CONCRETE CAN BE BEAUTIFUL - THE ROLE OF SURFACE FINISHES IN ENHANCING THE APPEARANCE OF CONCRETE

    CONCRETE CAN BE BEAUTIFUL - THE ROLE OF SURFACE FINISHES IN ENHANCING THE APPEARANCE OF CONCRETE Christopher C. Stanley*, Concrete Consultant, Ho...
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CONCRETE CAN BE BEAUTIFUL - THE ROLE OF SURFACE FINISHES IN ENHANCING THE APPEARANCE OF CONCRETE Christopher C. Stanley*, Concrete Consultant, Hong Kong. 32nd Conference on OUR WORLD IN CONCRETE & STRUCTURES: 28 - 29 August 2007, Singapore

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32nd Conference on OUR WORLD IN CONCRETE & STRUCTURES: 28 – 29 August 2007, Singapore

CONCRETE CAN BE BEAUTIFUL - THE ROLE OF SURFACE FINISHES IN ENHANCING THE APPEARANCE OF CONCRETE Christopher C. Stanley*, Concrete Consultant, Hong Kong.

Abstract An increasing number of new & beautiful concrete buildings & structures are now gracing our city skylines. But despite recent advances in both architectural design & construction the concrete jungle image, first perceived as a result of uncontrolled urban sprawl in the early 1960's is hard to shake off. Concrete - multi purpose in its functions offers an extensive range of appearances & complexions. Concrete has the ability to be moulded into an almost limitless variety of shapes or forms to satisfy the imagination of any designer. A surface to suit the structure & a surface to match the purpose is the target at which the design of concrete finishes should aim. Used properly concrete can make a significant contribution to our 21st century lifestyle. But it is not only the shape or form that is important; the appearance of the finished product must also not be neglected. Many new airports, civic amenities & houses are often spoilt because in some areas, just for the sake of saving a few dollars, the concrete is left unfinished. But concrete finishes don’t have to be costly & they can greatly contribute to the appearance of even the most basic & mundane buildings & structures. An infinite range of finishes are available that can be adapted to suit any situation. But what is more important - form or function? The answer is the right form for the right function. This paper draws attention to some of the basic types of finish available, in the hope that it will make future designers aware of the best way of fulfilling even their wildest aspirations. Never forget that used properly concrete can be a very beautiful & creative medium. A concrete surface finish is more than cosmetic; it is a building or structure's skin through which its personality is expressed. The choice of the texture & colour of the concrete should always relate to the overall design, in order to bring closer together that function of harmony of purpose & appearance, combined with a realistic sense of climate & surroundings upon which all fine forms of construction ultimately depend.

Introduction Concrete does not have to be an ugly black-stained austere material daubed in graffiti. It can be made to look beautiful & blend aesthetically into the environment into which it is placed. Many years ago it was considered unnecessary, & sometimes even unacceptable, to paint concrete. The writer of this

