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Jointless Bay

Summary

A jointless bay is a large pour with no induced joints.

Technical Information

The aspect ratio of a jointless bay has to be less than 1:1.5, though ideally no greater than 1:1.2, to allow uniform shrinkage as it is one single large slab. Typically fibre-reinforced, although steel or a combined reinforcement can be used, jointless bays can be ground or pile supported.

The term “jointless” can be misleading as formed free-movement joints are installed between each pour.  The maximum size of a jointless bay should not exceed 35x35m. Bays larger than this will have increased shrinkage and cause unacceptably wide openings at the formed free-movement joints. Isolation joints are provided to keep the slab detached from restraining features; such as walls or columns.

The positioning of the joints should take into account the racking and pile head layout.

Shrinkage and restraint are minimised in the construction of a jointless bay by:

  • Ensuring excellent isolation around any immovable features, e.g. columns.
  • Providing a well-laid slip membrane beneath the slab.
  • Using a concrete mix less prone to shrinkage.
  • Delaying the loading of the slab until the concrete has undergone most of its early shrinkage.

For jointless pile supported floors, there will be:

  • Free-Movement Joints – to minimise the restraint from when adjacent bays undergo shrinkage.
  • Isolation Joints – to minimise restraint from fixed elements in the floor when the slab is undergoing shrinkage.
  • An additional layer of slip membrane placed over each pile head, extending past the edge by 300mm in each direction.

Resources

  1. The Concrete Society: Technical Report 34; 11.1.1 

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