The construction of monolithic ceilings with a thickness of 150 millimeters is one of the most common solutions in private housing and the construction of commercial facilities. This thickness is considered the standard for spans up to 4.5-5 meters long, providing the necessary load-bearing capacity and sound insulation. However, the strength of the design directly depends on the quality of work on the creation of a reinforcement frame that perceives stretching loads.
Two-row reinforcement allows you to evenly distribute the forces throughout the height of the cross section, preventing the formation of cracks and deformations under the influence of the weight of furniture, people and the structure itself. In this article, we will discuss in detail how to correctly calculate the amount of materials, choose the diameter of the rods and perform a grid knitting so that your overlap will last centuries.
Principles of work of reinforcement in monolithic plate
Concrete, being the main material for floors, perfectly resists compression, but is practically powerless before stretching. That is why the concrete is placed inside the volume steel-frameWho takes all the stretching efforts. In a plate thickness of 150 mm, complex stresses arise: the upper part can experience compression, and the lower part can experience stretching, especially in the span part.
With two-row reinforcement, the lower mesh perceives the main loads in the span, preventing the deflection of the structure down. The upper mesh, in turn, is necessary to compensate for the loads over the supports, where a negative bending moment occurs. Without the upper layer of the reinforcement in the area of leaning on the walls or beams, cracks can go, which will critically reduce the durability of the building.
It is important to understand that the valve must be strictly in the body of concrete. The release of metal outwards or its too deep entrenchment disrupts the operation of the structure. The protective layer of concrete at the bottom and top should be at least 20-25 mm for internal floors and 30 mm for external or wet rooms. This prevents metal corrosion and ensures fire-resistance design.
⚠️ Note: The use of fittings with visible signs of corrosion or mechanical damage (cracks, fractures) is strictly prohibited. Even a small defect of the rod can become a source of destruction of the entire overlap under load.
Calculation of the diameter of the valve and the step of the cell
The choice of the diameter of the rods and the distance between them (step) is made at the design stage and depends on the length of the span and the expected load. For plates with a thickness of 150 mm, hot-rolled rebar of the periodic profile of the A400 (A500C) class is most often used. The standard diameter of the working rods varies from 10 to 14 mm, while for the distribution valves, a diameter of 8-10 mm can be used.
The reinforcement step is usually 150 to 200 mm. Reducing the step (regularity) increases the load-bearing capacity, but increases the consumption of metal and laborious work. For residential buildings with a standard load of 200-300 kg / m2, the optimal solution is the scheme of 200x200 mm or 150x150 mm. In areas of high loads, for example, under heavy columns or fireplaces, the step is reduced to 100 mm.
To accurately determine the required amount of metal, engineers use special tables or software complexes that take into account the brand of concrete and the class of reinforcement. However, there are empirical rules that allow us to estimate the order of the numbers. For example, for a 150 mm plate, the minimum percentage of reinforcement is about 0.15-0.2% of the concrete cross-section area.
The following is a table indicating the flow of fittings for various plate reinforcement schemes of 150 mm thick:
| Rebar diameter (mm) | Grid step (mm) | Consumption per 1 m2 (kg) | Recommended application |
|---|---|---|---|
| 10 | 200x200x200x200x200x200x200x200x200x200x200x200x200x200x200x200x200x200x200 | 7.7 | Light flooring, interior spaces |
| 12 | 200x200x200x200x200x200x200x200x200x200x200x200x200x200x200x200x200x200x200 | 11.1 | Standard residential flooring |
| 12 | 150x150 | 14.8 | Enhanced spans up to 5 meters |
| 14 | 150x150 | 20.1 | Commercial premises, large spans |
Technology of creation of lower and upper belt
The process of forming a reinforcement frame begins with laying the lower layer. The rods are laid out on the fixers ("chairs" or "frogs") with the observance of the protective layer. It is important that the reinforcement does not lie directly on the formwork or waterproofing, otherwise the concrete will not envelop the metal from all sides. Docking of the rods is carried out on the floor, the length of which is usually 40-50 diameters of the valve (for d12 it is about 50-60 cm).
After the crochet of the lower grid, special height retainers are installed, which separate the two levels of reinforcement. As such fixers, bent P-shaped elements from the same valve or ready-made plastic / metal "tables" are often used. The height of these elements is calculated so that the upper grid is at the desired level (usually 20-30 mm from the top of the future plate).
