PAPER PRESENTATION
ON
“ADVANCED SESMIC RETROFITTING
TECHNIQUE – FIBER COMPOSITE”
By
NAGA RAJU.P
III/IV ME
KOTESWARARAO.R
Email :nagaraju_oct22@yahoo.co.in
III/IV ME
kotesh320@yahoo.com
DEPARTMENT OF MECHANICAL ENGINEERING,
GUDLAVALLERU ENGINEERING COLLEGE,
GUDLAVALLERU-521356, KRISHNA Dist.
TO
VIGNAN ENGINEERING COLLEGE
‘ VIGNAN MAHOSTAV-06’
vadlamudi,Guntur
ABSTRACT:
This paper discusses a novel technique of rehabilitation of earthquakeaffected structures and retrofitting of structures against possible earthquakes using
fiber composites. This technique has been successfully applied in earthquake-affected
Gujarat; it introduces high-strength non-metallic fibers along with polymeric resins in
repair. As non-metallic fibers are hitherto unused in structural repairs in India, a brief
account on these materials has been included. Design methods, field application
techniques and their suitability have also been discussed.
The Gujarat earthquake on January 26, 2001 has caused
widespread damage of structures and a substantial portion of them require extensive
structural rehabilitation.
The structural rehabilitation community is in search of
techniques that are reliable, fast, cost effective and easy to implement. In addition, the
earthquake has exposed the vulnerability of the existing structures, especially in
highly seismic regions. A large number of unaffected structures in the region require
retrofitting to avoid future loss of property. A vast majority of these structures is
reinforced concrete (RC) buildings. Existing practices of repair go little beyond
cosmetic treatment of the structure. Such methods neither strengthen the structure nor
extend its life. This paper discusses a novel rehabilitation and retrofitting technique
that has been successfully implemented in rehabilitation and seismic qualification of
RC buildings in the Gujarat region. The method has been in use in other seismically
active regions of the world.
INTRODUCTION:
An earthquake generates ground motion in both the horizontal and the
vertical directions. Due to the inertia of the structure, the ground motion generates
shear force and bending moments in the structural framework. Most failures in
earthquake-affected structures are observed at the joints. Moreover, due to existing
construction practice, a construction point is placed in the column very close to the
beam-column joint, Fig. 1(a). This leads to shear or bending failure at or very close to
the joint. The onset of high bending moments may cause yielding or buckling of the
steel reinforcement. The high compressive stress in concrete may also cause crushing
of the concrete. If the concrete lacks confinement, the joint may disintegrate and the
concrete may spall, Fig. 1(b) and (c). All these create a hinge at the joint and if the
number of hinges is more than the maximum allowed to maintain the stability of the
structure, the entire structure may collapse. If the shear reinforcement in the beam is
insufficient, there may be diagonal cracks near the joints, Fig. 1(d). This may also
lead to failure. Bond failure is also observed in cases where lap splices are too close
to the joints.
The conventional strengthening methods for reinforced
concrete structures attempt to compensate the lost strength by adding more material
around the vulnerable sections. These methods (column retrofitting by concrete and
steel jacketing, L & T beam retrofitting, foundation by rebar) include section
enlargement, polymer modified concrete filling and polymer grouting.
The methods that involve concrete in strengthening are time
consuming, dusty and laborious.
They require a long time to implement, and
therefore, a longer period of evacuation. They also increase the dead load on the
structure. In some cases, especially in bridges, external post-tensioning bonded steel
plates and steel jacketing have been used.
These techniques often apply steel
reinforcement that remains exposed to environmental attack. Therefore, they are
vulnerable to corrosion that limits their lives.
Moreover, the quality of the
strengthening depends heavily upon the skill of the personnel.
It is difficult to
strengthen complex areas such as beam-column connections using these methods.
Recent developments in Fiber Reinforced Composites (FRC)
can solve many of these problems. These materials are extremely strong, with high
ultimate strain. They are chemically inert and corrosion resistant. Moreover, they are
very light and that facilitates easy implementation at site with less supporting
structures. These methods are cleaner and the materials used cure very quickly. This
leads to shorter down time of the affected structure. As these materials are relatively
new to concrete users, a brief description has been given below.
1. FIBRE REINFORCED COMPOSITES:
FRCs have two components-matrix and fiber, Fig. 1.
In the
present context, thermosetting resins like epoxy or polyethylene are used as matrix,
while aramid, carbon and glass fibres reinforce the matrix and lend strength to the
composite. The resin coheres and gives shape to the object, while fibres reinforce it.
The result of such combination is a light, flexible and strong composite material.
Unlike
homogeneous.
conventional
materials,
composites
are
not
Their properties are dependent on position and angle under
consideration. Generally, composites are elastic up to failure and exhibit no yield
point or region of plasticity. The properties are dependent on fibre and matrix, their
relative quantity and orientation of fibre.
If all the fibres are aligned in one direction then the composite
becomes very stiff and strong in that direction but it will have low strength and low
