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Void forms are sacrificial formwork members designed to provide a space between the concrete foundation and expansive soil, i.e., to protect the foundation from the heave of the underlying soil. They also offer a temporary support formwork for recently poured concrete till it achieves adequate strength to support its loads.

Moreover, void forms are utilized to decrease soil excavation and reduce the quantity of concrete. The depth of void forms under foundations ranges from 5 cm to around 60 cm based on the underlying soil’s potential for upward movement.

Furthermore, several types of void forms are available to use, like degradable void form, non-degradable void form, collapsible void form, and non-collapsible void form. These void forms can be constructed from corrugated paper, expanded metal, metal sheet, molded plastic, molded paper, Styrofoam, and wood.   

Why are Void Forms Used in Foundation Construction?

  1. To absorb expanding soil volume
  2. To reduce the amount of concrete  
  3. To create a more stable slab
  4. To reduce soil excavation

Void Form Materials

  1. Corrugated Paper
  2. Expanded Metal
  3. Metal Sheet
  4. Molded Plastic
  5. Molded Paper
  6. Stryrofoam
  7. Wood

Types and Applications of Void Forms

1. Degradable Void Forms

Degradable void form is manufactured from easily degradable materials like molded paper and corrugated paper. It is suitable for a construction site where there is a sufficient water supply in the environment to degrade the void form material before soil expansion occurs.

Degradable Void Forms
Figure-1: Degradable Void Forms

The degradable void forms are used to provide a space between expansive soil and grade beams (Figure-2), slab on grade (Figure-3), and separate foundation piers from the surrounding soil.  

Degradable Void Forms Used for Grade Beam Construction
Figure-2: Degradable Void Forms Used for Grade Beam Construction
Degradable Void Forms Used Under Slab
Figure-3: Degradable Void Forms Used Under Slab

They are strong enough to support fresh concrete until it achieves sufficient strength to carry its self-weight. The form would degrade and lose its integrity due to moisture and ultimately leave a space between the foundation and expansive soil.

Degradable void forms are an ideal solution for concrete construction where the removal of formwork is impossible. They are also used underneath a concrete slab to prevent severe damages caused by expansive soil.

Prevent the placement of materials like plastic sheathing or layer of gravel under carton forms; otherwise, the capillary water movement is obstructed, and the void form would not degrade.

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Void forms should be stored in a dry place. Void forms are also required to be protected from moisture until the concrete is poured. Plastic sheeting should be installed above degradable void forms. The degradable void forms are lightweight, easy to install, cut easily, and available in standard strength and sizes.

Schematic Representation of Void Form Placement
Figure-4: Schematic Representation of Void Form Placement

2. Non-Degradable Void Forms

As can be observed from the name, this type of void form is produced from materials that are not degraded easily, like metal, wood, Styrofoam, and plastic. Non-degradable void forms remain in their place for a long time, so foundation designers should keep that in mind before using it.

Non-Degradable Void Forms
Figure-5: Non-Degradable Void Forms

3. Collapsible Void Forms

Collapsible void forms are manufactured from materials which are damaged under the pressure of expansive soil. They should be strong enough to withstand fresh concrete pressure but considerably weak to resist expanding soil pressure.

The force of expansive soil is sometimes adequate to lift the structure and its foundation. The collapsible void form should not be used under the reinforced concrete slab.

Collapsible Void Forms
Figure-6: Collapsible Void Forms

4. Non-Collapsible Void Forms

Non-collapsible void forms are suitable for locations where the foundation’s strength is adequate to resist uplift forces of soil expansion exerted on the forms.

Non-Collapsible Void Form
Figure-7: Non-Collapsible Void Form

A non-collapsible void form system should be used under grade beam only. This type of form should not be utilized under a slab area with a long span between stiffening beams and thin unreinforced slab elements.

Installation of Collapsible Void Forms
Figure-8: Installation of Collapsible Void Forms

FAQs

What is a void form in concrete foundation construction?

Void form is a sacrificial element used underneath a foundation to create a space between the foundation and heave of expansive soil. Additionally, provide a temporary support for fresh concrete until it achieves a specified concrete strength.

What is a typical space between a foundation and underlying expansive soil to install void forms?

The spacing between the concrete foundation and expansive soil underneath ranges from 5 cm to around 60 cm based on the potential upward movement of the soil.

What are the different types of void forms?

1. Degradable void forms
2. Non-degradable void forms
3. Collapsible void forms
4. Non-collapsible void forms

What is the type of void form suitable for concrete slab?

Degradable void form is the only type that is appropriate to be installed under a concrete slab. The combination of dead and sustained live load and inherent stiffness of slab is not adequate to collapse both collapsible and non-collapsible void forms. As a result, the heave force will be transferred to the slab.

What are the advantages of void forms?

1. Absorb expanding soil volume
2. Reduce the quantity of concrete
3. Create a more stable slab
4. Reduce soil Excavation

Read More

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Frost Heave – How It Works, Effects, and Prevention

10 Most Common Reasons of Foundation Failure

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