STATE AND DEVELOPMENTS OF PRECAST CONCRETE BUILDING CONSTRUCTION IN EUROPE. Arnold Van Acker, Belgium INTRODUCTION

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1 STATE AND DEVELOPMENTS OF PRECAST CONCRETE BUILDING CONSTRUCTION IN EUROPE Arnold Van Acker, Belgium INTRODUCTION Many ongoing developments in building activity and in prefabrication are inspired by social and economic factors that have marked our society in recent decades: Strong competition between construction materials and techniques: concrete versus steel, precast concrete versus in situ concrete or steel. Fluctuating economical activity Growing environmental consciousness Higher demands as regards labour circumstances and general comfort The key words governing the evolution of the construction activity in general and precast concrete in particular are efficiency, durability and respect for the environment. Prefabrication possesses a large potential in this field to be a leading actor in the building activity of the future. The paper describes the evolutions in the market demands and the solutions developed by the precast concrete industry in the field of materials, products, systems and processes. MARKET DEMANDS Before getting into technical details, it is interesting to explain the meaning of the most prominent factors behind these new market demands with respect to building and prefabrication. Structural efficiency: The ideas here are to design structures and develop systems so that the building offers maximum efficiency to the user. For example create maximum exploitation capacity of the available building space by using more slender building components like in slim floor structures etc. Competition between different construction materials and systems is more and more judged in terms of performances and costs. Systems offering more floor area inside the building volume are increasingly competitive. Flexibility in use: Certain types of buildings are frequently required to be adaptable to the user s needs. This is especially the case with offices, but housing might also need to be more adaptable in the future. The most suitable solution to this effect is to create a large free internal space without any restriction to possible subdivisions with partition walls. Optimum use of materials: Each construction material possesses specific properties and optimum applications. Until recently, the structure of a building was mostly built in the same material. Now, the tendency is to use a combination of different construction materials that are best suited for the particular function within the project. Speed of construction: Because of the slowness of traditional in-situ construction methods, long construction delays have been accepted. Today, the demand for a speedy return on investment is becoming more and more important: the decision to start the work is postponed until the last moment, but the initially agreed construction delay has to be met. In addition, projects are getting more complex, which is not in favour of short construction delays. One of the solutions taken by the general contractor is to put more responsibilities on precasters. Quality consciousness: Quality has a broad meaning. Not only do the quality of materials and execution have to respond to higher standards than before, but also the quality in the domain as regards user friendliness, 1299

2 XIII Congreso Nacional de Ingeniería Estructural Puebla, Pue., México 2002 comfort and aesthetics is becoming more important. Several precasting companies have already obtained the ISO-9000 label, and a lot of work is being done in the field of European Standardisation. Adaptability: In the future, there will be much less demolition of entire buildings and more demands to adapt older buildings to the new market requirements. The principal reasons for this attitude will be the high costs for demolition due to regulations regarding noise, dust, traffic problems and many other inconveniences. On the other hand, after 30 to 50 years, an office building becomes less attractive for renting, and the owner will look for renovation, for example getting a more modern façade for his building. The design concept should facilitate such renovations, without demolition of the rest of the structure. The initial concept of the entire building will therefore have to take account of the life span of the different components of the building: load bearing structure: up to 100 years and more; facade: 30 to 60 years; services: 20 years. Consequently everything apart of the main structure should be designed replaceable and renewable within the normal building life to avoid early termination. Periodic refurbishment, major modifications, replacements and improvements throughout the building life shall be possible. Protection of the environment: in many countries the design of buildings and structures continues to be determined solely by the factors of "capital" and "labour" employed by ecologically unregulated market economies. This will become expensive when "nature" is added as the third market-determining factor with the full cost required to be paid. Preserving the environment is becoming globally increasingly important. In Europe, some governments are already imposing regulations as regards the disposal of plastic, recovering of packaging, recycling of waste, ground pollution by chemicals, etc. It is expected that more severe restraints will be imposed in construction, e.g. with respect to emissivity of materials, shortage of raw materials, waste dumping, noise and dust, energy consumption etc. SOLUTIONS OFFERED BY PRECAST CONCRETE Prefabrication has often been the prelude to modernisation in many fields: working conditions, advanced manufacturing technology, speed of construction and environmental friendliness. The latest developments to respond the market needs are described below. NEW MATERIALS High performance concrete, with compressive stresses exceeding 80 MPa, is now being used routinely in precast concrete. Initially, the applications were for heavy loaded columns, where the cross-section can be significantly reduced. Other applications include heavy bridge beams, long span roof beams, products with high durability requirements, etc. However, we see that the applications in Europe are limited. The reason lies probably in the fact that prefabrication is already working with much higher concrete strengths than cast insitu construction and that there is less need for higher strengths. Self-compacting concrete, is a new and very promising solution for prefabrication, with far better opportunities than high strength concrete. Whereas high strength concrete essentially focuses on improved product performances (strength and durability), self-compacting concrete has a serious beneficial impact on the production process. Self compacting concrete needs no vibration and thus opens a lot of advantages as low noise level during casting, less mould pressure, rapid casting, easy casting when using dense reinforcement or when having thin or complicated cross-sections, less air pores at the surface and easy to pump. In some precasting factories self-compacting concrete is already used for 15% of the daily production, but we expect that this will increase up to 50 % and even more in a near future. However, one of the consequences is that we will have to redesign the factory systems for transport of fresh concrete, moulds etc. PRODUCTS Hollow core elements are now the most widely used type of precast flooring: in Western Europe the annual production is about 20 million m². This success is largely due to efficient design and production methods, 1300

