Cracks in-buildings-3568

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Cracks in-buildings-3568

  1. 1. CRACKS IN BUILDINGS:CRACKS IN BUILDINGS: Some Remedial MeasuresSome Remedial Measures Dr. J. N.JhaDr. J. N.Jha Professor and Dean (Consultancy)Professor and Dean (Consultancy) Department of Civil EngineeringDepartment of Civil Engineering Guru Nanak Dev Engineering College,Guru Nanak Dev Engineering College, LudhianaLudhiana
  2. 2. IS THAT CRACK SERIOUS?IS THAT CRACK SERIOUS? • Simplest questions to ask.Simplest questions to ask. • One of the most difficult to answer.One of the most difficult to answer. • Prof. Mealcom Hollis-”Surveying buildings is anProf. Mealcom Hollis-”Surveying buildings is an art, verifying the cause of failure is a science”art, verifying the cause of failure is a science”
  3. 3. Classification of CracksClassification of Cracks Structural crackStructural crack Non structural crackNon structural crack Incorrect designIncorrect design Faulty constructionFaulty construction OverloadingOverloading Internal induced stress inInternal induced stress in building materialbuilding material
  4. 4. Continue…Continue… • Non Structural CrackNon Structural Crack Penetration of moisture through crackPenetration of moisture through crack Weathering actionWeathering action Result in corrosion of reinforcementResult in corrosion of reinforcement Structure become unsafe (structural crack)Structure become unsafe (structural crack)
  5. 5. Classification of cracksClassification of cracks (Based on width)(Based on width) TypeType WidthWidth ThinThin MediumMedium WideWide < 1 mm< 1 mm 1-2 mm1-2 mm > 2 mm> 2 mm
  6. 6. Common sight of crackCommon sight of crack VerticalVertical HorizontalHorizontal DiagonalDiagonal StraightStraight ToothedToothed SteppedStepped Map patternMap pattern RandomRandom UniformUniform throughoutthroughout Narrow at oneNarrow at one end and graduallyend and gradually widening at thewidening at the otherother
  7. 7. Internal stress in BuildingInternal stress in Building componentcomponent • CompressiveCompressive • TensileTensile • ShearShear • Building materialBuilding material • 1. Masonry, Concrete, Mortar1. Masonry, Concrete, Mortar 2. Weak in tension/shear2. Weak in tension/shear 3. Causing3. Causing tension/shear cracktension/shear crack
  8. 8. Present trend in constructionPresent trend in construction • Modern StructureModern Structure - Tall, slender, thin wallTall, slender, thin wall - Designed for higher stressDesigned for higher stress - Constructed at fast paceConstructed at fast pace • More crack proneMore crack prone
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  13. 13. Cause of cracks in buildingsCause of cracks in buildings • Moisture changeMoisture change • Thermal variationThermal variation • Elastic deformationElastic deformation • CreepCreep • Chemical reactionChemical reaction • Foundation movement and settlement of soilFoundation movement and settlement of soil • VegetationVegetation
  14. 14. Moisture MovementMoisture Movement • Reversible MovementReversible Movement - Material expands on absorbing moistureMaterial expands on absorbing moisture contentcontent - Shrinks on dryingShrinks on drying • Irreversible movementIrreversible movement -- Material undergo some irreversible movementMaterial undergo some irreversible movement due to initial moisture changedue to initial moisture change
  15. 15. Types and causes of cracks inTypes and causes of cracks in ConcreteConcrete • Before hardeningBefore hardening DryingDrying - Plastic shrinkagePlastic shrinkage - Settlement shrinkageSettlement shrinkage - BleedingBleeding - Delayed curingDelayed curing
  16. 16. Continue…Continue… ConstructionalConstructional - Formwork movementFormwork movement - Excess vibrationExcess vibration - Subgrade settlementSubgrade settlement - FinishingFinishing Early frost damageEarly frost damage
  17. 17. Continue…Continue… • After hardeningAfter hardening  Unsound materialUnsound material  Long term drying shrinkageLong term drying shrinkage  ThermalThermal  Moisture movementMoisture movement  BiologicalBiological  Structural design deficienciesStructural design deficiencies  ChemicalChemical  Corrosion of reinforcementCorrosion of reinforcement
  18. 18. Plastic shrinkage cracksPlastic shrinkage cracks • Concrete surface loses water faster than theConcrete surface loses water faster than the bleeding action brings it to topbleeding action brings it to top • Quick drying of concrete at the surface results inQuick drying of concrete at the surface results in shrinkageshrinkage • Concrete at the plastic state can not resistConcrete at the plastic state can not resist tensiontension • Crack Depth 5 to 10 cm, width 3 mmCrack Depth 5 to 10 cm, width 3 mm • Once developed difficult to rectifyOnce developed difficult to rectify
  19. 19. Measure to reduce plastic crackMeasure to reduce plastic crack • Moisture the sub grade and form workMoisture the sub grade and form work • Erect the temporary wind breakerErect the temporary wind breaker • Erect the temporary roof to protect greenErect the temporary roof to protect green concrete from hot sunconcrete from hot sun • Reduce the time between placing & finishingReduce the time between placing & finishing • In case of delay in finishing cover the concreteIn case of delay in finishing cover the concrete with polythenewith polythene
