Retrofitting of existing structure

Retrofitting is the process of addition of new features to older buildings, heritage structures, bridges etc. Retrofitting reduces the vulnerability of damage of an existing structure during a near future seismic activity.

Retrofitting of RCC structural members is carried out to regain the strength of deteriorated structural concrete elements and to prevent further distress in concrete. Strength deficiency of concrete structural members can be due to poor workmanship, design errors, and deterioration due to the aggression of harmful agents.

Need of Retrofitting

There are several problems that structural members experience and needed to be tackled among them some common problems include:

  • Structural cracks

  • Damage to structural members

  • Excessive loading

  • Errors in design or construction

  • Modification of the structural system

  • Seismic damage

  • Corrosion due to penetration- honeycombs

Methods of Retrofitting of Building

The following are the most common method of retrofitting of a building

1. Adding New Shear Wall:

  • This is a frequently used technique for retrofitting of building of non-ductile reinforced concrete frame buildings.

  • The elements can be either cast-in-place or pre-cast concrete elements.

  • New elements preferably are placed at the exterior of the building.

  • This method id not preferred in the interior of the structure to avoid interior moldings.

2. Adding Steel Bracing

  • Steel bracing is an effective solution in the retrofitting of building when large openings are required.

  • Potential advantages due to higher strength and stiffness and opening for natural light can be provided.

  • The amount of work is also less so foundation cost may be minimized and adds much less weight to the existing structure.

Retrofitting of structure by steel bracing

3. Wall Thickening Technique

  • The existing walls of a building are added a certain thickness by adding bricks, concrete, and steel aligned at certain places as reinforcement.

  • The weight of the wall increases and it can bear more vertical and horizontal loads.

  • Also, it is designed under special conditions that the transverse loads do not cause sudden failure of the wall.

  • Rust can be developed on reinforcement if not covered properly by mortar.

Wall thickening technique

4. Base Isolation Technique

  • Isolation of superstructure from the foundation is known as base isolation. It is the most powerful method for passive structural vibration control techniques.

  • When building isolates from the ground it causes lesser seismic loads, hence lesser damage to the structure and minimum repair of the super-structure.

  • The main demerit of this method is, it cannot be applied to structures like other retrofitting and expensive in the budget.

  • This method is inefficient for high rise buildings and not suitable for buildings rested on soft soils.

Base isolation technique

5. Mass Reduction Technique

  • In mass reduction technique, for instance, by removal of one or more storey’s as shown in the figure.

  • In this method, it is evident that the removal of the mass will lead to a decrease in the loading, which will lead to an increase in the required strength.

Mass reduction Technique

6. Jacketing Method:

  • It is most used method of retrofitting o f building.

  • Jacketing is the most popularly used method for the strengthening of columns and beams of a building.

  • Jacketing consists of added concrete with longitudinal and transverse reinforcement around the existing columns.

  • It improves the axial and shear strength of the column and major strengthening of the foundation may be avoided.

  • The amount of work is less as foundation strengthening does not require and increases the shear strength of the column. It also increases the confinement of concrete in circular columns.

  • Steel jacketing does not increase the significant weight of the column and also saves construction time.

7. Fiber Reinforced Polymer (FRP)

  • A fiber-reinforced polymer is an axial strengthening system which used to improve or enhance the capacity of reinforced concrete beams.

  • It can be used for both circular as well as a rectangular-shaped column but it is more effective in former shape.

  • FRP increases the ultimate load-carrying capacity of reinforced concrete members and improves the shear capacity of the reinforced concrete element.

  • Also, the ductility of a reinforced concrete column is increased considerably.

  • Composite must be dried before repair because all resins and some fiber absorb moisture.

8. Epoxy Injection Method

Epoxy injection is an economical method of repairing of non-moving cracks in the concrete walls, slabs, columns, and piers. In retrofitting of building, epoxy injection is used to fill and repair the structural fault.

It is capable of restoring the strength of concrete to its pre-cracked strength.

The epoxy tensile bond to the concrete is stronger than the tensile strength of concrete.

Strengthening is provided by installing additional reinforcement across the failure plane in combination with the epoxy resin injection.

9. External Plate Bonding

  • Strengthening of reinforced concrete beams with external plates or strips is a conventional method and has been utilized for many decades.

  • The external plate bonding method can be used for increasing the shear strength of reinforced concrete beams by completely or partially wrapping steel plates at the joint of a column and beam.

  • An external plate providing perpendicular to potential shear cracks is effective to increase the shear strength of the concrete reinforced member.

  • The additional shear strength is achieved, but depending on beam geometry, existing concrete strength, and applied the wrapping method.

10. Section-Enlarging Reinforcing Method

  • The enlarged cross-section method is used to increase the components of the reinforcement area, which enhance the bearing capacity and also increase its cross-section stiffness and change the natural frequency of vibration.

  • This method is widely used in the reinforced concrete structure of the beam, slab, column, etc.

  • The enlarged cross-section method is suitable for the reinforcement of reinforced concrete bending and compression member.

Section enlargement technique