Feature

For Sedimental Reasons

One of the most visually appealing characteristics of walls made of cast-in-situ concrete is the opportunity for continuous, unjointed surface and structure. Techniques for placement and compaction avoid creating cold joints between layers of fresh concrete as they are systematically built up in the formwork, minimising the impact of abrupt changes in tone or colour on the exposed surface. For high-quality visual concrete, the location of day-work and construction joints are also controlled, often hidden or otherwise expressed as a shadow gap feature.

But there are notable projects where the layering process of casting concrete has been deliberately expressed – some of which challenge the established methodology for concrete construction. In combination with earth-coloured pigments, the results echo the horizontal strata of natural rock formations and tap into a concrete aesthetic that is more natural than industrial. Neatly executed horizontal day-work joints are clearly visible in the concrete facades of the Foro Boca concert hall in Veracruz on the Gulf of Mexico (CQ 266) by Rojkind Arquitectos, where storey-high bands of concrete of varying tones are integral to the architectural expression of the building, its form and coastal location.

CQ Lightbox ImageThe layers between day-work joints are much shallower in the rammed concrete walls of the Secular Retreat in Devon – as less concrete can be placed each day using this more labour-intensive construction method (see case study, below). For the external walls of the Yorkshire Sculpture Park’s new visitor centre and gallery (CQ 264), designed by Feilden Fowles, distinctive horizontal layers were also created using shallow layers of site-batched concrete with different pigment and aggregate combinations.

Rather than ramming the concrete, slender walls were constructed using fluid concrete, which could therefore be reinforced. In order to provide a bond, each layer was cast before the one below was fully cured, with four layers achievable in each 1.2m lift of formwork. Various techniques were used to add surface texture, enhancing the horizontal definition and natural appearance. Facades reminiscent of geological layers are on show on a larger scale at the Forum :terra nova exhibition venue near Elsdorf, Germany by Lüderwaldt Architekten.

Secular Retreat, Devon

Living Architecture, the holiday property company set up by writer and philosopher Alain de Botton, has always invited daring design. It has teetered a barn by Dutch architect MMVRDV hair-raisingly over a Suffolk hillside, with a child’s swing dangling from the overhang; it has built a fantastical cottage in an Essex meadow, a shrine to a fictional character dreamed up by Grayson Perry and FAT Architects. But in some ways its most recent project, Secular Retreat in south Devon, is the most adventurous of the lot.

After all, none of the others involved two years of trialling just to work out how to build the walls. Designed by legendary Swiss architect Peter Zumthor, Secular Retreat stands on a hilltop surrounded by Monterey pines. In many respects, it is the essence of a modern villa: a flat roof slab projects over glazed walls, revealing open-plan interiors of bespoke furniture. What holds it all together is something surprisingly monumental, and almost geological: a structure of 700mmthick rammed-concrete walls that, like layers of prehistoric rock, bear the history of their making.

Despite the technique never having been used at this scale in the UK, it was integral to the design almost from the start. Zumthor, for whom this is a rare foray into houses and his first permanent UK project, had initially turned down Living Architecture’s entreaties but was persuaded by the possibilities of the site. “That was the thing that excited him,” says Mark Robinson, director of Living Architecture. “He had a very strong idea from the start based on the hilltop, and inspired by the stony outcrops on Devon’s moors. His early concepts involved literally piling stones, and that then manifested itself in the idea of layered concrete.”

While Zumthor had used rammed concrete before, notably on the Bruder Klaus Field Chapel in Germany (CQ 264), it was a voyage into the unknown for the local contracting team. “It took nearly two years of development and hundreds of samples,” says Robinson. “We built a full-scale mock-up of part of the house, to examine details such as where the rammed walls meet the poured roof slab. It was a laboured process but that’s what we had to do to make sure that the guys were confident and that Peter was happy with the approach.” It is easy to see why this project was 10 years in the making.

The walls were built up in 500mm layers. A dry, loose concrete mix – white cement, sand from the south-west coast and local limestone aggregate – was shovelled into the formwork in 150mm layers and tamped down to about 100mm, a process that was carried out five times. The concrete was left to dry overnight before the sliding phenolic ply formwork was struck, raised and the next layer added.

It was important, Robinson says, that the shuttering was 600mm high, as this meant that the site team couldn’t just level theconcrete off at 500mm. “Most contractors think  concrete has to look perfect, so it was about changing the mentality and saying, this is handmade, just tamp it down, and if it’s uneven that’s fine. You make rammed concrete by not thinking too hard about it, not trying to be too precious.” If that makes it all sound a bit easy, he adds: “At the same time, you can’t make it deliberately uneven.

