HILTI HUS4 Concrete Screw Anchor Instructions
- June 4, 2024
- HILTI
Table of Contents
HILTI HUS4 Concrete Screw Anchor Instructions
ABSTRACT
Modern fastening technology is becoming 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. At Hilti, we are committed to bringing better, safer, and more reliable fastening system solutions to our designers and contractors. Drawing on over 80 years of expertise, and a passion for innovation, we’re excited to announce our latest innovation yet:
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 your daily design.
GENERAL
Given the large variety of anchor systems available on the market today, design or installation professionals may find it difficult to select the appropriate anchor for a specific solution. In general a distinction between mechanical anchors (expansion anchors e.g. Hilti HST 3, drop-in anchors e.g. Hilti HKD, undercut anchors e.g. Hilti HDA and concrete screws e.g. Hilti HUS3) and chemical anchors (e.g. capsule anchor systems Hilti HVU2 and injection systems e.g. Hilti HIT-RE 500 V4) can be made. Furthermore, there are no general pros or cons but only individual anchoring system characteristics that matter, as the fastening systems should be selected considering the jobsite and construction conditions.
Fig. 1 below provides roughly the main perceived differences between mechanical and chemical anchors
In general, the benefit of mechanical anchors is seen as to be much simpler to
install compared to chemical anchors.
In addition, mechanical anchors are also perceived as more economical although
the edge and spacing distances are perceived as relatively large.
In contrary, the benefit of chemical anchors is clearly based on the
perception that (1) these can also be installed in poor concrete quality
without lacking performance, (2) they yield higher loads and (3) the mortar
can protect the boreholes from humidity compared to mechanical anchoring
systems.
On the other hand, the installation process is perceived as much more complex
due to the influence of environmental conditions like temperature, using of
dispenser, cleaning effort etc.
Consequently, it can be said that there is a demand for an anchoring technology combining the benefits – as much as possible of both anchoring technologies – mechanical anchors, and chemical anchors.
The Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener) does just that, coming along with a new European design concept and a new European assessment process.
Fig. 1 Perceived characteristics of mechanical vs. chemical anchoring
systems
WHAT IF
we could create an anchor that combines the advantages of both?
The Hilti HUS4 bonded screw fastener does just that!
The new European assessment document EAD 332795 for “bonded screw fasteners for use in concrete” allows for the qualification of the new technology which maybe denominated as a hybrid between a concrete screw (anchoring system based on mechanical interlock or undercut) and a chemical system (anchoring based on adhesion and micro-keying). The bonded screw anchoring system employs a concrete screw with a hexagonal head or outer thread in conjunction with a foil capsule filled with the constituent bonding materials or an injection system, see Fig. 2.
For example, the new Hilti HUS4-MAX is delivered in conjunction with foil capsules. In such a case the capsule contains polymer resin, hardener, and aggregates in a defined mix ratio. A foil capsule is placed in a drilled hole, where the drilled hole could have been produced by hammer drilling or diamond core drilling. The concrete screw is driven through the capsule. When driving the concrete screw into the hole the foil capsule is likewise shredded but also compressed, the resin hardener, aggregates are mixed and the annular gap around the concrete screw and the thread cut into the wall is filled with the polymer matrix, simultaneously cracks around the anchor are filled with resin, see Fig. 3.
The bonded screw fastener load transfer mechanism is based on mechanical interlock, friction, and chemical interlock, while the polymer increases the area of the mechanical interlock of the threads cut into the borehole wall. Consequently, a perfect bond, undercut and friction quality can be concluded, and the quantity of polymer materials contained is such that considering hole tolerances.
What is happening in the borehole?
Fig. 3 Thread cut by Hilti HUS4 screw and Hilti HUS4 bonded screw
fastener (Hilti HUS4 concrete screw with Hilti HUS4-MAX capsule)
An intellectual game for understanding the bonded screw fastener
To have an idea about the bonded screw anchor and why this anchoring technology is combining the benefits of mechanical anchors and chemical anchors to a certaindegree, the following intellectual game can be played:
If the polymer/mortar would be completely removed the residual anchoring system would be a pure concrete screw transferring the load by mechanical interlock via the thread cut into the wall, see Fig. 4.
