Hilti HUS4 Bonded Screw Fastener: A New Anchoring Technology

Abstract

Modern fastening technology is increasingly important in civil structural engineering worldwide. Every fastening element is designed for optimal performance for a specific application. When a fastening element is used for an application for which it is not intended, its performance can be negatively affected. Hilti is committed to bringing better, safer, and more reliable fastening system solutions to designers and contractors. Drawing on over 80 years of expertise and a passion for innovation, Hilti announces its latest innovation: a novel anchorage technology for designing and executing applications in concrete construction.

Hilti bonded screw fastener: Hilti HUS4 concrete screw with HUS4-MAX capsule

A new European assessment document EAD 332795 "Bonded screw fasteners for use in concrete" [1] was developed for this new fastening technology. EAD 332795 [1] is instrumental to a design method in line with EC2, part 4 [2], and regulated through the additional provisions given in a Technical Report issued by EOTA, the EOTA TR 075 [3]. This document coordinates the design method in its general aspects, however for the technical approach it endorses sections of the Eurocode 2, part 4 [2]. All these aspects are brought together in this paper to provide an understanding about the working principal, the assessment process, the design process, and the advantages concerning applications of this new anchoring technology for daily design.

General

The wide variety of anchor systems available on the market can make it difficult for design or installation professionals to select the appropriate anchor for a specific solution. Anchors are generally distinguished between mechanical anchors (e.g., Hilti HST 3 expansion anchors, Hilti HKD drop-in anchors, Hilti HDA undercut anchors, and Hilti HUS3 concrete screws) and chemical anchors (e.g., Hilti HVU2 capsule anchor systems and Hilti HIT-RE 500 V4 injection systems). There are no general pros or cons; only individual anchoring system characteristics matter, as fastening systems should be selected considering the jobsite and construction conditions.

Figure 1 provides a comparison of the main perceived differences between mechanical and chemical anchors. Generally, mechanical anchors are perceived as simpler to install and more economical, although they require larger edge and spacing distances. Chemical anchors, conversely, are perceived as better suited for poor concrete quality without performance loss, yielding higher loads, and protecting boreholes from humidity. However, their installation process is perceived as more complex due to environmental conditions like temperature and the need for dispensers and cleaning.

There is a demand for an anchoring technology that combines the benefits of both mechanical and chemical anchoring technologies. The Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener) offers this combination, along with a new European design concept and a new European assessment process.

What If: Creating an Anchor with Combined Advantages?

The Hilti HUS4 bonded screw fastener achieves this by combining the benefits of mechanical and chemical anchors. The new European assessment document EAD 332795 for "bonded screw fasteners for use in concrete" allows for the qualification of this hybrid technology, which combines a concrete screw (mechanical interlock or undercut) with a chemical system (adhesion and micro-keying). The system uses a concrete screw with a hexagonal head or outer thread, in conjunction with a foil capsule filled with bonding materials or an injection system (see Figure 2).

For example, the Hilti HUS4-MAX is delivered with foil capsules containing polymer resin, hardener, and aggregates in a defined mix ratio. The capsule is placed in a drilled hole (produced by hammer drilling or diamond core drilling), and the concrete screw is driven through it. As the screw is driven, the capsule shreds and compresses, mixing the resin, hardener, and aggregates. The annular gap around the screw and the thread cut into the wall is filled with the polymer matrix, simultaneously filling cracks around the anchor with resin (see Figure 3).

The bonded screw fastener's load transfer mechanism relies on mechanical interlock, friction, and chemical interlock, with the polymer increasing the mechanical interlock area of the threads within the borehole wall. This results in a perfect bond, undercut, and friction quality, with the polymer quantity optimized for hole tolerances.

What If: Designing Structural Connections with Combined Benefits?

A safe anchorage requires detailed planning and design, considering that the anchor system performs reliably under ordinary and adverse job-site conditions. In Europe, to fulfill this objective, construction product performance and characteristics are evaluated based on qualification requirements in a European Assessment Document (EAD). The new fastening technology "bonded screw anchors" provides a new qualification procedure and a specific design method.

A comparison between the new assessment procedure for bonded screw anchors and existing EADs for mechanical and chemical anchors is presented. As the bonded screw can be considered a hybrid system between a concrete screw and a chemical system, it cannot be assessed according to other existing EADs. An assessment method covering various combinations of both functioning principles is needed. Consequently, the assessment for bonded screw anchors includes verification aspects related to the mechanical functioning of the concrete screw (EAD 330232 [4]) and the chemical strength and durability of the mortar (EAD 330499 [5]).

4.1 Qualification of bonded screw fasteners for use in concrete

The European Assessment Document EAD 332795 [1] provides testing requirements and assessment criteria to verify system suitability, specify admissible conditions of use (e.g., loading type, environmental exposure), and determine performance characteristics (anchor resistance) needed for fastening point design. The evaluation outcomes are published in a European Technical Assessment (ETA).

