When considering different fastening technologies, several factors affect the decision making process to ensure the optimal solution for an application. Here, Hilti Group outlines how to choose the ideal anchoring technology by taking into account the application, conditions, as well as other factors.
Anchors are often used to connect structural elements to masonry and concrete in buildings and structures, however choosing the ideal anchoring technology depends on a number of things. For example, adhesive anchors offer large amounts of application versatility by utilising a wide range of diameters and embedment depths. Alternatively, mechanical anchors offer immediate loading capabilities that may be needed for an attachment.
Overall there are seven types of anchors, six that are post-installed and one that is cast-in-place. An 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, as well as the adhesive and the base material.
Torque controlled adhesive anchors are post installed anchors that are 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 expansion forces after a torque has been applied to the anchor once the adhesive has fully cured.
In addition, dual action anchors are post installed anchors that are inserted into a drilled hole in hardened concrete. Loads are transferred to the base material by keying and bonding.
As well as these, there is the screw anchor – 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. There is also the expansion anchor – 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. In addition there is the under cut anchor – 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.
Finally there is also the cast-in-place anchor, which is traditionally a headed bolt, headed stud or hooked bolt installed before placing concrete.
Given the variety of anchor systems available on the market today, understanding the different anchor types and working mechanisms can support designers in selecting an appropriate anchor for their specific application. Understanding the boundary conditions of certain types of anchors such as load capacity, sensitivity to temperature, or installation parameters, to name a few, can help designers decide on the appropriate anchor for their application.
Regardless of the technology, each application offers unique demands. Anchors designed for use in concrete and masonry develop resistance to loading based on a number of mechanisms including friction, keying, bonding (adhesion), dual action (keying and bonding), torque controlled adhesive anchor, as well as shear resistance.
Post installed mechanical expansion anchors, where friction is the mechanism to resist loads, is one of the most commonly used in the construction industry.
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).
Another mechanism, keying, is when 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 minimised 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.
Adhesive anchor systems utilise the bonding mechanism that takes place between the adhesive and the anchor element, as well as the adhesive and the concrete/masonry, 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 utilise micro-keying – flowing into the imperfections of the wall of the hole. Injection anchor systems offer unparalleled flexibility and high bond resistance for a wide variety of anchoring applications.
A new load transfer method has also 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 drilled concrete hole that has a capsule placed inside it. The capsule is made up 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 to the bottom of the borehole. 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.
The 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 as well as filling the annual space around the screw. Following the full cure of the adhesive there is friction interlock between the screw and the cured adhesive.
In addition, torque controlled adhesive anchors are used to 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.
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.
To assist in designing with these various anchoring solutions, Hilti offers its software, PROFIS Engineering Suite. Hilti PROFIS Engineering is a comprehensive design software for anchor design into concrete, masonry and concrete-over-metal-deck base materials and includes post installed rebar design into concrete-to-concrete connections as well as metal deck diaphragm design.
The software includes a versatile load engine, various options for base plate analysis, integration with third party software, as well as customised design templates. PROFIS Engineering also includes the Anchoring to Concrete provisions of the ACI 318 Building Code Requirements for Structural Concrete, as well as the anchorage provisions of CSA A23.3 Annex D.
Having spent a decade in the fastener industry experiencing every facet – from steel mills, fastener manufacturers, wholesalers, distributors, as well as machinery builders and plating + coating companies, Claire has developed an in-depth knowledge of all things fasteners.
Alongside visiting numerous companies, exhibitions and conferences around the world, Claire has also interviewed high profile figures – focusing on key topics impacting the sector and making sure readers stay up to date with the latest developments within the industry.
