After removal from the form the element is allowed to harden to a uniform hardness. The product is submerged in an acid solution and scrubbed to remove the cement surface and to show the natural colour of the aggregates in a smooth sand-textured surface. It is used with great success for any type of decorative element.
Reasons for Use
- Acid etching produces a surface closely resembling natural stones such as limestone, brownstone and sandstone.
- Combining aggregate size and colour, matrix colour with texture from the acid etching provides flexibility for the final finish.
- Detail is not damaged with acid etching.
- Bright and deep colours are achieved with acid etching.
- The “pre-weathered” surface will remain consistent over very long periods.
- Retention of detail is best achieved with acid etching.
- Acid etching minimizes surface crazing by removing the cement skin on the surface.
- This surface is not as easy to patch as deeper etches.
- Imperfections in the form show up more readily than on deeper etches.
- The process attacks the cement surface, so higher quality.
- Concrete has to be used together with surface protectors.
- The acid solution has to be controlled regularly and has to be recycled (closed loop) by direct filtration and waste water treatment.
- The colour and texture of the sand used is very important with acid etched elements. It is deciding factor in controlling the appearance and colour of pre-cast elements.
- This finish is used to achieve a bright, sparkling natural stone look.
- Surface protection improves water repellence and provides Anti-Graffiti-properties. Pollutants cannot penetrate into the concrete, surfaces will remain clean for years and cleaning is easy using nothing more than water.
Advantages of Acid Etching over Sandblasting
- Exposed sand retains more “sparkle” with acid etching than with sandblasting because the aggregate is not ground but merely exposed.
- Acid etching achieves a finer sand texture than sandblasting.
- Sandblasting hardened concrete is time consuming and expensive. The deeper the blasting, the higher the cost.
- The exposure is influenced by the size of the sandblasting sand and the skill of the sandblaster. It is quite impossible to ensure long-term uniformity of surfaces. With acid etching the surface loss is only defined solely by the concentration of the acid and length of application.
Advantages of Acid Etching over Chemical Retardation and Water Washing
- The surface of water washed concrete is controlled by the retardant in the mould. Therefore, any variations in the exposure cannot be easily corrected later. Acid etched surfaces can be easily repaired without affecting the finish.
- Vertical, radius or complicated surfaces are difficult to cast uniformly because the retardant is subject to movement during casting on these surfaces and may result in variations in water washed etching. Acid etching achieves homogeneous surfaces for all these geometries.
Advantages of Acid Etching over “Out of the Mould”
- The surface of out-of-the-mould concrete will mirrors the surface of the form it is cast from, showing up even minor imperfections in the form. Acid etching eliminates seams and other small surface faults.
- Since the colour of out-of-the-mould concrete is basically controlled by the cement, variations between the pieces and within pieces will be more pronounced. The colour of acid etched elements is defined by the aggregates together with the matrix colour. Acid etching eliminates the upper layer, so the influence of colour from the mould is also removed.
- The out-of-the-mould surface is difficult to repair. Acid etching enables repairs to be done with a perfectly uniform surface finish.
- Pockets are usually more noticeable on the out-of-the mould surfaces.
- Out-of-the-mould surfaces may develop crazing (fine spider lines), there may also be shadowing by aggregate or reinforcing showing through the surface. Colour differences due to different mould materials have also been observed. An acid etched surface with its living texture of aggregates eliminates these visual blemishes.
Another plastic material available in clear and coloured sheet materials; breakable but does have a harder surface than polycarbonate. Stiffer than the same thickness of polycarbonate it is often used in retail type signs.
Aluminium comes from the most common of metallic ores, bauxite. It is energy expensive to extract but once extracted can be easily re-cycled; generally 60% of all aluminium used is actually from recycled material. Aluminium is normally used in one of three forms: Extrusions, Sheet, Casting (see separate entries for the explanation of these forms.)
Aluminium - Anodising
An electrochemical method of finishing aluminium to offer a hard robust finish to high quality new extruded aluminium. Colours are achieved by the use of non-organic dyes and as such are UV stable. These dyes can be susceptible to produce varying shades of the same colour across different section sizes.
As a rule, we would suggest darker browns or black are the most consistent and superior finishes closely followed by bronze & blue-grey colours.
Anodising for longer produces ‘hard anodising’ giving a thicker hard sealed surface more resistant to damage and scratching.
