Void formers

Banister Fletcher, 1948, p. 251. The beautiful drawings record the use of standard amphora as void formers to lighten the concrete shell but also special interlocking cylinders in the upper section of the dome. 

A void former below a cast in situ floor slab (which can be ground bearing or designed as suspended). The void former simply supports the wet concrete xmtil it is set, and leaves a void below the slab. This is a common method of construction in warehouses or industrial buildings... 

There are several different types of void former in use but the main ones are either polystyrene, geocomposites or pipes and open graded gravel. 

Figure 7.7 Polystyrene void former (photograph reproduced with permission of Cordek Ltd., Horsham, UK)...

Table 7.4 Typical air flow rates and permeability of void formers (continued) ...

Positive pressurisation is a particular form of active system that works by pumping air into the void or void former below a floor slab to create a zone of... 

Passive subfloor ventilation (venting layer can be a dear void or formed using gravel, geocomposites, polystyrene void formers, etc.T Very good performance 2.5 Ventilation performance in accordance with Annex A of the British Standard. If passive ventilation is poor this is generally unacceptable and some form of active system will be required... 

Subfloor ventilation with active abstraction/ pressurisation (venting layer can be a clear void or formed using gravel, geocomposites, polystyrene void formers etc.) 2.5 There have to be robust management systems in place to ensure the continued maintenance of any ventilation system Active ventilation can always be designed to meet good performance Mechanically assisted systems come in two main forms extraction and positive pressurisation... 

Passive ventilation performance can be estimated using simple calculations. The most commonly adopted approach is described and examples are given for open voids and air flow through a void former. 

The design of the ventilation system must provide this volume of air flow to provide sufficient dilution of the gas emissions. This applies to open void or void former systems. 

For passive venting of void formers the approach adopted requires two calculations. One to ensure there is sufficient air flow tiirough tiie geocomposite and the second to size the vents, in the same way as for a normal ventilated void (see Box 8.6). 

Box 8.6 Calculation of air flow through a geocomposite or polystyrene void former... 

Active systems can also be used on lower gas regime sites to enhance the air flow through void formers below large buildings and avoid the need for high-level stacks. However, the increased risk implications associated with a lack of maintenance should be carefully assessed and all active systems should be able to act passively in the event of fan failure. 

Performance testing required, e.g. for membranes or void formers Yes D/No ... 

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