STONE FACADE CASE STUDY

Photos courtesy of HOFMANN FACADES

Project: Kulmbach Hospital
Location: Kulmbach, Bavaria, Germany
Architect: H2M Architects and Sander Hofrichter Architects

The Project: Kulmbach Hospital is a progressive and successfully run municipal hospital of care level II with 500 beds and 13 bed-managing departments, as well as centers for geriatric traumatology, breast cancer, colon cancer, endoprosthetics, and diseases of the spine. It has undergone a multiphase construction and upgrade project overseen by H2M Architects and Sander Hofrichter Architects. The work includes more than 120,000 square feet of facade area being designed and installed to create a cohesive appearance to this large health-care facility.

The Challenge: The facade design is based on a series of windows that are surrounded by opaque limestone. This geometry means that the limestone is actually a series of three-dimensional cruciform shapes jointed together in the field from a scaffold. This poses a major challenge to the fabrication tolerances and field tolerances. It is almost impossible to achieve this form without visible out of plane offsets at the vertical, horizontal, and diagonal joint lines between the sections and in the center where the pointy edges of eight surfaces meet. The low bidder on the project determined that it could not fabricate this shape within acceptable tolerances, and the project was turned over to the second-lowest qualified bidder.

The Design Solution: In solving this problem and to minimize the issues of fabrication and field tolerances, two different approaches were chosen. One was to divide the entire cruciform in two equal parts and carve each piece out of a solid block of stone. The other was to fabricate eight flat limestone panels but to join them together in the factory as a unitized mega panel held together by either a strong back steel frame system or a precast backing system made of ultra-high-performance concrete (UHPC).

The dimensional stone approach, while extremely accurate in its dimensions, proved to be very costly due to high waste factor in limestone cutting the elements directly from a block using expensive machinery. This approach required two lifts per cruciform element. The strong back approach proved to be cost-effective in the fabrication of the stone, but it was very fragile and difficult to transport, leaving large risk for individual panels to get damaged or broken. It also proved to be very costly to set the panels accurately in the shop, achieving tight panel-to-panel tolerances. This approach was therefore quickly abandoned.

The most cost-effective, low-risk solution able to meet the original design intent was the UHPC limestone veneer. Eight flat limestone panels are integrated into one cruciform shape of UHPC backer panel. This solution reduced the risk of breakage during transport, offered precise control of joint sizes, allowed for sanded silicone joints finished in the factory, and created an easy installation using a crane with one lift per panel. The weight of this approach is slightly heavier than the strong back option but more lightweight than the dimensional stone approach.

Most important in this approach is the thermal separation between UHPC and limestone to prevent cracks from thermal expansion and contraction due to the different thermal expansion properties of limestone and concrete. This is analogous to conventional precast stone veneer with a bond breaker and a minimum of three mechanical anchors for each stone panel.

The Results: The hospital is now covered with a tightly and precisely jointed silicone sanded limestone facade with unitized cruciform shapes. This allowed for an accelerated construction program and faster turnaround on-site. The remaining column and transom areas in the facade were still set by hand from a scaffold. For the next phase of hospital expansion, these areas will also be assessed for a unitized approach so that a scaffold can be entirely omitted.

Sintered Stone Facades

Sintered stone is a proven material that has been used on facades both in the United States and in Europe for some time. Sintering refers to the atomic diffusion of particles, which occurs most quickly at higher temperatures. Applying this to sintered stone produces a thin, lightweight, and very strong material with properties similar to, but better than, porcelain ceramic tile. The primary difference is that sintered stone products are made from selected natural minerals and only the atmospheric humidity that they contain. The manufacturing process relies on the use of natural raw materials that are reduced to fine powders. The powders are then arranged by color and pattern and subjected to pressures on the order of 15,000 psi. The resulting thin slabs are then fired in a kiln producing large-format panels that can be 4 feet by 12 feet or 5 feet by 10 feet, among others. Thicknesses are generally available in ¹⁄₈ inch, ¹⁄₄ inch, and ¹⁄₂ inch.

