Abstract: A steel spiral reinforcing rod which is a high-strength bar having a yield strength of over 55kg/mm.sup.2 and with the spiral grooves in the surface thereof, which after being wound into a spiral having the desired dimension can be axially compressed for delivery to the construction site, and when released from the fully-compressed state, the coil will expand to the desired due to its own elasticity and will not suffer plastic deformation even if stretched by an external force. For easy workability, the steel bar is heated in a heating furnace or an induction-heating coil to a specified temperature and then wound in the heated state on a winding machine of specified configuration into spiral form with the coils at the desired pitch. When the bar made of a material which is relatively easy to work, it is desirable that helical winding be done by cold working, followed by heating to a specified temperature in a furnace to remove the stress developed in the bar when it is worked.

Abstract: A steel rod for prestressing concrete which is enclosed in concrete to introduce prestress in said concrete (hereinafter called a P. C. steel rod). The P. C. steel rod has a circular section around which a plurality of spiral grooves have been formed by means of a die. One or more flat surfaces can be formed in the axial direction or the rod for greater adhesion to the concrete. The P. C. steel rod has a far shorter bond length than a conventional P. C. Steel rod and therefore it has greater adhesion to the concrete and when tensioned, the concrete does not develop longitudinal cracks, thus assuring reliable introduction of prestress in the concrete.