When to remove formwork from a strip foundation

In the construction process, removing formwork from a strip foundation is an essential step. It guarantees that the concrete has strengthened to the point where it can sustain the loads it will carry in addition to its own weight. For the structure to remain safe and intact, the timing of the formwork removal is crucial.

It takes time for concrete to harden and cure. Numerous variables, such as the kind of cement utilized, the surrounding environment, and the foundation’s unique design specifications, can have an impact on this process. These factors need to be taken into account by builders when deciding when to remove formwork.

Formwork can usually be removed when the concrete has reached approximately 70% of its ultimate strength. This typically occurs in a few days, but it’s crucial that you adhere to the project’s particular requirements. A formwork that is removed too soon can cause structural problems, and a formwork that is left in place too long can be expensive and inefficient.

It is essential to watch the curing process. Tests of the concrete’s strength are frequently used by builders to determine when formwork removal is safe. Before moving on to the next stage of construction, these tests help guarantee that the foundation is sturdy and secure by providing precise data on the concrete’s development.

Requirements and standards

The formwork for vertical panels is removed when the concrete base has gained 70% of its strength, or 50% if there are suitable grounds for doing so, in accordance with SNiP 3.03.01-87 "Bearing and enclosing structures." The requirements stated in SP 70.13330.2012 are exactly the same.

Additionally, if the air temperature was at least 0 degrees, the formwork can be disassembled when it has regained 50% of its strength, per the European regulation ENV 13670-1:20000. Other requirements are outlined in the American standard ACI 318-08; if the humidity and air temperature meet the standards, it is advised that the wooden frame be removed after seven days.

As you can see, practically all standards discuss the humidity, temperature, and other factors that affect the concrete base’s "maturation." As a result, the strength gain will happen more quickly or more slowly depending on how many degrees the surrounding temperature varies. This leads us to the conclusion that knowing the precise moment at which the concrete mass will gain strength is sufficient to determine when the wooden formwork on the strip foundation can be removed.

A strip foundation’s formwork removal is an important construction step that has to be timed precisely to guarantee the concrete has strengthened enough to support the weight. Formwork can usually be taken down in 7 to 10 days, depending on the type of cement, the environment, and the mix design. If removed prematurely, the foundation’s integrity may be jeopardized, and if left too long, the project may be delayed. Any building can have a strong, dependable foundation by being aware of these timing subtleties.

How much concrete is gaining strength

Four weeks after filling, concrete is thought to be reaching its maximum strength (more than 70%). But in this instance, +5 degrees Celsius of air temperature is given. This means that after two weeks, at a temperature mode of +10 degrees, you can remove the formwork. Consequently, if you use temperature indicators, you can create a rough schedule for concrete hardening while keeping in mind that, in accordance with American and European standards, 50% of hardening is sufficient.

This table leads us to the conclusion that, if hot weather is worth it, the formwork can be removed after two days.

Wholesome The first week is when concrete strengthens the most; the base then continues to harden for the next two years, during which time it can strengthen up to 250%.

But these terms increase if you used horizontal cast structures for formwork:

In order to help you decide when to remove the formwork more precisely, it is also advisable that you consider the following advice:

• If the length of the structure is more than 6 m, then you need to wait until the base hardens by 80%.
• Horizontal inclined elements can be removed when 50-70% of the strength is reached.
• Vertical elements are recommended to be removed when the strength indicators are 0.2-0.3 MPa.

However, as was said earlier, the strength of concrete is affected by many factors, so you also need to take into account the humidity of the air, as well as the appropriate treatment of the monolithic surface. That is why the first 2-3 days the fresh foundation is covered with a film and moistened with water 2 times a day. If moisture evaporates too quickly, then the hydration of the concrete will occur incorrectly, as a result of which the bearing capacity of the monolith will decrease.

Furthermore, it is worthwhile to take into account a "phenomenon" like concrete crystallization. The concrete mass’s ability to harden is also impacted by this chemical process.