Consequently, it can be said
that conditions influencing the bond behavior are absorbed by the concrete
screw. In case of bonded anchors these are borehole conditions like cleaned
vs. uncleaned boreholes, drilling process (hammer drilling vs. diamond
coring), temperature of the base material and the long-term behavior of the
mortar type. This being said, qualification tests with Hilti HUS4 bonded screw
fastener underline that this system is less to zero influenced by the
parameters which are in general valid for bonded anchors systems, only. Of
course, this statement is only true, if the combination of thread geometry and
characteristics of the adhesives are balances as provided with Hilti HUS4
bonded screw fastener. Other systems with reduced interlock in favor of higher
bond provided by the chemical, are assumed to show higher sensitivities
comparable with a fully bonded threaded rod.
This time, we assume that the thread of the concrete screw would disappear as much that they are not touching the wall but still large enough to have an interlock between t he mortar and the screws but not between concrete and the threads. Under these assumptions we would have a chemical anchor system where the load is transferred by adhesion and micro keying, see Fig. 5. Consequently, we can say that conditions influencing the mechanical behavior (e.g. concrete quality) are absorbed by the polymer mortar.
you could design your structural connections combining the best of both worlds?
A safe anchorage requires not only detailed planning and design but also the consideration that the anchor systems performs reliable under ordinary and adverse job-site conditions. In Europe, to fulfil such an objective, the performance and characteristics of a construction product are evaluated based on the qualification requirements included in a European Assessment Document (EAD) issued by European Organization for Technical Assessment (EOTA). With the new fastening technology “bonded screw anchors” a new qualification procedure and a specific design method are provided to the engineering community.
A comparison between the new assessment procedure for bonded screw anchors and
the pre-existing, wellestablished EADs for mechanical anchors and chemical
anchors is briefly presented in the following. As the bonded screw can be
denominated as a hybrid system between a concrete screw and a chemical system,
the system cannot be assessed according to other existing EADs, since an
assessment method to cover various ranges of combination of both functioning
principles is needed.
Consequently, the assessment for bonded screw anchors includes aspects of
verification related to the mechanical functioning of the concrete screw EAD
330232 [4] as well as to the chemical strength and durability of the used
mortar type EAD 330499 [5].
Qualification of bonded screw fasteners for use in concrete
The European Assessment Document EAD 332795 [1] provides testing requirements and assessment criteria intended to verify the suitability of a system, to specify the admissible conditions of use (e.g. loading type, environmental exposure among others) and ultimately to determine the characteristics of performance (i.e. resistance of the anchor) which are needed for the design of a fastening point. The outcomes of the evaluation with all relevant parameters for the construction product are then published in a European Technical Assessment (ETA), as usually foreseen by the EOTA CE-marking path.
The structure of the new assessment document is similar to those of EAD 330232
[4] (mechanical fasteners) and EAD 330499 [5] (bonded fasteners). The
conduction of the required test program allows to derive in detail the
essential characteristics of the product for all the potential failure modes
to be found in the European Technical Assessment (ETA) of the related product.
The main element of novelty in the EAD 332795 [1] for bonded screw fasteners
is the assessment of the failure mode combined pull-out and concrete failure.
It starts from the definition of the baseline tension resistance of the bonded
screw fastener. Afterwards, several sensitivity and robustness tests provide a
verification of the mechanical response of the entire system. In addition, to
complete the assessment of the combined pullout and concrete resistance, a
qualification of the sensitivities and durability of the bonding material
itself must be realized. These tests are conducted with the condition of
fastener diameter and embedment depth, that is identified having the largest
contribution to performance from the bonding material in uncracked concrete.
This is evaluated through a comparison between the basic resistance of the
bonded screw fastener with that of the same screw installed without the
bonding material.
With regards to the parameters for concrete cone resistance, it is considered that the presence of bonding material can compensate the potential wear of the screw thread at the tip after installation ensuring the load transfer to occur at the deepest embedment point. To verify this, tests are performed to verify the effective embedment depth to be used for the calculations of the concrete cone capacity (and related failure modes). This depth maybe equal to the nominal embedment of the fastener. Values of effective embedment depth between the nominal embedment and the equation of hef for concrete screws may also be assessed.
