Abstract: An anchoring unit is made up of a screen-type sleeve and an insert sleeve to be placed into the screen-type sleeve. The unit is anchored by a hardenable mass. After the screen-type sleeve is inserted into a borehole in a structure with internal open spaces, the hardenable mass is injected into the screen-type sleeve. When the insert sleeve is placed into the screen-type sleeve, the hardenable mass is forced outwardy through openings in the screen-type sleeve into the open spaces in the structure. The two sleeves each have an inside surface and an outside surface with alternating projecting surfaces and recessed surfaces to facilitate a positive lock with the hardenable mass. Further, the alternating surfaces improve the distribution of the hardenable mass. Preferably, the two sleeves are formed of a plastics material. A fastening element can be inserted into the insert sleeve for securing an object to the structure.