Retrofitting is a process used to restore the strength of deteriorated structural concrete elements and prevent further damage. The process is carried out on RCC (reinforced concrete) structural members. The lack of strength in these members can result from poor workmanship, design errors, or the deterioration caused by harmful agents.
The retrofitting process should begin with an investigation and diagnosis of any cracks. Once the issue has been identified, the appropriate retrofitting technique can be selected, along with compatible materials. Several techniques are available for retrofitting structural members, including section enlargement, external plate bonding, external post-tensioning, grouting, and fibre reinforced polymer composites.
Choosing the appropriate retrofitting technique will depend on the severity of the damage and the required capacity needed to regain the structure’s strength. Therefore, a proper retrofitting technique must be specified and implemented. By doing so, the structural element can be restored to its original strength and prevent any further damage.
When do RCC Structural Members Need Retrofitting?
RCC (Reinforced Concrete Cement) structural members are prone to various issues that require attention to ensure their stability and longevity. One of the most common problems encountered is the occurrence of structural cracks, which can significantly impact the durability and strength of the structure. These cracks can be caused by several factors such as external loads, shrinkage, temperature changes, or improper construction practices.
Another problem that RCC members face is the damage caused to them due to various reasons like exposure to harsh weather conditions, chemical reactions, or impact from heavy objects. It is imperative to repair or replace damaged components to maintain the overall stability of the structure and prevent further damage.
Excessive loading on structural members can also lead to problems such as deformation or failure. It is essential to ensure that the structure is designed and constructed to withstand the expected loads and stresses to prevent such issues.
Errors in the design or construction process can also lead to structural problems. It is crucial to follow the recommended standards and guidelines to ensure the structural integrity of the RCC members. Moreover, any modification to the existing structural system should be carried out carefully, taking into account the impact it may have on the overall structure.
Seismic damage is another challenge that RCC structures face. Structures located in earthquake-prone areas need to be designed and constructed to withstand seismic loads.
Lastly, corrosion due to penetration, commonly referred to as honeycombs, can affect the structural members’ strength and durability. Proper measures must be taken during the construction phase to prevent the formation of honeycombs and to address any existing ones.
Retrofitting Methods for RCC Structural Members
Fiber Reinforced Polymer (FRP) composites are a type of material that has been increasingly used in the field of civil engineering for its superior mechanical properties, durability, and corrosion resistance. One of the common applications of FRP composites in civil engineering is external plate bonding, which involves the use of FRP sheets or plates to strengthen and repair existing structures such as bridges, columns, and beams.
Another technique for reinforcing structures with FRP composites is through the use of Near Surface Mounted (NSM) FRP bars or strips. NSM involves embedding the FRP bars or strips into a slot cut into the surface of the structure and then bonding them to the structure using epoxy resin. This technique is often used in concrete structures such as slabs and beams to increase their flexural and shear strength.
Section enlargement is another technique that involves increasing the size of a structural member by attaching an FRP composite to its surface. This method is commonly used in columns and beams to increase their load-carrying capacity.
External post-tensioning is a technique used to strengthen concrete structures by applying tension to the concrete with the help of post-tensioning tendons. In this technique, FRP tendons are used instead of traditional steel tendons to enhance the durability and corrosion resistance of the structure.
Grouting is a technique used to fill gaps or voids in structures to improve their load-carrying capacity. In the context of FRP composites, grouting is often used to fill the gap between an FRP plate and the surface of a structure to increase its bond strength.
Epoxy injection is a technique used to repair cracks or voids in structures. In the context of FRP composites, epoxy injection is often used to repair the bond between an FRP composite and the surface of a structure. This technique involves injecting epoxy resin into the crack or void and then allowing it to cure to form a strong bond.
Factors Governing Selection of Retrofitting Methods
There are several important factors to consider when it comes to enhancing the strength of existing concrete structures. The first factor is the current strength of the concrete, as this will impact the magnitude of enhancement that can be achieved. Additionally, accessibility to work areas must be taken into account, as it can impact the ability to perform the necessary work.
Another key factor to consider is the cost of construction and maintenance, as enhancing the strength of existing concrete structures can be a costly endeavor. Time constraints may also play a role in determining the feasibility of such a project, particularly if there are tight deadlines or other scheduling considerations that must be taken into account.
