Technology: The Renaissance of Lightweight Concrete in Architecture
Lightweight concrete Architectural and architectonic experimentation with the self-weight of concrete dates back nearly two thousand years. One of the best known lightweight concrete structures is the Pantheon in Rome (ca. AD 128). The construction of its hemispherical dome, with an interior diameter of more than 43 metres, marks a climax in the progress of architecture and engineering. The Pantheon is a feat of structural engineering which was designed with great creativity and experience in terms of the details of the construction, building operations and building materials. The dome is made from a lightweight concrete mix which was obviously designed to reduce the self-weight of the structure. It was the advanced knowledge of the use of opus caementitium that encouraged the designers to plan this unique project and, indeed, to build it (ill. 3). Lightweight concrete in modern architecture More attention was directed to possibilities of lightweight concrete again beginning in the mid-twentieth century. Frank Lloyd Wright's Guggenheim Museum vividly demonstrates the potential of lightweight concrete structures. In Germany, lightweight concrete's architectural applications were developed primarily in the 1970s with the aim of coming up with a low-tech, single-leaf wall system for residential and commercial construction - by utilising its strength and thermal conductivity. The motivation for this early phase of construction with lightweight concretes was predominantly oriented to building technology aspects, a conscious adaptation of lightweight concrete in architecture was limited to a small number realisations here and there. Lightweight concretes can be used - as can the heavier normal concretes - as exposed concrete and in the design of complex structures. And lightweight concrete's significantly reduced self-weight also makes attenuated structures possible. Due to the tightening of codes governing energy performance of buildings in the wake of the 1970s energy crisis, the requirements could no longer be fulfilled with the usual wall thickness. As a result, using lightweight concrete in single-leaf walls was subsequently only feasible for special designs and programmes. The popularity and implementation of lightweight concretes dropped significantly. Adapting to new requirements Since the 1990s, building with concrete was not only rethought technically in numerous German and European codes and regulations, the material technology has also progressed significantly. The main developments occurred in building methods which had a strong presence in the market, because successful technical planning requires experimentation, and matures in practical application. Through this process of differentiated approach to the technical characteristics, the use of lightweight concretes was also reconsidered, but the number of realised projects was relatively small. The goals were to further reduce the unit density and to improve the thermal insulating properties. With classic lightweight concretes, the density (compared to normal concrete at approximately 2350 kg/m3) can be reduced to approximately 1600 kg/m3, while maintaining the standard strength specifications. With the current technology - at a strength level appropriate for current building practices - as little as 800 kg/m3 is being implemented. At the same time lightweight concrete's improved thermal-insulation characteristics were verified by the supervising authorities. Such lightweight concretes have provided inspiration to architects because they have opened the door to designs and realisations of single-leaf, monolithic walls. Its use hinged on architects' command of the building material's characteristics. Fair-face concrete In these applications, however, it became clear that with these monolithic versions, the exposed concrete surfaces cannot attain the standards of the prevailing school of thought dictating smooth, flawless surfaces; instead lightweight display deviations, as, for example, increased incidence of pores. Above all the uniqueness and the otherness of exposed concrete surfaces attained with ultra-lightweight concretes should be taken note of with respect to design, since as the unit density decreases, their ability to meet the German expectations for exposed concrete surfaces is diminishing. The advent of coated plywood sheets in the early 1960s enabled the economic realisation of large expanses of concrete with extremely smooth concrete surfaces - a new characteristic at the time - and gave exposed concrete a significantly improved image. During the 1980s and 1990s, the surface of smooth exposed concrete was structured primarily with tie holes and formliner seams. If we take a glance at the origins of exposed concrete, when its materiality was at the fore and architects tended to accept its inherent qualities rather than to actively rework it, the question arises, whether specifying flawless surfaces is still timely, and whether it should be continued in this manner. Although at the moment this preference appears to be intact, in architecture a strong tendency to turn away from this overarching philosophy can be detected; following this long phase of nearly one-dimensional surface treatment there is a dearth of fresh alternatives. Building monolithic walls with lightweight concretes - which possess a poignant authenticity - mobilises building-material synergies, and can lead to a renewal of design philosophy. This is apparent in the reactions to the publication of new projects in Switzerland and in Germany. Masonry blocks and precast elements A wide range of concrete-like building materials are designated lightweight concrete. Among them are aerated autoclaved concretes (AAC) and no-fines lightweight concretes, which, due to their favourable thermal insulating properties - and sufficient strength - are used as masonry blocks and precast elements (ill. 1). Structural lightweight concrete Structural lightweight