Two tales of architectural concrete
Two construction sites show how different architectural-concrete requirements are planned in detail and implemented in practice
Data & Facts
- 1. Suhrkamp-Verlagshaus, Berlin,
- 2. Baden-Wuerttemberg Cooperative State University, Stuttgart
- 1. Bundschuh Architekten
- 2. 3XN, Wenzel + Wenzel
- 1. Ernst G. Hachmann GmbH, Berlin
- 2. Bundesbau Baden-Württemberg, Schwäbisch Gmünd
- 1. KoHa Bauausführungen und Immobilien GmbH, Berlin,
- 2. Ed. Züblin AG, Stuttgart
- MEVA systems
- Engineering and support
- MEVA Schalungs-Systeme GmbH, Germany
Cleverly adapted formwork
Concrete is the material used in building construction. The mixture of aggregates, binding agents and water is ideal for different applications in the construction industry, above all due to its strength properties. However, textured wooden facings with different absorption properties often give the concrete surface a somewhat rustic appearance.
With the development of modern concrete recipes it has become possible to pour higher, more filigree and densely reinforced components. In addition, wooden facings with different coatings are an option to achieve good concrete finishes. All-plastic facings such as alkus also produce excellent results when used in applications with high cycle rates.
Form, colour, joints
Besides choosing suitable materials, the coordination of all parties involved also plays an important role when working with architectural concrete. In order to ensure that architects, principals, planners and construction companies share a common vision, design characteristics such as form, texture, colour, joint patterns, etc. need to be defined as precisely as possible beforehand.
Can they be planned? Absolutely!
With the objective of clearly defining the final appearance, international associations and organisations have produced their own guidelines for the purpose of creating a uniform planning process. The German Concrete and Construction Engineering Association (DBV) provides important information on this subject in a fact sheet. Similar regulations in other countries serve the same purpose. The DBV classification into four architectural-concrete classes provides an overview of the requirements ranging from low to high as well as the corresponding characteristics. For special design requirements such as those found in façade construction or representative elements, the DBV recommends defining the desired architectural-concrete finish using a sample surface. This enables the quality that can be produced to be determined under the actual conditions found at the construction site. The technical procedure required to produce this architectural concrete is thus developed and guaranteed in the long term. When doing so, the corresponding effort must be taken into account, especially for high-quality concrete finishes.
Art in the building industry
However, the main priority is not always to make the level concrete surfaces smoother and more uniform. Architects often skillfully use architectural concrete as a design element when planning new construction projects. The discussion on whether modern buildings transcend the border between housing and art can be left to the experts. But one thing is certain: the implementation of increasing design requirements requires experience and a large degree of skill on the part of the construction companies.
Stuttgart: Curved and smooth
On the basis of two construction projects in Germany, it is possible to demonstrate varying manifestations of the requirements on architectural concrete. In Stuttgart the Engineering Faculty of the Baden-Wuerttemberg Cooperative State University (DHBW) is presently under construction. In the middle an atrium made of SB4 architectural concrete projects its light onto a spiral staircase that meanders upwards from storey to storey with lateral offsets. The challenging geometry is thus supplemented by special requirements on the concrete finish. This has to be exceedingly smooth with a uniform hue as well as a minimum number of construction joints.
Berlin: Straight and matt
With the new office building for the German publishing company Suhrkamp Verlag in Berlin, a counterpart in architectural-concrete terms is being created for which the requirements could hardly be more different. The new publishing house is dominated by wide façades made of grey, roughly formed architectural concrete from which generously glazed office storeys branch off on both sides. The required concrete finish differs in detail from the familiar architectural concrete classes and stands out, above all, through matt surfaces with few cavities.
Single-sided formwork, two-sided tying
The requirements on the formwork joints and the frame imprints for the construction of the building in Berlin were precisely defined beforehand and meet the special stipulations for architectural concrete defined in the DBV fact sheet. The various surfaces on the sides of the L-shaped Suhrkamp building are to be 1.80 m high throughout. The position of the tie holes was also precisely specified in order to produce a uniform appearance over the entire height. However, these surfaces are not load-bearing walls but rather pure façade elements. As a result, they had to be produced using single-sided formwork. In order to integrate the desired tie holes into the architectural-concrete pattern, they were already taken into consideration during the initial planning. “As a precaution, empty anchor sleeves were inserted into the load-bearing walls of the building to replace the missing tie holes. The wall itself thus acted to a certain extent as initial formwork, making two-sided tying possible,” explains MEVA engineer Patrick Schmidt.
