Reinforcement work: professional advice, techniques and secrets

Recommendation points

• Reinforcement types
• Reinforcement of structures
• Reinforcement SNiP
• Reinforcement range
• Reinforcement class
• Reinforcement calculation
• Reinforcement scheme
• Rebar machine
• Rebar welding
• Rebar knitting

In this article we will tell you about the different types of structural reinforcement and reveal some of the secrets of the reinforcement profession. Simplified calculations, descriptions of documentation, reinforcement schemes will also be given. In the article you will find practical advice and recommendations for reinforcement work..

Reinforcement types

Reinforcement is an integral part of the structure, the material of which provides for the transition from a liquid to a solid state. This process is called setting or hardening. Reinforcement methods are distinguished:

1. Dispersed – adding fiber fibers or metal shavings to a liquid solution. Gives rigidity and abrasion resistance to the monolithic area. Used in the device of floors, screeds. Can be used in combination with the rod method.
2. Rod – in the volume of concrete or mortar, a system of rods (mesh, frame) is included, which distributes the load within the structure. Used for load-bearing and detached building elements.
3. Layer (layer reinforcement) – a mesh is included in the layer of liquid solution or putty to give stability to the finishing layer. Used for finishing and repairing surfaces.

In this article, we will look at the reinforcement of structures using a frame and meshes..

Reinforcement of structures

The hardened concrete can withstand high compressive loads – up to 1000 kg / cm², but unstable to break, break and stretch. Moreover, its production is relatively inexpensive..

The reinforcing bar carries significant tensile loads, but is not resistant to compression and bending. In addition, the cost of production is high, given that it includes the cost of mining the metal..

Since any load-bearing structure is subject to combined loads, a material is needed that meets several requirements. The combination of rebars and concrete gives a combination of their properties. The result is reinforced concrete that is resistant to compression, bending and fracture..

Since all reinforced concrete products are conditionally subdivided into factory and local production, fittings work in them in different ways. Most of the factory products are manufactured using prestressed reinforcement. Before placing concrete in the mold, the rods are pre-stretched (strained) with a special device. After hardening, the stress in the rods remains – the reinforcement, as it were, “presses” the entire element along them, which significantly improves the mechanical properties of the part. For example, a beam or slab with prestressed reinforcement can withstand higher bending loads (+ 40-60%) than conventional ones.

In high-rise buildings, the reinforcement cage serves as the basis for the entire structure. The rods move from one element to another, which makes them interconnected and gives the required rigidity to the building frame. This effect makes it possible to build skyscrapers in a relatively small area..

Reinforcement SNiP

In the construction of critical buildings and structures, the calculation of the cross-section and the number of rods is one of the main ones. Reinforcement standards are regulated by documents – SNiP 2.03.01–84 “Concrete and reinforced concrete structures” and the appendix to it “Reinforcement of elements of monolithic reinforced concrete buildings. Design Guide “. These documents describe in detail the calculations, tolerances and requirements for structures in which reinforcement is applied..

Operating conditions and requirements for the rods themselves are standardized by the document GOST 10884-94 “Steel for reinforced concrete structures”.

Deep calculations are necessary in the construction of large and complex objects – high-rise buildings, bridges, towers, dams. To calculate the reinforcement of structures in private construction, it is enough to adhere to the basic rules that are relevant for all applications of reinforcement.

Reinforcement range

Another useful document is assortment. It contains all the possible characteristics of reinforcing products – the weight of a running meter and its dependence on the diameter, the cross-sectional area of ​​the bar and steel grade, and many others. This data is required for more complex calculations – monolithic floors, tanks or buildings with more than 3 floors.

Reinforcement class

As a rule, the most common brands and diameters of rods are privately used. This set can be conventionally called the “optimal discharge”. It includes rods with a diameter of 6 to 18 mm. Classes of valves of optimal discharge according to GOST 5781:

1. A1 (A240). Smooth rod O 6-12 mm – in coils (bobbins, coils), 12-40 mm – in rods (circle).
2. A2 (A300). Has helical ribs. Diameter 10-12 mm – in coils, 12-40 mm – in rods.
3. A3 (A400). The transverse ribs diverge “herringbone” from the longitudinal rib. Ø 6–12 mm – in coils, 12–40 mm – in rods.

Other grades are rare – mainly in high-demand facilities, these products are made to order from higher quality steel.

There are only two types of concrete reinforcement in terms of construction – a flat mesh (can be bent) or a spatial frame. The mesh is used for recumbent slabs and screeds, the space frame is used for volumetric elements – beams, lintels, armored belts, columns, walls, etc. In this case, two grids, arranged at a stable distance from each other, already represent a frame (for example, wall).

Reinforcement calculation

When the shape of the product (element) and its size have been determined, it remains small – to determine the diameter and pitch of the frame cell. In construction with low requirements, it is optimal to use an effective system of adapted calculation. The principle of using reinforcement of different diameters is simple – the more load the element carries, the thicker the rods are needed.

Indicators of frames and meshes for different structures:

Item name	Rebar brand	Rod diameter, mm	Cell pitch, mm	Note
Concrete, blind area	A1, A2, A3	8	150-250	Unloaded areas
Reclining plate, recumbent beam (armopoyas)	A2, A3	12-16	150-200	No deeper than 50 mm from the top of the plate
Foundation beam, hanging beam, hanging plate	A3	16-18	100-160	Depending on the presence of reinforcements and attachment points, load
Column, thrust wall	A3	14-18	100-160	Depends on applied load
Sideboard	A2, A3	12-16	120-160	No significant load
Building wall	A3	sixteen	100-160	Depending on the binding

In the adapted calculation, the general principle can be applied – a sufficient cell pitch will be equal to the diameter of the bar multiplied by 10. In critical places – abutments and connections of elements – reinforcements should be added, that is, additional rods should be installed.

