arrangement conceals the pipe and reduces the chance of damage by water due to a stopped pipe bursting at a joint. Cast iron, aluminium, and rigid plastic pipes are used for rainwater dispersal.

18Finishes

18.1Floor finish considerations

The following basic factors should be considered in choosing a suitable floor finish to a given area:

•Durability — the floor finish must have the ability to withstand the traffic. Maintenance appropriate to its location is essential for longevity.

•Appearance — which may be of prime importance or of little consequence depending upon location.

•Cost — capital, maintenance and the expected lifetime are the key interdependent parameters of total economic choice.

•Surface finish — highly polished surfaces will show

variations in surface levelling; anti-slip properties may be important, particularly in areas of spillage, e.g., kitchens, laboratories, etc.

•Acoustic properties — sound absorption may be

required in certain areas, e.g., control rooms, offices, conference rooms, corridors, etc.

•Fire resistance — in areas of high fife risk, low flame spread and low smoke emission materials may be of particular importance.

•Access to services — specially raised access floors may be required in computer rooms, cable areas,

,etc.

•Special requirements — floors used in conjunction with under-floor heating, or in areas of potential

chemical or radiation contamination will require special materials..

• Thermal movement — suitable expansion and con­ traction joints may be required where differential expansion characteristics of sub-floor and floor finish may be experienced.

18.2 Types of floor finish

Having assessed the requirements of a floor finish in a particular location a selection can be made and some of the more common types available are as follows:

Trowelled concrete This is a means of obtaining a cheap, reasonable finish to a concrete slab. The con­ crete is trowelled smooth by hand or by machine (power floating) and the surface can be. treated with a proprietary anti-dust silicate sealant. This method can provide a suitable surface for the application of other sheet or liquid resin floor finishes. Its tendency to dust

Finishes

under traffic can be resisted by the application of cheap penetrative sealants.

Granolithic finish This finish is applied to concrete floors and consists of a layer of concrete made from fine granite chippings and cement. The granite aggregate is graded in size from chippings down to dust. It gives a superior finish to concrete, being harder and less absorbent. This finish can be applied to set concrete, but is not recommended due to inherent problems of adhesion. It is better laid on ‘green’ concrete, or integrally with the concrete, to ensure a good bond and to avoid differential shrinkage of the floor and finish. The thickness laid varies from 50 mm to 75 mm and depends upon the location on the floor. The thickness can be reduced if it is laid monolithic with the concrete, which will also ensure the most positive bond with the base concrete.

Terrazzo This is an expensive finish consisting of marble chippings set in white cement. The surface is ground and polished to a flat surface after the cement has hardened. It can be laid in-situ or as tiles. This finish is very impermeable and hygienic but is so brittle that cracking occurs very easily. Expansion and con­ traction joints must be very carefully located. Specialist armoured terrazzo tiles are manufactured which are quite suitable for turbine hall floors and other heavy duty areas, albeit expensive.

Quarry and other vitreous tiles These are made from burnt clay and have good acid-resisting properties. Tiles can be obtained in a variety of colours with rough or smooth surfaces. Purpose-made tiles can be obtained to form coves, skirtings and other similar features. They can be set and jointed in cement or, alternatively, in a proprietary jointing compound which remains slightly plastic after setting and thus prevents failure by differential movement of the finish and slab. The fullyvitrified tiles are impermeable and are suitable for areas subject to contamination in reactor buildings.

Wood Hardwood boards or blocks are the timbers usually used for floor finishes in power stations, although softwood boards are sometimes used to pro­ vide a base for a further covering. Timbers vary and although good results can be obtained from the best hardwoods such as maple boards or oak blocks, other types of wood flqors are not suitable for heavy traffic. Wood blocks laid with their grain vertical make an excellent floor finish for workshops and similar loca­ tions. The end grain gives good wearing characteristics and the nature of the surfaces means that there is a smaller chance of damage to castings and similar items placed on or moved across the floor. These blocks are often set in pitch and should not be used where the risk of fire is high.

Wood fibre blocks These are made from recon­ structed timber and are very similar to wood blocks.