The structure of the new assessment document is similar to EAD 330232 [4] (mechanical fasteners) and EAD 330499 [5] (bonded fasteners). The required test program allows for detailed derivation of essential product characteristics for all potential failure modes.

A key novelty in EAD 332795 [1] is the assessment of the combined pull-out and concrete failure mode. It starts with defining the baseline tension resistance of the bonded screw fastener. Sensitivity and robustness tests verify the mechanical response of the entire system. To complete the assessment of combined pullout and concrete resistance, the sensitivities and durability of the bonding material must also be qualified. These tests are conducted with the fastener diameter and embedment depth that have the largest contribution to performance from the bonding material in uncracked concrete. This is evaluated by comparing the basic resistance of the bonded screw fastener with that of the same screw installed without the bonding material.

Regarding concrete cone resistance, the presence of bonding material can compensate for potential screw thread wear at the tip after installation, ensuring load transfer at the deepest embedment point. Tests verify the effective embedment depth for concrete cone capacity calculations. This depth may equal the nominal embedment depth. Values of effective embedment depth between the nominal embedment and the equation for hef for concrete screws may also be assessed.

Resistance against hydrogen embrittlement and anchor capacity under shear loading are carried out following the same protocols as EAD 330232 [4].

The seismic assessment of bonded screw fasteners for categories C1 and C2 is conducted according to existing requirements for mechanical and chemical anchors. Performance under fire exposure can be taken from the resistance of the concrete screw without bonding material.

Table 1 provides an overview of the main technical parameters and their consideration in different EAD processes. Due to its hybrid nature, the bonded screw fastener is assessed concerning sustained loading (creep behavior) and temperature range.

What If: Designing Structural Connections with Combined Benefits? (Continued)

4.2 The new design concept for bonded screw fasteners

EC2, Part 4 [2] applies for fastening in concrete using anchors carrying an ETA. The respective assessment document specifies the design code and/or technical report to be followed. The Technical assessment for bonded screw anchors according to EAD 332795 [1] is instrumental to a design method in line with EC2, part 4 [2], and regulated through the additional provisions in EOTA TR 075 [3]. This document coordinates the design method in general aspects, endorsing sections of Eurocode 2, part 4.

The EOTA TR 075 [3] is needed because the product type "bonded screw fastener" is not explicitly mentioned in Eurocode 2 - part 4 [2], requiring guidance on the design path. Equations and criteria had to be adapted to receive a characteristic resistance force value (NRk,p) as input for calculations, instead of a characteristic bond strength (TRL) used for bonded fasteners in Eurocode 2, part 4 [2].

Additionally, EOTA TR 075 [3] addresses verification for seismic loading and fire exposure. The text from Eurocode 2 part 4 is adapted to receive the correct parameter from the technical assessment. In both cases, for assessment according to EAD 332795 [1], reference is made to procedures for mechanical anchors.

In summary, there is no technical deviation between the design method of EOTA TR 075 [3] and the design method for bonded fasteners provided in Eurocode 2, part 4 [2]. The main design provisions for anchors included in the Eurocode, the technical reports issued by EOTA, allow more flexibility in the delivery of state-of-the-art criteria to the engineering community. This complementary regulatory setup supporting the design practice is effective in ensuring both an adequate verification of the safety level for the fastening applications and the immediate transfer of new research findings.

A Technical Note: Addressing Specifier Concerns

Some designers express concerns about screw anchors potentially "self-loosening" or "unscrewing" themselves, especially under vibration, which could lead to significant issues over time. Hilti has addressed these concerns by developing the Hilti HUS4 screw and Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener). A complex set of design parameters, including thread characteristics, has been arranged to optimize and verify resistance against unscrewing under approved load parameters.

While the EAD process does not explicitly address this topic, Hilti developed a new test setup based on the Junker test philosophy and adapted to DIN 65151 for concrete anchorages. The Hilti HUS4 screw and Hilti HUS4 screw with HUS4-MAX capsule were tested under a given displacement amplitude and varying frequencies. The baseplate was installed on a concrete block and subjected to simulated vibrations at a 90° angle to the screw. The system's rotation was measured by marking the head/nut. After 15 minutes, no signs of unscrewing were detected, confirming that the Hilti HUS4 screw and Hilti HUS4 screw with HUS4-MAX capsule bonded screw fastener is optimized against unscrewing due to its complex thread design parameters.