Clearance issues may also arise, particularly if the structure in question is located in a tight or cramped space. This can impact the ability to access certain areas of the structure for maintenance or construction purposes. Seismic effect consideration is also an important factor to consider, particularly if the structure is located in an area prone to earthquakes or other natural disasters.
Finally, environmental aspects must also be taken into account, particularly if the construction or maintenance of the structure could have a negative impact on the surrounding environment. Overall, there are many factors to consider when enhancing the strength of existing concrete structures, and careful planning and consideration is necessary to ensure a successful outcome.
Retrofitting Strategy
Retrofitting of structural members involves ensuring that the members meet the required performance standards. To achieve this, it is necessary to specify the performance requirement for the member that needs to be retrofitted.
The retrofitting process should begin with an inspection phase, which includes assessing the structural element that requires retrofitting. Once the inspection is completed, the design of the retrofitting structure can be developed, followed by the selection of an appropriate retrofitting method.
It is necessary to evaluate the performance of the structural element based on the findings of the inspection work to determine whether it meets the required performance standards. If it does not, and retrofitting is desired, the design of the retrofitting structure should proceed.
When selecting a retrofitting method, it is essential to specify the materials to be used, the structural specifications, and the construction method. The performance of the structure should be evaluated after retrofitting to verify that it meets the required performance standards.
If it is determined that the retrofitting structure is capable of fulfilling performance requirements with the selected retrofitting and construction methods, the retrofitting work can be implemented.
How to Investigate and diagnose cracks?
When cracks appear in the reinforced concrete (RCC) structural members, it is important to identify the underlying cause of the cracks. If it is determined that the cracks occurred due to corrosion of steel, then additional field investigations and testing are necessary. This includes both destructive testing methods such as core testing as well as non-destructive testing methods like Rebound Hammer, Ultrasonic pulse velocity method, and rebar location.
To accurately assess the severity of the cracks, it is necessary to evaluate the degree of cracking, spalling of concrete cover, and corrosion of steel in each member. A classification system for cracks has been developed based on the width of the crack. This information is used to determine the appropriate repair and maintenance measures required to address the issues with the concrete structure. By identifying the root cause of the cracks and accurately assessing their severity, engineers can take appropriate steps to ensure the safety and longevity of the structure.
Table 1 Classification of Cracks based on Crack width
| Crack Width | Classification of crack |
| Upto 1mm | Thin cracks |
| 1 to 2 mm | Medium cracks |
| More than 2mm | Wide cracks |
The task at hand is to evaluate the condition of concrete, specifically looking at porosity, segregation, and thickness, as well as the condition of the cover. Additionally, it is important to specify the extent of damage to the reinforcement bars. Lastly, it is necessary to investigate any previous repairs and determine the cause of their failure, if any.
To begin with, the concrete needs to be assessed for its porosity, which refers to the amount of air pockets or voids in the material. Segregation, which is the separation of the concrete mixture into its individual components, is another factor to consider. The thickness of the concrete is also important, as this affects its overall strength and durability. Moreover, the condition of the cover must be examined, which refers to the protective layer of concrete that covers the reinforcement bars.
In addition to evaluating the condition of the concrete, it is crucial to determine the extent of damage to the reinforcement bars. These bars are typically made of steel and are used to provide additional strength to the concrete structure. Any damage to the reinforcement bars can significantly compromise the structural integrity of the building.
Finally, it is essential to investigate any previous repairs that have been made to the concrete structure. By doing so, it is possible to determine the cause of the failure of the previous repairs and avoid making the same mistakes. Overall, a thorough evaluation of the concrete and reinforcement bars is necessary to ensure the safety and longevity of the building.
Materials Used for Repair of concrete cracks
The following materials are generally used for repairing of cracks and rehabilitation of RCC structures :
1. Portland Cement
There are several methods for repairing gaps, pores, and cracks in concrete structures. One common technique involves injecting cement slurry into these imperfections, either with or without the use of polymers. This can effectively seal the damaged areas and prevent further deterioration.
Another approach to repairing damaged concrete surfaces is through the use of motor. This material can be applied with or without plasticizers and can serve as a replacement for concrete cover or surface coating. Motor is typically used for thin applications and can provide a smooth, durable finish.
For more extensive repairs, microcrete may be used. This involves guniting or shotcreting a layer of concrete over the existing surface. Microcrete can be applied as a replacement for concrete or as a cover for existing concrete. It is a versatile solution that can be customized to suit specific repair needs.
