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Latest Developments in Precast Concrete Connections

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Gary Connah  
Technical and Development Manager,
CRH CA & Halfen Moment, Singapore

Abstract: With about 60,000 homes required per year to house the current number of houseless families in India’s rapidly growing population, it makes sense that people are turning to precast and prefabrication as a way to meet the demand of the country.

As part of the building materials group CRH, Halfen Moment Group is ideally positioned to facilitate the transition towards a more efficient precast output; thanks to its local engineering presence and access to state-of-the-art connection technologies from its sister companies and partners in USA and Europe.

With all innovations, there are usually no standards or codes to follow, so more faith needs to be placed in prequalification through sound engineering principles and targeted testing regimes as the basis for the design methodology.

The connection of precast components on construction sites has always been a difficult challenge due to manufacturing tolerances and the need to transfer loads through the structure;even more so when you factor in a required resilience to a seismic events. Precast components are inherently stiff, so it is essential that the connection components are robust enough to allow the structure to absorb the energy of the earthquake, whilst providing enough robustness to avoid collapse.

This paper investigates the various innovative technology solutions offered by Halfen Moment Group of Companies and cross references them with any international or local standards and approval systems that exist as a method of prequalification.

Introduction

With about 60,000 homes required per year to house the current number of houseless families in India’s rapidly growing population, it makes sense that people are turning to precast and prefabrication as a way to meet the demand of the country.

As part of the building materials giant CRH, Halfen Moment Group is ideally positioned to facilitate the transition towards a more efficient precast output; thanks to its local engineering presence and access to state-of-the-art connection technologies from its sister companies and partners in USA and Europe.

With all innovations, there are usually no standards or codes to follow, so more faith needs to be placed in prequalification through sound engineering principles and targeted testing regimes as the basis for the design methodology.

The connection of precast components on construction sites has always been a difficult challenge due to manufacturing tolerances and the need to transfer loads through the structure; even more so when you factor in a required resilience to a seismic events. Precast components are inherently stiff, so it is essential that the connection components are robust enough to allow the structure to absorb the energy of the earthquake, whilst providing enough robustness to avoid collapse.

There are a number of ways of doing this such as via grouted couplers to connect walls and columns to foundations, wire rope box connectors to connect walls together and dowel systems for connecting slabs to walls. Here, we will look at each of these technologies in turn to highlight the challenges they need to overcome with reference in parts to New Zealand. The Canterbury Earthquake in New Zealand on 22nd February 2011, was a magnitude 6.3 earthquake that took the lives of 185 people. This earthquake is of particular interest as the New Zealand market relies heavily on the use of precast concrete in its construction methods and the author strongly recommends reading the royal commission to ensure that the lessons from New Zealand are learnt throughout the world.

State-of-The-Art Technologies
Grouted Coupler

These type of systems are traditionally used in the bottom of a column or wall as a means to connect on to starter bars protruding from the foundation. Although they might often be designed as a pin connection, the reality is that this will transfer moment from the foundation in to the wall or column and should therefore always be made of a ductile material that does not compromise the ductility of the steel reinforcement that it is coupling together. To that end, the coupler should always be of a “bar break” type where there is a considerable factor of safety between the bar failure and the coupler failure capacities.

 

Since the Canterbury Earthquake, the New Zealand standard committee that writes NZS 3101:2006 [2], have chosen to tighten up their requirements for coupler systems, by insisting on compliance with ISO 15835:2009 [1] as well as a tests to demonstrate resistance to brittle fracture (such as a Charpy V-Notch test) so that the rapid accelerations generated in an Earthquake can be safely resisted. In addition, the New Zealand Transport Agency has chosen to expressly ban the use of cast-iron couplers in its latest revision of their Bridge Manual – 3rd Edition. That being said, fully compliant machined steel couplers would also provide the required behaviour and can safely achieve the Charpy test results providing the correct steel and heat treatment processes are selected.

Wire Rope Loopboxes

Wire rope loop boxes are prefabricated components with a constant rebate geometry and 1 or 2 high strength (c. 1600-1800MPa) wire rope loops. Loops from adjacent elements then overlap during installation of the panels and are secured using a reinforcing bar and a high early strength grout to complete the connection.