paper came close to being dismissed from his employment for even daring to suggest that concrete should be painted! Indeed his suggestion was treated with derision by many Architects, & he was politely informed that he was not qualified to make such public statements - but why not? After all we paint wood & steel so why should concrete be any different. Indeed today most concrete buildings in Singapore & many other Asian countries are now painted. But 20 years ago that was not the case. The first attempts at painting concrete were to paint it white by using a type of paint specially formulated for concrete called Snowcem. It was later followed with a series of polymer paints of many colours called Sandtex & the era of concrete decoration was truly born. Even today there is considerable resistance against painting concrete. Opponents say painting creates a maintenance problem. However what they don’t realise is that the maintenance problems of unprotected concrete may eventually end up being far greater & more expensive to resolve than the cost of an occasional coat of paint. Many buildings & structures are still finished in plain, as-cast concrete finishes. Any contractor, who has tried to produce a flat plain uniform grey, or coloured, blemish-free concrete finish will testify as to the near impossibility of such a task. Indeed, in the writer's opinion such finishes should never be specified. Even if such a finish is produced, often at great expense, & probably after many confrontations between the Architect & the Contractor, it won't stay in a pristine condition for very long. Lime exudation & weathering will take their toll after only a few years exposure, even in fairly mild environmental conditions. A completely plain surface therefore is not likely to succeed unless it is painted. So there is a need to design & specify a finish that can blend into, weather uniformly, & adapt to the environment in which it is to be used. This does not mean that all concrete has to be painted. In fact there is a wide variety of what are referred to as direct & indirect finishes available for enhancing the appearance of concrete surfaces. Direct & Indirect finishes Direct finishes are defined as those that are 'as-struck' finishes; that is the finish is achieved with the minimum of extra work, after removal of the formwork. Such finishes include modeled or profiled finishes & some types of exposed aggregate or tiled finishes. Indirect finishes are those which require some additional work to create the concrete finish. For example by mechanically removing the surface of the hardened concrete in order to expose the underlying aggregate; or by painting or treating the concrete in order to change its as-struck appearance. Some finishes are equally appropriate to in-situ & pre-cast concrete & some are better suited to one or the other. Specifications Concrete finishes tend to be subjective & as yet there is no universally adopted method of specifying them. In the British Standard Code of Practice for Structural Concrete, BS8110, surface finishes are grouped into three categories. Class 2, is for general finishes exposed to view, but where appearance is not critical. Class 1, is for most surfaces exposed to view, including the surfaces of industrial & domestic buildings, & Special is appropriate to the highest standards of appearance, such as might be found in Cathedrals & other prestigious buildings, where it is possible to justify the high cost of production. Concrete for internal use is further subdivided into Type A, B, & C finishes, where the Type C finish involves first producing a Type B finish & then rubbing down & filling noticeable surface blemishes to match the colour of the original concrete. The UK Department of Transport Specification for Road & Bridge Works defines finishes from F1 to F5. (The latter specified as being free from any blemishes & imperfections such as discoloration & fins). These categories often give rise to considerable disputes between contractors & engineers on site as to what is an 'acceptable' finish. There are also categories for 'unformed finishes' such as road & floor surfaces ranging from U1 - U3.

Many other specifications exist around the world; however, few are in overall agreement. None give sufficient guidance in order to avoid disputes occurring on site. In the end sensible engineering or architectural judgement, combined with a basic understanding of concrete material science, is what is required, but this is often lacking in most of the disputes that arise. Reference samples In order to avoid misunderstandings & later confusion taking place on site it is essential to start off by casting a reference sample of any proposed finish. Such a sample should not be a hand held block of concrete, but a full-size mock up of an intended column or wall panel as an example. The people on site who will be responsible for the concrete finishing aspects of the project should prepare the sample. This will also give them the necessary practical experience & they will then know what is required in terms of the finish. Another important aspect is to decide what is an acceptable distance from which to view the proposed finish. This will depend whether it is situated some distance up the side of a building or structure or forms part of a prestigious building that will be viewed from comparative close up. Once a sample has been produced which is considered to be of acceptable quality, this can then be used as a basis on which to judge subsequent finishes by comparison. Plain concrete finishes As mentioned above, plain flat concrete finishes should be avoided wherever possible. This is because a continuous uniform grey or coloured surface is almost impossible to achieve in practice. This is due to the fact that small colour changes will be caused by even the slightest changes in the batch proportions, the absorbency of the formwork, or the presence of joints, & any variation in the compaction & subsequent curing procedure. The situation is even worse where pigmented coloured concrete is used. The worst colours to produce as a uniformly coloured surface are black, blue, & green. Even if a fairly uniform colour is produced, the effect may only be short -lived, because the concrete surface will rapidly be affected by the formation of efflorescence & possible fading. Red & buff are fairly permanent colours, which are based on iron oxide pigment, but other colours tend to be less stable, especially when used externally. Exposed aggregate finishes Coloured concrete can however easily be produced by using exposed aggregate finishes, as described below, where the principal colour comes from the aggregate. In such a finish a coloured matrix can be used between the stones, because even if it does vary slightly, such an effect will be masked by the overall appearance of the surface finish. Exposed aggregate finishes also have the advantage, that they will maintain the colour of the rock for the life of the finish. Finishes can either be produced in-situ, or by using pre-casting techniques. Some finishes are more suitable for use in-situ than for pre-cast concrete & some can be employed equally well in both types of construction. Exposed aggregate finishes are widely used throughout the world, but for some unknown reason, have yet to gain universal acceptance in the Far East. In Hong Kong & Singapore their use has been very limited. This is surprising because of the wide variety of local aggregate types & options that are available. Importance of the concrete mix proportions Because the emphasis is on appearance, when making an exposed aggregate finish, careful attention to the development of the mix design is essential. The grading of the aggregate must be such as is required to produce a dense uniform packing & distribution over the entire surface. At the same time, the mix must have sufficient fines in order to prevent segregation & the formation of honeycombed areas during the compaction process. Particle packing is a science in itself. For example if three spheres are placed together, touching each other, then the size of the sphere required to fit between them is one with 1/7 of their diameter. So to fill all the voids, each sphere has to be 1/7 of the diameter of the next largest one & so on. Since we can seldom have the luxury of using perfectly spherical aggregates, the voidage between aggregates can either be assessed using the British Standard Angularity test, which assesses the voidage relative to 33%. (The voidage between perfect single- sized spheres), or the optimum packing of the

