boards for forming thin shell-roofs. particularly of dome, barrel-vault, or hyperbolic paraboloid design. The roof shape is first set up on tem. porary braces with a minimum of steel supporting wires, to which boards of expanded polystyrene can be fitted, provided that there is no excessive curvature. Although this technique has so far been applied to only a few roofs, it has obvious potential advantages.
Floors
Expanded polystyrene is suitable for insulating concrete floors. and here again its low moisture absorp tion characteristics and resistance to fungal attack are an advantage. For ground floors it should be laid as boards over site concrete which has been treated with a vapour seal or covered with a damp-proof membrane it is not regarded as sufficient damp-proof course in itself). The boards should then be screeded over to a depth of 2 in.. care being taken that they are not compressed by ex- cessive local loading from feet or wheelbarrows. For intermediate floors, the boards can be used either to line the shuttering and provide a permanent insulating skin on the underside of the structure
or slab. or they can be laid on top of the slab and. as before, covered with a screed.
This latter form of construction may offer an advantage in providing some reduction in transmission of impact sound through the floor. when a special pre-compressed board is used. particularly if the screed is isolated from the structural walls by a strip of expanded polystyrene round the perimeter. However. con- siderable doubts have been expressed about the long-term behaviour and resistance to creep of the material under sustained loading, and more experience is required of its continu- ed resilience under periodic vibra. tion. In one case, where some floors were taken up after several months in use. no compaction was observed. but a thorough investigation would be necessary before it could be stated with certainty that there is no signi. ficant compaction to impair the im pact-absorbing efficiency of such a layer. As far as airborne sound is concerned, a massive structure is the basic requirement to provide a sub- stantial reduction in transmission.
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but continuous cavities double-leaf construction) add moderately to this insulation, and so also can floating floors on proper resilient layers,
Where expanded polystyrene is used to provide thermal insulation bencath under-floor heating cables. it should be separated from these by a layer of screed. It may be possible to provide this by working some of the topping screed under the heat- ing cables, as it is being placed, but it is better practice to lay a separate screed before placing the cables. The separation should not be pro- vided simply by the use of chicken wire which could, in the event of overheating. cut into the cable covers and short-circuit the system. The final screed cover above the ex- panded polystyrene should be about 2 in. thick. which is adequate to distribute any mechanical loading that might be applied in domestic circumstances and so prevent per- manent compression of the insulat ing hoard; this could otherwise oc- cur. particularly at the temperatures reached during heating if additional thermal insulation. in the form of carpets and rugs. were to be laid subsequently.
Other applications
Expanded polystyrene has recently appeared on the 'do-it-yourself* market. in sheets of only a few mil limetres thickness, to be fixed with paste under wall-paper in much the same way as thin sheets of cork are used. While the contribution of such a lining to the total thermal insula- tion of a room is likely to be com- paratively small, in a room which is only heated intermittently it will permit a more rapid rise in tempera- ture to an acceptable level, and it may also prevent condensation when this occurs under marginal condi tions.
is also
Expanded polystyrene available as pre-formed insulating jackets. to fit standard sizes of cylinders and tanks in plumbing in- stallations. Except in systems with carefully regulated temperatures, it is not advisable to use it on hot water cylinders or pipes that could cverheat because of its comparative- ly low heat resistance. For domestic cold water pipes and tanks it is a very suitable material, and the fact that it is not harmed by wetting from
condensation or splashing, is again an advantage. It is too rigid to be bent for bends in pipes but can be cut to shape very easily.
As mentioned above, in some cir- cumstances, expanded polystyrene may reduce impact sound. A num- ber of acoustic tiles' made from this material are also marketed for the purpose of sound absorption but be- cause of the closed cell nature of the material it is necessary to make per- forations in it to achieve any useful effect, and this is, in fact. limited. There is little advantage, therefore. apart from ease of fixing. in using expanded polystyrene for this pur- pose.
Unfused beads have been used for two applications where polystyrene seems barely competitive with such lightweight materials as exfoliated vermiculite or expanded perlite--as a loose fill for cavities and as light- weight aggregate for concrete. In the first of these applications, such a filling, if used for an external cavity wall. bridges, the cavity and could provide a path for rain to penetrate to the inner leaf; unlike other loose- fill materials, however, expanded polystyrene does not absorb water and does not, therefore, lose its thermal insulating value when wet.
As an aggregate for lightweight concrete, it can contribute little to strength, which depends mainly on the proportion of cement and sand in the mixture. In general, the proper- ties of a lightweight concrete made with expanded polystyrene aggregate may be compared with those of a similar concrete made with expanded perlite, and similarly proportioned mixes are used. The manufacturers recommend wetting the beads before adding cement to the mix, but the material has so very little weight that it might well prove very difficult to handle on a building site. In the event of a fire, it is probable that the polystyrene beads would collapse and, while no evidence is available on this point, it would be wiser not to use it in structural walls in the absence of further information.
One application of expanded poly- styrene that seems to be unique is its use for the production of holes and channelling in concrete.
It can be readily preformed or carved from solid blocks to the shape of hole re- quired for inserts or channels; it is
THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 6