5 Things to Know Before Buying Concrete Slab Formwork

Poured concrete is a viscous solution that can take up to 48 hours to properly set. During the setting time, the concrete needs to be contained in a concrete form. Concrete forms or formwork are the supporting materials or molds that are used when working with concrete. They also determine the final shape of the concrete, because liquid takes on the shape of its container.

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However, not just any material can be used as a concrete form, as formwork needs to be able to hold the concrete’s weight and shape. For the formwork to be useful, it should have strong joints to prevent leaks, a rigid structure so that it doesn’t bend or break under the weight of the concrete, and it should also be able to be used with horizontal and vertical bracing. Keep reading for more details and important information about how to use concrete forms.

Related: Cement vs. Concrete: What’s the Difference?

1. Concrete forms are necessary to ensure that the concrete sets properly during a construction project.

Concrete is used regularly in construction projects. For example, it can be poured to create supports for a fence or gate or as a foundation for a new shed. However, when concrete is initially mixed, it isn’t a stiff compound that will hold a shape without help. It is a thick, viscous liquid that is difficult to mix, and it’s hard to manage without using concrete forms.

A concrete form or formwork is a support or mold that the concrete is poured into. Temporary or permanent, the formwork is built into the shape that the user wants the concrete to take when the liquid concrete begins to dry.

2. Timber is the most commonly used material for formwork.

There are several different materials that are used to make formwork, but the most commonly used material is timber. This is because timber is cost-effective, lightweight, durable, and flexible, allowing the concrete to shift while it sets without pieces breaking off or putting strain on the formwork.

For the best results, invest in timber formwork that is easy to work with, lightweight, and dried thoroughly. Also, make sure the timber doesn’t have any loose knots that could pop out and allow the wet concrete to seep through the hole. While there is the chance that wood formwork will warp if it absorbs the water from the concrete, this effect can be limited by treating the timber forms with a sealant.

3. While timber is most common, there are other materials that are used for concrete forms, including steel, plywood, and fiberglass.

In addition to timber, there are a few other common materials used to make concrete forms. Steel is regularly used in heavy civil construction to create thin steel plates, which can be held together with clamps or with fasteners to support broad concrete projects. This durable material is more expensive than timber, but it’s also stronger and can be reused or sold.

Another type of material used to make formwork is resin-bonded plywood. These sheets of plywood will typically be attached to timber frames to create large, inexpensive concrete forms. However, plywood is less durable and rarely reusable. Fiberglass concrete forms are best for projects with curved surfaces, though they are expensive and often need to be custom ordered.

Related: 13 Things All DIYers Should Know About Plywood

4. A good concrete formwork needs to be both strong and lightweight with a rigid construct that retains its shape under stress.

In order for a concrete formwork to be suitable for a construction project, it needs to be strong enough to hold the weight of the concrete without slipping, cracking, or bending. Any significant damage to the formwork will allow the concrete to pour out, or it may just ruin the intended shape if the concrete is partially cured.

However, the concrete forms also can be made with a lightweight material that are used without significant physical effort on the part of the user. Timber and plywood are great examples of material that is strong, flexible, and relatively lightweight, yet rigid enough that the concrete forms will keep the same shape, allowing the concrete to set in the desired form.

5. Concrete forms are necessary to create high-quality concrete, but with careful planning, measuring, and cutting, users can save on the overall cost of formwork.

Many people either don’t know about concrete forms or they don’t want to have to pay for timber or plywood to set up formwork because the extra cost is too high. Unfortunately, without the use of formwork, it’s all too easy for construction projects to go awry, leaving the concrete slab for the new shed or workshop uneven and difficult to use as support. So, while it’s a bad idea to go without concrete forms, there are ways that can help save on the cost of these materials.

Plan out the project carefully so that pieces of the formwork can be reused for multiple sections and it’s also a good idea to limit the number of slender sections to avoid wasting small pieces of timber or plywood. By reducing the number of cuts that need to be made in the concrete forms, there is less timber or plywood needed in total. However, users should keep in mind that while formwork typically costs about $1.10 per square foot of the concrete area, a rounded or contoured framework is generally more expensive.

Related: How To: Cut Concrete

6. One of the benefits of using steel is the long-lasting durability that allows it to be reused and even resold to help save on the cost of the project.

The initial cost and skill required to make steel concrete forms are higher than what’s needed for timber or plywood. Due to the impressive strength, durability, and resilience of steel, the steel concrete forms can be used on multiple projects without damage. For companies or independent professionals that regularly work with concrete, this is a way to cut costs on a project.

DIYers also can benefit from the long-lasting strength and flexibility of steel concrete forms by using this formwork on more than one project. If the steel formwork is just sitting in storage in the garage, consider selling it to another avid DIYer. A professional also may be interested in a bargain price for used concrete forms to help reduce the total cost of a project.

7. Formwork helps to ensure that the concrete sets properly, so it’s necessary for a range of projects including building a fence, installing a gate, pouring a foundation, and constructing a home. 

