If you’re pouring concrete in forms then you might be wondering if there is an issue with leaving the forms on for too long (or for too short). If you’re a contractor and frequently work with forms, then you’ll want to get them from one job to the next as quickly as you can, because as we all know time is money.
In short, there is no such thing as leaving concrete forms on for too long, as they will simply continue the curing process as the forms remain in place. However, there is no real need to leave forms on for longer than necessary. After a short period of time, the concrete is dry enough to retain its shape, and the curing process is far enough along to remove the forms.
As for leaving the concrete forms on for too short a period, this can definitely be a concern. Forms are needed to let the concrete dry enough to remain solid, but they are also useful for ensuring the concrete retains the necessary moisture as it cures.
Let’s take a look at how long to leave concrete forms in place for when the ideal time is to remove them, and the issues with taking them off too early.
How Long to Leave Concrete Forms in Place?
Forms can be removed once the concrete has dried enough to support its own weight. This is typically between 24 and 48 hours after the concrete is poured, depending on the type of project, the concrete mixture, as well as the climate.
Standard concrete mixture is dry enough to walk on after 24 hours, but this doesn’t mean you should always remove the forms after this point. You generally want to leave forms in place while the concrete begins the curing process, as this will result in a more durable, long-lasting final product.
According to the American Concrete Institute, the removal times for concrete framework for concrete with ordinary Portland cement is as follows:
|Type of Formwork||Removal Time|
|Walls, columns & vertical beams||24 to 48 hours|
|Props supporting slabs||14 days|
|Props for beams and arches||14 to 21 days|
These are just general recommendations though, and the best ideal time to remove concrete forms will depend on a multitude of factors that influence concrete drying and curing times.
Temperature is a serious concern whenever you’re mixing and pouring concrete.
High temperatures increase the speed of the chemical reaction between the cement and the aggregate resulting in a more rapid curing process. In extreme cases, this can actually cause the concrete to dry too rapidly, causing excessive moisture loss and cracking and spalling on the surface.If you’re working with concrete in hot temperatures, there are several steps you can take to prevent excess moisture loss. Physical barriers like sunscreens, tarps, and evaporation retardants will work to trap moisture where it’s needed. You can also finely mist the concrete surface daily as the concrete cures, which will add back some lost moisture to the surface where it’s most likely to crack.
Leaving the forms in place will also work to trap moisture and prevent excess evaporation, so this should be considered before removing concrete forms. Wood forms can be sprayed with water to keep them moist.
Cold temperatures slow down the speed of concrete drying and curing, and when they drop below freezing can actually reduce the strength of the final product by up to 50%. Unlike in hot environments, cold will slow down the chemical reaction, resulting in a slower curing process.
This means you may need to increase the time you leave your forms in place in colder climates. To prevent freezing, concrete insulation blankets, heating coils, or electrical heated forms or pads can be deployed.
If too hot or too cold can both create problems, then what is the ideal temperature to pour and form concrete?
Generally, you’ll want to pour concrete between 60 and 80 degrees Fahrenheit. Of course, this isn’t always possible, so if you’ll be pouring outside that range make sure to use some of the mitigation methods above.
Another key component to concrete curing correctly is using the correct amount of water in the mix. Be sure to follow the directions on the back of the bag for how much water to add to the mix.
Too little moisture in the mix will lead to a weakened final product, while too much can cause excessive shrinking and cracking while the concrete dries.
Rain can also be an issue, especially within the first 24 hours of concrete drying. If you’re expecting a downpour, then make sure to have some tarps on hand to cover your mixture while it dries.
Of course, the type of concrete mix you’re using will have a significant impact on the drying and curing times. Quick-dry concrete products like Quikrete’s Fast-Setting Mix are designed to dry enough to bear weight in just 4 hours, making them ideal for setting fence posts, mailboxes, and other applications where you can’t afford to wait around.
Standard concrete mix takes significantly longer to dry, drying enough to bear weight in 24 hours, and then curing to 70% of its final strength in 7 days.
Curing Time Tradeoffs
When you’re using forms on a concrete project, you’ll want to consider the tradeoffs of leaving them in place longer vs. taking them off a bit earlier.If you’re a contractor or concrete professional, then leaving them in place longer means they won’t be available for use on another job. Taking forms off after 24 hours may be acceptable for some applications, but it can present issues with the concrete curing correctly and long term-durability.
If you’re going to take off the forms earlier, say after just 12 hours, then it’s a good idea to consistently mist the concrete surface to prevent excess moisture loss. You can also use a curing compound on the concrete surface to help lock moisture in.
Generally, the rule of thumb is to leave concrete forms for 48 hours before removing them. This will trap moisture and increase the curing time within the key time period following the initial drying.
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