A formwork for providing a concrete frame with horizontal reinforcing bars (101, 102) at least partially inserted in the concrete frame, wherein said horizontal reinforcing bars (101, 102) have ends that protrude from or are embedded in the concrete frame, comprising: - a bottom base the shape of which in plan view corresponds to said concrete frame, said base having a plurality of sides; - at least one containment wall which stands up vertically from said bottom base to make a casting chamber for said concrete, said at least one containment wall comprising a plurality of first openings, distanced from said bottom base, for the insertion from above of a plurality of longitudinal reinforcing bars, said first openings of the containment wall being provided with inflatable elements which are configured to transition from a deflated configuration, in which said bars can freely slide in said first 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, characterized in that said formwork is suitable for providing a plinth with said concrete frame delimiting at least one inner space in which said horizontal reinforcing bars (101, 102) are exposed and suspended thereon and said ends protrude from the concrete frame, and wherein: - each side of said base is provided with inner and outer lateral containment walls (12, 13), said casting chamber being defined between said inner and outer lateral containment walls (12, 13) and said at least one space being defined by said inner lateral containment walls; - each wall of said inner lateral containment walls comprising said plurality of first openings for allowing insertion from above of a plurality of longitudinal reinforcing bars; - at least one wall of said outer lateral containment walls being provided with a plurality of second openings, distanced from said bottom base, for the insertion by transverse sliding of a plurality of transverse reinforcing bars; - said first openings and said second openings being provided with said inflatable elements configured to transition from said deflated configuration, in which said bars can freely slide in said first and second openings (14, 15) and in which said plinth can be extracted from the formwork from above, to said inflated configuration, in which said bars are locked in position and said casting chamber is watertight. - a bottom base the shape of which in plan view corresponds to said concrete frame, said base having a plurality of sides; - at least one containment wall which stands up vertically from said bottom base to make a casting chamber for said concrete, said at least one containment wall comprising a plurality of first openings, distanced from said bottom base, for the insertion from above of a plurality of longitudinal reinforcing bars, said first openings of the containment wall being provided with inflatable elements which are configured to transition from a deflated configuration, in which said bars can freely slide in said first 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, - each side of said base is provided with inner and outer lateral containment walls (12, 13), said casting chamber being defined between said inner and outer lateral containment walls (12, 13) and said at least one space being defined by said inner lateral containment walls; - each wall of said inner lateral containment walls comprising said plurality of first openings for allowing insertion from above of a plurality of longitudinal reinforcing bars; - at least one wall of said outer lateral containment walls being provided with a plurality of second openings, distanced from said bottom base, for the insertion by transverse sliding of a plurality of transverse reinforcing bars; - said first openings and said second openings being provided with said inflatable elements configured to transition from said deflated configuration, in which said bars can freely slide in said first and second openings (14, 15) and in which said plinth can be extracted from the formwork from above, to said inflated configuration, in which said bars are locked in position and said casting chamber is watertight.