When the existing structure deteriorates or has to be modified due to the fact that the structure may not achieve its purpose, reinforcement of the concrete structure must be considered. Attention must be paid to existing materials (usually in a deteriorated state), loads during strengthening, and existing geometry. In some cases, it is also difficult to reach areas that need strengthening. When strengthening concrete structures, all failure modes must be evaluated. Strengthening flexure structures may result in shear failure rather than providing the desired increased bearing capacity. It should also be noted that not only the failure mode of the stiffener is important. If one key component in the structure is strengthened, another component can become a key component. Because the stiffness of the uncertain structural system changes, the entire structure must be studied. The reinforcement design should also consider minimizing maintenance and repair requirements. When designing reinforcement, the consequences of loss of reinforcement effect caused by fire, damage, collision, etc. must also be considered.

The concrete structure needs to be strengthened for the following reasons:

The load is increased due to high live load, increased wheel load, installation of heavy machinery or vibration.

Damage to structural components due to aging of building materials or fire damage, reinforcement corrosion and/or vehicle impact.

Improved serviceability due to deflection limitations, reduced reinforcement stress, and/or reduced crack width.

The structural system was modified because walls/columns and/or openings through the floor were eliminated.

Planning or construction errors due to insufficient design dimensions and/or reinforcement.

The reinforcement methods of concrete structures can be divided into two categories: direct reinforcement and indirect reinforcement. The direct reinforcement methods are as follows:

1. Enlarged section reinforcement method: applicable to beams, slabs, columns, walls and general structures.

2. Replacement concrete reinforcement method: applicable to beams and columns with low concrete strength or serious defects in the compression area.

3. Bonded wrapped steel reinforcement method: also known as wet wrapped steel reinforcement method.

4. Stick steel plate reinforcement method.

5. Stick fiber reinforced polymer(FRP) reinforcement method: it has the advantages of corrosion resistance, moisture resistance, durability, etc.

6. Wire winding method: applicable to the reinforcement of concrete structural members with insufficient diagonal section bearing capacity.

7. Anchor bolt anchoring method.