How Long Should You Wait to Remove Concrete Formwork?
Engineered formwork is a type of mold that concrete is poured into to create walls, slabs, columns, and many other types of cement structures. Engineered formwork is a metal mold, typically made of steel or aluminum, gives contractors the option to create structures of many different shapes and sizes. It is a cost-effective alternative to traditional timber formwork; engineered formwork can be reused as many as 2,000 times if it’s properly maintained.
Once the concrete has hardened and cured inside the molds, the formwork can be removed. The process is sometimes called stripping or shuttering.
If you’re working on a project that requires a good deal of forming, you may be wondering how you can speed up the concrete formwork removal process. It’s a great idea to know approximately how long you should wait and under what conditions removal without damage is possible. It can help you better plan removal on a timeline that doesn’t slow your project down.
The exact length of time formwork must remain in place will vary on every jobsite. Things like the type of cement, climate, admixtures, and size of the forms and structure will all have an impact on the finished product.
Mistakes to Avoid
There are many reasons concrete fails to set correctly, or ends up with cracks and structural issues. The good news is that it is preventable, you just need to be aware of the common mistakes made when pouring concrete.
Here are a few:
- Not ordering enough concrete
- Using the wrong type of concrete or admixtures
- Pouring on an unprepped site
- Using the wrong water/cement ratio
- Not having finishing tools on hand, or leaving the site without finishing the freshly laid concrete
- Being impatient and removing formwork before the concrete is cured
General Rules of Thumb
The curing process requires monitoring and maintenance of moisture and temperature within the formwork. It is crucial that you leave the formwork in place as long as possible. The engineer should always be present during the removal process, and all state or local mandates should be followed.
Generally, the American Concrete Institute recommends the following:
- ASTM C 150 Type I cement: 7 days
- ASTM C 150 Type II cement: 10 days
- ASTM C 150 Type III cement: 3 days
- ASTM C 150 Type IV or V cement: 14 days
- ASTM C 595, C 845, C 1157 cements: Varies
We can examine some guidelines for specific structures a little more closely. Always check with your site engineer to confirm the exact timeline your concrete forms require to cure properly. The following can be are general rules for Ordinary Portland Cement (OPC):
- Walls and columns can be removed after about 24-48 hours
- Slabs, with their props left under them, can typically be removed after 3-4 days
- Soffits, with their props left under them, can be removed after one week
- Props supporting slabs under 15 feet can be removed after one week
- Props supporting slabs over 15 feet can be removed after two weeks
- Props to supporting beams and arches under 20 feet can be removed after two weeks
- Props to supporting beams and arches over 20 feet can be removed after three weeks
When using Portland Pozzolana Cement (PPC), the amount of time before formwork can be removed is 10/7 of the times above.
Read about how to safely remove concrete formwork here.
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Updated March 8, 2019.