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December 2001 * The Indian Concrete Journal1

Seismic retrofitting techniqu

using fibre composites

This paper discusses a novel technique ofrehabilitationof earthquake-affected structures and retrofitting of

structures against possible earthquakes using fibre

composites. This technique has been successfully

applied in the earthquake-affected Gujarat; itintroduceshigh strength non-metallic fibres along with polymeric

resins in repair. s non-metallic fibres are hitherto

unused in structural repairs in !ndia a brief accountonthese materials has been included. "esign methods# field

application techniques and its suitability have alsobeendiscussed.

The Gujarat earthquake on $anuary % %''( has caused

widespread damage of structures and a substantial portion

of them require e)tensive structural rehabilitation. The

structural rehabilitation community is in search of techniques

that are reliable# fast# cost effective and easy to implement. !n

addition# the earthquake has e)posed the vulnerability ofthe e)isting structures# especially in high intensity earthquakeregions. large number of unaffected structures in the regionrequire retrofitting to avoid future loss of property. vastmajority of these structures is reinforced concrete *+,buildings. )isting practices of repair of buildings go littlebeyond cosmetic treatment of the structure. Such methodsneither strengthen the structure nor e)tend its life. This paperdiscusses a novel rehabilitation and retrofitting techniquethat has been successfully implemented in rehabilitation andseismic qualification of +, buildings in Gujarat region. Themethod has been in use in other seismically active regions ofthe world(.Conventional methodsThe conventional strengthening methods for +, structuresattempt to compensate the lost strength by adding more

bhijit /ukherjee and /angesh 0bhijit /ukherjee and/angesh 0bhijit /ukherjee and /angesh 0bhijit

/ukherjee and /angesh 0bhijit /ukherjee and /angesh0. $oshi. $oshi. $oshi. $oshi. $oshi

material around the vulnerable sections. These methods

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 increas

the dead load on the structure. !n some cases# especially in

bridges# e)ternal post tensioning# bonded steel plates and

steel jacketing have been used%#1. These techniques often asteel reinforcements that remain e)posed to environmental

attack. Therefore# they are vulnerable to corrosion that li

their lives. /oreover# the quality of the strengthening dep

heavily upon the skill of the personnel. !t is difficult to

strengthen comple) areas such as beam-column connections

using these methods.

recent development in fibre reinforced composites

*2+, can solve many of these problems3# 4. These materials

are e)tremely strong with high ultimate strain. They are

chemically inert and corrosion resistant. /oreover# they arvery light and that facilitates easy implementation at sitewith less supporting structures. These methods are cleanerand the materials used cure very quickly. This leads to shodown time of the affected structure. s these materials arerelatively new to the concrete users a brief description habeen given below.Fibre reinforced composites2+,s have two components5matri) and fibre# 2ig (. !nthe present conte)t# thermosetting resins like epo)y orpolyethylene are used as matri)# while aramid# carbon andglass fibres reinforce the matri) and lend strength to thecomposite. The resin coheres and gives shape to the object#while fibres reinforce it. The result of such combination islight# fle)ible and strong composite material


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The Indian Concrete Journal * December 20

7nlike conventional materials# composites are not

homogeneous. Their properties are dependent on position

and angle under consideration. Generally# composites are

elastic up to failure and e)hibit no yield point or region of

plasticity. The properties are dependent on fibre and matri)#

their relative quantity and orientation of fibre. !f all thefibresare 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.

"ue to their malleability# fibre reinforced plastics are easy

to fabricate. +ecent development in this field has indicated

that they can be used as highly efficient construction materials

in various civil engineering activities. 2ibre reinforced plastic

composites *2+8, have already been successfully used in

industries like aerospace# automobile and shipbuilding.

+ecently# civil engineers and construction industry have

begun to realise that these materials have potential to provide

remedies for many problems associated with the deterioration

and strengthening of infrastructure. ffective use of these

materials could significantly increase the life of structures#

minimising the maintenance requirements.

