In areas with expansive soils, carton forms are often used to provide a temporary support for the placement of concrete structural slabs and grade beams. They provide a cavity into which soil can expand, preventing the transfer of uplift pressures into the structural slab or beam above.
The defining factor of an effective carton form is its ability to transition between being strong enough to support concrete load and weak enough to collapse as the concrete sets and becomes self-supporting.
The reinforcing steel in structural concrete allows it to span between supporting elements that are substantially or relatively unaffected by expansive soil movement, such as drilled piers, driven piles, and intermittent footings. The areas or sections between structural concrete should be “void” of any continuous support, as whatever it can hold up, it can also push up.
When properly used and protected from premature moisture absorption, these offer the greatest loss of strength that is needed to ensure complete structural breakdown after the concrete is placed.
Careful planning and jobsite preparation must be considered when utilizing these types, since they are susceptible to precipitation and moisture in the air and surrounding soil.
If extreme water is prevalent or in inclement weather, it becomes difficult to utilize materials that absorb moisture to properly function by design. Thin, flexible plastic components that are impervious to water become the best option.
Although frequently used to mitigate damage caused by frost heaving or soil expansion, solid blocks of EPS foam do not provide a sufficient cushion to allow for significant uplift. Foam-based materials increase resistance as they compress, transferring energy into the structure above. They must be engineered having the majority of the material removed to create void cavities between support elements.
