked matrix is held by the reinforcing bars. Existence of one phase (i.e. steel or
concrete) does not improve the basic strength characteristics of the other phase and
consequently the overall performance of the traditional reinforced concrete composite is
dictated by the individual performance of the concrete and steel phase separately.
1.2.1 FIBRE REINFORCED CONCRETE
Fibre reinforced concrete is a concrete mix that contains short discrete fibers that
are uniformly distributed and randomly oriented. Fibre material can be steel, cellulose,
carbon, polypropylene, glass, nylon, and polyester. The amount of fibres added to a
concrete mix is measured as a percentage of the total volume of the composite (concrete
and fibres) termed Vf.
Vf typically ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing
fibre length (l) by its diameter (d). Fibres with a non-circular cross section use an
equivalent diameter for the calculation of aspect ratio. The primary role of the fibres in
hardened concrete is to modify the cracking mechanism. By modifying the cracking
mechanism, the macro cracking becomes microcracking. The cracks are smaller in width,
3. 3
thus reducing the permeability of concrete and the ultimate cracking strain of the concrete
is enhanced. The fibres are capable of carrying a load across the crack.
The percentage of Steel fibre used is about 1.5% is taken constant for all the
specimens, the steel fibre length ranges 30mm and aspect ratio ranges about 60.
The ratio of polypropylene fibre is varied from 0.2% and 0.4%. The length is
about 20mm and the aspect ratio is about 2500 & coir pith of 3%. The addition of fibre
cocktails does not significantly increase compressive strength, but it increases the tensile
toughness, and ductility. It also increases the ability to withstand stresses after significant
cracking (damage tolerance) and shear resistance.
1.3 BEAM-COLUMN JOINT
The joint is defined as the portion of the column within the depth of the deepest
beam that frames into the column. In a moment resisting frame, three types of joints can
be identified viz. interior joint, exterior joint and corner joint (Fig.1.1). When four beams
frame into the vertical faces of a column, the joint is called as an interior joint. When one
beam frames into a vertical face of the column and two other beams frame from
perpendicular directions into the joint, then the joint is called as an exterior joint. When a
beam each frames into two adjacent vertical faces of a column, then the joint is called as a
corner joint. The functional requirement of a joint, which is the zone of intersection of
beams and columns, is to enable the adjoining members to develop and sustain their
ultimate capacity. The joints should have adequate strength and stiffness to resist the
internal forces induced by the framing members.
(a) Interior Joint (b) Exterior Joint (c) Corner Joint
Fig.1.1 Types of Joints in a Moment Resisting Frame
The severity of forces and demands on the performance of these joints calls for
greater understanding of their seismic behaviour. These forces develop complex
mechanisms involving bond and shear within the joint.
4. 4
1.3.1 FORCES ACTING ON EXTERIOR BEAM-COLUMN JOINT
The pattern of forces acting on a joint depends upon the configuration of the joint
and the type of loads acting on it. The effects of loads on the three types of joints are
discussed with reference to stresses and the associated crack patterns developed in them.
The forces on an Exterior joint subjected to gravity loading can be depicted as shown in
Fig.1.2.
Fig.1.2. Gravity loading in Exterior joint
The tension and compression from the beam ends and axial loads from the
columns can be transmitted directly through the joint. In the case of lateral (or seismic)
loading, the equilibrating forces from beams and columns develop diagonal tensile and
compressive stresses within the joint.
Cracks develop perpendicular to the tension diagonal in the joint and at the faces
of the joint where the beams frame into the joint. Concrete being weak in tension,
transverse reinforcements are provided in such a way that they cross the plane of failure
to resist the diagonal tensile forces. The shear force in the joint gives rise to diagonal
cracks thus requiring reinforcement of the joint. The detailing patterns of longitudinal
reinforcements significantly affect joint efficiency. Some of the detailing patterns for
exterior joints are shown in Fig. 1.3(a) and Fig. 1.3(b). The bars bent away from the joint
core (Fig1.3 (a)) result in efficiencies of 25-40 % while those passing through and
anchored in the joint core show 85-100% efficiency.
5. 5
a. Poor detail b. satisfactory detail
Fig.1.3 Exterior Beam-Column Joint
However, the stirrups have to be provided to confine the concrete core within the
joint. The forces in a corner joint with a continuous column above the joint (Fig.1.3 b)
can be understood in the same way as that in an exterior joint with respect to the
considered direction of loading
Special and careful detailing is required to avoid failure of such joints so that the
strength of adjacent members could be developed. The stress resultants from the framing
members are transferred into the joint through bond forces along the longitudinal
reinforcement bars passing through the joint and through flexural compression forces
acting on the joint face.
The joints should have enough strength to resist the induced stresses and sufficient
stiffness to control deformations. Large deformations of joints result in significant
increase in the storey displacement.
1.4 MAJOR DESIGN FACTORS
There are four design requirements that control the amount of transverse
reinforcement to be provided in the vicinity of the joint region. They are Shear strength,
prevention of bucking of compression bars, confinement of compressed concrete in
potential plastic hinge regions or over the full length of lapped bars splices. In order to
achieve ductile plastic hinge behaviour it is essential to avoid sudden failure of the
concrete when it reaches its compressive strength.
6. 6
1.4.1 CONFINEMENT OF CONCRETE
Concrete can be made to act in a ductile manner by providing adequate transverse
confining reinforcement in the form of arrangement of spirals, hoops, stirrup ties or cross
ties. The concrete becomes confined when at strains approaching the unconfined strength
the transverse strains becomes very high and the concrete bears out against the transverse
reinforcement which then applies a passive confining pressure. The strength and ductility
of concrete is considerably improved by confinement. The confinement arises because of
arching of the concrete between the transverse bars and the longitudinal bars. The cover
concrete including that concrete outside the arching forces. Is not confined and will be
lost as in the case of unconfined concrete.
