Technology for reinforcing openings in monolithic reinforced concrete slabs in the domestic regulatory documentation lit rather sparingly. In the design manual "Reinforcement of monolithic elements reinforced concrete buildings"(Moscow, 2007) in the section Reinforcement at holes it says: Openings of significant sizes (greater than or equal to 300 mm) in monolithic reinforced concrete walls and slabs must be bordered by additional reinforcement with a cross-section not less than the cross-section of the working reinforcement (of the same direction), which is required by the calculation of the slab as continuous. Holes up to 300 mm are not edged with special rods. The knitted working and distribution reinforcement around such holes is thickened - the two outer rods are placed with a gap of 50 mm. When reinforcing the slab welded mesh It is recommended to cut holes up to 300 mm in the reinforcement locally, and it is advisable to bend the cut rods into the body of the slab.
In the Design Guide reinforced concrete structures with beamless floors (Moscow, 1979) in paragraph 3.13. said: Single holes with a maximum size of up to 700 mm are installed in the ceiling without local thickening of the slab. The weakening of the slab by the hole should be compensated for by additional reinforcement laid along the edges of the hole. If concentrated forces are applied to the edge of the slab adjacent to the hole, and also in cases where the precast slab is significantly weakened by holes (by 50% or more), it is recommended to reinforce the slabs along the edges of the holes with rigid reinforcement or provide for thickening of the slabs, or edging the holes with ribs. The rigidity of the bordering ribs must be no less than the rigidity of the section of the slab section occupied by the hole. It is recommended to thicken (strengthen) the part of the heel adjacent to the hole based on the condition that the rigidities of the section weakened by the hole are equal and without taking into account the weakening. At rectangular holes At the corners of these holes in the slab, 2 - 4 reinforcing bars with a diameter of 10 - 14 mm should be laid, placing them in plan at an angle of 45° to the sides of the hole.
The requirement for indirect reinforcement of the corners of openings to accommodate longitudinal loads in slabs and prevent the formation of cracks is contained in the guide to the design of reinforced concrete products (S. N. Sinha Handbook of Reinforced Concrete Design, 2008. Round openings in slabs are also subject to indirect reinforcement.
Foreign regulatory documentation (Swedish building code VVK 04, Polish building code PN-B-03264) provides the following requirements for the reinforcement of holes and openings in monolithic reinforced concrete slabs:
Holes and openings with a diameter (side) of 150 mm or less do not require reinforcement. Holes from 150 to 450 mm require reinforcement with U-shaped clamps (transverse reinforcement) around the perimeter of the opening, connecting two layers of reinforcement. In foreign sources, the length of the clamps is defined as three slab thicknesses, and in domestic sources as two slab thicknesses (SP 63.13330.2012 Concrete and reinforced concrete structures. Basic provisions. Updated edition of SNiP 52-01-2003, clause 10.4.9). Holes (openings with a diameter) (side) of 450 mm to 900 mm require framing the opening with double condensed reinforcement around the perimeter and laying indirect corner double reinforcement. Holes or openings with a side of more than 90 cm require reinforcement of the slab with internal hidden beams or retaining beams.
Maximum size the opening according to different sources can be up to 1/4 longest side slabs, or no more than 1/3 of the smallest side of the slab. Minimum allowable thickness
Ladder - important element designs multi-story building. It must, of course, be as reliable and safe as possible during operation. To increase its load strength, it is imperative to reinforce the staircase opening (flight). This will increase the stability of the staircase itself, protect the structure from sagging and stretching, and prevent chips and cracks.
Before starting work, you need to pay attention to the type of structure of the future staircase and take into account the level of loads that it will receive daily. Under intense loads, the structure will begin to experience pressure from above, where the concrete will begin to compress, while tension will occur from below, which will lead to a loss of concrete strength. Therefore, reinforcement of the lower part of the march is especially important. When reinforcing a staircase opening, staple-bending products and flat reinforcing cages are used, less often - reinforcing mesh, but their effectiveness in these structures is practically zero. When assembling frames and laying reinforcement, they use additional elements– channels laid on the sides of the formwork, and reinforcing angles for reinforcing the edges of the steps.
It is necessary to reinforce the staircase opening from top to bottom, because the main pressure is exerted from above, which means that the staircase must be strengthened from reverse side. Dimensions flat frames both upper and lower ones must match. Simple monolithic stairs, which do not have platforms in their design, do not need reinforcement of the lower part; only the upper part will be sufficient. The reinforcement will increase the rigidity of the stairs and protect it from possible impacts and damage. Double-flight staircases must be securely fastened to the walls, since their monolithic platforms accept huge loads from the weight of the same ladder. For this purpose, special reinforced concrete crowns are used. If the house is being built using monolithic casting technology, the construction and reinforcement of stairs should be taken up during the construction of the walls themselves.
