a substantially hollow cylindrical body extending along and around a longitudinal anchor axis (2.7; 30.7), which extends from an outer end arranged in the installation position to an inner end,

the outer end having a radially outwardly projecting, seating shoulder (2.1; 30.1), which has a through opening (2.3; 30.3) extending along the longitudinal axis of the anchor and having an internal thread, which is designed to be placed around a blind hole on an outer surface of a joining partner, and

wrench flats (2.2; 30.2) on an outer circumferential surface for rotationally fixed connection to a tool, in particular a setting tool (17), and

wherein a plurality of arm-shaped and spreadable spreading arms (2.4; 30.4) extending parallel offset to the longitudinal axis of the anchor are formed by the seating shoulder (2.1; 30.1),

wherein an undercut shoulder (2.5; 30.5) projecting radially outwardly is formed at the inner end of each of the spreader arms (2.4; 30.4),

in a joining partner, in particular a concrete or composite slab, the method comprising:

drilling a blind hole in the joining partner, which extends from a blind hole upper end at an outer surface of the joining partner to a blind hole lower end in the joining partner; and

forming an undercut in the joining partner at the bottom of the blind hole, and

wherein the undercut is formed only by rotating and spreading the undercut anchor (2; 30).