You can’t be random about it.” As the long development period suggests, the rudimentary construction technique belies some extremely careful detailing. The uppermost layers of concrete are more compacted, with slightly more water in the mix, so that they act as an aesthetic bridge to the poured-concrete roof slab. Likewise, because the structure was built as two unreinforced 300mm walls around 150mm of insulation, the team could use a slightly wetter mix to give a softer, more refined finish to the inner surface.

The outer wall is rougher and more porous, effectively acting as an extremely thick rainscreen to the waterproof insulation behind. For Robinson, one of the most appealing things about rammed concrete is that it brings its own narrative. “We can read our building by the concrete,” he says. “Because we used a dry mix, it was always affected by whatever the weather was doing. If it was a dry day or a wet day, or if it started dry and then got wet, you can read it in a 500mm lift.

What Peter wanted was that unpredictability. The guys can probably point to the walls they did because everybody had a slightly different way of doing it. It’s a testament to them – they’ve handcrafted a building.”

Photos Jack Hobhouse/Living Architecture, Adolf Bereuter, Mole Architects

Ready-mixed concrete using different pigments was supplied, placed and compacted to create reinforced walls two storeys high, the order and location of colours carefully pre-planned. Each layer had to be firm enough before the next was placed to prevent them from fully blending, but still fluid enough so that vibration could be used to avoid discontinuity between the layers. The whole surface was “ground” to further blur the junctions.

Varying the surface texture was used to give the impression of sedimentary layering in the sloping walls of Snøhetta’s Lascaux IV visitor centre in south-west France (CQ 260). Long, horizontal strips of rough concrete were created by sand-blasting the smooth as-struck surface using a stencil – a more precise and controllable process than layering during construction.

At the Islamic cemetery in Altach, Austria, by Bernardo Bader, varying depths of timber boarding were used as formwork, the natural variation between timber planks creating horizontal tonal variation in the resulting surface. As the following projects demonstrate, the logistics of designing pigmented mixes, orchestrating their placement and predicting curing times is quite a challenge, requiring commitment from supplier and contractor. Carefully controlled trials in advance are essential, but it will never be possible to exactly predict the final outcome.

After all, concrete’s free-flowing nature and its inherent tonal variations are an integral part of its appeal – and so they are to be embraced.

Plot A3 building, Lyon

Lyon’s Presqu’île is both its historic heart and home to some of its most interesting new architecture. This spit of land between two of France’s mightiest rivers is now graced by the Herzog & de Meuron-masterplanned Confluence quarter, as well as Coop Himmelb(l)au’s eyepopping, crystalline Musée des Confluences at its southern tip. In this company, an eight-storey office block that doesn’t even have a name wouldn’t usually be of much interest – were it not for its intriguing concrete structure.

The first three storeys of the building on Plot A3, designed by Swiss architect Christian Kerez, are dominated by a series of 90cm-diameter columns that stretch from the exterior all the way through the curtain-walled interiors. What makes these elements so striking is their layered, almost geological aesthetic – a product of the béton estampé casting method.

The columns were cast in situ using formwork of thin vertical timber boards. The concrete was placed from above and tamped down by hand to create layers of 10-20cm. A smaller tool was required to tamp around the columns’ inner ring of reinforcement. Each layer was left to cure for up to two days before the next was added.

The only variation in the mix was the grade of the aggregate, which was all sourced from a local limestone quarry. The use of the columns as such a prominent feature was inspired by the neoclassical architecture of the Presqu’île’s Cours Charlemagne, the western border of the Confluence quarter. “In classical buildings you always have the pedestal at the bottom, which is often heavy, with enormous brickwork elements,” says Kerez.

The idea was to embrace this heaviness by using a structural system that relied on hefty masonry elements. “Because [béton estampé] is less dense and can only be densified by applying pressure from the top, not from all sides, the dimensions have to be much bigger.”

Kerez was happy to let the architecture be led by the engineering, with testing of the concrete focused solely on the structural performance. The result was a grid of columns with an unusually monumental scale: “When we did a mock-up at 1:1, it was really striking how differently a human body relates to the scale of 90cm-diameter columns.”

When it came to the finish, Kerez was again happy to let the architecture be guided by the production process. The columns were left asstruck. “We don’t find it interesting to work from a picture,” he says. “The finish is the aesthetic consequence of a change in fabrication method.

We were just hoping to make it look natural and that it would fit with the buildings from previous times.” When the formwork was removed, Kerez was delighted to find that the concrete had a stone-like quality that echoed the Presqu’île’s traditional buildings. “The pressure from the process is so big that the aggregate is pushed to the outside and it begins to look like a terrazzo. We were quite amazed to see this surface.”

Photos Maxime Delvaux