Construction

Centering and Shuttering / Form Work


Shuttering or form work is the term used for temporary timber, plywood, metal or other material used to provide support to wet concrete mix till it gets strength for self support. It provides supports to horizontal, vertical and inclined surfaces or also provides support to cast concrete according to required shape and size. The form work also produces desired finish concrete surface.   

Shuttering or form work should be strong enough to support the weight of wet concrete mix and the pressure for placing and compacting concrete inside or on the top of form work/shuttering. It should be rigid to prevent any deflection in surface after laying cement concrete and be also sufficient tight to prevent loss of water and mortar form cement concrete. Shuttering should be  easy in handling, erection at site and easy to remove when cement concrete is sufficient hard.  

Steel plates for Steel Shuttering

Generally there are three types of shuttering. 

  • Steel Shuttering
  • Wooden Planks Shuttering
  • Temporary Brick Masonry Shuttering

Steel Shuttering
Steel shuttering plate is the best type of shuttering because this is water tight shuttering which can bear the load of cement concrete placed on it. This shuttering can be used for horizontal, vertical or any other shape required for the work. It gives leveled surface which has good appearance. This shuttering gives good appearance and pattern work according to architectural drawings.  If the plaster is required, the thickness of plaster will be less. Being water tight shuttering, the strength of concrete with steel shuttering is comparatively higher.   

Shuttering with the help of Steel Plates.

  

Note: As this is water tight shuttering, it is considered the best shuttering.

Wooden Plank Shuttering
Generally wooden planks shuttering is used by contractors because this shuttering is cheap and easily available. But this type of shuttering effects the strength of concrete and have some disadvantages which are given below.

Disadvantages

  • This is not water tight shuttering as the size and thickness of planks differ and are not of same size. Due to this difference the water and cement flow to the ground from joints and reduce the strength of concrete.
  • Bottom level of RCC slab is not in straight line and the surface being uneven, the thickness of plaster is more which remains weak.
  • Due to leakage of cement slurry through joints, earth work below ’'Ballies’ may settle and create problems.
  • In some cases wooden planks can not bear the weight of concrete. Due to low strength there is bending or deflection in wooden planks. Sometimes the planks may break.

Shuttering done with the help of  wooden ballies and batten.


Temporary Brick Masonry Shuttering
In some cases labor contractor uses Temporary Brick Masonry in mud for vertical support of sides of beams, fascia etc. This shuttering should be avoided. This type of shuttering reduces the strength of cement concrete by soaking cement slurry. Also no proper compaction is made as this shuttering does not bear the pressure of vibrator. The surface of cement concrete given by this type of shuttering is uneven and the thickness of plaster is increased.


Precautions for Vertical Supports for RCC Beams, Slabs etc

  • The ’'Ballies’ used for vertical support should not be less than 6'' dia and these should be in one length without joints.
  • Never allow bricks support of more than one or two bricks below a 'balli' to make required height.
  • Cross Ballies or bracing should be done for better support to beam as well as slab.
  • The wooden batten used below the plate should not be less than 5'' in height.
  • At the time of concreting one carpenter with helper having spare ballies, nails etc. should be deputed for watching any disturbances in ballies under shuttering.

Precautions for Cantilever slabs and Beams

  • While doing shuttering of a cantilever part, outer edge of shuttering is 1” to 2'' higher than inner edge with the wall.
  • The bracing of vertical supports for cantilever portion should be tied to vertical supports of internal slab.        
  • The concrete should be laid on cantilever portion very gently.
  • The shuttering should be removed after 28 days.

Recommended Period for Removal of Shuttering

  • 48 hours for sides of foundations, columns, beams and walls. 
  • 7 days for underside of slab up to 4.5 meter span
  • 14 days for underside of slab, beams, arches above 4.5 meter up to 6 meter span.  
  • 21 days for underside of beams arches above 6 meter span and up to 9 meter span.
  • 28 days for underside of beams arches above 9 meter span.  

 Defects Found In Shuttering/Form Work

  •  The supports of form work are not in plumb and are not cross braced. 
  • The ground supports of ballies are poor and therefore settle the form work. 
  • There is insufficient thickness of shuttering plates/planks unable to bear lateral pressure imposed by wet concrete especially in columns.
  • Shuttering plates are not cleaned and oiled or oiled with dirty oil.
  •  There are many insufficient and loose connections in centering and shuttering.
  • The form work is removed before time.  The work is not planned and designed properly.
  • In case of beam shuttering proper provision for retaining side is not made. Hence the side of beam is not in proper line.
  •  The shuttering is poorly made with cracked and warped timber planks having lots of holes and knots.
  •  Through bolts for RCC walls form work for an underground tank is used. Later these holes made by bolts are not plugged.
  • ‘Ballies’ are resting on bricks or brick pillars
  •  Ballies are not in one piece. Small ballies are used and these are not properly jointed. Also no additional cross bracing is provided at the joint.
  • The supports under shuttering plates are not properly tight.
  • The earth work under supports is not properly compacted before starting shuttering work.
  • The bottom of ‘ballies’ are in wedge shape, not having proper base.
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