Crystallization of concrete

The phases in which the foundation components are located must be considered in order to calculate the amount of time needed for concrete to set:

• Phase 1 – immediately after pouring the solution. During this period, the solution is considered supersaturated and the process of mixing with water occurs in it. It lasts only 1-2 minutes, but during this time a "union" of ettrigint and calcium hydroxide is formed in the concrete mass.
• Phase 2 – 1 hour after mixing the cement-concrete mixture. During the induction period (3-6 hours), the cement-sand mixture turns into a cement paste, characterized by mobility and impermeability to water.
• Phase 3 lasts from 12 to 36 hours, after the formation of the cement paste. During this period, cementation of the stone occurs, due to which the so-called spatial lattice is formed from the “scales” of cement.

It is not advised to remove the frame earlier than two days after phase 3 is finished because doing so could alter the concrete’s chemical and physical characteristics.

The proportion of cement to water in the mixture is another important detail because it affects how quickly foundation strength increases.

It is also important to consider the technology and order of formwork removal.

How formwork is dismantled

It’s time to begin removing the wooden frame from the corners and the parts of the foundation base that stick out. The concrete hardens most intensely in these regions. Furthermore, the central portion of the foundation will continue to strengthen as you remove sections of the formwork from the corners.

To preserve the integrity of the entire structure, the boards must be disassembled with extreme caution. As a result, you shouldn’t use tools to quickly break off or tear off the boards.

It is necessary to remove the formwork in the following order, starting at the top:

1. Loosen the bolts and remove the upper metal ties.
2. Remove the bolts of the lower ties (leave the ties in the concrete mass).
3. Carefully move the panels away from the posts.
4. Remove the supports.
5. Remove the wooden frame.

Even if you "froze" the construction until the following summer, the formwork still needs to be taken down. This is due to a few factors:

• If you leave a wooden structure in the rain and snow, the wood will quickly absorb moisture.
• It is better to dismantle the formwork and waterproof the foundation before the wooden frame cracks when all the moisture absorbed into it melts in the fall.

Despite the assertions of "poor advisers" who are certain that a foundation that has survived the winter will only grow stronger as a result, it is also not worthwhile to leave the foundation alone for the winter. The structure can be easily pushed out of the ground if an empty concrete base is left for the winter (especially if the soil is heaving). Furthermore, it will be extremely challenging to evaluate a foundation that has survived the winter without waterproofing in order to ascertain whether damage caused by the expansion of water within the mass has occurred.

Conversely, it makes no sense to load the foundation in advance. The best course of action is to schedule the work so that, after four weeks, the concrete base will have had enough time to strengthen and you will have enough time to load the base.

The success of your construction project depends on when formwork on a strip foundation should be removed. Understanding a number of variables, including the kind of concrete used, the surrounding circumstances, and the particular needs of your project, is necessary for the process. To preserve the integrity of the foundation, make sure the concrete has sufficiently strengthened before removing the formwork.

The curing time is one of the important indicators. Generally speaking, concrete needs to set sufficiently for formwork removal in at least 24 to 48 hours, but this can vary depending on the mix and weather. Warmer weather can expedite the curing process, while colder temperatures might necessitate longer times. A more precise schedule for the safe removal of the concrete can also be obtained by measuring its strength using compression tests.

The type of formwork that is used is another crucial factor to consider. Compared to contemporary metal or plastic systems, which are intended to remain in place longer and offer superior support during the curing process, traditional timber formwork may need to be removed sooner. Always adhere to the manufacturer’s instructions, and seek professional advice if you’re unsure of the right time.

In conclusion, when determining whether to remove formwork from a strip foundation, patience and close observation are essential. You can make sure that your foundation achieves the required strength and stability by taking into account the concrete mix, environmental factors, and formwork materials. Your construction project will be more durable and safe in the long run if you take this cautious approach.

Video on the topic

Dismantling the formwork

when to remove the formwork from the foundation

How long should the foundation settle When can construction continue on a poured foundation

mistakes when dismantling the formwork of a strip foundation