Hilti HUS4 Screw with HUS4 MAX capsule

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, HUS4 concrete screw with HUS4-MAX capsule

As post-installed fastening systems in concrete cannot be regulated via prescriptive standards as it is done, say, with high strength structural bolts 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.

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 V3) can be made. In general, there is no pros or cons as the anchor systems should be selected to consider the jobsite and design conditions. Table 1 below provides roughly the main perceived differences between mechanical and chemical anchors.

Fig. 1 Perceived characteristics of mechanical vs. chemical anchoring systems

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 while 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 on 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 does just that, coming along with a new European design concept and a new European assessment process.       
 

What if we could create an anchor that combines the advantages of both?

The Hilti HUS4 SCREW with HUS4- MAX CAPSULE does just that!

The new European assessment document EAD 332795 on “bonded screw fasteners for use in concrete” allows for the qualification of the new technology which may be 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.

Fig 2 Hilti HUS4 MAX System, anchoring with a) hexagonal head and b) external thread

For example, the new Hilti HUS4 MAX is delivered in conjunction with foil capsules. In such a case capsule contains polymer resin, hardener, and aggregates in a defined mix ratio. A foil capsule is placed in a drilled hole, 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.

Fig 3 Thread cut by concrete screw only e.g. HUS 4 (a) and bonded screw fastener (Hilti HUS4 and Hilti HUS4 MAX Capsule), thread cut into the wall and filled with polymer matrix increasing the area of the mechanical interlock and filling the cracks in the concrete (b)

 
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 certain degree, the following intellectual game can be played: If the polymer/mortar would be completely disappeared 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 condition (cleaned vs. uncleaned), 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 SCREW with HUS4-MAX CAPSULE 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 SCREW with HUS4- MAX CAPSULE. 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.

Fig 4 Separating the simultaneously acting load transfer mechanism

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 the 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.

Fig 5 Separating the simultaneously acting load transfer mechanism Left side

What if you could design your structural connections combining the best of both worlds?

A safe anchorage requires not only detailed planning and design but also anchor systems that function reliably under ordinary and adverse job-site conditions. In Europe, to fulfil such 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, well-established EADs for mechanical anchors and chemical anchors is briefly presented in the following. As the bonded screw can be denominated as a hybrid between a concrete screw and a chemical system, the system cannot be assessed according to other existing EADs, since an HUS4 CONCRETE SCREW with HUS4- MAX CAPSULE If we “remove” the thread height to that point where they only touch the sides of the drilled hole, we have still a bonded anchor system Fig 5 Separating the simultaneously acting load transfer mechanism Left side 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].

EAD 332795 “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,..) 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. As usual for EAD processes, the concept requires first to evaluate the basic performance of the fastener in ordinary/normal conditions. Secondly, the potential sensitivity of the resistance to adverse conditions both at the time of installation and during the service life, is verified. Some potential reductions may be obtained by these tests and then applied to the basic performance to achieve the published resistance data 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. Optional tests with a group of four anchors are carried out in order to verify that the effective embedment depth to be used for the calculations of the concrete cone capacity (and related failure modes) is 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.

With regards to steel resistance, the qualification procedure receives the requirements already in place for the concrete screw anchors. The resistance against hydrogen embrittlement and the anchor capacity under shear loading, for example, are carried out following the same protocols of EAD 330232 [4] (mechanical fasteners).

The assessment of other parameters for splitting under tension loading and for the concrete pry-out or edge failures under shear loading is unvaried compared to the existing qualification procedures. 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 Comparison of technical parameters as considered in the related EAD

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), behavior temperature range.

The new design concept for bonded concrete screws, EOTA TR 075 [3] and EC2, Part 4 [2]

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] was needed firstly 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 would be required to avoid inconsistent assumptions by the existing manufacturers. 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 new design method for bonded screw anchors requires in general to follow the design method for bonded fasteners given in Eurocode 2, part 4 [2]. As a consequence of this approach, on one hand the effective embedment depth hef may be increased to match the nominal embedment of the anchor hnom, on the other hand the tensile resistance of a group of anchors is partially limited by the overlapping areas also in the case of pullout failure. In this way the method covers systems ranging between the two extremes of a concrete screw without bonding material to an almost fully bonded fastener with a small thread cutting in the concrete.

