Retrofitting of RCC Structural Members, Types, Selection, and Procedures

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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.

The retrofitting process shall start with investigation and diagnosis of cracks and then applying suitable retrofitting technique and compatible materials.

There are several techniques which are used to retrofit structural members such as section enlargement, external plate bonding, external post-tensioning, grouting, and fibre reinforced polymer composites. Based on the severity of the damage and required capacity to be regained, a proper retrofitting technique is specified and implemented.

When do RCC Structural Members Need Retrofitting?

There are several problems that RCC structural members experience and needed to be tackled. Common problems include:

1. Structural cracks.
2. Damage to structural members.
3. Excessive loading.
4. Errors in design or construction.
5. Modification of structural system.
6. Seismic damage.
7. Corrosion due to penetration- honey combs

Retrofitting Methods for RCC Structural Members

1. Fiber Reinforced polymer(FRP) composites.
2. External plate bonding.
3. Near Surface Mounted FRP bars or Strips.
4. Section enlargement.
5. External post-tensioning.
6. Grouting.
7. Epoxy Injection

Factors Governing Selection of Retrofitting Methods

1. Existing concrete strength.
2. Accessibility to work areas.
3. Magnitude of strength to be enhanced.
4. Cost of Construction and maintenance.
5. Time constraints.
6. Clearance issues.
7. Seismic effect consideration.
8. Environmental aspects.

Retrofitting Strategy   

1. Specify the performance requirement for the structural member that needs to be retrofitted.
2. Then, set an overall plan from inspection phase to retrofitting method selection, the design of retrofitting structure, and execution of retrofitting work.
3. After the finalization of plan, inspect the structural element that needs to be retrofitted.
4. Evaluate the performance of the structural element based on the findings of the inspection work.
5. Check whether the structural element fulfills performance requirements.
6. If the structure does not fulfill performance requirements, and if continued use of the structure through retrofitting is desired, proceed with the design of the retrofitting structure.
7. Select an appropriate retrofitting method.
8. Specify materials to be used, structural specifications and construction method.
9. Evaluate the performance of the structure after retrofitting and verify that it fulfills performance requirements.
10. If it is determined that the retrofitting structure is capable of fulfilling performance requirements with the selected retrofitting and construction methods, implement the retrofitting work.  

How to Investigate and diagnose cracks?

1. After the appearance of cracks in RCC structural members, it is necessary to diagnose the root cause of cracks.
2. If it is ascertained that the cracks in concrete have occurred due to corrosion of steel, further field investigation and testing are required such as destructive (core testing) and non-destructive testing (Rebound Hammer, Ultrasonic pulse velocity method and rebar location etc.).
3. Determine the degree of cracks, spalling of concrete cover and corrosion of steel for each member. The following table gives the classification of crack with crack width:

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

4. Determine the condition of concrete i.e. porosity, segregation, and thickness and condition of cover.

5. Specify the extent of damage to the reinforcement bars.

6. Investigation about failure of previous repairs if any.

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

Cement slurry injections with or without polymers to seal the gaps, pores or cracks, Motor with or without plasticizers for replacement of concrete cover or surface coating, Microcrete: Guniting / shotcrete as replacement of concrete or cover concrete, and Concrete with or without plasticizers as replacement of existing concrete.

2. Polymer modified concrete (PMC)

Polymer modified concrete or mortars with the help of polymer latex such as acrylates and SBR (Styrene Butadiene Rubber).

3. Epoxy Resins

Epoxy resins with or without addition of filler materials such as quartz sand for injection or concrete repairs. Polymer resins with or without addition of filler materials for concrete repairs.

Diagnose Structural Damages and Their Repair

1. Retrofit Corroded Structural Member

If corrosion has started, following process is adopted:

1. Remove weak concrete and expose reinforcement all around.
2. Clean the rust of steel by wire brushes or sandblasting.
3. Apply rust removers and rust preventers.
4. Provide reinforcement to supplement rusted steel if required with anchorage i.e. shear connectors.
5. Apply tack coat (bonding coat to provide a bond between old concrete and new concrete) of polymer or epoxy based bonding material.
6. Use one of the patching technique to restore concrete to the original surface level. Polymer modified mortars are very good. This can be used with or without guiniting.
7. Injection of cement slurry or polymer modified slurry or epoxy to fill up pores or internal cracks or honeycombing.
8. Apply a suitable protective coating.

2. Retrofit Severely Damaged Concrete

In case the condition of original concrete is very bad and injection grouting is not able to rehabilitate the section to take the required loading, RCC Jacketing of concrete section is to be provided.

1. Provide the required supporting system to the structure.
2. Remove weak concrete.
3. Clean the surface and clean the rust of steel.
4. Apply rust removers and rust preventers.
5. Provide additional steel all around the section.
6. Provide required formwork.
7. Provide a polymer based bonding coat between old and new concrete.
8. Place the concrete of required thickness and grade and workability admixed with plasticizers.