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Concrete Undercut Anchors

There are many post-installed mechanical anchors types, but concrete undercut anchors are regarded as the strongest and can resist higher loads than mechanical expansion anchors. As a result, concrete anchors are always used in structures like nuclear power plants, thermal power stations whereby the heat source is typically a nuclear reactor, meaning that human safety is very important since failure can result in catastrophic consequences.

Generally, the suitability concrete to resist action load transfer from the application is normally decisive. “Concrete capacity” – The base material where a fixing has been fixed in to determines how strong it will be. However, the way the mechanical anchor will transfer the action load also has a great impact on the capacity of the concrete to resist the action or load.

Design of Anchors in Concrete

As their name suggests, mechanical expansion anchors work by expanding into a hole that has been drilled into concrete. The friction that the expansion creates provides the resistance to that tensile action.

Generally, the higher the tensile action, the more the horizontal forces required. High expansion forces often create high stress/compression zones in the concrete. This detrimentally affects how the concrete is capable of resisting the action loads.

Concrete expansion anchors work differently; they create a positive key in the self-cut or pre-cut void inside the concrete. Expansion anchors concrete either create their own void then key into it, as seen in Liebig Ultraplus BLS anchors or open out into the pre-cut void as seen in the Liebig Ultraplus. They don’t rely on friction and expansion and thus they don’t create such high stress or compressive zones in the concrete. Therefore, concrete floor anchor can make better use of concretes capacity, hence they can resist much greater tensile loads.

On top of the usual tensile action loads, the compressive stress zones reduction in the concrete and the positive mechanical interlock make heavy duty anchors for concrete more capable of managing the action loads they are created in shock, seismic, dynamic, plus other high impact application conditions like that.


Installation generally differs depending on the undercut anchor type used. When installing self-undercut anchors, such as Liebig SUPERPLUS BLS, you don’t need special tools. The anchor will create the undercut. When BLS anchors are expanded into pre-drilled holes, keys at the base of the expansion will create voids at a depth inside the concrete that’s predetermined. The keys will lock into the voids and end up providing resistance to tensile action.

When it comes to the Liebig Ultraplus installation, a special tool called split drive concrete anchor is used to create the void at a depth inside the concrete that’s predetermined. The spring action of Ultraplus anchors after installation forces keys to start expanding into pre-cut voids. When using this anchor, that’s how the tensile action resistance is created.

Split drive anchors are often used to anchor into solid concrete base materials. The working end of this tool will be inserted through the fixture, then into a hole in the concrete that has been pre-drilled. As the anchor is being driven into the hole, the halves that have been pre-expanded at the bottom of that anchor will push against the concrete continuously and strive to regain their initial shape. It’s this force that is being continually exerted against the concrete walls that provides the excellent holding values for this tool.

features of a m6 m16 anchor bolt

Uses of Undercut Anchors

Cracked concrete anchors perform just as good as non-cracked ones. This is because of the positive keying action that’s created during installation. Positive keying enhances their capability of maintaining their resistance as the crack in the concrete is created or start expanding. Mechanical expansion anchors, on the other hand, need significant follow-up expansion ability. This is why undercut anchors are commonly used where seismic qualification is taken into consideration or on structures that are located in areas that are prone to earthquakes.

Other Undercut Anchors Benefits

As aforementioned, the way an undercut anchor transfers tensile action into concrete and enhances the concretes capacity to start resisting the tensile actions, allows a significant increase in terms of load capabilities. This system of operation brings additional benefits as well. Lower stress/compressive forces in the concrete typically means that you can use undercut anchors closer to the concrete edges and you can install them closer to anchors of the same type. This reduces materials by allowing smaller baseplates to be used, hence overall installation costs for applications will be significantly reduced.