Strengthening of building structures - MyCourses · PDF fileExternal plate bonding ... proper...

Strengthening of building structuresMon 14.03.2016

Rak-43.3313 Repair Methods of Structures, exercise (4 cr)Esko Sistonen & Fahim Al-Neshawy

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Introduction& Conservation

Repair methodsof the roofing

structures

Repair methodsof the windowand the door

structures Buildinginformation

modelling BIM

Strengthening ofbuilding

structures

Repair methodsfor woodenstructures

Repair of firedamage

The use of ICT forcondition assessment of

structures

Course Content

HVAC-renovation

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Outlines

• Strengthening– The need of structural

strengthening– Factors affecting selection of

strengthening method– Strategies for structural

strengthening• Strengthening techniques

– Section enlargement– External plate bonding– External Post-Tensioning– Ferrocement laminates– Sprayed concrete

• Strengthening of concretestructures– Foundations– Slabs– walls– Beams– Columns– Masonry walls

• Strengthening using FRP– Sprayed FRP– FRP plates / strips

• Case study: Sectionenlargement using SCC


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Strengthening

The aim of strengthening isto increase the capacity of

an existing structuralelement.

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 the structural system

The new load-bearing material:• High quality concrete• Reinforcing steel bars• Thin steel plates and straps• Post-tensioning tendons• FRP composite materials

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The need of structural strengthening

• Load increases due to:– higher live loads,– increased wheel loads,– installations of heavy machinery– vibrations.

• Damage to structural parts dueto:– aging of construction materials– fire damage,– corrosion of steel reinforcement– impact of vehicles.

• Improvements in suitability foruse due to:– limitation of deflections,– reduction of stress in reinforcement– reduction of crack widths.

• Modification of structuralsystem due to:– elimination of walls/columns– openings cut through slabs.

• Errors in planning orconstruction due to:– insufficient design dimensions– insufficient reinforcing steel

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• Magnitude of strength increase• Effect of changes in relative

member stiffness• Size of project

– methods involving specialmaterials and methods may beless cost-effective on smallprojects

• Environmental conditions– methods using adhesives might be

unsuitable for applications in high-temperature environments

– external steel methods may not besuitable in corrosive environments

• In-place concrete strength andsubstrate integrity– the effectiveness of methods relying

on bond to the existing concrete canbe significantly limited by lowconcrete strength

• Dimensional/clearanceconstraints– section enlargement might be

limited by the degree to which theenlargement can encroach onsurrounding clear space

• Accessibility

http://theconstructor.org/practical-guide/

Factors affecting selection of strengthening method

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• Operational constraints– methods requiring longer

construction time might be lessdesirable for applications in whichbuilding operations must be shutdown during construction

• Availability of materials,equipment, and qualifiedcontractors.

• Construction cost, maintenancecosts, and life-cycle costs

• Load testing to verify existingcapacity or evaluate newtechniques and materials

http://theconstructor.org/practical-guide/

Factors affecting selection of strengthening method

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Strategies for structural strengthening

1. Design faults– Lower performance before taken

into service– Strengthening slightly above the

desired performance level

2. Damaged due to an accident– Collision, fire or overload that

damaged the system to a levelwhere performance requirementswere not fulfilled

– Repaired and strengthened to anew performance level

3. Structure demands changes– Higher load bearing capacity was

required

– Strengthening to meet theperformance level

– possible to keep the structurein service

Performance History of a Structure

http://theconstructor.org/structural-engg/strengthening-structures

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Strategies for structural strengthening

Strengthening techniques• Section enlargement• External plate bonding• Post-Tensioning• Ferrocement laminates• Sprayed concrete• FRP

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Section enlargement (Jacketing)

• Jacketing– Adding R.C. jacket section– achieve the desired section

properties and performance

• Advantages– Increasing of the load-carrying

capacity– Increasing of the stiffness.

• Disadvantages– Increase in the concrete

member size obtained after thejacket

– The need to construct a newformwork.

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i. Temporary supportsii. Removal of the deteriorated

concreteiii. Corrosion removal from the

exposed reinforcementiv. Surfaces cleaning and

preparation to ensure bondingwith the repair material

v. Replacement or addition of thesupplementary reinforcement

vi. Reinforcement protection (insome cases)

vii. Formwork and applying of therepair material

Stages of section enlargement

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External plate bonding

• Steel plates or steel flat bars arebonded to the structural elements

• Widely appear in strengtheningof bridge structures.

• The bonding is ensured by:– the use of epoxy adhesives– additional fastening by means

of dowels or bolts glued to theholes drilled in the concretemembers.

• Disadvantages:– it can be applied only to the

relatively sound structures.

http://www.chemcosystems.com/tech_platebond.html

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Post-Tensioning

• Effective in increasing the flexuraland shear capacity

• Applied to reinforced and pre-stressed concrete members

• The post-tensioning forces aredelivered by:– Standard pre-stressing tendons– High-strength steel rods

• Usually located outside the originalsection.

