Concrete is easily the most used material in the construction world. It is simple to produce, solid, durable, relatively affordable, and incredibly adaptable.
Concrete forms hold wet concrete in place as it cures and hardens. Concrete hardens and takes the shape of whatever it is poured into, but that form or mold has to be solid enough to hold the concrete’s weight after it has been set.
And it must remain in place until the concrete hardens and cures. So, concrete must be correctly mixed, poured, and cured into durable shapes to erect buildings and other structures that will stand the test of time.
If the forms or molds are removed too soon, the structure or final product may be compromised. If removed too late, the concrete will become too hard to complete correctly.
Read on if you have been searching for information on how long concrete should cure before removing forms.
Factors Affecting Concrete Formwork Striking Times
The strength of the structural element determines how long it takes for the concrete formwork to be struck. The growth of a concrete member’s strength is based on the following:
Concrete Grade
If the grade of the concrete is higher, it gets strong quicker, which means that it gets strong in less time.
Cement Grade
When the cement grade is higher, it takes less time for the concrete to get stronger.
Cement Type
How strong concrete gets depends on the type of cement used.
Temperature
Since the temperature of concrete is higher when it is being poured, it gets stronger in less time. During the winter, the time it takes for concrete to get stronger gets longer.
Also, the formwork helps to insulate the concrete from its surroundings. This means that the longer the formwork is in contact with the concrete, the less heat is lost, and the rate of strength gain is rapid.
Size of the Concrete Member
The size of the concrete member also influences how strong it gets. Larger sections of concrete get stronger more quickly than smaller sections.
Accelerated Curing
You can also use Accelerated to speed up the rate of strength gain during the curing process.
When to Remove Concrete Form
The right amount of time to let concrete cure depends on factors like the proportion, strength specified, size, shape, and form of the concrete member, weather conditions at the time, and future exposure conditions.
The American Concrete Institute (ACI) says concrete should cure until it has at least 70% of the specified compressive strength. Most of the time, the seven-day curing time means that the material is about 70% as strong as it was supposed to be.
When concrete cures at higher temperatures or with certain combinations of cement and additives, it may reach the 70% strength threshold more quickly.
Also, different combinations of materials and lower curing temperatures may need more time. Because of this, the ACI Committee and ASTM suggests the following minimum curing times:
|
NO |
Cement Type |
Time |
|---|---|---|
|
1 |
ASTM C 150 Type I |
7 days |
|
2 |
ASTM C 150 Type II |
10 days |
|
3 |
ASTM C 150 Type III |
3 days |
|
4 |
ASTM C 150 Type IV |
14 days |
|
5 |
ASTM C 595, C 845, C 1157 cements |
Varies depending on cement |
|
NO |
Type of Formwork |
Minimum Period Before Striking Formwork |
|---|---|---|
|
1 |
Walls and columns vertical Supports |
24-48 hours |
|
2 |
Slabs, with their props left under them |
3-4 days |
|
3 |
Soffits, with their props left under them |
7 days |
|
4 |
Props supporting slabs under 15 feet |
7 days |
|
5 |
Props supporting slabs under 15 feet |
14 days |
|
6 |
Props supporting arches& beams over 20 feet |
21 days |
How to Properly Remove Formwork
When the formwork is taken down, it must be done so that the concrete doesn’t get too much shock or vibration. The striking of the formwork must not hurt the building in any way. Therefore, the formwork must be removed with the following considerations in mind.
- Ensure the concrete’s strength is developed by doing the appropriate tests (mainly NDT).
- When dismantling a form, starting with the vertical members is best.
- Slab or soffit supports should be removed only when it is certain that the structure can withstand its weight and any external loads.
- Using crowbars to pry up the formwork from the sides might cause harm to the concrete’s perimeter.
- Edge and corner loose concrete or other materials must be eliminated.
- Before the formworks are removed, the site must pass an inspection by the safety experts to verify it is safe.
- Before removing formwork from high places, ensure there are enough braces and work platforms to keep workers safe.
- Those in charge of tearing down forms on a construction site must know what risks are there and what steps to take to avoid them.
