Pouring concrete can be a difficult task, but with the correct information and the right tools, it can be made much easier.
Large-scale pours need to be completed by professionals, but on a smaller scale, it’s well within the capabilities of a DIYer. Getting the concrete shaped and set properly relies heavily on formwork. Knowing when to remove concrete forms is the difference between a successful concrete pour and a failed one.
In this article, we’ll explain all about when to remove concrete forms.
How Long Should You Keep Concrete Forms in Place?
A form must be kept in place until the concrete has set enough to support itself. The amount of time before a form can be removed will vary depending on certain factors, which we will discuss further in this article. The below figures are a general guide and should be adjusted according to the situation at hand.
Even when concrete appears hardened, it’s not as strong as it needs to be. There is a process after hardening called curing. You have to wait for the concrete to cure before weight and stress can be put on it. This means if you’ve just laid a driveway, you’ll be able to walk on it before putting your car on it.
Interestingly, concrete never really stops curing, and as time goes by, concrete gets stronger. However, there is a point where the concrete is hard enough to be safe for the job you need. As noted, there are a lot of factors that change the speed at which concrete will cure. But as a guideline, in natural drying conditions, concrete will have 50% of its needed strength in 3 – 7 days, 75% in 12 – 14 days, and after 28 days it will have 90% of its total strength.
Additionally, there are different types and grades of concrete, which also determine how long the concrete takes to cure. As a guideline, the American Concrete Institution outlines these time scales:
| Cement Type | Time Scale |
| ASTM C 150 Type I | 7 days |
| ASTM C 150 Type II | 10 days |
| ASTM C 150 Type III | 3 days |
| ASTM C 150 Type IV or V cement | 14 days |
| ASTM C 595, C 845, C 1157 | Varies |
It can be more useful to understand the timescale requirements of the actual structure you are creating. For example, a load-bearing pillar must be cured longer than a concrete worktop. It’s important to remember that an engineer must confirm all structural elements, timelines and guidelines have been followed. Below are rough guides for when you can remove forms from different structures.
| Structure | Time |
| Walls and columns | 24 – 48 hours |
| Slabs with props | 3 – 4 days |
| Soffits with props | 7 days |
| Props for slabs under 15’ | 7 days |
| Props for slabs over 15’ | 14 days |
| Props for supporting beams and arches under 20’ | 14 days |
| Props for supporting beams and arches over 20’ | 21 days |
How Long Should You Leave Forms in Place If Using Quikrete?
Quikrete is a concrete product that’s designed to harden and cure much quicker than normal concrete. In fact, this fast-setting concrete mix can harden in 25-40 minutes. Alongside this, Quikrete Fast-setting Concrete can be walked on within two hours and can take heavy weight after four hours.
6 Factors Affecting How Long Concrete Forms Should Stay On
A form should stay on long enough for the concrete to cure properly. As explained in this section, there are multiple factors that influence curing time.
1. Curing Method
The way concrete is cured will impact when the forms can get taken off. For example, if there is an accelerant in the mix, the concrete will cure quicker. However, the final product may not be as strong. The choice to use an accelerant will depend on how quickly you need the concrete to cure and how strong it needs to be, finding a balance between these two will help define the type of mix you want.
2. Grade and Type Of Cement
As we outlined in a table earlier in this article, the grade, and type of cement will change the curing time of the concrete. For example, Type III cement might cure in three days, whereas Type V cement can take up to 14 days.
3. Heat
If it’s too hot and dry, concrete will crack. Over a long period of heat, for instance, during a heatwave, concrete can crack quite quickly. If you are pouring concrete in this weather, try to keep it shaded and moist to counteract the dry conditions.
4. Freezing Weather
If the temperature gets cold enough to freeze, this will also have a big impact on the curing process. Within the first 24 – 48 hours of a pour, you want to avoid the temperature dropping to freezing levels, so it’s important to plan ahead and keep an eye on the weather. You can also erect a shelter and insulate the concrete. If the concrete freezes or cools too quickly, then there is a risk of it cracking.
5. Moisture
Curing is all about moisture content. Concrete needs to keep moisture within it long enough for the chemical reaction between water and cement to take place. As long as there is moisture within the concrete, it will be getting stronger. This is extremely important for the initial curing process. If there is less water in the mix, the cement will cure quicker because there is less water for the cement to bind to. Striking the balance of just enough moisture is key to a successful pour.
However, too much water can also create a weak mixture. The concrete can take a long time to cure and the top layer may start to flake.
6. Type of Structure
The type of concrete structure you are creating will change the amont of time you need to leave forms on. As we outlined in the table earlier on in this section, the larger the structure, the longer the forms must stay in place.
What Are the Risks of Removing Concrete Forms Too Soon?
Removing concrete forms too soon can be a major hazard and a big setback in a project. Three of the biggest risks are that:
- The concrete hasn’t cured
- The concrete is not strong enough to support itself
- Time and resources are wasted
Can You Leave Concrete Forms on for Too Long?
Depending on the type of material you use for the forms, you can leave the forms on for too long – especially if using wood. However, if you are using other types of formwork, like block or brickwork, the forms can be permanent. If you are laying foundations, you can also pour the concrete into dug trenches, which obviously eliminates the need for form removal.
What Are the Risks of Removing Concrete Forms Too Late?
If you remove a concrete form too late, you create a number of risks. If you are in an area that has termites, they are a significant hazard when using wooden forms. Alongside this, wood forms left in place create a fire risk and an obstruction for further construction.
Can You Leave Concrete Forms in Place?
If your forms are made from wood, they shouldn’t be left in place. As explained in the previous section, this is because it creates a risk of mold, termites, and fire. Alongside this, the appearance can look ugly especially when the wood starts to rot. If there is more work to do after the concrete has been poured it can also make it harder to work.
For more about this, read this article.
Summary
Getting the timing right for removing concrete forms can be a bit of an art. You must leave the forms on long enough for the concrete to cure properly but you need to take them off as soon as possible to allow for the project to continue.
If conducting a concrete project, check with an engineer that you comply with all legal requirements.