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Mixture that stands the test of time

If longevity is any measure of success for a construction material, then mastic asphalt is surely a winner, as it can boast a history dating back to Old Testament times. Importantly, though, the material has managed to maintain its place as the waterproofing material of choice throughout the intervening millennia, and is still specified today for many of the UK's most prestigious buildings.

Mastic asphalt is a mixture of limestone and bitumen commonly used for roofing, tanking, paving and flooring. The bitumen and closely graded stone are added together to form a molten mass that is fluid enough to be laid by wooden float yet sets swiftly to create a voidless block that is impermeable to water, gas and contaminants.

By far the biggest market for mastic asphalt in the UK is roofing, which accounts for up to 60 per cent of the industry's output.

Flooring makes up 25 per cent, paving 10 per cent, and tanking the remaining five per cent.

The asphalt used in each of these applications varies a little in composition, although all combine limestone and bitumen in a ratio of about 9:1 by volume. For tanking and waterproofing applications the composition is softer or 'richer in bitumen', while flooring and paving require a harder mix.

Mastic asphalt application is one of the few construction activities still regarded as a 'craft trade'. All installers serve a three-year apprenticeship that includes studying at college. They must achieve at least NVQ Level 2, although the industry's trade body, the Mastic Asphalt Council (MAC), offers a £1,000 incentive to those who complete the extra eight weeks at college that will give them a Level 3 NVQ.

Mastic asphalt has a reputation as a traditional material, so it is hardly surprising to find that Buckingham Palace, the Houses of Parliament and the Bank of England all have mastic asphalt roofs that have lasted for more than 100 years. But it is equally likely to be found on modern buildings - particularly town- centre and large shopping developments built using traditional construction methods. Both Bluewater in Kent and Birmingham's Bullring - the largest retail development in Europe - have mastic asphalt roofs, as does Foster's landmark Swiss Re building in London - inside the glass.

The industry does face competition from other materials in the roofing sector, although the alternatives tend to be as expensive and have nowhere near the durability. This was highlighted in an independent report from the Building Performance Group in 2000, which stated that mastic asphalt outlasts its competitors, and without higher initial costs.

Independent testing by the Building Research Establishment puts the life expectancy of mastic asphalt in roofing applications at well in excess of 50 years - more than twice that of other flat-roofing materials.

As a result, says John Blowers, director of the Mastic Asphalt Council, the material comes into its own when life-cycle costs are taken into consideration. 'Designers should not just be looking at the installation costs, ' he says, 'they should be looking at the cost of recycling and replacing materials over the lifetime of the building.'

A recent technological advance has been the development of polymer-modified asphalts - proprietary products that involve adding a polymer to the basic limestone and bitumen recipe to give the material greater flexibility at low temperatures and more rigidity at high temperatures. 'Asphalt is a therma-plastic. It expands in the sun and shrinks when temperatures cool, ' says Blowers. 'Polymers enable the asphalt to return to the original shape, which makes the material more durable and longer lasting.'

The polymers have particular benefits where the asphalt is exposed to thermal movement such as in roofing. For flooring and tanking applications such modification is unnecessary.

The second important change to the industry in recent years is the introduction of 'hot-charge machines' that bring the asphalt to site ready to lay - much like readymixed concrete. This saves time on site, where previously the mixing process dictated the rate of progress. With a hot-charge machine, a single team of installers can apply up to 40 tonnes of mastic asphalt per day on car parks, large floors and roofs.

Blowers says the provision of hot charge also opens up the choice for mastic asphalt to be used more frequently for bridge-deck waterproofing, as it is elsewhere in Europe.

In Germany and France - the two largest users of mastic asphalt in Europe - the material is seen more as a road material than for roofing or domestic flooring. In France it is also used frequently for terracing, footpaths, podiums, accessways and pedestrian areas.

Blowers says: 'They are more outgoing in their approach than we are in the UK.'

He hopes the development of coloured asphalts - created using suitable colour-stable pigments - will encourage architects to specify the material for a wider variety of applications. 'Architects are very colour conscious, ' he says. 'At the moment they might opt for a spray-applied system instead of mastic asphalt because they think they can get a wider choice of colours.Architects are waking up to mastic asphalt's broad spectrum and realising they can have colour without sacrificing durability. But we still need to get the message out to effect the culture change.

