The reinforcing bars are surrounded by a layer of a plastic mass which is formed by adding an additive to the plasticizing water used in the mass surrounding the reinforcing bars. The additive used is e.g. a retrding agent or an accelerator, and is used exclusively in the lower part of the lement where the reinforcing bars are situated. The additive may be e.g. a hydroxide, phosphate, silicate, carbonate, Mg(OH)2, Zn(OH)2, an amine, a mercaptan, a slat of uric acid, a sulphide or an aldehyde. The prods. have good resistance to fire and corrosion. Prestressed concrete parts made using this process are less prone to stress corrosion, esp. if the stone and/or water used contains chlorides.