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Fig. 3.17 Prestressed concrete
material always remains in compression when subjected to flexing up to the maximum working load.
The tensile forces within the steel tendons act on the concrete putting it into compression, such that only under excessive loads would the concrete go into ten- sion and crack. Two distinct systems are employed; in pre-tensioning, the tendons are tensioned before the concrete is cured, and in post-tensioning, the tendons are tensioned after the concrete is hardened (Fig. 3.17).
Pre-tensioning
Large numbers of precast concrete units, includ- ing flooring systems, are manufactured by the pre-
tensioning process. Tendons are fed through a series of beam moulds and the appropriate tension is applied.
The concrete is placed, vibrated and cured. The ten- dons are cut at the ends of the beams, putting the concrete into compression. As with precast reinforced concrete it is vital that prestressed beams are installed the correct way up according to the anticipated loads.
Post-tensioning
In the post-tensioning system the tendons are located in the formwork within sheaths or ducts. The concrete is placed, and when sufficiently strong, the tendons are stressed against the concrete and locked off with special anchor grips incorporated into the ends of the concrete. Usually reinforcement is incorporated into post-tensioned concrete, especially near the anchor- ages, which are subject to very high localised forces.
In the bonded system, after tensioning the free space within the ducts is grouted up, which then limits the reliance on the anchorage fixing; however, in the unbonded system the tendons remain free to move independently of the concrete. Tendon ducts are typ- ically manufactured from galvanised steel strip or high-density polythene.
Post-tensioning has the advantage over pre- tensioning that the tendons can be curved to follow the most efficient prestress lines. In turn this enables long spans of minimum thickness to be constructed. During demolition or structural alteration work, unbonded post-tensioned structures should be de-tensioned, although experience has shown that if demolished under tension, structures do not fail explosively. In alteration work, remaining severed tendons may sub- sequently require re-tensioning and re-anchoring to recover the structural performance. However, the use of post-tensioning does not preclude subsequent struc- tural modifications.
Fig. 3.18 High-quality visual concrete—St John’s College, Oxford. Architects: MacCormac Jamieson Prichard. Photograph: Courtesy of Peter Cook
The appearance of visual concrete is affected by four key factors:
the composition of the concrete mix;
the formwork used;
any surface treatment after casting;
the quality of workmanship.
DESIGN CONSIDERATIONS
The satisfactory production of large areas of smooth concrete is difficult due to variations in colour and the inevitability of some surface blemishes, which can be improved, but not eradicated, by remedial work.
Externally smooth concrete weathers unevenly due to the build-up of dirt deposits and the flow of rainwater.
Therefore, if concrete is to be used externally as a visual material, early design considerations must be given to the use of textured or profiled surfaces to control the flow of rainwater. Generally, the range of finishes and quality control offered by precasting techniques are wider than those available for in situ work, but fre- quently construction may involve both techniques. The
use of external renderings offers an alternative range of finishes for concrete and other substrates. Figure 3.19illustrates the range of processes available in the production of visual concrete.
PRECAST CONCRETE
Precast-concrete units may be cast vertically or hor- izontally, although most factory operations use the latter, either face-up or face-down, as better quality control can be achieved by this method. Moulds are usually manufactured from plywood or steel. Whilst steel moulds are more durable for repeated use, ply- wood moulds are used for the more complex forms;
they can also be more readily modified for non- standard units. Moulds are designed to be dismantled for the removal of the cast unit and must be manu- factured to tight tolerances to ensure quality control on the finished product. As high costs are involved in the initial production of the moulds, economies of construction can be achieved by limiting the number of variations. This can have significant effects on the overall building aesthetic. Fixing and lifting systems
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Fig. 3.19 Types of visual concrete according to formwork and surface treatment
for transportation must be incorporated into precast units, usually in conjunction with the steel reinforce- ment. In addition to visual concrete panels, units faced with natural stone, brickwork or tiles extend the range of precast architectural claddings (Figs. 3.20 and 3.21).
The document PD CEN/TR 15739: 2008 categorises the range of precast concrete finishes according to flat- ness (P), texture (T) and colour (C).
