26.Strengthening of Structural Elements by Ordinary Reinforcement (New One)

7/29/2019 26.Strengthening of Structural Elements by Ordinary Reinforcement (New One)

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Strengthening of structural elements byordinary reinforcement


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Strengthening:

Strengthening is modification of structure , not necessarily a

damaged one, with the purpose of increasing its load carrying

capacity or stability with respect to its previous condition.

Strengthening of structure should be designed and constructed in

accordance with appropriate building codes.


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Many buildings that originally were constructed for a specific use

now are being renovated or upgraded for a different application

that may require higher load-carrying capacity.

As a result of these higher load demands, existing structures need

to be reassessed and may require strengthening to meet heavier

load requirements.


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Strengthening can be achieved by....

Replacing poor quality or defective material by better quality

material

Attaching additional load bearing material

Redistribution of the loading actions through imposed

deformation of structural system


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Strengthening of column.....


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General :

The load carrying capacity ofa column can be increased byencasing the existing columnwith concrete or providing a

supplementary steel structurewhich will relieve the originalof part of its load or make thecolumn stiffer.


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Strengthening of column required when:

The load carried by the column is increased due to eitherincreasing the number of floors or due to mistakes in thedesign.

The compressive strength of the concrete or the percentand type of reinforcement are not according to the codesrequirements.

The settlement in the foundation is more than theallowable.


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Construction stages:

Step- 1 :

Before this technique is carried out, we need to reduce oreven eliminate temporarily the loads applied to the

column; this is done by the following steps:

Putting mechanical jacks between floors.

Putting additional props between floors.


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Stage-2 :if corrosion in reinforcementfound then,

Remove the concrete cover andexpose rusted reinforcement.

Identify extent of corrosion ofreinforcement and removeconcrete 150mm beyond rusted

length. Remove plaster or

coating,100mm beyond the edgeof cut concrete.

Hack back to sound concrete and

roughen exposed concrete.


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Stage-3:

Cut and remove any severelycorroded and detachedreinforcement.

Clean the steel bars using awire brush or sand

compressor. Provide new supplementary

reinforcement as directed.

Protect old and newreinforcement and seal

gaps.


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Add steel connectors into theexisting column in order tofasten the new stirrups of the

jacket in both the vertical andhorizontal directions at spacesnot more than 50cm.Thoseconnectors are added into thecolumn by making holes 3-

4mm larger than the diameterof the used steel connectorsand 10-15cm depth.

Fill the holes with anappropriate epoxy materialthen insert the connectors


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Stage-3:

Do the Coating on the existing

column with an appropriate

epoxy material that would

guarantee the bond between

the old and new concrete.

Pour the concrete pack of the

jacket before the epoxy

material dries. The concrete

used should be of low

shrinkage and consists of

small aggregates,sand, cement and additional

materials to prevent shrinkage.

Plaster the area or apply

protective coat.


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Different repairs techniques for column:

1.)Damaged area is in between

but not involving corners

Steel bar is corroded but intolerable state and concrete is

sound


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Stage 1 & 2 of repairs

1. Removed concrete2. Existing steel protective

coated3. New stirrups protectivecoated existing stirrups notshown

4. Quick setting fixing aid

5. Bond coat6. Hand pack concrete7. New protective plaster

Enlargedview ofdamagedarea afterstage 1&2 ofrepair


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Stage 3 & 4 ofrepairs

Enlarged detail at E


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2.)Damaged area involving one corner

Severe steel corrosion , concrete core sound or unsound

Stage 1 & 2 ofrepairs

Enlarged viewof damaged

area after stage1&2 of repair


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1. Removed concrete2. Existing steel protective coated3. New stirrups protective coated

existing stirrups not shown

4. New additional bars5. Fixing aid6. Bond coat7. Hand pack concrete8. Guide strip

9. New protective plaster


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Stage 1 & 2 of

repairs

Enlarged view ofdamaged areaafter stage 1&2 ofrepair

3.) Damaged area involving two corners


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1. Removed concrete2. Existing steel protective

coated3. New stirrups protective

coated existing stirrups notshown

4. New additional bars5. Fixing aid6. Bond coat

7. Hand pack concrete8. Guide strip9. New protective plaster


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4.) Damaged area involving three or all corners

Enlarged view of damaged area

after stage 1&2 of repair


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1. Removed concrete

2. Existing steel protectivecoated

3. New stirrups protectivecoated existing stirrups

not shown4. New additional bars5. Fixing aid6. Bond coat7. Hand pack concrete8. Guide strip9. New protective plaster


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General stepsshowingincreasingcross-sectionarea of columnand increasingits strength


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Strengthening of beams:


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General:

Reinforced concrete beams needsstrengthening when the existing steel bars in the

beam are unsafe or insufficient, or when the loadsapplied to the beam are increased. In such cases,there are different solutions that could be followed.