To calculate the amount of required reinforcement, you need to compare indicators such as the length of the flight of stairs, the distance between the rods, the minimum height of the working slabs, and the diameter of the reinforcing bars. The calculations should be entrusted to professionals, people who are knowledgeable in this matter. Even if you decide to do reinforcement yourself, you cannot do this without a well-developed reinforcement design scheme.
The most reliable (but not always appropriate) option for interfloor slabs is a monolithic slab. It is made of concrete and reinforcement. Read about the rules for installing monolithic floors in this article. Analysis of the characteristics of types and applications, installation of monolithic floors.
In what cases is it necessary to install monolithic floors?
Monolithic reinforced concrete flooring is the most reliable, but also the most expensive of all existing options. Therefore, it is necessary to determine the criteria for the feasibility of its design. In what cases is it advisable to install monolithic floors?
- Impossibility of delivery/installation of prefabricated reinforced concrete slabs. Subject to conscious refusal of other options (wooden, lightweight Terriva, etc.).
- Complex configuration in plan with an “unfortunate” location interior walls. This, in turn, does not allow laying out a sufficient number of serial floor slabs. That is, a large number of monolithic sections are required. Costs for crane, and the formwork is not rational. In this case, it is better to immediately move on to the monolith.
- Unfavorable operating conditions. Very heavy loads, extremely high humidity values, which cannot be completely solved by waterproofing (car washes, swimming pools, etc.). Modern floor slabs are usually prestressed. Tensile steel cables are used as reinforcement. Due to their very high tensile strength, their cross-section is very small. Such slabs are extremely vulnerable to corrosion processes and are characterized by a brittle rather than ductile nature of destruction.
- Combining overlap functions with function monolithic belt. Supporting precast concrete slabs directly on lightweight block masonry is generally not permitted. A monolithic belt is required. In cases where the cost of the belt and prefabricated floor is identical to or exceeds the price of a monolith, it is advisable to focus on it. When resting it on masonry with a depth equal to the width of the belt, the installation of the latter is usually not required. An exception may be difficult soil conditions: type 2 subsidence, seismic activity, karst formation, etc.
Determining the required thickness of a monolithic floor
For bending slab elements, over decades of experience in the use of reinforced concrete structures, the value of the ratio of thickness to span has been experimentally determined. For floor slabs it is 1/30. That is, with a span of 6m optimal thickness will be 200mm, for 4.5mm - 150mm.
An underestimation or, conversely, an increase in the accepted thickness is possible based on the required loads on the floor. At low loads (this includes private construction), it is possible to reduce the thickness by 10-15%.
VAT of floors
For determining general principles When reinforcing a monolithic floor, it is necessary to understand the typology of its operation through analysis of the stress-strain state (SSS). The most convenient way to do this is with the help of modern software systems.
Let's consider two cases - free (hinged) support of the slab on the wall, and pinched one. Slab thickness 150mm, load 600kg/m2, slab size 4.5x4.5m.
Deflection under the same conditions for a clamped slab (left) and a hinged slab (right).
The difference is in the moments of Mx.
The difference is in Mu's moments.
The difference is in the selection of upper reinforcement according to X.
The difference is in the selection of upper reinforcement according to U.
The difference is in the selection of lower reinforcement according to X.
The difference is in the selection of lower reinforcement according to U.
Boundary conditions (nature of support) are modeled by imposing the corresponding connections at the support nodes (marked in blue). For hinged support, linear movements are prohibited; for pinching, rotation is also prohibited.
As can be seen from the diagrams, when pinched, the work of the near-support section and the middle region of the slab is significantly different. IN real life any reinforced concrete (prefabricated or monolithic) is at least partially clamped in the body of the masonry. This nuance is important when determining the nature of the reinforcement of the structure.
Reinforcement of a monolithic floor. Longitudinal and transverse reinforcement
Concrete works great in compression. The reinforcement is tensile. Combining these two elements we get composite material. Reinforced concrete, which involves strengths each component. Obviously, the reinforcement must be installed in the tensile zone of concrete and absorb tensile forces. Such reinforcement is called longitudinal or working. It must have good adhesion to the concrete, otherwise it will not be able to transfer the load to it. For working reinforcement, periodic profile rods are used. They are designated A-III (according to the old GOST) or A400 (according to the new one).
The distance between reinforcing bars is the reinforcement pitch. For floors it is usually taken equal to 150 or 200 mm.