For more information on anodised aluminium for architectural application click here
Anti – Graffiti Coating
Coatings should really be considered in two applications:
- Stone or concrete products
- All other materials
On stone and concrete there a number of proprietary systems that can be used (AKS is our preferred system). A three layer system is applied, the prime coat that bonds to the concrete substrate, a protection coat and a final sacrificial coat. This final coat is removed when paint or graffiti is removed and a new coat needs to be re-applied after cleaning.
There are single coat polyurethane systems that offer some protection but they produce a very shiny surface, often sloppy, that offers limited protection.
For more information on anti-graffiti coatings for concrete/stone click here
On all other materials (and that often includes signs) the Ritec, rite coat 2200 system is a 2 part wet applied resin system. The first part actually prepares the surface of the item and removes contaminants; the second part forms a shiny non-stick coating.
Castings can be produced in a variety of methods and grades of aluminium. LM6 grade is normally used for marine applications; whether as a sand casting (offering low tool cost but relative costly castings) or pressure die-castings (high tool cost but cost effective components).
Concrete is a composite construction material composed primarily of aggregate, cement and water. There are many formulations that have varied properties. The aggregate is generally a coarse gravel or crushed rocks such as limestone, or granite, along with a fine aggregate such as sand. The cement, commonly Portland cement, and other cementitious materials such as fly ash and slag cement, serve as a binder for the aggregate.
Various chemical admixtures are also added to achieve varied properties. Water is then mixed with this dry composite which enables it to be shaped (typically poured) and then solidified and hardened into rock-hard strength through a chemical process known as hydration. The water reacts with the cement which bonds the other components together, eventually creating a robust stone-like material. Concrete has relatively high compressive strength, but much lower tensile strength. For this reason is usually reinforced with materials that are strong in tension (often steel).
Concrete can be damaged by many processes, such as the freezing of trapped water.
For more information on concrete specification click here
Contamination rust staining is very often caused when contaminated finishing media have been used or when ferritic atmospheric dust or other corrosive element is deposited on the material surface. This is a particular risk in multi-metal fabrication shops unless special precautions are not taken to avoid the spread of contamination.
During installation it is not uncommon for the dust from paving cutting discs to settle on the stainless steel street furniture resulting in orange staining. Any such contamination should be removed with clean water and suitable stainless steel cleaner before the surface becomes corroded.
Weathering steel, best-known under the trademark COR-TEN steel and sometimes as ‘Corten ‘, is a group of steel alloys developed to form a stable rust-like appearance, when exposed to external weathering conditions, that eliminates the need for painting or additional treatment. The finish of Corten steel changes considerably over time and may take several months to stabilise after installation.
Embossed/Debossed Lettering - Signage
These types of signs can be produced as laser cut and 3 dimensional signs or laser cut profiles let into paving, cladding or seating.
It is important when incorporating into paving that the material used (for instance stainless steel) has sufficient slip resistance. The detail on this is covered by British Standard.
Emeri / Idigbo - Timber
Occurs in Equatorial Guinea, Sierra Leone, Liberia, Ivory Coast, Ghana, southern Nigeria, in parts of the rain forest and throughout the deciduous forest areas.
A tall tree with a buttressed trunk attaining a height of over 30m and commonly 1.0m or more in diameter. The buttresses are broad and blunt, but the bole usually is clean and straight, 20m or more above the buttress.
A plain, pale yellow to light brown coloured wood. Sometimes relieved by a zonal figure originating in the growth rings, suggesting plain oak. There is little distinction between sapwood and heartwood, though the latter is somewhat darker in colour. The grain is straight to slightly irregular, and the texture is somewhat coarse and uneven. It is soft to medium hard, and weighs about 560 kg/m³ when dried. The weight is often variable, due to a prevalence of lightweight brittle-heart, particularly in large, over-mature logs. It may vary from 480 to 625 kg/m³ but for general assessment, the average dry weight is as given. Idigbo dries readily and well, with little distortion and splitting, and shrinkage is small.
Idigbo dries readily and well, with little distortion and splitting, and shrinkage is small.
It has excellent strength properties, being as strong and stiff as English oak in bending, although considerably softer and less resistant to shock loads. It splits easily and has been used in West Africa for roof shingles. When converting large logs the heart should be boxed out as the brittle- heart has very much lower strength properties than the normal wood. In freshly converted stock, brittle-heart may often be recognised by a distinctive pinkish colour which may develop after exposure to light for a few days. Natural compression failures, often referred to as "thunder shakes' usually accompany brittle-heart.