1.4.2 TRANSVERSE REINFORCEMENT
The instability of compression bars, particularly in the potential plastic hinge
zone, must be prevented Some yielding of the column bars both in tension and
compression, may be expected in the end regions of “elastic” columns, even through full
development of plastic hinge will not occur. When yielding occur in both tension and
compression there is a reduction in the tangent modulus of the steel at low stress levels
which makes the bars more prone to buckle. Therefore transverse stirrup ties, sometimes
referred to as anti-bucking reinforcement, should be provided in the end regions of all
columns of the frames in the same way as for the end regions of beams.
1.4.3 NECESSITY OF 1350 HOOK
It is required that transverse steel should be anchored by at least a 1350
hook
around a longitudinal bar plus an extension of at least eight transverse bar diameters at the
free end of the bar into the core concrete of the member. Alternatively, the ends of the
bars should be welded. Note that the loss of concrete cover in the plastic hinge region, as
a result of the cover concrete spalling during plastic hinge rotation, means that the
transverse steel must be carefully detailed. However it is considered in that the 900
bend
is undesirable since the extension of bar beyond the 900
is not embedded in the concrete
core. Thus, when the cover concrete spalls the 900
bend anchorage may become
ineffective.
7. 7
Fig 1.4 Necessity of 135o
hook as per IS 13920-1993
When a plastic hinge develops adjacent to the joint, with the beam bars entering
the strain hardening range, yield penetration into the joint core and simultaneous bond
deterioration, as discussed, is inevitable. Consequently, after a few cycles of inelastic
loading, anchorage forces for tension will be redistributed progressively to the hook
except for very deep columns. Bond loss along a straight bar anchored in an exterior very
deep columns. Bond loss along a straight bar anchored in an exterior joint would result in
complete failure. Therefore, beam bars at exterior joints, which can be subjected to yield
in tension during an earthquake should be anchored with a hook or with other means of
positive anchorage.
1.5 PERFORMANCE CRITERIA
The moment resisting frame is expected to obtain ductility and energy dissipating
capacity from flexural yield mechanism at the plastic hinges. Beam-column joint
behaviour is controlled by bond and shear failure mechanisms, which are weak sources
for energy dissipation. The performance criteria for joints under seismic actions may be
summarized as follows:
1. The joint should have sufficient strength to enable the maximum capacities to be
mobilized in the adjoining flexural members.
2. The degradation of joints should be so limited such that the capacity of the
Column is not affected in carrying its design loads.
3. The joint deformation should not result in increased storey drift.
8. 8
1.6 SCOPE FOR THE STUDY
The main objectives of this study are
To evaluate the performance of fibre reinforced exterior beam- column joint
with conventional detailing as per IS 456:2000 & SP34 (O1, O2, F11, F12,
F21, F22, F31, F32)
Conventional reinforcement exterior beam column joint with seismic detailing
as per IS 13920:1993 (S1, S2).
To do a comparative study between conventional joint and cocktail fibre
reinforced joint through experimental investigation.
Study of behaviour of Exterior Beam-Column joint under cyclic loading and
reverse cyclic loading.
9. 9
CHAPTER 2
REVIEW OF LITERATURE
2.1 GENERAL
The behaviour of beam-column joints subjected to seismic loading has been
studied over the past four decades. The area investigated varied from design procedure
and connection details to analytical modelling of beam-column sub assemblages and
frames.
2.1.2 EFFECT OF STEEL FIBER
Melvin R.Ramey (1989) had made experimental investigation on ten beam-
column joints were tested to determine whether increases in joint hoop spacing in
conventional concrete type 2 joints (seismic joints) could be achieved using steel fiber
concrete in place of conventional concrete in the joint region. Two cyclic loading
sequences were applied to each beam-column specimen. The properties of ductility,
ultimate strength, energy dissipation capacity, and joint stiffness of conventional concrete
specimens were compared with those for steel fiber concrete specimens having differing
increases in joint hoop spacing. It was determined that steel fiber concrete specimens with
joint hoop spacing of up to 1.7 times the spacing recommended by ACI-ASCE
Committee for conventional concrete type 2 joint had the same or better ductility,
ultimate strength, energy dissipation capacity, and joint stiffness. This preliminary study
suggests that type 1 joints (non seismic joints) with steel fibre concrete could be
considered for use in place of type 2 joints in seismic zones. The fibre concrete specimens
exhibited very little or no spalling of the concrete, whereas the conventional concrete
specimens showed extensive spalling of the concrete.
2.1.3 EFFECT OF STEEL FIBER IN DEEP BEAMS
Vengatachalapathy and Ilangovan (2010) The properties of ductility, ultimate
strength, energy dissipation capacity, and joint stiffness of conventional concrete
specimens were compared with those for steel fiber concrete specimens having differing
increases in joint hoop spacing. It was determined that steel fiber concrete specimens with
joint hoop spacing of up to 1.7 times the spacing recommended by ACI-ASCE
10. 10
Committee for conventional concrete type 2 joint had the same or better ductility,
ultimate strength, energy dissipation capacity, and joint stiffness.
This preliminary study suggests that type 1 joints (non seismic joints) with steel
fiber concrete could be considered for use in place of type 2 joints in seismic zones. The
fiber concrete specimens exhibited very little or no spalling of the concrete, whereas the
conventional concrete specimens showed extensive spalling of the concrete.This
experimental study deals with the behaviour and ultimate strength of steel fiber
reinforced concrete (SFRC)deep beams with and without openings in web subjected to
two point loading, nine concrete deep beams of dimensions 750mm×350mm×75mm
thickness were tested to destruction by applying gradually increased load. Simply
supported conditions were maintained for all the concrete deep beams. The percentage of
steel fiber was varied from 0 to 1.0.The influence of fiber content in the concrete deep
beams has been studied by measuring the deflection of the deep beams and by observing
the crack patterns. The investigation also includes the study of steel fiber reinforced
concrete deep beams with web reinforcement with and without openings. The ultimate
loads obtained by applying the modified Kong and Sharp’s formula of deep beams are
compared with the experimental values. The above study indicates that the location of
openings and the amount of web reinforcement, either in the form of discrete fibers or as
continuous reinforcement are the principal parameters that affect the behavior and
strength of deep beams.
