A formwork for providing a plinth with concrete frame and with exposed horizontal reinforcing bars, comprising a base the shape of which in plan view corresponds to said concrete frame, each side of said base being provided with inner and outer lateral containment walls which extend vertically from said base to make a casting chamber for said concrete, said inner walls comprising a plurality of openings for the insertion from above of a plurality of longitudinal reinforcing bars, at least one of said outer walls being provided with a plurality of openings for the insertion by transverse sliding of a plurality of transverse reinforcing bars, said openings being provided with inflatable elements which are configured to transition from a deflated configuration, in which said bars can freely slide in said openings, to an inflated configuration, in which said bars are locked in position and said chamber is watertight in order to prevent concrete from coming out during casting.