Focus: Lit Concrete

A light-transmitting concrete that also resists corrosion has been used uniquely to full effect in a spa’s thermal pool

Words by Francis Pearce

The very idea of solid concrete continues to be challenged – or cheated – by means such as incorporating fibre optics into the material to make it translucent. Light transmitting concrete that also resists corrosion has been used to create a first-of-its-kind, self-illuminating and colour-changing shell for a thermal baths in the Bavarian city of Bad Staffelstein. Krieger Architekten Ingenieure renewed a 170 sq m pool at the Obermaintherme with underwaterjets, a waterfall and a channel as a connection to an existing outside pool.

As part of the project, architect Sonja Baumeister designed a cave for the pool with a galvanised steel frame clad with 20 sq m of light-transmitting Lucem concrete to create a stylised salt crystal. Within its doublewall construction some two million embedded optics carry light from programmable LEDs. The result is a structure of panels that appear to transform from heavy natural stone to a surface that is light in both senses.

The design process was complex and included Ingenieurgesellschaft Lievens und Partner CAD modelling the anodised aluminium mesh for the LED lighting.

Transparent polycarbonate spacers 2cm thick were inserted between the steel frame and the light transmitting concrete panels to minimise the shadow of the frame. The concrete can be lit with single or gradients of colour that can change to music. Although the structure measures just 7m x 5m the finished 3D model generated nearly 600 drawings for parts and production details and 130 assembly drawings.

The water in the giant Obermaintherme pool wells up from 1,600m below ground at Bavaria’s hottest and saltiest natural source; at 3.5 per cent its salinity matches that of seawater. Corrosion was prevented by means including filling the LED modules with epoxy resin and all the joints with flexible natural stone silicone.

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