aggregates can be assessed using a Voidmeter. This is an apparatus where the aggregates are packed into an airtight cylinder & the voids are assessed by measuring the partial vacuum between the particles, when the air is extracted using a u-tube manometer partially filled with water. It follows therefore that this test will not work on highly porous or permeable aggregates. Initially the test is carried out using separate single sized aggregates, which can then be recombined to obtain their optimum packing ratio. The smallest voids are eventually filled with the paste of water & cement. This is the purist's approach. In practice, it is normally done on a trial & error basis based on an assessment of the aggregate grading of the component single sized aggregates, which are then mathematically combined together in order to produce the optimum overall grading. A good exposed aggregate surface can therefore only be produced using an individual single size aggregate or the optimum proportions of re-combined single size materials. An 'all-in' 40mm-5mm or 20mm-5mm graded aggregate will clearly not therefore suffice for use in concrete where there is a primary requirement for the production of an exposed aggregate surface finish. Direct finishes Given that we have arrived at a suitable mix for producing an exposed aggregate finish; how is the finish best achieved? There are many ways of doing this & the decision is usually based on providing the most cost-effective solution, unless the finished appearance is of the most paramount importance. This may be the case, for example, in a prestigious building or structure where the cost is less of a critical factor. Wash & Brush finishes By far the simplest approach is the wash & brush finish. This is mainly a pre-cast technique, although it is often also used to provide a finish on flat in-situ slabs, such as those used on car parks & large paved areas including patios & terraces. After the concrete has been compacted & finished, it is left for an hour or so to stiffen. The surface is then gently sprayed with water, whilst at the same time it is brushed in order to expose the underlying aggregate. Timing is critical; brush too soon & the surface may be damaged. Brush too late & the surface may be difficult to expose. Between about ten & eighteen hours after casting the surface can also be exposed using a wire brush. When the concrete has fully hardened, the surface needs to be rinsed with a dilute 10% solution of hydrochloric acid, in order to remove slight traces of cement from the aggregate surface & to enhance the colour of the finish. It is essential that the concentration of the acid solution does not exceed the 10% concentration otherwise it will encourage future staining & efflorescence to occur. A similar finish can be prepared on vertically cast concrete surfaces by treating the form faces with a surface retarder before the mould is assembled. The retarders are generally sugar based chemicals & the concrete surface is sufficiently retarded so that the surface can be washed & brushed soon after the removal of the formwork in order to expose the aggregate. Using this technique, it is essential that the mould is filled near the centre of the concrete section that is being cast & the concrete is then encouraged to flow towards the formface. In addition any internal vibrators used for the compaction process must be kept away from the formface whilst the concrete is being compacted. Sand-Bed finishes Where absolute uniformity of the finished appearance is required, this can best be accomplished by using the sand-bed technique. In this method, which is used in pre-cast concrete, a layer of fine sand is first spread in the bottom of a mould. The depth must not exceed one third of the maximum size of the aggregate being used. Then individual pieces of stone are hand placed in the bottom of the mould & pressed firmly into the sand. A mortar is then spread over the stones & allowed to stiffen for about 30min, before the backing concrete is placed over it & compacted. Care has to be taken not to disturb the finish whilst the backing concrete is being placed. The size of stones used in this method normally ranges from 40mm, to sometimes in excess of 100mm. When the concrete has hardened the panel is stripped from the mould & any loose sand is brushed off the face in order to expose the finish. It is also possible to produce this type of finish in situ, by pre-casting the finish & then using it as an insitu permanent formwork liner against which the structural concrete backing is then cast. Such panels need a lightweight steel mesh to be incorporated in the mortar layer both to prevent damage due to handling & the hydraulic pressure of the concrete backing acting against the panel during the subsequent casting process.

