How to select the right post-installed bonded anchor for the required application

Post-installed anchors are widely used in the construction industry for fixing steel structures to the existing concrete members. They are classified depending on their working mechanism; mechanical (mechanical interlock/keying, friction) and bonded anchors (adhesive bond). In this article, we focus on bonded anchors, namely injection mortars, capsule anchors. Mechanical anchors will be covered in another article.

# Fig. 1.1: Bonded anchor working mechanism

2. Working mechanism and classification of bonded anchors

These anchors utilise the property of the adhesive to form a bond between a chemical adhesive-concrete interface (mechanical interlocking via bond) and a chemical adhesive-anchor interface (combination of micro-interlock and chemical adhesion), thereby developing the load-carrying capacity (Fig. 1.1 and Fig. 2.1). The adhesives may be organic (e.g., epoxy, polyester, vinyl ester) or inorganic (i.e., cement based). Adhesives usually have resin and a hardener component. They can be delivered in injectable cartridge/foil pack systems or in glass/foil capsule systems. When the two components are mixed, the adhesive hardens and achieves its bond properties.

The adhesive is placed in a drilled, cleaned hole and the fastening element (e.g., threaded rod, sleeve with internally threaded rod etc.) is then inserted. These systems can be loaded only after the adhesive has cured and hardened. The curing time may differ from product to product and environmental conditions (mainly temperature), and it is specified by the manufacturer. Post-installed bonded anchors offer high flexibility in design and can be tailored to a wide range of diameters and embedment depths.

Fig. 2.1: Sample stress field generated by bonded capsule anchor in concrete under tension loading

Based on adhesive properties, injection mortars for bonded anchors are primarily either epoxy based which is known as “Slow cure” due to longer curing time or hybrid type (blend of epoxy and vinyl ester) known as “Fast cure” due to its shorter curing time.

Capsule anchors come with a foil filled with chemical adhesives and used with steel anchor rods.

3. Epoxy based bonded anchor and Hilti portfolio

An epoxy-based bonded anchor uses a two-part epoxy resin as the bonding agent to secure steel elements (like threaded rods) into drilled holes in hardened concrete. For main epoxy based bonded anchor (HIT-RE 500 V4), the curing time which is required to fully harden and reach its design strength is about 7 hours at 20°C temperature and the base material temperature is in the range of -5°C to 40°C. The working time known as installation time which is the period after mixing during which the adhesive remains workable, and the anchor rod must be inserted into the hole is around 30 mins at 20°C temperature. The main Hilti epoxy based bonded anchors with features and advantages are presented in Table 3.1.

Table 3.1: Hilti epoxy based bonded anchor portfolio

4. Hybrid mortars bonded anchor and Hilti portfolio

A hybrid mortar for bonded anchors combines the properties of epoxy and vinyl ester resins to deliver a balance of strength, speed, and versatility. Epoxy helps with high bond strength and durability whereas vinyl ester ensures fast curing and good chemical resistance. For popular hybrid bonded anchor, HIT-HY 200 A/R V3, curing time is only 60 mins and working time is 9 mins at 20°C temperature. It can be installed within a base material temperature range of 10°C to 40°C.The main Hilti hybrid mortar bonded anchors with features and advantages are presented in Table 4.1.

Table 4.1: Hilti hybrid mortar bonded anchor portfolio

5. Capsule bonded anchor and Hilti portfolio

The capsule bonded anchor consists of a capsule filled with adhesive and a steel anchor rod. The capsule is pre-filled with mix of resin and hardener and inserted into a drilled hole in concrete. The anchor rod is driven into the hole, and it breaks the capsule and activates the adhesive by mixing the resin and hardener part. The mixed adhesive fills the gap between anchor rod and concrete. The capsule anchor (HVU2) can be installed instantly and working time is negligible. Curing time at 20°C temperature is around 5 mins which is also less in comparison to other available bonded anchors.

The main Hilti capsule bonded anchors with features and advantages are presented in Table 5.1.

Table 5.1: Hilti capsule bonded anchor portfolio

6. Bonded expansion anchor- working principle and Hilti portfolio

Bonded expansion anchor is a combination of torque-controlled expansion anchor and bonded anchors and work on the combined principle as micro-keying (bonding) of the injected mortar and expansion forces due to the helix design of the rod. The fastening element used is a special threaded rod with multiple steel cones in the bottom portion of the rod (see Fig. 6.1). It has a coating that allows the bond to break at the concrete-adhesive interface with the application of “predefined” torque after an initial loading which is resisted by the bond. After the breaking of the bond, the hardened adhesive acts as multiple expansion clips between each cone and concrete.

The popular bonded expansion anchors are Hilti HIT-Z and HAS-D anchor rod (Fig. 6.2) used in combination with injection mortar HIT-HY 200-A/R V3 and Hilti HAS-TZ anchor rod in combination with capsule Hilti HVU-TZ.

The adhesion principle is provided by injection mortar whereas expansion without an expansion sleeve is generated by the applied torque that causes small cracks in the mortar at the closest annular distance between the rod and the bore hole surface. This allows the elements to act as an expansion anchor with multiple-concentric cones: The pressure generated on the concrete bore-hole surface ensures that the design load capacity is reached even without cleaning the hole.

The two working mechanisms combined for this anchor also result in a higher pull-out resistance compared to traditional anchor technologies.

Fig. 6.3: HY 200 + HIT-Z for steel-to-concrete baseplate application

The different bonded expansion anchors with features and advantages are presented in Table 6.1.

Table 6.1: Hilti HIT-Z and HAS-D with HY 200 A/R V3 bonded expansion anchor portfolio

7. Conclusion

In conclusion, post-installed bonded anchors can be a preferred choice for many critical structural or non-structural applications as different types of anchors can address different requirements like dry and wet concrete, small edge and spacing, different loading types and higher design life demand etc. Failure to select the correct anchor or follow installation guidelines (Instruction for Use) can lead to severe structural damage, serious safety hazards and legal consequences. The performance, safety and durability of any structure depends on connections, i.e., anchors. Always consult Hilti technical documentation and local regulations .

8. References

[1] S2C Handbook: Steel to concrete connections using Post-installed systems, Schaan: Hilti Corporation, 2024.

[2] Fastening Technology Manual; Version 2023, Hilti Corporation, 2023.