| There are three basic working principles by which an anchor develops its "holding" power in concrete: friction, keying, and bonding. | Friction: The tensile load, N, is transferred to the base material by friction, Ffr. An expansion force Fexp is necessary for this to take place. It is produced, for example, by driving an expansion plug into an HDI anchor. |  |  | N |
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| Keying: The tensile load, N, is in equilibrium with the bearing forces, Fb, acting on the base material, such as with the HUC undercut anchor. | | Fb | | |  | | N | | Fb | | |
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Bonding: A synthetic resin fills the annular space around the anchor and provides adhesive bonding to the anchor rod and the wall of the drilled hole. Transfer of the tensile load, N, takes place through shear stresses,  ,into the concrete. |  | | N |
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Combination of Working Principles
Anchors may derive their holding power through a combination of these working principles. In an expansion anchor, for example, an expansion force is exerted by an anchor against the wall of the hole as a result of the displacement of a cone relative to a sleeve. This causes the longitudinal force to be transmitted from the anchor to the concrete by friction. At the same time, the expansion force causes a permanent local deformation of the concrete. This allows a keying of the sleeve into the base material, giving a second method of holding power.
For adhesive anchors, there is, in addition to the bonding, a local keying as the adhesive infiltrates into any pores of the base material. |