The resistance against hydrogen embrittlement and the anchor capacity under shear loading, are carried out following the same protocols of EAD 330232 [4] (mechanical fasteners).
The seismic assessment of bonded screw fasteners for the categories C1 and C2 is conducted in accordance with the existing requirements regulating mechanical and chemical anchors. It should be noted that the performance of the bonded screw fasteners under fire exposure may be taken from the resistance of the concrete screw without bonding material.
Table 1 provides an overview about the main technical parameters and how these are considered in the different EAD process. Due to the combination as hybrid system, the bonded screw fastener must also be assessed concerning sustained loading (creep behavior under sustained stress) and behavior temperature range.
The new design concept for bonded screw fasteners
EC2, Part 4 [2] applies for fastening in concrete using anchors carrying an ETA as the anchor parameters for calculating the resistance must be obtained from the respective assessment document. The respective assessment document specifies which design code and/or technical report must be followed. The Technical assessment for bonded screw anchors according to the 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 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.
The EOTA TR 075 [3] is needed because the construction product type “bonded screw fastener” is not explicitly mentioned in the Eurocode 2 – part 4 [2] and instructions on which design path to follow are required. In addition, some equations and criteria had to be adapted considering the outcome of the technical assessment for the new product type, as described in the EAD 332795 [1]. The equations of Eurocode 2, part 4 [2] regulating bonded fasteners consider a characteristic bond strength (tRk) as an input for the calculations to be applied for the design at the specific anchorage configuration. Therefore, those had to be adapted to receive as an input for calculation directly a characteristic resistance force value (NRk,p) in case of bonded screw fasteners.
Additionally, the EOTA TR 075 [3] addresses the verifications to seismic
loading and to fire exposure to explicitly regulate those design cases as
well. In both cases the text from the Eurocode 2 part 4 is simply adapted to
receive the correct parameter out of the technical assessment.
In both cases, for the assessment according to the EAD 332795 [1], a reference
to the procedures for mechanical anchors is used.
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]. With 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. Table 2 provides an overview about the main equations between the 3 fastening types.
A TECHNICAL NOTE
to the main concerns from the specifier community
Some designers have concerns that screw anchors might show risk of “self- loosening” and may “unscrew” themselves e.g. in case of vibration and it is clear that nobody wants to have a turning concrete screw under loading for the next 50 years or even 100 years, which may lead to other problems. Hilti developed the Hilti HUS4 screw and Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener) where a complex set of design parameters involving the characteristics of the thread was arranged such that the resistance against unscrewing under the approved load parameters is optimized and of course verified by tests.
As there is no clear request concerning that topic in the EAD process Hilti developed a new test setup which is based on the philosophy of the Junker test and takes into account DIN 65151 which was adapted to conditions found in anchorages to concrete conditions. Hilti HUS4 screw and Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener) were tested under a given displacement amplitude considering different test frequencies. The baseplate was installed on a concrete block and exposed to simulated vibrations at a 90° angle to the Hilti HUS4 screw and Hilti HUS4 bonded screw fastener. To measure any rotation of the system, the head/nut was market as shown in Fig. 6. Even after 15 min no signs of unscrewing was detected which ensures that Hilti HUS4 screw and Hilti HUS4 screw with HUS4-MAX capsule bonded screw fastener is optimized against unscrewing within the scope of the application due to a complex set of design parameters involving the characteristics of the thread.
DESIGNER BENEFITS
of Hilti bonded screw fastener depending on the applications
The Hilti bonded screw fastener (Hilti HUS4 screw with HUS4-MAX capsule) carries an assessment according to EAD 332795 [1] and is therefore qualified for the usage in conjunction with EOTA TR 075 [3] / EC2, Part 4 [2] and may provide you as engineer design advantages due to the following reasons:
(a) Add structural safety since conditions influencing the bond behavior (e.g.
temperature) are absorbed by the concrete screw. Conditions influencing the
mechanical behavior (e.g. concrete quality) are absorbed by the mortar.