Medium - The timber works easily with most hand and machine tools. It has little dulling effect on cutting edges and a clean finish is obtained in most operations. There is a tendency however, for the grain to pick up when quarter-sawn material is planed, and a reduction of cutting angle to 20° or less is advisable where smooth surfaces are required. Idigbo turns well and has fairly good nail and screw holding properties and will take glue well; stains effectively and reacts well to finishing treatments.
- Moderately durable
- Extremely difficult
Density (mean, Kg/m³)
- 560 kg/m³
- Available at specialist timber merchant
- Stains yellow in contact with water, stains in contact with iron when wet. Acidic and may corrode ferrous metals.
- White/cream, Light brown
Engraving & Backfilling – Signage Material
Stainless steel or aluminium is engraved and then filled with paint to provide a robust fingerpost option.
Extrusions can be anodised, silver or coloured, painted or left as untreated or mill finish if the final appearance is not important.
The beauty of extrusions is that high levels of detail can be included, removing the need for expensive fabrications, and achieve a high degree of accuracy. The designer needs to understand the design process and its limitations; aluminium is not as strong as steel for instance.
Materials (such as stainless steel) can be let into the surface of the stone using water cut stone with laser cut templates. (See Cabot Circus Project).
A wide variety of glazing materials can be used but glazing used in the Public Realm is covered by BSEN 6001? – Final selection must be compliant with this standard.
Iroko - Timber
C. excelsa has a wide distribution in tropical Africa, from Sierra Leone in the west, to Tanzania in the east.
C. regia is confined to West Africa, where it occurs from Senegal to Ghana. There does not appear to be any significant difference between the timber of the two species.
C. excelsa attains very large sizes, reaching 45m or more in height and up to 2.7m in diameter. The stem is usually cylindrical and mostly without buttresses. It occurs in the rain, and mixed deciduous forests.
When freshly cut, or when unexposed to light, the heartwood is a distinct yellow colour, but on exposure to light it quickly becomes golden-brown. The sapwood is narrow, being about 50mm to 75mm wide, and clearly defined. The grain is usually interlocked and the texture is rather coarse but even, and the wood weighs on average 660 kg/m³ when dried. Large, hard deposits of calcium carbonate called 'stone' deposits are sometimes present in cavities, probably as a result of injury to the tree. They are often enclosed by the wood and not visible until the time of sawing, though the wood around them may be darker in colour, thus giving an indication of their presence.
The timber dries well and fairly rapidly, with only a slight tendency to distortion and splitting.
Iroko has excellent strength properties, comparing well with teak, though weaker in bending and in compression along the grain.
Medium to difficult - Iroko works fairly well with most tools, though with some dulling effect on their cutting edges, especially when calcareous deposits are prevalent. On quarter-sawn stock, there is a tendency for grain to pick up due to interlocked grain, and a reduction of cutting angle to 15° is usually necessary to obtain a smooth surface. An excellent finish can be obtained if the grain is filled. It takes nails and screws well, and can be glued satisfactorily.
- Extremely difficult
Density (mean, Kg/m³)
- 660 kg/m³
- Available at specialist timber merchant
- Occasional deposits of stone may occur
- Yellow brown
Glass – Toughened
Glass is formed into sheets then heated in an oven to produce toughened glass. The glass is heat-sinked (kept and a high temperature for a long period) to ensure that inclusions in the sheet are excluded. These inclusions if not removed may very slowly rise to the surface of the glass panel and cause the panel to explode.
Glass – Laminated
Two sheets of glass are interleaved with a pliable resin that bonds the shards together in the case of breakage of the glass.
Liquid Stone (Ultra High Performance Concrete - UHPC)
UHPC is a class of concrete defined by its exceptionally high strength and durability. It was developed in the 1980s for specialized applications that demanded superior strength and corrosion resistance – marine anchors, piers and seismic structures. Over the last three decades, use of UHPC has expanded to applications requiring its high strength in narrow profiles, such as bridge spans and building façades. In these cases, the material’s strength, wear resistance, lighter weight and lower life cycle costs have been the driving determinates. Escofet is the first Spanish company to unite formulation, manufacturing and design to create UHPC products that address the requirements of the architectural and design profession.
What is Liquid Stone?
Liquid Stone is a new ultra high performance concrete (UHPC) with unlimited potential in the architecture, landscape and product design industries. Liquid Stone’s unique formula and manufacturing process, developed with our research partners in iMat-Construction Technologic Center Polytechnic and Polytechnic University of Catalonia yield a UHPC with superior strength, durability and colour saturation that can be cast in a wide variety of shapes and patterns
Liquid Stone is a new UHPC distinguished by its high matrix density. In partnership with UHPC specialists in Polytechnic University of Catalonia we’ve developed a formulation optimizing the particle size of each material to ensure a densely packed concrete matrix. The tight packing of particles creates stronger chemical bonds and lower water absorption, which yields extremely high compressive, tensile and flexural strength. The resulting material exhibits a beautiful surface with integral pigment that stands up to water, salt and corrosive environmental contaminants.