'In France and Germany paving is extremely big business. They even imprint it with grid lines so it looks like block paving.'

A number of UK councils - including the City of Westminster and the London Borough of Croydon - already use mastic asphalt for footpaths. These pavements are particularly attractive as they cut down on accidents caused by loose slabs or blocks, and potentially very costly legal actions.

Blowers predicts that the next few years will see applications of mastic asphalt become far more diverse as markets change. In Germany, for example, asphalt is used for insulated internal floors to help with both soundproofing and thermal insulation. 'There are moves over here to have more insulated floors, ' says Blowers, 'and research is under way to develop a specification that could be used over board insulation on floor screeds.'

Another new market could come from proposals currently under consultation to tighten up the Building Regulations covering ingress of moisture and contaminants such as methane and radon. 'Mastic asphalt is the ideal barrier, ' says Blowers. 'The more stringent that requirement is, the more likelihood that asphalt will be seen as the ideal solution.'

One sector he is keen to expand is highquality internal flooring, where the material can be coloured, ground and polished to produce a waterproof, hard-wearing surface that rivals marble or terrazzo in appearance.

'I have seen a floor made with red asphalt and limestone chips that was polished with a grinding machine and looked beautiful - just like marble, ' he says, 'perfect in the entrance to a hotel, mall or shopping centre.

'We have to be able to offer architects something different as well as the traditional products they know. We have the tools and we have the materials to do that.'

Roofing

Roofing is by far the most significant use for mastic asphalt, accounting for about 60 per cent of the industry's output.

There are three main types of design that incorporate mastic asphalt: cold roofs, warm roofs and inverted roofs.

Cold roofs are most suitable where there is no occupied space beneath - for example, a cantilevered balcony. The mastic asphalt functions as a waterproofing layer laid directly on top of the roof structure. Usually, it is then covered with paving or gravel for decoration as well as solar protection to reduce thermal gain.

Prior to the introduction of the recently revised Part L of the Building Regulations, cold roofs were also frequently found above occupied spaces, but the need to incorporate high-value insulation into roof construction led to the development of warm roofs.

In installation, a layer of insulation is laid on the roof deck, topped by a mastic asphalt layer directly above, before being covered or paved.

Exposure to thermal movement in warm roof applications has greatly speeded the adoption of polymer-modified asphalts, according to John Blowers, director of the Mastic Asphalt Council.

'With mastic asphalt laid on top of higher-value roofing insulation, the material is exposed to heat in the summer and cold in the winter, so the asphalt is much more thermally active. The new polymer-modified products are designed to accommodate that movement.'

Increasingly popular, though, are inverted roofs, where the asphalt is laid first on the deck - usually screeded concrete - followed by the insulation and finally paving, gravel or landscaping. The insulation is designed to carry water through to the mastic asphalt layer, and can also be cut to falls, if necessary.

In inverted roof construction, the mastic asphalt acts both as a waterproofing layer and as a vapour-control barrier to prevent condensation in the building below. And because it is protected from the sun, the mastic asphalt is not subject to thermal movement, and can be guaranteed for the lifetime of the building.

Blowers says that this inverted form of construction now accounts for 80 per cent of new-build roofs.

Flooring

Although flooring represents only 25 per cent of the UK's overall mastic asphalt use, the percentage is far higher outside the south-east of England, and in the north there is a thriving market for the material as a damp-proof material on the ground floor of domestic properties. It is also popular in industrial and specialist applications where a hardwearing, durable surface is required, including shop floors, police cells and hospitals.

Four different flooring grades are available. Grade I is for hard flooring, laid in one coat between 15mm and 20mm thick.

This is designed for specialist applications such as hospital wards, schools, shop floors, offices and domestic floors where the surface must be totally indent-proof under any point load, even when the environment is warm.

Grade II asphalts are slightly softer, and suitable for light-duty flooring, where point loading will not occur, or where shallow indentations are acceptable. They are used as an underlay for other floor finishes such as carpet or tiles, in domestic situations.

Medium-duty industrial floors usually call for Grade III asphalt, which is designed primarily for heavy foot traffic in areas such as hospital corridors and sports halls.

