It may seem a little incongruous today, but in 18th century London the discovery of a spring in Bermondsey resulted in the area becoming a spa leisure resort. This changed dramatically as localised industrial boom and bust followed throughout the next two centuries, until in more recent times the area had become something of a forgotten backwater. Since 2000 however, and due primarily to the construction of the Jubilee Line extension, Southwark Council has been working hard on the regeneration of the Bermondsey Spa area. It is a multi-award winning scheme, involving fifteen key sites across a 50-acre area and will result in over 2,000 new homes, health centres, offices, shops and newly landscaped open space.
Post-tensioned concrete frames
The latest phase of construction, just off the Jamaica Road, where Willmott Dixon is the main contractor, involves four buildings up to ten storeys high. These are post-tensioned concrete frames and 220mm deep, two-way post-tensioned flat slabs, the total slab area on the project being around 28,500m2. With concrete having such a high thermal conductivity, the prevention of thermal bridging at connectivity points throughout the project is a critical consideration if necessary government standards are to be met. Regulations insist that there must be a major reduction in local heat loss and CO2 emissions, but there are other factors too. If low internal temperatures around the thermal bridge are below the dew point of the air, condensation will form. This in turn can lead to structural integrity problems with absorbent materials. Worse still, it encourages mould growth, which has serious implications for building occupants and may cause them to develop certain medical conditions such as respiratory problems and dermatitis.
The design of these latest four blocks is unusual in that some of the residential units are conventionally insulated on the outside, and others insulated on the inside. This form of internal insulation is commonly found in France, and has the peculiarity of needing thermal breaks not only at the balconies, but also around the perimeter of the inner slab, due to the thermal bridge created by the break in the internal insulation at these points. Left untreated, these large thermal bridges result in an unacceptably high loss of energy and risk of damage.
Schöck and Freyssinet considered the options
Schöck and post-tensioning specialists Freyssinet, considered the options, and initially a possible solution was the type Q Isokorb® for concrete-to-concrete connectivity. However, this is not a product that incorporates the required nominal restraint for such an application. Schöck prides itself on its ability to draw on specialist product solutions from other countries, and on this occasion the UK design team looked to their French colleagues.
In France, due to internal insulation being the most common form of construction, a special Isokorb® derivative, known as Rutherrma DF, has been developed for such situations. The Rutherma DF incorporates Schöck technology found in the Isokorb® range of thermal breaks, and has a technical certification issued by the French Centre Scientifique et Technique du Bâtiment (CSTB). The DF module has a high density insulation body and uses reinforcement bars with both normal and stainless steel welded together. The use of stainless steel not only guarantees corrosion protection, but also reduces the thermal conductivity of the bars. The DF incorporates bars that bear the load of the inner slab, and additional bars for further rigidity and restraint.
For the inner envelope of the building, fire protection is a crucial consideration and the Rutherma DF range has been tested and certified by the CSTB, up to a rating of REI120 (Resistance, Entegrity, Insulation). Other thermal bridging requirements on the project involve the reinforced concrete external balconies and walkways, which are separated from the PT slabs by Schöck Isokorb® type Q thermal break modules.
The co-ordination of the tendon stressing anchors with the thermal breaks is relevant to post-tensioned slabs. This might be considered problematic because Freyssinet would normally stress from the edge of the slab, and if a thermal break module is present at that edge, it could prove difficult. However, the tendons only need to be stressed at one end, so the primary solution is to coordinate the stressing anchors with the modules. Where this doesn't solve the issue (Schöck thermal break modules on opposite edges for example), the use of a Freyssinet stressing pan allows the tendon to be stressed from the top surface.
In addition to the special capabilities of the Rutherma, the conventional UK Isokorb® range is unique in allowing connections to be made between concrete-to-concrete, concrete-to-steel and steel-to-steel – and it also provides BBA Certification and LABC Registration. The range also comfortably exceeds the UK regulation (BRE IP1/06), which requires that the temperature factor used to indicate condensation risk (fRSI), must be greater than, or equal to, 0.75 for dwellings, residential buildings and public buildings.
There is also compliance with the Government Standard Assessment Procedure, SAP 2009, concerning CO2 emissions from buildings and respectively heat losses through non-repeating thermal bridges. Here, the lambda values of the Schöck Isokorb® enable energy loss in various connective situations to be reduced by as much as 84% to 91%.
The Oxfordshire based Schöck design team is on hand to provide specifiers, structural engineers and contractors with a high service support level. This now includes an in-house thermal-modelling capability and additional support in the form of a complete library of information available as hard copy, or downloadable information from the website. There is a 20 page 'Specifiers Handbook', which provides an overview and introduction to the Schöck Isokorb®; and a comprehensive 236 page 'Technical Guide' which displays in detail the complete range of thermal break applications for all construction types.
For your free copy of the Specifiers Guide and /or the Technical Guide contact Schöck Ltd on 01865 290 890.
Artists impression courtesy of Levitt Bernstein and The Hyde Group
Architect: Levitt Bernstein
Client: The Hyde Group
Principal Contractor: Willmott Dixon
Structural Engineer: Clancy
Frame Contractor: Mitchellson
Post-tensioning Contractor: Freyssinet Ltd