Liquid Stone’s performance on all fronts is extraordinary. Exceptionally high compressive, tensile and flexural strength; durability in a wide range of conditions and climates; highly saturated integral colour – all achieved with minimal waste and environmental impact makes Liquid Stone a leader in performance.
Traditional precast cast concrete and GFRC panels often fail because water and salt (marine and road) are absorbed over time into its pores. The water then freezes and cracks the concrete from the inside out; the chloride ions penetrate the matrix and eventually break down steel reinforcements. Most GFRC is coated to prevent infiltration of the concrete matrix by water and harmful environmental contaminants. In contrast, Liquid Stone is so dense and has so few pores that it absorbs almost no water over time and does not require a coating to enhance durability; therefore, it has excellent freeze-thaw performance and is highly resistant to salt. This extremely dense material matrix has the added benefit of protecting the pigments and improving UV performance.
The colour of Liquid Stone is integrated throughout the material. By adding pigment to the mix, Liquid Stone’s colour is consistent throughout, unlike many other materials that are colour-coated. A wide range of colours and hues can be achieved and offer rich saturation and colour fastness. Our pigments have been tested and selected as a result of their UV performance, stability and visual appeal. In addition to the standard colours developed by our designers and engineers, we can create and match a variety of custom colours.
UHPC Technical Properties
Concrete consistency fluid / liquid, with a minimum cement content 700kg/m3 with organic fibres, inorganic or metal and siliceous aggregates smaller than 1mm.
- UHPC 90-150 MPA
- UNE-EN 12390/2001
- UHPC 16-35 MPA
- UNE-EN 12390/2001
ABSORPTION OF WATER
- UHPC 6.50% without the use of repellents
- UNE-EN 1339/2004
RESISTANCE TO FROST
- UHPC 0 Kg/m2 Loss of mass after 28 cycles of freezing / thawing (with dicing salts)
- UNE-EN 1339/2004
WEAR RESISTANCE OF FRICTION
- 18.3 mm UHPC (Average width of the track)
- UNE-EN 1339/2004
- UNE-EN 127748-1/2006
Another dry electrostatic spray coating using polyamide resins to produce a durable slippery coating; not easy to repair on site and should be used on protected rather than raw steel. It is difficult to paint over as a maintenance routine due to its finish, ideally needs to be abraded and use special paints that chemically bond to the nylon.
Q. petraea produces the sessile or durmast oak, while the pedunculate oak is produced by Q. robur: both species occur throughout Europe including the British Isles, and extend into Asia Minor and North Africa.
Both species reach a height of 18m to 30m or a little more depending upon growth conditions which also affect the length of the bole. When drawn up in forests at the expense of their branches, this may be 1 5m or so in length, but in open situations, the tree branches much lower down. Diameters are about 1.2m to 2m.
There is no essential difference in the appearance of the wood of either species. The sapwood is 25mm to 50mm wide and lighter in colour than the heartwood which is yellowish-brown. Quarter-sawn surfaces show a distinct silver-grain figure due to the broad rays. The annual rings are clearly marked by alternating zones of early-wood consisting of large pores, and dense late-wood. Conditions of growth accordingly govern the character of the wood to a great extent; for example, in slowly grown wood the proportion of dense late-wood is reduced in each annual growth-ring, thus tending to make the wood soft and light in weight. The growth conditions in the various countries which export oak vary considerably.
Baltic countries, including northern Poland, produce oak which is generally hard and tough, but further south in Poland the growth conditions become more favourable to the production of milder, more uniformly-grown oak, the rich black soil of south-east Poland producing the famous Volhynian oak, the character of this type of wood changing but little in countries in Central Europe such as Czechoslovakia and Hungary, but being generally a little milder in character in Yugoslavia, from whence Slavonian oak is shipped. The weight of oak varies according to type; that from the Baltic area, western Europe, and Great Britain being about 720 kg/m³ and that from Central Europe about 672 kg/m³ on average after drying.