The following conclusions can be drawn from the experimental results:
Web openings may be provided in the compression zone of the beams and
fiber content of 0.75%by volume may be added to improve the strength of the
structure.
The opening in the tension zone weaken the beam.
Fiber content of 0.75%by volume of the beam improves the ultimate load and
the first crack load of the beam.
Additional of steel fibers increase the tensile strength of concrete matrix and
also increase in the flexural rigidity of the beam.
11. 11
2.1.4 STEEL FIBER REINFORCED ULTRA-HIGH STRENGTH CONCRETE
Marko Orgass, Yvette Klug (1995) had made an investigation on Steel fibre
reinforced Ultra-High Strength Concretes. The paper presents the influence of the short
and long steel fibres in respect of ductility and size effect on the mechanical properties of
ultra-high performance concrete. Experiments were carried out on specimens with
different geometries. The influence of steel fibers was evaluated with the flexural strength
and the post crack behaviour.
It was observed that highest compressive and flexural strength was obtained on
smallest specimens (prism).
In vibrated concrete the compressive strength decreased with the increase of the
specimen slenderness.
This phenomenon was not observed in self compacting ultra-high performance
concrete.
The flexural strength of ultra-high performance concrete without coarse
aggregates is higher than that of ultra-high performance concrete with coarse
aggregates.
Flexural strength increased almost proportionally with the increase of the fibre
volume fraction.
2.1.5 EFFECT OF ANCHORAGE BARS
C.V.R.Murty (2001) summarizes two issues in the design of joints, namely the
anchorage of beam bars into the column and the control of shear stresses in the joint
region. The earthquake-resistant structures are designed to resist strong ground shaking
by large inelastic actions so that the input seismic energy can be dissipated.
Earthquake-resistant structure is required
To possess good ability to absorb seismic energy through inelastic deformations.
To develop a favorable collapse mechanism in the event of strong earthquake
shaking.
Ductility of a section is enhanced through proper confinement of concrete and
flexure failure modes occur before the shear failure modes.
12. 12
2.1.6 EFFECT OF DEVELOPMENT LENGTH
Prabhakar.N (2001) summarizes 70 percent of cities fall under the seismic zones
of III. The paper says stirrups spacing of the column shall not exceed d/4 or 100 mm
minimum for a distance of 2d from supports, and d/2 for the remaining portion, where d is
the effective depth of beam. This requirement is very much at variance with the minimum
spacing of 0.75d as per IS 456. The beam-column joint anchorage length provided Ld +
10db (Development length + 10x bar diameter).
2.1.7 EFFECT OF FRC IN BEAM COLUMN REGION
Mustafa, Ilhan Eren (2002) had made study on ductile behavior of beam-column
joints, closely spaced transverse reinforcement is required by earthquake codes. To
carryout the experimental works for four specimens considered. The specimen 1&2 were
produced to understand the importance of closely spaced stirrups in joints. The two
specimen 3&4 were produced by steel fibre reinforced concrete in joints was intended to
minimize the difficulties. These four full scale specimens were tested under reversed
cyclic loading. The results of the experiments were evaluated with respect to strength,
damage and energy absorption.
The SFRC used in the critical regions of beam-column joints increased the
strength capacity for bending moment and shear forces.
The SFRC specimen have high total and accumulated energy, dissipated energy
and stored energy compare than others.
The ductility and strength capacity could be increased by using SFRC and
decreasing the stirrups in the joint and confinement regions of the beam and
column.
The usage of SFRC can reduce the cost of steel reinforcement and its installation,
and the difficulties in placing and consolidating the concrete in the regions of the
beam-column joints.
Ductile behavior and the strength capacity of beam-column connections depend
on the volume content, aspect ratio of the fibers, fiber type, the regions of SFRC
used in joints, the strength of the concrete, and fiber dispersion in the concrete
mix.
13. 13
2.1.8 EFFECT OF FIBER REINFORCED POLYMERS IN RC JOINTS
Costas P.Antonopoulos and Thanasis (2003) had carried out an experimental
investigation on behavior of shear-critical exterior reinforced concrete (RC) joints
strengthened with fiber reinforced polymers (FRP) under simulated seismic load, are
presented in this study. The role of various parameters on the effectiveness of FRP is
examined through 2/3-scale testing of 18 exterior RC joints. Conclusions are drawn on
the basis of certain load versus imposed displacement response characteristics,
comprising the strength (maximum lateral load) the stiffness, and the cumulative energy
dissipation capacity.
The composite wrap increased the shear capacity of the joints by 35%.
Both the strength and the dissipated energy increase considerably with-but not
proportionally (due to premature debonding) to-the number of FRP layers.
The effectiveness of FRP increases as the transverse steel reinforcement in the
joint decreases.
2.1.9 EFFECT ON DUCTILITY USING FIBER REINFORCED CONCRETE
Andre Filiatrault et.al (2004) an experimental investigation is presented on the use
of steel fibre reinforced concrete to provide ductility in beam to column joints during
earthquake excitation. Four full scale exterior beam-column joints, part of a prototype
building designed according to the National Building Code of Canada were tested under
cyclic loading. Experimental results indicated that FRC is an appealing alternative to
conventional confining reinforcement.
Steel fibre bridging across cracks in the concrete mix increase the joint shear
strength and can diminish requirements for closely spaced ties.
The performance of a joint is closely related to the volume content and aspect
ratio of the fibres.