The strengthening of beams may be required for the

following:a.) Flexural strength

b.) Shear strength


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1.)when exposed steel corroded but in tolerable state,concrete core is sound (nominal repair)

Stage 1 & 2 of repairs shown in elevation (nominal repair)

Stage 3& 4 of repairsshown in section

) h d t l d d b t i t l bl t t


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2.)when exposed steel corroded but in tolerable state,concrete core is sound (repair of deep cavity )

Form work details Stage 3 & 4 of repairs shown in section

) i i b d t d


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3.) corrosion in bars severe and concrete core sound orunsound (when damages to part span)

Stage 1 of repair shown in elevation

Stage 2 of repairs shown in elevation


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Form work details

Section 3 &4 of repairsshown in elevation


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4.) Rebar corrosion sever concrete sound or unsound

(damage along entire span)

Stage 1& 2 of repairs shown in elevation


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Form work details


shown in section


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5.) Increasing both the reinforcing steel bars and the cross-sectional area of concrete

( damage along entire span)

1. Supporting the beam

2. The concrete cover is removed for both the upper andlower steel bars. The steel bars are well cleaned and

coated with an appropriate material that would preventcorrosion.

3. Holes are made, in the whole span of the beam under

the slab, 15-25cm apart, a diameter of 1.3cm and extendto the total width of the beam.


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4. The holes are filled with an epoxy material with lowviscosity.

5. Install steel connectors for fastening the new

stirrups. Steel connectors are installed into thecolumns in order to fasten the steel bars added to thebeam.

6. The added stirrups are closed using steel wires and

the new steel is installed into these stirrups.7. The surface is then coated with a bonding epoxy

material. The concrete cover is poured over the newsteel and the new stirrups.


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Strengthening of

beam by increasingthe cross-sectionalarea and bars


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Strengthening of slab:


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General

Due to increase in the applied loads on slabs or theirunsafe design, or corrosion of the reinforcing steel bars,or cracks in the slabs, strengthening of slab is required.

If the slab is unable to carry the negative moment and thelower steel is sufficient, upper steel mesh should beadded with a new concrete layer.

If the slab is unable to carry the positive moment or whenthe dead load is much less than the live load carried bythe slab , a new concrete layer on the bottom of the slabshould be added.


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Procedure:

Support the slab, removing the concrete cover, cleaningthe steel bar and coating them with epoxy.


I t ll i d i d di t ib ti t l t i d


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Install required new main and distribution steel at requiredspacing.

Enlarged detailing at E


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Do the coating of epoxy, pour the new required layer ofconcrete.

Prospective view of slab subjected to repair


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Strengthening ofslab by increasing its

depth from bottom


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Strengthening of slabby increasing its depthfrom bottom


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Strengthening of foundation:


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General:

Columns foundations need strengthening in the case ofapplying additional loads. Widening and strengthening ofexisting foundations may be carried out by constructing aconcrete jacket to the existing footings.

The new jacket should be properly anchored to theexisting footing and column neck in order to guaranteeproper transfer of loads. The size of the "jacket" shall be

selected such that the averagemaximum foundation pressure does not exceed therecommended allowable value.


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Attention shall be given during construction in order thatthe excavations for the new "jackets" do not affect theexisting adjacent foundations.

An isolated footing is strengthened by increasing the sizeof the footing and the reinforcement steel bars.


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Procedure:

Excavating around the footing


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Cleaning and roughening the concrete surface ,Install dowelsat 25-30cm spacing in both directions using an appropriateepoxy material.

Fasten the new steel bars with the dowels using steel


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Fasten the new steel bars with the dowels using steelwires. The diameter and number of steel bars should beaccording to the design.

D th C ti f ti f ith b di t


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Do the Coating on footing surface with a bonding agentin order to achieve the required bond between old andnew concrete. Pour the new concrete before the bonding

agent dries. The new concrete should contain a non-shrinkage material.


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Specifications:

Concrete strength to be 20 N/mm2 (minimum)

Cover to bars to all sides: Beams 35mm

Slabs 25mm

Columns 40mm

Ground beams 50mm

Foundations 75mm

Bar spacing to be restricted to 200mm maximum. All bars

to be tied with links. Jacketing thickness should be 100mm minimum. Aggregate

size should be restricted to 10mm.


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Documents

26.Strengthening of Structural Elements by Ordinary Reinforcement (New One)