So-called brown oak is the result of fungus attack in the growing tree. The fungus, Fistulina hepatica, causes the wood first to assume a yellow colour, then a richer brown or reddish-brown. A yellow-coloured streak sometimes appearing in oak is the result of another fungus, Polyporous dryadeus, but since very few tree diseases persist after the tree is felled, dried timber is no different from normal coloured wood, indeed, brown oak is often preferred for its decorative appeal.
Oak dries very slowly with a marked tendency to split and check, particularly in the early stages of drying, and there is considerable risk of honeycombing if the drying is forced, especially in thick sizes. End and top protection must be provided to freshly sawn stock exposed to sun and drying winds, and sticker thickness should be reduced to about 12mm for stock piled in the open air during early spring and onwards until winter.
Both the sessile and pedunculate oaks have well known and high strength properties, and those hybrid oaks developed from both types and common throughout Europe, are similar in their strength properties.
Medium to difficult - The working and machining properties of oak vary with the mild to tough material which either machines easily or with moderate difficulty. These basic properties are concerned with growth conditions, but they may be exaggerated by indifferent drying methods which allow plain-sawn boards to cup, or severe case-hardening to develop, causing excessive wastage in planing and moulding, cupped stock in resawing, and a greater degree of blunting of cutting edges. These must be kept sharpened, particularly where cross grain is present, and especially in planing highly-figured quarter-sawn surfaces where there may be a liability for the grain to tear out at the juncture of the wide ray-figure thus producing a shelly appearance. In general, oak finishes well from the planer or moulding machine although in some cases a reduction of cutting angle to 20° is preferable. The wood can be stained, polished, waxed, and glued satisfactorily, takes nails and screws well, except near edges, when the wood should be pre-bored, and takes liming and fuming treatments well.
- Extremely difficult
Density (mean, Kg/m³)
- 720 kg/m³
- Medium to coarse
- Readily available at timber merchant
- Iron staining may occur in damp conditions, similarly corrosion of metals.
- Yellow brown
Paint – Micaceous Finishing
A painting system as part of the highways agency standard painting system offering a purely protective finish.
A green or brown film on the surface of bronze or similar metals, produced by oxidation over a long period or a gloss or sheen on wooden furniture produced by age and polishing.
Permeability refers to how well water flows through a material - this is controlled by how large and how well connected the pores in the material are.
Polyurethane – Engineering Grade (Ferrocast)
Urethane resigns, normally used with a structural steel core, offering a wide variety of through colours, good mar resistance and the ability to produce a variety of shapes since it lends itself to casting process.
Polyester Powder Coating
What is powder coating?
Powder coatings are used when a hard finish which is tougher than conventional paint is required. Basically anything capable of holding a small electrical charge can have powder applied to it. It is usually used for coating metals, however, newer technologies allow other materials such as MDF and even glass and ceramics to be coated using different coating methods.
So how does it work?
Powder application is very simple. The powder is applied to an object using an electrostatic gun. Compressed air pushes the powder out of the gun, past an electrode which gives the powder a positive charge. The object being coated is grounded so the positive powder particles are attracted to it.
When the object is completely covered, the ground is removed and the object is put into the oven to be baked. While in the oven, the powder goes through a chemical change where it becomes free flowing and forms a ‘skin’ around the object. After cooling, the powder is ‘set’ and forms a hard finish. Due to the process needing ovens to cure the coating the restriction on the item to be coated is governed by the physical dimension of the oven, normally 4m length.
What can I apply powder to?
Powder can be applied to items kept inside; including washing machines and lighting and also to exterior items including architecture and fencing. Powders are capable of withstanding extreme environments, thanks to our intensive testing procedures. The levels of powder durability are dependant on the mix of ingredients used.
The advantages of powder coatings over conventional paint
Powder coatings :
- contain no solvent.
- emit no VOC’s (volatile organic compounds)
- can produce much thicker coatings without running or sagging
- overspray can be recycled and reused (no wastage)
- production lines produce less hazardous waste than conventional liquid coatings
Some powder coatings are coatings guaranteed for up to 20 years (Akzo Nobel D3000 system) and are currently in the testing cycle for 10 years external exposure in Florida
"AkzoNobel’s Interpon D2000 product has already surpassed 15 years on a California hospital (Mission Hospital, Viejo) in 2006.
A quote from this project is stated as:
"Measurements of its gloss and colour showed they had hardly changed since installation in 1991, making re-painting or re-cladding a far-away prospect."
Polyester Powder Coating - Zinc Rich Primer
Woodhouse specify a zinc rich PZ coating (Akzo Nobel Interpon process).
