4 retrofitting of bridges

8/7/2019 4 Retrofitting of Bridges

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Retrofitting of BridgesRetrofitting of Bridges

Technical paper on

a Case Study

By

Saket P Borgaonkar


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Need of RetrofittingN

eed of Retrofitting Upgrading of seismic coefficients as a result

of revised seismic zone

Updating design criteria due to revision ofcode

Bridges not designed to seismic force

Bridges that damaged in earthquakes

Deterioration and aging


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WhyN

ot Demolition and ReconstructionW

hyN

ot Demolition and Reconstruction New structure needs large cost.

New alignment necessitates acquisition of land,

property.

Large quantity of material required for reconstructionthat has to be obtained from nature by destroying itand thus a loss of ecological balance.

New routs add to inconvenience of public and otherliving beings.

The residual utility of old structure goes to waste.


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Methods of Retrofitting


SuperStructure

Most common retrofit for superstructures is

installation of unseating prevention devices

Sub Structure

steel jacketing, reinforced concrete

jacketing, FRP jacketing.


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Bearings. Replacing steel bearings by base isolation bearings.

Bearing seat extension. Provision of stoppers and devices to prevent jumpingof girders.

Abutment.A unique retrofit for tilt of abutments in the frontdirection is to replace some part of the back-fills withexpanded polystyrene (EPS).


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Case StudyCase StudyRetrofitting of the Bridge

across Vishwaganga River

between Malkapur & Nandura


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Salient Features of the BridgeSalient Features of the BridgeYear of construction 1964

Carriageway 7.5 m

Superstructure RCC girder deck system

Substructure CR masonry

Foundation open

No. Of spans 10 spans of 13.72 m,Thickness of deck slab 215 mm


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Salient Features of the BridgeSalient Features of the BridgeNumber of girders

3 main girders at 2.25m c/c with size 300mm

and 1400mm deep and 450 x 300 mm bottomflange and

2 cross girders at 4.40 m c /c

Wearing courseRCC wearing course followed by bituminouswearing course


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History

History

Repairs were carried out twice in 1994

and 2001.

In 1994 mostly cement grouting was

done which could not have served the

purpose.

In 2001, 3 spans were tackled by epoxygrouting, but pot holes to slabs were

not treated.


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Field ObservationsField Observations Considerable holes in deck slab with spalling

of concrete and virtually no bond between

concrete and steel. Reinforcement broken atmany places.

At number of places in bottom bulb of girders,

concrete had not reached due to congestion ofreinforcement & heavy cracks observed ingirders with porous & spalled concrete. Heavyvibrations observed with vehicle movement.


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Field ObservationsField Observations In the bottom surface of cantilever portion,

steel was corroded thereby cracking & spalling

of concrete was observed.

The wearing course was totally damaged withheavy leakages in rainy season thereby further

damaging deck slab.

Weep holes provided for draining of rain waterover the bridge had been blocked.


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Span wise visual observationsSpan wise visual observationsSPAN MAIN GIRDERS DECK SLAB CANTILEVERS

Span 1(N) Crack Honey

Combing

Mapping Cracks Corroded Reinforcement

Span 2 MinorHoney

Combing

Mapping Cracks Corroded Reinforcement

Span 3 Cracked Holes Corroded Reinforcement

Span 4 Cracked Concrete Spalling Corroded Reinforcement

Span 5 Cracked MinorHoles Repaired but No Bond with Old

Concrete

Span 6 Steel Exposed

Cracked

Holes Developed Repaired but No Bond with Old

ConcreteSpan 7 Minor Cracks Severally Damaged Corroded Reinforcement

Span 8 Slightly Damaged Holes Developed Corroded Reinforcement

Span 9 Ok Concrete Spalling Corroded Reinforcement

Span 10 Cracked Concrete Spalling Corroded and Spall

ing


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Non Destructive TestsNon Destructive Tests

To decide exact strength of existingconcrete ultrasonic testing and

rebound hammer testing of girders wascarried out.

Concrete found to be porous with

pulse velocity from 2.5 to 3.0 km/sec.

Rebound hammer test indicated

average strength of about 14.0 N/mm2

.


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Restoration MeasuresRestoration Measures

The quality of concrete was insufficientin strength to withstand dynamic loads

resulting from movement of very heavyvehicles. As a matter of fact due to holesin deck slab, cracks in girders a totalretrofitting was necessary to enhancethe life of structure.

the retrofitting work was divided inseveral activities member wise.


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Main and cross Girders

Girders were strengthenedby injecting low viscous

epoxy grout (SLV grout).Holes were drilled at 150 mmc/c along the cracks byelectric drill and nipples,then cracks were sealedwith epoxy patchingcompound. After grouting,welded mesh was fixed togirders & entire girders were

shotcreted.


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Deck Slab

For portion where Reinforcedsteel was broken, loose

concrete of wearing courseand deck slab was removed.Additional reinforcement waswelded to girder reinforcement.Anticorrosive coating was

applied to rebar. super fluidmicro concrete was pouredwith 3 days compressivestrength of 250 kg/cm2


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Cantilever

The loose cover concrete wasremoved by chisel. Near

corroded reinforcement EPCOKP 100 was applied along theexposed concrete surface. Theexposed and corroded steelwas cleaned with wire brush

and the bars were coated withIPN BR as anticorrosivecoating. The welded meshwas fixed. EPI bond epoxy was

applied and the surface wasshotcreted 50 mm thick.


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Wearing Coat

The Wearing Coat was removed from half

portion i.e. one lane up to deck slab. Holeswere drilled @ 1000 c/c and grouted with

polymer grout. The removed wearing course

was redone with M 35 with superplisizers,

micro silica and polypropelene fibers.

The entire lane was relaid with bituminous

concrete.


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ConclusionConclusion

The girders, which were cracked severely haveshown improvement through out section ofgirder. Integrity of girder and strain carrying

capacity has shown significant improvementwith vibrations reduced to great extent.

The effectiveness of restoration process wasascertained by same NDT techniques.

The average velocity was 3.75 km/s withimprovement of 25 to 30% and concrete qualitywas good.

The average compressive strength was 25.0

N/mm2

4 retrofitting of bridges

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