RESTORATION OF THE HISTORICAL CONCRETE-GLASS WINDOWS OF THE TOWN-HALL IN AACHEN

Transcription

1 RESTORATION OF THE HISTORICAL CONCRETE-GLASS WINDOWS OF THE TOWN-HALL IN AACHEN Sergej Rempel, Stephan Geßner, RWTH Aachen University, Institute of Structural Concrete, Miesvan-der-Rohe Str. 1, Aachen, Germany Abstract: Textile-reinforced concrete (TRC), an innovative composite material, creates new opportunities for the refurbishment of historic buildings. For example, filigree elements, such as the historic concrete-glass windows of the Aachen City Hall, benefit from the corrosion-resistant carbon textile reinforcement. The 60 year old windows had to be repaired because of the corroded steel reinforcement and concrete spalling. Therefore, the glass stones embedded in the concrete were separated and reinstalled in reconstructed concrete-glass windows. The steel reinforcement was replaced by carbon reinforcement. Thus, the historic monuments protection authorities terms were respected while avoiding future corrosion damage. INTRODUCTION The gothic Aachen City Hall is adjacent to the Aachen Cathedral, the most distinctive building in the imperial city of Aachen. It is one of the most important medieval state buildings in Germany. Erected on the foundations of the King's Hall of Charlemagne from the 8th century, this palace was built for the coronation ceremonies of German kings. The palace has been adapted and restored gradually to comply with contemporary requirements or demands. In 2014, the City of Aachen celebrated the 1200th anniversary of Charlemagne s death, for which parts of the old City Hall were refurbished. The focus of the renovation work was on the Marien tower (fig. 1, left), the main apse of the former Kingdom Hall. Particularly striking features of this structure are the spire and the concrete-glass windows, which should reflect the appearance of the quarry stone walls (fig. 1, right). As part of the reconstruction of the tower in the 1960s due to its destruction in Second World War, 19 unique concrete-glass windows were designed and installed by the architects G. Graubner and U. Fuß. The concrete-glass windows of Aachen City Hall vary in appearance and size, and consist of glass stones embedded in a 3,5 cm thick concrete layer. The distance between the rows of glass stones is about 2,5 cm, reinforced with 5-mm diameter horizontal steel bars. The windows are surrounded by a steel frame, which is required to anchor the windows in the wall and served as lost formwork. In the production of the old concrete-glass windows, the glass stones were positioned on a formwork panel within the steel frame and then concreted. Due to the small concrete cover (around 1,5 cm), concrete spalling occurred and the corroded reinforcements surfaced. Consequently, the refurbishment of the historic win- 359

2 dows was essential. The regulatory constraints of the historic monuments protection authority for the refurbishment of the concrete-glass windows were to maintain the appearance of the original windows. Identical glass stones were to be used, reusing those already used in the old windows. In addition, the position of the glass stones in the windows was required to remain the same. As a consequence, the thickness of the concrete layer (3,5 cm) could not be changed. Thus, the thin concrete cover must be maintained. This excludes a reapplication of the steel reinforcement, which would not be sustainable. The corrosionresistant carbon reinforcement used in the textile-reinforced concrete enables the prescribed concrete thickness of 3,5 cm. Figure 1: View of Aachen town-hall with Marien tower on the right side WINDOW-SETUP AND MATERIALS Fig. 2 shows the setup-up of a concrete-glass window. The load transfer in the windows is uniaxial, parallel to the glass stone rows and concrete rows. At the lateral ends, the loads are transferred via the perforated plate into the frame. By connecting the frame to the masonry, the loads are transferred into the masonry. 360

3 Figure 2: Sectional view of a concrete-glass window Textile reinforcement Due to the thin concrete cover in the old concrete-glass windows, the steel reinforcement corroded and concrete spalling occurred. These damages were to be avoided in the new windows. The required size of the concrete cover could be realized by using textile reinforcement. As textile reinforcement for the concrete-glass windows of Aachen City Hall, a scrim of carbon filaments was used. The cross-sectional area of the textile reinforcement mesh is 110 mm²/m, with 4,18 mm² per roving. To increase the ultimate stress of the roving, the fabric was impregnated with epoxy. By doing so, tensile stress of more than N/mm² can be achieved, corresponding to a tensile force of 12,5 kn per roving. In addition, the impregnation improves the handling and the workability of the reinforcement. The 90- degree direction (fill direction) is the main bearing direction of the window. Since the glass stones enable a continuous installation of the textile reinforcement mesh in only one bearing direction, the mesh was fragmented and the rovings were installed between the glass stones in the concrete rows. The 0-degree direction (warp direction) is not required for the load transfer in the concrete-glass windows. Concrete The composition of the concrete used for the original windows is not known, and could not be reproduced. For this reason, a fine-grained composition commonly used for textilereinforced concrete with a maximum grain diameter of 5 mm was utilized. The concrete has a compressive strength of 87 N/mm² and a bending tensile strength of 10,6 N/mm². The high material strength is particularly suitable for components that have to remain uncracked in use. 361

