Void forms serve as sacrificial members for formwork and are specifically designed to create space between the concrete foundation and expansive soil. The primary function of this space is to protect the foundation from the heave that may occur in the underlying soil. Furthermore, void forms provide temporary support to recently poured concrete until it becomes strong enough to support its intended loads.
In addition to their protective function, void forms also help to reduce soil excavation and the amount of concrete needed for construction. The depth of void forms placed beneath foundations typically ranges from 5 cm to 60 cm, depending on the potential for upward movement of the underlying soil.
A variety of void forms are available for use in construction, including degradable and non-degradable forms, as well as collapsible and non-collapsible forms. These forms can be made from a range of materials, including corrugated paper, expanded metal, metal sheeting, molded plastic, molded paper, Styrofoam, and wood.
Why are Void Forms Used in Foundation Construction?
There are several reasons why one may consider using an alternative material to concrete when building a slab. One reason is to accommodate expanding soil volume. By using a material that can adjust to changes in soil volume, one can prevent damage to the slab and ensure its longevity. Another reason is to reduce the amount of concrete used in the construction process. Concrete can be expensive and using less of it can lead to cost savings. Additionally, using an alternative material can create a more stable slab, which is important for structures that need to support heavy loads. Another benefit of using an alternative material is that it can help reduce the amount of soil excavation required during construction, which can lead to cost savings and reduce the environmental impact of the project.
Void Form Materials
There are several materials that are commonly used for packaging and construction purposes. One of these materials is corrugated paper, which is often used for making cardboard boxes and other types of packaging. It is a lightweight and inexpensive material that is also environmentally friendly, as it can be easily recycled.
Another material that is commonly used in construction is expanded metal. This material is made by expanding a sheet of metal, creating a mesh-like pattern. It is often used for building fences, as well as for reinforcing concrete structures.
Metal sheets are also frequently used in construction. They are durable and strong, making them ideal for building roofs, walls, and other structures. They come in various thicknesses and can be cut to size, making them versatile and easy to work with.
Molded plastic is a popular material for making a wide range of products, including toys, household items, and automotive parts. It is lightweight, durable, and can be molded into a variety of shapes and sizes. Molded paper is a similar material, made by pressing paper fibers into a mold. It is often used for making packaging and other disposable items.
Another lightweight material commonly used in packaging is Styrofoam. This material is made from expanded polystyrene foam, and is often used to protect fragile items during shipping. It is also used for insulation and as a cushioning material in furniture.
Wood is a versatile material that has been used in construction for centuries. It is strong and durable, and can be used for everything from building houses and furniture to making paper and other products. Wood is also renewable and can be sustainably harvested, making it an environmentally friendly choice.
Types and Applications of Void Forms
1. Degradable Void Forms
Degradable void forms are specifically designed for construction sites where there is ample water supply in the surrounding environment. These void forms are made from easily degradable materials, such as molded paper and corrugated paper. The primary advantage of using degradable void forms is that they naturally degrade over time when exposed to water, allowing the soil to expand without causing any structural damage. This makes them an environmentally-friendly choice for construction projects that prioritize sustainability. The use of degradable void forms is especially ideal in areas where there is sufficient water supply available to facilitate the natural degradation process before the soil expands.
Figure-1: Degradable Void Forms
Degradable void forms are commonly employed as a means of creating a gap between expansive soil and various structural elements, such as grade beams (as depicted in Figure-2), slab on grade (as shown in Figure-3), and foundation piers. These void forms are designed to eventually break down over time, leaving a void or empty space that helps to prevent direct contact between the structural elements and the surrounding soil. This can be particularly useful in situations where expansive soils are present, as it helps to minimize the potential for soil movement and subsequent damage to the structural elements. By acting as a barrier, degradable void forms can help to enhance the durability and stability of these structural components, contributing to the overall longevity and performance of the construction project.