2+8,s offer many advantages over other materials usedin construction and rehabilitation9 *see bo).FRPCs in structural application2ig % shows different applications of 2+8,s in structures:. !tcan be seen that composite materials are used in a variety offorms both in new construction and repairs. owever# in thispaper the discussion is restricted to non-prestressedapplications of 2+8,s repair and retrofitting of structures.This form is most interesting in the conte)t of earthquake-resistant constructions of Gujarat.!n non-prestressed applications 2+8,s can be used in thefollowing forms.8lates8lates8lates8lates8lates


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December 2001 * The Indian Concrete Journal#

tension reinforcement in beams and slabs to replace

the steel bars.

SheetsSheetsSheetsSheetsSheets ontal and vertical direction. "ue to the inertia of the

structure the ground motion generates shear forces and

bending moments in the structural framework. !n earthquake

resistant design it is important ensure ductility in the

structure# that is# the structure should be able to deform

without causing failure. The bending moments and shear

forces are ma)imum at the joints. Therefore# the joints nee

to be ductile to efficiently dissipate the earthquake force

/ost failures in earthquake-affected structures are observe

at the joints. /oreover# due to the e)isting construction

practice# a construction joint is placed in the column very

close to the beam-column joint# 2ig 3*a. This leads to sheor bending failure at or very close to the joint. The onset

high bending moments may cause yielding or buckling of

the steel reinforcements. The high compressive stress in

concrete may also cause crushing of concrete. !f the concret

lacks confinement the joint may disintegrate and the concre

may spall# 2ig 3*b and *c. ll these create a hinge at thjointand if the number of hinges is more than the ma)imum

allowed to maintain the stability of the structure the enti

structure may collapse. !f the shear reinforcement in the b

is insufficient there may be diagonal cracks near the joint

2ig 3*d. This may also lead to failure. =ond failure is alobserved in case lap splices are too close to the joi

!ndian codes suggest methods that attempt to delay all

these failures through a sound reinforcement detailing('.

owever# in many structures these details have not been

followed due to perceived difficulties at site. !n most of

structures in Gujarat lack of confinement and shear cracks

Fi ' (a) Failure at construction oint (b) Crushin ofconcrete (c) $pallin of concrete (d) Diaonal shear crac&(a)(b)(c)(d)


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(a)

(b)

(c)

Fi # (a) Pre+cured CFRPC plate (b) ,FRPC rebar (c) ,lass-bre roll

The Indian Concrete Journal * December 20

have been found to be most common causes of failure.

rehabilitation and retrofitting strategy must alleviate thesedeficiencies from the structures.

Rehabilitation and retro-ttin .ith FRPC

The two main advantages of 2+8, in earthquake resistant

applications are its high strength and high ultimate strain.

"ue to its high strain at failure 2+8, wrapped columns e)hibit

high level of confinement and shear strength. "ue to its

corrosion resistance 2+8, can be applied on the surface of

the structure without worrying about its deterioration due

to environmental attack. s 2+8, sheets are malleable theycan be wrapped around the joints very easily. n e)haustive

test programme has been undertaken at the !ndian !nstitute

of Technology *!!T# =ombay to evaluate the efficacy of 2+8,

in structural strengthening# with collaboration from the

8ennsylvania State 7niversity and ,old +egions +esearch

and ngineering ?aboratory# 7S((#(%. detailed account of

the research is beyond the scope of the present paper.

owever# the strengthening achieved using 2+8, wrap is

highlighted here. 2ig 4 presents a typical a)ial stress versus

strain curve of cylindrical specimens wrapped with 2+8, of

varying number of layers. !t may be noted that with one

layer of 2+8, wrap the ultimate strength of the specimensincreased by a factor of %.4. The ultimate strength went on toincrease up to 9 times when 9 layers of the wrap were used.The ultimate strain increased by & times with one layer ofwrap. This feature is particularly attractive for earthquake-resistant structures. "ue to higher ultimate strain the ductilityof the structure also increases.!t may be noted that the ultimate strain of the specimensis insensitive to the number of layers of wrap. Therefore# forearthquake resistance a thin wrap that offers high ultimatestrain but low stiffness is desirable. Glass fibres that haveconsiderably lower stiffness than the carbon fibres and higherultimate strain are desirable. The unfavourable creepbehaviour of glass fibre poses little adversity in earthquake-resistant applications as earthquake forces are seldomencountered. /oreover# glass fibre is much less e)pensive

than carbon fibre. Therefore# glass fibre has been used in

rehabilitation and retrofitting of structures in Gujar

The resin must be able to hold all the fibres together.