• The repair system supplementsminimal additional load to thestructure thus being an effectiveeconomical strengtheningtechnique. External Post-Tensioning

Internal Post-Tensioning

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Stages of the post-tensioning strengthening

i. If there are existing spallspatching must be done, becausethis repairs must ensure that thepre-stressing forces aredistributed uniformly across thesection of the member

ii. The existing cracks must berepaired by means of epoxyinjecting or other knownmethods

iii. The tendons are connected tothe structure at anchor points,typically located at the ends ofthe member

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Ferrocement laminates

• Thin composite material made ofcement mortar reinforced with wiremeshes.

• The wire meshes are uniformlydistributed in continuous layerswith relatively small diameters

• The Ferro cement is used toreplace the damaged concrete

• Strengthening with Ferro cementimproves:– cracking resistance– flexural stiffness– the ultimate loads compared to

the original un-strengthenedelement.

• These improvements depend onthe full composite action betweenthe Ferro cement layers.

Helza et al. 2014, State-of-the Art Review: Strengthening of ReinforcedConcrete Structures – Different Strengthening Techniques

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Sprayed concrete (shotcrete)

• It is one of the oldest materials andthe most common techniques ofrepairing and strengthening ofreinforced concrete structures

• Sprayed concrete has been usedin that field for almost 90 years.

• There are two processes forapplying sprayed concrete– dry mix sprayed concrete in

which most of the mixing water isadded at the nozzle

– wet mix sprayed concrete inwhich the ingredients , includingwater, are mixed beforeintroduction into the delivery hose.

• Both dry mix and wet mix sprayedconcrete are used in concreterepair / strengthening work, but theuse of dry mix sprayed concrete ismore common.


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Wet injection methodDry injection method

Shotcreting

Strengthening using FibreReinforced Polymer (FRP)

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Strengthening using Fibre Reinforced Polymer (FRP)

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• Plastic and polymer materials thatare reinforced with structural fibersuch as

– fiberglass (GFRP)– carbon fiber (CFRP)– aramid fiber.

Common methods• Externally bonded FRP (EBR).

– FRP plate bonding.– FRP Confining or jacketing

• Sprayed fiber reinforced polymers

http://eprints.qut.edu.au/26889/1/26889.pdf

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http://eprints.qut.edu.au/26889/1/26889.pdf

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Strengthening using sprayed FRP

• Sprayed carbon or glass choppedfiber with vinyl ester resin uponconcrete structures

• Benefits of using vinyl ester resin:– it takes shorter time to harden

the resin than epoxy resin– the mechanical properties of

vinyl ester resin are the sameas the one of epoxy resin

Surface curingprocesses

Fibers spraying

Sketch illustrates sprayedFRP strengthening

technique for differentstructural element

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Typical FRP strengthening applications

http://www.construction-innovation.info/images/pdfs/Research_library/ResearchLibraryC/Project_Reports/Review_of_Strengthening_Techniques_using_Externally_Bonded_Fiber_Reinforced_Polymer_Composites.pdf

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Flexural strengthening of slab

Typical FRP strengthening applications

Flexural strengtheningof beam

shearstrengthening

and confinementof column

wrappingof concrete

tank

shearstrengthening of

beam-column joint


Strengthening of concretestructures

• Foundations• Slabs• walls• Beams• Columns• Masonry walls

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Strengthening the foundations

ShotcretingConcrete casting

Ref. www.lemcon.fi

Piling

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• Columns foundations needstrengthening in the case ofapplying additional loads.

• Widening and strengthening ofexisting foundations may be carriedout by constructing a concretejacket to the existing footings.

• The new jacket should be properlyanchored to the existing footing andcolumn neck in order to guaranteeproper transfer of loads.

• This can be accomplished bydrilling holes into existing concreteof footing and epoxy grouting thelongitudinal reinforcement of jacket. http://theconstructor.org/practical-guide/

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Betoni 3/2006: www.betoni.com/Download/21971/BET0603_s22-24.pdf

Column strengthening bycasting new foundation Temporary supporting with steel columns


WeeGee-house, ESPOO

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Betoni 3/2006: www.betoni.com/Download/21971/BET0603_s22-24.pdf

WeeGee-house, ESPOO

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Strengthening of R.C. slabs

Adding new steel bars to RC slabusing section enlargement technique.(a) from bottom, (b) from top

Ferrocement strengtheningtechnique by adding wiredsteel mesh to R.C. slab

strengthening of reinforcedconcrete slab by removingthe concrete cover andapplying new bottom steelrods to flexural zones ofslab to enhance itspositive flexural capacity

applying new top steelrods to flexural zonesof cantilever slab toenhance its negativeflexural capacity


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Strengthening of R. C. slabs

Strengthening a slab by increasing its depthfrom top

Strengthening a slab by increasing itsdepth from bottom

http://theconstructor.org/structural-engg/strengthening-techniques-r-c-slab/

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Strengthening of R. C. slabs

• Strengthening the slab withstressing method

• Increasing punching shearcapacity of the slab– Bolting through the slab– Strengthening hollow-core

slabs

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Strengthening the R.C.walls

The dimensions of the wall and itsreinforcement are in creased by thefollowing steps:1. Roughing the total area of the concrete

surface.2. Installing steel connectors for the whole

surface at 25-30cm spaces in bothdirections.