Degradable void forms are an excellent solution for concrete construction where removing formwork is not feasible. These forms are strong enough to support fresh concrete until it gains sufficient strength to bear its own weight. However, they are designed to degrade over time and may lose their integrity due to moisture, leaving a gap between the foundation and expansive soil.
To ensure proper performance, it is important to avoid placing materials such as plastic sheathing or a layer of gravel under carton forms, as this can obstruct capillary water movement and prevent the void form from degrading effectively. It is also crucial to store void forms in a dry place and protect them from moisture until the concrete is poured. One way to do this is by installing plastic sheeting above the degradable void forms.
These void forms are lightweight, easy to install, and can be easily cut to fit specific requirements. They are available in standard strength and sizes, making them a convenient choice for various concrete construction projects. By following proper storage and installation practices, degradable void forms can effectively support fresh concrete and prevent damages caused by expansive soil, making them an ideal solution for challenging construction scenarios where formwork removal is not possible.
2. Non-Degradable Void Forms
Void forms are essential elements in building foundations, and they serve as placeholders that provide temporary support for the structure until the concrete sets. There are different types of void forms available for use, each with its unique characteristics and advantages. One such type is non-degradable void forms, which are made from materials that do not break down easily, such as metal, wood, Styrofoam, and plastic.
Unlike other void forms, non-degradable ones have the ability to remain in place for extended periods without deteriorating, making them a popular choice for builders. However, it’s crucial for foundation designers to keep in mind the long-term effects of using non-degradable void forms. Since they can last a long time, their placement should be carefully considered to ensure that they do not interfere with any future construction or maintenance work. By taking the necessary precautions and designing foundations accordingly, builders can successfully utilize non-degradable void forms in their projects.
3. Collapsible Void Forms
Collapsible void forms are designed to be manufactured from materials that are capable of being damaged under the pressure of expansive soil. The purpose of this design is to ensure that the material is strong enough to withstand the pressure of fresh concrete but weak enough to resist the pressure of expanding soil. It is important to note that the force of expansive soil can be powerful enough to lift both the structure and its foundation. As a result, collapsible void forms should not be used under a reinforced concrete slab.
4. Non-Collapsible Void Forms
Non-collapsible void forms are ideal for areas where the foundation has sufficient strength to withstand the uplift forces caused by soil expansion on the forms.
Figure-7: Non-Collapsible Void Form
When constructing a building, it is important to carefully consider the type of form system used. In particular, for grade beams, a non-collapsible void form system is recommended. This type of form system should only be used specifically for grade beams, and not for slabs with long spans between stiffening beams and thin, unreinforced slab elements. It is crucial to avoid using non-collapsible void forms in such areas, as they may not provide sufficient support or stability for the thin slab elements due to their inability to withstand the weight and pressure over long spans. Therefore, it is imperative to use appropriate form systems based on the specific structural requirements of the construction project, ensuring the safety and integrity of the building.
FAQs
What is a void form in concrete foundation construction?
Void form is a structural component that is commonly used to provide additional support for foundations that are built on top of expansive soil. The purpose of this sacrificial element is to create a space between the foundation and the heave of the soil, which can help prevent damage or distortion to the structure over time. By providing this buffer zone, void form can help ensure the stability and longevity of the foundation.
In addition to its role as a preventative measure, void form can also serve a more immediate purpose during the construction process. Freshly poured concrete can be susceptible to damage or deformation before it has fully cured and achieved its maximum strength. In order to protect the concrete and provide support while it sets, temporary void forms can be put in place to help distribute the weight of the concrete and ensure that it sets properly. This can help prevent cracking or other types of damage that could compromise the strength and stability of the foundation over time.
What is a typical space between a foundation and underlying expansive soil to install void forms?
The spacing between the concrete foundation and the expansive soil beneath it can vary significantly. The distance typically ranges from 5 centimeters to approximately 60 centimeters. The amount of space required depends on the potential upward movement of the soil. This movement can cause significant damage to a structure if not properly accounted for during the building process. By leaving the appropriate amount of space between the foundation and the soil, builders can help prevent damage and ensure the long-term stability of the structure.
