3 calculation controls for partial stripping of floor slabs
You have just concreted a slab, a few days have passed, and you don’t know if you can do a partial stripping? How can you justify, or calculate, that you can do a partial stripping?
Partial formwork stripping is the operation, carried out 3 days after the concreting of a slab, in which the entire formwork surface and most of the resistant structure of the formwork that supports it is recovered, generating a constructive clearing process.
After this operation, the slab behaves as a continuous beam, which is still supported on the part of the formwork structure that we have left. There is an acting load on it that includes, in addition to its own weight, the execution overload necessary for its execution (formwork weight, workers…)
The resistant justification of this beam is to verify that it is capable of withstanding the stresses to which it will be subjected. You can calculate the worst stress that occurs (maximum bending moment) and with that, using the bending formulas, obtain the value of the flexural strength that the slab needs in order to support itself. Since all the strengths are related to each other, to facilitate their control you can find their equivalence with the compressive strength value by following the formulas included in the standards.
Now, once the required characteristic compressive strength for the slab has been established, compare it with the estimated characteristic compressive strength of the cast-in-place concrete, at 3 days, to verify that its value is equal to or higher than the required value.
How do you know if a given concrete at a short age (3 days) will have this strength or not? What is the control parameter?
The most direct parameter is that obtained by testing specimens under site conditions. Failing that, we can estimate values according to maturity testing.
What is maturity testing? It is the technique that allows to assess the compressive strength of concrete in real time and for a given temperature. It is a widely used technique that complies with ASTM 1074-98 (1999) and the CEB-FIP Model Code (1993). Based on it, for example, it can be known at a reference level that concrete HA-25 at 16º gives a strength of 10 Mpa, which is 40% of 25 Mpa.
First control: cracking
It is essential to prevent slab cracking at a young age and the safest way to prevent such cracking is when the stresses that occur in the slab are resisted only by the concrete, without the contribution of the reinforcing steel.
In the construction phase, your primary fuse will be the flexural strength of the concrete (generally very low), without needing to evaluate the contribution of the reinforcement or the effect that the bond between concrete and steel may have. Look for its equivalence to compression and you will obtain a first strength value that you will have to meet.
Second control: strength
In addition to the above effect, it should be taken into account that when partial formwork is stripped, the structure starts to work and assume a percentage of the slab load, which is transferred to the building columns themselves. You will have to assess whether the slab is able to support the percentage of load it receives.
This parameter leads to setting a second value of the characteristic compressive strength that the concrete should have. This value is conditioned by the ratio of the construction load assumed by the slab in this operation to its design load. Under normal load and temperature conditions for conventional buildings, its value is 40% of the characteristic strength considered at 28 days.
Compare the two strength values obtained and take as reference the highest value, which will be the safest for this stripping operation.
Third control: Avoid a very low deformation Module E
If the control was only on strength, having a concrete with a high initial strength, which would give 40% strength at 1 or 2 days, would it allow you to remove the formwork at that time?
In this control, the aim is to work with a concrete that cannot produce deformations greater than those foreseen, and for this purpose it must have a well developed module E. At 3 days under normal conditions, its value is around 60%, but at 1 and 2 days, its value plummets and can generate adverse effects. It is therefore advisable to carry out this operation at least 3 days after concreting the slab.
Be careful! Red lines of caution
It is generally accepted by the market that this operation can be carried out as long as 3 days have passed and the concrete has acquired 40% of its characteristic strength.
But can it always be applied? No, there are a series of red lines that tell us when this criterion is not applicable, and if they occur, the calculations should be modified. What are they?
- The first is temperature: at low temperature, the response of concrete is worse. Does anything happen to concrete when it is below 16 degrees? We know that it will not reach 10 Mpa (40% fck) and, in general, we will have to increase the period to 4 or 5 days.
- When doesn’t this work either? If the design conditions of the beam change, having a higher value of acting load (stockpiles, earth…), or having left a greater separation between supports, going from being at a distance of 2 m to being at 3 or 4 m. This significantly increases the stresses and the strength value necessary to avoid cracking will increase.
- The third red line, which is usually believed or mistaken on site: the inertia of the slab. Which partial stripping do you think may be more critical: that of a very heavy edge beam or that of a very thin slab with very little weight? You always have to think that the strength we need to flexural strength in concrete is determined by the formula relating momentum and inertia. For the same bending momentum, if you have an edge beam with a high inertia you will need a low strength value and if you have a 8-12 cm thick slab you will need a high strength value. For this purpose, slender floor slabs are always much more critical than large edge beams. In any event, the appearance of any of these requires the calculation to be checked.
So now you know: partial stripping is possible and advisable, but it must be done in a calculated and controlled manner. Never improvise…