3 choice of unit depth and capacity, surface finish and last but not least structural efficiency. For many years, the maximum depth of prestressed hollow core floor units was 300 mm. However, recent developments in Northern Europe have led to a 500 mm deep unit, with spans up to 21 m. In the context of sustainable construction, precast hollow core scores better than other types of floors, because of the rational use of the materials. The presence of longitudinal voids in the cross-section leads to a 50% saving in concrete compared with a plain slab, and at the same time cuts the amount of prestressing steel by 30 % because of the lower self-weight. Hollow core products suit also very well the purpose of flexibility and adaptability because of the large span/performance capacities. Presently, it is common practice to realise with a hollow core floor of 400 mm thickness, a span of 17 m for a live load of 5 kn/m². In Finland, a 500 mm thick unit achieves already a span of 21 m with a live load of 5 kn/m². In Sweden, the concept to span from one facade to the other without intermediate support is currently applied in office buildings. Light roof units. One of the perceived inconveniences of concrete is its self-weight. Consequently the developments are focusing on lighter products to enable a better competition with steel structures. The latest developments for roof units are slender beams with large openings and complex wafer-form cross-sections, with a wall thickness of 20 mm. The realisation of such thin concrete sections is now possible with the help of fibre reinforced concrete. As a result, roof units spanning 6m to 30m can now be made with a self-weight of only 100 to 200 kg/m². SYSTEMS Fig. 1 Prestressed roof unit with large span capacity and low self-weight Multi-storey precast concrete frames are constructed with columns and beams of different shapes and sizes, stair and elevator shafts, and floor slabs. The system is widely used for multi-storey buildings up to 20 storeys and above. The structural frame is commonly composed of rectangular columns over one or more storeys height. The beams are normally rectangular, L-shaped or inverted T-beams. They are single span beams, simply supported and pin-connected to the columns or concrete corbels, or by specially developed hidden connection details. Hollow core floor slabs are by far the most common type of floor slabs in this type of structure. New types of hidden corbels Column corbels often are inconvenient, especially in residential and administrative buildings. Recent developments go towards hidden corbels. There are different solutions. One of them consists of a steel box, which is incorporated in the beam (Fig.2). A sliding plate fits into a rectangular slot in the box. A notch at the end of the plate fits in another steel box cast into the column and is anchored over a lip at the bottom of the box. The system can be used for rectangular and round columns. The advantage of the solution is that the intersection between beam and column is neat, without an underlying corbel. The connection is also attractive from the aesthetic point of view. 1