What are the different types of void forms?
In the construction industry, there are various types of void forms used for different purposes. One type of void form is degradable, meaning that it will break down over time and eventually disappear. These types of void forms are often used in applications where the formwork will be difficult to remove, such as in cast-in-place concrete walls. The degradable void forms can be made from materials such as paper or cardboard.
On the other hand, non-degradable void forms are designed to remain in place indefinitely. These types of void forms are often used in applications where the formwork will be easily accessible, such as in precast concrete structures. The non-degradable void forms can be made from materials such as plastic or foam.
Another classification of void forms is collapsible. These types of void forms are designed to compress and collapse under the weight of the concrete, leaving behind a void. Collapsible void forms are often used in applications where the formwork will be exposed to high stresses, such as in bridge decks. These forms can be made from materials such as corrugated cardboard or paper tubes.
Lastly, non-collapsible void forms are designed to maintain their shape under the weight of the concrete. These types of void forms are often used in applications where the formwork will be enclosed and the formwork will not be easily accessible, such as in tunnels. Non-collapsible void forms can be made from materials such as rigid foam or plastic.
What is the type of void form suitable for concrete slab?
When it comes to installing void forms under a concrete slab, the only appropriate type to use is a degradable void form. This is because the combination of dead and sustained live load, along with the inherent stiffness of the slab, is not sufficient to collapse both collapsible and non-collapsible void forms. If these types of void forms were used, the heave force would end up being transferred to the slab.
To avoid this problem, it’s crucial to use a degradable void form, which will naturally degrade over time and prevent any heave force from being transferred to the slab. This type of void form is designed to break down and collapse as the slab settles and compacts over time. As a result, it ensures that the slab remains stable and secure, without any risk of heaving or shifting.
Ultimately, choosing the right type of void form for a concrete slab installation is essential for ensuring the long-term durability and stability of the structure. By selecting a degradable void form, builders and engineers can rest assured that the slab will remain secure and in place for years to come, without any risk of heaving or other structural issues.
What are the advantages of void forms?
One approach to addressing the challenges posed by expanding soil volume is to design structures that are able to absorb this expansion. This can involve using materials or techniques that allow for flexibility or movement, such as incorporating expansion joints or using specially formulated concrete mixes with higher elasticity. By accommodating the natural expansion of the soil, these design strategies can help prevent cracking or damage to the structure.
Another way to mitigate the impact of expanding soil volume is by reducing the quantity of concrete used in construction. This can be achieved through various means, such as optimizing the design of the structure to minimize the amount of concrete needed, or using alternative construction materials that are more lightweight. This not only helps to reduce costs, but also reduces the overall weight of the structure on the soil, which can help alleviate pressure on the expanding soil and reduce the risk of damage.
Creating a more stable slab is another important consideration in construction projects dealing with expanding soil volume. This can involve using techniques such as reinforcing the slab with steel bars or fibers to increase its strength and stability, or incorporating techniques such as post-tensioning, which adds tension to the slab to counteract the forces exerted by the expanding soil. A stable slab is less likely to experience cracking or other structural issues caused by soil expansion.
Reducing soil excavation can also be a beneficial approach in mitigating the challenges posed by expanding soil volume. Excavating soil can disturb its natural state and potentially exacerbate issues related to soil expansion. By minimizing the amount of soil excavation required during construction, such as through techniques like shallow foundations or soil stabilization methods, the potential for soil expansion-related problems can be reduced.
In summary, addressing the challenges posed by expanding soil volume in construction projects can involve incorporating design strategies that allow for soil expansion, reducing the quantity of concrete used, creating a stable slab, and minimizing soil excavation. These approaches can help mitigate the risks associated with expanding soil volume and contribute to more resilient and durable construction projects.