Types of joints and their uses

Why joints are necessary

Buildings are exposed to temperature changes, moisture, shrinkage, settlement and earthquake movement. Joints give room for these movements so that the structure does not crack. Joints also make it possible to cast concrete in stages.

Types of joints and their uses

1. Expansion joint

An expansion joint is a gap of 20-25 mm provided through the full depth of the structure above the foundation to allow expansion and contraction due to temperature, so that cracks do not form. It is used at intervals of 30-45 m in buildings, and in long walls, slabs, roofs and parapets.

   Expansion joint in a wall/slab
    slab |  25 mm gap  | slab
    =====|             |=====
    wall |  filler     | wall
         |  (bitumen)  |
         | sealant on top
   foundation continuous below

The gap is filled with a compressible filler (bitumen-impregnated fibre board, cork, rubber) and sealed on top with a sealant. The two parts are fully separated, often by a double column or wall.

2. Construction joint

A construction joint is made where concrete placing is stopped and resumed after the earlier concrete has set, because the structure cannot be cast in a single operation. It must be located where shear and bending are small, and it must give good bond between old and new concrete.

Positions (IS 456 / IS 11817):

• Slabs: in the middle third of the span, at right angles to the main reinforcement.
• Beams: in the middle third of the span (where secondary beams frame into girders, the joint in the girder is offset by at least twice the width of the secondary beam).
• Columns: at the top of the footing, and about 75 mm below the lowest beam/slab soffit (or below the haunch/capital).
• Walls: at openings, at junctions, and in the middle third.

  Slab/beam                 Column
  ___________|___________    |slab|
  |<-L/3->|joint|<-L/3->|    | beam
                             | col  ---- joint just below beam
                            ===footing=== joint at top of footing

3. Contraction (control) joint

A groove cut or formed in the concrete at fixed spacing (e.g. 3-6 m in pavements and floors) so that shrinkage cracks form along the joint line.

4. Sliding joint

Allows horizontal movement between a slab and the wall supporting it, using a bitumen layer or plain sheets.

5. Settlement joint

Provided between parts with different foundations or heights so that differential settlement does not crack the building.

6. Isolation (separation) joint

Provided between a slab and a column or wall, so that each can move independently.

7. Seismic (separation) joint

A wider gap (as per IS 4326 / NBC 105) between adjacent buildings or blocks to prevent pounding in an earthquake.