concrete's characteristics differ from the two aforementioned types; its appearance, in the completed building, cannot be distinguished from normal concrete. Concretes are designated structural lightweight concrete whose matrix of reinforcing steel is tightly surrounded and protected from corrosive influences in the same manner as is the case in normal concrete. These are simply known as lightweight concretes and are, with respect to the standards, equivalent to the normal concretes. Beginning with the implementation of the most recent codes for concrete and reinforced concrete construction, normal and lightweight concretes are covered in the same codes. Structural lightweight concrete/normal concrete In terms of material and technological details, there are some significant differences between lightweight and normal concrete. Normal concrete is essentially classified by its strength, durability and by the specifications for surface quality. These criteria also apply to structural lightweight concrete. The latter is additionally characterised by a lower unit density as compared to normal concrete. The lower unit density gives the lightweight concrete significantly improved thermal insulating properties, which do not, however, attain the values of AAC and no-fines lightweight concrete. Structural lightweight concrete does not differ in appearance from normal concrete. This is not surprising, since producing structural lightweight concretes essentially involves substituting the heavy normal aggregate (e.g. quartzite gravel) with lightweight aggregate, for example lightweight expanded-clay. In most cases this substitution is limited to the coarse aggregate. Natural sand continues to be used for the sand component. Such lightweight concretes are used primarily for engineering structures (such as bridges), for floor slabs, interior walls in residential construction and in larger parking garages utilising precast elements. In the areas mentioned, weight reduction is the fundamental issue. By substituting the natural sand with a lightweight sand, the concrete unit density can be reduced a further 200 kg/m3. However, for the implementation of these lightweight concretes the significantly improved thermal insulation is decisive, because lightweight aggregate is a poorer heat conductor than normal aggregate. Characteristics of structural lightweight concretes By exploiting the possibilities available today for concrete technology (modern plastifiers and concrete aggregates) the pure lightweight concretes can be further optimised. The varied performance in terms of strength of the three lightweight concretes is evident in illustration 6. The values given depict the limit of the technical possibilities. Illustrations 7 and 8 show the relationship between the concrete unit density and the attainable thermal conductivity. The charts clearly show the advantage of lower permissible values, compared to the standard values, which leads to specification of thinner walls. Both aspects - strength and thermal conductivity - are important in the selection of lightweight concrete. The relationship is given in simplified terms via the concrete unit density. Strength and thermal conductivity For lightweight and ultra-light concretes, three areas of utilisation and synergy result from the combination of required structural strength and the thermal conductivity necessary to meet building physics specifications. • Office building facades with a large num-ber of openings, and thus involving greater stresses on the structural system, are executed at a unit density of 1.3-1.6 kg/m3. • Facades which are less complicated structurally, in other words, which have either thicker walls, fewer storeys or few openings, will be at a unit density of 1.0-1.3 kg/m3. • The best thermal-insulation, ultra-lightweight concretes are used increasingly for luxurious buildings in the residential sector. The walls of these buildings are executed at a unit density as low as 0.8 kg/m3, and thereby use Germany's allowable range for structural lightweight concrete to their full advantage. For the wall thicknesses specified by the building-physics requirements, the corresponding attainable strength is, as a rule, sufficient to guarantee with ease the structural capacity of typical buildings. Durability independent of strength Structural engineers sometimes express their concerns with regard to lightweight concretes in lower strength classes, because in their work with normal concrete they have become accustomed to there being a relationship between strength and durability: the greater the compression strength category of a normal concrete, the greater its resistance to corrosive influences. However, this causal relationship does not apply to lightweight concretes. Although the same influences apply to normal und lightweight concrete's durability, the strength of lightweight concrete is determined to a greater extent by the fine aggregate. Hence, lightweight concretes in lower strength classes with the correct composition can also easily fulfil the exposure class requirements. Advantages of single-leaf assembly The decision to employ a single-leaf lightweight concrete wall usually stems from the desire to create - assisted by a material which can be formed at will - unique architecture and, at the same time, exploit the design potential of exposed concrete. Using normal concretes, due to thermal performance requirements, this wish can only be fulfilled with multi-component assembly, and thermal insulation sandwiched between different components. In contrast, lightweight concrete provides the opportunity to execute exposed concrete with a single-leaf, monolithic wall assembly. This solution has a number of advantages for the architect. • The homogeneous wall assembly precludes defrost water from collecting, as might be the case in a multi-component wall system. • Executing curved walls is simpler than in multi-component systems. • Connections for glazed facade elements are uncomplicated. Exposed concrete