assessment and retrofit of concrete structures

8/13/2019 Assessment and Retrofit of Concrete Structures

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By

V.Parameswari

A0104394J


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The process of deterioration of reinforcement due to

chemical or electrochemical change caused in presence of

moisture.

It is a process of returning of a metal to its original

state(ore) by oxidation.

2Fe(s) + O2(g) + H2O(l) Fe(OH)2

Removal of damaged concrete as it may contain cracks,

delamination, etc

If the reinforcing member has rust should be cleaned and aprotective coat may be applied.

Highly corroded steel area which needs replacement.

Replace the removed concrete which prevents the ingress of

corrosion.


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DIAGONOSIS

PHYSICAL

INSPECTION

STUDY OF

DOCUMENTS

ESTIMATION OFLOAD ACTING

ESTIMATINGENVIRONMENTAL

EFFECTS

RETROSPECTIVEANALYSIS

IS DIAGONOSIS

CONFIRMED

ESTIMATION FOR

FUTURE USE

ASSESMENT OF

STRUCTURAL

ADEQUACY

LOAD TEST

YES

NO

MATERIAL TEST

CHOICE OFACTION


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One of the cause for corrosion in underground MRT tunnelis water infiltration.

The cracks and rust strains are formed on reinforced

concrete lining segments may be due to

Chlorine attack

Carbonation

Sulfate attack

Alkali-aggregate reaction (AAR)

Delayed ettringite formation(DEF)


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Soils containing sulphates of calcium sodium, potassiumand magnesium are the primary source of sulphate ions in

groundwater.

A high sulphate content in water is generally indicative of

the presence of magnesium and alkali sulphates. In

combination with ammonium, chloride, nitrate andcarbonate ions the sulphate ion attack is more severe.

The sulfate attack on concrete manifests itself in the form

of expansion, cracking, loss of mass and/or disintegration.

As far as corrosion in underground tunnel DEF will be the

major problem. It is a special case of sulphate attackwhere sulphate ions are formed within the concrete. It

causes swelling of concrete and thus forming map

cracking.


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In Singapore, MRT are operated all throughout the day.

The possibility of executing the repair strategies in during

the night time from 12am to 5.30am.

Since the testing also done in the night temperature limits

are not taken into account.

If temperature is considered there could be moreconcentration of corrosion.

And can also be done at the intervals of two trains arrival

about 10 min gaps.

The regions which needs repairing can be separated into

different parts so that job can be carried out day by daywithout interruption of traffic.

Planning and scheduling the repairing works for each day.

Involving a small group of skilled labours to complete the

work in time, economically and efficiently.


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Half cell potential

Ultra sonic pulse velocity (UPV)

Measured by using a digital voltmeter and corrosion of

reinforcements are studied at several points.


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The results are plotted to get

the contour map of corrosion

of reinforcement.

In tunnels ,indirect transmission method can be followedbecause only one face of the concrete is accessible.

T R

KEY

T-Transmitter

R-Receiver


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The dynamic modulus of elasticity of concrete is measured bymeasuring the velocity of ultrasonic pulse through concrete.

The test equipment has provisions for generating ultrasonicpulse, transmitting it to concrete, receiving and amplifyingthe pulse and measuring and displaying the pulse travel time.

Good acoustic coupling between the transducers andconcrete is to be established for correct measurement of thespeed.

By this technique one can assess the quality of concrete suchas honey combing & compaction

UPV value in km/sec (V) Concrete quality

V greater than 4.0 Very good

V between 3.5 and 4.0 Good, but may be porous

V between 3.0 and 3.5 PoorV between 2.5 and 3.0 Very poor

V between 2.0 and 2.5 Very poor and low integrity

V Less than 2.0 and reading fluctuating No integrity,

large voids suspected


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Ultra sonic pulse velocity method is used to find out the

internal cracks and depth of visible cracks causes by

sulphate attack.


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Strategy 1:

Corrosion inhibitors

Using high grade mix proportion with silica fumes as

mortar

Quick setting cement

Epoxy injection and grouting

Strategy2:

Epoxy coating of reinforcement

Strategy 3: Using zinc primer coating

Strategy 4:

Cathodic protection


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The surface of the damaged area is removed as there will

be map cracking due to sulphate attack. As the concrete cover is removed the reinforcement is

exposed.

