RETROFITTING OF CONCRETE REINFORCED STRUCTURE

1. INTRODUCTION

1.1 GENERAL

       In the current situation, concrete structure deterioration is an international issue. Numerous factors contribute to this, including the occurrence of natural disasters like earthquakes, a lack of awareness of various crucial statutory requirements in the field of building, inadequate oversight, etc. These elements cause constructions to lack strength. Structure overloading can occasionally result in extreme deformations and corrosion, both of which demand today's attention. Today's building industry often calls for strengthening, retrofitting, and repair work to counteract all these effects on reinforced concrete structures.

All of these potential ways to produce damage will call for a range of potential repair methods, from which the most efficient one will be selected in each specific situation. It is the most eco-friendly method and helps to extend the life of buildings. In some circumstances, even recently constructed structures need to be repaired and strengthened to remove flaws caused by mistakes in the design or construction. Repairing a concrete structure or one of its components has always been highly difficult, and effective solutions frequently required substantial work. To deal with structural elements damaged by unexpected events like fire, earthquake, foundation movement, impact, and overload, specialized strengthening, stiffening, and repair procedures are required.

Numerous existing bridges, industrial buildings, urban transportation hubs, marine structures, and earth-retaining structures require maintenance or improvement. The reinforced concrete constructions must undergo some alterations and advancements during their service life for a variety of reasons. Retrofitting or replacement are the two options available in this situation. Whole structures must be replaced, which has drawbacks including expensive labour and material prices. Therefore, if possible and practical, it is preferable to repair or update the structure rather than replace the entire thing.

2. RETROFITTING

2.1  GENERAL

Retrofitting is the strengthening of existing structures or structural elements to enhance their performance with new technology, features and components. It is the process of giving anything a component or feature that was not included during manufacture or by adding something that was not present at the time of original construction. Retrofitting RCC structural members are done to restore the strength of the weakened concrete element structure.

2.2  PROBLEM FACED IN CONCRETE STRUCTURES

•  Damaged to structural members
• Structural cracks
• Corrosion due to penetration
• Seismic damages
• Errors in design or construction

Excessive loading

2.3  GOALS OF RETROFITTING

The main goal of retrofitting is to stabilize the current structure of buildings and make them earthquake-resistant. Retrofitting is now spreading uncontrollably over the globe as a substantial portion of recorded, public, and private noteworthy designs get old and weaken with time. Retrofitting is likely the best option for securing a weak structure against potential threats or other natural forces. Retrofitting is the process of adding new features to older structures. The major goals are:

1.     To increase the lateral strength and stiffness of the building.

2.     To increase the ductility and aims to avoid the brittle modes of failure. 

3.     To increase the integral action and continuity of the members in a building.

4.     To eliminate the effects of irregularities

2.4   STAGES OF RETROFITTING

Retrofitting of structures shall proceed as follows:

• Select the objective of retrofit by the determination of performance criteria for the existing structure that has to be retrofitted and create a comprehensive plan that includes inspection, technique selection, structural design, and implementation of the retrofitting work.
• Reviewing and examining the initial consideration of the structure that needs to be retrofitted.
•  Assess the structure's performance, and age based on the inspection's findings, and make sure it complies with the performance standards.
• Evaluate the seismicity of that particular area where is building situated.
• If the structure does not fulfil performance requirements, and if continued use of the structure through retrofitting is desired, proceed with the design of the retrofitting structure.
• Select an appropriate retrofitting method and establish the materials to be used, structural specifications and construction method.
• Evaluate the performance of the structure after retrofitting and verify that it will fulfil performance requirements.
• Implement the retrofitting work if it is assessed that the retrofitting structure will be able to satisfy performance requirements using the chosen retrofitting and construction methods.
• Maintaining and monitoring the retrofitted structure.

2.5  CLASSIFICATION OF RETROFITTING

a)     Global retrofitting

b)     Local retrofitting

2.5.1  Global Retrofitting

Global retrofitting is also named as Structure-Level retrofitting. To provide increased lateral stiffness and strength to the building as a whole and to ensure that a total collapse of the building does not occur. This technique targets the seismic resistance of the structures. It includes adding shear structure, adding steel bracings, adding infill structure and base isolation. Shear structures can be introduced in a building with flat slabs or flat plates. These can be provided in the exterior frames with the least disruption to the building’s use.

