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PLASTICS
VS IRON - THE TRUE COMPARISON
The advantages and disadvantages of the various materials used for waste water and drainage products have been the cause of much debate in the building and architectural industries for many years. Materials properties, including the effects of chemical and biological activity, fire; longevity, recycling and cost are regularly compared. Cast iron and plastics are currently the two leading materials, although concrete, steel and aluminium are also used. This article discusses why these are the two most specified, highlighting the merits and downfalls of both. It is only in terms of their application that the two materials can be realistically compared. In any installation, the choice should be governed by the performance requirements of the system and the potential consequences in the event of failure. In most domestic or small commercial applications, the failure of a drainage system is likely to affect comparatively few people, result in little consequential loss and require relatively low repair costs. On cost alone, PVC-u is likely to prove the more appropriate material. Different factors apply to multi-storey commercial or residential installations. Take first chemical or biological activity within the system. Cast iron systems can be supplied with specially treated pipes and couplings capable of resisting temperatures of up to 95¡C, acids, bacterial type deposits and detergents. Plastics too are capable of transporting certain fluids, such as oil, potable water and gas, but would not be capable of handling highly corrosive materials and many are subject to expansion or contraction under extreme temperatures. In terms of fire safety, cast iron is seen to have distinct advantages. Non-combustible, cast iron requires no special safety measures such as intumescent fire collars to resist the transference of fire from one floor to another, or expensive cladding. Cast iron is flame proof for four hours and also acts as a barrier to flame expansion. Whilst PVC-u is difficult to ignite, when it does burn it emits a number of toxic fumes. In buildings where noise reduction is a key issue, thought has to be given to the ability of water and waste systems to transmit noise. Here the density of its surface mass - the higher this density, the better the sound deadening qualities - should guide the choice of material. Cast iron has a density of 21.5kg/m? compared with approximately 4kg/m? for plastic materials such as PVC and polyethylene. Independent tests have found that sound emissions from plastic pipes were almost four times higher than cast iron. To achieve noise reduction with plastic systems, one has to look at twin-walled systems; foam filled outer skins or external insulation. All of these techniques are effective, but can add to the cost or complexity of installation. On environmental grounds, Greenpeace argues strongly against the use of plastics building materials, pointing out that the production, use and disposal of PVC results in the creation and release of large amounts of toxic chemicals which contribute towards the levels of persistent toxic chemicals in the environment. It might be argued that there is potential for pollution in the manufacturing process for cast iron too and indeed foundries of the past were responsible for significant levels of air pollution. However, in the UK today, the foundry industry conforms to highly stringent emissions legislation with regard to the manufacturing process. Cast iron is produced from virtually 100% recycled material and is equally capable of recycling. As a thermoplastic material, PVC is also capable of being recycled, although this is not yet common practice. When considering the life cycle analysis, cast iron is generally described as a fit and forget material. Its strength gives it durability and it has minimal maintenance requirements. In areas where ground disturbance or extra loading is likely, plastics may need additional protection, such as a covering concrete slab or concrete surround. Cast iron offers superior resistance to ground movement and is capable of bridging minor voids caused by settlement. In accessible parts of the drainage system, cast iron systems are more resistant to accidental or deliberate damage than plastics. Where cost is the major consideration and risks from failure of the system are negligible, there is no doubt that PVC systems will be more frequently specified. In a below ground application, if the cost is expressed against the total cost of the drainage installation (including trenches, backfill, earthwork support etc), the cost of modern cast iron manufactured to the European Standard EN877 adds 47% compared to PVC-u. Above ground, again taking into account the total drainage installation cost, modern cast iron is 62% more expensive than PVC-u. However, the type of building and the location of the systems should be carefully considered and the consequential loss in the event of failure taken into account when choosing drainage materials. In terms of a cost in use analysis, cast iron, because of its ease of installation, longevity and inherent performance benefits, is the prime option. |
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