Fig. 3.21 Reconstructed stone cladding—Experian Data Centre, Nottingham.Photograph: Courtesy of Trent Concrete Ltd.
Precast aircrete panels
Precast aircrete panels, 100 mm thick and to standard storey heights, are suitable for the inner leaf of stan- dard cavity construction and internal walls. Tolerances are close, similar to those required by the equivalent
Fig. 3.20 Slate surfaced precast concrete cladding—Swansea Museum.Photograph: Courtesy of Trent Concrete Ltd.
thin-joint masonry system. Maximum dimensions are 600×3000 mm with a standard thermal conductiv- ity of 0.11 W/m K. The thin mortar jointing system with 2 mm joints is used for fixing the panels. Larger units, 200 mm thick, are manufactured for commercial projects.
IN SITU CONCRETE
The quality of in situ visual concrete is heavily depen- dent on the formwork as any defects will be mirrored in the concrete surface. The formwork must be strong enough to withstand, without distortion, the pres- sure of the fresh concrete, and the joints must be tight enough to prevent leakage, which can cause honeycombing of the surface. A wide range of timber products, metals and plastics are used as formwork, depending on the surface finish required.
The Millau Viaduct in France (Fig. 3.22), completed in 2004, is an elegant cable-stayed bridge supported by seven slender piers 270 m over the Tarn Valley.
The main columns of 1.5 m thick concrete were cast in situ with self-climbing steel formwork externally and by crane hoist internally. The concrete piers are surmounted by 90 m steel pylons, which support the steel box section road platform. Each pier is sup- ported by four reinforced concrete piles splayed at the foot to spread the loading. During construction work of the 2.46 km viaduct, rapid-hardening con- crete was being placed at the rate of 80 m3 per hour and ultimately a total mass of 205,000 tonnes was used.
CONCRETE FINISHES Smooth finishes
In direct as-cast concrete, the surface texture and water absorbancy of the formwork or any formwork lining directly determine the final exposed fairfaced finish.
A high level of quality control is therefore required to ensure a visually acceptable finish. Hard, shiny, non- absorbent formwork materials, such as steel, glass-fibre reinforced polyester (GRP) or plastic-coated plywood, can give surfaces which suffer frommap crazingdue to differential shrinkage between the surface and under- lying bulk material. Additionally,blow-holescaused by air bubbles trapped against the form face may spoil the surface if the concrete has not been sufficiently vibrated. Where the absorbency of the formwork varies, because of the mixing of new and reused form- work, or variations within the softwood timbers, or because of differing application of release agent to the formwork, permanent colour variations may be visible on the concrete surface. Release agents prevent bond- ing between the concrete and the formwork, which might cause damage to the concrete on striking the formwork. Cream emulsions and oils with surfactant are typically used as release agents for timber and steel, respectively. Formwork linings with controlled poros- ity can improve the quality of off-the-form finishes, by substantially reducing the number of blow-holes.
The linings allow the escape of air and excess mois- ture but not cement, during vibration. A good-quality direct-cast concrete should exhibit only a few small blow-holes and modest colour variation.
Fig. 3.22 Concrete columns—Millau Viaduct above the Tarn Valley, France.Architect: Foster + Partners.Photograph: Arthur Lyons
L I M E , C E M E N T A N D C O N C R E T E 9 3 The application of paint to off-the-form concrete
will emphasise the surface blemishes such as blow- holes. These become particularly noticeable if a light- coloured gloss paint is used. Surface defects must therefore be made good with filler before priming and subsequent painting of the concrete.
Textured finishes
A variety of textured finishes can be achieved by the use of rough-sawn boards as formwork. The grain effect can be enhanced by abrasive blasting, and a three- dimensional effect can be achieved by using variations in board thickness. Plastic materials, such as glass-fibre reinforced polyester (GRP), vacuum-formed thermo- plastic sheeting, neoprene rubber and polystyrene, can be used as formwork linings to give different pattern effects. Colour variations are reduced by the use of matt finishes, which retain the mould release agent during compaction of the concrete. The number of blow-holes is reduced by the use of the slightly absorbent mate- rials such as timber and polystyrene. Concrete panels cast face-up can be textured by rolling or tamping the concrete whilst it is still plastic.