4 XIII Congreso Nacional de Ingeniería Estructural Puebla, Pue., México 2002 Pipe floor Fig. 2 Hidden beam-to-column connection A recent development in The Netherlands concerns the introduction of the so-called Pipe Floor. All grooves and passages for ventilation and draining pipes within the screed are made in the precast factory during the production of the hollowcore floors. On site, all pipes can be installed in one go, including the drains and sewers for the bathroom and ventilation pipes. Afterwards, the remaining grooves are filled with normal sand or a cement finishing coat. In this way, construction time is greatly reduced, as the complete floor is supplied on the building site. Twin skin façade system Fig. 3 Principle of the Pipe Floor Sandwich façade panels are currently used in Northern European countries for residential and administrative buildings. The inner leaf of the elements carries the floors and the above elements, whereas the outer leaf is only decorative and assures the water tightness. The thickness and material of the insulating layer depends obviously on the climate: in Nordic countries, the current practice is to use 150 mm rock wool; in the more Western part of Europe, the normal insulation thickness is 50 mm polyurethane foam or glass wool. In the context of sustainable construction major efforts are made to reduce the energy consumption and the emission of carbon dioxide for heating of the buildings. Sandwich panels have excellent physical building properties, because of their thermal capacity and acoustic insulation. A recent development is to incorporate a cavity in-between the outer leaf and the insulation. Its principal role is to avoid the penetration of rainwater into the insulation and the inner surface. Seeping water is evaporating in the ventilated cavity or evacuated at the horizontal joint. 1302

5 One of the drawbacks of sandwich panels is the limitation of flexibility in façade design, due to the rigid rectangular shape of the elements and the fact that the inner configuration of the façade has to be identical to the outer one. It results in a rather rigid joint pattern in the façade. The problem can be solved by using brick tiles overlapping the joints, or to design the joints in a staggered position. More recent developments are the split-wall façade and the jointless façade. The split structure facade concerns a facade construction in which the two leaves (inside leaf and outside leaf) are fabricated separately and erected separately. The load-bearing leaf of the facade consists of simple framed panels placed with the smooth moulded side towards the interior of the building. The precast floor units are supported on these elements. Afterwards an airtight joint sealing is applied and an insulation layer is attached to the exterior face of the wall panels and finally the exterior cladding panels are erected. The exterior cladding can be made in precast concrete or in other materials. Very often the window frames are placed over several storeys giving a large architectural freedom to the design The solution offers numerous advantages over the classical sandwich panels: Large flexibility in the design of the exterior façade. Complete freedom with respect to size and shape. Possibility to use other materials for the exterior cladding. The precast units are simple. The internal panels are rectangular storey-high elements with a large repetition. The exterior cladding is mostly made of flat panels. The connections are very simple. The insulation is continuous over the façade without any single weak thermal point, inclusive at the panel joints. The façade comprises a ventilated cavity between the exterior cladding and the insulation. The outlook of the façade can be totally different from one building to another, without major difference in the interior structure and used elements. The joint pattern in the façade is unobtrusive. Jointless precast facades Precast facades are often characterised by a rigid joint pattern, especially in the case of sandwich façade panels. This technique is often applied in Nordic countries and there is a growing resistance against the architectural outlook. To overcome the problem, a new type of jointless sandwich façade is being developed now. There are two techniques. In the first solution, the precast façade panels comprise the inner load bearing leaf and the insulation. The exterior facing is applied on site in the classical way. In the second solution, the contour edges of the exterior leaf of the precast sandwich panels have a re-entrant profile with projecting plastic reinforcement. After erection, the grooves are filled and the whole surface is rendered with a special coating. There are already some realisations in Finland. Product integrated services In the future, precast products will be more sophisticated for example with integrated techniques. There are already some realisations, such as heat storage in hollow core floor slabs. The ventilation air circulates through a labyrinth made in the hollow cores and enables to store or recuperate heat in the concrete mass of the floor. Another example concerns the incorporation of ducts and pipes in the core of the floors. PRODUCTION PROCESSES In the coming decades, the challenge for the precast concrete industry will be automation. There are already good examples in the domain of pipes and small products with large series, such as terrazzo tiles, railway sleepers etc. In the area of building components, most has still to be done. Among the precursors are the hollow core slab, lattice girders and some small products. The most suited solution for the main reinforcement of beams and floor units is prestressing on long beds. The advantage of prestressing compared to mild steel reinforcement is that it gives high capacity for low labour input. The only remaining obstacle is the automatic 1303