In case of pinching, a supporting moment occurs in the support zone. It generates tensile force in the upper zone. Therefore, working reinforcement in monolithic floors is placed both in the upper and lower zones of concrete. Special attention should be paid to the lower reinforcement in the center of the slab, and the upper reinforcement at its edges. And also in the area of support on internal, intermediate walls/columns, if any, this is where the greatest stresses arise.
To ensure the required position of the upper reinforcement during concreting, transverse reinforcement is used. It is located vertically. It can be in the form of supporting frames or specially bent parts. In lightly loaded slabs they perform a structural function. Under heavy loads, transverse reinforcement is involved in the work, preventing delamination (cracking of the slab).
In private construction, transverse reinforcement in floor slabs usually performs a purely structural function. Support shear force(the “shear” force) is perceived by the concrete. The exception is the presence of point supports - racks (columns). In this case, it will be necessary to calculate the transverse reinforcement in the support zone. Transverse reinforcement is usually provided with a smooth profile. It is designated A-I or A240.
To support the upper reinforcement during concreting, bent U-shaped parts are most widely used.
Pouring the floor with concrete.
Calculation of a monolithic floor example
Manual calculation of the required reinforcement is somewhat cumbersome. This is especially true for determining deflection taking into account crack opening. The standards allow the formation of a crack in a tensile concrete zone with a strictly regulated opening width. They are completely invisible to the eye, we are talking about fractions of a millimeter. It is easier to simulate several typical situations in a software package that performs calculations strictly in accordance with current building codes. How to calculate the installation of monolithic floors?
The following loads were taken into account in the calculation:
- Self-weight of reinforced concrete with a calculated value of 2750 kg/m3 (with a standard weight of 2500 kg/m3).
- The weight of the floor structure is 150 kg/m2.
- The weight of the partitions (average) is 150 kg/m2.
General view of the calculation scheme.
Scheme of deformation of slabs under load.
Diagram of Mu moments.
Diagram of moments Mx.
Selection of upper reinforcement according to X.
Selection of upper reinforcement according to U.
Selection of lower reinforcement according to X.
Selection of lower reinforcement according to U.
The spans were assumed to be 4.5 and 6 m. The longitudinal reinforcement was specified:
- class A-III fittings,
- protective layer 20mm
Since the area of support of the slab on the walls was not modeled, the results of selecting reinforcement in the outer plates can be ignored. This is a standard nuance of programs that use the finite element method for calculations.
Pay attention to the strict correspondence of the spikes in moment values with the spikes of the required reinforcement.
Monolithic floor thickness
In accordance with the calculations performed, we can recommend, for the installation of monolithic floors, in private houses, a floor thickness of 150 mm, for spans up to 4.5 m and 200 mm up to 6 m. It is not advisable to exceed the span of 6m. The diameter of the reinforcement depends not only on the load and span, but also on the thickness of the slab. The often installed fittings with a diameter of 12 mm and a pitch of 200 mm will form a significant reserve. Usually you can get by with 8mm at 150mm pitches or 10mm at 200mm pitches. Even this reinforcement is unlikely to work to the limit. The payload is assumed to be 300 kg/m2 - in a home it can only be formed by a large closet completely filled with books. Really effective load V residential buildings, as a rule, significantly less.
The total required amount of reinforcement can be easily determined based on the average reinforcement weight coefficient of 80 kg/m3. That is, to install a floor with an area of 50 m2 with a thickness of 20 cm (0.2 m), you will need 50 * 0.2 * 80 = 800 kg of reinforcement (approximately).
In the presence of concentrated or more significant loads and spans, the diameter and pitch of the reinforcement specified in this article cannot be used for constructing a monolithic floor. Calculations for the corresponding values will be required.
Video: Basic rules for constructing monolithic floors
Monolithic floors
When redevelopment involves combining rooms vertically, as well as when installing communications between floors, it becomes necessary to construct and strengthen openings in the ceilings.
This type of work is potentially hazardous to the structures of the house and living in it, therefore it requires development and approval project documentation based on a technical opinion on the possibility of redevelopment.
All work on dismantling and subsequent strengthening of the opening in the floor slab must be carried out only by qualified specialists experienced in construction.
How to make an opening in the ceiling?
When creating openings in floors, you should use special equipment that does not transmit shock vibrations and vibrations to the concrete. Using professional ones allows you to avoid the formation of cracks that occur when working with jackhammers and rotary hammers.
