a method and apparatus is provided for anchoring threaded bars , such as bolts or similar elements , to hardened concrete , cement , or stone . an anchoring hole and a side hole are cut in the solid structure , and a sprag is then positioned within the side hole . the sprag is in the form of a sleeve and has a screw - threaded opening which extends through it and which is adapted to receive a threaded anchoring bar . the angle between the longitudinal axis of the sprag and the opening is equal to the angle between the anchoring bar hole and the side hole which are formed in the structure . the sprag is positioned within the side hole until its threaded opening is in line with the anchoring bar hole , so that the anchoring bar can be screwed into it .