3. Installing steel connectors into the wallfootings, with the same number anddiameter of the main vertical steel bars,using an epoxy material.

4. Installing the steel mesh and fasten it bysteel wires to the steel connectors.

5. Coating the surface of the wall with anappropriate epoxy material.

6. Pouring the concrete jacket using lowshrinkage concrete before drying of theepoxy material.

http://theconstructor.org/structural-engg/strengthening-of-r-c-walls/

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Strengthening of R. C. beams

• Reinforced concrete beams needstrengthening when the existingsteel reinforcement or crosssection in the beam is unsafe orinsufficient

or• when the loads applied to the

beam are increased due to thereasons mentioned before

Strengtheningprocedure of RCbeam by addingnew steel bars tothe bottom of thebeam

Strengtheningof RC beam byusing steelplates


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• Adding reinforcement steel bars tothe main steel without increasingthe beam’s cross sectional area

• Increasing both the reinforcingsteel bars and the cross –sectional area of concrete

• Tensile and/or shear capacity is tobe increased by gluing steelsheets / carbon fiber reinforcedlaminates into a lower and/orside surface of the beam

• Reinforced concrete beam is to bestrengthened by adding new steelbars

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ADDING STEEL PLATES TO THEBEAM1. Roughing and cleaning the

concrete surfaces where theplates will be attached.

2. Coating the concrete surfaceswith a bonding epoxy material.

3. Making holes in the concretesurfaces and plates.

4. Putting a layer of epoxy mortaron top of the plates with a 5mmthickness.

5. Attaching the steel plates to theconcrete using bolts http://theconstructor.org/structural-engg/strengthening-of-

reinforced-concrete-beams/

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http://theconstructor.org/structural-engg/strengthening-of-reinforced-concrete-beams/

Reducing the load on the beam using steel beam

• Concrete beam is to besubstituted with steel beam

• Steel profile is to be fixedinto a concrete beam

• Steel profile is to bewedged below the beam

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Share and FlexuralStrengthening ofBeam Using FRP

Post-tension byexternal tendons

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Strengthening of R. C. columns

• Strengthening of reinforcedconcrete columns:– Load increasing due to due to the

increasing in floors number– Load increasing due to design

mistakes– Compressive strength of concrete

is not according to the coderequirements

– Percent or type of reinforcement isnot according to the coderequirements

– Column inclination is more thanallowable

– foundation settlement is more thanallowable.

installation of newsteel cage around concrete

column during strengtheningprocess using RC jacketing

the concretecolumn after

strengtheningprocess

Beams supportingwith metallic supports during

column strengthening


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• Entirely or partly jacketing• Strengthening the column with the

steel form• Increasing cross sectional surface

with steel profiles• Substituting the concrete column

with the steel column

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ref: www.norcure.com

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Strengthening of R. C. columns (Video)

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Strengthening of masonry walls

http://theconstructor.org/building/strengthening-of-masonry-walls

Strengthening of Masonry Wallsby Application of Single and

Double sided reinforced concrete(RC) jackets

Strengthening of Masonry Walls using FRPStructural Repointing; (a) Grinding of masonryjoints, (b) Masking of masonry to avoid staining,(c) Application of epoxy based paste to masonryjoint, (d) Installation of GFRP Rods

Case study: Sectionenlargement using SCC

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Case study: Section enlargement using SCC

Moulding work

Condition after repairing

Repairing process of the supporting walls, parapet and slab of the balconyusing self-compacting concrete. (Batch 1, October 2004, laboratory condition)

Condition before casting

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Repairing process of the supporting column and slab of the balcony usingself-compacting concrete (Batch 2, November 2004, outdoors condition)

Moulding work

Condition after repairingCondition before casting

Case study: Section enlargement using SCC

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Case study: conclusions

• An objective with the work was to findthe minimum cover depth of the self-compacting concrete (SCC) suitable forthe repair method studied.

• Fresh and hardened concrete tests forthe self-compacting concrete usedshowed that the concrete fulfilled therequirements for balcony repairing.

• The results of slab frost resistancetests show the scaling and flexuralstrength change of self-compactingconcrete was very small after 300cycles and the changes in ultrasoundtransmission time were small, so thereis no sign of inner deterioration.

• Drying shrinkage of self-compactingconcrete was higher than thetheoretical shrinkage for the normalconcrete because self-compactingconcrete contained more fineaggregate. Using reinforcement meshreduces the shrinkage of self-compacting concrete

• The bond between the old concrete,which has a lower strength than therepairing self-compacting concrete, ispoor specially, when casting concrete inthe lower surface of concrete slab.

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Summary

Lecture summary– Introduction– Strengthening techniques– Strengthening of concrete

structures– Strengthening using FRP– Case study: Section

enlargement using SCC

Next Events:– 21.03.2016: First excursion to

the renovation site (Dipoli)– 04.04.2016: Midcourse seminar– 11.04.2016: Second excursion

to the renovation site(Parliament house)

Next Lecture:– 18.04.2016– Building information modelling

BIM

Strengthening of building structures - MyCourses · PDF fileExternal plate bonding ... proper...

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