CAUSES OF CRACKS IN CONCRETE.


The principle causes of cracks are discussed below. 

1. Temperature and plastic shrinkage
2. Thermal variation
3. Drying shrinkage
4. Weathering conditions
5. Poor construction practices
6. Construction overloads

1. Temperature and plastic shrinkage :
Plastic shrinkage cracking occurs when subjected to a very rapid loss of moisture caused by high temperature, relative humidity and wind velocity at the surface of the concrete. These factors can combined to cause high rates of surface evaporation in either hot or cold weather.
When moisture evaporates from the surface of the freshly placed concrete faster then it is replaced by bleed water. the surface concrete shrinks. Due to the restrain provide by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete. Resulting in shallow cracks of varing depth which may form a random. Polygonal pattern or may appear essentially parallel to each other. These cracks are often fairly wide at surface. Plastic shrinkage cracks are begin as shallow cracks but can become a full depth cracks.

Remedies to prevent plastic shrinkage cracks :

  1. Shading the surface of concrete.
  2. Cooling the concrete in warm weather.
  3. Using plastic sheeting after finishing operation.
  4. Use of fog nozzles to saturate the air above the surface of concrete.
  5. Proving adequate temperative reinforcement.
  6. Mortar should not be richer than what is necessary.
  7. Work done in winter is less liable to shrinkage.
  8. Brink should not be overburnt.
  9. Providing 8 to 10 mm wide groove in external plaster finish at 0.5 to 1.0 meter in both the direction.

2. Thermal variations :
It ie well known phenomenon of science that all materials expand on heating and contract on cooling. Magnitude of movement however varies from different materials depending on their molecular structure and other properties.
Temperature differences within a concrete structure may be caused by portion of the structure losing heat of hydration at different rates or by the weather conditions cooling or heating portion of the structure to a different degree or at at a different rate than another portion of structure. These temperature differences result in differential volume changes. These cracks are occur in mass concrete in case of large columns, piers, footings and dams.

3. Drying shrinkage :
Drying shrinkage caused by the loss of moisture from the cement paste constituent, Which can shrink by as such as 1 percent. Fortunately, aggregates provide internal restraint that reduces the magnitude of volume change to about 0.06 percent. On wetting, concrete tend to expand.
These moisture induced volume change are a characteristics of concrete. If the shrinkage concrete could take place without restraint, the concrete would not crack. It is the combination shrinkage and restraints that causes tensile stresses to develop. When tensile strength of concrete if exceeds. It will crack.

4. Chemical reaction :
Concrete may crack with time as the result of slowly developing expansive reaction when the aggregate contains active silica and alkalies. The alkali-aggregate reaction results in the formation of a swelling gel, Which tends remove water from other concrete ingredient. This causes local expansion and accompanying stresses, and mass eventually result in the complete deterioration of the structure. 

Remedies to prevent chemical reaction :
  1. Proper selection of aggregate.
  2. Use of low alkali cement.
  3. Use of pozzolanas.
5. Wheathering condition :
Weathering processes that can causes cracking include freezing and thawing, wetting , drying, cooling and heating.
Damage from from freezing and thawing is the most common weather related physical aeterioration concrete may be damaged by freezing of water in the paste, in the aggregate or in both. 
Damage in hardened cement paste from freezing is caused by the movement of water to freezing sites and by hydraulic pressure generate by the growth of ice crystals.
Concrete is best protected against freezing and thawing through the use of the lowest practical water cement ratio and total water content, durable aggregate and adequate air entertainment. allowing the structure to dry after curing will enhance its freezing and thawing durability.

6. Poor construction practices:
A wide variety of poor construction practices can result in cracking in concrete structures foremost among these are:
  • Common practice of adding water to concrete to improve  workability 
  • Lack of curing.
  • Inadequate formwork supports.
  • Inadequate compaction.
  • placement of construction joints at points of high stress.

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