Feature

Return to Slender

A kind of sublime structural geometry enables the thin shell structure of L’Oceanogràfic in Valencia (1994-2002), by architect Félix Candela and structural engineers Alberto Domingo and Carlos Lázaro. It is one of many dramatic long-spanning, curved structures where naturally self-supporting hyperbolic and curved forms are constructed in a concrete that seems impossibly thin. Geometry is also key to the slender profiles achieved at Acoustic Shells, Littlehampton (CQ 254), whose sprayed concrete structure is as thin as 100mm in some places.

Geometry aside, the thickness of a reinforced concrete structure is influenced by the size and location of steel reinforcement bars and by the depth of concrete cover they require. This is a response to exposure conditions, design life and fire performance. In the UK, contemporary construction standards typically lead to a greater overall thickness than was common in the 1950s and 60s for comparable structures. But this trend is now being reversed with innovation in various fields, particularly in alternative types and uses of reinforcement, providing opportunities for thinner concrete and chiming with the recent focus on resource efficiency.

The post-tensioning (PT) of in-situ slabs has increased dramatically in the UK over recent decades. Pre-compressing the concrete element – so that when it flexes under applied loads it still remains in compression – allows a more efficient structural design, offering a potential reduction in floor slab depth of 50-75mm. This is a major benefit for high-rise development, where an extra floor can be gained for every ten storeys. Prestressing, or pre-tensioning in the manufacture of precast floor slabs, is also commonplace in the UK. The “pre” in pre-stressing describes the stress applied before any normal loads. The “post” in posttensioning refers to the tensioning of the strands after the concrete has been cast and when it has gained sufficient strength to be compressed in an equal and opposite reaction to the tensioning of the strands. For more information, see The Concrete Centre guide, Post-tensioned Concrete Floors.

Catharina Bridge, Leiden

The Catharina Bridge in Leiden is the longest bridge in high-performance concrete in the Netherlands. The pedestrian and cycle bridge, designed by Delft-based architect DP6, is 36m long and 6m across.

The slender design has a slight S shape and is supported by two piers rising from the water. Ultra high-performance concrete was specified to allow for a long span between the piers and a slim deck with a shallow gradient, thereby ensuring the free flow of traffic both by road and by water.

The middle of the bridge deck is 275mm deep.

Photo: hi-con.com / Gerda van Ekris

Progress in fibre reinforcement over recent years has also led to thinner forms of concrete (see House in Two Days by Cornish Concrete, below). Ultra-high performance concrete (UHPC) is a form of fibre-reinforced concrete used for factory-produced or precast products. It is poured into moulds like conventional concrete, but achieves greater bond, shear and compressive strengths of 100MPa - 350MPa. The use of steel fibres also enhances the Young’s modulus, tensile strength and ductility of the concrete, providing an opportunity to create thinner structures than previously possible (see Catharina Bridge, left), or types of structure that would not have been possible at all, as exemplified by the delicate laced concrete screen of Mucem, Marseille (CQ 245).

Examples in the UK have so far been seen in staircases, interiors and cladding (see Gasholders, below), as well as the very specific requirements for the repair of Hammersmith flyover in west London. In Denmark, there are numerous examples of ultra-slim balconies using a version called CRC (compact reinforced composite – see overleaf), and elsewhere in Europe product development includes ultra-thin insulated sandwich panels and even precast whole-house solutions.

Glass fibre reinforcement is used in a thin cement-based composite material, glass-fibre reinforced concrete (GRC).

House in Two Days, Cornwall

Cornish Concrete Products (CCP) provided all of the structural components, except for the roof, for a two-storey private residence near Falmouth, and erected them in just two days.

The four-bedroom detached property has been built with crosswall construction, using a system known as Slimcrete. The mineral fibre reinforcement used in the concrete offers greater tensile strengths than polypropylene or steel fibres, as well as being non-corroding.

Because of the strength of the fibres, thinner panels are possible – an important consideration in the residential market. The house comprises a 100mm reinforced structural skin, with 150mm insulation – to achieve a 0.15W/m2K U-value – and a 50mm reinforced external skin.

Photo: Cornish Concrete Products

This shares many of the aesthetic qualities of architectural precast concrete, but is used for non-load bearing applications. Its slenderness, around 15-20mm depending upon application, results from the use of strands of alkali-resistant glass-fibre reinforcement. In UK construction, it is commonly used for facade cladding panels or features such as sills, cornices and parapets where lightweight forms have a distinct construction advantage. The hollow units creating the white “veil” facade of The Broad art musuem in Los Angeles by Diller Scofidio + Renfro (CQ 254) are one such example. GRC can be either wet-cast into moulds or sprayed, offering great variation in shapes and profiles. Since they are subject to greater thermal and moistureinduced movement than standard concrete, GRC elements should be designed as an independent layer or object, with flexible support fixings. The International Glassfibre Reinforced Concrete Association (GRCA) provides guidance on codes of practice and national standards covering its manufacture and performance.

Gasholders King’s Cross, London

Ultra high-performance concrete was used to clad the internal atrium of the Wilkinson Eyre-designed Gasholders project in King’s Cross, north London.

The Gasholders comprises three luxury residential towers – of eight, nine and 11 storeys respectively – constructed within the refurbished cast-iron structures of the Victorian Pancras Gasworks. In the central open atrium, formed from the space where the three cylinders meet, precast-concrete panels form a pale, controlled backdrop to the Victorian girders and columns.

Thorp Precast installed 180 of the UHPC panels, which have a self-finished texture created using a Reckli rubber formliner.

Photo: Peter Landers

Ferrocement is another way to create superthin, load-bearing structures – a notable recent example being the roof canopy over the Stavros Niarchos Foundation Cultural Centre in Athens (CQ 260). There it met the desire for a visually very light and fine structure which could be completely un-jointed – unlike steel cladding – combining structure, waterproofing and finish in one element. The ferrocement skins were created with layers of fine diameter stainless-steel mesh embedded in a cementitious paste. More traditional forms, such as those used for boat building, are built up in layers of ferrous steel mesh in a very cement-rich matrix.

Non-corrosive reinforcement requires less depth of cover than conventional steel reinforcement. Various alternatives are in development, such as polymer reinforcement and bars made of carbon fibre, but none are yet available as a mainstream solution. It will be interesting to discover which other innovations might result in slimmer concrete, enabling the creation of remarkable forms that benefit from concrete’s aesthetic, its low maintenance requirements and its ability to be both structure and finish.

Fælledudsigten, Copenhagen

The Fælledudsigten residential project is located in Orestaden in the southern part of Copenhagen.

Hi-Con supplied the project with 46 balconies in compact reinforced composite (CRC). These measure 3.55- 3.85m long, 1.5-1.8m deep, 2.6m high and 70-85mm thick.

The high strength of CRC and the very small cover required for the dense matrix of steel-fibre reinforcement makes it possible to produce light and slender structures while retaining the workability of concrete.

Photo: Hi-Con, hi-con.com