modulus in the transverse direction.
Due to their malleability, fibre reinforced plastics are easy to fabricate. Recent
developments in this field have indicated that they can be used as highly efficient
construction materials in various civil engineering activities.
Fibre Reinforced
Polymer Composites (FRPC) have already been successfully used in industries like
aerospace, automobile and shipbuilding. Recently, civil engineers and construction
industry have begun to realize that these materials have potential to provide remedies
for many problems associated with the deterioration and strengthening of
infrastructure. Effective use of these materials could significantly increase the life of
structures, minimizing the maintenance requirements.
FRPC MATERIAL:
GLASS FIBRE:
E-glass fibre sheets that have a minimum tensile strength of 1700 MPa
and an average elastic modulus of 75000 MPa with a density 900 g/m2 are used.
Sheets of width 250 mm and 500mm and a length of 50 m were found to be
convenient to use and they also resulted in very little wastage.
RESINS:
Resin impregnation is necessary to obtain good mechanical properties
for glass fibre. For standard fibre wrapping, resin is impregnated at the construction
site under ordinary temperature and pressure.
One of the important properties
regarding the workability of resin is optimum viscosity that simultaneously enables
good impregnation into the fibres and keeps the fibres in place. A viscosity of around
1000 cps was found to be suitable.
2. FRPCs IN STRUCTURAL APPLICATIONS:
Fig.3 shows different applications of FRPCs in structures. It
can be seen that composite materials are used in a variety of forms—both in new
construction and repairs. However, in this paper the discussion is restricted to nonprestressed applications of FRPCs in repair and retrofitting of structures. This form is
most interesting in the context of earthquake resistant constructions of Gujarat. In
non-prestressed applications FRPCs can be used in the following forms.
Plates: - These pre-cured FRPC members are used mainly to increase the bending and
shear capacity of concrete sections, Fig.
3(a). These sections are produced by
pultrusion in factories with high reliability of performance. However, the shape of the
FRPC element must be known at the time of its production. It is unsuitable when the
FRPC element needs to be bent at site.
Bars: - These are also produced in factory by pultrusion Fig. 3(b) . These bars can be
used as near-surface reinforcement with little risk of corrosion, as tension
reinforcement in beams and slabs to replace the steel bars.
Sheets: - These are uncured fiber tapes with unidirectional fibers or bi-directional
woven roving, Fig. 3(c). The main advantage of this form is that it can be laid in any
form at site. Therefore, they are most suitable in wrapping around deteriorated
concrete members. The main application of sheets is in wrapping around concrete
sections to increase confinement and shear strength. However, their strength is not as
reliable as that of the plates and the bars. The FRP sheets have been used most widely
in Gujarat.
There are a few other less popular forms of FRPC such as grids, cells
and honeycombs. These are beyond the scope of this paper.
3. REHABILITATION AND RETROFITTING WITH FRPC:
The two main advantages of FRPC in earthquake resistant
applications are its high strength and high ultimate strain. Due to its high strain at
failure, FRPC wrapped columns exhibit a high level of confinement and shear
strength. Due to its corrosion resistance, FRPC can be applied on the surface of the
structure without worrying about its deterioration due to environmental attack. As
FRPC sheets are malleable, they can be wrapped around the joints very easily. An
exhaustive test programme has been undertaken at the Indian Institute of Technology
(IIT), Bombay to evaluate the efficacy of FRPC in structural strengthening, with
collaboration from the Pennsylvania State University and Cold Regions Research and
Engineering Laboratory, USA. A detailed account of the research is beyond the scope
of the present paper. However, the strengthening achieved using FRPC wrap is
highlighted here.
Fig. 4 presents a typical axial stress versus strain curve of
cylindrical specimens wrapped with FRPC using a varying number of layers. It may
be noted that with one layer of FRPC wrap, the ultimate strength of the specimens
increased by a factor of 2.5. The ultimate strength went on to increase up to 8 times
when 8 layers of the wrap were used. The ultimate strain increased by 6 times with
one layer of wrap. This feature is particularly attractive for earthquake resistant
structure. Due to higher ultimate strain the ductility of the structure also increases.
It may be noted that the ultimate strain of the specimens is insensitive
to the number of layers of wrap. Therefore, for earthquake resistance a thin wrap that
offers high ultimate strain but low stiffness is desirable.
The unfavorable creep
behavior of glass fibre does not pose problems in earthquake-resistant applications as
earthquake forces are seldom encountered.
Moreover, glass fibre is much less
expensive than carbon fibre. Therefore, glass fibre has been used in rehabilitation and
retrofitting of structures in Gujarat.
The resin must be able to hold all the fibres together. It is also important that the resin
maintains a bond between the concrete and the FRP.
4. PREPARATION OF SUBSTRATE:
The procedure of fibre wrapping is shown in, Fig. 5.
application of wrap, the substrate has to be prepared.
Before
In the case of damaged
members, the first step is to rebuild the damaged member. The step in rebuilding
consists of:

removing all loose materials and exposing the concrete surface

treating all internal cracks and voids with suitable grouts

replacing the spelled concrete with epoxy mortar or epoxy concrete

Preparing a smooth concrete surface that is suitable for wrapping.
One must remember that the FRPC layer is very thin. Therefore, it is
extremely important to prepare a smooth convex surface of concrete before the
wrapping is begun. The FRPC becomes ineffective if it is not in contact with the
surface of concrete.
Care must be taken to avoid wrinkles, voids and sheet
deformation. Moreover, sharp edges and corners are potential zones of fibre breakage
due to stress concentration. Therefore, all projections are removed and all corners are
rounded off.
A corner radius of 25mm is found sufficient to avoid stress
concentration.
5. FIBRE SHEET WRAPPING:
After preparation of the surface a low viscosity primer is applied on the
concrete surface to improve bond between the fibre sheet and the concrete, Fig. 7(a).
Fibre sheets are cut to required sizes. An allowance for the length of lap joint must be
given while cutting the sheets. The lap length is determined based on test results in
the laboratory and the precision that can be maintained in construction. The cut fibre
sheets are rolled on a circular spindle to make them easy for wrapping.
It is very important to choose the right epoxy resin for wrapping
applications. The resin must be viscous enough to hold the fibres in place. On the
other hand, the resin must wet the fibre thoroughly and there should not be any dry
pockets.
The viscosity of the resin, therefore, is a trade off between these two
contradicting requirements. The resin is usually a two-part mix.
The mixing of the
parts must be thorough. The resin should not entrap air during mixing. Therefore, the
speed of the stirrer and the duration of stirring are extremely important parameters.
The mixed epoxy resin is applied on to the concrete surface that is to be wrapped.
There are two methods of laying dry lay up and wet lay up. In the dry
lay up, the dry fibre sheet is applied on the concrete surface freshly coated with epoxy
resin.
In the wet lay up, the fibre sheet is wetted with epoxy resin before wrapping.
Although wet lay up ensures a better wetting,
lay up, especially in the hot climate of Gujarat.
in the present work.
it is not always convenient to use wet
Therefore, dry lay up has been used
The sheet should not be slack at the time of wrapping and care
must be taken to maintain the intended fibre direction. The sheet is rolled by serrated
Teflon rollers,
so that the resin oozes out through the sheet and wets the sheet
properly. Rolling must always be in the direction avoid any defect in bond.
Spreading some extra resin on the lap area is a good idea. The wrapping must be
completed within the pot life period of the resin that is usually 20 to 30 minutes.
Therefore, it is advisable to mix small quantities of resin at a time. A thin coat of
resin is applied after the wrapping is over. After the resin is completely cured
(usually 24 hours), the wrap is inspected to rule out any defect. A micaceous
polyamide topcoat is applied on the wrapped surface to protect the resin from
deterioration from exposure to ultraviolet rays. The wrapped column is shown in Fig.
6. STRENGTHENING OF BEAMS:
Due to the forces of earthquake, the beams may weaken in shear;
bending or they may have crushing of the concrete due to a lack of confinement.
Beams require separate treatments for strengthening the above aspects. While the
treatment required improving confinement is largely the same as that for columns, the
flexural and shear strengthening require separate discussion.
6.1 FLEXURAL STRENGTHENING:
Flexural strengthening of beams and slabs is necessary when the
tension steel has yielded or it has deteriorated due to corrosion. Flexural members
that are found to have inadequate reinforcement can also be strengthened by this
method.
In order to improve the flexural capacity of beams and slabs, continuous
fiber sheets or plates are bonded to its tension and compression faces. This is the
simplest method of improving flexural capacity of a structural member. However, the
stiffness of the FRPC is of great importance in this case. The allowable transverse
deflection of the flexural members is very small. As a result, we need a stiff FRPC
layer for effective improvement of the flexural capacity. The bond between concrete
and FRPC is also of immense importance here. Therefore, the adhesive must be
chosen with great care.
The method of application of the FRPC in flexural strengthening,
however, is the same as that in the case of wrapping. The only difficulty one faces in
flexural strengthening is that often the application is overhead.
To resist the
displacement of FRPC due to gravitational forces, a thyrotrophic adhesive is often
used. However, in Gujarat, the same glue that is used in wrapping has been used in
flexural strengthening. The application of FRPC also impedes moisture ingress and
further corrosion of steel.
6.2 SHEAR STRENGTHENING:
The shear capacities the beams can be improved by placing
FRPC on the webs the beams. The same wrapping techniques as that given for
columns is employed to strengthen the beam.
Wherever possible, the beam is
wrapped on all four sides. Along with improving the shear capacity, it improves the
confinement the concrete. That, in turn, delays the failure of concrete. For T-beams,
where full wrap is not possible due to obstruction from slab, U-wraps are provided.
The method of application of shear wraps is identical to that of column wraps.
6.3 STRENGTHENING OF BEAM-COLUMN JOINTS:
In earthquake-affected structures, most of the failures are found
at the beam-column junctions, and are combinations of the three primary types of
failures discussed earlier. Therefore, a combination of all the above strengthening
methods is to be used. Using FRPC sheets, a simple and fast method is developed and
employed to strengthened beam-column connections. The step-by-step procedure is
explained in fig. 9.
7. FRPC – ADVANTAGES:

FRPCs are non-metallic. Therefore, they are resistant to corrosion.

They have high strength to weight ratio. Therefore, for the same strength FRPC is
considerably lighter.
This eliminates requirements of heavy construction
equipment and supporting structures.

FRPCs are available in rolls of very long length. Therefore, they need very few
joints, avoiding laps and splices. Its transportation is also very easy.

They have a short curing time; therefore, the application takes a shorter time. This
reduces the project duration and downtime of the structure to a great extent.

Application of FRPC does not require bulky and dusty materials in large quantity,
therefore, the site remains tidier.

FRPCs have high ultimate strain; therefore, they offer ductility to the structure,
and they are suitable for earthquake resistant applications.

They have high fatigue resistance. So they do not degrade, which easily alleviates
the requirement of frequent maintenance.

They have low thermal conductivity.

They are bad conductors of electricity and are non magnetic.
Due to their lightweight prefabricated components, they can be
easily transported. They encourage prefabricated construction; reduce site erection,
labour cost and capital investment requirement.
8.LIMITATIONS:
The exact analysis of concrete members with FRP is
computationally involved and not warranted for a designer 9, 10. It is important to
develop simple design methods that are compatible with the existing Indian codes of
practice.
CONCLUSION:
In this paper a novel technique for repair and retrofitting
structures with emphasis on earthquake resistance is described. The method is fast
emerging and replacing the conventional methods of repair. The durability tests on
the technique have been extremely encouraging. The method has been successfully
applied in rehabilitation of earthquake-affected structures in Gujarat. The technique
requires understanding the behavior and properties of a new set of materials such as
glass, carbon and Kevlar fibers and thermo sets such as epoxy, polyvinyl and
polyester resins. In this connection, it must be mentioned that the technique demands
a different set of skills than that available with most rehabilitation contractors.
Figures:
Fiber Composite
Material
Fiber Phase
Filter
Matrix
Phase
Resin
Carbo
n
Glass
Aramid
Boron
S-glass
Kevla
High tensile Eglass
r
Strength
(1860
Strength
Strength
-7070 Mpa) (3400
(4500
High cost
Mpa)
Mpa)
High cost
Most
popular
Less
cost
Thermos
et
Thermoplas
tic
Highly crossed
linked non
recyclable for
example,epox
y,poly ethane
Not crossed
linked
Recyclable
for
example,pol
y propylene
Fig. Fiber Composite Materials
FRFC application
Stand
alone
Hybrid FRP
concrete
Prestresse
d
New
Bonde
d
Repair and retrofitting
Unbounde
d
Platin
g
Fig.
Non-Prestressed
New
Wrappin
g
Applications of FRPC
Repair and retrofitting
Platin
g
Wrappin
g
Preparation of
surface
Resin mixing
Application if resin
primer
Spreading of resin
Sheet wrapping and resin
impregnation
More
layer
Yes
No
Initial
curing
Inspection and
repair
Top coating
coating
Fig. Flow Diagram of Wrapping Procedure
Fig. Details of FRP Wrapping
Fig. Shear Strengthening
REFERENCES:
1. Abhijet Mukerjee and M. V. Joshi (2002), “Seismic Retrofitting Technique Using Fiber
Composites”, Indian Concrete Journal (New Delhi), Vol. 76 No. 8, p.p. 496 – 516.
2. www.icjonline.com
3. www.google.com