Through two centuries the area has seen industrial boom and bust, and in more recent times had become something of a forgotten backwater. Since 2000 however, 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.
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. 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 post-tensioning specialists Freyssinet, considered using 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 and the Schöck UK design team decided to involve their French colleagues.
In France, internal insulation is the most common form of construction and a special Isokorb® derivative, 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). It has a high density insulation body and uses reinforcement bars with both normal and stainless steel welded together. The stainless steel guarantees corrosion protection and reduces the thermal conductivity of the bars.
Fire protection is a crucial consideration concerning the inner envelope of the building, 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.
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.
In addition, 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.
Architect: Levitt Bernstein
Client: The Hyde Group
Principal Contractor: Willmott Dixon
Structural Engineer: Clancy
Frame Contractor: Mitchellson
Post-tensioning Contractor: Freyssinet Ltd