Reinforcement scheme

As a rule, two types of elements are arranged from reinforced concrete – beams and slabs. In 80% of cases, two positions will be enough to complete a frame of any complexity:

• working rods – reinforcement rods of Ø 12–18 mm, arranged along the structure;
• distribution (structural) elements – products made of wire Ø 6–8 mm, which are distributed in space and fix the working rods with a given pitch.

Of course, you need knitting wire..

Beam reinforcement scheme: 1 – reinforcement of recumbent, foundation beams and armored belt; 2 – reinforcement of hanging beams, foundation; 3 – protective layer 40 mm; 4 – auxiliary working rods; 5 – main working rods; 6 – clamp

If the beam is supposed to be hanging, all the rods in it must be of the same section (at least 16 mm). For a recumbent beam, auxiliary rods can be of smaller diameter.

Plate reinforcement scheme: 1 – recumbent plate; 2 – hanging plate; 3 – “frog”; 4 – distribution fittings; 5 – working fittings

The hanging slab frame consists of two mirrored grids. The equal distance between them is kept with the help of limiters.

Rebar machine

In order to make elements of the “clamp” or “frog” type, you will need a special device – a bending machine. If a tangible amount of concreting is expected, one should start with the manufacture of this machine from the material at hand. It is a workbench on a steel frame, securely installed in a horizontal position.

To assemble a machine for reinforcement on site, you will need an improvised material – scrap metal, among which there should be two corners 40×40 or 45×45.

Work order:

1. The main element of the machine is a stop with a sleeve. In the middle of the workbench we weld a rod 8-10 mm long vertically and select a steel tube that can be easily put on it.
2. We weld a lever to the tube – the best corner is a horizontal shelf to the tube. If there is no corner, then the stop is 100 mm from the welded rod.
3. We weld a comfortable handle to the outer edge of the lever.
4. We lay the reinforcement of the largest diameter (but not more than 18 mm), which must be bent parallel to the long edge of the workbench.
5. We weld the stop to the workbench – the corner is best.

The machine can be of any design. The basic idea is that the force is applied at three points through the levers.

On sale you can often find factory hand tools for rebar bending, but they rarely withstand heavy loads and are intended for home use. For large volumes, you can purchase an electric bending machine 220 or 380 V. Using an electric machine, you can bend rather complex elements, which are also used in artistic forging. The price of a new electric bending machine up to 40 mm starts from 70,000 rubles.

Rebar welding

The most common mistake when performing reinforcement work is the use of electric welding to connect the frame elements. The reasons why this should not be done:

1. Overheating of the metal. Steel with a relatively high carbon content is used in the production of valves of classes A1, A2, A3. This means that after heating it loses up to 50% of its strength properties. This is especially important for angled connections..
2. Incorrect load distribution. The rigidly fixed (welded) section of the rod is, as it were, isolated from it and works separately from the rest of its part. For this reason, abnormal stresses arise, concentrated in places of rigid fixation (welding) instead of being distributed along the entire length.
3. An incorrectly assembled frame will only have to be thrown away (cannot be redone).
4. Danger to other workers – possible accidental electric shock.
5. Electricity costs.

However, there are cases when welding is not only irreplaceable, but also required:

1. Installation of embedded parts (ZD). ZD – priority elements on which a large load is concentrated. They are welded into the frame for better load transfer to the rods.
2. Welding of longitudinal joints (overlaps). Overheated reinforcement retains up to 70% of its tensile properties. In addition, it is doubled on the overlap. Butt welding of longitudinal rods makes no sense.
3. Fastening in place to already existing ST or steel elements (during the reconstruction of buildings).

Rebar knitting

Fastening intersecting rods to each other is a painstaking and time-consuming job. But it cannot be avoided when reinforcing structures. To do this, use a soft knitting wire with a thickness of 0.5 to 2.5 mm. The device for work – the hook of the fitter – each specialist chooses for himself. There is a small assortment of factory models, but in the overwhelming majority of cases the hook is made on site from a rod of wire Ø 8–12 mm. To do this, you need to bend it in a convenient shape and sharpen it at one end. You can put a plastic tube on the opposite end of the hook bar. Also, the hook can be installed in a cordless screwdriver, which will greatly facilitate the work.

To facilitate the work of the fitter, there are developed forms of a crochet hook:

1. Factory reinforcement hook. A bearing is installed between the handle and the hook shaft.
2. Automatic hook. Rotates due to the spring in the handle connected to the sting.
3. Knitting device (gun). The operation is automated, the gun itself presses the rods and knits the wire.

When creating frames for different elements, a different knitting step is used. The more critical the site, the denser the nodes will be.

Step of nodes in different frames:

Item name	Cell pitch, mm	Node step, cells along x cells across
Concrete, blind area	150-250	3 x 3
Reclining plate, recumbent beam (armopoyas)	150-200	2 x 3
Foundation beam, hanging beam	100-160	every intersection
Hanging slab (ceiling, balcony)	100-160	2 x 2
Column, thrust wall	100-160	2 x 2
Sideboard	120-160	3 x 3
Building wall	100-160	2 x 2

Reinforcement work often involves the installation of formwork, which is often oiled to facilitate dismantling. Make sure that the oil does not get on the rods – this will lead to a lack of adhesion between the concrete and the reinforcement. The use of highly oxidized fittings is categorically undesirable..