Mechanical vs. Adhesive Anchors

Anchors designed for use in concrete and masonry develop resistance to loading on the basis of one or more of the following mechanisms:

Friction: This is the mechanism used by most post-installed mechanical expansion anchors to resist loads, including the KWIK Bolt TZ, HSL-3 and HDI anchors. The frictional resistance resulting from expansion forces generated between the anchor and the wall of the drilled hole may also be supplemented by local deformation of the concrete. The frictional force is proportional to the magnitude of the expansion stresses generated by the anchor. Torque-controlled expansion anchors like the KWIK Bolt TZ and HSL-3 anchors use follow-up expansion to increase the expansion force in response to increases in loading beyond the installation torque-induced preload or to adjust for changes in the state of the base material (cracking).

Keying: Undercut anchors, screw anchors, and, to a lesser degree, certain types of expansion anchors, rely on the interlock of the anchor with deformations in the hole wall to resist the applied loading. The (bearing) stresses developed in the base material at the interface with the anchor bearing surfaces can reach relatively high levels with minimized crushing due to the triaxial nature of the state of stress. Undercut anchors like the Hilti HDA offer much greater resilience to variations in the base material conditions and represent the most robust solution for most anchoring needs.

Bonding (adhesion): Adhesive anchor systems utilize the bonding mechanism that takes place between the adhesive and the anchor element, and the adhesive and the concrete, to transfer the applied load from the anchor element into the concrete. The degree of bonding available is influenced by the condition of the hole wall at the time of anchor installation. To some extent, adhesive anchors also utilize micro-keying – flowing into the imperfections of the wall of the hole. Injection anchor systems like Hilti’s HIT-HY 200 offer unparalleled flexibility and high bond resistance for a wide variety of anchoring applications.

Hybrid anchor elements for adhesive applications, like the Hilti HIT-Z threaded rod, combine the versatility of an adhesive anchor system with the working principle of a torque-controlled expansion anchor by providing follow-up expansion capability. This results in increased reliability under adverse job-site conditions.

Shear resistance: Most anchors develop resistance to shear loading via bearing of the anchor element against the hole wall. Shear loading also induces secondary tension in the anchor element. Because of variability in the installation torque-induced tensile preload, most shear designs rely on bearing between the attachment and the anchor, rather than friction between the attachment and the base material.

There are five common types of anchors, four that are post-installed and one that is cast-in-place.

Adhesive anchor is a post-installed anchor that is inserted into a drilled hole in hardened concrete, masonry or stone. Loads are transferred to the base material by the bond between the anchor and the adhesive and the adhesive and the base material.

Cast-in-place anchor is traditionally a headed bolt, headed stud or hooked bolt installed before placing concrete. Additionally, cast-in-place internally threaded inserts are a form of cast-in-place anchors.

Expansion anchor is a post-installed anchor that is inserted into a drilled hole in hardened concrete or masonry. Loads are transferred to the base material by bearing, friction or both.

Screw anchor is a post-installed anchor that is inserted into a drilled hole, typically smaller in diameter than the anchor, in hardened concrete or masonry. Loads are transferred to the base material by keying.

Undercut anchor is a post-installed anchor that derives holding strength by the mechanical interlock provided by undercutting the concrete at/near the back of the hole, achieved either by a special tool or by the anchor itself during installation.



To assist you in designing with these various anchoring solutions, Hilti offers best-in-class design software, PROFIS Engineering. Hilti PROFIS Engineering is comprehensive design software for anchor design in concrete, masonry, and concrete-over-metal-deck base materials and includes a versatile load engine and various options for base plate analysis. PROFIS Engineering includes the Anchoring to Concrete provisions of the ACI 318 Building Code and the ACI 349 Nuclear Code and the Anchorage provisions of CSA A23.3 Annex D.

Included within PROFIS Engineering software are a number of powerful features, exclusive to Hilti, that greatly enhance the user's choice and efficiency. Users can design with Hilti mechanical and adhesive anchor systems as well as cast-in-place headed studs and headed bolts. Tutorials explain how to navigate within PROFIS Engineering. The PROFIS Engineering Design Guide is an innovative, interactive tool that explains ACI 318 Chapter 17 strength design calculations and PROFIS Engineering design assumptions.

Looking for a someone to lay it all out? Check out our Anchor Selector Chart for a thorough overview of chemical anchors, steel elements, and mechanical anchors.