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Cracks in-buildings
This document discusses various types and causes of cracks in buildings. It classifies cracks as either structural or non-structural and further categorizes them based on their width. Common causes of cracks include moisture movement, thermal variation, excessive loading, and foundation settlement. Plastic shrinkage, bleeding, delayed curing, and use of poor quality materials can lead to cracks in concrete before it hardens. Thermal expansion and contraction from temperature changes is another major cause of cracks. Various remedial measures are proposed to prevent or reduce cracking in structures.
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Cracks in-buildings
- 1. CRACKS IN BUILDINGS: SOMEREMEDIAL MEASURES
- 2. IS THAT CRACKSERIOUS? Simplest questions to ask. One of the most difficult to answer. Prof. Mealcom Hollis-”Surveying buildings is an art, verifying the cause of failure is a science”
- 3. CLASSIFICATION OF CRACKS StructuralcrackStructural crack Non structural crackNon structural crack Incorrect designIncorrect design Faulty constructionFaulty construction OverloadingOverloading Internal induced stressInternal induced stress in building materialin building material
- 4. CONTINUE… Non StructuralCrack Penetration of moisture through crack Weathering action Result in corrosion of reinforcement Structure become unsafe (structural crack)
- 5. CLASSIFICATION OF CRACKS (BASEDONWIDTH) TypeType WidthWidth ThinThin MediumMedium WideWide < 1 mm< 1 mm 1-2 mm1-2 mm > 2 mm> 2 mm
- 6. COMMON SIGHT OFCRACK 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. INTERNAL STRESS INBUILDING COMPONENT Compressive Tensile Shear Building material 1. Masonry, Concrete, Mortar 2.Weak in tension/shear 3. Causing tension/shear crack
- 8. PRESENT TREND IN CONSTRUCTION Modern Structure - Tall, slender, thin wall - Designed for higher stress - Constructed at fast pace More crack prone
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- 13. CAUSE OF CRACKSIN BUILDINGS Moisture change Thermal variation Elastic deformation Creep Chemical reaction Foundation movement and settlement of soil Vegetation
- 14. MOISTURE MOVEMENT ReversibleMovement - Material expands on absorbing moisture content - Shrinks on drying Irreversible movement - Material undergo some irreversible movement due to initial moisture change
- 15. TYPES AND CAUSESOF CRACKS IN CONCRETE Before hardening Drying - Plastic shrinkage - Settlement shrinkage - Bleeding - Delayed curing
- 16. CONTINUE… Constructional - Formwork movement -Excess vibration - Subgrade settlement - Finishing Early frost damage
- 17. CONTINUE… After hardening Unsoundmaterial Long term drying shrinkage Thermal Moisture movement Biological Structural design deficiencies Chemical Corrosion of reinforcement
- 18. PLASTIC SHRINKAGE CRACKS Concrete surface loses water faster than the bleeding action brings it to top Quick drying of concrete at the surface results in shrinkage Concrete at the plastic state can not resist tension Crack Depth 5 to 10 cm, width 3 mm Once developed difficult to rectify
- 19. MEASURE TO REDUCEPLASTIC CRACK Moisture the sub grade and form work Erect the temporary wind breaker Erect the temporary roof to protect green concrete from hot sun Reduce the time between placing & finishing In case of delay in finishing cover the concrete with polythene
- 20. SETTLEMENT SHRINKAGE Ifconcrete is free to settle uniformly, no crack occurs Obstruction to uniform settlement creates voids/cracks (Reinforcement/Aggregate) Settlement crack (common in deep beam)
- 21. MEASURE Pouring ofconcrete in layers with proper compaction Revibration, if possible
- 22. BLEEDING Upward movementof water when concrete settle downs Internal bleeding - Bleeding water trapped below reinforcement & aggregate - Affects bonds between reinforcement and concrete - Interface, prone to micro cracking - Weak link in concrete - Further loading propagate cracking
- 23. CONTINUE… External Bleeding -Upward movement of water emerged at top surface - After evaporation : surface – Porous and abrasion resistance very little.
- 24. GENERAL OBSERVATIONS Masonfloats concrete when bleeding water still standing. Results - Downward movement of coarse aggregate - Upward movement of fine particles (cement and water) Top surface - Presence of fine materials develops crack and craziness (Craziness: occurrence of closely spaced crack at surface)
- 25. DELAYED CURING Commonpractice - Delayed curing - Interruption in continuous curing - Curing not done for required period Major cause for shrinkage Minimum 7 to 10 days curing required
- 26. CONSTRUCTIONAL EFFECTS Crack/Deformationof plastic concrete (After Compaction) -Lack of rigidity of formwork (Remains unnoticed) -Use of high consistency concrete (present trend) : Pumping requirement, Use of superplasticizer -Avoid segregation by proper vibration Segregated Concrete Mix- Exhibits high shrinkage crack
- 27. UNSOUND MATERIAL Useof crushed sand very common now Amount of dust (<75 micron): Very high Presence of excess dust : - Causes crack formation - Interfere with setting time, Shrinkage and Bond Maximum %age of fine = 3% (Coarse aggregate)
- 28. SOME MEASURES TOAVOID CRACKS
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- 40. THERMAL EXPANSION AND SHRINKAGE Assume characteristics compressive strength of concrete -25 MPa Modulus of elasticity – 5000x √fck N/mm2 - 5000 x √25 = 25 x 104 N/mm2 Flexural strength = 0.7 x √fck = 3.5 N/mm2 Coefficient of thermal expansion of concrete = 10 x 10-6 / c◦ Diurnal Variation in temperature = 20 c◦
- 41. CONTINUE… Thermal shrinkagestrain = 20 x 10 x 10-6 Modulus of elasticity = Stress/Strain 2.5 x 10 4 = Stress/200x10-6 Tensile Stress = 5.0 N/mm2 Tensile Strength of concrete = 3.5 N/mm2 Sure to cause crack
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- 49. CRACK DUE TOVEGETATION…
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