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Void Boxes
April 3, 2023
What are void boxes and how do they impact construction and schedules?
What
Void boxes, also referred to as void forms or carton forms, are made of a paper, fiberboard, or cardboard material that is intended to disintegrate over time. If you think back to a time you left paper outside, you can image what will happen to these boxes once they are buried in the dirt and exposed to the elements.
What
Why
So why would we spend thousands of dollars on a material we know will deteriorate? The purpose of the void box is to temporarily support a concrete foundation. Once the grade beam or slab has been poured and the concrete has cured, the goal is that the paperboard material will disappear, leaving a void between the dirt that it was sitting on and the concrete above it.
Where
This void is desirable in areas with expansive soils because as the soil heaves, it is moving into the void rather than putting pressure on the foundation. The result of using void boxes is reduced cracking of the foundation.
When
Sounds good until you are trying to use them in the rainy season! Ideally, the void boxes should stay completely dry until the concrete goes in. This is more crucial for slabs on void forms, because the construction crew will need to walk on top of the boxes in order to lay out the rebar, and the boxes could be crushed under a person's weight if they get wet. On the other hand, void boxes in a grade beam trench can probably handle a drizzle since no one is walking on top of them.
Some subcontractors might wrap the boxes in plastic in case they get wet, but this is inconsistent with their purpose. More weather resistant boxes are available as well, but again, with disintegration as the goal, that is not always recommended. Ideally, they could be installed during a dry season or protected during the installation process.
Who
Concrete subcontractors may be weary of a job with a lot of void boxes. “Who is going to pay for all these void boxes if they get destroyed before we pour concrete?” they may ask. Therefore, limiting the use of void boxes could be worth discussing with the structural engineer.