Visually, sintered stone panels can be created that look like natural stone, a solid surface, textured surfaces, or custom looks in a wide range of colors. As a very dense, lightweight, and sustainably produced material (at least one manufacturer is carbon neutral as of 2019), sintered stone has been regarded by many architects worldwide as a facade material of choice suitable for the most demanding exterior building projects, whether commercial or residential. Its physical properties make it a high-performing, low-maintenance, easy-to-clean product that resists scratches, abrasion, graffiti, and ultraviolet light. It has been tested as a noncombustible product that does not propagate fire, heat, or smoke. It also exhibits a very low porosity (less than 0.08 percent), making it virtually waterproof without the need for additional coatings or sealers. As such, it can contribute directly to the longevity and resilience of the building facade.

Photo courtesy of Neolith®

Sintered stone panels provide a lightweight, durable, easy-to-work-with facade material that is available in a wide range of colors, textures, and finish treatments.

From a construction standpoint, sintered stone panels are lightweight and easy to work with. They can be securely applied relatively quickly, creating efficiencies in the construction process compared to other materials. Specifically, for facade systems, there are three common methods of attachment:

• Exposed mechanical cleating: Recognizing the common desire for providing ventilated rainscreen style facades, sintered stone panel manufacturers have worked to provide systems to suit these situations. Comprised of a self-supporting metal structure originally designed to support ceramic tiling in different formats and thicknesses, it has been refined to suit sintered stone panels too. This approach is based on a visible mechanical fixing system comprised of supports made from vertical T- or L- shaped profiles and safety clamps to which the sintered stone panels are attached.
• Hidden chemical fixation: In cases where concealed fastening is preferred, hidden support systems are available that use a longitudinal elastic chemical adhesive with vertical aluminum supports. Those supports can be in either T- or L-shaped profiles depending on whether they are used to coincide with the joints between the sintered stone panels or to reinforce the center of them.
• Hidden mechanical (hybrid) fastening: At least one manufacturer has created a mixed (chemical and mechanical) hidden profile system that works due to the pressure created by the system in the rear of a sintered stone panel. This is achieved with a double groove at opposing 45-degree angles (like a dovetail in carpentry terms), where two aluminum profiles are inserted and fixed with structural adhesive to secure the channels to the panels. The connected channels are then secured to the aluminum support system holding up the whole facade. The panels can hang and be levelled either to align with adjacent panels or be staggered without needing to increase the number of vertical supports. In all cases, this method of attachment allows the support system to become invisible due to the concealed fastening and aluminum members. It also allows for easy removal and replacement of panels if ever necessary.

In addition to the above, there are other aspects of sintered stone that make it an exceptional choice for facades in urban environments. A significant new coating process (supplied by a third party) is now available to reduce outdoor air pollution. This coating uses an additive with titanium dioxide in a nanoparticle-based treatment that can be specified and baked-in during the manufacture of the sintered stone slabs. The treatment uses the process of photocatalysis in an ongoing chemical reaction between the added titanium dioxide and sunlight falling onto the facade. When sunlight (or some LED lights) shine on the surface, the titanium dioxide particles are activated using the light energy to transform moisture in the air into oxidizing agents. This destroys nitrogen dioxide particles and other contaminant compounds, and chemically transforms them into harmless water vapor and salt. This photocatalysis process is repeated millions of times per second until all contaminants are destroyed, meaning the surface as well as the surrounding air are constantly being self-treated and cleaned.

Tahir Demircioglu, principal architect of the firm Builtd, chose treated sintered stone panels for a facade in a recently completed New York skyscraper. He explains, “The initial selection of the sintered stone panels was for their lightness and aesthetic qualities, but once we found out that they came with this air-cleaning element, we said, ‘Of course!’ It is a huge plus that these treated panels also limit the amount of maintenance required. Instead of using chemicals and power-washing the facade, the material is hydrophilic, so it cleans itself.”