Designer Benefits of Hilti Bonded Screw Fastener

The Hilti bonded screw fastener (Hilti HUS4 screw with HUS4-MAX capsule) is assessed according to EAD 332795 [1] and qualified for use with EOTA TR 075 [3] / EC2, Part 4 [2]. It offers engineers design advantages due to the following reasons:

• Structural Safety: Conditions influencing bond behavior (e.g., temperature) are absorbed by the concrete screw, while conditions influencing mechanical behavior (e.g., concrete quality) are absorbed by the mortar.
• Design Flexibility: Allows for design changes or future design requirements. Adding the capsule can increase performance by up to 30%, or it can be fully removed for solutions requiring minimal edge and spacing distances.
• Installer Benefits: Using the Hilti HUS4 screw with HUS4-MAX capsule eliminates the need for curing time, cleaning, torqueing, mandatory accessories, and restrictions on drilling techniques.

These benefits are particularly relevant for the following applications:

Low to medium duty primary structures and their secondary members

Suitable for normal weight cracked and non-cracked concrete (C20/25 to C50/60) under static, quasi-static, and seismic (C1) loading. The highlight is the double holding function (undercut & adhesion), which adapts performance to application requirements without overdesigning, allowing selection between the Hilti HUS4 screw and the Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener).

Double holding function for higher safety/robustness. The combination of mechanical and chemical fastening systems ensures higher safety of the fastening point due to lower sensitivity to environmental conditions and design assumptions.

Medium duty, non-structural but safety relevant applications

For normal weight cracked and non-cracked concrete (C20/25 to C50/60) under static, quasi-static, and seismic (C1) loading. Even if non-structural, these applications have safety relevance, as fastening failures could lead to structural collapse, posing risks to human life or causing significant economic loss. This category includes elements supported by or attached to buildings, such as handrails, roofs, and lightweight steel structures.

For these applications, the Hilti HUS4 screw and Hilti HUS4 bonded screw fastener allow for smaller edge and spacing distances, similar to chemical anchors. The Hilti HUS4 bonded screw fastener also protects the borehole against standing water. Furthermore, even with the mortar capsule, the Hilti HUS4 bonded screw fastener remains completely removable (see Figure 8).

Double holding function for higher safety/robustness. The combination of mechanical and chemical fastening system ensures higher safety of the fastening point due to lower sensitivity to environmental conditions and design assumptions.

Dismantlable & Reusable

Protected borehole against standing water and moisture

Designing building equipment, building systems, and machinery

For applications in normal weight cracked and non-cracked concrete (C20/25 to C50/60) under static, quasi-static, and seismic (C1) loading. Poor fastening performance is a major contributor to damage and business interruption. The double holding function and removability offer flexibility for functional changes. The Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener) and Hilti filling set provide a substance-tight solution for coated floors.

Double holding function for higher safety/robustness. The combination of mechanical and chemical fastening system ensures higher safety of the fastening point due to lower sensitivity to environmental conditions and design assumptions.

Dismantlable & Reusable

Protected borehole against standing water and moisture

Protected borehole against various substances for sealed industrial floors

Summary

The new HUS 4 is the fourth generation of a post-installed, self-locking concrete screw using mechanical interlock for reliable load transfer in cracked and non-cracked concrete, fresh concrete, and other base materials with top edge productivity. The new Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener) adds mortar to the concrete screw, combining the advantages of chemical anchors with those of mechanical anchors. It reduces sensitivity to design conditions and assumptions while maintaining high productivity.

Conditions influencing bond behavior, such as temperature, are absorbed by the concrete screw. Conditions influencing mechanical behavior, like concrete quality, are absorbed by the polymer mortar. The system offers greater flexibility for design changes; adding the capsule can increase performance by up to 30%, or it can be removed for solutions fitting the smallest edge. Contractors also benefit, as Hilti HUS4 screw with HUS4-MAX capsule eliminates the need for curing time, cleaning, torqueing, mandatory accessories, and drilling restrictions.

This system is assessed according to EAD 332795 "Bonded screw fasteners for use in concrete" [1] and can be designed in line with EC2, part 4 [2], regulated through EOTA TR 075 [3].

The best of both worlds for designers and contractors brought to you by Hilti.

References

• [1] European Organisation for Technical Assessment (EOTA): European assessment document, EAD 332795-00-0601 Bonded screw fastener for use in concrete, in preparation
• [2] DIN EN 1992-4:2019-04: Eurocode 2 - Bemessung und Konstruktion von Stahlbeton- und Spannbetontragwerken - Teil 4: Bemessung der Verankerung von Befestigungen in Beton; Deutsche Fassung EN 1992-4:2018
• [3] European Organisation for Technical Assessment (EOTA): EOTA Technical Report, Design of bonded screw fasteners for use in concrete, TR 075, in preparation
• [4] European Organisation for Technical Assessment (EOTA): European assessment document, EAD 330232-01-0601 Mechanical fasteners for use in concrete, December 2019
• [5] European Organisation for Technical Assessment (EOTA): European assessment document, EAD 330499-01-0601 Bonded fasteners for use in concrete, December 2018
• [6] EN 206:2013+A1:2016 Beton - Festlegung, Eigenschaften, Herstellung und Konformität, 2013