Cracks can occur due to chemical reactions in construction materials, changes in temperature and climate, foundation movements and settling of buildings, environmental stresses like nearby trains, earth quakes etc. Faulty design, bad quality materials, wrong method of construction, weather effects and lots of wear and tear can create cracks in walls, floors and ceilings. Here are given various reasons of cracks and their prevention techniques.
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When the walls are unevenly loaded, due to variation in stresses in different parts of wall the cracks are formed in walls. When two materials having wide different elastic properties are built together under the effect of load, different shear stresses in these materials create cracks at the junction. Dead and live loads cause elastic deformation in structural components of a building.
PreventionCreate slip joints under the support of RCC slab on walls. Masonry work on RCC slabs and beams should not be started before drying RCC slab and beam. Provide horizontal movement joints between the top of brick panel and RCC beam/slab.
All materials expand on heat and contract on cool. Thermal movement in components of structure creates cracks due to tensile of shear stresses. It is one of the most potent causes of cracking in buildings and needs attention.
PreventionConstruct joints such as construction joints, expansion joints, control joints and slip joints. The joints should be planned at the time of design and be constructed carefully.
Chemical reactions in building materials increase their volume and internal stress causes cracks. The components of structure also weaken due to chemical reactions. Some common instances of chemical reactions are following.
- Sulphate attack on cement products
- Carbonation in cement based materials
- Corrosion of reinforcement in concrete
- Alkali aggregate reaction
PreventionUse dense and good quality concrete i.e. richer mix of cement concrete 1:1.5:3 to prevent cracks. Repair corrosive cement concrete surface by ‘guniting’/ injecting technique after removing all loose and damaged concrete and cleaning reinforcement from all rust also.
Most building materials expend when they absorb moisture from atmosphere and shrink when they are dry. Cement made materials shrink due to drying up of the moisture used in their construction. The factors causing shrinkage in cement concrete and cement mortar and their preventions are following.
Excessive WaterThe quantity of water used in the mortar mix can cause shrinkage. Vibrated concrete has less quantity of water and lesser shrinkage than manually compacted concrete.
PreventionUse minimum quantity of water required for mixing cement concrete or cements mortar according to water cement ratio. Never allow cement concrete work without mechanical mix and vibrator.
Quantity of CementAs a general rule, the richer the mix is, the greater the shrinkage/drying will be.
PreventionDo not use excessive cement in the mortar mix.
Un-graded AggregateAggregate can cause shrinkage also. If un-graded and fine material/aggregate is used in cement concrete and cement mortar which requires more water and can cause greater shrinkage.
PreventionUse largest possible aggregate and ensure good grading of materials. The use of water according to required workability has less shrinkage because of reduction in the porosity of hardened concrete.
CuringAfter laying cement concrete mix, the hardening of cement takes place, causes reduction in moisture and creates shrinkage. This causes cracks in concrete work.
PreventionProper curing should be started as soon as initial setting has taken place and be continued for at least seven to ten days. When hardening of concrete takes place under moist environment, the shrinkage due to drying is comparatively less.
Excessive Fine materialsFine materials take more surface area and require more water for mix. The use of excessive fine materials i.e. silts; clay and dust in aggregate create more shrinkage.
PreventionDo not use fine materials containing silt, clay and dust. Use coarse sand/fine aggregate in cement concrete and cement mortar mix which has silt and clay less than 4%. Use coarse aggregate and fine aggregate after washing to reduce silt contents.
Foundation Movement and Settlement of Soil
Shear cracks occur in buildings when there is large differential settlement of foundation due to any of following causes.
- Unequal bearing pressure under different parts of the structure
- Bearing pressure being in excess of safe bearing strength of the soil
- Low factor of safety in the design of foundations
- Local variation in the nature of supporting soil
The design of foundation must be based on sound engineering principles and good practice.
Crack may occur due to sudden shift in lower layer of the earth. The voids in the earth might have suddenly collapsed and be filled with soil from the above. Many geological events can trigger earth movements but is continuous movement. This results in cracks.
Construct the foundation of buildings on firm ground while doing construction. Tie up the building with connecting beams at foundation level, door level and roof level.
The roots of trees located in the vicinity of a wall can create cracks in walls due to growth of roots under foundation. The cracks occur in clay soil due to moisture contained by roots.
Do not let trees grow too close to the buildings, compound walls etc. Remove any saplings of trees as soon as possible if they start growing in or near of walls etc.