Architectural Concrete with UHPC for façades and interior design-recent application in Germany

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1 Architectural Concrete with UHPC for façades and interior design-recent application in Germany Thomas Drössler 1 1: Benno Drössler GmbH & Co. Bauunternehmung KG, Siegen, Germany The building company Benno Drössler has been using UHPC as a building material since This paper includes a report on three projects which are recent examples of the application of UHPC. 1. New construction of the headquarters of the Ferchau company in Gummersbach. In this project, about 900 m² of UHPC fair-faced concrete elements have been installed in a curtain wall construction. These concrete elements are about 4 cm thick and 3.0 m long, with the height spanning from floor to floor. 2. Cloakroom steles in the Bible Museum Frankfurt, Main These cloakroom elements, which measure 2.95 m x 1.05 m x 0.05 m, had to be clad in fair-faced concrete allround, which was achieved by casting the concrete into the vertical formwork gap from the narrow 5 cm wide side. 3. Staircase exhibit by Dyckerhoff AG made of white UHPC for the Bau 2011 trade exhibition in Munich: The angulated structure of this staircase, which is only 29 mm thick, has been produced using white UHPC. Subsequently the precast concrete elements were bonded between two laminated safety glass cheeks using an epoxy resin adhesive. Keywords: façades, structural analysis, design, production 1 New construction of the Ferchau headquarters in Gummersbach: fair-faced curtain wall in UHPC concrete Design The design for the new Ferchau headquarters on the old Steinmüller site in Gummersbach was produced by Gerber Architects, Dortmund, and features a basic rectangular footprint. The rooms are grouped around a central inner courtyard, which is open at the top, and an enclosed four-storey entrance foyer. The openness on the inside contrasts with the austere glass and fair-faced concrete element patterns of the façade. The fair-faced concrete elements consist of approx. 3.0 m high pilaster strips and approx m long parapet elements, with a total of about 1,120 individual parts (the area measures about 900 m² in total), forming the main design element of the outer façades as well as the façades around the open internal courtyard and the entrance foyer (see Figures 1 and 2). Figures 1 and 2: Elevation of north façade and inner courtyard

2 Concept and detailed design The architects Ahlbrecht, Felix, Scheidt, Kasprusch were in charge of the façade design, the overall detailed design and site supervision. According to the scheme design, the façade specification called for a conventional curtain wall façade with a thickness of up to 12 cm. The grey colour of the fair-faced concrete was intended to blend in with the neighbouring buildings on the Steinmüller site, which were also built in fair-faced concrete. However, during the initial discussions with contractors, the following difficulties emerged: Firstly, it is very difficult to reach the inner courtyard and, especially, the covered four-storey foyer which has conventional, and hence very heavy, precast concrete components. Secondly, the space available for the façade panels on the very slender shell structure is very limited and is further encroached upon by bonding work at the window areas. This makes the installation of precast concrete curtain wall façades with conventional fixing systems very difficult. One solution to the problem is to minimise the thickness of the concrete panels in order to facilitate the installation in the areas with difficult access (the inner courtyard and roofed-over areas) and in order to reduce the width required for fixing the panels on the supporting structure. Having seen sample panels, the architects and the client opted for a design with 4 cm thick elements made of smooth, grey UHPC with an identical surface finish as conventional fairfaced concrete (see Figure 3). Figure 3: Elevation of roofed-over entrance area (internal) Technical details The façade elements are produced and monitored in accordance with DIN V The structural calculations for the façade and supporting structure are based on DIN 1055 and DIN In order to comply with structural requirements, the concrete material must have high bending tensile strength and be tested accordingly. In this case the the structural calculation demanded a present bending tensile strength >3.1 N/mm² according to DIN , 12.99

3 Architectural Concrete with UHPC for façades and interior design-recent application in Germany including a 75% statement of probability and a triple factor of safety which was verified by a test certificate according to DIN V and DIN EN No reinforcements or fibres were used. The elements are fixed to the shell structure using anchor pins and specially dimensioned holding and supporting brackets in stainless steel (see Figure 4). Figure 4: Façade fixing detail Production Owing to the large number of elements required it was possible to produce 20 to 30 elements per production day. The elements are cast in prefabricated forms which are placed horizontally on steel tables. Once the panels have been removed from the forms using vacuum lifting equipment, they are placed in specially designed boxes and stored where they are protected from the weather. Then they are secured for transport and shipped to the building site. Characteristic properties of UHPC Self-compacting UHPC is a very sensitive construction material which during mixing and in production requires nearly constant environmental conditions and production processes. The very dense surface shows a very accurate imprint of the formwork surface. Furthermore, some tests with different forming oils have revealed reactions with resulting discolourations of the concrete surface. The average tensile strength and compressive strength of the UHPC / HPC varies according to the composition of the concrete recipe and the requirements of the special projects. For the structural calculation of façades elements according to DIN V and DIN , the bending tensile strength and the ultimate anchor load are relevant and verified by test certificates. Installation of elements Once on site, the holes for the anchor pins are drilled, the elements are lifted into place on heavy-duty scaffolding, using suitable lifting apparatus, and they are then fixed manually to the shell structure with the anchor dowels. The reduced weight of the elements made it much easier to reach and clad the façades of the inner courtyard and the roofed-over four-storey entrance foyer (see Figure 5 to 7).