6 XIII Congreso Nacional de Ingeniería Estructural Puebla, Pue., México 2002 incorporation of stirrups of different shapes and sizes. The solution probably lies in fibre concrete, using steel fibres or other types yet to be developed. Automatic production of round columns An interesting method for full automatic production of round columns in architectural concrete has been developed in Finland (Fig.4). The manufacture takes place by shotcreting a from 40 to 130 mm thick, reinforced shell around a turning element blank. After adequate setting, the products can be trowelled, and after hardening, the surface can be finished with other techniques, for instance polishing. Fig. 4 Automatic production of round columns It is possible to produce a variety of surface textures by using coloured concrete and different types of aggregates. In the latter case, only the final coat has to be of this more expensive material. Grey concrete can be used in the inner part. Load-bearing or decorative columns are the main applications. The columns are generally one storey high. Their maximum height is 4 m and the maximum diameter 1.2 m. Also conical shapes can be produced. One machine enables to produce 40 columns per day in a double working shift system. The structural connections between columns and other components are realised with bars anchored after production inside the cavity of the columns. Automatic measuring technique The system is already currently applied in other industries, and is now being introduced also in precast concrete production. The slide shows a mould table for wall elements. A laser spot from a computer, hanging on a movable beam indicates the position of the side moulds. Waste treatment In several countries, waste dumping is discouraged by high taxation. For precast concrete plants, rejected elements, concrete debris, waste concrete, residual slurry from cleaned mixers or material from sawing the concrete, grinding and other processes, will therefore have to be processed. Waste concrete and slurry are already recovered and recycled in several factories. They are ideal for recycling because their origin is known. Research reveals that the proportion of coarse recycled material in structural concrete can be up to 20% of the total amount of aggregates. 1304

7 EXAMPLES OF APPLICATION A. INDUSTRIAL AND COMMERCIAL BUILDINGS Systems, using frames composed of columns and beams, are still widely applied. The most prominent current changes are larger spans, lighter structures, and new types of connections. A new system for industrial buildings is using load bearing sandwich walls in architectural concrete and long span lightweight TT roof units. The latter have a self weight of 180 to 200 kg/m² and span up to 30 m between facades. The system offers a more rational, economical and aesthetic construction. B. CAR PARKS Car parks are well suited for prefabrication because of the large spans and fast construction. Other requirements for modern car parks are aesthetics, efficient use of space, water tightness, fire safety, low maintenance etc. There are complete design solutions for different types of layout and forms: ramped floor, flat deck and split-level. C. HOUSING AND APARTMENTS Large wall panel systems combined with hollow core floors are still currently used for housing in Northern European countries. The multi-family houses include complete outer walls, hollow core floors, lift shafts and stairs, balconies and roofs. The design of the building is not fixed by rigid concrete elements and almost every building can be adapted to the requirements of the architect or the builder. Generally no load-bearing internal walls are needed in the system. By using the hollow core elements with spans up to 11 m extending across the house, floors with very large and unobstructed areas are obtained enabling the greatest possible range of uses and longest service life. D. OFFICE BUILDINGS Current trends in office buildings point towards more prefabrication, more efficiency e.g. through a flat under surface for floors without underneath beams and corbels, reduced site activity by incorporating ducts and conduits in the floor elements, and safer and faster construction. In Sweden and Finland, the trend goes towards long spans exceeding 20 m and a minimum of internal supporting columns. The long spans give the designers great freedom of positioning office areas inside the building. It also allows flexibility for later changes in the building's use. Architectural concrete is still often used for high quality facades of prestigious buildings. Precast concrete architectural cladding offers an extremely wide range of visual appearances. Architectural concrete facades can give the impression of being constructed with brickwork, natural stone cladding or other finishes. The possibilities are illustrated by the following slides. e. Stadia Stadia and grandstands are often constructed in precast concrete because of the extra short construction delays. There are numerous good examples in many countries, e.g. a large arena for ice hockey for people in Helsinki, constructed in 6 months. CONCLUSION Prefabrication has a great potential to respond to new market demands. Possible solutions lie not only within the classical advantages related to working conditions, technology and speed of construction, but also in new developments of materials such as high performance and self-compacting concrete, buildings systems such as mixed structures, manufacturing technology, automation, service integrated products and others. 1305