Welding a rebar allows you to quickly make a strong frame of any size. However, the use of this technique is not always acceptable. The solution depends on the material of the rods and the load acting on them.
What is rebar?
Concrete has high compressive strength, but performs poorly in tension. In order to avoid cracking under these loads, steel bars with a circular cross-section – reinforcement – are embedded in it. Their additional function is to strengthen the concrete in compression.
Armature varieties:
1. Smooth rods made of hot-rolled quiescent, semi-quiescent or boiling St3 steel.
2. Rebars of this and the following classes have a periodic profile – protrusions which provide a firm grip on concrete. The bars with diameter less than 40 mm are made from steel (calm and semi-calm).
3. This variety is an improved version and is marked by applying an indelible white paint on the ends. Unlike the first, it can be joined by electric welding; the rods have increased strength due to cold drawing. This is one of the most common types of reinforcement. It is used in critical structures, including prestressed ones.
Two types of reinforcement are distinguished in reinforced concrete products:
Longitudinal. Is the main one, holds the calculated load.
Transverse. Serves to connect the belts of the longitudinal reinforcement into a 3-dimensional framework. Provides a bond between the compressed concrete and the stretched bars, absorbs the forces of thermal expansion.
Features of rebar welding
The round shape of the rods led to the emergence of a special method – tubular. It is used in butt welding, when the ends of the elements are adjacent to each other.
Basic requirements
At the point of welding, the metal should not lose its strength and ductility. Some steels is forbidden to join by this method, as the material in the area of the weld becomes brittle. Frames of such rods are assembled with binding wire or spring steel clamps.
Before welding, dirt and dust are removed from the reinforcement. For small amounts of work, it is cleaned to a metallic shine, which has a positive effect on the quality of the weld. This is not done on large construction sites, as such preparation entails a significant increase in the cost of the process and stretches it out in time. The thickness of the electrode and the welding current are chosen so as to avoid burning through the rod.
Fittings welding methods
Use manual arc welding, automatic and semi-automatic. There are several ways of positioning the rods in relation to each other.
Overlapping
This method is used in areas that are not subject to heavy loads.
Butt joint
This type of connection is used for rods of 20-100 mm in diameter. Welding is carried out by the bathing method. The disadvantage is the high consumption of electrodes. To fill a bath between bars with 28 mm thickness, 3 4-mm consumables are needed. If they melt in series, the master is required to know how to change them quickly. It is easier to clamp all 3 electrodes in a special comb and weld them simultaneously.
Spot contact
Such welding is used in automated production. Its advantage is high productivity. Its disadvantages: high power consumption; The need for special large-sized equipment (can be used only in the conditions of an industrial enterprise).
Cross-shaped
Welding is carried out strictly horizontally or vertically. This ensures firm contact of the fittings and prevents the molten metal from spilling out. The electrode is held at an angle of 30-45° to the plane of the bars.
Reinforcement welding technology
As an example, it is useful to consider the connection by the bath method. Proceed as follows:
- The rods are trimmed with a guillotine or a bolt cutter to the desired length. You can use a welding machine, but it is a more expensive way.
- Dirt and dust are removed and the ends are ground to a metallic luster at a length of 3-4 cm. Then they are washed and degreased.
- Install the valve in the designed position. The maximum permissible deviation from the alignment is 5% of the rod thickness. The size of the gap between the ends depends on the voltage: 1.5D (diameter of the electrode), if 220 V; and 2D, if 380 V.
- Fasten in the connection area the pads or the bracket, preventing the metal from flowing out. Steel parts have to be left, which leads to more expensive work. Removable ceramic or graphite pads are used many times. They are fixed with a wire or wire clamp.
- If metal pads are used, they are screwed to the armature.
- Clean one of the ends with a consumable and melt it, making oscillatory movements between the bars of the reinforcement. The burnt electrode is quickly replaced by a new one, and so on until the bath is full.
- Wait for the weld to cool and check its quality.
Checking the strength of the finished construction
The quality of the joints is checked:
- Visually. Examine the connection for cracks, through-burns, undercuts, non-cutting, pores and craters.
- Gamma-ray inspection. A method for detecting internal defects (blind holes, pores, microcracks, non-metallic inclusions).
- By tapping with a hammer or by dropping the frame on the floor from a height of 1-1.5 m (strength test).