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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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Stage 3 & 4 of repairs
Enlarged detail at E
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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Stage 3 & 4 of repairs
Enlarged detail at E
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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Stage 1 & 2 of repairs
Stage 3 & 4 of repairs
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
Stage 3 & 4 of repairs
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.
Stage 1 & 2 of repairs
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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