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LOOKING TO THE FUTURE

AJ SPECIFICATION - FOCUS ON: CLADDING & CURTAIN WALLING

Environmental concerns are nothing new. The world is all too aware that with oil production set to peak in about 2015, there is an urgent need for alternative fuels and materials. This informs all elements of construction but perhaps none more so than a building's skin; sustainable development is, without question, the thrust behind modern facade design. And the responsibilities are huge.

Consider the new Part L, due in early 2006, which will require the calculation of total carbon emissions for a building.

Artificial light in office buildings accounts for a large part of the overall energy consumption. For facade designers, the emphasis has to be on providing a good daylight performance but the real challenge is combining this with good thermal insulation and solar control.

In material innovation, too, the pressure is on. The burgeoning fashion for structural stone will raise issues regarding the sustainability of quarrying and the supply of large blocks of stone. The biggest leap in procurement is in developing markets such as China, a vast untapped source of traditional building materials. We have to ensure that procurement remains ethical, especially with regard to labour conditions and health and safety issues. Going for the cheapest price will not ensure this.

Alternative composite materials (eg fibre-reinforced plastics), currently used for niche applications such as moulded complex geometry, also have potential. Could replacing oil-based polymers with plant-based versions be a potential solution?

And what of the critical pressure on landfill and increases in landfill tax?

The driver to dismantle a building and reuse or recycle its constituent parts will also grow. The use of reclaimed materials and components may be possible on smallersized projects but requires a huge effort on the part of the design team. Hurdles such as availability, the procurement process and product condition and residual life all need to be addressed. Changes in legislation and taxation could change the extent to which reuse of elements is fi nancially viable.

DESIGNING FOR DAYLIGHT In the past, for a facade to have good solar control, it required a dark glass or a shading device - thus jeopardising good light performance.

However, products can now achieve a combined solar-light performance using special glass coatings able to select the visible wavelengths and screen the infrared part of the solar radiation. These have become so popular that they are now supplied by all major European glass manufacturers and their properties continue to be improved.

They can now reduce more of the infrared solar radiation and allow in more daylight, keeping a neutral and relatively transparent appearance. These coatings can be applied to a glass surface enclosed in a sealed cavity, in a laminated single glass or on a film to be applied to the glass surface or suspended within a cavity.

Daylight systems applied to the facade can also enhance the penetration of natural light.

The principle is simply the reflection of the direct light when it reaches the facade at a certain angle. This is achieved using different systems:

prismatic elements, laser-cut panels, reflective slats, light shelves, etc. The aim of these products is to screen the perimeter area from direct light (thus avoiding glare) by redirecting it towards the ceiling in order to create a brighter and more comfortable environment. More advanced systems foresee the use of movable parts to track the sun along its path and exploit the available daylight.

NO NEED TO CLEAN With the growing trend for complex surfaces, self-cleaning coatings are becoming more widespread. Some are well known but are more commonly associated with glass - Pilkington Activ or Saint-Gobain Aquaclean for example. But there is plenty of research into products for use on concrete, metals and other materials.

Photocatalytic paints and products can reduce noxious gases in the air. In Japan, roadside concrete slabs developed by Noxer help block and absorb pollution. The noxious gases are converted to nitric acid by UV radiation from sunlight and the acid can then be neutralised or simply washed away by the rain.

A factor that is driving their use in Europe is the requirement for EU member states to reduce their nitrogen oxide levels by 2010.

ELECTROCHROMIC GLASS Switchable glazing technologies have been around, in various guises, for the past decade.

From internal privacy control, such as Saint-Gobain's Privalite, to specialist products for the aircraft industry, the technologies have not yet achieved a commercially viable product for facade use - mainly because of durability concerns. The latest advance is solid-state electrochromic glass, based on semi-conductor technology developed in the photovoltaic industry.

Fascinating research carried out by Claes-Goran Granqvist and Arne Roos at the Angstrom Institute at Uppsala University in Sweden has produced a 'smart window', with variable transparency through the use of electrochromic foil. Although still at prototype stage for small-scale projects, the glass could potentially replace glare blinds in the future. For further information on this new development, visit www. uu. se or email Arne Roos at arne. roos@Angstrom. uu. se TRANSLUCENT STONE Translucent stone is becoming popular for external, as well as internal applications. The laminated composite panels can be glass-stone-glass, produced by Fiberstone for example, or stone-stone from Stone Panels. The advantage for cladding use includes, for example, an increased strength-to-thickness ratio and the ability to use a less-durable stone (in the case of glass-stone-glass) and larger panel sizes. Potential pitfalls for external use, obviously dependent on prevailing environmental conditions, could be delamination, water ingress or discolouration from resins.

Used most recently in a unitised system for Foster's Singapore Law Courts, Arup is currently exploring possibilities for a 20,000m 2 translucent stone wall, which will be the largest application of translucent stone to date.

NON-TRADITIONAL:

TITANIUM Facade design has traditionally been influenced by other industries, such as aerospace and vehicle design, especially their use of materials. Architects tend to use their research and apply it directly to construction.

Adoption in this way tends to go in 3 stages:

1. replacement for traditional;

2. popular choice regardless of whether appropriate for application;

3. suitability or benefit found.