The upper layer of the reinforcement is knitted similarly to the lower, strictly perpendicular or parallel to the lower rods, depending on the design scheme. Particular attention is paid to the areas of the plate resting on the walls. The upper armature should be sufficient. anchorage beyond the support or be reliably bent to transfer the forces to the supporting structures.
To provide the perfect protective layer, use ready-made plastic star fixers or cones. They are cheap, do not rust and ensure that the rebar does not pop up when pouring concrete.
Rules for mating and rod junction
The main way of connecting reinforcement rods in private construction is wire knitting. Welding is less common, as thermal action weakens the metal at the point of connection, making it brittle. For mating, annealed wire with a diameter of 1.2-1.4 mm is used. The process is carried out using a knitting hook or a special pistol.
The nodes are knitted at all intersections of longitudinal and transverse rods. There are several ways of knitting, but the most reliable is a double knot, which eliminates the slipping of wire when pouring concrete. It is important not to pull the knot so that the wire does not burst, but also not to leave it weak, otherwise the frame will "float".
When the length of the rods (docking) is increased, the overhang should be uniform throughout the length. You can not do a cross section more than 50% of the total number of fittings - the joints should be placed in a chess order. This avoids the formation of a weak line in the design.
⚠️ Note: When knitting the valves, be sure to use protective gloves. The cut ends of the wire and sharp edges of the reinforcement bars can cause serious hand injuries.
Strengthen the plate in the areas of support and holes
The design of the overlap is heterogeneous in its loads. In places of placing the plate on walls, columns or beams, stress concentration zones arise. A standard grid might not be enough. The project often involves the installation of additional reinforced rods (amplification with "chess" or additional clamps) at the top of the plate above the supports.
If the plate provides technological holes (for ventilation, chimneys, stairs), their edges also require reinforcement. The reinforcement should not be interrupted abruptly at the edge of the hole. The rods circle the opening, forming a reinforced frame, or are trimmed with the addition of additional diagonal rods at the corners of the hole to prevent cracking.
For the design of holes of small diameter (up to 150 mm), the rebar is often simply pushed apart, without violating the integrity of the rods. For large openings, additional rods of the same diameter as the main fitting are installed, with a 100 mm increment around the perimeter of the hole. This creates a kind of “arched” reinforcement.
Do I need to reinforce the plate under the partitions?
If the partition is light (gypsum board), special reinforcement is not required. However, if a heavy brick partition is planned, an additional reinforcement or beam must necessarily pass under it, since the point load from the wall can push a standard plate 150 mm.
Quality control and preparation for concrete
Before pouring concrete, it is necessary to conduct a thorough acceptance of reinforcement work. Checks the compliance of diameters, grid pitch and length of the covers with the design data. Particular attention is paid to the purity of the reinforcement: the rods should not have oil, paint, ice or a large amount of loose rust, which can worsen the grip on the concrete.
The most important step is to check the height of the reinforcement. Used probes or simple measurements ruler at several points. If the top grid is too low, it will not work on stretching in the area of the supports. If too high, the protective layer will decrease, which will lead to corrosion. Permissible deviations are strictly regulated by SNiP (usually ± 5-10 mm).
After checking the reinforcement frame, the sides of the formwork are installed (if they are not already installed) and the final cleaning of the bed from debris is performed. Before applying concrete, the formwork is recommended to be moistened so that dry wood or plywood does not draw moisture from the concrete mixture, which could lead to the formation of microcracks.
☑️ Checking before pouring concrete
Frequently Asked Questions (FAQ)
Can I use a composite rebar instead of a steel 150 mm plate?
The use of composite (basalt or fiberglass) reinforcement is possible, but requires a special approach to calculations. It does not work on kink in the same way as steel and has other adhesion characteristics with concrete. For residential buildings, classic steel reinforcement A500C is more often recommended due to the predictability of the structure’s behavior during overloads.
Do I need to cook the rebar or knit enough?
For private construction and most industrial facilities, knitting is preferable. Welding is permissible only for special grades of reinforcement (with the index "C", for example, A500C), but even in this case, the knitting is considered more reliable, since it does not create zones of thermal influence that weaken the metal.
How long after the filling can I remove the formwork?
It is possible to remove formwork only after a set of concrete of sufficient strength (usually 70-80% of the design). At a temperature of +20 ° C, this takes about 7-10 days. Full load on the plate can be given no earlier than 28 days, when the concrete will gain 100% strength.
The quality of reinforcement is more important than the brand of concrete. Even the most expensive concrete will not save the plate if the reinforcement frame is associated with violations or has an insufficient protective layer.