also important that the resin maintains a bond between the

concrete and the 2+8. The details of materials for this wo

are presented below.

FRPC material

,lass -bre

-glass fibre sheets that have minimum tensile strength of

(6'' /8a and average elastic modulus of 64''' /8a with

density :'' gm@m% are used. Sheets of width %4' mm and

4'' mm and a length of 4' m were found to be convenient to

use and they also resulted in very little wastag

Resins

+esin impregnation is necessary to obtain good mechanical

properties for glass fibre. 2or standard fibre wrapping# re

is impregnated at construction site under ordinary

temperature and pressure. Ane of the important propertiesregarding the workability of resin is optimum viscosity tha

simultaneously enables good impregnation into the fibres

and keeps the fibres in place. viscosity of around *('''

was found to be suitable.

Preparation of substrate

The procedure of fibre wrapping is shown in# 2ig &. =efore

application of wrap the substrate has to be prepared. !n ca

of damaged members the first step is to rebuild the damaged

member. The steps in rebuilding are consist ofones of fibre breakagedue to stress concentration.

Therefore# all projections are

removed and all corners are

rounded off. corner radius of %4

mm is found sufficient to avoid

stress concentration# 2ig 6.

Fibre sheet .rappin

fter preparation of the surface a

low viscosity primer is applied onthe concrete surface to improvebond between the fibre sheet andconcrete# 2ig 9*a. 2ibre sheets arecut to required si>es# 2ig 9*b. nallowance for the length of lap jointmust be given while cutting thesheets. The lap length is decided ontest results in laboratory andprecision that can be maintained inconstruction. The cut fibre sheetsare rolled on a circular spindle tomake them easy for wrapping.!t is very important to choosethe right epo)y resin for wrappingapplications. The resin must beviscous enough to hold the fibresin place. An 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

trade off between these two contradicting requirements. The

resin is usually a two-part one. The mi)ing of the parts mu

be thorough. The resin should not entrap air during mi)ing.

Therefore# the speed of the stirrer and the duration stirri

are e)tremely important parameters. The mi)ed epo)y resin

is applied on to the concrete surface that is to be wrappe

There are two methods of laying 5 dry lay up and wet

lay up. !n the dry lay up the dry fibre sheet is applied on

concrete surface freshly coated with epo)y resin. !n the we

lay up the fibre sheet is wetted with epo)y resin before

wrapping. lthough wet lay up ensures a better wetting it i

not always convenient to use wet lay up# especially in the

climate of Gujarat. Therefore# dry lay up has been used in

present work. 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 rol

2ig 9*c# so that the resin oo>es out through the sheet and

wets the sheet properly. +olling must always be in the dire

of fibre. The lapped ends must be pressed thoroughly toavoid any defect in bond. Spreading some e)tra resin on the

Fi 3 Details of FRP .rappin


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Fi 4 (a) $urface preparation b5 rindin (b) Cuttin of ,FRP sheet (c) !pplicationof ,FRP sheet (d) 6rapped column

(a)(b)

(c)(d) The Indian Concrete Journal * December 20

Fi 7 $hear strenthenin

Fi 10 $trenthenin procedure in beam+columnconnections

lap area is a good idea. The wrapping must be completed

within the pot life period of the resin that is usually %' to 1'

minutes. Therefore# it is advisable to mi) small quantities of

resin at a time. thin coat of resin is applied after thewrappingis over. fter the resin is completely cured *usually %3 hours

the wrap is inspected to rule out any defect. micaceous

polyo)ide topcoat is applied on the wrapped surface to protect

the resin from deterioration from e)posure to ultraviolet

rays. The wrapped column is shown in 2ig 9*d.