The process involves:
- It dramatically reduces risk of thermal distortion
- No cleanup and fettling required as powder layer is even
The PZ and APP layers can be powder coated or wet painted to obtain final colour
This also has a range of benefits over the galvanising process:
A final coat system offering a colour finish option on top of other paint systems allowing colour match whilst still offering a protective paint system.
This paint system is used often for painting of lighting columns and is normally a spray or brush applied wet paint system therefore not limiting the size of the object painted. Marine grade finishing is also available in this system.
A plastic material, normally clear but opal is available, that offers protection from penetration and is arguably unbreakable.
It is flexible (so it needs supporting more often than toughened glass for example) and soft so is easily scratched. Harder surface finished product is available, albeit more expensive. Cheaper product is susceptible to ultra violet light degradation causing yellowing and weakening.
Porous Materials – Porosity
Porosity is the amount of void space in a rock or other earth material (like a sand deposit) - i.e. how much water a material can hold.
Sapele - Timber
Listed in the IUCN Red List of Threatened Species as VU – Vunerable: at risk of extinction.
Not listed in CITES. Believed available from well-managed sources. Check certification status with suppliers.
It is found in the rain forests of West Africa from the Ivory Coast through Ghana and Nigeria to the Cameroons, and it extends eastwards to Uganda and Tanzania.
A very large tree with cylindrical bole and small or no buttresses. Grows to a height of 45m or more, and a diameter at breast height of 10m or slightly more.
The sapwood is pale yellow or whitish, the heartwood pinkish when freshly cut, darkening to typical mahogany colour of reddish-brown. Sapele is characterised by a marked and regular stripe, particularly pronounced on quarter-sawn surfaces. Occasionally mottle figure is present, it is fairly close textured, and the grain is interlocked. It is harder and heavier than African mahogany. Weighing about 640 kg/m³ when dried, it has a pronounced cedar-like scent when freshly cut.
The timber dries rapidly with a marked tendency to distort. Quarter-sawn material is less liable to degrade in drying.
Sapele is much harder than African or American mahogany, and in resistance to indentation, bending strength, stiffness, and resistance to shock loads, is practically equal with English oak.
Medium - Works fairly well with hand and machine tools, but the inter-locked grain is often troublesome in planing and moulding, and a reduction of cutting angle to 15° is needed to obtain a good finish. It takes screws and nails well, glues satisfactorily, stains readily, and takes an excellent polish.
- Moderately durable
Density (mean, Kg/m³)
- 640 kg/m³
- Available at specialist timber merchant
- Pink/pale red (when freshly cut), Reddish brown (typical mahogany colour)
Sheet Material Extrusions
Sheet materials can be used as cladding panels, combined with other materials to give composite panels or used as the raw materials to produce fabrications.
Proprietary products are available with textured aluminium facings; sheets can be textured or perforated using automatic punch presses, laser or water cutting.
Signs cover a huge variety of applications so here we are talking about the actual graphic signs, whether maps, finger signs or general information. Generally we are looking at static signs. Electronic information systems are wide ranging and specific to application so cannot be covered here.
The method of carrying these signs needs to be robust and suitable for the public realm external environment; signs that are designed for indoor shopping centres for instance do not have to be weatherproof and often have a close security presence allowing the use of less robust and costly signs.
Stainless steel, aluminium and galvanised painted steel are materials normally used for the supports and we will cover those materials later.
Silk Screen Printing – Signage
Similar in process to vitreous enamel this produces cost effective signs but they do have a limited life; even using exterior quality inks they do fade in a relative short time (2-3 years) and are susceptible to scratching and graffiti.
The use of stainless steel for today’s urban architecture provides sustainable, safe and elegant products.
Stainless steel is an ideal material for these applications for the following reasons:
- Good tensile and impact strength. This enables the use of lightweight posts, without compromising the safety of pedestrians or buildings that the bollards are designed to protect.
- A range of smooth finishes can be applied that optimise the corrosion resistance, minimise the adherence of dirt and promote.
- In-situ cleaning by rainwater.
- Stainless steel can be fully recycled.
Stainless steel also has a unique feature:
It is self healing due to the alloying elements in the stainless steel; a thin, transparent "passive layer" is formed on the surface.
Even if the stainless steel surface is scratched or otherwise damaged, this passive layer, which is only a few atoms thick, instantaneously reforms under the influence of oxygen from air or water.
This explains why stainless steel does not require any coating or other corrosion protection in service. It is important to bear in mind that stainless steel must be exposed to a continuous supply of oxygen to form the protective layer otherwise ‘crevice’ corrosion will occur. This can take place in any component or joint from which oxygen is excluded.
