What Is Retrofitting?
Retrofitting is the method of treatment of a structure to increase its usable life. The retrofitting may include a number of procedures depending upon the structure, degree of damage, purpose of retrofit, etc. Before moving ahead let us understand why structures fail.
Structures are sometimes subjected to loads for which they were originally not designed. These loads may occur due to earthquakes (seismic load), changes in loading patterns, etc. Hence, the structure experiences extraordinary stress and fails under it.
The other reason for the failure of structure is the completion of its’ design life. An RCC structure is designed for 40-50 years of life during which it can resist the loads. Afterward, the strength and durability of the structure become questionable and hence need proper investigation and retrofitting.
The current article shall develop your understanding of the various retrofitting methods used for RCC structures.
Types Of Retrofitting
Retrofitting can be broadly divided into four categories as given below.
1. Preservation Retrofitting
The term preserve means to maintain originality. The same applies to preservation retrofitting. In this method, the treatment is done to prevent the structure from potential damage without affecting the integrity of the structure, material, and value. It is a preventive method.
2. Rehabilitation Retrofitting
The usage of structure may change over time. Hence, the initial design may not be compatible with the new proposed usage thus, rehabilitation retrofitting is required.
3. Restoration Retrofitting
The structures deteriorate over time and lose their strength, durability and aesthetics. Restoration retrofitting is a method of bringing back a structure to its original form.
The process of creating a replica of a structure is called reconstruction.
Methods of Retrofitting
The methods of retrofitting depend upon the purpose of their application. Therefore, the methods have been divided into subheads of concrete repair, structure repair, cracks repair, surface treatment, etc.
Methods of Concrete Member Retrofitting (Local Methods)
The methods discussed in this section correspond to the treatment of a specific structural member. Therefore, these methods are called local methods. Let us discuss them.
The jacketing is a process of providing extra strength to the existing RCC member by providing a casing (jacket) over it. The jacket may be of fiber-reinforced plate or steel sheet. There are two methods of jacketing-
The prestressing involves the binding of prestressing wires to the structural members. This method substantially improves the load-carrying capacity of the member. The cost of prestressing is higher and needs skilled manpower.
3. Repaving Method
The replacement of existing structural members by precast or in-situ casted members is called repaving. This method is expensive as it involves the removal of an old member and adding a completely new member.
Methods of Structure Retrofitting (Global Methods)
The structure retrofitting methods are for the treatment of complete structure. Therefore, they are called global methods as they cover the entire structure.
1. Beam Addition
The secondary beams transfer the load to the main beams and the main beam transfers the load to the column. Due to asymmetric loading between the two beams, one main beam may be subjected to more stress than the other. In order to distribute the load evenly to both beams, a secondary beam can be introduced between the two main beams.
2. Shear Wall Addition
An RCC shear wall is used as a connection between two columns to control their buckling. Hence, higher loads can be supported. The shear wall makes the structural system more rigid and improves the structural integrity.
3. Support Point Addition
The longer span beams are susceptible to deflection. In order to limit the deflection, a support system can be designed at a suitable position along the span.
A support point is also used where the load on supports is to be redistributed. Moreover, it also increases the load-carrying capacity.
4. Seismic Isolation Method
In an earthquake, horizontal forces act on a structure. The isolation of the base of the structure by introducing the bearings improves the seismic performance of the structure.
Extensive research has shown the structures treated with seismic isolation methods are relatively more stable and safer against collapse. This technique is most effective for relatively stiff buildings with low rises and heavy loads.
5. Crack Fill Method
Applying low viscosity resin and ultra-fine cement by forcing into the cracks in existing concrete members to seal the crack.
The introduction of dampers to the structure limits the seismic vibration of the structure. It improves structural stability. With the advancement in technology and extensive research, there are multiple types of dampers available.
The elastoplastic damper, lead extrusion damper, tuned liquid damper, and friction damper are the types of dampers used in structures.
7. Foundation Retrofitting
There are various foundation retrofitting methods. Some of them are discussed below.
Underground wall (beam) addition method:
In this method, the cast in-site diaphragm walls and underground beams are connected to distribute stress ensuring the stability of the whole system.
Pile/ Or footing addition construction method:
Foundation improvement method:
Cement improvement materials are used to improve the bearing capacity of soil and horizontal foundation resistance. It also prevents excessive pore water pressure and liquefaction.
Steel sheet-pile coffering construction method:
In this method of retrofitting, sheet-piles are placed around the periphery of the footings and bonding them to the footings to improve the bearing capacity
and horizontal resistance.
Foundation Compacting Method:
This method is adopted when insufficient foundation bearing capacity is a
concern due to scouring. So using concrete the ground around the foundation is compacted in order to restore bearing capacity.
Ground anchor method:
8. FRP Strengthening
A Fibre Reinforced Polymer (FRP) typically consists of high tensile continuous fibers used to enhance the shear, flexural, compression capacity, and ductility of the deficient member. In FRP strengthening, glass fibers are the most commonly used.
FRP strengthening method is commonly adopted for structural applications. It is a quick, effective, and aesthetically pleasing method to rehabilitate
reinforced as well as prestressed concrete structures.
9. Use of Steel Bracing
Concentric or eccentric bracings can be used in the selected bays of an RC frame to increase the lateral resistance of the structure. Such method does not require an intervention of the foundation because steel bracings are usually installed between existing members.
Advantages Of Retrofitting
- Increases the life of the structure.
- Increases the strength, stability and sustainability of the structure.
- Time saving
- Economical than re-construction
- No need for new location clearance and liasoning
- No statutory approvals
- Original value of the structure is retained
- Historical importance and integrity remain intact
Disadvantages Of Retrofitting
- The main disadvantage is the unavailability of skilled worker.
- Small irresponsibility or negligence can cause further damage.
- Reduction of internal space due to the addition of components.
- Retrofitting methods are somwhow difficult than reconstruction.
- No Indian standard code for guidance.
- Lack of education.
Retrofitting has its’ own pros and cons and hence the feasibility differs from project to project. The suitability of the retrofitting method must be assessed beforehand by necessary testing. Structural testing methods can be destructive or non-destructive.
Due to limited knowledge of the retrofitting methods, the involvement of a subject-matter expert shall optimize the process.
Article By- Shazeb Ali Khan