Saturday02 August 2014

Undulating concrete wall shows potential of rubber formwork

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Walter Jack’s Crushedwall celebrates Cornish mining heritage

Designer Walter Jack Studio
Location Pool, Redruth, Cornwall
Completed April 2012

Walter Jack Studio has completed a 40m-long undulating concrete wall to mark one of the entrances to the Heartlands project in Pool, Cornwall, an 8ha outdoor visitor attraction that celebrates the area’s mining heritage.

“Heartlands sits over a very special geology,” says artist Walter Jack. “The 19th century miners dug an astonishing 1,000m down through granite to extract tin ore. When we were asked to design a retaining wall, the geological connection seemed serendipitous. Retaining walls hold back the geology — they are the boundary between our above-ground world and the world of mining.”

The project started with a sheet of rubber, some plywood and a cement mixer in Jack’s back garden — inspired by watching builders pour concrete on to a crumpled damp-proof membrane while building his house extension.

Jack decided to scale up the effect of these soft folds, using a single sheet of rubber, 6mm thick and glued into one long 40m roll weighing about a tonne. Using an agricultural machine, the rubber was spooled off and bolted to plywood backing by hand, using padded wooden washers to prevent it from tearing.

The wall changes in height along its length, with the rubber hung like a curtain at one end, and the 4m height getting progressively more crumpled down to 1.5m at the lower end, making the surface “much stormier”, as Jack says.

Layers of expanding polyur-ethane foam were sprayed on to the rear of the folded rubber and long polystyrene fins attached to strengthen the formwork. It was then sliced into six pieces and the edges reinforced with fibreglass to ensure the folds would remain under the hydrostatic pressure of casting.

The six shutters were then taken from the barn in Gloucestershire, where they were made, to Ladds Concrete, only half a mile from the site. The pieces were cast using Bardon Cemflow, a free-flowing self-compacting mix of an architectural grade, used to achieve very high-quality finishes.

The concrete flows into place, avoiding the need for further compacting, which would have altered the fluid folded shape. It also forms a high spec surface, eliminating extra remedial work and post finishing.

Each piece, weighing around 20 tonnes, took three to four weeks to cast and was then delivered to site and craned to place onto 1m-deep foundations, with the whole wall assembled in just one day.

Creating the formwork

Building the wall used 30m of formwork from plywood and a long sheet of rubber, which was crumpled and folded to create a giant jelly mould in which the concrete was set.

Walter Jack Studio

Source: Simon Burt

The formwork was then cut into six sections and transported to Ladds Concrete where the concrete was poured into the moulds.

Shaping the wall

Walter Jack Studio

Source: Simon Burt

A 40m roll of 6mm-thick rubber sheeting was pinned back to plywood shuttering by hand, using padded wooden washers to prevent it from tearing. A scale model was used to map out the principal contours, with extra folds added by hand.

Walter Jack Studio

Source: Simon Burt

Several layers of expanding polyurethane foam were applied to the rubber mould to strengthen the formwork.

Walter Jack Studio

Source: Simon Burt

Polystyrene fins were embedded into the foam to provide further stiffness — shown here as the formwork was removed.


Designer Walter Jack Studio
Client Heartlands
Concrete and reinforcement Ladds Concrete
Structural engineering Structural Solutions
Lifting South West Cranes
Landscape design Cathy Lowe, Cornwall Council
Surveying Dando Surveying
Concrete consultant David Bennett


Readers' comments (1)

  • Alex Henderson

    This is what architecture is about! Taking the simple aspects of a structure or building and designing them in such a way to make them interesting.

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