What Are Box Voids and Void Forms?
Concrete is one of the most durable and versatile materials used in construction. However, when poured directly onto expansive soils, even the strongest concrete slab can crack or shift. This is why box voids and void forms are essential components of concrete foundations. These specialized structures protect slabs by creating a void space between the concrete and the surrounding soil, which allows room for soil movement without applying damaging pressure to the structure above.
In this article, we’ll explore what void forms are, the different types available, and how box voids help protect concrete slabs in a variety of soil and weather conditions.
According to Deslauriers, a void form is a temporary structure used during concrete forming to create an empty or void space beneath a slab, beam, or foundation. Once installed and exposed to moisture from the surrounding soil or wet weather, the form gradually loses its supportive strength, allowing it to collapse or degrade after the concrete has cured.
This intentional collapse ensures that the void area remains intact, creating a space where expansive soils can swell without applying pressure to the concrete slab above. Without this space, soil expansion can push upward, leading to cracks, uneven settlement, and long-term foundation damage.
Box voids, sometimes referred to as box void forms or void boxes, are a specific type of void form often made from double-faced corrugated fiberboard, wax-coated cardboard, or molded paper products. These materials are designed to provide temporary supportive strength during fresh concrete placement and curing.
Contractors value box voids because they are:
- Lightweight and easily assembled on-site
- Installed without special tools
- Available in several types, sizes, and materials
- Cost-effective and reliable for preventing foundation damage
What Are the Different Types of Void Forms?
There are three main types of void forms used in concrete construction: collapsible, degradable, and non-collapsible/non-degradable. Each type is engineered to behave differently depending on soil conditions, moisture exposure, and structural requirements. Choosing the right type ensures your concrete foundation remains stable long term.
1. Collapsible Void Forms
Collapsible void forms are designed to temporarily support fresh concrete and then collapse after exposure to moisture. These are typically made from double-faced corrugated fiberboard, molded paper, or wax-coated cardboard. These materials provide enough supportive strength during the pour, but intentionally weaken when wet.
As soil moisture increases or wet weather sets in, these forms lose their strength, forming a void area beneath the concrete. This void space gives expansive soils room to swell without exerting pressure on the slab or load-bearing beams above.
2. Degradable Void Forms
Degradable void forms also create space beneath slabs, but unlike collapsible versions, they degrade gradually rather than collapsing immediately. These are typically made from treated cardboard, corrugated paper, or fiberboard designed to maintain strength for long enough for fresh concrete to fully cure.
Once the concrete has gained adequate strength, the void form begins to decompose due to moisture exposure, slowly creating the necessary void space underneath. This gradual process minimizes the risk of early collapse and helps maintain the integrity of the concrete during curing.
3. Non-Collapsible Void Forms / Non-Degradable Void Forms
Non-collapsible and non-degradable void forms are permanent fixtures that retain their shape and strength indefinitely. These are typically constructed from plastic, metal, or wood and are used in applications where the void space must remain open and stable over the life of the structure.
Instead of degrading, these forms provide a permanent separation between the concrete foundation and the soil. They are especially useful in non-expansive soils or in situations where engineers require a fixed cavity for ventilation, drainage, or design purposes.
How Do Box Voids Help Protect Concrete Slabs?
Box voids play an essential role in protecting concrete slabs from the destructive effects of expansive soil. Here’s how:
1. Allowing for Soil Expansion
By creating a void space beneath the slab, box void forms give expansive soils room to swell during periods of high soil moisture. This space absorbs soil movement and prevents pressure from transferring to the concrete structure above.
2. Preventing Slab Heaving and Cracking
Without a void, soil pressure due to expansion can cause slabs to lift, shift, or crack, which compromises foundation integrity. Box voids prevent this by maintaining separation between the slab and soil, allowing the structure to remain stable even when soil conditions fluctuate.
3. Reducing Long-Term Maintenance Costs
Foundations built without void forms are far more susceptible to cracking, settlement, and moisture-related damage. Using box voids minimizes these risks, which in turn lowers long-term repair costs and ensures a more durable concrete foundation.
4. Supporting Fresh Concrete During Pouring
During installation, box voids temporarily hold the weight of fresh concrete, acting as a supportive form until the concrete hardens. Once cured, the voids begin to degrade or collapse, ensuring the slab is left with the necessary void area for soil expansion.
Ready to Protect Your Foundation?
Box voids and void forms are small components that make a massive difference in the longevity and performance of concrete structures. By providing a controlled void space, they protect foundations, slabs, and beams from the unpredictable behavior of expansive soils and moisture.
At Deslauriers, we specialize in engineered void form solutions that help contractors and engineers safeguard their concrete foundations from soil-related damage.
Contact us today to discuss your project requirements and get expert guidance on the best box void forms for your site conditions.
