23-03-2011, 10:40 AM
presented by:
SHABIR MALIK
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INTRODUCTION
Cracks result in when applied forces are greater than those which the building or its part can withstand. These forces may have emerged externally to the building or internally within the building or have been developed in the materials of the building as a result of Chemical changes. There may be a single force or a combination of forces having a single cause or several causes.
This is a residential building at San Francisco. Major cracks have developed on the external facade of the building. The cracks running across the entire facade of the building are diagonally directed and diagonal cracks are generally structural cracks. They occur due to structural problems in the buildings. Structural cracks could be due to major or minor problem in the structural design. Intensity of structural problem is determined on the basis of the intensity of cracks developed.
LOOPHOLES OF CRACKS
1. BUILDING BECOME UNSAFE
2. IT SPOIL THE APPERANCE OF BUILDING
3. DAMPNESS IN THE BUILDING
4. REDUCE IN MARKET VALUE
5. INCREASE IN MAINTAINENCE COST
Most cracks occur due to drying of construction water.
• These are generally superficial
• Do not affect the serviceability of the building
• Can be easily repaired
If cracks are more significant, repair work may be repeated periodically.
Structural significance of the cracks tends to be exaggerated by a natural reaction of the owner.
Many cracks do not have any effect on the stability of structure, because of “high factor of safety”.
Cracking has adverse effects on appearance. Sometimes several cracks go unnoticed for years which affect the performance of the structure.
Rain penetration through a crack depends on:
1. Construction of a building
2. Degree of exposure
3. Capillarity of fine cracks
4. Wind driven rain penetration through wide cracks
CAUSES OF CRACKS
Structural cracks
Due to incorrect design
Faulty construction
Overloading
Non Structural cracks
Moisture changes
Thermal movement
Elastic deformation
Chemical reaction
Foundation movement
Settlement of soil
Vegetation
CHANGE IN MOISTURE
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 are following.
Excessive Water
The 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.
Quantity of Cement
As a general rule, the richer the mix is, the greater the shrinkage/drying will be.
Un-graded Aggregate
Aggregate 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.
Curing
After laying cement concrete mix, the hardening of cement takes place, causes reduction in moisture and creates shrinkage. This causes cracks in concrete work.
Excessive Fine materials
Fine 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.
Temperature:
Concrete made in hot weather needs more water for same workability hence results in more shrinkage
Thermal Movement
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
Elastic Deformation
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.
Chemical Reaction
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
Certain chemical reaction in building materials result is appreciable change in volume of resulting products and internal stresses are set up which may result in outward thrust and formation of cracks.
Soluble sulphate reacts with tricalcuium aluminates in cement and hydraulic lime and form products which occupy larger volume and ends in developing cracks. An example of cracking of a floor due to coming in contact of the sub base made of brick khoa with heavy sulphate content and water can be seen in fig
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
Buildings on expansion clays are extremely crack prone. The soil movement in such clay is more appreciable upto a depth of 1.5 to 2M and this cause swelling and shrinkage and results in crack in the structure. The cracks due to settlement are usually diagonal in shape. Crack appearing due to swelling is vertical