The results of reverse cyclic loading tests performed on full-scale exterior beam–
column joints indicate that fibre-reinforced concrete is an appealing alternative to
conventional confining reinforcement to provide ductility.
Steel fibres bridging across cracks in the concrete mix increase the joint shear
strength and can reduce, or even eliminate, the requirements for closely spaced
ties. The performance of a joint is closely related to the volume content and
aspect ratio of the fibres.
14. 14
2.1.10 EFFECT ON STRENGTH AND STIFFNESS
Jia-Yih Yen and Hsien-Kuang Chien (2004) had made a study on two-
dimensional RC beam-column joint retrofit under cyclic loads. Two steel plates were
glued to the lateral surfaces of the beams and passed continuously through the beam-
column joint. Some of the steel plates were bolted or stiffened with steel strips. Steel
plates were affixed to the beams in three different ways. The first method used epoxy to
glue the steel plates onto the concrete beams. The other two methods involved preparing
additional stiffening devices between the steel plates and concrete beam (bolts anchored
or welded steel strips).
Strength and stiffness improvement in an RC beam subjected to cyclic loads using
bonded steel plates on both web sides with or without additional bolts or welded
strips.
This unsuitable design was improved by providing additional stiffening devices
such as anchored bolts or welded steel strips. Good strength and satisfactory
ductility were exhibited in every specimen so rehabilitated.
2.1.11 EFFECT ON ULTRA HIGH STRENGTH FIBER
Yung-chin wang and Ming-Gin Lee (2007) had made experimental investigation
on Reinforced concrete (RC) structure strengthened with ultra-high steel fiber reinforced
concrete (UFC) is introduced in the study. Interior RC beam-column joint sub-
assemblages strengthened by means of joint replacement with UFC are tested cyclically
to observe their seismic performance. Prior to frame testing, UFC mechanical properties,
for example the compressive, flexural, rebar bonding, and slant shear strengths, and
durability are examined and discussed. The material test results indicate that the UFC
displays excellent performance in terms of mechanical and durable behavior. The frame
test results show that the UFC-replaced joint frame behaves very well in seismic
resistance.
The abrasion resistance of UFC is about 8 times higher than that of normal
strength concrete, and about 4 times higher than that of HSM.
The UFC higher steel bond strength and better durability than either the NC or the
HSM & twice the shear resistance as normal concrete.
The good durability of UFC is also shown by its high resistance to freeze-thaw
reactions and lower weight loss during abrasion testing.
15. 15
2.2.0 EFFECT OF COCKTAIL FIBER IN BEAM COLUMN JOINTS
Rajadurai.R (2007) focuses on behaviour of M25 concrete in exterior beam-
column joint subjected to cyclic loading, by using various fibre properties with steel fibre
1.5% and polypropylene fibre as 0.2%, 0.4%, 0.6% and 1.5%. The specimens detailed as
per IS 456 and IS 13920 were cast and tested under cyclic loading after 28 days curing.
The parameters analysed are ductility, energy absorption, load Vs displacement curve,
beam-column reinforcement strain, crack control and reduction of shear reinforcement in
the joint region.
From the experimental investigation, it is found that fibre reinforced exterior
beam-column joint with cocktail fibre (1.50% steel fibre+0.2% polypropylene fibre)
performs better ductility, load carrying capacity, stiffness and strength by 10% than
conventional reinforced exterior beam-column joint. The increased spacing of the spacing
reinforcement in the beam-column joint region will make practical advantages in placing
and compacting the concrete.
16. 16
CHAPTER 3
FIBRE PROPERTIES AND TYPES
3.1 GENERAL
Fibre is a reinforcing material which is discontinuous, discrete and can be
uniformly dispersed in concrete. There are many types of fibres available like steelfibre,
glass fibre, carbon fibre etc.
3.2 SIZE, SHAPE AND ORIENTATION OF FIBRE
Fibre is a small piece of reinforcing material possessing certain characteristic
properties. They can be circular or flat. The fibre is often described by a convenient
parameter called aspect ratio. The aspect ratio of the fibre is the ratio of length and its
diameter. Typically aspect ratio ranges from 30 to 150. Fibre efficiency increases with
increase in “Aspect ratio”.
Fig.3.1 Dimensions and Shape of Steel Fibre
It has been reported that up to aspect ratio of 75 increases in aspect ratio increases
the ultimate strength of the concrete linearly. Beyond 75, relative strength and toughness
is reduced. The table shows the effect of aspect ratio on strength and toughness.
Plain concrete possess a very low tensile strength, limited ductility and little
resistance to cracking. Internal micro cracks are inherently present in the concrete and its
poor tensile strength is due to the propagation of such micro cracks, eventually leading to
brittle fracture of the concrete. In plain concrete and similar brittle materials, structural
cracks (micro-cracks) develop even before loading, particularly due to drying shrinkage
or other causes of volume change. The width of these initial cracks seldom exceeds a few
microns, but their other two dimensions may be of higher magnitude.
17. 17
Table.3.1 Effect of Aspect Ratio on Strength and Toughness
( Jules, October 1995, “Seismic Behavior of Steel-Fiber Reinforced Concrete Interior
Beam-Column Joints”, ACI Structural Journal, pp. 543-552. )
When loaded, the micro-cracks propagated and open up and owing to the effect of
stress concentration, additional cracks form in places of minor defects. The structural
cracks proceed slowly or by tiny jumps because they are retarded by various obstacles,
changes of direction in bypassing the more resistant grains in the mix.The development of
such micro cracks is the main cause of inelastic deformations in concrete. They have been
recognized that the addition of small, closely spaced and uniformly dispersed fibers to
concrete would act as crack arrester and would substantially improve its static and
dynamic properties. The difference between conventional reinforcement and fibre
reinforcement is that in conventional reinforcement, bars are oriented in a direction, while
fibres are randomly oriented.