The Aggregate Transfer method This type of finish can also be made for in-situ or vertically pre-cast concrete, using the aggregate transfer method. Essentially the technique is the same as that of the sand-bed technique, except water-soluble glue is first mixed with the sand. The stones are set in the water-soluble glue mixture, which is then allowed to set before the formwork is assembled & the in situ concrete is placed. Care has to be exercised that the facing is not dislodged during the subsequent placing & compaction of the in situ concrete. The concrete surface is then washed down with a spray to remove the sand layer & expose the aggregate. Grouted aggregates Where large rocks are used, a system using grouting can be employed. First large stones or rocks are placed inside the formwork. Then the voids between the rocks are filled by injecting a mortar into the mould starting at the bottom & working upwards. Soon after completion the formface can be removed & the surface washed to expose the aggregate. Indirect finishes Most of the other methods of producing an exposed aggregate finish achieve this by the indirect process & we are in the realms of mechanical tooling, whereby the surface of the hardened concrete is chipped away in order to expose the underlying aggregate & produce the finish. The tools work by distributing the point of impaction over greater or smaller areas depending on the depth of indentation & surface exposure that is required. The tools used to provide the exposed finish may consist of a simple hammer & chisel. But this is a very slow & labour intensive process & therefore most methods use tools that are either electrically operated, or driven by compressed air. When exposing the underlying surface with mechanical tools, it is good practice to leave a 25mm wide margin around the edges or corners of the unit being exposed in order to prevent unsightly spalling of the edges taking place. The margin can either be left as a feature or finished off gently later using a hammer & chisel. Mechanical tools Typical tools used to expose the concrete, are point tools, where a deep, rugged texture is required; the triple scaling hammer, mainly designed for use in cleaning steel surfaces, which when used on concrete, can produce a uniform regular fairly flat finish. Another tool is the needle gun or Jason's' Pistol, This tool is best used to expose the aggregate when the concrete is not more than 24hrs old, whereas the others need the concrete to be at least 3days old in order to be most effective. Grit blasting Another method used for exposing aggregate is grit blasting. This can be started as soon as the concrete is self-supporting. It is most economical to use within the first three days after casting. The surface is abraded away using fine hard grit, propelled against the concrete by compressed air. This system has the advantage that it can expose around sharp corners & arrises without spalling them. It is therefore also useful where complex shapes need to be exposed. If the surface is very hard the cost of grit blasting can be prohibitive; therefore the concrete should be treated as soon after removal of the formwork as possible. An important aspect to note is that an exposed finish will not mask any defects in the original concrete such as leaking formwork joints; on the contrary it will tend to make them more obvious & pronounced. Grinding & polishing Flat polished surfaces are used both for floors & for the surfaces of cladding panels. Since the emphasis will be on the colour & distribution of the aggregate, again attention to the mix design is essential in order to produce a uniform surface. Where the surface is to be polished the hardness of the aggregate should match that of the matrix in order to achieve the best results. Where very hard aggregates are used, if the matrix is not hard enough, the matrix surrounding the aggregate will be ground away more than the aggregate, leaving the stones slightly protruding. For this reason limestone & marble type rocks are most ideally suited for this purpose. After the surface has hardened sufficiently, the surface is given a preliminary grind, using rotating grindstones. The surface is then examined & any blowholes are filled in before the final grinding process takes place. The final grind is carried out using a very fine grindstone, after which the surface is cleaned & sealed in order to enhance the colour & appearance.