A universal joint cutter is usually used for cutting floors. This device consists of a frame with a trolley and a motor with a belt drive, which rotates the cutting disc at a given speed. It has variable cutting depth and does not need to be fixed on special guides. As work surface The joint cutter uses a disc coated with segments of industrial diamonds.
Sometimes an opening in the ceiling is made in a different way - using large-diameter diamond core bits.
Usually the dismantling process itself goes like this: the marked opening is removed in small parts, carefully lowering each with a winch steel cable on bags or tires. In some cases, the required area is removed entirely.
When making openings in floors, reinforcement with metal structures is required according to the design in order to compensate for the decrease in their strength and the increase in load on them.
Strengthening openings in ceilings
Before dismantling work on the ceiling begins, it is unloaded using temporary supporting supports.
When strengthening a small opening in a monolithic reinforced concrete slab, channel framing is used around the perimeter. The metal is welded to the reinforcement protruding from the ceiling and caulked with mortar.
When strengthening a large opening, metal supports are used, attached to the lower load-bearing walls(channels, I-beams or angles). This structure is mounted before cutting the hole. On brick walls The reinforcement beams are inserted at both ends into grooved grooves, and are attached to monolithic ones with special locks. Interval between metal elements and the overlap is caulked with mortar.
If for some reason it is not possible to attach metal structures to the load-bearing walls, permanent columns are installed under the affected floor.
When reinforcing a small opening in panel slabs, a channel or other type of profile is brought in from below and connects the slab to the opening with intact slabs. Another metal jumper is placed on top and pulled together with pins from the bottom.
Example of an opening in a ceiling:
Even in professional schemes When laying out floors, a monolithic section between slabs is often found in buildings with complex configurations. Concreting this piece is much easier than casting a solid slab, since the lower and upper levels are set by default, there is no side formwork, the lower panel is sufficient. One option is to use prefabricated monolithic floor SMP.
Monolithic floor section technology
Slabs are more often used in individual construction standard height 220 mm. This must be taken into account when reinforcing makeshift plot, providing the minimum possible protective layer of 15 - 30 mm. If the monolithic section between the floors protrudes above the adjacent ones, an increase in the thickness of the screed will be required when finishing the floors.
Factory floors have voids in which it is convenient to stretch electrical cables. IN homemade stove communications must be walled up before pouring, so as not to chisel the concrete later. This technique is often used to make hatches. If openings for stairs are cut out in industrially manufactured slabs, the reinforcement pattern is disrupted and the structure loses bearing capacity, becomes dangerous to operate.
Formwork
The monolithic section between the slabs is poured onto a shield, which must be supported from below with racks. The simplest calculations of lumber sections are the most a budget option for an individual developer, show that boards and timber with minimal dimensions can be used for formwork:
In this case, the structure will support the weight concrete floor without sagging or geometry changes.
By default, the monolithic section between the floors has side formwork, which is the ends of the reinforced concrete products laid in place. All that remains is to place the boards under the bottom surface, placing their edges under the existing PC boards, to check the flatness and absence of deflection in any direction. To do this you need to follow these steps:
After that, the remaining pillars are mounted between the outer posts, ensuring the horizontality of the beams, purlins, and deck boards. When choosing grade 2 wood, the bending strength of the lumber is insufficient. In addition to the bottom strapping of the pillars with 25 mm boards, which is necessary to prevent shifting during pouring, a similar strapping is additionally used at a level of 1.3 - 1.5 m. All pillars are stitched crosswise and lengthwise with an inch, forming a rigid spatial structure.
To facilitate stripping, extendable racks are used:
- they are manufactured smaller than the design height
- are built up in pieces in the upper part, which just needs to be unscrewed when dismantling
When stripping, first the lower bars of the racks are dismantled, then the beams with the upper pieces of the racks are removed. After which, the deck with the purlins screwed to it is dismantled. In the future, all lumber is suitable for construction rafter system. If you choose grade I wood, you can reduce the cost of inch boards for tying the posts in the middle part.
If it is necessary to fix the formwork elements to existing walls It is better to use anchors with metal sleeves. They are easily removed from the masonry after stripping, unlike dowel-nails, the plastic elements of which are almost impossible to remove from the wall.
Deck
At this stage, the monolithic section between the slabs is equipped with a deck on top of the purlins. The edges of the boards are placed under the existing floor slabs, the middle lies on the beams, which ensures the rigidity of the structure.
The gaps between the boards are foamed from the inside of the formwork (from above), the boards are covered plastic film. This will retain water in the concrete, facilitate stripping, and prevent cracking of the floor slab. The plank design is convenient for wiring engineering systems– holes of any diameter can be drilled with crowns and drills without problems in any area.