An example of this three-stage development of material adoption is titanium.

Currently regarded by many as a relatively 'new' material to architecture, it has been used for more than 30 years. Current developments in primary material production (extraction of the material from the ore) promise potential price reductions, which in turn are likely to result in increased material usage. So rather than just being used for its aesthetic appeal, the benefits of titanium itself - its low thermalexpansion coefficient for example - may be exploited.

RECONSTRUCTED STONE The drive for sustainable facades is leading to a desire for a reduction in the amount of materials used. Techrete offers polished reconstructed granite and stone finishes which compare favourably with natural stone, with the extra benefit of the structural capability of precast panels.

A smooth and modern look can be achieved through a careful selection of naturally coloured sands and aggregates. If you do prefer the real thing, stone cladding is already available in considerably thinner panels and the push now is for it to be attached to thinner substrates, producing a very lightweight system.

FIBRE-REINFORCED PLASTIC Fibre-reinforced plastic (FRP) elements can be mass produced, using a process known as pultrusion. Their structural properties are similar to aluminium and they have low thermal conductivity. With this combination of properties, pultruded FRP has a potential advantage compared with aluminium extrusions because the material is inherently better at insulating. Aluminium extrusions need to incorporate thermal breaks in order to minimise heat loss through the facade and the thermal performance is therefore limited.

Pultruded FRP does not require thermal breaks and could potentially provide much better thermal performance than extruded aluminium systems. However, products currently available are limited to the domestic windows market.

FRPs are not readily recyclable because the polymer matrix cannot be melted and recovered. Currently, the only feasible method for recycling FRP is to grind it into powder and reuse it as filler in other materials.

There is, however, a strong drive in the automotive industry to use FRP materials for their weight-saving benefits, so other solutions are in development. A thermoplastic matrix could be melted and recovered but this presents other problems - processing, material strength and durability. And, in construction, this would simply not be appropriate, because it softens at a relatively low temperature. A more feasible alternative is to use the naturally derived fibres and matrix material.

PLANT-BASED RESINS Plant-based resins are currently being developed by some of the larger chemical companies although they are still at an early stage in terms of product development. They can be used as plastics, adhesives and as part of fi bre-reinforced polymer composites. They have the same inherent characteristics as conventional GRP products, including advantages of weight to strength, and they can be formed into complex shapes.

However, according to CIRIA, we do not know how the plantbased products will behave in the type of aggressive environments that traditional GRP can be used in - for example, in marine applications. Hemp, sisal and other natural fibres are also being researched for use in FRP. There are a few barriers to plant-based substitutes and these relate to cost, a lack of large-scale production and lifecycle/durability issues.

Another material has been discovered by researchers at Cornell University in the US, who have made a polystyrenelike plastic from citrus fruit and carbon dioxide. Visit www. chem. cornell. edu/ for more information.

In terms of the availability of these materials - production is small scale at present and, unless taken up by commercial producers, they are likely to remain a niche product.

DESIGNING FOR DISPOSAL At the present time the awareness of what will happen to the building at the end of its life is increasingly being considered and incorporated into designs. This includes how it is to be demolished, whether elements can be reused as they are or if they require some treatment, or if they can be recycled. Possible approaches include using fixing methods that can be undone, use of standard-sized components and marking elements with their product details. For example, the Arup Campus is a steel-frame construction with bolted connections and precastconcrete plank-floor construction to enable future dismantling and reconfiguration if required. The environmental impacts of reuse are much lower than recycling, so energy would be saved and pollution avoided if a steel beam, for example, could be reused and not recycled. In the future, all the elements of a building could be labelled and installed so that they could be removed easily at the end of the building's life.

Existing buildings could then become a store of future building elements.

DYNAMIC FACADES Today, media facades are capable of displaying dynamic images. It should be possible to take the technology a step further to make the facade appear transparent by displaying dynamic images onto it showing the view from the opposite side of the building. This effect has already been researched for military applications, for example for active camouflage.

In principle, this effect could be done using the combination of arrays of sensor pixels and arrays of display pixels, coupled to a computer capable of processing the images in real time. Such technology could effectively cloak the facade and open up a new range of possibilities for architects. For example, engineers could strive to reduce the depth of floor slabs to achieve minimal structure for an architect - facade cloaking could make the slab appear to be transparent.

Most of the things we hope will happen are based on optimising the facade capabilities and enhancing the building's performance.

For instance, micro-structured surfaces can use reflected light from the facade and direct it either away from surrounding buildings or back into the building to create and store energy there.

Dynamic facades could employ textiles that absorb or reflect heat, thus keeping the internal temperature steady;

using external sensors that affect the internal environment:

electronic sun sensors, light sensors, or blind controls. Used in conjunction with computercontrolled monitoring, the building could be constantly assessed and adjusted to reduce total carbon emissions.

Ultimately, we have to be interested in long-term performance. The advanced materials and analytical tools are the means by which our designs may be realised and enhanced but they are only a means to an end.

With thanks to the materials and facade specialists at Arup Facade Engineering: Bruno Miglio (stone);

Graham Dodd (glass); Nille JuulSorensen (product design); Clare Perkins (sustainability) and the Building Envelope Physics team.

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