Combinations of direct & indirect finishes A technique that is both an indirect & a direct finish is the striated finish. This was first used on the exterior of the Elephant House in London's Regent Park Zoo in the early 1960's. The walls offer a hint of elephant skin with more than a hint of elephant strength. But while the metal roof of the building is streaked & stained with rain, the strong vertical ribbing of the finish has kept the effects of differential weathering of the concrete to a minimum. This type of finish is therefore particularly good at masking the effects due to weathering especially in an urban environment where the strong vertical ribbing will keep unsightly weathering to a minimum. Firstly tapered battens are fixed vertically to the formface. Such battens are stopped short of a construction joint, because it is virtually impossible to continue the profile across a joint line, either in line up or keeping the joint leak proof. About a month after the ribbed concrete has been cast, or when the concrete strength has reached at least 20Mpa, the ends of the ribs are broken off, by going down the ribs & hitting them alternately on each side with a steel club hammer. Modeled & profiled surfaces Apart from exposed finishes, complex decorative shapes can be obtained by casting concrete against rubber formliners. This technique, sometimes referred to as a modeled surface is often used for creating profiled or artificial rock surfaces, where liquid rubber is first poured against natural rock surfaces to create the outline that is subsequently reproduced in concrete. Careful use of pigments & stains can faithfully reproduce the appearance of natural stone. Such surfaces when they weather & produce growths of lichen are often indistinguishable from natural stone. Rubber form liners can be used to produce a wide variety of finishes & such formliners can, with care, be re-used many times. Vegetable oils must be used on rubber surfaces as release agents because mineral oils will deteriorate the rubber. Plastic formliners can also produce a wide variety of shapes. Flat thermoplastic sheets can be warmed & then vacuum-drawn against a master profile. Such finishes tend to produce a dense, cement rich surface. This may have a tendency to produce fine crazing cracks; especially if the formwork in left in position for several days before removal. These cracks are more noticeable where white cement is used; especially when the cracks fill with dirt. If fine crazing cracks are evident, the surface of the concrete should be sealed with a silane waterproofing solution, in order to maintain the appearance, before dirt & grime enter the cracks & tend to emphasise them. Foamed polystyrene can be cut, or melted with soldiering irons, to produce detailed & complex shaped formwork for a single use operation. When designing such formwork attention has to be focussed on whether it can easily be removed from the hardened concrete after casting. Timber & hardboard form liners are also widely used to provide many profiled concrete surfaces. Again such formliners often tend to be for single or a limited number of uses only. Stenciled surfaces A relatively cheap method for creating decorative finishes on horizontal surfaces such as floors & terraces is to provide an imprinted surface on the face of the concrete using a stencil. A flat slab is cast using conventional concrete & then a stencil is placed on the slab & a thin topping is then cast monolithically. It provides a cheap way of providing a surface, for example resembling the pattern of stone blocks or sets. A typical example can be seen within the Star Ferry concourse at Wanchai in Hong Kong. This system is widely used in the USA but has only been tried at a few locations in Asia. Most of the stencils are card type materials intended for single use only. Tile clad surfaces Tiled finishes are becoming increasingly popular & apart from being available in a wide variety of colours, tend to be self-cleansing & maintain their original appearance. Where tile surfaces are used it is essential to specify the provision of both horizontal & vertical movement joints. This is because the underlying concrete frame will move more than the tiled surface, due to the combined effects of creep, shrinkage & thermal expansion. This may cause both the tiles & render to fall off unless the joints are properly detailed. It is also important to ensure that any bedding render to which the tiles are affixed is securely bonded to the concrete substrate, & preferably contains an integral waterproofer to help stabalise it & prevent stains & efflorescence exuding from the tile joints. Larger tiles, ie over 100mm square, tend to be more successful on external elevations, rather than the smaller mosaic tiles. Tile