Fig. 3.2 Types of Fibre Orientation in Concrete
Types of concrete Aspect ratio Relative strength Relative toughness
Plain concrete with
randomly dispersed
fibres
0 1.00 1.00
25 1.50 2.00
50 1.60 8.00
75 1.70 10.50
100 1.50 8.50
18. 18
3.3 TYPES OF FIBRE
There are different types of fibres used in the concrete for experimental studies
and research. Each type has its characteristic strength and properties and limitations.
Some of the fibres that could be used are steel fibres, polypropylene, nylons, asbestos,
coir, glass and carbon. Polypropylene and nylon fibres are found to be suitable to increase
the impact strength. They posses very high tensile strength, but their low modulus of
elasticity and higher elongation do not contribute to the flexural strength. Asbestos is a
material fibre and has proved to be most successful of all fibres; it can be mixed with
Portland cement. Tensile strength of asbestos varies between 5600 to 9800 kg/sq.cm.
The composite called asbestos cement has considerably higher flexural strength
than Portland cement. For unimportant fibres like coir, jute, cane splits are also used. For
various applications considered here, use of steel fibres and polypropylene fibres are
having the advantage of being alkali resistant, inert and free from changes of corrosion,
light in mass, and cheaper than steel fibres. Made of monofilaments or extrusion film,
these fibres are of length 6 to 54mm and 400 to 500 mm.
Engineering data and application details of steel fibre reinforced concrete are
widely reported and seem to be well accepted. Polypropylene fibres were of more recent
development. Its applications were restricted mainly for crack arrest in fresh concrete,
more as a replacement of welded wire fabric reinforcement. However, considerable data
on its properties are forthcoming which shows that polypropylene fibres can be adopted
in many engineering applications. Experimental results show that polypropylene fibres
increases the toughness (or energy absorbing capacity) of mortars substantially more than
steel, glass or asbestos fibres. Combined use of steel and polypropylene fibres (hybrid
combination) is also resorted to in some situations. Significant application of
polypropylene fibres in relatively greater volumes (0.5 to 1.0%) includes shortcrete for
tunnel and rock support, concrete overlays and sheeting, slabs on grade and industrial
floors.
19. 19
3.4 TYPES OF FIBRES AND PROPERTIES
The various types of fibres that can be used in cement- based composites are steel,
glass polypropylene, asbestos and natural fibres. Typical properties of the fibres are listed
in the table 3.2.
Table.3.2 Types of Fibres and Properties
Types
Specific
gravity
Tensile
strength
MPa
E,GN/m2
Elongation
at failure,%
Common
Vf, %
Steel 7.86 400-1200 200 3.5 <2
Polypropylene 0.91 550-700 3.5-6.8 21 <2
Glass 2.7 1200-1700 73 3.5 4-6
Asbestos 2.55 210-2000 159 2-3 7-18
Polyester 1.4 400-600 8.4-16 11-13 0.065
Coirpith 1.21 200-300 2.2-3.4 5-7 0.095
Concrete for
compression
2.4 2-6 20-50 - 0
3.5 USES OF FIBRES
There are many types of fibres available. The following are the uses of fibres.
Steel fibres: It is one of the most commonly used fibres. Generally, round fibres are used.
The diameter may vary from 0.25 to 0.75 mm. Use of steel fibre makes significant
improvements in flexural, impact and fatigue strength of concrete. The steel fibre is likely
to get rusted and lose some of its strengths. But investigations have shown that the rusting
of the fibres takes place only at the surface.
Polypropylene and Nylon: These are found to be suitable to increase the impact
strength. They possess very high tensile strength, but their low modulus of elasticity and
higher elongation do not contribute to the flexural strength.
20. 20
Asbestos: It is a mineral fibre and has proved to be most successful of all fibres as it can
be mixed with Portland cement. The composite product called asbestos cement has
considerably higher flexural strength than the Portland cement paste.
Glass fibre: It is recent introduction in making fibre concrete. It has very high tensile
strength. Glass fibre which is originally used in conjunction with cement was found to be
effected by alkaline condition of cement.
Carbon fibre: It posses very high tensile strength and young’s modulus. The use of
carbon fibres for structures like cladding, panels and shells.
Table. 1.2. Types of Fibers and Properties
S.No Properties Steel fiber Polypropylene fiber Coir pith
fiber
1 Length 30 mm 20 mm 30 mm
2 Diameter 0.50 mm 0.008 mm 0.5 mm
3 Aspect ratio 60 2500 75
4 Modulus of elasticity 210000 Mpa 240000 Mpa 18000 Mpa
5 Tensile strength 1100 Mpa 2000 Mpa 800
21. 21
CHAPTER 4
MATERIALS AND METHODOS
4.1 MATERIALS USED
Following materials were used to make the ordinary conventional concrete, In
addition with cocktail fibers like steel, polypropylene and coir pith fiber makes the fiber
reinforced concrete to increase the tensile strength and compressive strength by using
various proportion fibers that gives when compare with ordinary conventional concrete.
Cement (53 grade OPC)
Fine aggregate
Coarse aggregate
Water
Cocktail Fibers
(Steel & Polypropylene fiber)
In addition with coconut coir pith
4.2 CEMENT
Cement is a binding material used in the preparation of concrete. It binds in the
aggregate and fine aggregate with help of water, to a monolithic matter. And also it fills
the fine voids in the concrete. There are two intrinsic requirements for any cement in the
concrete mix design. That is compressive strength development with time and attainment
of appropriate rheological characteristics, type and production of concrete. Vibration in
the chemical composition and physical properties of cement affect the strength
parameters. The various tests for cement conducted in laboratory are:
Standard consistency test
Setting time test
22. 22
4.2.1 Standard consistency test
For finding out initial setting time, final setting time and soundness of cement and
strength parameter known as standard consistency has to be used. The standard
consistency of a cement paste is defined as that consistency which will permit a Vicat
plunger having 10mm diameter and 50mm length to penetrate to a depth of 33-35mm
from the top of the mould.