(b) Be more flexible concerning design changes or even anticipate for future
design requirements as adding the capsule increase the performance up to +30%
or take advantage of full removability and find design solutions even for the
smallest edge and spacing distances.
(c) Even for installers, using the Hilti HUS4 screw with HUS4-MAX capsule
(bonded screw fastener) means you no longer must deal with curing time,
cleaning, torqueing, mandatory accessories, and restrictions of drilling
technics.
We see these benefits especially for the following application:
Low to medium duty primary structures and its secondary members
in normal weight cracked and non-cracked concrete (C20/25 to C50/60) under static, quasi-static and seismic (C1) loading. The highlight is provided by the double holding function (undercut & adhesion) adapting the performance to the application requirements without overdesigning by choosing between Hilti HUS4 screw and Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener).
Medium duty, non-structural but safety relevant applications in normal weight
cracked and non-cracked concrete (C20/25 to C50/60) under static, quasi-static
and seismic (C1) loading. Even if the application is non-structural this does
NOT mean there is NO safety relevance.
The EOTA TR 075 [3] / EC2, part 4 [2] is intended for safety related
applications in which the failure of fastenings may result in collapse or
partial collapse of the structure, which could cause risk to human life or
lead to significant economic loss. In this context it also covers non
structural elements supported by, or attached to new or existing buildings
like handrails, roofs, and lightweight steel structures. For such applications
Hilti HUS4 screw and Hilti HUS4 bonded screw fastener provides you the
possibility of designing with smallest edge and spacing distances as used with
chemical anchors while also the Hilti HUS4 bonded screw fastener protects the
borehole against standing water. In addition, even in conjunction with the
mortar capsule, the Hilti HUS4 bonded screw fastener is still completely
removable, see Fig. 8.
Designing building equipment, building systems and machinery with Hilti HUS4 screw and HUS4-MAX capsule (bonded screw fastener) in normal weight cracked and non-cracked concrete (C20/25 to C50/60) under static, quasi-static and seismic (C1) loading. Poor performance of fastenings for equipment, and systems is the greatest contributor to damage and business interruption. The double holding function and removability when changing the position of the equipment provides you the advantages of being more flexible against functional changes. Hilti HUS4 screw with HUS4-MAX capsule (bonded screw fastener) and Hilti filling set, is a substance tight solution for coated floors.
SUMMARY
The new HUS 4 is the fourth generation of a post-installed, self-locking concrete screw using mechanical interlock for a reliable load transfer mechanism 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 the advantages of mechanical anchors and reduce the sensitivity to design conditions and design assumptions by having the same productivity focus.
This is because conditions influencing the bond behavior, such as temperature, are absorbed by the concrete screw. While conditions influencing the mechanical behavior, like concrete quality, are absorbed by the polymer mortar. It also provides you with greater flexibility to deal with design changes. By adding the capsule, you can increase the performance up to 30%, or you can choose to fully remove the capsule for a solution to fit the smallest edge. Also, contractor benefits from your design as Hilti HUS4 screw with HUS4-MAX capsule by means contractors no longer must deal with curing time, cleaning, torqueing, mandatory accessories, and restrictions of drilling technics.
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], and regulated through the additional provisions given in a Technical Report issued by EOTA, the EOTA TR 075 [3].
The best of both worlds for designers and contractors … brought to you by Hilti.
References
- European Organisation for Technical Assessment (EOTA): European assessment document, EAD EAD 332795-00-0601 Bonded screw fastener for use in concrete, in preparation
- 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
- European Organisation for Technical Assessment (EOTA): EOTA Technical Report, Design of bonded screw fasteners for use in concrete, TR 075, in preparation
- European Organisation for Technical Assessment (EOTA): European assessment document, EAD 330232-01-0601 Mechanical fasteners for use in concrete, December 2019
- European Organisation for Technical Assessment (EOTA): European assessment document, EAD 330499-01-0601 Bonded fasteners for use in concrete, December 2018
- EN 206:2013+A1:2016 Beton – Festlegung, Eigenschaften, Herstellung und Konformität, 2013
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