- Building typology and scope
- Structural design calculations
- Calculation of the reinforcement of a monolithic slab
- Installation of formwork of various types
- Reinforcement and strapping
- Pouring concrete
- Next steps
Casting a monolithic interfloor floor is not the easiest, but really versatile and time-tested method. In this article, we will talk about the main structural features and stages of the overlapping device, as well as the types of formwork, including non-removable.
Building typology and scope
The main areas of application for monolithic floors are buildings with bearing walls made of bricks, block masonry or concrete panels, as well as domed houses. The requirements for the solidity of the floor may be due to:
- non-standard building plan;
- the need to significantly increase the bearing capacity of the floor;
- increased requirements for hydro and noise insulation;
- the need to provide a free layout;
- reducing the cost of interior decoration.
Pouring is performed, as a rule, after the completion of the construction of the walls of the first floor. However, there are options for pouring monolithic ceilings already in buildings with a roof, if weather or other conditions require it. In this case, I-beams are mounted on the masonry of the lower floor and a crown is poured along the perimeter of the bearing walls to the height of the ceiling. Also, to strengthen mechanical ties, embedded reinforcement is released from the inner side of the crown to 40-50 cm. Its total section cannot be less than 0.4% of the section of the longitudinal section of the crown.
Structural design calculations
When choosing the span length, it should be related to the slab thickness as 30: 1. However, with an independent design, it makes little sense to make an overlap thicker than 400 mm, since the load-bearing capacity of the structure increases along with its own weight and static stresses. Therefore, the permissible load on self-made floors rarely exceeds 1500-2000 kg / m2.
The situation can be corrected by including I-beams in the supporting structure, laid on the concrete-leveled masonry surface of the bearing walls. Another way to increase the span while maintaining relative freedom of the layout is to support the floor on columns. With a monolithic structure thickness up to 400 mm and a span length in four directions from the columns up to 12 meters, the support cross-sectional area is 1–1.35 m2, provided that the cross-section of embedded reinforcement in the column is not less than 1.4%.
Calculation of the reinforcement of a monolithic slab
In general, the thickness of the slab is determined by the amount of reinforcing steel that is embedded in it. The density of the reinforcement, in turn, depends on the maximum permissible load and crack resistance. Avoiding special cases, we can give a general example of a design that demonstrates full compliance with regulatory requirements with a sufficiently high safety factor..
In private construction, reinforced concrete is reinforced with reinforcement with a periodic profile of the A400 class, aka A-III.
Diameter of rods in slabs with thickness:
- up to 150 mm – not less than 10–12 mm;
- from 150 to 250 mm – not less than 12-14 mm;
- from 250 to 400 mm – not less than 14–16 mm.
The reinforcement is laid with two nets with a mesh size of 120–160 mm, the thickness of the protective concrete layer from the edges of the slab is at least 80–120 mm, and at least 40 mm above and below. The direction of laying four rows of reinforcement, starting from the bottom: along, across, across, along. For dressing, galvanized wire with a thickness of at least 2 mm is used.
Installation of formwork of various types
The formwork must withstand a load of 500-1100 kg / m2, including the dynamic impact of falling concrete. To create a formwork plane, you can use:
- Reusable formwork plastic sheets.
- Moisture resistant plywood 17-23 mm thick.
- OSB 20-26 mm thick.
The edges of the slabs must adhere tightly to the walls; it is not allowed to use formwork with gaps at the joints of more than 2 mm, unless it is planned to cover the surface with a waterproofing film.
Sometimes it is reasonable to make the formwork fixed, using profiled sheets for this, orienting them with a narrow shelf down. They are placed along the slab so that the waves during pouring form numerous stiffeners. The thickness is calculated from the bottom rib, thus saving the concrete mix is 20–25%. In this case, the height of the ridge should not exceed one third of the total thickness of the slab. If the formwork is not planned to be removed, self-tapping screws with a rubber washer are screwed into it and tied with a thin wire to the reinforcement.
The installation of the formwork begins with the placement of racks: it can be either steel telescopic racks with a tripod and a unilateral, or wood without defects with a cross section of at least 100 cm2. Each strut should be tied to two adjacent slant rails from an inch board. The racks are mounted along the lines of the beams, the distance between which, depending on the thickness of the plate 150-400 mm, is:
- 190–240 cm with plywood thickness up to 20 mm;
- 210-260 cm with plywood thickness from 21 cm.
In this case, the distance between the posts of one beam, depending on the gap between them, is:
- from 140 to 200 cm with a span of up to 150 cm;
- from 120 to 180 cm with a span of 160–210 cm;
- from 100 to 140 cm with a span of 210–250 cm.
The main beams are usually made of 100×100 mm timber. Secondary beams, which have a cross-section of 50% of the main ones, are laid on them across with a step of 500-650 cm. If the formwork is made of profiled sheet, the spacing of the secondary beams is 3.5 distances between the waves.
The vertical formwork is assembled from retaining panels attached to the outer wall of the building. Often, blocks of aerated concrete with a thickness of 80-100 mm are laid along the perimeter to hide the floor belt.
Reinforcement and strapping
After the installation of the formwork, it is lubricated with an anti-adhesive compound and the installation of reinforcement begins. On the crowns and supporting ribs, the rods are tied into a square, keeping the minimum permissible protective layer on all sides. The main slab is reinforced with a mesh. The bottom layer is laid on plastic “crackers”, which control the preservation of the bottom protective layer. The net is tied at the intersection of every third rod.
After strapping the lower mesh, intermediate clamps are installed on it every 100 cm in a checkerboard pattern. To strengthen the support, end clamps are mounted on the walls. These elements help maintain the design distance between two reinforcement planes..
The assembled upper mesh is tied to the lower connecting brackets. After completion of installation, the reinforcing structure should be as a whole and easily take the load from people walking on it.
Monolithic ceilings are poured with concrete of the B20-B30 brand, prepared in the factory. The filling of monolithic floors should be carried out in one stage, therefore filling the space with small doses is not recommended. If it is impossible to complete the entire volume of work at once, sections of the slab must be cut with a mesh with a cell of 8-10 mm.
The supply of the mixture to the ceiling can be carried out by a concrete pump or a volumetric bucket lifted by a crane. After feeding up, the mixture is evenly distributed, seated by vibration and left to solidify.
Concrete gains sufficient strength after 4 weeks, all this time it needs periodic wetting and protection from rain for the first 2 days. After drying, the formwork can be removed and the walls can be erected..