4.2.2 Setting time test
An arbitrary division has been made for the setting time of cement as initial
setting time and final setting time. The initial setting time is regarded as the time elapsed
between the moment that the water is added to the cement, to the time that the paste starts
losing its plasticity. Final setting time is the time elapsed between the moment the water
is added to the cement and the time when the paste has completely lost its plasticity and
has attained sufficient fineness to resist certain definite pressure.
4.3 AGGREGATES
Aggregates are the important constituent in concrete. They give body to the
concrete, reduce shrinkage and effect economy. Earlier aggregates were considered as
chemically inert materials but now it has been recognized that some of the aggregates are
chemically active and also that certain aggregates exhibit chemical bond at the interface
of aggregate and paste.Aggregates are divided into 2 categories from the consideration of
size ie coarse aggregate and fine aggregate. The size of aggregate bigger than 4.75 mm is
considered as coarse aggregate and whose size is 4.75 and less is considered as fine
aggregate.
23. 23
4.3.1 Grading of aggregates :(As per IS 2386-1963)
The particle size distribution of an aggregate as determined by sieve analysis is
termed as grading of aggregate. If all the particles of an aggregate are of uniform size
,compacted mass will contain more voids, where as aggregate comprising particles of
various sizes will Give a mass contains lesser voids. The particle size distribution of a
mass of aggregate should be such that the smaller particles fill the voids between the
larger particles. The proper grading of an aggregate produces a dense concrete and need
less quantity of fine aggregate,cement paste. Therefore it is essential that the coarse
aggregate be well grade to produce quality concrete. The grading of an aggregate is
expressed in terms of percentage by weight retained or passing percentage through a
series of sieves taken in order of 4.75mm,2.36mm,1.18mm,600microns,300microns,and
150microns for fine aggregate and 80mm,40mm,20mm,10mm,4.75mm for coarse
aggregate
4.3.2 Specific gravity test
The specific gravity of an aggregate is defined as the ratio of the mass of solid in a
given volume of sample to the mass of an equal volume of water at the same temperature.
Since the aggregate generally contains voids, there are different types of specific gravity.
4.4 WATER
Water is an important ingredient of concrete as it actively participates in the
chemical reaction with cement. The strength of cement concrete comes mainly from
the binding action of the hydration of cement get the requirement of water should be
reduced to that required chemical reaction of un-hydrated cement as the excess water
would end up in only formation undesirable voids (or) capillaries in the hardened
cement paste in concrete. It is generally stated in the concrete codes and also in the
literature that the water fit for drinking is fit for making concrete.
24. 24
4.5 MIX DESIGN (ACI METHOD)
M60 - CONCRETE GRADE
DESIGN STIPULATION
Target strength = 60Mpa
Maximum size of aggregate used = 12.5 mm
Specific gravity of cement = 3.15
Specific gravity of fine aggregate (F.A) = 2.6
Specific gravity of Coarse aggregate (C.A) = 2.64
Dry Rodded Bulk Density of fine aggregate = 1726 Kg/m3
Dry Rodded Bulk Density of coarse aggregate = 1638 Kg/m3
Calculation for weight of Coarse Aggregate
From ACI 211.4R Table 4.3.3 Fractional volume of oven dry Rodded C.A for 12.5mm
size aggregate is 0.68m3
Weight of C.A = 0.68 X 1638 = 1108.13 Kg/m3
Calculation for Quantity of Water
From ACI 211.4R Table 4.3.4
Assuming Slump as 50 to 75mm and C.A size 12.5 mm the Mixing water = 148 ml
Void content of FA for this Mixing water = 35%
Void content of FA (V)
V = {1-(Dry Rodded unit wt / specific gravity of FA X 1000)} X 100
= [1-(1726/2.6 X 1000)] X 100
= 34.62%
Adjustment in Mixing water = (V-35) X 4.55
= (34.62 – 35) X 4.55
= -1.725 ml
Total water required = 148 + (-1.725) = 146.28 ml
Calculation for weight of cement
From ACI 211.4R Table 4.3.5(b)
Take W / C ratio = 0.29
Weight of cement = 146.28 / 0.29 = 504.21 kg/m3
25. 25
Calculation for weight of Fine Aggregate
Cement = 504.21 / 3.15 X 1000= 0.1616
Water = 146.28 / 1 X 1000= 0.1462
CA = 1108.13 / 3 X 1000= 0.3690
Entrapped Air = 2 / 100= 0.020
Total = 0.7376m3
Volume of Fine Aggregate= 1-0.7376
Weight of Fine Aggregate= 0.2624 X 2.6 X 1000= 683.24 kg/m3
Requirement of materials per Cubic meter
Cement = 504.21 Kg/m3
Fine Aggregate = 683.24 Kg/m3
Coarse Aggregate = 1108.13 Kg/m3
Water = 146.28 Kg/ m3
Final Ratio Becomes
Cement : Fine agg (kg/m3) : Coarse agg (kg/m3) : Water (l/m3)
1 : 1.35 : 2.19 : 0.29
Normal concrete mix design
Cement : Fine agg (kg/m3) : Coarse agg (kg/m3) : Water (l/m3)
1 : 1.42 : 2.07 : 0.35
Fiber concrete mix design
Cement : Fine agg (kg/m3) : Coarse agg (kg/m3) : Water (l/m3)
1 : 1.31 : 1.92 : 0.34
26. 26
CHAPTER 5
RESULT AND DISCUSSION
5.1 GENERAL
The beam-column joints with normal reinforcement detailing, seismic
reinforcement detailing and with steel fiber (1.5%), by varying polypropylene fiber and
0.4%& 3% coir pith were investigated. Conventional concrete with Cocktail fiber
reinforced concrete (steel fibers (1.5%), polypropylene fibers (0.4%) and coir pith fiber
(3%)) were investigated. From the experimental investigation, it is found that fiber
reinforced with cocktail fiber (1.50% steel fiber + 0.4% polypropylene fiber + 3% coir
fiber) performs better ductility, load carrying capacity, energy dissipation and strength by
10% than conventional concrete. Chart below shows the load carrying capacity between
the conventional concrete as well as fiber reinforced concrete.