Ribbed and profiled finishes
Ribbed concrete is typically cast in situ against vertical timber battens fixed to a plywood backing. In order to remove the formwork, without damage to the cured concrete, the battens must be splayed and smooth. A softer ribbed appearance is achieved by hammering off the projecting concrete to a striated riven finish.
Profiled steel formwork and rope on plywood produce alternative finishes. Where deep profiles are required, expanded polystyrene and polyurethane foam can be carved out to produce highly sculptural designs.
Abraded, acid etched and polished finishes
Light abrasion with sand paper may be applied to in situ or precast concrete. Acid etching is normally limited to precast concrete due to the hazards asso- ciated with the use of acids on site. Both techniques remove the surface laitance (cement-rich surface layer) to create a more stone-like finish with some exposure of the aggregate. Polishing with carborundum abra- sives produces a hard shiny finish, imparting full colour brightness to the aggregate. It is, however, a slow and therefore expensive process.
Exposed aggregate finishes
The exposure of the coarse aggregate in concrete, by removal of the smooth surface layer formed in con- tact with the formwork, produces a concrete with a more durable finish and better weathering character- istics, which is frequently aesthetically more pleasing.
Smooth, profiled and deeply moulded concrete can all be treated, with the visual effects being largely depen- dent on the form and colour of the coarse aggregates used. While gap-graded coarse aggregates can be used in both precast and in situ exposed aggregate finishes, precasting gives additional opportunities for the uni- form placement of the aggregate. In face-down casting, flat stones can be laid on the lower face of the mould, which can be pretreated with retardant to slow the hardening of the surface cement. In face-up casting, individual stones can be pressed into the surface either randomly or to prescribed patterns without the use of retardants. Alternatively, a special facing mix can be used on the fairfaced side of the panel, with the bulk material made up with a cheaper standard mix. The aggregate has to be exposed by washing and brush- ing when the concrete has cured sufficiently to be self-supporting. The use of a retarder applied to the formwork face enables the timing of this process to be less critical. The surface should be removed to a depth no more than one third of the thickness of the aggre- gate to eliminate the risk of it becoming detached. An alternative method of exposing the aggregate in both precast and in situ concrete involves the use of abrasive blasting. Depending on the size of grit used and the hardness of the concrete, a range of finishes including sculptural designs can be obtained.
Tooled concrete finishes
A range of textures can be obtained by tooling hardened concrete either by hand or mechanically.
Generally, a high-quality surface must be tooled as blemishes can be accentuated rather than eliminated by tooling. Only deep tooling removes minor imper- fections such as blow-holes and the effects of slight formwork misalignment. Hand tooling is suitable for a light finish on plain concrete and club hammering can be used on a ribbed finish. Where deep tooling is anticipated, allowance must be made for the loss of cover to the steel reinforcement. The exposed aggregate colour in tooled concrete is less intense than that pro- duced by wash-and-brush exposure due to the effect of the hammering on the aggregate. Standard mechanical
Fig. 3.23 Tools for indirect visual concrete finishes
tools are the needle-gun, the bush hammer and the point-tool (Fig. 3.23). A range of visual concrete fin- ishes is illustrated inFig. 3.24.
Weathering of concrete finishes
The weathering of exposed visual concrete is affected by the local microclimate, the concrete finish itself and the detailing used to control the flow of rainwater over the surface. It is virtually impossible to ensure that all sides of a building are equally exposed, as inevitably there will be a prevailing wind and rain direction which determines the weathering pattern. It is therefore likely that weathering effects will differ on the various eleva- tions of any building. Some elevations will be washed regularly, whilst others may suffer from an accumula- tion of dirt which is rarely washed. However, this broad effect is less likely to cause unsightly weathering than the pattern streaking on individual facades.
The choice of concrete finish can have a significant effect on the weathering characteristics. Good-quality dense uniform concrete is essential if patchy weather- ing is to be avoided, and generally a rougher finish is likely to perform better than a smooth as-cast finish. Profiling and the use of exposed aggregates have the advantage of dictating the flow of rain- water, rather than letting it run in a random man- ner, but dirt becomes embedded in the hollows. Dark aggregates and bold modelling minimise the change in appearance on weathering, but generally, exposed non-absorbent aggregates are likely to give the best
weathering performance. Horizontal surfaces may be subject to organic growths and this effect is increased by greater surface permeability.
