PolyVoid & SuperSlab Design



Objective


To offer a compelling and competitive concrete ground slab for residential dwellings in the Australian home building market. The ground slab will address the issues facing the builder with variable and difficult site conditions by the use of existing design principles, available technology, building materials and local engineering expertise.


Goals


Reduce the total system cost of a residential concrete ground slab where site conditions warrant the use of current local market labour and material intensive designs.


Solution


SuperSlab design principles utilising a suspended slab together with PolyVoid void formers will provide a practical and cost effective method of structural design and construction that adds flexibility and cost benefits to engineers and builders. Utilisation of existing, readily available building materials allow for existing trade and supply relationships to be maintained.


SuperSlab Design Principles


Clay soils change in volume to varying degrees in response to changes in their moisture content. They increase in volume upon wetting and decrease in volume when dried out. Clay soils therefore change in volume with the seasons, the change being manifested in seasonal vertical and horizontal movement of the soil.


Generally, potentially expansive clays will only swell and shrink if the prevailing climatic conditions lead to significant seasonal wetting and drying. The greatest seasonal heaves will, therefore, occur under semi-arid climatic conditions (such as South Australia) where pronounced short wet and long dry periods lead to major moisture changes in the clay.


The swelling of the clay that occurs at the edge or centre of the slab is dependent on the initial moisture of the clay. Further, this swelling increases gradually and occurs in the short term over several months and in the long term over several years.


The PolyVoid is designed to fail at a prescribed load due to its unique design and material composition. Due to its creep characteristics the PolyVoid will continue deforming/collapsing under a constant load.


Testing of the PolyVoid has confirmed that total failure occurs at an applied load of 800kg over an area of 1.19m2 (1.09 x 1.09), which is equivalent to a pressure of 6.73kpa. Simply put once the clay swells and puts pressure on the PolyVoid in excess of the above it will simply collapse and fail at a greater rate. For more information regarding the testing process and results, please click here to make an enquiry:

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During the period of this upward pressure the slab and piers need to resist this swell less the self-weight of the slab.


The slab is designed as suspended in accordance with AS3600 combining the normal dead and live loads and the upward swell of the clay with the dead loads. The analysis will also indicate that the piers need to be designed for tension as well.


As a final note the slab is fully suspended with soil swell having little impact and as a result AS2870 would not be applicable.