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Expansion and construction joints, positions

Short answer

Joints are planned breaks for movement or staged concreting. Expansion joints are 20-25 mm gaps filled with compressible filler and sealant, placed every 30-45 m in long walls, slabs, and roofs. Construction joints are placed where concreting stops, in low-shear zones: slabs and beams in the middle third of the span, columns at the top of footing and 75 mm below the beam soffit.

Describe the joints in the structure. Illustrate the types of joints. Explain briefly the expansion and construction joints and their probable positions in the structure.

A joint is a planned break or gap in a structure, provided to allow movement or to stop concreting. It reduces random cracking.

Types of joints

  1. Expansion joint: allows thermal expansion.
  2. Contraction (control) joint: controls shrinkage cracks.
  3. Construction joint: at a stopping place of concreting.
  4. Settlement joint: between parts that settle differently.
  5. Sliding joint: between slab and wall, for horizontal movement.
  6. Seismic joint: between blocks, to avoid pounding.

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.

Positions: at 30-45 m intervals along long buildings; where the plan changes shape (L, T, U); where the height or foundation level changes; between wings, and between old and new portions; in long parapets, roofs, retaining walls and pavements.

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

✓ Answer checked by Claude Opus 5.5 (high effort)

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