Sheet floorings These include linoleum, rubber and PVC. They are usually Secured to a timber or cement

Civil engineering and building works

scored base by means of a suitable adhesive. They give a good impervious hygienic finish which is not difficult to keep clean but they are not suitable lor areas of heavy traffic.

Plastjc tiles These are similar to sheet floors but in tileform. They are more easily laid, but hase the disadvantage of many joints which are vulnerable to moisture spillage and consequent loss of adhesion, together with an inherent tendency to curl in lesser quality tiles.

In-situ composition floors 'lhese include asphalt, magnesium oxychloride, epoxide resins and many other types of finish. The quality and type of finish can be varied to resist acids, alkalis and other contaminants. Such special materials normally fall outside the architect’s or civil engineer’s experience and specialist advice is sought before adoption.

18.3 Finishes to walls and ceilings

For many power station buildings of brick and concrete construction a fair finish and flush pointing to the brick­ work together with a good finish to the concrete is adequate. Plastering, rendering and similar treatments to walls and ceilings are called wet processes. Although •satisfactory results are usually obtained, the time taken by application of these finishes together with the drying out periods which must be allowed before decoration can commence have led to the development of dry finishes. These include ceilings constructed by plastic sheet, insulation board, hardboard, metal trays and other materials which can be supported by hangers from the roof to provide a space above the ceiling for ducts and services. Insulation boards on the inside of sheet cladding and unit partitions in office blocks tire typical examples of this type of finish. Many such panels can be supplied ready decorated if required.

Special finishes such as vermiculite spray can be applied in thicknesses usually between 25 mm and 50 mm to concrete, steelwork or other surfaces. Although occasionally used for insulation, the main function of these finishes is the protection of steelwork against fire.

18.4 Wall tiling and other special finishes

In kitchens, toilets, medical centres or similar locations where walls are subject to contamination by grease, dirt or corrosive substances and a high degree of hygiene is necessary, the conditions and frequent washings re­ quire the use of a better surface than is obtainable with a paint finish. In these circumstances glazed or unglazed

. wall tiles, in-situ terrazzo or terrazzo tiles, plastics or similar materials are used for the finish to wall surfaces. Particular attention to such decontaminabie finishes is paid in the design of those areas of nuclear stations vulnerable to radioactive contamination.

Chapter 3

18.5 Internal painting

Apart from protecting the steelwork, internal painting is used mainly for decorative reasons and is usually carried out in accordance with schemes prepared by the architects. For painting on timber, concrete, brick­ work, plaster, metal or similar surfaces, the basic system is usually as follows:

(a)Cleaning down and surface preparation Before painting, all loose material should be removed and

■all surfaces thoroughly cleaned. A vacuum plain should be provided lor the removal of dust. Clean­ liness of the base prior to application of any protective coating is of paramount importance.

(b)Priming coat This seals and prepares the surface and also provides a key for subsequent coats of

paint. It can also be used to protect a material during the constructional stages before the final coatings are applied near to station completion. Aluminium or conventional wood primers are normally used for timber, and alkali-resisting primers for the surfaces of new plaster, concrete, brickwork and similar surfaces. For ferrous metals a zinc phosphate or other approved rust inhibiting primer is suitable, and for galvanised surfaces an etch primer followed by a zinc phosphate primer should be used.

(c)Undercoats One or two coats of paint is the usual specification for undercoats, using a paint which is compatible with the primer and finishing coat.

(d)Finishing coats These arc normally alkyd-based resin paints or similar materials which give a hard durable finish. They can be supplied to give a matt, semi-gloss or high gloss finish.

(e)Other paints A more economic but less durable finish can be obtained on brickwork, plaster, concrete and similar surfaces by the use of emul­ sion paints.

18^6 External painting

Although the decorative aspect of external painting must receive some consideration especially on the main buildings, offices, welfare blocks and similar buildings, there are many locations such as precipitator steelwork, coal gantry steelwork, external plant and other similar items where protection is of prime importance and appearance of less importance. External painting is carried out using preparations and materials similar to those used for internal work with the addition of micaceous iron oxide, chlorinated rubber, polyurethene, epoxy, acrylated rubber, vinyl acrylic and glass flake systems as required, depending on the local climate and in-service environment envisaged for the protective coating.