A device is for tensioning a rod having a threaded portion and includes an actuator adapted to provide a tensioning axial force and a tie-rod adapted to transmit the axial force from the actuator to the threaded portion of the rod. The tie-rod includes a plurality of jaws, the jaws being movable radially between a closed position at which the jaws are engaged with the rod threaded portion and an open position at which the jaws are disengaged from the threaded portion, each jaw having an outer surface. A locking element includes a sleeve slidable axially along the tie-rod and disposable about the jaws, the sleeve having an inner frustoconical surface contactable with the outer surfaces of the jaws.