 2.5.2   Local Retrofitting

Local retrofitting is also named as Member-Level retrofitting. To avoid failure of the components, and also thereby enhance the overall performance of the structure. A local retrofitting technique focuses on a member's seismic resistance. The local retrofit technique involves jacketing structural members, such as beams, columns, foundations, and beam-column joints, with concrete, steel, or fibre-reinforced polymer. By adding a fresh layer of concrete with longitudinal reinforcement and evenly spaced links, concrete jacketing is accomplished. The jacket improves the column's or the beam's flexural strength in both directions.

3. METHODOLOGY

3.1  GENERAL

Retrofitting is the modern methodology of strengthening an existing structure. It also includes the addition of new features or technology to the old system. Retrofitting is making changes to an existing building to protect it from flooding or other hazards such as high winds and earthquakes. Selecting the appropriate treatment strategy is a great challenge involved in the retrofit process and must be determined individually for each project. Depending on project objectives, the preservation and renovation of buildings may involve an array of diverse technical considerations, such as fire life safety, geotechnical hazards and remedies, weathering and water infiltration, and structural performance under earthquake and wind loads.

3.2  METHODS OF RETROFITTING OF REINFORCED CONCRETE STRUCTURE

3.2.1       Global Retrofit Strategies

Addition of Infill Walls

     The infill wall is the supporting wall that closes the perimeter of a building constructed with a three-dimensional framework structure (generally made of steel or reinforced concrete). Therefore, the structural frame ensures the bearing function, whereas the infill wall serves to separate inner and outer space, filling up the boxes of the outer frames. The infill wall has the unique static function to bear its weight and usually increases the lateral stiffness of a storey. The infill wall is added in this method between two columns by proper anchorage. The use of masonry infill walls, and to some extent veneer walls, especially in reinforced concrete frame structures, is common in many countries. The use of masonry infill walls offers an economical and durable solution. They are easy to build, attractive for architecture and have a very efficient cost-performance.

Addition of Shear Walls

     Shear walls are commonly used for non-ductile reinforced concrete buildings. The addition of shear walls is one of the most common methods used for strengthening existing structures. This method is efficient in controlling global lateral drift, thus reducing damage to frame members. A new shear wall can be cast in-situ or precast concrete elements It can be placed on the exterior wall of the building. Increase the lateral strength, ductility and stiffness of the building. During the design process, attention must be paid to the distribution of the walls in plan and elevation (to achieve a regular building configuration), transfer of inertial forces to the walls through floor diaphragms, struts and collectors, integration and connection of the wall into the existing frame buildings and transfer of loads to the foundations. Added walls are typically designed and detailed as in new structures.

Addition of Shear Walls

     Shear walls are commonly used for non-ductile reinforced concrete buildings. The addition of shear walls is one of the most common methods used for strengthening existing structures. This method is efficient in controlling global lateral drift, thus reducing damage to frame members. A new shear wall can be cast in-situ or precast concrete elements It can be placed on the exterior wall of the building. Increase the lateral strength, ductility and stiffness of the building. During the design process, attention must be paid to the distribution of the walls in plan and elevation (to achieve a regular building configuration), transfer of inertial forces to the walls through floor diaphragms, struts and collectors, integration and connection of the wall into the existing frame buildings and transfer of loads to the foundations. Added walls are typically designed and detailed as in new structures.

Base Isolation

     Isolation of the superstructure from the foundation is known as base isolation. It reduces the impact on the building structures, hence lesser damage to the structure and minimal repair of the superstructure. It is suitable for hard soil only. Seismic isolation is mostly adopted for the rehabilitation of critical or essential facilities, buildings with expensive and valuable contents and structures where performance well above performance levels is required. A seismic isolation system significantly reduces the seismic impact on the building structure and assemblies. Generally, the isolation devices are inserted at the bottom or the top of the first-floor columns. Retrofitting mostly requires traditional intervention; in the first case the addition of a floor to connect all the columns above the isolators while in the second case the strengthening of the first floor columns (enlarging of the cross-sections, addition of reinforcing bars or construction of new resistant elements). Nevertheless, inserting an isolator within an existing column is not so simple because of the necessity of cutting the element, temporarily supporting the weight of the above structure, putting in place the isolators and then giving back the load to the column, without causing damages to persons and structural and non-structural elements.

