By Phase 3 Helical were facing some challenges to their projects’ programme, site logistics and budgets. It was decided that an offsite approach could address these so Thorp was asked to provide an offsite facade solution which still incorporated the various high-quality brick, natural stone and reconstituted stone ‘lace faience’ facing materials to produce an exceptional finished detail.
Figure 1: Phase 3, Barts Square
Having previously delivered the earlier phases of the development in a more traditional manner, Thorp took the original traditional handset masonry design intent and incorporated all these aspects into a bespoke face sealed cladding design, similar to a unitised curtain wall approach. Whilst the design is not a standard system, it does take its form and principles from all the good practice developed within BS 8297 (architectural design, manufacture and installation of architectural precast concrete).
The facings are laid face down in the moulds with differing methods of mechanical key, allowing the concrete to be poured over the facing material to make an extremely robust monolithic cladding panel. Although a bespoke design, using the application of longstanding good practice gives assurance that the facade performance criteria will be achieved. This was later confirmed by a suite of offsite CWCT sequence B testing, in advance of full production through the Thorp factory in Stoke-On-Trent.
Early in the design process Thorp identified that the finesse of the detailing, programme and logistical constraints would preclude the use of traditional reinforced precast concrete.
The application of UHPFRC over traditional reinforced concrete allowed us to achieve these goals and remain within the original handset facade’s deadload allowance. Therefore, allowing the reinforced concrete structure which had already been substantially designed to remain unchanged.
UHPFRC is not a lightweight material; its density is the same as traditional concrete. But unlike traditional reinforced precast concrete, UHPFRC has a tensile capacity that allows us to use thinner sections or plate sizes. This allows us to maintain the integrity and quality the construction industry has become accustomed to when procuring precast concrete cladding facades. UHPFRC uses mild or stainless steel micro fibres instead of traditional reinforcement bars, the fibres are protected by a cementitious paste matrix which has a designed compressive strength in excess of 150N/mm². The omission of aggregates from the mix also results in durability and porosity properties which are greatly enhanced above that of more traditional C40/50 concrete mix designs.
Figure 2: Panel in storage awaiting window installation off-site.
Thorp worked hard with the client design and construction teams, developing cladding fixings back to the primary structure that could be accessed in a safe and controlled manner internally from the floor slabs. This removed the need for external scaffolding during the facades construction, unlocking valuable space from an already constrained site, and greatly improving productivity of the facade installation.
Figure 3: Installation onsite
The manufacture of panels off-site allows the client team early access to Thorps ISO accredited quality and environmental controls. The ability to carry out Inspections and Test Plans (ITP’s) and benchmarking of quality in factory conditions offers certainty to all stakeholders. In the case of St Barts this included the ITP’s for the glazing and balustrade packages, with QA/QC happening well in advance of on-site operations. These were then followed-up by separate on site ITP’s to check the installation through to final handover of each individual panel and then by elevation or phase to the main contractor, incorporating any minor cleaning and snagging activities.
From the outset Premier Guarantee were reassuringly vigorous in their review of Thorps off-site offering including the designs, methods and materials that are employed within the finished products. Whilst the focus was on the UHPFRC products, PG also reviewed the traditional RC panels which were also being applied in isolated areas on St Barts and other projects. This included historic and current third-party durability testing of the different facing materials, and was combined with off-site CWCT testing and onsite water testing by UKAS accredited organisations which encompassed panel to panel joint performance and adjacent trade package interfaces.
Their initial appetite for a rounded but detailed understanding of innovative Modern Methods of Construction (MMC) production techniques and materials were well received and set the stage for an excellent and open working relationship. This has since allowed any questions that might have arisen throughout the project to be addressed swiftly and concisely.
This proactive early engagement process, after a further period of PG audit reviews, led to Thorp being awarded with a PG product approval for both traditional RC and UHPFRC offerings. Since the start of St Barts – Ph 3, we have engaged with Premier Guarantee on several other projects and look forward to many more in the future.
There are many major UHPFRC civil, structural and architectural project examples around the world and Europe. Thorp have been working with UHPFRC since 2012 and have successfully applied UHPFRC face sealed facades in the UK to more than a dozen commercial and residential projects, alongside our standard façade offering. Many of which, to date, have been high-rise buildings up to 26 storeys. Given the benefits of the Thorp’s UHPFRC deigns and façade products generally, we are seeing a strong sustained interest by clients, specifiers and main contractors who have previously worked with us. Providing us with a strong order book throughout 2020 into 2021, including many potential UHPFRC projects and are happy to report PG are joint stakeholders on some of those projects.