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
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
- 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.