There are many different construction projects that utilize concrete and most of them require some type of formwork to ensure that the concrete sets according to plan. Goals can range from making a few simple steps to lead up to the porch to pouring a complex foundation to make insulating concrete forms for the home. Insulating concrete forms are essentially concrete walls that are poured between two layers of insulating material to create a strong and insulated foundation.

Other common uses for concrete forms include pouring a simple concrete slab to act as the foundation for a shed or workshop, securing the posts of a fence or a gate in the ground with a concrete base, or making simple paving stones for the yard with a shallow formwork mold.

Related: How To: Repair Concrete Steps

Concrete is a durable, versatile, and liquid building material that is a viable choice for many construction projects. Without proper forming, however, concrete is useless for most construction projects.

This article will cover what concrete forming is, how to use it in concrete construction, how it works, and much more.

Let’s delve in:

1. What Is Concrete Forming?

A concrete form is a temporary or permanent barrier that molds concrete into a specific shape until it hardens enough to retain that shape.

Concrete forming is crucial to the construction of concrete structures. It allows concrete to dry fully in the correct shape without cracking or sagging.

Although concrete forming is not new, it has evolved over time.

Concrete forming dates back to ancient Roman times when they first used reeds, then graduated to wood, stacked rocks, and bricks. Well-known structures like the Colosseum and Pantheon were both built with the help of formwork.

Since then, we’ve designed entire concrete forming systems that are much less rudimentary.

2. How Is Concrete Forming Used?

So, how are concrete forms used in erecting a concrete project?

A concrete slab is the easiest and most well-known concrete product you can create using a form. However, there is so much more that you can do with the help of concrete forms.

Various components are built with the help of concrete forms and joined together to create a complete concrete structure.

These components include:

• Foundations
• Walls
• Ceilings
• Beams
• Columns
• Paths
• Walkways
• Sidewalks
• Driveways

Vertical Formwork

You can form concrete from the ground up or from one structure to another. These are vertical and horizontal formworks.

Use vertical formworks to create foundations, walls, and columns. You’ll have to anchor these to the subsoil. Vertical formworks need tie rods or wires to keep the concrete from expanding the form outward.

Horizontal Formwork

Horizontal formwork creates concrete forms off the ground. These are useful for components like ceilings and beams.

These need much more rigid support from columns, braces, struts, or fully formed walls to hold the form in place until the concrete sets.

3. How Does Concrete Forming Work?

Concrete contractors will prepare the building site by removing debris and excess soil with an excavator.

They then erect the formwork at the job site before pouring concrete. Contractors can choose which type of formwork to use based on the project.

They use these forms to keep the concrete in the desired shape until set. One project will often require the use of many concrete forms to complete.

Forms are erected and dismantled strategically to ensure the supporting concrete is stable before adding more on top.

Components of Concrete Formwork

While there are many different types of formworks available, most share several common components.

Each component plays an integral role in holding the concrete in the correct shape and allowing it to gain strength and durability.

Formwork Sheeting

Formwork sheeting refers to the pieces that touch the concrete and hold it into the form. These can be aluminum, acrylic, fiberglass, or plywood sheets.

Formwork sheeting is also known as formwork shell; using it can add texture to decorative concrete.

Formwork Bearers

Formwork bearers are support beams for the sheeting to help with the weight of the concrete. These are usually made of steel beams and clamps to quickly build a concrete wall form.

Formwork Ties

Another reinforcement used in formwork is formwork ties. These consist of steel tie wire strung around the bearers and woven through the form boards.

This wire is then tightened by twisting it with a hand tool. Formwork ties help to strengthen the form as a whole.

Other common ways to increase the strength of the form are to use fasteners, latches, and joints.

Rebar

Steel stakes called rebar go inside the formwork to reinforce and strengthen the structure before pouring the concrete.

4. How Do You Know What Type of Concrete Form to Use?

There are many types of concrete forms on the market, and each has its benefits and downfalls.

When determining which formwork to use for your project, you must consider the formwork’s cost.

The most inexpensive formwork is more labor intensive, less efficient, and more susceptible to collapse. However, it will save you a ton of money.

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An expensive premade system is safer, more labor efficient, and improves productivity on the project.

While the initial investment is significantly higher, much of the cost is regained on labor and the longer life cycle of the formwork itself.

Rent or Buy?

Many project owners who don’t foresee the need to own formwork in the future will choose to rent engineered formwork systems. Rentals require little maintenance since the rental company will care for the system.

However, if multiple projects need completing, buying formwork would be more economical than renting it.

Of course, owning formwork systems requires storage and regular maintenance in order to maintain their strength.

Six Major Types

There are six significant formwork types that you can use to mold concrete.

Keep reading to understand their differences and what each is best for.

1. Timber Forms

The oldest and cheapest type of concrete form is the timber form.

Most contractors know how to build these forms. They are lightweight and can be made into any shape you desire.

They do have a couple of pitfalls. First, they are the most labor-intensive type of form to build. Erecting timber forms can take as long as building the structure itself.