surfaces can tend to be prone to workmanship problems, which is the reason why their external application was banned in Singapore a few years ago following a series of high profile failures. Elsewhere in Asia they are still widely used & quite safe providing sufficient care is taken to design of the surfaces to which they are to be applied & attention is paid to the workmanship during application. Greater control of uniformity of tile surfaces can be obtained with the use of pre-cast concrete, where the tiles are cast face down in the mould. This system is used extensively for the finishes on cladding panels. Since the backing concrete can be cast directly against the tiles, this process eliminates the need for the use of either render or tile adhesives, which are the cause of most of the problems when tiles are applied to in situ concrete walls. Reconstructed Stone Rocks, particularly sandstone & limestone can be crushed to a maximum size of 5mm & re-combined with white or coloured cements to produce reconstructed stone surfaces more economically than the use of cut stone blocks. This technique has been widely employed in recent years, both in Singapore & Hong Kong, in order to successfully reproduce economically, the appearance of stone clad buildings. This technique is also employed for restoration work. For example, in Athens, Greece, part of the ancient Parthenon temple has been restored using pre-cast concrete column sections, which are hard to distinguish from the original structure. Appearance & weathering In selecting any type of surface finish, consideration needs to be given both to the exposure conditions & the weathering characteristics of the surface. For example it is pointless to specify a white concrete finish in a heavily polluted city environment. Equally, many finishes are spoilt, because architects pay scant attention as to how water flows across a surface. This often results in unsightly staining due to uncontrolled or uneven water flow. Detailing a building or structure to handle water run off is almost as important as the choice of the finish itself. Where the long - term appearance of a concrete surface is important, its appearance can be maintained, by the use of an integral waterproofing admixture, incorporated in the concrete mix during the batching & mixing process. Alternatively, when the hardened concrete has dried out, it can be sprayed with a Silane waterproofing solution. There is a common misconception that concrete is waterproof on its own & does not need to be waterproofed. That is definitely not the case; all concrete needs to be waterproofed no matter how good the quality is alleged to be. The importance of waterproofing concrete cannot be over emphasised, especially where the surface appearance needs to be maintained in a pristine condition. Waterproofing concrete also helps to reduce the black staining caused by mould & algae formation, which tends to happen, especially when the surface pH of the concrete is reduced, due to a combination of both acid rain & carbonation. Landscaping with water Buildings or structures can often be landscaped with water in order to enhance their appearance. Both pre-cast & in situ concrete can be used to modify the landscape or produce a water feature. There are some good examples surrounding some of the new housing developments in Asia as well as the new Casinos constructed in Macau. The water can also be designed to flow in a pattern or be turbid in order to add to the attraction. Appearance matters Careful selection of finishes can do so much to enhance the appearance of concrete buildings & structures. It should become an essential part of the design process, & not an afterthought, as is so often the case. A well-chosen finish can increase the value of a development as well as enhancing its appearance. Many low-cost public housing estates have been abused or defaced by their tenants, as a direct result of a lack of consideration being taken over their finished appearance. If the buildings present an attractive appearance the occupiers will take a pride in living in & also the upkeep of the property & this will also greatly reduce future maintenance costs. Concrete can be beautiful. All it needs is a little more careful thought & imagination at the conceptual stage. The comparatively recent application of glass & other types of chopped fibres added into

concrete can also enable the production of delicate or complex shapes to be achieved which were not previously attainable by any other method. Concrete does not have to be dreary or boring in its appearance. Perhaps we need a 'Beautiful Concrete Award' in order to stimulate future innovation in the use of concrete finishes. It could potentially have far more benefit to society than winning an Oscar!

Plate 1 Concrete's main advantage is that it can be moulded into any shape or form. The moulds for this sculpture were carved out of polystryene.

Plate 2 Brasilia, Brazil. The congress building. Even plain looking towers can be blended into the environment given careful thought and design.

Plate 3 Causeway Bay, Hong Kong. High land prices and shortage of space can lead to a concrete jungle. Trees and landscaping should form part of any development, particularly with the current emphasis on global warming.

Plate 4 Use of lighting and water can emphasize or highlight buildings and structures

Plate 5 Concrete mushrooms - emphasis on the human scale!

Plate 6 Concrete retaining walls can be enhanced with the use of murals.

Plate 7 The slender design and use of white and black exposed aggregate concrete add visual appeal to this water tower.

Plate 8 Landscaping with concrete paving and water can enhance the environment

Plate 9 House in Kuwait City made from Ferro-cement designed to resemble a typical Arab tent.

Plate 10 Natwest Tower, London, using vertical fins to emphasize height.

Plate 11 A hospital landscaped to provide a pleasant environment for patients.

Plate 12 Concrete exposed aggregate paving. Still in good condition more than 50 years after it was laid.

Plate 13 Concrete chess pieces and sculptures.

Plate 14 Eye catching Concrete Wall.

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