Construction joints in structure

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 of construction joints

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

Methods of joining new concrete to old concrete

1. Cleaning: remove all loose material and laitance from the old concrete, and roughen the surface by hacking, wire brushing or sand blasting to expose the aggregate.
2. Wetting: saturate the old surface with water for several hours, and remove the free water before concreting.
3. Bonding coat: apply a layer of neat cement slurry or 10-15 mm of rich cement mortar (1:2) just before placing the new concrete.
4. Bonding agent: apply an epoxy or latex (SBR) bonding agent for better adhesion.
5. Shear keys or dowels: form a groove (key) in the old concrete or fix dowel bars across the joint for load transfer.
6. Placing new concrete: place it in layers, compact it thoroughly with a vibrator, and cure it well.
7. Waterstops (PVC or rubber strips) in water-retaining structures.