~ Article courtesy of Hilti North America

When considering different fastening technologies, several factors affect the decision-making process to ensure the optimal solution for an application. Anchors are often used to connect structural elements of base material to masonry and concrete in buildings and structures.

Choosing the ideal anchoring technology depends on the application and the conditions. There are also certain features to consider. For example, mechanical anchors offer different advantages than adhesive or hybrid ones, which combine mechanical and bonding elements.

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

Friction
This is the mechanism used by most post-installed mechanical expansion anchors to resist loads. 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 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 offer much greater resilience to variations in the base material conditions and represent the most robust solution for most anchoring needs.

Bonding
Adhesive anchor systems use 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 is influenced by the condition of the hole wall at the time of the anchor installation. To some extent, adhesive anchors also use micro-keying — flowing into the imperfections of the wall of the hole. Injection anchor systems offer unparalleled flexibility and high bond resistance for various anchoring applications.

Dual-action: Combining keying and bonding
A new load-transfer method has been introduced to the industry of anchor fastening, which combines the working principles of a mechanical screw anchor with the working principles of an adhesive anchor.

Coined the “dual-action” anchor, the system works by driving a concrete screw anchor into a hammer/core drilled concrete hole, which has a capsule inside. This capsule is made of a polymer resin, hardener, and aggregate in a defined mix ratio.

When driving the concrete screw into the hole the adhesive capsule is shredded and compressed. The resin hardener and aggregate are mixed and fill the annular gap around the concrete screw as the screw thread cuts into the wall. Simultaneously, cracks around the anchor are filled with resin.

This load-transfer mechanism is based on mechanical interlock and friction. The adhesive from the capsule increases the area of the mechanical interlock of the threads as they cut into the surrounding borehole and fill the annual space around the screw.

Following the full cure of the adhesive, there’s a friction interlock between the screw and the cured adhesive.

Torque-controlled adhesive anchor: Bonding and expansion
Torque-controlled adhesive anchors combine the use of an adhesive anchoring system with a proprietary anchor element that typically has a cone-shaped helix portion that is embedded in the adhesive. Following the full curing of the adhesive, the anchor element must be torqued to set the anchor.

A special coating on the helix portion of the anchor allows the anchor to slip a fraction of an inch and exert expansion forces into the concrete, similar to an expansion anchor. With the inclusion of expansion forces and bonding forces nature of this anchor, higher tension loads can often be achieved.

This results in increased reliability under adverse job-site conditions, including core-drilled holes.

Shear resistance
Most anchors develop resistance to shear loading from the 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 six types of anchors of which five are post-installed and one is cast-in-place.

• Torque-controlled adhesive anchor is a post-installed anchor that’s inserted into a drilled hole of hardened concrete or fully grouted masonry. Loads are transferred to the base material by the bond between the adhesive and the base material, and the expansion forces, once a torque has been applied to the anchor after the adhesive has fully cured.
• Dual-action anchor is a post-installed anchor that’s inserted into a drilled hole in hardened concrete. Loads are transferred to the base material by keying and bonding.
• Screw anchor is a post-installed anchor that’s 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.
• Expansion anchor is a post-installed anchor that’s inserted into a drilled hole in hardened concrete or masonry. Loads are transferred to the base material by bearing, friction or both.
• Undercut anchor is a post-installed anchor that derives holding strength from the mechanical interlock provided by undercutting the concrete at/near the back of the hole, achieved by a special tool or the anchor itself during installation.
• Cast-in-place anchor is conventionally a headed bolt, headed stud, or hooked bolt installed before placing concrete. Cast-in-place internally threaded inserts are also a form of cast-in-place anchors.

Given the variety of anchor systems available on the market, understanding the different anchor types and working mechanisms can support designers in selecting an appropriate anchor for a specific application.

Also, knowledge of the boundary conditions of certain types of anchors such as load capacity, sensitivity to temperature, or installation parameters, can help designers decide on the ideal anchor for their job site.

 

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Filed Under: Fastener Engineering • Fasteners, Fastener adhesives, FASTENING • JOINING • locks • latches • pins
Tagged With: hilti
 

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