Strand tensioning system

Claim 1

THE COMBINATION OF AN ELONGATED CASTING FORM FOR CASTING CONCRETE SLABS, STEEL REINFORCING STRANDS DISPOSED IN SAID FORM FOR LONGITUDINAL PRESTRESSING OF SAID SLABS AND MEANS FOR TENSIONING SAID STRANDS, SAID MEANS INCLUDING AT LEAST ONE HYDRAULIC JACK, SUCH JACK HAVING A DOUBLE ACTING PISTON AND PISTON ROD WITH HYDRAULIC FLUID CONNECTIONS TO EACH END OF SAID PISTON FOR SECURING SUCH DOUBLE ACTION, SAID PISTON HAVING A PISTON ROD EXTENDING FROM ONE SIDE THEREOF, WHICH PISTON ROD REDUCES THE EFFECTIVE AREA OF SAID PISTON AS COMPARED TO THE FULL AREA OF THE OTHER SIDE OF SAID PISTON, A PLURALITY OF SOURCES OF HYDRAULIC FLUID UNDER SEPARATE, DIFFERENT, PREDETERMINED HIGH PRESSURES, A SINK CONSTITUTING A SOURCE OF FLUID AT ATMOSPHERIC PRESSURE, MEANS INCLUDING FLUID LINES FROM SAID FLUID SOURCES FOR SELECTIVELY CONNECTING ANY TWO OF SUCH FLUID SOURCES ON OPPOSITE SIDES OF SAID PISTON, THE DIFFERENCES IN PRESSURES BEING SUFFICIENTLY GREAT SO THAT THE PISTON ROD SIDE OF SAID PISTON IS SUBJECT TO A TOTAL FORCE WHICH EXCEEDS THE FORCE ON THE OTHER SIDE OF SAID PISTON