Faculty Of Built Environments Construction Essay

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The main benefits of precast concrete over other cladding materials are its good strength-to-weight ratio, its mobility and, because it is a non-combustible material, its fire performance. Precast concrete came into its own for use in cladding during the 1950s and early 1960s with the development of high-rise housing. There are three (3) main advantages of using precast as against in-situ concrete which are speed of erection, freedom from shuttering support on site, and better quality and variety of surface finish because panels are manufactured in controlled factory situations.

This is dependent on the degree of exposure to weather (which is BS 8110 : 1985 lists as mild, moderate, severe and very severe), the grade of concrete used (25-30N/m²), and whether a facing mix is required to obtain a special finish. Figure 1.2 shows the depths to be allowed for panel web thickness for two types of concrete mix.

The depth of external and internal cover should be determined according to the grade of concrete to be used and the conditions of exposure. Normally, external cover is either 40 or 50 mm and internal cover 20 or 25 mm. A zone of 25 mm should be allowed for the reinforcement, stiffening bars and variability of placing the steel in the mould. For most conditions, a nominal thickness of 100 mm can be used for web thickness in design. Using a span : depth ratio of 1:27, the web would then span 2.7 m.

The depth of the horizontal support nib is determined from the load bearing and fixing requirements. The most important consideration is that there should be a minimum bearing of 100 mm on the structural slab plus an allowance of 25 mm for any inaccuracies in the edge of the slab and any danger of spalling. Figure 1.3 shows the minimum dimensions of the horizontal support nib and its bearing upon the type of fixings to be used. Allows for 175 mm with dowel fixings and 125 mm using cleat fixings.

In addition to providing space for reinforcement the height of the nib is usually affected by the type of fixing used and whether or not it is necessary to provide cast-in fixing is used, the suggested height of nibs is 150 mm. For a dowel fixing, this can be reduced to 125 mm. Concrete corbels and stainless steel bearing angles are extensively used to restrict the height of the gravity support device to fit under raised floor.

The depth of the vertical strengthening ribs is related to the span between the support, and should be determined from BS 8110 : 1985. For coffered panels the web is designed using a span : depth ratio of 1:27, whereas the ribs can be designed as beams with a span : depth ratio of 1:17. Sufficient depth is often necessary to accommodate the elements of an open-drained joint (See figures 1.4 and 1.5), especially where the profile incorporates a drainage groove or where an upstand is provided at the head of the panel. Figure 1.4 shows vertical strengthening ribs of 250 mm to coincide with the depth of a panel needed to incorporate an upstand joint.

Sealant joints are designed to provide a complete watertight barrier in the form of a single-stage joint positioned towards the panel face. The main types of sealant joint for use with concrete cladding are poly-sulphide (one- and two-part) acrylic and low modulus silicone. Sealant joints are often used for highly profiled units such as spandrel or parapet panels. Rectangular- or circular-section sealant backing of closed-cell polyethylene foam is used to ensure the correct depth of sealant and to separate it from incompatible materials, which could cause its breaking down (See figure 1.5).

In single-skin form, GRC can be mounted onto a metal stud framing (Fig. 2.1). This framing provides support and stiffness against out-of-plane forces, and permits expansion and contraction of the GRC skins. This approach has been extensively used in Germany and the USA and increasingly in the UK and continental Europe. The choice of the type of construction (single skin, profiled or sandwich) will be governed by a combination of requirements that need to be satisfied: fire, thermal, acoustic, weight etc. Table 2.0 shows the relationship between various types of wall construction and the performance requirements for spans, fire resistance and weight, which are usually the most critical.

Gaskets are effective only where positive pressure is available to deform or compress them. A typical joint incorporates a top-hat section with captive nut to pull back a metal cover strip against a neoprene gasket. Advantage can be taken of the quality of the edge profile, which is normally possible in GRC, to use pushin fir-cone gaskets, as used, for example, at the UOP Fragrances factory (architects: Piano and Rogers) (Fig. 2.2). These gaskets incorporate barbed legs, which can be pushed into position in the joint. One problem in using such gaskets is that the moulded GRC nibs to receive the gasket have only one mould face, and however well the back face is compacted, minor variations in thickness will occur, which can cause points of weakness for an effective weatherproof seal. These nibs can be wrapped in tape to take up any tolerances in surface defects resulting from manufacture, in order to gain a tight fit before applying the compression gasket.The UOP Fragrances factory also used a push-in gasket on the inside face; some difficulty may be experienced in sealing past the adjustable clamps in such a detail.

Various mastic sealants, including polysulphides, polyurethane and silicone rubbers, have been used, but their success depends upon good surface preparation and the use of correct primers. The sealant surface may be exposed to ultraviolet light, and its long-term attributes are therefore questionable. Manufacturers' recommendations for sealants must be closely followed at all times. Sealants designed to bond to smooth surfaces should be specified, and care should be taken to ensure that any silicon face sealer to the panel is not carried round the edges of the panel, as this inhibits adhesion of the sealant.