What Are Concrete Void Boxes?

Concrete void boxes are hollow containers strategically placed underneath a concrete foundation to reduce the stress of soil movement. 

Why Do I Need Concrete Void Boxes?

When designing a concrete foundation, it’s important to factor in all the potential stressors. One of the common stressors for a concrete foundation is soil movement below the foundation. Particularly in heavy clay soils, the soil can heave (push upwards) when it becomes wet. Soil heave can be so severe that it will actually push a concrete foundation upwards several inches or more, damaging the alignment of the foundation.

One way to counteract upward soil force is to place voids boxes underneath the foundation. Void boxes create an empty space for soil to “heave into”. In other words, rather than pushing up the foundation, the soil can simply bulge into designated air spaces underneath the foundation.

How Are Void Boxes Made?

The void box we use is typically made with cardboard. While you might not think of cardboard as being a strong material, these boxes are designed to withstand the weight of the concrete after it’s poured. The void box is a concrete form that shapes the underside of the concrete foundation.

Over time, soil moisture from below the foundation will soften the cardboard void box, making it collapse. In the place where there used to be a strong cardboard form, there is now an air space where heaving soil can move upwards in a controlled manner.

Other Types of Void Boxes

Cardboard is not always the best type of void box, especially when forming concrete during wet weather. Void boxes can also be made with polystyrene (foam) or polyethylene (plastic). These void boxes are either designed to be soft enough to compress over time, or strong enough to act as a permanent void box below the concrete.

How Are Void Boxes Placed?

Void boxes are normally spaced evenly throughout the foundation, creating a regular pattern over the length of the foundation. The quantity of void boxes depends on the site and the soil type. Typically the more expansive the soil, the thicker the void boxes are, most common void boxes range from 6-12” deep.

Final Thoughts

Designing the perfect foundation comes down to understanding the stressors that the concrete will face, and designing the foundation accordingly. Void boxes are a great way to reduce the upward heave stress, and improve the long term stability of your foundation.

Whether you need a new residential or commercial foundation, our expert team is ready to design the perfect foundation for your next project.