appointed after fulfilling a number of requirements. Thus, removal of the side elements of the formwork, which do not bear the load from the weight of the structure, is allowed only after the concrete has achieved strength that ensures the safety of the surface and edges of the corners.
More stringent requirements are imposed on the removal of load-bearing formwork of reinforced concrete structures, which can be removed only after the concrete reaches the design strength value:

• load-bearing structural elements with a span of up to 2 m – 50%;


• load-bearing structures of beams, crossbars, purlins, slabs and vaults with a span of 2-6 m – at least 70%;


• load-bearing structures with a span of more than 6 m – at least 80%;


• load-bearing structures reinforced with load-bearing welded frames - at least 25%.

Approximately, we can assume that after 3 days, Portland cement concrete will gain strength of about 30%, after 7 days - about 60%, and after 14 days - about 80% in relation to 28-day strength. However, concrete hardening continues even after the onset of 28 day old. So, by 90 days of hardening, concrete can gain an additional 30-35% strength.
Standard conditions for concrete hardening are: temperature 20±5ºC ​​and air humidity above
90%. It should be borne in mind that in practice, as a rule, real conditions do not correspond to standard standards, and the concrete hardening process either slows down or accelerates. For example, at a temperature of 10ºC, after 7 days concrete will gain 40-50% strength, and at 5ºC - only 30-35%. If hardened at a temperature of 30-35ºC, concrete will gain 45% strength within 3 days. At negative temperatures Concrete without special additives does not gain strength at all. Therefore, the decision to strip the formwork and load the structure should be made after testing the concrete for strength.
The time frame for concrete to achieve a given strength is determined by the construction laboratory based on test results. control samples or methods non-destructive testing. At sites with a total volume of work less than 50 m3, receiving ready-mix concrete from factories or installations located at a distance of no more than 20 km, it is allowed to assess the strength of concrete according to the laboratory of the manufacturer concrete mixture without making test samples at the installation site. However, this instruction does not apply to critical paired and thin-walled structures: beams, columns, floor slabs, as well as monolithic joints of prefabricated structures.
Of course, when constructing suburban residential buildings, concrete strength measurements are usually not taken, since most construction companies There are simply no construction laboratories working in the private housing construction sector. Therefore, in this case, you will have to rely on the laboratory data of the concrete mix manufacturer. Additionally, you can conduct your own concrete strength testing. To do this, you need to take a metal ball with a diameter of at least 20 mm and throw it from the same height to concrete surface: control and subject. Based on the height of the ball’s rebound, it will be possible, I’ll make a reservation right away - with a big stretch, to determine whether the strength of the concrete has reached the required value.
Full design load in stripped form reinforced concrete structure can be allowed only after the concrete has acquired its design strength.
A metal floor beam in the form of an I-beam has a number of undeniable advantages. So a metal I-beam can cover large spans with a significant load. In addition, the metal steel beam is absolutely non-flammable and resistant to biological influences. However, a metal beam when exposed to aggressive environment may corrode, so a protective coating must be applied to it.
In most cases in private housing construction, a metal beam has hinged supports - its ends are not rigidly fixed, for example, since in a frame steel structure. The load on the floor with steel I-beams, taking into account its own weight, should be calculated without a screed of 350 kg/m2 and 500 kg/m2 with a screed.
It is recommended to make the step between I-beams equal to 1000 mm, however, in order to save money, you can increase the step between the metal beams to 1200 mm.
The table below shows the choice of the number of an I-beam metal beam for different pitches and lengths of purlins.

Span3 m			Span4 m			Span6 m
I-beam number at step			I-beam number at step			I-beam number at step

As can be seen from the table, with a total load of 500 kg/m2 and a span length of 6 m, you should have chosen an I-beam of a higher number and chosen a smaller beam installation step.

Added: 05/26/2012 08:21

Discussion of the issue on the forum:

We poured the ceiling between the first and second floors along I-beam No. 12, span 6 meters with an outlet 1 meter from load-bearing wall first floor. The distance between the I-beams is 2 meters, from below between them a mesh of cell 20 is connected from reinforcement No. 12, on top of mesh No. 5, cell 10 cm. Question: after how many days can the formwork be removed and after how many days can the walls be laid, including at the outlet?