 3.2.1       Local Retrofit Strategies

Concrete Jacketing

     Concrete jacketing is one type of retrofit for the beams. To increase the flexural and/or shear capacity, extra longitudinal bars and stirrups must be added together with an additional layer of concrete around the current beam. The inquiry on reinforcing beams with bottom bars that are interrupted at joints is described in the current work. Concrete jacketing is required to increase bearing load capacity after a structural design revision or to restore structural design integrity following a structural member failure. This method is applied on vertical surfaces like walls, columns, and other combinations like the bottom and sides of beams. As discussed earlier, columns are one of the structural elements that are often in need of retrofit in both buildings and bridges. This method has been used for several years and was, for example, widely applied after the 1985 earthquake in Mexico City. In this technique, the structure dimensions are increased by adding new concrete to the original web. Concrete jacketing increases the flexural and shear capacity, extra longitudinal bars and stirrups must be added together with an additional layer of concrete around the current beam. It involves drilling holes in the existing beam. But drilling holes for the stirrups at closing spacing damages the beam.

Steel Jacketing

     Increases in basic strength capacity can also be achieved by the use of steel jacketing. Steel jacketing not only offers adequate confinement but also stops shell concrete deterioration, which is the primary cause of bond failure and longitudinal bar buckling. Steel jacketing is the process of covering a part with steel plates and filling the space with grout that won't shrink. It is a very efficient way to fix flaws like insufficient shear strength and poor longitudinal bar splices at crucial points. But it could be expensive, and you have to think about how fire-resistant it is. Steel strips and angles are the most widely utilized reinforcing method in real life. Steel jacketing appears to be useful for retrofitting columns since it helps to restore the strength, ductility, and energy absorption capacity of columns. Additionally, the steel jacket contributes to the lap-spliced column's increased flexural strength and ductile behaviour, improving the lateral performance of columns. Steel jackets can be used to retrofit both columns and joints. Column retrofit will be discussed first. After the 1971 San Fernando Earthquake, reinforced columns were recognized as a structural element that needed more attention. The retrofit of columns using steel jackets has been extensively studied in the 1990s, mainly in the context of bridge columns. The principle behind this technique is that the steel jacket acts as a passive confinement reinforcement. The jacket will prevent the concrete from dilating, forcing it into lateral compression and increasing its compressive strength, its effective ultimate compressive strain, and its ductility. For circular columns, the method uses two semicircular half-sections that are field welded along the entire height of the jacket.

 Fibre Reinforced Polymer (FRP) Sheet Wrapping

     Fibre-reinforced polymer (FRP) composite materials have received increasing attention in the past few decades as a potential material for retrofitting existing structures due to their high strength, lightweight, and ease of application. FRP has desirable physical properties like high tensile strength and corrosion resistance. These are thin and flexible. The increase in thickness of the wrapping improves the strength and ductility. FRP builds the ultimate load-carrying capacity and shear capacity of reinforced concrete members. FRP laminates, sheets or rods can be used [2]. The most common composite used in civil engineering applications are jackets or sheets and the fibres might be pre-stressed to increase the efficiency of retrofit. The use of FRP composites offers also a faster and easier retrofit alternative, especially when the evacuation of the entire building during the retrofit is not possible, in that case, FRP will provide the required strength without interrupting the use of the building. When using composite, the general expectations are lightweight, high stiffness or high strength-to-weight ratio, as well as corrosion resistance, durability, low thermal expansion (at least in the fibre direction), and low maintenance. They can be used in marine environments and are usually applied without much disruption to the building or its occupants (often the structure does not have to be closed). The largest disadvantage is the high initial material cost.

3.3  ADVANATGES OF RETROFITTING

•  Increase the life of the building.
• Economical.
• Prepares the building to bear different weather conditions.
• Helps in strengthening and enhancement of the structure.

3.4  DISADVANTAGES OF RETROFITTING

• Small irresponsibility can cause further damage.
•  Need critical and expert analysis before retrofitting.
• Manpower involved needs to be of good expertise

4. CONCLUSION

The studies performed on the various methods of retrofitting and the importance and necessity of retrofitting techniques are presented.

• The combination of engineering, machines and year of experience makes this possible to develop the technology of retrofitting
• At present day, retrofitting has a very lucrative market in developed and as well as developing countries.
• It provides a no of ways to improve the damaged structure and allows it to expand the lifespan of a structure, and ductility, increasing its functioning and safety.
• To find more effective sustainable retrofitting approaches, new techniques are being developed. The most popular methods are those that were just stated.
• The retrofitting of a structure affects how that structure responds to risks other than those related to flooding, such as wind hazards, and the architect, engineer, or code official must be aware of this.