Finally, concrete can also be used to replace existing concrete. This material can be mixed with or without plasticizers, depending on the desired properties. It is a durable and long-lasting solution that can provide structural support and improve the appearance of damaged concrete surfaces.
2. Polymer modified concrete (PMC)
Polymer modified concrete and mortars are types of construction materials that are made by incorporating polymer latex, such as acrylates and SBR (Styrene Butadiene Rubber), into the mixture. These materials are known for their improved properties and performance compared to traditional concrete and mortars.
Polymer modified concrete and mortars are preferred over traditional materials due to their increased durability, strength, and resistance to cracking. The polymer latex acts as a bonding agent between the aggregates and the cement matrix, resulting in a stronger and more cohesive mixture.
Acrylates and SBR are two commonly used types of polymer latex in the construction industry. Acrylates are known for their excellent water resistance and high bonding strength, making them ideal for use in waterproofing applications. SBR, on the other hand, is known for its flexibility and ability to absorb energy, which makes it suitable for use in applications where impact resistance is important.
In summary, polymer modified concrete and mortars are a popular choice in the construction industry due to their superior properties and performance. Polymer latex, such as acrylates and SBR, play a crucial role in improving the durability, strength, and resistance of these materials to ensure they meet the demands of modern construction.
3. Epoxy Resins
The use of epoxy resins for injection or concrete repairs is a common practice in construction and maintenance industries. These resins can be used with or without the addition of filler materials such as quartz sand. When used with fillers, the resulting material can provide improved strength and durability, as well as enhanced resistance to wear and tear. Epoxy resins can be applied to concrete surfaces, either manually or with the help of specialized equipment, depending on the specific needs of the project.
In addition to epoxy resins, polymer resins are also used for concrete repairs. Similar to epoxy resins, these polymers can be used with or without the addition of filler materials. The use of fillers can improve the properties of the resulting material, such as its strength and resistance to abrasion. Polymer resins can be applied to concrete surfaces using a variety of methods, including injection, spraying, or manual application.
Overall, the use of epoxy and polymer resins, with or without fillers, provides a versatile and effective solution for concrete repairs and maintenance. The choice of which type of resin to use, and whether or not to use fillers, will depend on the specific needs and requirements of each project.
Diagnose Structural Damages and Their Repair
1. Retrofit Corroded Structural Member
When corrosion has begun, it is necessary to follow a specific process to address the issue. First, weak concrete must be removed, and the reinforcement should be exposed all around. This will allow for a thorough assessment of the damage caused by the corrosion.
After exposing the reinforcement, the rust on the steel should be cleaned off using wire brushes or sandblasting. Rust removers and rust preventers should then be applied to prevent any further corrosion from occurring.
If required, reinforcement should be added to supplement the rusted steel, and anchorage, such as shear connectors, should be used. A tack coat made of polymer or epoxy-based bonding material should be applied to provide a bond between the old and new concrete.
One of the patching techniques, such as polymer modified mortars, should be used to restore the concrete to its original surface level. This can be done with or without guiniting. Additionally, injection of cement slurry or polymer modified slurry or epoxy can be used to fill up pores, internal cracks, or honeycombing.
Finally, a suitable protective coating should be applied to ensure that the concrete is well protected from any future damage caused by corrosion. By following this process, corrosion can be addressed effectively and efficiently, and the integrity of the structure can be maintained.
2. Retrofit Severely Damaged Concrete
In situations where the original concrete of a structure is in very bad condition and injection grouting is unable to rehabilitate the section to withstand the required loading, an alternative solution is to provide RCC jacketing. This involves placing a layer of reinforced concrete around the section to strengthen and support it.
To carry out the RCC jacketing, the first step is to remove any weak or damaged concrete from the section. This is done to ensure that the new layer of concrete can adhere properly and provide the required support. Once the old concrete has been removed, the surface must be cleaned thoroughly, and any rust on the steel reinforcement should be removed. Rust removers and rust preventers are then applied to the steel to prevent further corrosion.
To further reinforce the section, additional steel is placed all around it. The required formwork is then provided to ensure that the new concrete is poured in the correct shape and size. Before pouring the new concrete, a polymer-based bonding coat is applied between the old and new concrete to ensure proper adhesion.
The concrete used for the jacketing must have the required thickness, grade, and workability, and should be admixed with plasticizers to enhance its properties. Once the concrete is poured, it is allowed to cure properly before any further loading is applied to the structure.