Cracks in Concrete Structure RRS
- 1. Repair& Rehabilitation of Structures RamanuJ Jaldhari Asst. Prof. KITE, Jaipur
- 2. CRACKS IN CONCRETE Why does concrete crack? •Most cracks occur as a result of shrinkage of concrete. •Shrinkage is simply a reduction in the volume of concrete as it hardens. •If this reduction in volume were unrestricted, then a crack would not occur. However, in reality, ground friction and a number of things such as structural connections inhibit free shrinkage and thus cause cracks.
- 3. CRACKS IN CONCRETE Distress Can Be Broadly Classified As : •Structural Causes STRUCTURAL CRACKS •Non-Structural Causes NON-STRUCTURAL CRACKS Structural distresses are generally caused by faulty design, faulty construction, and/or overloading. Non-structural distresses are caused by internally induced stresses in building components.
- 4. Non-structural Cracks Not every crack threatens the structural safety of a building. In fact, in many instances, cracks are merely cosmetic in nature. These cracks are typically seen in flat work such as driveways, patio, walkways and curbs. Typical causes of these cracks are •Poor workmanship •Inappropriate joint detailing •Higher shrinkage of concrete
- 5. Structural Cracks A majority of structural cracks occur as a result of the following conditions: •Design deficiency •Construction deficiency •Settlement or heaving of soil •Reinforcement corrosion Sometimes structural cracks manifest themselves with some side effects. Doors and windows do not open and close easily. Floors feel uneven.
- 6. Cracks Classification of the cracks is done on the basis of their widths. Thin - less than 1 mm in width Medium - 1 to 2 mm in width Wide - more than 2 mm in width
- 7. Cracks According to IS:456-2000, crack width is as follows: In general the crack width should not exceed 0.3 mm in members, where cracking is not harmful and does not have any serious effect upon the preservation of reinforcing steel nor upon the durability of concrete. Cracking in tensile zones of the member is harmful due to the exposure to the effect of weather or continuously exposed to moisture or in contact with soil or ground water. In such situations the maximum upper limit of the crack width is suggested as 0.2 mm. For aggressive environment such as severe category of exposed conditions, the surface width of cracks should not in general exceed 0.1 mm.
- 8. Factors Contributing To Cracks in Concrete WATER CEMENT AGGREGATE BLEEDING IMPROPER CURING EXPOSURE COVER
- 9. Types of Cracks in Concrete SULFHATE ATTACK CRACK Sulfate attack can be 'external' or 'internal'. External: due to penetration of sulfates in solution into the concrete from outside. Internal: due to a soluble source incorporated into concrete at the time of mixing.
- 10. Types of Cracks in Concrete External Sulfate Attack This is the more common type and typically occurs where water containing dissolved sulfate penetrates the concrete. A fairly well-defined reaction front can often be seen in polished sections; ahead of the front the concrete is normal, or near normal. Behind the reaction front, the composition and microstructure of the concrete will have changed. These changes may vary in type or severity but commonly include: Extensive cracking Expansion Loss of bond between the cement paste and aggregate Alteration of paste composition
- 11. Types of Cracks in Concrete Internal sulfate Attack Internal sulfate attack occurs when a source of sulfate is incorporated into the concrete when mixed. The use of sulfate-rich aggregate, Excess of added gypsum in the cement or contamination. Proper screening and testing procedures should generally avoid internal sulfate attack
- 12. Types of Cracks in Concrete Sources of Sulfate Which Can Cause Sulfate Attack Include: Seawater Oxidation of sulfide minerals in clay Bacterial action in sewers - anaerobic bacterial produce sulfur dioxide Masonry - sulfates in bricks can be gradually released over a long period
- 13. Types of Cracks in Concrete LOADING CRACK •A loading crack is a result of the loading to which the structure is subjected. •A properly designed structure would not exhibit these cracks, but an improperly designed structure is very susceptible to this damage. •Vertical cracking at the end of a structure is typically due to a concentrated force being applied at the top of a structure. •This type crack typically maintains a tight appearance at the top and at the bottom but may show a wider gap at approximately mid-height of the structure.
- 15. Types of Cracks in Concrete PLASTIC SHRINKAGE CRACKS •When water evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. •Due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth. •These cracks are often fairly wide at the surface.
- 16. Types of Cracks in Concrete DRYING SHRINKAGE CRACKS •Because almost all concrete is mixed with more water than is needed to hydrate the cement, much of the remaining water evaporates, causing the concrete to shrink. •Restraint to shrinkage, provided by the sub grade, reinforcement, or another part of the structure, causes tensile stresses to develop in the hardened concrete. •Restraint to drying shrinkage is the most common cause of concrete cracking.
- 17. Types of Cracks in Concrete ALKALI AGGREGATE REACTION CRACK •Alkali-aggregate reactivity is a type of concrete deterioration that occurs when the active mineral constituents of some aggregates react with the alkali hydroxides in the concrete. •Alkali-aggregate reactivity occurs in two forms alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).
- 18. Types of Cracks in Concrete THERMAL CRACK Temperature rise (especially significant in mass concrete) results from the heat of hydration of cementitious materials. As the interior concrete increases in temperature and expands, the surface concrete may be cooling and contracting. This causes tensile stresses that may result in thermal cracks at the surface if the temperature differential between the surface and center is too great.
- 19. Types of Cracks in Concrete SETTLEMENT CRACK Loss of support beneath concrete structures, usually caused by settling or washout of soils and sub base materials, can cause a variety of problems in concrete structures, from cracking and performance problems to structural failure. Loss of support can also occur during construction due to inadequate formwork support or premature removal of forms.
- 20. Types of Cracks in Concrete CORROSION CRACK Steel reinforcement, in the alkaline environment provided by concrete, is in a stable condition because a protective oxide layer forms on the steel surface, which stops corrosion. Basic cracks are formed by two reactions in corrosion. •Carbonation •Chloride attack
- 21. Types of Cracks in Concrete PREVENTION OF CRACKING: Optimum water ratio Curing method Solid ground Proper usage of material Control joints Reinforcement steel Cover for reinforcement Coating on reinforcement Coating on concrete Corrosion inhibitor Cathodic protection
- 22. Types of Cracks in Concrete MEASURES ADOPTED TO RECTIFY THE CRACKING •Low-water cement ratio to reduce shrinkage and well compacted to prevent settlement due to shrinkage •The aggregates used should be well graded to give high quality concrete. •In order to avoid formation of hair cracks, dry cement should not be allowed to be sprinkled over the wet surface to absorb bleeding water, but finishing operation should be delayed till the evaporation of water is complete. •The curing of concrete should be allowed for a sufficient time •In order to avoid absorption of water from the concrete by the surface over which concrete is to be placed, the surface should be made sufficiently moist. This will avoid drying shrinkage. •Formwork used should be sufficiently strong.
- 23. Any Query???