Anchor attachments may be divided, depending on how they work, into undercut, spread and composite anchors although there are also anchor system that combine two of these methods, such as composite spread anchors.

Undercut anchors can be divided into self-undercutting anchors and those in which the undercut is made in a preceding step. The preceding undercut is made using a special undercut tool or a special drill, swinging the drill out in a circular motion. The anchor is then placed using a setting tool (e. g. Fischer FZA-K, Figure 7.6). With self­undercutting anchors the undercut in the concrete is made by hard metal cutting at

the anchors sleeve (e. g. Hilti HDA Figure 7.7). Undercut anchors transmit tension loads, even if there are wide cracks, such as in the event of an earthquake, as they are anchored in a form lock way within the concrete: so the undercut must be inspected very carefully. With the products available on the market today, the undercut can be checked via coloured markings which must be visible if the anchor is installed correctly.

Spread anchors can also be divided into path-controlled and force-controlled (or even torque-controlled) anchors. Path-controlled spread anchors are installed by being hammered in or driven in by machine and then checked by measuring the set depth. Anchors can be marked to ensure they are installed to the set depth required. Force — controlled spread anchors are inserted using a torque wrench, which applies the tensile or spread force required for the anchor to grip as it should (Figure 7.8).

Spread anchors transmit tensile loads via the grip between the spread anchor and the surrounding concrete.

Composite anchors consist of a composite mortar containing an embedded metal component. The composite material may be made of synthetic mortar, cement mortar or a mixture of the two. In practice, cartridge and injection systems are used. With cartridge systems, glass or synthetic capsules are fitted into the borehole. When the anchor is inserted, the cartridge is destroyed and the chambers it contains are mixed with the two components of the mortar.

Injection systems consist of the metal component to be inserted and a two-chamber injection cartridge. The composite material is mixed when it is expelled from the cartridge and injected into the borehole. The metal component is then inserted by hand or mechanically, depending on the anchor system involved.

Composite anchors work via the grip between the metal part and the composite mortar and the grip between the composite mortar and the borehole wall. The grip with the

concrete means that great attention must be paid to cleaning the borehole particularly thoroughly in accordance with installation instructions.

Composite anchors can also include combinations with undercut or spread anchors (Figure 7.9).

The loads in a composite spread anchor are transmitted via a combination of bonding and spreading, in which the spreading is also achieved through its particular shape. This enables it to bridge even broad cracks up to 1.5 mm and shows a ductile load-bearing behaviour.

The load-deformation diagram for a composite spread anchor shows the wavelike course of the load very well. This is caused by the individual spread cones penetrating into the composite mortar. With smaller crack widths, all spread cones in a anchor can be assigned to the individual waves in the load-deformation diagram.

With undercut composite anchors, the load is transmitted by a combination of the bonding of mortar to the borehole walls and the mechanical form locking of the mortar in the undercut of the concrete.