Suspended floor slabs

A site was underlain by 2 m of made ground that was inert soil with very occasional pieces of wood in it. The site was to be developed with a steel framed warehouse building with piled foundations and a reinforced concrete cast in situ suspended floor slab. Historical gas monitoring data was available but did not include flow rates. 

Low-rise housing Suspended floor slab Ventilated underfloor void... 

Many inban developments include basement car parks that are well venhlated to deal with exhaust gases from cars. They are often constructed with thick reinforced concrete retaining walls and cast in situ suspended floor slabs. The basement is in effect a very large and effective ventilated void that provides more than adequate protection against low levels of ground gas. Care should be taken, however, to ensure that features such as lift shafts or service risers which extend to the base of the car park are provided with adequate protection. 

Reinforced concrete cast in situ suspended floor slabs can give reasonable gas resistance, if they are detailed correctly with anti-crack reinforcement. They also need to be sealed at joints (possibly with water bars a design measure to ensure joints are watertight) and sealed at all service penetrations with water bars. Evidence from Building Research Establishmenf (BRE) research has shown that reinforced concrete rafts with a venting layer below give the best level of protection to gas ingress. 

Differential settlement occurred between a groimd-bearing slab and walls on piled foimdations. The membrane laid across the slab and through the walls was tom apart. This can be avoided by using suspended floor slabs or designing the slab to limit the amoimt of differential movement between it and the building stmcture... 

Points for inspection of gas protection measures (gas-resistant membrane on a cast in situ reinforced concrete suspended floor slab (provided by Cooper Associates (Nik Reynolds and Ben Hill), prepared for Liverpool City Council)... 

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... 

A site is proposed for a shopping centre development and has a maximmn gas concentration of 26% and borehole flow rate of 0.11/h. The source of the gas is a tiiick layer of alluvium with peat lenses and groundwater levels in flie area are known to rise and fall quickly in response to rainfall events. The development will have a suspended cast in situ reinforced concrete floor slab fliat is 200 mm thick. 

For characteristic situation 2 the development would require a reinforced concrete cast in situ floor slab (suspended, non-suspended or raft) with minimum 1200 g DPM. Underfloor venting, or pressurisation in combination with... 

Reinforced concrete cast in s/tu floor slab (suspended, non suspended or raft). Proprietary gas resistant membrane and ventilated or positively pressurised underfloor sub-space, oversite capping and in ground venting layer and in ground venting wells or barriers,... 

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