When the width of the void section is less than 1 m, technology without racks and beams is often used:
The deck is attracted by wire twists through the timber to the lower planes of the laid slabs, reinforced, and poured using standard technology. It is not recommended to punch holes for reinforcement at the ends of the slabs, as they weaken the structure of hollow PC products. The wire clamps are cut flush with an angle grinder when stripping the formwork; part remains inside the monolithic piece.
To increase the service life of the floor, reinforcement of at least A-III periodic section (hot rolled) with a diameter of 10 - 16 mm is used. The main nuances of reinforcement are:
To knit the joints of the cells, 1 - 2 mm wire is used, the knots are created with manual, mechanical hooks, homemade equipment installed in a screwdriver or a special knitting gun.
The area between the slabs can be reinforced with a ready-made mesh or knitted on site. In the first case, the dimensions of the longitudinal and transverse rods are taken, taking into account a 4 cm protective layer on each side. The nets are knitted on flat areas and laid on the deck on top of the film on spacers of 15 - 30 mm. More often, concrete blocks 10 x 10 cm are used or plastic coasters with cross-shaped slots for reinforcement.
These devices are not suitable for the top layer due to small sizes. Clamps, brackets, tables are used here different forms, designs. The main task of these elements is to support the upper mesh in the design position (15 - 30 mm below the plane of the slab).
Used for bending reinforcement homemade devices. For example, a piece of 50 - 70 cm pipe with a 10 - 15 cm mandrel welded to one edge will provide the required radius (5 rod diameters) and will reduce the force.
The area between the slabs may contain input nodes for engineering systems. Embeds and void formers are installed after or before reinforcement, depending on location, configuration, and size. For example, it is better to install an 11 cm sewer cross before laying the grids; sleeves for water pipe risers can be installed at any stage. 
Void formers complex shape necessary for specific communications. Therefore, they are usually made from polystyrene foam, polystyrene foam, cutting pieces of the same format to achieve the desired length from a 5 cm sheet.
For rigid fixation and absence of movement of light polymer fittings and polystyrene foam void formers when pouring the floor, the following technology is used:
- plugs are put on the fitting
- fixed with self-tapping screws from below through the deck
- or the plug is screwed on top
- then a fitting is put on it
These self-filled areas can support internal flights of stairs. For them you need:
- release the reinforcement of the lower mesh
- make a step for supporting a reinforced concrete flight structure with a counter seat
- install formwork for the staircase/hatch
To release the reinforcement, you will need to make cuts in wooden shield jumpers chain saw. Place the board on the reinforcement, inserting it into the cuts, and foam the remaining cracks. Steps and recesses are created by screwing narrow strips to the formwork from the inside.
Fill
Before laying concrete between floor slabs, it is recommended to prime the ends of existing slabs to improve adhesion. The main recommendations for concrete works are:
Concrete is contraindicated in solar ultraviolet radiation, hot dry weather, and frost. Covering with burlap, sawdust, and sand allows you to wet the surface without destruction. The film protects against sun rays, in winter it provides the principle of a thermos, preserving the heat generated by the hydration of cement with water.
The grade of concrete is selected in accordance with the standards SP 63.13330 for reinforced concrete structures:
- density – 1,800 – 2,500 kg/m3
- compressive strength – from B7.5
Water resistance and frost resistance are not particularly important for structures used indoors. At self-production concrete, it is necessary to take into account that the likelihood of cracking is sharply reduced if filler of different fractions with a continuous series of grains is used. Sand should not exceed 1/3 of the total volume of filler.
After pouring between the floor slabs, sagging may remain in the newly made area. They are polished with diamond equipment for an angle grinder (“grinder”) of a disc type. If the project includes a self-leveling, heated floor, or screed, alignment of the joints is not necessary. For better adhesion of two adjacent reinforced concrete structures, grooves can be made in the side faces of factory slabs if the appropriate tool is available.
When laying concrete, these recesses are filled with the mixture, the two slabs are almost monolithic. The quality of the bottom edge of the slab is usually inferior to factory analogues, so finishing with suspended, level ceilings is more often used. 
This technology is very convenient in the manufacture of hatches or staircases. These technological holes can be reinforced with diagonally placed rods near them, dramatically increasing the strength of reinforced concrete. If you cut out a hatch in a factory slab, the integrity of the reinforcing mesh, which weakens the default design. This is especially true when the opening is shifted to the middle of the slab.
Technology monolithic area homemade ceiling allows you to fill voids when laying out slabs without reducing structural strength. Even without pre-tensioning the reinforcement, the slabs have a high service life if the specified requirements are met.