The final category, Grade IV, is for heavy-duty flooring, and is laid between 30mm and 50mm thick. It is intended for situations where the floor is subjected to traffic such as forklift trucks.

All the grades are available in both red and black, and other colours can be achieved by adding paints. Different finishes can also be created by grinding and polishing or rubbing in a layer of sand after laying.

One major advantage of mastic asphalt as a damp-proof membrane is that it can be covered or trafficked within a few hours of laying. Certain grades are also resistant to a wide range of chemicals and industrial products, so they can be used to line plant rooms, tanks and storage vessels.

Paving

Currently in the UK, most mastic asphalt paving is associated with rooftop car parking, but elsewhere in Europe it is popular for a far wider range of applications. Footpaths, carriageway reinstatements, terraces, pedestrians zones and bus stops are common, and in Germany, the largest user of mastic asphalt in Europe, it is frequently used for road construction and bridge-deck waterproofing.

European specifiers also opt for a wider range of coloured asphalts for paving applications than their UK counterparts, as well as a variety of different finishes in the design of hard-landscaping schemes.

Mastic Asphalt Council director John Blowers says price seems to put specifiers off all but the basic black finish in the UK.

'Colours are achieved by adding pigments, which tend to make the product more expensive.We often find designers look into it as an option, but keep coming back to standard asphalts.'

But he believes coloured mastic asphalt does become competitive when compared with alternative paving systems. If specifiers get behind it, Blowers says he can see coloured asphalts becoming very popular as a way of delineating parking bays, bus stops or pedestrian walkways.

In some urban areas there is already a move to adopt mastic asphalt for footpath construction, as local authorities find themselves under increasing pressure from litigation over accidents caused by broken flagstones.

But the material is equally suitable for heavily trafficked areas such as car parks, bridge decks and even roads, and the introduction of polymer-modified asphalts has eliminated concerns about the effect of thermal movement in these situations.

Slip and skid resistance can be incorporated into the wearing course of the mastic asphalt by sand rubbing, surface crimping or by adding pre-coated chippings. The ultimate non-skid surface can be created by applying an epoxy-resin dressing with coloured bauxite chippings.

Different grades are available depending on the loading the asphalt is likely to be subjected to - so a harder grade would be used for bus bays than for footpaths or car parks.

Heavily loaded areas will require a thickness of 40-50mm, whereas rooftop car parks are usually 25-35mm thick, and footways require only 20-30mm of asphalt.

Tanking

The principal purpose of tanking is to protect underground structures from penetration of ground- or subsoil water.

Tanking represents the smallest market for mastic asphalt in the UK, accounting for 10,000 tonnes of output per year. But the Mastic Asphalt Council (MAC) is confident this is a growing market, and new opportunities will be generated by future changes to the Building Regulations, and increased building in areas with high groundwater where the pressure exerted has to be taken into consideration.

Mastic-asphalt waterproofing can be applied to either the inside or outside of an underground structure, depending on site conditions and other design considerations.

External tanking is preferable if conditions allow, because the groundwater forces will push the mastic asphalt against the surface of the structure. In internal tanking applications the converse is true, and a structural slab and loading walls have to be built inside the mastic asphalt layer to provide structural resistance against the anticipated water-pressure load.

A significant proportion of this market comes from repairs to existing structures, where mastic asphalt is being used to replace failed waterproofing systems, especially in London. 'There are a lot of other materials that are cheaper, ' says MAC director John Blowers, 'but mastic asphalt is the ultimate in tanking materials.'

In a lot of refurbishment work, internal mastic asphalt tanking is added as a secondary layer of defence. 'It's not that easy to put in after construction, so I would recommend using it from day one, and then you know the structure will be OK throughout its life, ' says Blowers.

Proposals out for consultation at present could see a change to the Building Regulations relating to moisture and contamination. Blowers believes this could result in a substantial increase in the use of mastic asphalt tanking, as the material is resistant to a wide range of common contaminants, including methane and radon.

In the meantime, though, a significant market for tanking-grade mastic is in the construction of soft-landscaped roof gardens, used to line the structure in which topsoil or turf is laid. In this situation the material behaves in exactly the same way as in the external lining of an underground structure.

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