Retrofitting is a process used to enhance the lifespan of a structure. It involves various procedures tailored to factors like the type of structure, its existing condition, and the purpose of retrofitting. Before delving into the methods, let’s understand why structures sometimes face issues.
Structures can face unexpected stresses, like earthquakes or changes in loading patterns, for which they were not originally designed. This can lead to structural failure. Additionally, once an RCC structure reaches the end of its designed life (usually 40-50 years), its strength and durability may become uncertain, necessitating investigation and retrofitting.
Retrofitting techniques can be broadly classified into four categories:
1. Preservation Retrofitting : Preservation retrofitting aims to protect a structure from potential damage without altering its original integrity, material, or value. It’s a preventive measure to maintain the structure’s current state.
2. Rehabilitation Retrofitting : As the purpose of a structure may change over time, the initial design might not be suitable anymore. Rehabilitation retrofitting is needed to adapt the structure to its new intended use.
3. Restoration Retrofitting : Over time, structures can deteriorate, losing strength, durability, and aesthetics. Restoration retrofitting is a method to bring a structure back to its original condition.
4. Reconstruction : This involves creating a replica of a structure.
The choice of retrofitting method depends on its intended application. These methods can be further categorized into local methods (specific to individual structural members) and global methods (for the entire structure).
Local Methods: Retrofitting Specific Structural Members
These methods are focused on treating specific structural elements:
1. Jacketing : Jacketing involves adding extra strength to an existing RCC member by placing a casing (jacket) over it. This jacket can be made of materials like fiber-reinforced plate or steel sheet.
2. Prestressing : Prestressing involves attaching prestressing wires to structural members, significantly improving their load-carrying capacity.
3. Repaving Method : This entails replacing existing structural members with either precast or in-situ casted members, which can be costly.
These methods cover the entire structure:
1. Beam Addition : By introducing secondary beams between main beams, the load distribution can be made more even, preventing overloading of any one beam.
2. Shear Wall Addition : Adding an RCC shear wall between two columns increases their load-bearing capacity and overall structural integrity.
3. Support Point Addition : This involves designing a support system at a strategic location along the span of longer beams to limit deflection and increase load-carrying capacity.
4. Seismic Isolation Method : Introducing bearings at the base of a structure improves its seismic performance, making it more stable during earthquakes.
5. Crack Fill Method : Low viscosity resin and ultra-fine cement are used to seal cracks in existing concrete members.
6. Dampers : Various types of dampers can be introduced to limit seismic vibrations and enhance structural stability.
7. Foundation Retrofitting : Several methods can be employed to strengthen the foundation, including underground wall (beam) addition, pile/footing addition, foundation improvement, steel sheet-pile coffering, foundation compacting, and ground anchor methods.
8. FRP Strengthening : Using Fibre Reinforced Polymers (FRP) can enhance the capacity and ductility of deficient structural members.
9. Use of Steel Bracing : Installing concentric or eccentric bracings in selected bays of an RC frame can increase lateral resistance.
Retrofitting offers both advantages and disadvantages, and its feasibility depends on the specific project. Assessing the suitability of a retrofitting method through structural testing is crucial. Involving a subject-matter expert can greatly optimize the process.