with pores The decision to use lightweight concrete is also coupled with limitations. And the desire to have exposed concrete surfaces typically also implies expectations of a flawless surface, nearly free of pores. This design decision necessitates - even when normal concrete is being used - extra care during the entire construction process; fair-face concrete is clearly more difficult to achieve with lightweight concrete. Due to its lower unit density, lightweight concrete absorbs the vibration energy stemming from compacting. The lower the unit density is, the greater the effort required for compacting the concrete. For concretes with very low unit density, even when extreme measures are taken, there is still no certainty that the desired surfaces can be achieved. The aim is, first and foremost, to achieve the greatest possible consistency of the surface through avoidance of blowholes and defects. Realisations The use of lightweight concrete in structural applications is illustrated at the Volkswagen Arena in Wolfsburg (ill. 5), in the upper and lower boundary elements of the upper tribunes. These elements rest on the ends of the cantilevering arms of the support structures. Due to their lower weight, as compared to normal concrete, the self-weight prone to swaying could be effectively reduced. Through this structural solution the upper tribune does not noticeably sway due to audience movement at pop concerts. The rigorous specifications for exposed concrete surface quality and the T- and H-shaped building-component geometry were met by employing a self-compacting lightweight concrete LC25/28 D1.6. Particularly for floor slabs in lightweight concrete, the weight of the structural members can be reduced and the desired architectural concept can be implemented. Two examples using lightweight concrete floor slabs are found in Munich. In the1970s, using a combination of a suspension structure and floors in lightweight concrete, spacious and highly flexible office space was created for the BMW high-rise (ills. 14, 15). At the new vocational school centre on Riesstraße, a lightweight concrete in strength class LC50/55 D1.6 was selected for the cantilevering floor slabs above the entrance areas, making it possible to do away with most of the columns. With lightweight concrete, the vertical load was reduced to such an extent that it can be directed through the building via the cross-walls. Facades of modern office buildings and commercial buildings are characterised by large expanses of glass. A visible structural system is often employed by architects to structure the facade. If the facade structure is to be executed in exposed concrete, a number of building physics details must be resolved. The wall must satisfy the minimum thermal performance requirements as set forth in DIN 4108. With normal concrete this would only be possible with interior thermal insulation. At the same time linear thermal bridges occur at the connections of the facade elements. For these reasons, with this type of facade, structural lightweight concrete is again being used increasingly. The structural requirements of strength class LC35/38 D1.4 are met, as are the thermal performance requirements, thanks to a U-value of less than 0.7 W/(m2K). One concept using this type of lightweight concrete was realised in two buildings in Frankfurt for the investor Ardi Goldman: Michael A. Landes selected a regular, clearly articulated frame structure for a building component of the Union Lofts. Thanks to the lightweight concrete, the connections between interior and exterior building components did not require thermal separation. For their UFO lofts and commercial building, Dietz Joppien Architekten employed a monolithic lightweight concrete facade in fair-face quality (see ill. 16, Detail 4/07 p. 370). The Berlin architects Bumiller & Junkers were faced with a programme placing high demands on thermal protection, while maintaining the concrete's load-bearing capacity, for the construction of the courthouse in Frankfurt/Oder (ills. 10-13). A multi-layer curtain facade could not have produced the compact, monolithic aesthetic. Because the surface finish plays a decisive role, the wall could not have irregularities or blemishes. At the same time, the office building - with a 60 to 90 cm thick facade assembly of lightweight concrete - fulfils a further requirement for flexible room sizes. According to the architect's concept, the facade had to provide numerous points of connection for the partition walls. The lightweight concrete which was selected, in strength class LC12/13 D1.2, has a thermal conductivity of only 0.45 W/(mK) and met the architect's expectations for the exposed concrete surfaces. Peter Böhm chose a similar lightweight concrete for the Anna Landsberger Youth Centre in Berlin, in order to clearly distinguish it - along with his design's sculptural qualities and the structure of the seemingly free-standing straight and curved wall segments - from the surroundings. Monolithic fair-face lightweight concrete is much better suited to creating curved walls than alternative assemblies in normal concrete or precast elements. A further advantage was the simplified structural solution for the connections for the facade elements. Some recent residential projects demonstrate the ultimate step in implementing buildings in lightweight concrete. In them lightweight concrete is used as structural insulation in fair-face quality. This was achieved by developing special, ultra-lightweight insulating concretes, in strength class LC8/9 D1.0 with a thermal conductivity of 0.36 W/(mK). The most well known examples are the residence designed by Patrick Gartmann outside Chur in Switzerland (ills. 4, 17, 18); and the house designed by Trager ap88 Architektenschaft - for himselve (ill. 19), and the residential house Schlaich, currently under construction in Berlin. These buildings impressively document the possibilities which open up for architects who opt to employ structural lightweight concrete.