They are usually used to connect 2 adjacent precast panels together and tend to be referred to as “Loop Boxes”, where the interlocking rebar and wire loops provides robustness in the event of an Earthquake to ensure that grout degradation would not result in complete collapse.

 

Currently this type of system does not have a harmonised standard which can be used for design, although there are some new sections to Eurocode 2 (EN 1992 [3]) some of which are currently at a pre-norm stage, and can be used to quantify most of the failure mechanisms that can occur.

Further information about the detailed design of these components can be obtained from the author, but it is not part of the scope of this paper.

Shear Dowel Systems

Shear dowel systems are always a challenge as they generally require very tight tolerances to ensure effective load transfer, so systems without grout and oversized holes are almost impossible.

But by keeping the grouting to a minimum, one such technology – Invisible Connections – has revolutionised the use of precast all throughout Northern Europe; it is a patented system,with its origins in Norway, to connect precast concrete without use of corbels or steel angles, and is used extensively throughout Europe due to the independently tested and certified European

Technical Approval (ETA [4]) that provides engineers with peace of mind about the capacities.

 

Product Range

The range consists of the following:

For simplicity, we will focus on the TSS version in this paper although further information on the rest of the range is available from the author.

The TSS is cast in to the slab (normally a stair landing) and a recess is cast in to the precast or insitu poured concrete wall. When erecting the concrete elements on site, the inner tube is pulled out with a wire to fit into the recess in the wall,together with a safety device to ensure the inner tube is correctly located in the recess. Additional items are available to ensure that lateral movements in Earthquakes do not result in the unseating of these dowels.

 

What is Traditional Technology?

Traditionally, precast slabs such as stair landings, used to be supported by a steel angle bolted to the wall or via a concrete corbel. Installation of angles is slow, requires high degrees of accuracy, ties up the crane with costly hook-time and can only be used on straight walls. Creation of corbels is time consuming and will only be economically viable in large quantities to ensure adequate reusability of the mould.

TSS telescopic connectors by Invisible Connections were developed specifically to address these problems and have become the proven alternative to traditional methods all throughout Europe.

 

 

 

Why Invisible Connections?

From a buildability perspective, the telescopic connectors appeal typically to precast concrete manufacturers where fewer bespoke moulds are required, insitu concrete frame contractors who appreciate the increased connection speed and architects due to the aesthetic appeal of ‘clean lines’ wherever concrete elements are joined.

Advantages of Invisible Connection

The Invisible Connection system has several advantages:

–    Corbel free connection between landings and walls – After installation, there is an invisible connection, with no visible supports.
–    Reduce impact sound from the stairways – The Invisible Connection System has several ways for sound insulation. From the more ordinary to the remarkable sound reduction.
–    Safe and simple installation on site – With a crane, the elements are safe and quickly located in the shafts. The correct method is to support the elements from beneath, level the element and pull out the inner tubes into the recesses. For the permanent connection, the recesses are then filled with mortar.
–    No welding or bolting on site – By using this system you avoid welding and bolting on site. Only filling up the recesses with mortar is needed to provide uniform performance.
–    Simple design of walls – Easy to plan the recesses in both precast walls and in moulds on site, due to standard location and sizes.
–    Less use of crane time on site – The installation time is shorter, compared to similar systems. Saving crane time and therefore money.
–    Standard boxes for wall recesses – The Invisible Connection System also has standard boxes for planning the recesses. These are corresponding to the size of the units. For steel moulds, they are magnetic. Additional elements and detailing are available to ensure robustness for seismic design.

Conclusion

There are many good technologies available, but a dependence on local testing and local reference projects will hinder and slow down their introduction. An increased readiness to accept the certification schemes for prequalification that exist overseas such as ETA and ICC certification where appropriate, will mean that the technologies can help keep pace with the requirements of the precast industry.

References

–    ISO 15835:2009 – Steels for the reinforcement of concrete — Reinforcement couplers for mechanical splices of bars
–    NZS 3101:2006 Part 1 – The design of concrete structures
–    EN1992-1 – Eurocode 2 – Design of Concrete Structures
–    ETA 11/0346 – European Technical Approval for TSS and RVK Staircase connections.

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