  20. 20. Settlement shrinkageSettlement shrinkage • If concrete is free to settle uniformly, no crackIf concrete is free to settle uniformly, no crack occursoccurs • Obstruction to uniform settlement createsObstruction to uniform settlement creates voids/cracks (Reinforcement/Aggregate)voids/cracks (Reinforcement/Aggregate) • Settlement crack (common in deep beam)Settlement crack (common in deep beam)
  21. 21. MeasureMeasure • Pouring of concrete in layers with properPouring of concrete in layers with proper compactioncompaction • Revibration, if possibleRevibration, if possible
  22. 22. BleedingBleeding • Upward movement of water when concrete settleUpward movement of water when concrete settle downsdowns • Internal bleedingInternal bleeding - Bleeding water trapped below reinforcement &Bleeding water trapped below reinforcement & aggregateaggregate - Affects bonds between reinforcement and concreteAffects bonds between reinforcement and concrete - Interface, prone to micro crackingInterface, prone to micro cracking - Weak link in concreteWeak link in concrete - Further loading propagate crackingFurther loading propagate cracking
  23. 23. Continue…Continue… • External BleedingExternal Bleeding - Upward movement of water emerged at topUpward movement of water emerged at top surfacesurface - After evaporation : surface – Porous andAfter evaporation : surface – Porous and abrasion resistance very little.abrasion resistance very little.
  24. 24. General ObservationsGeneral Observations • Mason floats concrete when bleeding water stillMason floats concrete when bleeding water still standing.standing. • ResultsResults -- Downward movement of coarse aggregateDownward movement of coarse aggregate - Upward movement of fine particles (cement and water)Upward movement of fine particles (cement and water) • Top surfaceTop surface - Presence of fine materials develops crack and crazinessPresence of fine materials develops crack and craziness (Craziness: occurrence of closely spaced crack at surface)(Craziness: occurrence of closely spaced crack at surface)
  25. 25. Delayed CuringDelayed Curing • Common practiceCommon practice - Delayed curingDelayed curing - Interruption in continuous curingInterruption in continuous curing - Curing not done for required periodCuring not done for required period • Major cause for shrinkageMajor cause for shrinkage • Minimum 7 to 10 days curing requiredMinimum 7 to 10 days curing required
  26. 26. Constructional EffectsConstructional Effects • Crack/Deformation of plastic concrete (AfterCrack/Deformation of plastic concrete (After Compaction)Compaction) -Lack of rigidity of formwork (Remains unnoticed)-Lack of rigidity of formwork (Remains unnoticed) -Use of high consistency concrete (present trend)-Use of high consistency concrete (present trend) : Pumping requirement, Use of superplasticizer: Pumping requirement, Use of superplasticizer -Avoid segregation by proper vibration-Avoid segregation by proper vibration • Segregated Concrete Mix- Exhibits highSegregated Concrete Mix- Exhibits high shrinkage crackshrinkage crack
  27. 27. Unsound MaterialUnsound Material • Use of crushed sand very common nowUse of crushed sand very common now • Amount of dust (<75 micron): Very highAmount of dust (<75 micron): Very high • Presence of excess dust :Presence of excess dust : - Causes crack formation- Causes crack formation - Interfere with setting time, Shrinkage and- Interfere with setting time, Shrinkage and BondBond • Maximum %age of fine = 3% (Coarse aggregate)Maximum %age of fine = 3% (Coarse aggregate)
  28. 28. Some measures to avoid cracksSome measures to avoid cracks
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  40. 40. Thermal expansion and shrinkageThermal expansion and shrinkage • Assume characteristics compressive strength ofAssume characteristics compressive strength of concrete -25 MPaconcrete -25 MPa • Modulus of elasticity – 5000x √fck N/mmModulus of elasticity – 5000x √fck N/mm22 - 5000 x √25 = 25 x 10- 5000 x √25 = 25 x 1044 N/mmN/mm22 • Flexural strength = 0.7 x √fck = 3.5 N/mmFlexural strength = 0.7 x √fck = 3.5 N/mm22 • Coefficient of thermal expansion of concrete =Coefficient of thermal expansion of concrete = 10 x 10-10 x 10-66 /◦c/◦c • Diurnal Variation in temperature = 20 ◦cDiurnal Variation in temperature = 20 ◦c
  41. 41. Continue…Continue… • Thermal shrinkage strain = 20 x 10 x 10Thermal shrinkage strain = 20 x 10 x 10-6-6 • Modulus of elasticity = Stress/StrainModulus of elasticity = Stress/Strain • 2.5 x 102.5 x 10 44 = Stress/200x10= Stress/200x10-6-6 • Tensile Stress = 5.0 N/mmTensile Stress = 5.0 N/mm22 • Tensile Strength of concrete = 3.5 N/mmTensile Strength of concrete = 3.5 N/mm22 • Sure to cause crackSure to cause crack
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  47. 47. Foundation settlement…Foundation settlement…
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  49. 49. Crack due to vegetation…Crack due to vegetation…
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  53. 53. Thanks…..Thanks…..

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