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).

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. Therefore, it had to be clarified that the assessed pullout resistance values are to be treated as resistance values to combined pullout and concrete failure.

Table 2 Comparison of the design methods of mechanical fasteners, bonded screw fasteners and bonded fasteners

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 [2]. The document is a guidance for a consistent use of the inputs for calculations given in the relevant ETA. 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. As in this case, the EOTA technical reports, allow to cover new technologies and avoid leaving gaps in the design provisions for anchors. 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, which is not covered in the EAD process, at least not yet

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 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 of course verified by tests.

Fig. 6 Test setup to measure the unscrewing mechanism of Hilti HUS4 SCREW and Hilti HUS4 SCREW with HUS4- MAX CAPSULE by going beyond the EAD requirements

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 account of DIN 65151 but was adapted to conditions found in anchorages to concrete conditions. The test setup is shown in the Fig. 6. Hilti HUS4 SCREW and Hilti HUS4 SCREW with HUS4-MAX CAPSULE were tested under a given displacement amplitude taking account of 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 SCREW with HUS4-MAX CAPSULE bonded concrete screw.

Fig.7  Hilti HUS4 SCREW and Hilti HUS4 SCREW with HUS4-MAX CAPSULE show no sign of unscrewing in the adapted Junker test

To measure any rotation of the system, the head/nut was marked as shown in Fig. 7 and Fig. 8. Even after 15min 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

Fig.8 No signs of unscrewing for Hilti HUS4 SCREW and Hilti HUS4 SCREW with HUS4-MAX CAPSULE from start of the test to the end of the test

Design benefits of the system depending on the applications

The Hilti bonded screw fastener Hilti HUS4 SCREW with HUS4-MAX CAPSULE carries and 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 reason:
(a)   Add structural safety as 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 polymer mortar.
(b)  Be more flexible concerning design change 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 the installer, using the Hilti HUS4 SCREW with HUS4-MAX CAPSULE 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

 

Fig 9 Hilti HUS4 SCREW with HUS4-MAX CAPSULE for low to medium duty primary structures, its secondary members, and the provided design advantages of having a total design flexibility and the benefit of the reduced sensitivity concerning environmental conditions and design assumptions as discussed before.

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 non-structural this does NOT mean safety relevant. 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, 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 SCREW with HUS4-MAX CAPSULE provides you the possibility of designing with smallest edge and spacing distances as used with chemical anchors while also the Hilti HUS4 SCREW with HUS4-MAX CAPSULE protects the borehole against standing water. In addition, even in conjunction with the mortar capsule, the Hilti HUS4 SCREW with HUS4-MAX CAPSULE is still completely removable, see Fig. 10.

Fig 10 Hilti HUS4 SCREW with HUS4-MAX CAPSULE for non-structural but safety relevant applications and the provided design advantages of smallest edge and spacing distances, borehole protection from humidity and full removability

Designing building equipment, building systems and machinery with Hilti HUS4 SCREW with HUS4-MAX CAPSULE 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 more flexible against functional changes. Hilti HUS4 SCREW with HUS4-MAX CAPSULE and Hilti filling set, is a substance tight solution for coated floors.

Fig 11 Hilti HUS4 SCREW with HUS4-MAX CAPSULE for fastening of equipment, and systems and the provided design advantages

In 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 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, the contractor benefits from your design as Hilti HUS4 SCREW with HUS4-MAX CAPSULE 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

[1] EAD 332795-00-0601 BONDED SCREW FASTENERS FOR USE IN CONCRETE
[2] EN 1992-4:2018 Design of concrete structures — Part 4: Design of fastenings for use
[3] EOTA TR 075 (in preparation) Design of bonded screw fasteners for use in concrete
[4] EAD 330232-01-0601 Mechanical fasteners for use in concrete
[5] EAD 330499-01-0601 Bonded fasteners for use in concrete
[6] EN 206:2013+A1:2016 Concrete: Specification, performance, production, and conformity