Clay heave void former

The key thing to remember is that Clayboard is designed to collapse under pressure when wet so it must be kept dry while supporting the weight of steel and wet concrete. The fact that it may rain when you have planned to install Clayboard does not mean you should avoid it but will require some vigilance. If there is standing water, Clayboard should not be laid. Any surface water should be removed. Pooling in trenches may require a sump and pump arrangement. Instead of sand, some users prefer a pea gravel blinding, as wet sand can hold the water longer.

The time between laying the product and pouring the concrete should be kept to a minimum to reduce the risk of Clayboard getting wet. Over large areas, consider wrapping in smaller sub-sections. Take care in carrying mesh, re-bar etc in case this rips the poywrapping.

While the Clayboard remains intact and under heave, that insulation will be held in place. There is a concern that if the ground subsequently dries out and shrinks away, what is holding the insulation in place. Two methods used are 1) to poke wire ties of suitable length through the insulation so it is standing proud on the upper face and attached to mesh or rebar or 2) use wide helix plastic screws in the upper face of the insulation, half embedded and half exposed to key into the concrete when poured.

The general rule of thumb is a minimum of one Voidpak per 24m2. Where internal beams subdivide the building footprint, then use at least one per enclosed area (more if each enclosed area is >24m2) of Clayboard.

No, the Clayboard is not removed by the irrigation method and remains in situ. The aim of the Voidpak pipe to introduce water is purely to weaken the honeycomb core. This will ‘switch off’ its compressive strength, taking it from 30kN/m2 down to a nominal 2-3kN/m2. This ensures that, when the ground heaves, the Clayboard yields with no resistance. This is in contrast to polystyrene voidformers with a narrow safe load/fail load window requiring the slab to be designed to resist uplift.

Yes. If we are inserting Clayboard into sections of lay flat tubing, a max number per section is 3 panels, due to practicalities of handling. In this case, one Voidpak pipe is required per bagged section, rather than one per 24m2.

No. The placement of a sand blinding is to provide a smooth, flat surface to avoid any point loadings to the underneath of Clayboard when applying the load of steel and wet concrete. Sand is preferred as it allows for draining of water, or sometimes a pea gravel is used. After the irrigation process, use a steel rod poked down the opening of each Voidpak pipe and break through the bottom skin to allow any excess water to drain through the blinding. If you have used a concrete blinding, you just need to go a bit further in knocking a hole through that as well.

This will always depend on the concrete mix, depth of pour etc The general rule of thumb is how long it takes for the concrete to achieve self-supporting strength: when this point is reached, the Clayboard is now redundant, as the slab is supported by piles and beams.