$trenthenin of beams

"ue to the forces of earthquake the beams may weaken in

shear# bending or they may have crushing in concrete due to

lack of confinement. =eams require separate treatments for

strengthening the above aspects. Bhile the treatment

required to improve confinement is largely the same# as that

in columns the fle)ural and shear strengthening requireseparate discussion.

Fleural strenthenin

2le)ural strengthening of beams and slabs is necessary when

the tension steel has yielded or it has deteriorated due to

corrosion. 2le)ural members that are found to have

inadequate reinforcement can also be strengthened by this

method. !n order to improve the fle)ural capacity of beam

and slabs continuous fibre sheets or plates are bonded to i

tension and compression faces# 2ig :*a. This is the simple

method of improving fle)ural capacity of a structural member

owever# the stiffness of the 2+8, is of great importance in

this case. The allowable transverse deflection of the fle)u

members is very small. s a result# we need a stiff 2+8,

layer for effective improvement of the fle)ural capacity. T

bond between concrete and 2+8, is also of immense

importance here. Therefore# the adhesive must be chosen

with great care.

The method of application of the 2+8, in fle)ural

strengthening# however# is the same as that in case of

wrapping. The only difficulty one faces in fle)ural

strengthening is that often the application is overhead. To

retain the displacement of 2+8, due to gravitational forces

thi)otropic adhesive is often used. owever# in case of Guj

the same glue that is used in wrapping has been used in

fle)ural strengthening. The application of 2+8, also impede

moisture ingress and further corrosion of steel

$hear strenthenin

!n earthquake-affected structures shear cracks are often

observed at the ends of beams and sometimes# at several

places through out the span of beam. The shear capacities o

the beams can be improved by placing 2+8, on the webs of

the beams. The wrapping techniques same as that given for

columns is employed to strengthen the beam. Bherever

possible the beam is wrapped on all four sides. long with

improving the shear capacity it improves the confinement of

concrete. That# in turn# delays the failure of concrete. 2o

beams where full wrap is not possible due to obstruction

from slab# 7-wraps are provided# 2ig :*b. The method of

application of shear wraps is identical to that as column w

$trenthenin of beam+column oints!n earthquake-affected structures most of the failures arefound at the beam-column junctions that are combinationsof the three primary types of failures discussed earlier.Therefore# a combination of all the above strengtheningmethods is to be used. 7sing 2+8, sheets a simple and fastmethod is developed and employed to strengthened beam-column connections. The step-by-step procedure is e)plainedin the 2ig ('Concludin remar&s!n this paper a novel technique for repair and retrofittingstructures with emphasis on earthquake is described. Themethod is fast emerging and replacing the conventionalmethods of repair. The durability tests on the technique habeen e)tremely encouraging(1. The method has beensuccessfully applied in rehabilitation of earthquake-affectstructures in Gujarat. The technique requires understanding


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the behaviour and properties of a new set of materials such

as glass# carbon and Devlar fibres and thermosets such as

epo)y# polyvinyl and polyester resins. !n this connection# it

must be mentioned that the technique demands a different

set of skills than that available with most rehabilitation

contractors. Therefore# services of specialised applicators may

be warranted for the technique. The e)act analysis of concrete

members with 2+8 is computationally involved and not

warranted for a designer:# ('. !t is important to develop simple

design methods that are compatible with the e)isting !ndian

codes of practices. fforts are underway at this !nstitute and

will be reported soon.

cknowledgement

The authors would like to acknowledge the encouragement

received by them from /r 7. S. wasthi# managing director#

!ndian 2armers 2ertilisers ,ooperative ,o. in carrying out

the field work.

+eferences

(.DTS7/T# .# DA=ES!# D.# /A+!T# S. and /TS7F# E. $apanese state of the

art on seismic retrofit by fibre wrapping for building structures

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