Concrete jacket reinforcement is a technique often used to strengthen building columns and is made of cast-in-place concrete or often shotcrete.
This method involves adding a layer of reinforced concrete in the form of a jacket using longitudinal steel stiffeners and transverse steel connections outside the perimeter. During the life of a building, reinforcement of the concrete jacket may be required due to earthquakes, severe impacts, fires, explosions, and situations involving changes in the performance of the structure.
What is a concrete jacket?
Concrete jacketing is the process of strengthening weak columns that have deteriorated over a period of time due to adverse weather conditions or due to poor maintenance of the structure. Among other reasons for the need to strengthen the concrete jacket, at the construction stage, we can mention design errors, low amount of concrete and improper execution of columns.
This strengthening process, unlike any other work in which steel elements are used, does not require specialized work. The simplicity of this process enables any company to do this easily. This method is usually done as follows
1) Adding a longitudinal reinforcement and stabilizer to the column
In this method of strengthening the concrete jacket, the strength of the column is increased and the load is distributed evenly. In this method, the overall goal is to create new columns or at least strengthen existing columns. To strengthen the concrete jacket, longitudinal steel rebar can be attached to the existing column.
Reinforcement with concrete jacket
2) Place the longitudinal steel bars of the concrete jacket in the slab
If it is necessary to reinforce floors with a concrete jacket, holes must be drilled in the slab to allow the longitudinal steel bars of the jacket to penetrate through them.
In the case of slab structures, this method is easy to implement and has no problems, but in structures with beams, due to the presence of intermediate steel reinforcement, longitudinal steel reinforcement should be used at the corners only.
3) Strengthening the interface
Proper façade preparation is important to create a strong bond between the main column and the additional concrete jacket so that the resulting component functions as a single unit.
This method involves increasing the surface roughness of the existing main shaft followed by the use of a bonding agent and in some cases steel fasteners.
The following steps are performed to prepare the interface between the column and the concrete jacket:
• Increased surface roughness
- Wetting the surface
- Use a binding agent
- Adding steel connections
Reinforcement with concrete jacket
4) Temporary strengthening of the structure
An important aspect of any concrete jacket strengthening technology is how it is constructed. In temporary strengthening of the structure, the main goal is to transfer the load on the column to other components, which can be done using hydraulic jacks.
5) Adding concrete
In general, the maximum volume of concrete added in this technology is about 2 mm, which is added due to the lack of space available for the use of jacket reinforcement and due to the reduction in thickness of the volume occupied by the presence of steel reinforcement. This is why self-compacting concrete (SCC) is often used in this technique.
The concrete jacket is covered with concrete, which is used to strengthen the concrete jacket by installing the mold around the column. The thickness of the concrete jacket is usually more than 10 cm, so that the concrete can be poured without creating openings and cracks.
In concrete jacket reinforcement, the thickness of the jackets is 7.5 cm or more to create a cover of sufficient thickness so that there is space between the longitudinal and transverse stiffeners of the structure, the new reinforcement and the existing member.
Step by step method for jacketing an RCC column
It is very important to prepare the surface of the column that will be reinforced with concrete jackets. In order to achieve a good bond with the jacket, it is essential that the concrete column is clean and sanitary. New concrete connects best with sufficient surface roughness and the presence of steel nails.
To prepare, new rebars and vertical steel columns were installed according to the designed dimensions and diameters. Since it is often not possible to bend the rings at a 135 degree angle, due to the thinness of the jacket, welding is usually required. Before strengthening with the concrete jacket, special attention must be paid to the joint areas, where the concrete jacket is to expand, and to be oriented.
In order to connect longitudinal reinforcement between adjacent floors, it is usually necessary to drill small holes in the concrete slab with a small hammer or portable electric drill.
Also, vertical holes must be made in the adjacent beams, which can only be done with an electric drill, so as not to cause serious damage to the beam during reinforcement with a concrete jacket.
Reinforcement with concrete jacket
Why use reinforcement with concrete jacket?
In the case of reinforcement using a concrete jacket, a significant increase in the bending and shear strength and vertical bearing capacity of concrete is achieved.
In addition, through the confining and bending resistance performance of the new stirrups, there is a significant increase in the shaft’s ductility and deformation capacity.
The bearing capacity and flexural strength of the column are increased due to longitudinal reinforcement with the concrete jacket, while improvement in shear strength and ductility are achieved through transverse reinforcement.
As a result, it is beneficial to use a concrete jacket in buildings that are undergoing major renovation.
Advantages of reinforcement with concrete jacket
The concrete jacket strengthening technique increases the uniform distribution in the structural structure and strengthens the strength and rigidity of structural components such as columns and beams. The resistance of the main components has also been greatly improved.
On the other hand, this strengthening method does not require specialized work. All these reasons make this technology an extremely valuable asset in building improvement and strengthening.