The public swimming pool “Obermaintherme” in the Bavarian city Bad Staffelstein is Bavaria’s hottest and strongest saltwater emerging from 1600 m deep drilled spring. More than 1.600 m² water surface and more than 15.000 m² sauna are inviting to relaxing and recovering.

Within a large renovation process of some areas in the main bath hall, the inside pool has been renewed, too. A new highlight has been designed with a cave in the shape of a salt crystal made with LUCEM translucent concrete, allowing the new shell to light in different colours along with the water. This ambient lighting concept, along with a special sound system in the Cave integrated with the massage jet nozzles in the water, creates a wellness feeling for everybody inside this installation. The light shines through the surface of the translucent concrete with more than 2 million fiber optics.

The former design of the interior pool included a cave made of natural stone and covered with artificial palms. This design was very popular over decades, therefore, the new design had to be a very special and an impressive one. As part of the modernization of the baths, the cave should visually pick up and become a stylized salt crystal, self-illuminating and bright inside-out.

The architectural firm Krieger Architekten Ingenieure GmbH from the city of Velbert/Germany, saw in the light transmitting concrete a perfect material for this use. Translucent concrete is a very elegant and visually appealing material, with the ability to transmit the light behind through thousands of fiber optics embedded within the panels. As a result, the panels look in one moment like a heavy grounded natural stone and when lit, they shine and glow and look like a hovering light material. The duality of this material gives different ambiance for the space, use and users.

The project manager of Krieger Architekten Ingenieure, Arch. Sonja Baumeister, and her team completely renewed a 170 m² large pool with underwater-jets, a waterfall and a channel as a connection to an existing outside pool. As part of this fundamental renovation, the architects designed for one area of this new pool a cave made of LUCEM light concrete, spanning in the form of a stylized salt crystal about 7 x 5 meters. For the cladding of this monument approx. 200 m² light transmitting concrete were used. Within the double wall construction on a steel frame there is a programmable LED lighting technology, that can be adjusted to different coloured mood lighting

Krieger Architects Engineers with over 50 years of experience in pool construction have realized this project as an absolute novelty – a world first time salt water application for light transmitting concrete. The salt concentration in this pool is one of the highest levels in brine water from local wells. The pool has a salt content of 3.5%, which is comparable to seawater. Accordingly, the steel frame inside the LUCEM installation had to be designed as a structure resistant to seawater. For this reason, hot dip galvanizing has been applied in accordance with DIN EN ISO 1461 in combination with a coating system for corrosion category C5-H – the highest level of corrosion protection, similar to open sea oil drilling platforms – according to DIN EN ISO 12944-5 to be used. The steel frame was manufactured as a welded structure of steel frame, then completely galvanized and then a layer thickness of 320 microns primer was applied. Finally, the steel frame was coated again with at least 320 microns twice, after the installation on site. The coating was carried out by a specialized company.

For the detail planning of the new light transmitting concrete salt crystal object LUCEM appointed Ingenieurgesellschaft Lievens und Partner mbH to be responsible. The first step was to combine the complex design approach with the local details on site and to bring this together in one 3D model. All joints of the concrete shell have been carefully studied and modelled along with the fixing details of each concrete slab including all screws and undercut anchors. Then the anodized aluminum mesh for fixing the LED lighting has been fully designed in this CAD model. From the finished 3D model of a relatively small but complex structure emerged a total of 571 drawings for single metal parts and production details and 130 assembly drawings, added with lists for all fixing materials such as stainless steel screws and concrete anchors for the fixing in the floor.

For an easier communication between client, architects and subcontractors, 15 large overview drawings and 27 sketches explaining fixing details have been displayed on site. To guarantee highest precision, the AutoCAD data files have been directly transferred to the CNC cutting, drilling and welding machines. Through the use of the 3D model it was guaranteed that all parts fit together perfectly, resulting in a smooth construction process on site. The structural engineers from B. Walter Ingenieursgesellschaft mbH, Aachen, calculated the statical properties of the complex steel frame as well as the behaviour of the LUCEM panels. Finally they suggested a thickness and design of the steel frame and a fixing system for the LUCEM panels, which allowed to build the full shell with 20 mm panels and it is allowed to enter the structure with 3 persons.

After installation of the steel frame on a concrete foundation, the outer shell made of light transmitting concrete was installed by using KEIL undercut anchors. Transparent polycarbonate elements in 2 cm thickness have been inserted as spacers between the steel frame and the light transmitting concrete panels in order to minimize the shadow of the frame. In the center of the 240 mm thick steel frame2-sided RGB LED modules were installed. The LED modules have been developed and produced by LUCEM GmbH. The modules have been fully filled with epoxy resin to guarantee a long life despite the high humidity and salinity in the air.

The RGB LEDs can be activated and controlled via software by computer or smartphone – lighting-scenarios ranging from static colours through changing colour gradient to moving colour play synchronously to music can be adjusted. The wiring of the LEDs with 4-pin wire proved to the basement cabinets. The passage of swimming hall to the technical room below the pool are sealed permanently waterproof and had to follow the stringent fire-protection requirements for all 50 cables running through the wall.

After a proper testing of the wiring and the installed LEDs, the inner shell of LUCEM light transmitting concrete panels has been fixed by stainless steel profiles around each panel. All joints were then sealed with flexible natural stone silicone.

With the new light transmitting concrete salt crystals LUCEM presents a unique world’s first design object. The impressive result shows how fascinating public pools can be designed today.

Technical Sheet  

Project: Renovation of Obermaintherme

Client: Zweckverband Thermalsolbad, Bad Staffelstein

Architect: Krieger Architekten Ingenieure GmbH, Velbert

Steelframe 3D Modelling: Lievens & Partner GmbH, Aachen/Germany

Structural Engineers: B. Walter IngenieurgesellschaftmbH, Aachen/Germany

Material: LUCEM LINE White

Format: 120 x 60 cm

Thickness: 20 mm

Surface of Light Transmitting Concrete: 200 sqm

Fixing sytem: Stainless steel in combination with galvanized and 3-times coated steel

Light Technology: RGB-LEDs

Installation Steelframe: Krummholz Stahl- und Metallbau, Coburg/Germany

Cutting of LUCEM panels and installation: efecto GmbH, Weitramsdorf/Germany