Cleverly adapted formwork
“The height and width of the specified joint pattern didn’t match any of the formwork systems on the market,” stresses Patrick Schmid with a glance at the model on his computer. In addition, the imprint of the frame formwork in the joint pattern had to be prevented by doubling up the facings at the joints where the panels meet. “I have been working almost exclusively with MEVA formwork systems for 12 years now because I can rely on its formwork and competent staff,” says construction manager Hartmut Matthäus. “Thus, if I have the choice, I know what I would opt for.” KoHa Bauausführungen und Immobilien GmbH decided to use StarTec wall formwork and adapted it to suit the specified tie hole and joint pattern. “We achieved the defined tie hole positions by drilling holes in the facings. The formwork was attached over its entire length using alignment rails,” explains Hartmut Matthäus. “This ensured that the flatness tolerance of 0.5 mm was met,” adds MEVA engineer Patrick Schmidt. In addition, the drilled alkus all-plastic facings can still be reused, as even large holes can be repaired using the same material.
As if cast in one piece
In Stuttgart the spiral staircase of the Baden-Wuerttemberg Cooperative State University meanders upwards from storey to storey with lateral offsets. Thus, there is no getting around MEVA’s Special Design Office. This is where the team led by Jochen Moosmann works. He explains that the detailed planning is primarily manual work. “The formwork consists of individual solid bodies which have to be designed in three dimensions and then split up again so that every curve and every angle are a perfect match,” says Jochen Moosmann. To build the spiral staircase, it was necessary to insert a joint between the ramp and the balustrade,” adds his colleague Alexander Schmid. “This harmonises well with the overall appearance of the staircase.”
Turning visions into reality
As the staircase is made of white concrete according to the specification for architectural concrete class SB4, it will look as if it has been cast in one piece upon completion. Especially for this light colour the DBV recommends planning the component geometry so that it can be poured easily and quickly in order to produce a uniform hue. Complicated component geometries should be avoided to allow the formwork tie rods to be tightened evenly. For a curved form this is correspondingly complicated. “We implement the architects’ visions,” says Alexander Schmid. “Here, we are bound by the specified form and have to consider how this can be best poured in individual parts and how we can support, tie and strip these.” Thus, the ramp and the balustrade of the staircase were formed separately. Only a low fresh-concrete pressure acted on the flat formwork of the ramp, allowing the dimensional accuracy to be met without tying. By tying above and below the balustrade, this also remained free of tie holes.
Keeping in shape
As the individual flights of stairs overhang the storeys and are offset relative to each other (see figure on page 8), a holistic concept to support the individual components was required. “Using the MT 60 shoring tower, we built a number of platforms in order to precisely support the special formwork,” explains Bernd Schwendeman, the construction manager for Züblin AG in Stuttgart. “Thanks to the detailed planning of the individual parts, it was possible to assemble the special formwork almost as easily as the shoring tower itself. MEVA really did a brilliant job here.” The constituent parts of the special formwork were planned precisely to the nearest millimetre and exactly labelled according to the construction plan in order to facilitate the assembly. The MT 60 shoring tower convinces with its lightweight parts, each with a maximum weight of 15 kg, and can be assembled both lying down and upright.
More than just black and white
In Berlin the architectural-concrete façade of the new building was designed to appear rough and vibrant. To achieve this, the focus was on producing a non-uniform colour. Various options were tested upfront in order to produce the desired result. In the end, the decision was taken to use an absorbent facing that produces a darker surface. At the same time, the absorption of the excess water creates a structured surface that is almost completely free of pores and cavities.
Spick and span
Deposits and impurities cause discolouration and uneven surfaces. Hence, when the requirements on the architectural concrete are high, the facing is usually thoroughly cleaned after every use. Thanks to clever planning, some of the components in the special formwork in Stuttgart were reused several times. “In the event of damage, we can still grind, recondition, clean and thus reuse the panels several times,” reports construction manager Bernd Schwendemann.
In Berlin, however, the doubled-up facing of the rough concrete façade could only be used once in order to meet the defined requirements across the board. Hence, it was not necessary to clean the facing afterwards. “If the absorbent wooden facings had been reused, unwanted discolouration would have occurred,” explains construction manager Hartmut Matthäus. “In order to protect the elaborate architectural-concrete surfaces, we thus leave the facing on the concrete during stripping to avoid scratching and denting during construction.”
The two works of art made of concrete can be seen at their respective locations. On completion of the respective construction work, the capital of Germany and the capital of the federal state of Baden-Wuerttemberg will both be all the richer for an architectural masterpiece.