Formed joints mark the boundaries between separate pours of concrete. Some reasons a constructor might introduce these joints are:
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.