If the reinforcement inside the old concrete had been

severely damaged or had lost substantial proportion of its

cross-sectional area by corrosion ,it may need to bereplaced by new bars.

Replacement is necessary if it had lost more than 10-20%

of the cross-sectional area.

Less corroded reinforcements are cleaned and worn out

reinforcements are replaced. The old concrete surface at the back of the reinforcements

are grouted with epoxy.


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The new concrete cover is made with a high grade of mixproportions with silica fumes and quick setting compoundlike calcium phosphate is used.

Quick Setting Concrete attains initial setting within15minutes under normal conditions and should be placed

immediately upon mixing. Leave at least 2 hours after

setting before applying any load. It gives a good

compressive strength at earliest time.

Corrosion inhibitors like calcium nitrite is also added tothe concrete.

These admixture forms a passive film (-FeOOH)around the

reinforcements and thus resisting from corrosion. Itprotects from chlorides and sulphates attack.


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Silica fumes are added to

Make the concrete dense and prevents from shrinkage cracks.

To reduce the chloride associated corrosion and sulphateattack.

To control alkali-silica reaction

to conserve cement

To produce ultra high strength of concrete.

Epoxy coating is a best way to prevent corrosion of

reinforcement if it is done at the early stage.

Since the reinforcement cannot be fully removed and

coated with epoxy this method also fails. One more disadvantage is that when the epoxy coating

breaks that acts as the anode and gets easily corroded.


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Apply zinc primer coating on the reinforcements wherezinc gets oxidized and acts as a sacrificial film.

Application of zinc primer fails as the reinforcement hasto be fully removed from the concrete structures.

It is costly and time consuming.

Cathodic protection can be also adopted to resist thecorrosion.

The damaged concrete cover is removed.

Sacrificial anodes (e.g.Zn,Mg) can be provided at definiteintervals along with the reinforcements.

Sacrificial anodes takes up the corrosion and they geteasily oxidized.

Thus preventing the reinforcement from corrosion.

This method is time consuming .


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Therepair material used should be studied on thefollowing properties to have a good bonding with the old

concrete structures:

compatibility requirements of new and old materials

Compatibility in dimensional stability

Shrinkage of concrete

Thermal expansion of concrete

and also some lab tests for bond strength , shear tests and

compression and shear test had to be done.


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The repaired works are monitored in a periodic intervals.

The concrete is checked whether it had attained a good

bonding with the old concrete.

If any small cracks are formed during the time of

monitoring, Drill in to the crack from the face of concrete

Water or solvent is injected to flush the dirt and foreign

matter.

The surface is allowed to dry

The Epoxy is injected until it floes out through the holes

So it helps in preventing cracks in the future which will

lead to corrosion.


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Fromthe above all methods using corrosion inhibitors inthe concrete is the economical way.

It does not require much skilled labours.

As quick setting cement is used it will definitely will not

hinder the traffic.

Less time consumption.

Number of labours required is also less.

Materials used are cost effective.


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REFERENCE MATERIALS:Structural renovation in concrete,by Zongjin Li,Christopher Leung,and Yunping

Xi published by Taylor & Francis 2009

Repair and protection of concrete structures, by Noel.Pmailvaganam,CRC

Press,1992

http://www.youtube.com/watch?v=RAlC75xG4qU

http://www.fhwa.dot.gov/bridge/tunnel/maintman00.cfm

Causes, Evaluation, and Repair of Cracks in Concrete Structures. In Lindsay

Kennedy (ed.) (1998), Concrete repair manual, pp. 453-474 (Causes, Evaluation,

and Repair of Cracks.pdf)

Diagnosis of Deterioration in Concrete Structures: Identification of Defects,

Evaluation and Development of Remedial Action. In Lindsay Kennedy (ed.)

(1998), Concrete repair manual, pp. 198-206 (Diagnosis of deterioration in

concrete structures.pdf)

Electrochemical Tests for Reinforcement Corrossion. In Lindsay Kennedy (ed.)(1998), Concrete repair manual, pp. 413, 424-431 (Electrochemical tests for

reinforcement corrosion.pdf)

Non-structural Cracks in Concrete. In Lindsay Kennedy (ed.) (1998), Concrete

repair manual, pp. 475-497 pt. 1 (Non-structural cracks in concrete pt.1.pdf)


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assessment and retrofit of concrete structures

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