Question: Plans for a small building show a 3- by 8-inch cardboard void form to be used between the soil and the bottom of a grade beam wall. These void forms can be kind of expensive. Is there any reason why we couldn’t simply use a 3- by 8-inch strip of polystyrene foam?
Answer: Johnny Gates with VoidForm Products responds: It depends on the reason the engineer specified the carton void forms. It seems likely that the engineer was concerned about damage due to expansive soil conditions. Carton void forms are designed to hold up the weight of fresh concrete long enough for the concrete to cure and support itself on piers or beams. Since it is a paper product, it absorbs the moisture of the surrounding soil and concrete and its supportive strength gradually diminishes. If the ground were to then swell, the carton forms would crush and leave the concrete structure undamaged. Foam does not really create a true void space. It will crush 20% to 25% of its original size but could result in concrete damage.