Stainless Steel Grades
The grade of stainless steel has a major influence on its performance and needs to be matched to the environment.
Optimum performance is achieved by taking into account these characteristics when designing in stainless steel.
The most commonly used stainless steel grades in the building industry are Cr-Ni alloys, technically termed “austenitic”.
Stainless steel is normally specified as 304 grade (EN 1.4301/1.4307) and in more corrosive environments 316 grades (EN 1.4401/EN 1.4404).
The Cr-alloyed (“ferritic" and magnetic) stainless steel type 430 grade (EN 1.4016) should be limited to interior applications.
Ferritic stainless steels have similar physical properties to carbon steels.
The Woodhouse quality system ensures the highest product consistency and fabrication possible avoiding problems such as:
- Poor quality of the weld seam: aesthetically poor and with compromised corrosion resistance.
- Use of bolts of an incompatible, low corrosion resistant alloy.
- General rust staining on the polished stainless square tubes.
An aggressive (e.g. coastal) atmosphere will compound the corrosion problems.
To reduce the risk of these problems occurring, the following points are considered and factored into the Woodhouse supplier assessment:
- More care during welding to avoid the uneven weld bead and spatter.
- Correct finishing of the weld.
- Use of matching grade stainless steel fasteners.
- Proper protection of all stainless steel components around the fabrication shop.
- Appropriate on-site cleaning with non-chlorine based product.
Stainless Steel Finishes
The influence of grade on the performance of stainless steel is fairly well known. However, it is not so well known that surface finish has an equally important role in determining corrosion resistance.
Poor quality fabrication and finishing can lead to disappointing performance of stainless steel.
Stainless Steel - Bead Blasted Finish
Bead and shot-blasted finishes are produced by the impact of a hard, inert medium onto the steel surface that results in non-directional, uniform matt surface with low reflectivity.
These finishes have both aesthetic appeal and can enhance the surface properties of stainless steels.
The texture of the blasted surfaces vary with the blast media and for architectural applications steel shot or glass bead peened finishes are a good choice for getting the a good combination of corrosion resistance from a non-directional surface 'sheen' finish.
Woodhouse specify the use a fine stainless steel shot which if fully recyclable and provides a smooth attractive uniform surface finish which is very durable. Unlike glass bead blasting this finish is very resistant to finger marks and very durable.
It is ideal for structures such as benches where a variety of materials and processes are used to achieve a consistent overall finish.
For added corrosion resistance, bead blasting together with final electro polishing provides an attractive finish with a brighter appearance.
Stainless Steel - Electro Polished Finish
Electro polishing is electro-plating in reverse. Instead of depositing a coating of another material on a surface, the action of electro polishing is to remove a surface layer, typically 20-40 micro-metres in depth in the case of stainless steel.
In the majority of cases, electro polished stainless steel surfaces are bright and highly reflective. This results from the removal of an often contaminated surface layer and the electrochemical action of micro-smoothing. The advantageous features resulting from such a bright and smooth surface are many.
The passive oxide layer, which is essential to prevent stainless steel from corroding, cannot be improved upon following electro polishing. By reducing the total surface area, a result of micro-smoothing, products are less likely to adhere to an electro polishing finish.
In the same way, surfaces can be cleaned and kept clean more readily. Friction is also reduced and the mechanism of preferentially removing surface high spots makes electro polishing suitable for eliminating fine burrs. The highly reflective bright finish is ideally suitable for a number of decorative applications, particularly where the shape of the item requiring polishing is extremely complex.
This finish is commonly specified for aggressive environments such as coastal installations.
Stainless Steel - Grained Finish
There is no accepted definition of an abrasive grain or grit size that differentiates grinding from polishing. As a guide, but not a definition, grit sizes of 80 and coarser would be associated with grinding, whereas grit sizes of 120 and finer are used in preparing polished finishes. Like polishing, which often involves using successively finer abrasive grit sizes to obtain the desired final finish, grinding can also involve more than one abrasion stage.
The final grit size used in both grinding and polishing does not fully define the finish and must not be used in an attempt to specify a ground or polished finish on stainless steel. Other parameters such as abrasive pressure, contact time, material feed rate and whether the operations are done dry or wet all affect the character of the finish produced.
Mechanical finishes merely described as 'satin', 'polished', 'dull', 'bright' or 'mirror' can vary quite significantly between mechanical finishing contractors. The Woodhouse approved ‘grained’ finish gives a fine, clean cut with minimal micro crevices which prevent the possibility of harbouring corrosive atmospheric elements.
