Formwork plays a critical role in construction, serving as a temporary mold to support freshly poured concrete until it hardens and gains the strength to stand on its own. But knowing when formwork can be removed is not just a matter of guesswork — it requires careful consideration of several factors to ensure the safety and integrity of the structure. Let’s take a closer look at what determines the right time to remove formwork.
Concrete Strength: The Most Important Factor
The main factor that dictates when formwork can be removed is the strength of the concrete. Concrete doesn’t achieve its full strength immediately; instead, it gradually gains strength over time, especially during the first few days after it’s poured.
For safety, engineers wait until the concrete has reached a certain percentage of its full design strength before removing the formwork. Typically:
- Vertical structures like walls and columns can have their formwork removed when the concrete reaches about 50% of its design strength.
- Horizontal elements like slabs and beams usually need to reach about 70% of their design strength before the formwork can come off.
The exact timing depends on the type of project and the safety standards being followed.
Type of Structure Matters
Different parts of a structure require different approaches:
- Beams and slabs, which are horizontal and bear more stress, need more time and strength before the formwork is removed.
- Columns and walls, being vertical and primarily under compression, often allow for earlier removal.
Curing Conditions Make a Difference
The environment plays a big role in how quickly concrete gains strength. Warm and humid conditions are ideal, helping the concrete cure faster. On the other hand, cold weather slows down the curing process, meaning the formwork may need to stay in place longer.
Concrete Mix Design
The composition of the concrete itself also determines how quickly it gains strength. Some mixes are designed for faster curing, using additives that speed up the process, while others take longer. For example, high-performance concrete or mixes with accelerators can shorten the waiting time before formwork removal.
Safety and Load Considerations
Even if the concrete has reached the required strength, it’s crucial to think about the loads the structure will need to handle after the formwork is removed. Will there be workers, equipment, or materials on it? Engineers ensure that the concrete can handle these loads without cracking or deforming.
Following Standards and Codes
Construction codes and standards, like those from the American Concrete Institute (ACI) or Eurocode, provide clear guidelines on when formwork can be removed. These standards ensure that safety isn’t compromised during construction.
Testing and Inspections
Before removing formwork, engineers often test the concrete to make sure it’s ready. Common methods include:
- Compressive strength tests, where concrete samples are tested to see how much pressure they can handle.
- Rebound hammer tests, a non-destructive method to check surface hardness.
- Visual inspections, to ensure there are no cracks or visible weaknesses in the concrete.
Site Conditions and Deadlines
Finally, practical considerations like project timelines and site conditions can also influence decisions. For instance, a tight schedule might push for the use of quick-curing concrete, while a more relaxed timeline might allow for longer curing periods.
Before concreting operations
To cast the new slab, the supporting slab immediately below should have sufficient capacity to carry the loads imposed on it during construction. The loads on the supporting slab are
• Self-weight of supporting slab (at 24 kN/m3).
• Any construction load on the supporting slab.
• Total construction load of the new slab, i.e.
◊ weight of concrete in new slab plus
◊ the self-weight of the temporary works plus
◊ construction live load on new slab.
If the supporting slab has sufficient capacity to carry the loads imposed, backpropping is not required. If it does not then backpropping is required
Before they are loaded, the concrete strength required in backpropped supporting slabs should be calculated using the determinations in Table 1.
It should be noted that w will be much higher than for the new slab
(see Table 2.) Determination 1 might be critical and this temporary condition might need to become a design load case agreed with the PWD.
Table 1: Determination of strength required for early striking of flat slabs

Determining the backpropping requirement
The construction load is determined as described above ‘Before concreting operations’. If the supporting slab has sufficient capacity to carry the loads imposed, no backpropping is required, but if not then backpropping is required.
Figure 1 gives a representation of typical backprops over different numbers of floors.

The number of floors to be backpropped depends on the load on the slab immediately below that being cast not exceeding the design service load. In practice there will be little benefit in having more than one level of backpropping, as can be seen from Table 2.

The assumptions made in arriving at the percentages in Table 2 are outlined below and are explained in detail.
They are:
- The slabs behave elastically.
- An imposed construction operations load is applied at all times.
- The density of normal-weight concrete is 24 kN/m3.
- The backprops between the floor slabs and the falsework shoring supporting the soffit formwork are not rigid, i.e. they change in length with load. (Props of different material e.g. steel and aluminium, MUST NOT be mixed. The percentages given
- in Table 3 are for aluminium props. If steel props are used, the load in the supporting slab may be reduced by 4%.)
- The load applied onto the supporting slab from the falsework is uniformly distributed and, further, the back-propping generates a uniformly distributed support system from underneath the slab. (This is reasonable if the backprops are spaced at least at one-third points along the supporting slab.)
- The load transferred through supporting slabs can be proportioned pro rata to the stiffness of the slabs considered. The percentages given in Table 3 are for slabs of equal stiffness.
- The backpropping is installed
- with zero pre-load. (Props are
- ‘finger tight’ to ensure stability
- in the unloaded condition.)
- The effects of temperature change are ignored.
In Summary
Deciding when to remove formwork is not as simple as following a set number of days. It involves assessing the concrete’s strength, understanding the type of structure, considering environmental conditions, and adhering to safety standards. Thorough testing and inspections ensure that formwork is removed at the right time, minimizing risks and keeping the project on track.
By carefully evaluating all these factors, construction teams can safely and efficiently move forward without compromising the integrity of the structure.