Careful detailing is necessary to ensure a dispersed and controlled flow of water over the washed areas. The water should then be collected or shed clear by bold details to prevent pattern staining below. Water col- lected onto horizontal surfaces should not be allowed to run down facades below, so copings, sills and string courses all should be provided with drips to throw the water off the building face; alternatively, water should be removed by gutters. Multistorey facades should be articulated with horizontal features to throw the water off, at least at each storey height. Only on seriously exposed facades where strong winds are likely to cause rain to be driven upwards, should small horizontal drip projections be avoided. Where concrete is modelled, due consideration should be given to the direction of flow and the quantity of rainwater anticipated.
EXTERNAL RENDERING
Renders are used to provide a durable and visually acceptable skin to sound but unattractive construction.
Renders can reduce rain penetration and maintain the thermal insulation of walls. The finishes illustrated in Fig. 3.25are all appropriate for external use. In each case it is essential to ensure good adhesion to the background. Where a good mechanical key, such as raked-out brickwork joints, is not present, an initial stipple coat of sand, cement, water and appropriate
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Fig. 3.24 Selection of visual concrete finishes
Fig. 3.25 Typical render finishes
bonding agent (e.g. styrene-butadiene-rubber) is required to create a key. Bonding is also affected by the suction or absorbency of the background; where suction is very high, walls may be lightly wetted before the rendering is applied. Metal lathing may be used over timber, steel or friable masonry to give a sound background. Two or three coats of rendering are nor- mally applied; in either case the successive coats are weaker by a reduction in thickness or strength of the mix. Smooth renders require careful workmanship for external work, as they may craze if finished off with a steel rather than a wooden float.
Generally, permeable renders are more durable than dense impermeable renders as the latter may suffer cracking and subsequent localised water penetration.
Sands for external renderings should be sharp rather than soft. The design detailing of rendering is impor- tant to ensure durability. The top edges of rendering should be protected from the ingress of water by flash- ings, copings or eaves details. Rendering should stop above damp-proof course level and be formed into a drip with an appropriate edging bead. Rainwater run-off from sills and opening heads should be shed away from the rendering to prevent excessive water absorption at these points, which would lead to deteri- oration and detachment of the rendering.Figure 3.26 illustrates the striking visual effect of the rendered blockwork student halls of residence at the University of East London adjacent to the Royal Albert Dock.
Roughcast render
Roughcast consists of a wet mix of cement (1 part), lime (0.5 part), sand (3 parts) and 5–15 mm shingle or
crushed stone (1.5 parts), which is applied to walls by throwing from a hand scoop.
Dry-dash render
A 10 mm coat of cement (1 part), lime (1 part) and sand (5 parts) is applied to the wall, and whilst it is still wet, calcined flint, spar or shingle is thrown onto the surface and tamped in with a wooden float.
Scraped finish
A final coat of cement (1 part), lime (2 parts) and sand (9 parts) is applied and allowed to set for a few hours, prior to scraping with a rough edge (e.g. saw blade) to remove the surface material. After it has been scraped, the surface is lightly brushed over to remove loose material.
Textured finishes
A variety of finishes can be obtained by working the final rendering coat with a float, brush, comb or other tool to produce a range of standard textured patterns.
Pargeting, in which more sophisticated patterns are produced, has its cultural roots in Suffolk and Essex.
Tyrolean finish
For a Tyrolean finish, cement mortar is spattered onto the wall surface from a hand-operated machine.
Coloured mixes may be used.
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Fig. 3.26 Rendered blockwork—University of East London, Docklands Campus.Architects: Edward Cullinan Architects.Photograph: Arthur Lyons
Painted rendered finishes
Most renderings do not necessarily need painting;
however, smooth renderings are frequently painted with masonry paint to reduce moisture absorption and give colour. Once painted, walls will need repainting at regular intervals.