This helps optimize the corrosion resistance and minimising dirt retention of the surface. These finishes are more suitable for external applications than regular none specific grained finishes, especially where service environments are aggressive course finishes would not retain their appearance.
Stone in Signage
Stone and concrete can be used as materials for seating; sometimes they have been incorporated into signs and historically concrete columns were used for street lighting.
Selection of stone for street furniture obviously needs to be non friable and finished in a suitable manner for its application. Sawn stone for monolithic signs is fine but in seating you would want to introduce radii and perhaps offer a polished finish.
Softer stones, i.e. limestone would not normally be used for seating due to the ease with which they can be carved with graffiti.
Polished – polishing the unit actually cuts through the aggregate and you see more of each stone completely changing the appearance of the finished article.
Engraving stone is possible (normally a silicon mask is applied to the finished stone surface and then blasted with aluminium oxide to eat into the unprotected part of the stone surface to the required depth.
Cast stone (really concrete with a mix of larger specific stone aggregates) offers a wide variety of colours and can be finished in a number of ways:
- Straight out of mould – normally used for very functional units or where low cost is needed.
- Acid Etched – exposing the aggregate by using an acid to remove the concrete from around the aggregates on the surface of the seat.
Vandal resistant materials
Opalescent sheet materials, reinforced with wire mesh, are available as a true vandal resistant material. They are however aesthetically unattractive and detract from the appearance of any structure that is glazed with them.
Timber & FSC
Timber is most often used in seats or kiosk type structures in the street furniture market, although wooden columns are available that offer the same strengths and lifetime of steel and aluminium poles.
FSC certified supplies would be normal for any supplier who has IS14001 certification.
Whilst timber slats for seats are available in a wide variety of wood types the selection normally depends on these three factors:
- Is it ethically sourced?
- Is it suitable for use as external seating (no splits or shakes in outdoor use, resistant to rot and insect attack, hard wearing, easily machined, available supply?) Some FSC timbers, for instance, Opepe and Jujoba have a limited supply and may not be available when required since they are a farmed product.
- Is it cost effective? English oak meets the criteria in 1 and 2 but is much more expensive than European oak.
The classifications quoted below refer to the resistance to fungal decay of the heartwood of the species only. The sapwood of most species is not durable or slightly durable and should not be used in exposed situations without preservative treatment.
Five natural durability to wood-destroying fungi classes are recognised in BS EN 350-1 Guide to the principles of testing and classification of the natural durability of wood:
- Class 1 very durable
- Class 2 durable
- Class 3 moderately durable
- Class 4 slightly durable
- Class 5 not durable
The classifications above are those given in BS EN 350-1 for the timbers included in this technical glossary.
Insect attack is generally of less significance in the UK. Information on the resistance of some timbers to insect and marine borer attack is given in BS EN 350-2 Guide to natural durability and treatability of selected wood species of importance in Europe.
Guidance on the need for preservative treatment in particular situations is given in BS 8417:2003. Note that BS 5268-5 and BS 5589 are now obsolete.
Vinyl – Signage Material
Vinyl Letters applied directly onto the sign structure. The life of the vinyl is normally either available in two standards, 5 years or 9 years.
Vinyl (Reverse Applied) – Signage Material
This process normally uses illuminated signs with the graphics applied internally to the inside face of the glass panel. It provides a crisp illustration during the day and a detailed night time map or information at night.
Vitreous Enamel – Signage Material
This process used a glass slurry silk screen printed onto a high carbon steel panel and fired between each colour. Artwork needs to be produced specifically for this process because the printing followed by the firing has to follow a pattern dictated by the colours used. Certain colours must be printed first and certain colours need to overlay other colour to provide clear crisp images.
Photographs, maps and complicated graphics are possible using this method.
The signs can be expensive, especially when using large colours, but offers unparalleled colour stability, resistance to graffiti and vandal damage.
To demonstrate the anti graffiti characteristics of Vitreous Enamel, the VE association sprayed it with industrial yellow line paint, which is, by design, made to be tough, resilient and difficult to remove.
It was then allowed to dry for 4 days. Even after these 4 days, the VE can be scrubbed clean with white spirits, but this was time consuming and effortful work. When using some domestic paint stripper, the graffiti came off immediately and a wash with clean water returned the sign to its former glory.
The same procedure, when applied to signs finished in paint, made of plastic, screen-printed stainless steel or aluminium; sustain permanent damage.