4.10/ P/ 1

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Cracks and Repairs R.C.C. Structures
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Cracks in building

• Structural cracks
• due to incorrect design
• faulty construction
• Overloading
• Non Structural cracks
• Moisture changes
• Thermal movement
• Elastic deformation
• Creep
• Chemical reaction
• Foundation movement and settlement of soil
• Vegetation

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Non Structural cracksMoisture Changes

• Building materials expand on absorbing moisture
  and shrink on drying. These are generally
  reversible. Shrinkage in concrete or mortar
  depends on a number of factors
• Cement concrete Richer the mix greater is the
  drying shrinkage.
• Water content More water in mix induces greater
  shrinkage
• Aggregates Large aggregates with good grading
  has less shrinkage for same workability as less
  water is used

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• Curing If proper curing starts as soon as
  initial set has taken place and continued for 7
  to 10 days shrinkage is comparatively less
• Excessive fines More fines in aggregate requires
  more water for same workability and hence more
  shrinkage.
• Temperature Concrete made in hot weather needs
  more water for same workability see Fig 8 and
  hence results in more shrinkage.
• Temperature Concrete made in hot weather needs
  more water for same workability hence results in
  more shrinkage.

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Initial Expansion

• An example of cracks of wall due to initial
  expansion of bricks is given in fig

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Some Measures for controlling shrinkage

• Shrinkage in plastering can be reduced by
  ensuring proper adhesion. The plastered should
  not be stronger than the back ground.
• Shrinkage cracks in masonry can be minimized by
  avoiding use of rich cement mortar and by
  delaying plastering till masonry has dried after
  proper curing and has undergone most of its
  initial shrinkage.

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Thermal Movement

• . The cracking of a typical structure due to
  thermal movement is given in fig

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In case of framed buildings due to thermal
movement frames are distorted and cracks may
appear as shown in fig
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Prevention of Thermal Cracks

• To prevent thermal cracks expansion joints,
  control joints and joints in case of change of
  shape and direction of wing in a structure are to
  be provided

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General guide lines to provide movement joints
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Elastic Deformation
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Creep

• Building items such as concrete and brick work
  when subjected to a sustained load not only
  undergo elastic strain but also develop gradual
  and slow time dependent deformation known as
  creep or plastic strain. The creep in brick work
  may stop after 4 months but the same in concrete
  continue upto a year or so. The creep in concrete
  may be 2 to 3 times of the elastic deformation
  and hence has to be fully care fully considered.

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  General measures for avoidance reduction of
cracks due to elastic strain, creep and shrinkage

• Water cement ratio is to be controlled.
• Reasonable pace of construction adopted.
• Brick work over load bearing RCC members should
  be done after removal of shutting giving a time
  gap.
• Brick walls between columns should be deferred as
  much as possible.
• Plastering of areas having RCC and brick members
  should be done after sufficient time gap say one
  month or suitable groves provided in junction.
• Shutting should be allowed stay for a larger
  period say 30 days or so for cantilevers which
  are bound to defect appreciably.

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Movement due to chemical 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
  aluminate 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

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Prevention

• If sulphate content in soil is more that 0.2 or
  in ground water more than 300 ppm use rich mix of
  concrete ant mortar has to be adopted.
• Avoid bricks containing too much soluble
  sulphates (more than 5 ) and use rich mortar in
  such cases.
• Use expansion and control joint at closure
  intervals 

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Corrosion of Reinforcement

• Corroded reinforcement expands and cracks the
  concrete cover. To avoid this phenomenon rich mix
  of concrete using proper quality of water and
  adequate cover should adopted.

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Foundation movement and settlement of soil

• Building 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 Fig

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Cracking due to vegetation

• Large trees growing in the vicinity of buildings
  cause damage in all type of soil conditions. If
  the soil is shrinkable clay cracking is severe

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Repair

• The repair for cracks may be undertaken after
  ascertaining the reasons for the appearance of
  the crack. A few basic principles if followed
  will be more effective
• 1.  Rendering of minor crack less that 1m wide
  may be done after observing the crack for some
  time and then sealing it with weak mortar of
  cement, lime and sand. 
• 2.   Cracks where width change with season should
  be filled up with elastic fillers like silicon or
  polyurethene compound.
• 3.   Where sheer crack are observed shear keys
  made of RCC concrete with at least 1.5 percent
  steel vein forcemeat may be provided at 1 to 1.5m
  intervals.
• 4.  If cracks are due to movement of soil in
  black cotton once, prevention of moisture
  penetration in the surrounding areas has to be
  ensured by providing a waterproof blanket around
  the plinth. The masonry wall below ground level
  should also be separated from the adjoining soil
  by replacing the existing soil with coarse grain
  material.

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Structural Cracks

• Structural cracks mainly occur due to
• a)      Defective design and defective load
  assumptions and perception of behavior of the
  structure.
• b)      In correct assessment of bearing capacity
  of foundation soil and soil properly.
• c)      Defective detailing of joints of
  components like roof with brick wall corner
  joints of walls
• d)      Defective detailing of structural
  detailing of steel reinforcement.
• e)      Lack of quality control during
  construction.