The parameters that were determined from the experiment are
1. Load vs. displacement cycle
2. Energy dissipation
3. Beam and column reinforcement strain
4. Load vs. displacement curve
5. Ductility
6. Crack pattern and
7. Joint distortion
5.2 LOAD VS DISPLACEMENT CYCLE
The load vs. displacement cycle is drawn for every specimen, the load is noted for
5mm displacement, for each cyclic and reverse cyclic of loading. The reversal of loading
is noted from the proving ring.
41. 41
Fig.5.9 Load vs Displacement cycle for F31
Fig.5.10 Load vs Displacement cycle for F32
Figure 5.1 to 5.10 shows the variation of load vs displacement cycle for various
beam column joints with various combinations at beam end displacement. The area for
each hystersis loop was calculated using this figures. From the figure 5.4 it is inferred that
specimen F21 and F22 (steel fibre 1.50% + polypropylene fibre 0.2%) has the load
carrying capacity (23kN) higher than the ordinary beam column joint.
42. 42
5.3 ENERGY DISSIPATION
Table 5.11 Energy Dissipation vs Deflection
Disp Energy dissipation for Specimens(kNmm)
O1 O2 S1 S2 F11 F12 F21 F22 F31 F32
5 0 0 0 0 0 0 0 0 0 0
10 18.52 22.4 14 10 12 18 20 14 24 20
15 30 42.22 38.32 40 34.02 26.5 30 40.25 42.7 28.6
30 84.25 138.23 96 114.6 156.5 164 148.5 148 130 108
45 208.41 218 266.25 278 318 286.4 344.2 352.5 276.32 232.5
-5 0 0 0 0 0 0 0 0 0 0
-10
-12.33 -22.08 -14
-
16.34
-
16.25
-
20.25
-
18.25 -18 -14.8
-
18.34
-15
-42 -34 -40.25
-
26.48 -40 -58 -24
-
38.25 -30.25 -30
-30 -
120.62
-
154.72
-
130.06 -106
-
166.5
-
176.4
-
104.6
-
106.2 -130
-
114.5
.
Fig. 5.11 Energy dissipation vs Displacement
The figure 5.11 shows the energy dissipated at each cycle for various specimens.
The energy dissipation capacity is calculated using the enclosed area of the load
deformation curve during each cycle of laoding (i.e. area of each hystersis loop). From
this figure it is inferred that energy dissipation for F21 and F22 (steel fibre 1.50% +
polypropylene fibre 0.2%) is more than other specimens
43. 43
5.4 BEAM AND COLUMN REINFORCEMENT STRAIN
To measure the beam and column strain, demec strain gauge was placed in the
beam bottom reinforcement and in the column top outer face reinforcement. The gauge
length 100 mm was maintained for all the specimens to fix the strain gauge.
5.4.1 BEAM BOTTOM REINFORCEMENT STRAIN
Table 5.12 Beam bottom reinforcement strain
Displacement
(mm)
Cycle Specimen ID
O2 S2 F12 F22 F32
0 0 0 0 0 0 0
5 0.5 0.014 0.008 0.01 0.002 0.002
-5 1 -0.022 -0.02 -0.006 -0.002 -0.012
10 1.5 0.006 0.014 0.018 0.026 0.014
-10 2 -0.022 -0.018 -0.01 -0.03 -0.01
15 2.5 0.016 0.026 0.02 0.038 0.016
-15 3 -0.046 -0.016 -0.01 -0.018 -0.016
30 3.5 0.018 0.028 0.016 0.034 0.028
-30 4 -0.05 -0.054 -0.018 -0.026 -0.038
45 4.5 0.014 0.046 0.01 0.048 0.03
For the specimen O2, S2,F12 and F32 the yield strain occurs at 5 mm (1st
cycle)
and ultimate strain at -30 mm (4th
cycle). In the specimen F22, yield strain occurs at 5 mm
(1st
cycle), ultimate strain at 45 mm (5th
cycle).
48. 48
Fig 5.15 Bar chart of Load carrying capacity for all specimens
The figure 5.15 gives the load vs dispacement values, it shows that the specimen
F21 & F22 (steel fibre 1.50% + polypropylene fibre 0.2%) has more load carrying
capacity than other specimens.
5.6 DUCTILITY
Ductility is an important characteristric of any structural element. It is described
as the capacity of a structural element to undergo deformation beyond yield without
loosing much of the load carrying capacity. Any type of brittle failure should be avoided,
as it does not show warning before failure. If the structure posses sufficient ductile
behaviour, it will be able to experience large deformation near ultimate loads. The
amount of this inelastic deformation is proportional to the amount of ductility of the
member.Ductility has generally been measured by a ratio called ductility factor. It is
usually expressed as a ratio of deflection (∆) at failure to the corresponding property at
yield, as shown below
Displacement ductility factor µ∆ = ∆u / ∆y
Where ∆u – Ultimate displacement
∆y – Yield displacement
Increase in deformations after reaching ultimate load condition was not considered
during experimentation. Hence the deformations at ultimate load have only been
considered. The values of displacement ductility factor are calculated from experimental
readings and listed in table 5.15
49. 49
Table 5.15 Displacement ductility factor
Sl.No Specimen ID
Displacement
Yield ∆y
(mm)
Ultimate-∆u
(mm)
Ductility
factor µ∆
1 O1 17.0 40 2.35
2 O2 16.0 39 2.43
3 S1 15.0 45 3.00
4 S2 14.5 45 3.10
5 F11 13.0 45 3.46
6 F12 12.0 45 3.75
7 F21 11.0 42 3.81
8 F22 11.5 43 3.64
9 F31 9.0 45 4.89
10 F32 10.0 45 4.50
The ductility for the various proportions were calculatedand presented in table
5.15. It is observed that the ductility property for the beam column joint with fibre mix is
more comparing to other specimens. The exterior beam-column joint F31 and F32 with
cocktail fiber (1.50% steel fiber + 0.4% polypropylene fiber) more ductility factor
compared to the other specimens.
