Oct. 21, 2024
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.
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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:
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.
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:
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 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.
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.
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 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 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.
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.
Steel stakes called rebar go inside the formwork to reinforce and strengthen the structure before pouring the concrete.
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.
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.
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.
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.
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.
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.
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.
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.
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:
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:
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.
You should be aware of some common mistakes made during formwork design so you can avoid repeating them on your project.
Here they are:
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.
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).
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.
Contact us to discuss your requirements of slab formwork. Our experienced sales team can help you identify the options that best suit your needs.
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.
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.
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.
You may also like: Is Steel Stronger Than Concrete? Which Is Best?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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concrete formwork question
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MVPs
(Structural)
(OP)
21 Apr 12 11:07I have a situtation where perimeter masonry walls support an outdoor patio. The owner wants to use a structural reinforced slab instead of slab on grade. I know there are ways around having to shore the slab (light gauge framing supporting slab, composite deck, steel joists, etc.), so this is just a general question. Have you ever been in this type situation where the formwork was required but would be enclosed by the new concrete structure and have to be... abandoned for lack of a better word? Thanks.
Replies continue below
(Structural)
21 Apr 12 11:24Typically use a 6" void form... this can be a cardboard type or my preference, a foamed plastic type. This is placed on moderately compacted granular fill (depending on the soils, sometimes no compaction) and the concrete is cast over it. You can still place a PEVB between the void form and the concrete slab.
Dik
(Structural)
21 Apr 12 11:58We've used the cardboard forms, foam, and metal form deck. All left in place. I'd be wary of leaving wood formwork abandoned near a house if you are in termite country.
(Structural)
21 Apr 12 12:52@bigengineer: We have done what we call structural slabs on grade for interior applications where for design purposes the slab is considered as a suspended structural slab. This is possible where the underlying soil/base is capable of supporting the weight of the structural slab and construction loads during the construction phase.
Your post does not mention about your region. If you are in a region where there are no frost protection requirements, then it is possible to place the slab directly on soil/base without using any sacrificial forms or frost protection measures.
(Structural)
(OP)
21 Apr 12 14:15Thanks for the replies. Actually, this is an elevated patio, about 5-6' above grade with perimeter masonry walls. The owner doesn't want to use fill inside the masonry walls and wants a free spanning r.c. slab spanning between the walls. There are no access doors, etc to remove form work later. And again, owner is requesting no fill material.
I can use a composite steel deck to achieve the spans he wants, but would be cheaper to do some metal decking (non-composite) as form work that is shored a couple/few times between permanent supports. Like JAE said, this would cause concerns with termites though with abandoned forms. Maybe form work from treated lumber?
(Structural)
21 Apr 12 14:37You can support metal deck on wood knee walls to cast the concrete and remove the temporary framing after the concrete is sufficiently hard. Metal deck forming can be less costly than plywood... and you may be able to obtain roof deck (not composite) from a local salvage supplier.
Dik
(Structural)
21 Apr 12 16:46Don't use composite metal decking. The decking would serve as the structural reinforcement but because it is an exterior deck, it will rust and eventually you would have a slab with no reinforcement.
Use form deck and reinforce with rebar or do one of the other ideas above.
(Structural)
21 Apr 12 17:45Agree with JAE. Use whatever deck will carry the load during construction, but reinforced the slab as if the deck did not exist.
(Structural)
21 Apr 12 19:22Thanks, guys... I should have been a little more emphatic...
Dik
(Structural)
21 Apr 12 20:52I have seen metal decking used as lost formwork on a footbridge project. The slab was reinforced with conventional reinforcement and the metal decking was lest to rust away over time.
Becareful with cardboard void formers. I used them once and they collapsed when the concrete was placed. Very costly.
(Structural)
22 Apr 12 11:45bigenginnerz:
You should have some openings for ventilation... Can have a louvred opening, or simply a steel grate/wire opening to keep birds and rodents out...
asixth:
Cardboard ones work OK, as long as they're kept dry prior to placing and contractors often wrap them in poly and provide a 1/8" or 1/4" masonite board on top to keep the rebar chairs from poking through.
Dik
(Structural)
25 Apr 12 17:46bigenginner
- I definitely agree with JAE & Hokie, do not depend on the metal deck for any composite action. Dik makes a good point about providing a couple of screened openings to ventilate the enclosed void. You don't mention the plan dimensions, but you may be able to lighten up the deck gauge by fabricating a simple truss from angle stock to hang off the walls and provide additional support at the middle of the deck span.- I definitely agree with JAE & Hokie, do not depend on the metal deck for any composite action. Dik makes a good point about providing a couple of screened openings to ventilate the enclosed void. You don't mention the plan dimensions, but you may be able to lighten up the deck gauge by fabricating a simple truss from angle stock to hang off the walls and provide additional support at the middle of the deck span.
Ralph
Structures Consulting
Northeast USA
(Structural)
25 Apr 12 17:58You could also build another central wall support or piers to cut down on the slab span.
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