13 computergesteuertes Schleifen eines Maschinenbauteils aus Beton 14 Betonieren einer Elementdecke bzw. der ersten Schale einer Doppelwand in der Fertigungsanlage 15 Kippstation zur Fertigung einer Doppelwand, der Verbund erfolgt im noch nassen Beton. 13 Computer-controlled smoothing of a concrete machine component 14 Concreting a floor-slab element or the first layer of a double-skin wall in the prefabrication workshop 15 Tilting station for fabrication of a double-skin wall; the bonding occurs with concrete in a wet state
13
Nowadays, precast concrete components are a common feature in building. In view of the broad spans they can cover and the scope they offer for serial production, they are widely used in industrial and commercial construction. But even in smaller structures, precast floor slabs, stair flights and balconies are not unusual and compete with in-situ work. The advantages of prefabrication are a shorter construction period, greater reliability in meeting deadlines, increased precision and a better surface quality. One must, however, be aware of the complex relationships involved. A collaboration with a structural engineer who has experience of prefabricated forms of construction is recommended in order to determine the optimum division of a building into individual elements, including non-loadbearing elements such as facade cladding. Other factors, such as transport costs, play an important role as well. Generally speaking, the more complex a constructional unit is, the greater will be the transport distance that can be justified. Another aspect is the space available on site, for to save time during assembly, storage areas and cranes will be needed. A division of the construction into viable elements should occur at an early stage, ideally during the design process. Maximum slab dimensions for economic transport are a length of 10 m and a width of 4 m. Formwork tables for prefabrication are, therefore, constructed to this size.
14
In the case of load-bearing walls, one can distinguish between the following types. • Solid, single-skin walls (Fig. 1 a) are the simplest form. If concrete wall elements of this kind have external thermal insulation or a facade layer in a composite form of construction, the surface finish will not usually play a role. With precast concrete cladding, a wide range of divisions, colours and textures are possible. The outer jointing pattern will depend entirely on the load-bearing elements. • Double-skin walls (Fig. 1 b) have been made in a combination of precast and in-situ concrete for roughly 20 years now. Walls of this kind consist of two structurally reinforced slabs with lattice bearers as distance pieces (Fig. 14). The bonding process takes place at the prefabrication works in a wet concrete state (Fig. 15). The walls are then transported to the site and assembled. Each of the two slabs will have a minimum thickness of 6 cm. Walls with an overall thickness of 18 – 40 cm are possible. A smooth surface finish is another advantage of this type of element. If a wall skin projects at the top or bottom, formwork will not be necessary to the edges of concrete floors (Fig. 1 b). • Sandwich slabs (Fig. 1 c) are an economic solution for walls with core insulation. They are concreted layer by layer, starting with the facing skin. The two layers of intermediate insulation should be fixed with stag-
15
62
gered joints. Depending on the requirements, mineral wool and expanded or extruded polystyrene are suitable materials for this purpose. Stainless-steel anchors create the requisite structural bond between the two layers of concrete. Glass-fibrereinforced plastic anchors help to improve the insulation properties. The maximum length of sandwich slabs is 10 m, but the outer facing skin must be divided at roughly 6 m intervals, since it is much thinner, and cracks and deformation could occur. The prefabrication of the load-bearing walls will, therefore, be evident in the external jointing. In the case of standardised prefabricated floor slabs, one distinguishes between elements with and without an in-situ concrete topping. Solid precast concrete floors are capable of bearing loads immediately after assembly. Their great weight, however, restricts the dimensions of the slabs in respect of transport and handling. More economical solutions have been developed, therefore, the most common of which are described below. • Prefabricated floor slabs (Fig. 2 a) consist of 5 –7 cm precast concrete units to which an in-situ concrete topping is added to obtain the requisite structural thickness. The bond with the in-situ concrete is achieved through the coarse surface finish and intermediate lattice bearers on the precast units. Elements of this kind also help to avoid elaborate formwork. At most, some support may be needed until the concrete topping has hardened. The width of floor slabs ranges from 80 to 300 cm, and the maximum span is 11 m. Joints between slabs should be filled to obtain a homogeneous underside. Units of this kind are of limited suitability where exposed soffits are involved. • Prestressed concrete slab elements (Fig. 2 b) do not require support during assembly. This can be of advantage where great room heights exist. Prestressing increases the live loads that can be borne and reduces the deflection of the slabs. Spans are similar to those for non-tensioned floor units. • TT floor slabs (Fig. 2 c) are used mainly in commercial structures and where very great live loads occur. Slabs of this kind are usu-