5.7 CRACK PATTERN
In the beam-column joints, compression and tension developed in joint region
during cyclic loading and the bond between concrete and reinforcement were reduced
consequently. The first crack occurred near the beam-column joint and with further
increase in loading, the cracks propogated and initial cracks started widening. The crack
patter n of the specimens will be discussed as below.
50. 50
Fig. 5.16 Crack pattern for ordinary joint (O1 & O2)
Figure 5.16 shows the crack pattern for ordinary joint. In this the first crack
occurred vertically at 7mm deflection in second cyclic loading. When load is applied at
the bottom of the beam. The second crack occurred at -10 mm deflection at third reverse
cycle of loading. The third crack occured diagonally at beam column joint at 25 mm
deflection at fourth cyclic of loading. The major crack occurred at joint region at 40mm
deflection at fifth cyclic of loading.
Fig. 5.17 Crack pattern for seismic joint (S1 and S2)
The figure 5.17 shows the crack pattern for the seismic detailed joint. In this joint,
while applying second cycle loading, the first crack occurred vertically at 5mm deflection
in second cyclic loading. The second crack occurred at -15 mm deflection at third reverse
cycle of loading. The third crack occured diagonaly at beam column joint occour in 30
mm deflection at fourth cyclic of loading. The major crack occurred at joint region in 45
51. 51
mm deflection at fifth cyclic of loading. In this joint the crack width is small compare to
ordinary joint, so it behaves better than ordinary joint.
Fig. 5.18 Crack pattern for joint with steel fibre 1.50% (F11 and F12)
Fig. 5.19 Crack pattern for joint with steel ,polypropylene fiber&Coir
(1.50%+0.4%+3%)(F21 and F22)
By comparing the figure 5.16 to 5.20, we can observe that the width of the crack
is reduced and the ductility is increased as showed in table 5.15. From the figure 5.18 &
5.20 the second, third, fourth cracks were only hair cracks. In the fibre reinforced
specimens closely spaced finer cracks were developed and width of such cracks was
smaller than those developed in conventional reinforced concrete joint. It was observed
that the use of fibre reinforced concrete in the joint core could increase the joint stiffness
and minimise damage to the concrete.
52. 52
By comparing the figure 5.16 to 5.20 , we can observe that In ordinary specimens
wide cracks were developed at the joint and the crack width was more concentrated at the
joint. But in fibre reinforced specimens exhibts finer cracks were developed and width of
such cracks was smaller than those developed in conventional reinforced concrete joint.
5.8 JOINT DISTORTION
The Dial gauge was used for measuring joint distortion in beam- column joint
while applying cyclic load. The joint distortion is calculated from the formula
Joint distortion = (((e1+e2) / 2) x (D/ (hxb)))
Where
e1, e2 – Changes in length of the diagonal joint region in mm
D – Initial diagonal length in mm
H – Depth of the joint region in mm
B – Breadth of the joint region in mm
Model Calculation:
Changes in length of the diagonal joint region (e1) = 138 mm
Changes in length of the diagonal joint region (e2) = 145 mm
Initial diagonal length D = 148 mm
Depth of the joint region h = 110 mm
Joint distortion = (((e1+e2) / 2) x (D/ (hxb)))
Joint distortion = 0.2115 mm
53. 53
Table 5.16 Joint Distortion
S.No Specimen Joint distortion (mm)
1 O1 0.2107
2 O2 0.2122
3 S1 0.2113
4 S2 0.2143
5 F11 0.2115
6 F12 0.2008
7 F21 0.2093
8 F22 0.2034
9 F31 0.1945
10 F32 0.1938
The above table 5.16 shows the joint shear stress vs. joint distortion. In this while
comparing the values of joint distortion, the beam-column joints made with fiber has less
joint distortion compared to the beam column joints made with ordinary concrete and
concrete with seismic detailing.
54. 54
CHAPTER 6
CONCLUSIONS
6.1 CONCLUDING REMARKS
Based on the experimental study, the following conclusions have been drawn.
The exterior beam-column joint F21 and F22 with cocktail fibre (1.50% steel fiber +
0.4% polypropylene fiber + 3% coir pith) performs better than the beam-column joint
without fibers, as stated below.
10% Higher load carrying capacity
Finer cracks in the joint region with fibres
15% More energy dissipation
The exterior beam-column joint F31 and F32 with cocktail fibre (1.50% steel fibre
+ 0.4% polypropylene fibre) performs better than the beam-column joint without fibers as
stated below.
Very less joint distortion in fibre joint
More ductility factor
The fibre concrete specimens exhibited very little or no spalling of the concrete,
where as conventional concrete specimen showed extensive spalling of the
concrete.
Fibre reinforced concrete increase the ultimate shear strength when compared to
conventional reinforced concrete.
Steel fibres bridging across the cracks in the concrete and restrain the crack
propagation.
Evaluation of performance of cocktail fiber reinforced concrete with conventional
concrete as per IS 456:2000 that gives more compressive strength.
By using the cocktail fibre, the spacing of hoops provided in the core of beam-
column joint can be increased while maintaining ductile behaviour.
The practical difficulties in placing and compaction of the concrete in beam-
column region can be avoided.
Thus fibre reinforced concrete can be seen as an appealing alternative to
conventional confining reinforcement in all aspects.
55. 55
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