Textile-reinforced concrete roof shells with a component thickness of just 2cm
Fits like a glove
A demonstration building made of textile-reinforced concrete shells was built once again at RWTH Aachen University. Wheras last year the material thickness of the textile-reinforced concrete pavilion of the Fuculty of Civil Engineering was 6 cm, it is just 2 cm now.
The textile-reinforced roof elements of the new bicycle parking area of the Institute of Textile Technology (ITA) have an extraordinary delicate appearance with dimensions of 4.40 m x 2.14 m and a component thickness of 2 cm. The innovative building material textile-reinforced concrete makes this possible, which is the material the scientists of the two university institutes, the Institute of Solid Building (IMB) and the Institute of Textile Technology (1TA), have been dealing with tot quite some time. As part of a current research project at RWTH Aachen, they have now been developing in cooperation with a precast component manufacturer a method to manufacture and assemble single-curved textile-reinforced precast components. „The positional accuracy of the textile reinforcement layers made of carbon fibers is in the range of millimeters owing to the minor component thickness of the shell, constituting a new dimension tot structural concrete products!" as Alexander Scholzen reports. He accompanied the project as scientist of the IMB and received his doctorate in textile-reinforced concrete structures under Prof. Josef Hegger. In this context, he calculated the necessary shell thickness according to the finite element method, and in accordance with the findings he dimensioned the same very slender.
The reinforcement scrim made of thin carbon fibers looks like black strands of halt crossing each other with knots being fixed with white string. The textile scrim, that was developed at ITA headed by Prof. Thomas Gries, actually consists of high-strength carbon fibers. The key characteristic of the textiles used for the precast components developed is the aspect that a textile reinforcement does not corrode and consequently does not require high concrete covers. In addition to that, the reinforcement can be adjusted to single-curved building component geometries.
Subsequent research utilized
The research project utilizes the findings of the former special research project carried out on the issue of „textile-reinforced concrete" ai the RWTH Aachen University as well as the experience gained last year in the realization of the textile-reinforced concrete pavilion of the Faculty of Civil Engineering. Dr. Rostislav Chudoba, as head of the working group at the Institute of Solid Building, supported together with Mr. Scholzen both projects in structural terms. The remarkably minimized material thickness of the filigree roof elements is the significant difference between both projects. Whereas the thickness of the umbrella-like structural shells of the textile-reinforced concrete pavilion still was 6 cm owing to the large span of the future room for seminars and events measuring 7 x 7 m, the thickness of roof shell for the bicycle stand roofing amounts to just 2 cm consistently. The concrete surface has another visual appearance too. Although a high-strength fine concrete was used in both cases, the first demonstration building - the pavilion - has a somewhat coarser character, as the result of the even finer maximum grain size. Due to the fact that the roof shells could be manufactured with a PVC formliner in a precast concrete plant, it was moreover possible to further improve the surface quality.
Optimized production of the concrete shells
The concrete mix design was optimized by Durapact GmbH as well as die overall five roof shells made of textile-reinforced concrete were manufactured in their precast concrete factory in Düsseldorf- Haan. The company has many years of experience in the field of fiber-reinforced concrete and already manufactured the textile-reinforced concrete façade of the building expansion of ITA some years ago. The manufacturing methodology of the roof elements was developed in an intensive collaboration with the scientists of IMB in order to realize the high dimensional requirements, in particular, concerning the positional accuracy of the textile reinforcement that is relevant in terms of static’s. In the manufacturing process, the fine-grained concrete matrix was placed in layers in a spraying process and the textile reinforcement layers were laminated gradually.
Following a curing phase of several weeks in the protective production hall, the precast concrete components were turned for striping the forms and each element was placed on a separate wood support. During the curing process, the supporting structure ensured the dimensional accuracy of the shells. Due to the composition of fine-grained concretes used, the shells have a much higher tendency to creep and shrink than conventional concretes. Owing to the high accuracy of the shells, it was later possible to fit them in the steel structure without restraint. The substructures, furthermore, served for transport securing purposes allowing the textile-reinforced concrete shells to be loaded on a truck by means of a fork-lift and safely supplied to die construction site.
