Formed joints mark the boundaries between separate pours
of concrete. Some reasons a constructor might introduce these joints are:
- They can only place a limited amount of
concrete in one session.
- They are limited by the design constraints of
using certain types of slab i.e. in a jointless bay slab.
- They are using the long strip method of
construction to tighten the flatness tolerance of their slab – construction
joints are provided between the strips.
Formed joints can act as free-movement joints,
restrained-movement joints or tied joints, depending on the required capacity
for movement in the local area of the slab; specialist racking systems may need
a lower tolerance for movement than a jointless bay, for example.
Because construction joints are always situated on the
edge of a pour, the arris on both sides of the joint may have been less well
worked by the power trowel. This means that the concrete at the arris is likely
to consist of less aggregate and excess cement paste, making the arrises less
resistant to damage caused by traffic across the joint. Therefore, it is
advisable to armour construction joints if they are going to be trafficked significantly,
especially if they are also free-movement joints.
If a construction
joint is unarmoured, extra care is needed when removing the temporary formwork
to ensure no damage is inflicted on the arris.