When to remove forms from concrete patio slabs?

Concrete is one of the strongest and most widely used construction materials to build everything from patios to walls and columns. For any concrete structure to be durable, the way the concrete is produced, poured, and most importantly, the curing process is all important.

Knowing when to remove formwork is an important part of curing concrete. In this short read, we will discuss when to remove forms from concrete patios and other structures.

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What is formwork?

Fresh concrete is poured into a container or mold of sorts and compacted. When the concrete has hardened completely, this mold is removed, leaving the concrete in the shape of the mold. This is called formwork and is a very important part of pouring concrete slabs.

The process of removing concrete formwork is called strike-off. This happens only when the concrete has achieved sufficient strength. Special care is taken during strike-off to ensure that the remaining formwork is not disturbed during the process.

When is the right time to remove concrete forms?

As we mentioned earlier, the concrete needs to cure properly and should have gained sufficient strength before the forms are removed. The amount of time this takes depends on the following factors.

Concrete grade

The higher the grade of concrete, the higher the rate of development of strength. This means that higher grade concrete will gain strength sooner than poor grade concrete.

Cement grade

As with concrete, a higher grade of cement will set and gain strength faster as well.

Cement type

The type of cement used also makes a difference in the amount of time the concrete takes to gain strength. For example, ordinary Portland cement will take longer to gain strength than rapid hardening cement will. 

Timeframe for removing formwork

The ASTM (American Society for Testing and Materials) has defined in how much time formwork must be removed based on the type of cement. While Type I and Type II cement need 7-10 days to gain enough strength, Type III, which is high early strength cement, will be strong enough for forms to be removed after 3-4 days.

Similarly, the American Concrete Institute also has recommendations about the minimum time period for concrete curing before formwork removal based on the type of structure.

• Vertical supports for walls and columns can be removed in 24-48 hours.
• In the case of concrete slabs with their props under them supporting slabs, you can remove the form boards in 3 to 4 days.
• Forms for soffits with their props left under them can typically be removed after one week.
• The time period is the same for the removal of props supporting slabs under 15 feet, while props supporting slabs above 15 feet need to be left in place for at least 14 days.
• Formwork supporting arches and beams that are over 20 feet in height ought to be left in place for 21 days.

Formwork removal tips

The removal of formwork is dangerous work, as with all construction work. It must be done only by those with enough training and experience with concrete work. If done wrong, the results could be hazardous to both the structure as well as the people working on-site. Here are some tips to keep in mind while removing forms.

1. Ensure NDT is conducted on the concrete to ensure its strength and quality.
2. While removing forms, always start from the vertical supports first.
3. The forms supporting slabs and soffits must be removed only after testing that the structure can handle its own weight and the weight of external stresses.
4. Never lodge crowbars into the side of structures to remove forms. You are liable to end up damaging the concrete structure.
5. Ensure safety engineers check the site to ensure the concrete has had enough time to cure before removing the forms.
6. Always ensure the people working on the forms wear adequate safety gear while working.

Read more: How to remove paint from concrete

Written by Narayan Shrouthy. August 26, 2022

Narayan is a content writer for Kukun. With experience in both content writing and editing, Narayan mixes his writing and love for reading with his passion for music. As an active musician, He aims to be a key player in Kukun's content experience.