Steel frame lined with grouting mortar
The textile-reinforced roof shells of the bicycle stand roofing have a supporting steel structure. The steel construction company Krings & Sieger based in Düren was responsible for planning and construction of the same. The four columns, the two large longitudinal beams, the transverse bracings and the shell supports were prefabricated and supplied to the construction site by a low-bed trailer. In advance, two columns each were welded to one longitudinal beam to make a frame, which then only needed to be placed on the intended foundations. In preparation, the craftsmen had drilled two holes each into the foundations, into which they cemented threaded rods using epoxy resin mortar. Then, the two steel frames were first lifted above the threaded rods with the aid of a mobile crane, lowered and afterwards screwed to the foundations. After a final height adjustment, the low ends of the steel frame were lined with grouting mortar. As an essential part in preparation for the installation of the roof shells, the inclined shell supports resting on the longitudinal beams were adjusted in the next step. These are the later support points for the textile-reinforced concrete shells. For this purpose, both the shell supports and the horizontal double- T beam profiles were designed with slotted holes tot an accurate adjustment.
Installation of the textile-reinforced concrete shells
The installation of the textile-reinforced concrete roof shells onto the steel structure was carried out by Oehmen, an installation company located in Düren as well. The manufacturer of the shells supplied the roof elements already the previous day, enabling the removal of the supporting structure on the points of support and thus uncovering the anchorage points. On the day of the installation, the live shells were lifted to their final position by means of a mobile crane. To do this with the minimum impact on the concrete elements, IMB erected a supporting structure of rugged double- T beams arranged in a cross-like configuration. Two bow-shaped loops were suspending from the structure, and the workers placed them around each shell beside the later mounting points. The crane then carefully lifted the precast element weighing about 420 kg and swung it across the future supports. There, one worker each at every four points was ready to have the precast element lowered into the mounting support in an accurately fitting way tot screwing them afterwards to the substructure. The inclination of the L-shaped bearing points of the textile-reinforced concrete shells is 40° in relation to the horizontal line, resulting from the tangential line to the shell curve at the level of the mounting points. The lower edges of the concrete elements lie flush within the two sides of the L-shaped bearing points over a width of 110 mm on permanently elastic elastomer bearings. These bearings that are oriented in a tangential and radial direction to the shell allow a reliable transfer of the occurring compressive forces to the shell support.
Metal batten strips were set in concrete at the appropriate parts in the 20 mm thick shell edges in order to equalize the bearing pressure. In this regard, it was important that die strips just lie flat and are not embedded as a mounting rail, because this inevitably would have led to spreading, i.e. cracking of the concrete shell at this point, owing to the load prevailing. Such fastening was however not necessary here, since the points in their final shell position are under pressure anyhow and thus being kept in the position. The lifting forces acting on the shell, in particular, due to wind loads are transferred through the connecting devices at the supports. This constructional detail was designed by IMB so as to absorb the compressive forces through the elastomer bearings and not through the connecting devices. Scholzen pointed out that the high compressive strength of the textile-reinforced concrete could be ideally used for load-bearing purposes in this way, hence allowing the thin walled shell intended at all.
Textile-reinforced concrete without surface coating
As above mentioned, a high-strength fine-grained concrete was used for the textile-reinforced concrete, which is impermeable to water and weather resistant without any coating due to its dense structure and its smooth concrete surface. Although the shell crowns are aligned horizontally and the curves are anyhow exposed to the W-rich sunlight in approximately vertical position, a protective coating is not considered to be necessary and thus not taken into account either. According to the functional concept, rainwater simply drains along the shells into six box-shaped rainwater gutters, where it is collected and guided to downpipes. These are properly connected to the sewer system.
Protection against vandalism not needed
Although the 2 cm thick concrete shells seem to be extremely fragile, they demonstrate a sufficient stability at their final position. They are even in the position to absorb high imposed loads which might occur owing to the selected geometry of the roof shells, e.g. by snow accumulations and wind. The planning engineers are convinced that such loads are absorbed reliably by the textile carbon reinforcement. Because several material tests were carried out, the load-bearing capacity was calculated and the stability was subsequently proven by means of a prototype in a large-scale test in advance as part of the research project.
The question of the author, whether the shells should not be protected against vandalism when stored for a short time on ground level at the construction site, was just waved aside by the engineers being aware of the fact that the shell is able to bear more than 10 t under vertical loads. They actually stood themselves on the prototype without anything happening. At the end, all parties involved agreed on the fact that the lightness of the concrete shell has an impressive appearance.
Robert Mehl, Aachen