The second pitfall is that plywood sheeting is not the most durable and won’t have as long of a life cycle as steel or plastic forms.

2. Engineered Forms

These prefabricated, engineered forms are the safest and strongest form for concrete available on the market.

They are also one of the most expensive, but they are easily put together and last a long time when well maintained.

3. Plastic Forms

Plastic forms are usually modular to allow for the creation of different shapes. They’re also incredibly lightweight but surprisingly powerful and easy to keep.

4. ICF Forms

ICF, or insulated concrete forms, are permanent forms that stay with the structure even after the concrete has dried completely.

These forms are easily set up, add extra insulation, and eliminate any fears of weakness in the concrete due to incorrect forming.

5. Stay-in-Place Forms

Stay-in-place forms are made from different materials that don’t corrode. They stay in the concrete even after it has fully dried.

These are similar to ICF forms but don’t provide much insulation.

The types of stay-in-place forms include:

• Polyvinyl chloride (PVC)
• Fiber-reinforced polymer (FRP)
• Galvanized coiled sheet
• Fabricated steel
• Carbon/epoxy thin shell

6. Flexible Forms

Flexible forms are just as the name implies. They can bend and twist to create fluid concrete forms such as curves and domes.

As you can see, each type of formwork has its pros and cons, and every project has unique needs. While one project could use multiple forms, you cannot use every form for every project.

Choosing the type of concrete forms for your project is a balance of:

• Cost
• Efficiency
• Your contractor’s proficiency with the form
• And the value it will bring to the project itself

5. How Do You Design Concrete Forms?

Once you’ve chosen your concrete form type, you must design the form to create your concrete structure.

You’ll need to determine your form’s square footage and ensure you have enough material to create it.

You also need to be sure that your formwork is strong enough to support the load of the pour and the vibration after.

The pressure of the concrete against the formwork is the most critical issue. The height of the pour, the concrete pour rate, the concrete’s weight, and the air temperature at the pouring time all affect the pressure.

You should do a total calculation of the load and pressure before designing your formwork. This article explains how to do it, sharing the methods and specific formulas to use.

Use the proper thickness of sheeting-plywood grade, steel, or plastic sheeting to ensure that your formwork will support your pour based on your calculations.

You must also use the proper amount of props, beams, and supports to hold the vertical and horizontal loads as well as the lateral pressure. All joints, including isolation joints, need to be leakproof.

Most forms will require the addition of rebar to reinforce the concrete and add stability. This quick read will help you determine when to include rebar in your formwork design.

6. What Are Some Common Mistakes to Avoid?

You should be aware of some common mistakes made during formwork design so you can avoid repeating them on your project.

Here they are:

Not Caring for Formwork Properly

Damaged formwork can cause collapse during a pour. That is why it is essential to properly care for formwork before, during, and after each use.

To keep your formwork in pristine condition, you should clean and oil the formwork after each use.

It’s important to know how to properly care for and maintain formwork down to the type of oil to use and how to clean it correctly.

Removing Formwork Too Early

Patience is critical during the construction of concrete structures. Removing the formwork too early can cause a weakening in the structural integrity of the concrete (and the entire structure).

Incorrect Assembly of Formwork

It is also vital that you assemble your formwork correctly. When using pre-engineered formwork, directions should come with it. Follow these to a tee.

Use the correct amount of bearers and ties and space them correctly to support the pressure of the concrete. This includes expansion joints and anchor bolts.

Not Following Shoring Safety Guidelines

You’ll risk injury to your workers or property damage if you don’t enforce safety guidelines.

Regarding formwork, the best way to keep everyone safe is by following shoring guidelines.

Safway Services outlines shoring safety guidelines here .

7. Can You Reuse Concrete Forms?

Formwork can be a very costly part of your project but reusing them saves you a lot on material costs.

Some forms are permanent, like ICF or stay-in-place forms, and so won’t be reused, leaving only temporary forms for reuse.

Of course, you can only reuse certain temporary forms. Pre-engineered forms are created to be able to be set up and taken down repeatedly, so they are easily reusable.

You can also reuse precast forms (and plywood or acrylic) with the help of a special addition.

Apply Release Agent

To help you reuse formwork, you can apply a release agent to the formwork sheeting before pouring the concrete.

You can also use plastic sheeting or form liners that you can peel off to keep the concrete from sticking to the formwork.

Plywood panels for forms have an exterior layer of resin applied to help repel moisture and add stability for easier release.

This article can help you learn more about release agents and keeping concrete from sticking to your forms so you can reuse them.

Even with the help of release agents and other helpful tools, the strength of formwork decreases with time and each use. You should also inspect formwork carefully before each use.

Conclusion

Hopefully, these answers helped squash any doubts about the use and purpose of concrete forming in your next concrete projects. By following the methods and applying the information in this guide, your concrete formwork should be top-notch.

Proper concrete forming is crucial for the most sound, stable, and long-lasting concrete structures.

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