4 Figures 5 and 6: View of corner installation and completed corner Figure 7: View of north-western façade corner Project details: Building owner: FFI Frank Ferchau Immobilien GmbH Scheme design: Gerber Architects Façade design, detailed design and site supervision: Ahlbrecht, Felix, Scheidt, Kasprusch Structural calculations, production and installation: Benno Drössler GmbH & Co. Bauunternehmung KG

5 Architectural Concrete with UHPC for façades and interior design-recent application in Germany 2 Cloakroom steles in the Bible Museum Frankfurt, Main Design As part of the re-design of the new reception and cloakroom area in the Bible Museum in Frankfurt, Main, the architect Peter Harroider included 8 concrete steles measuring 2.95 x 1.05 x 0.05 m with a fair-faced finish on all surfaces (see Figures 8 and 9). Figures 8 and 9: View of extension to the Bible Museum and Interior Technical details and production The architect s requirements (thickness of only 5 cm and fair-faced finish on all sides) can only be fulfilled with self-compacting UHPC concrete. The material is filled into the vertical formwork from the 1.05 m high and 5 cm wide side. The 5.0 cm wide side, which is used for filling, is slightly larger; this makes it possible to grind down this side and thus achieve an identical appearance to the other sides with the smooth formwork pattern. Originally, the design called for a very dark, anthracite-coloured surface. During production tests it was discovered, however, that the pigmented UHPC is very prone to form pores and cavities when cast in vertical position. It was therefore decided to produce the steles with a nano-scaled white cement in natural white. The mixture does not contain reinforcements or fibres (see Figure 10). Figure 10: Interior view of detail Projects details: Building owner: Frankfurter Bibelgesellschaft e.v. Design: Peter Harroider, Dreieich Distribution and structural certificates: durcrete GmbH, Limburg Production: Benno Drössler GmbH & Co. Bauunternehmung KG, Siegen

6 3 Staircase exhibit by Dyckerhoff AG made of white UHPC for the Bau 2011 trade exhibition in Munich Design The design of the staircase exhibit by Luis Ocanto-Arciniegas (Ourstudio Architecture and Design) is intended to demonstrate the capabilities and yet lightness of the new material UHPC. The monolithic angulated structure of this filigree precast concrete staircase is only 29 mm thick. The angulated structure of the stair flight and landing have been bonded between two 20 mm thick staircase cheeks of laminated safety glass. The exhibit is 4.0 m long and 2.0 m high and comprises three parts: the lower flight, the landing and the upper flight. Technical details and production The intention was that, if possible, the angulated structure of the staircase and landing would have a smooth surface like that of the formwork. This meant that the steps of the staircase had to be cast in vertical position, with material poured in from the 29 mm wide side and the face on that narrow side being ground smooth afterwards. As the opening for filling in the concrete was very narrow, a new mixture of self-compacting UHPC had to be developed. Using the Nanodur Compound 5941 (white), a fine-grained mixture with a proportion of 1 volume % of micro steel fibres was developed. The micro steel fibres are intended to increase the ductility of the material and counteract stresses in case of cracking. In this fine-grain mixture, the 0/2 mm pit sand and 2/5 mm split material is replaced by < 0.5 mm quartz sand. The material is cast monolithically in vertical formwork (see Figure 11). Figure 11: Vertical concrete casting process Bonding the glass staircase cheeks The required tensile bending strength is reached after a curing period of seven days. After that period it is possible to attach the laminated safety glass cheeks with epoxy resin in order to stabilise the staircase and provide a safety structure. Load test In order to verify the loadbearing capacity of the structure, which was calculated as part of a theoretical feasibility study, a load test was carried out on a test sample. For this purpose, a load of 100 kg was applied to each step and a further weight of 100 kg was suspended from the left-hand and right-hand glass cheek (see Figures 12 and 13). Once this load test had been passed successfully, a further test was carried out in which the test sample was loaded until breaking point was reached. To do that, two precast concrete components with a total weight of

7 Architectural Concrete with UHPC for façades and interior design-recent application in Germany approx. 7.4 tonnes were placed on a timber structure which was installed on 4 steps. Even under this weight of 7.4 tonnes, the staircase did not develop any cracks in the angulated concrete structure or the glass cheeks or show any failure of the bonded joints (see Figures 12 and 13). Meanwhile another load test was carried out at the TU Dresden where the glued connection collapsed under the weight of approx tonnes (approx. 2 tonnes per step). The scientific analysis is not yet completed. Figure 12: Load test with weights Figure 13: Load test with precast concrete components Exhibit at the trade fair The three staircase elements were assembled to form the staircase at the trade fair stand. With its combination of UHPC and glass it offers attractive design options for the future, especially using the new bonding technology for joining (see Figure 14).

8 Figure 14: Staircase exhibit at the trade fair Project details: Idea and Nanodur Compound 5941: Dyckerhoff AG Design: Ourstudio, Dortmund Structural considerations: G.tecz, Kassel White laminated safety glass: Schott AG, Mainz Prefabricated staircase components and bonding: Benno Drössler GmbH & Co. Bauunternehmung KG, Siegen 4 Conclusions The examples of UHPC concrete applications given here are only a small part of the visionary potential of this material. However, further research and development will be required in order to develop the full potential of UHPC and make it a realistic choice for commercial and industrial applications. Further standardisation of applications will only be possible with the close cooperation of research establishments, together with representatives from industry and users.