A stay-in-place form comprises a plurality of elongated panels connectable to one another in edge-to-edge relationship. The plurality of panels comprises first and second panels connectable to one another in edge-adjacent relationship by a connection which comprises a contact joint. The first panel comprise a longitudinally extending first seal-retaining projection shaped to project outwardly from the outer surface of the first panel at a location spaced apart from a first outer-surface transverse edge of the first panel in a first transverse direction. The second panel comprising a longitudinally extending second seal-retaining projection shaped to project outwardly from the outer surface of the second panel at a location spaced apart from a second outer-surface transverse edge of the second panel in a second transverse direction opposite the first transverse direction. The first and second seal-retaining projections and the outer surfaces of the first and second panels defining at least a portion of a seal-receiving concavity which opens outwardly from the form when the connection is made.

A grid beam slab formwork comprising of primary beams, secondary beams, multi-level drophead unit, and props. The multi-level drophead unit offers up to or more than six connection points for the primary beams and the secondary beams. The multi-level drophead unit allows the construction of an additional structure that otherwise requires a separate drophead or an adaptor thereby reducing an overall cost associated with the formwork. Moreover, the multi-level drophead unit enables the creation of the platform wherever needed. On the other hand, the adapter prevents entrapment of the sheathing members, such that the beams can be removed when the concrete has achieved required strength and the load can be taken by the props.