styrofoam is a material used for various uses; insulation, void filler, etc. voidform is used under foundations to prevent damage from frost heave or expansive soils. It can also be used for other applications, but mainly for the above.
Void form will crush under a specific load, so that it can support the weight of the concrete placement and then when frost heave or soil expansion occur, the upward force of the soil crushes the void form and allows the soil to expand.
Styrofoam is not designed to crush in that manner.
dgkhan, void form is like a small cardboard box. At first it is pretty strong, then you let it sit with moisture (wet concrete, soil, humidity) it will eventually lose its strength and decomposed.
I generally spec a material called 'Frost Cushion'; this is a foam insulation material that is cut to reduce the compressive capacity. I've had problems with grade beams on void form and the cavity left, filling with water and freezing and actually lifting the beam off the pile foundation. I've also used aerofoam skins which are the scraps sliced thin and taped together with success. The problem with foam is that it has a compressive capacity of several psi. Aerofoam skins about 4 psi... this is still 500 psf... Other foam can be typically 8 or 10 and specially formulated foam (Dow Hi60) for example can be 6o psi based on load-deformation.
I have seen the term Void Form used on a drawing to simply indicate a formed void in the concrete, not necessarily speaking to the type of material used. In our TX office, the cardboard forms are usually called Carton Forms, but I do not know if that is a trade name or trademark. Styrofoam is a registered trademark as far as I know, and comes in many flavors and capacities. Some is rated for forkwork and void forming, some is not explicitly rated but still has adequate capacity.
In what context do you ask your question? Are you trying to form a permanent void in an inaccessable location where sacrificial formwork is needed or are you trying to form a void on grade? Or another application?