4 Fastening elements The historical windows were surrounded by steel frames. The frames were used to fix the fastening elements and ensured that the loads were directed into the masonry. The required connection of the concrete to the steel frame was secured by hooks. The existing mechanism for the force transmission into the masonry should be maintained. Only the material of the frame has been replaced by stainless steel to prevent corrosion. In addition, the hooks were replaced by a perforated plate that was welded to the frame. Thus, a steady load distribution should be achieved in the frame. The refurbished windows were attached to the masonry via welded screw sleeves. Therefore, stainless steel screws were used screwed halfway into the sleeves. The other half of the screws was inserted into a modified bearing rail, which was anchored in the masonry. After the installation, the bearing rail and all interstices were filled with color-adjusted mortar. To enable strains, the windows were installed on an expansion strip. MANUFACTURING The manufacturing of the new concrete-glass windows was carried out in accordance to the production process of the old windows 60 years ago. In the first step, the historical windows were documented in detail, as the appearance of the windows was to remain unchanged. Therefore, the windows were surveyed and photographed; the glass stones were labelled. In the next step, the historical windows were carefully disassembled. In this work, the damage of the historical window became obvious. After the steel frame was dismantled, the concrete crumbled after simply tapping on it. This had advantages in that the majority of the glass stones remained intact and could be reused. This step revealed that the damage was severe and renovation was urgently needed. Simultaneously, custom-fit anchoring rails made of stainless steel were manufactured. These replaced the steel frame, but without completely surrounding the windows, as the steel frame had done. Instead, the anchoring rails were installed only on the bearing side of the window. The possibility to realize sharp-edged concrete components and the modified load transfer enabled the omission of a surrounding frame. Thus, the 2-meter high windows could be divided into portable segments, and then be mounted standing one above the other on-site to be installed without lifting equipment. This procedure had advantages in both the manufacturing as well as in the assembly. The sharp edges make the stacked segments look like a monolithic created window. The bigger spacing between some glass rows were already in existence in the old concrete-glass windows. 362

5 Figure 3: Concrete-glass window under construction with about 400 glass stones In the next production step, the glass stones were glued to the formwork with acrylic (up to 400 glass stones per window segment). Here, each glass stone had a specified position, corresponding to its position in the old window. The conglutination guaranteed the position of the glass stones during the compaction work, and ensured that no concrete covered the glass stones. The position of each glass stone was checked with a true to scale plan of the old windows on a transparency. Before concreting, the carbon reinforcement was built in and fixed on the perforated plates to ensure central positioning during concreting. No spacers were used. The placing of the concrete was done in the casting process; a table shaker was used for compacting. The stripping of the formwork had to be done very carefully, as the acrylic s adhesion was very strong. Finally, the windows were cleansed of the acrylic and the concrete surfaces were acidified. Finally, the window segments were reinstalled in Marien tower. For this purpose, bearing rails were installed in the masonry, into which the concrete-glass windows were inserted. In the final step, the bearing rails were plastered with a mortar of a similar color. 363

6 Figure 4:View of the inside of Marien tower (left) and installed concrete-glass windows seen from outside (right) CONCLUSION The refurbishment of the historic concrete-glass windows of Aachen City Hall was an extraordinary challenge. The use of innovative carbon reinforcement contributed significantly to the solution of the problem. With the use of epoxy-impregnated carbon textiles and high performance concrete, a corrosion-resistant solution was found, which also impressed the critical monument conservators. ACKNOWLEDGEMENT The authors thank the DFG and the BMWi for the financial support during SFB 532 period and the transfer project T08. A special gratitude goes to Hering Bau GmbH & Co. KG and solidian GmbH for their commitment to support the development of the new composite material TRC. 364