How Do Concrete Forms Work?
A Look at the Various Types of Concrete Forms and How They Help in Concrete Construction
Whether you’re building a concrete patio or a skyscraper, you’re going to need concrete forms to get the job done.
Without concrete forms, your concrete won’t hold its shape and will be at risk of cracking and losing its strength.
Concrete forms go hand-in-hand with other concrete placing techniques to ensure your concrete remains strong and durable.
So to guarantee your concrete construction projects go smoothly, order enough concrete products for the job and make sure you use concrete forms.
Here’s a look at how formwork is used, the principles behind it, and why it can be so helpful on the job site.
What Is Concrete Formwork?
Formwork, also known as concrete forms, is a temporary or permanent mould used to create concrete slabs and structures.
Concrete forms hold freshly poured concrete in place and the ideal shape until the concrete is strong enough to hold its own weight and shape.
To work effectively at supporting the concrete’s weight and shape, concrete forms must:
- Have tight joints so the wet concrete cannot leak through;
- Be rigid enough to prevent bowing under the weight of the concrete;
- Be strong enough to hold the concrete in place;
- Have proper horizontal and vertical support and bracing, such as with poles and stabilizers (also known as falsework);
- Be completely level; and,
- Be able to withstand the elements without warping.
With the proper construction and placement of concrete forms, you can ensure your finished concrete project will be the right shape with optimal strength and durability.
How Is Formwork Made?
Formwork is usually made from timber and plywood. But it can also be made from other materials, such as metals, ceramic, or glass fibre reinforced plastics.
Plywood formwork is also called shuttering. Shuttering is the most popular type of formwork and is created on site using water-resistant plywood and timber.
When on the job site, concrete contractors will erect plywood, steel, or aluminum formwork to create the moulds of concrete slabs, walls, and other structures.
Concrete formwork includes the following components:
The formwork that comes in direct contact with concrete is called the formwork sheeting, or the formwork shell. This sheeting will mould the shape of the concrete and have the most influence on the finished concrete surface.
The formwork bearers, which are usually made of timber or steel beams and clamps, support the formwork sheeting. These distribute the concrete forces through the falsework, supporting structures, formwork ties, and soil.
Formwork ties are usually made of steel wire and are used to tie two formwork faces together to support horizontal pressure from vertical formwork.
The steel wire is slung around the formwork bearers, guided through boreholes in the formwork sheeting, and tightened by twisting the wire.
Once the formwork is constructed, concrete contractors will place rebar inside the formwork to reinforce the concrete.
Next, contractors will pour concrete inside the forms. Once the concrete hardens, they will remove (strip) the forms from the concrete structure if using temporary formwork.
To prevent damage to the concrete and difficulties when removing formwork, it’s important to consider the placement of formwork and the sequence in which you the framework will be stripped.
Avoid using too many nails and supports (falsework) when building formwork, as this will be too difficult to strip later on. Only use as much as necessary to maintain support of the formwork.
How Long Do You Leave Forms On Concrete?
Temporary concrete forms should be left to support the concrete until the concrete has thoroughly dried and reached an ideal strength to support its shape. This usually takes about two days after pouring the concrete.
If you remove the formwork too soon, the concrete might begin to sag, crack, and collapse. Remember to be patient and let your concrete thoroughly dry before removing formwork.
What Is Formwork Used For?
Formwork is used for holding the shape of concrete while it dries. It is commonly used for building concrete:
- Driveways, and,
The type of formwork you use will depend on your project needs. A simple temporary plywood formwork, such as plank shuttering, is suitable for moulding concrete paths and driveways.
But if you’re building a concrete foundation for a home, you will want to use more sophisticated formwork, such permanent insulated concrete forms.
What Are Insulated Concrete Forms (ICF)?
Unlike traditional concrete forms, insulating concrete forms (ICF) are not removed once the concrete hardens. Instead, ICFs remain as permanent parts of the concrete structure.
While concrete creates the structure, the ICFs insulate the structure. Homes and buildings with ICF foundations are soundproof, airtight, and comfortable in all seasons. This insulation helps homeowners save on energy costs for heating and cooling throughout the year.
ICFs are often made from a combination of insulating materials, such as plastic foam beads and cement, or wood fibre and cement.
Another popular material for ICFs is expanded or extruded polystyrene. Expanded polystyrene is made by expanding plastic beads in a mould, similar to Styrofoam coffee cups.
Extruded polystyrene is made by expanding plastic resin and extruding through a die, similar to meat trays from grocery stores.
ICFs typically come in blocks or planks. ICF block systems are pre-formed with foam and metal or plastic ties. They are ready to stack and interlock when they arrive on site.
Plank ICFs are individual panels or planks of foam requiring assembly on site with ties.
Block ICFs save you time and labour costs, while plank ICFs save on shipping costs since they are more compact to ship.
These concrete forms usually provide at least two inches of insulation on both sides of a concrete wall.
ICFs are not only faster to place, but they also provide significantly better insulation compared to wood or steel frames. So if you’re planning to build a new home, consider using ICFs for a well-insulated foundation.
Cases Where Formwork Might Be Necessary
Formwork is necessary when there is no soil or other structural components to support freshly poured concrete and hold it in the desired shape.
Formwork is necessary for the construction of concrete and reinforced concrete structures, such as:
- Solid structures—e.g. foundations and columns;
- Reconstruction of structures;
- Structures with special functions—e.g. chimneys and containers;
- Bridges and towers;
- Public buildings; and,
- Structures with an irregular arrangement.
Vertical formwork is necessary for building concrete foundations, walls, and columns. Tie wires or tie rods are also used with this formwork to support the lateral pressure of concrete.
Vertical formwork must also be anchored to the subsoil to prevent lifting from buoyant forces in the concrete.
Horizontal formwork is necessary for constructing concrete ceilings and beams. This formwork must be supported against vertical and horizontal loads, using formwork load-bearers and columns, braces and struts, or a rigid connection to existing structures, such as walls and columns.
Concrete forms are an essential part of building sound concrete structures. When constructed correctly, formwork moulds concrete into the desired shape and supports the weight of concrete while it’s drying.
And most importantly, concrete forms allow concrete to reach its optimal strength and durability without sagging or cracking.
For more information on b