RETROFITTING OF RCC STRUCTURAL MEMBERS

Retrofitting of RCC structural members is necessary to prevent further distress in concrete. The retrofitting of RCC members should start with investigation and diagnosis of cracks and then by applying suitable retrofitting measures. Following are the steps involved in this process:

1. Investigation and diagnosis of cracks:

(i) After the appearance of cracks in RCC structural members, it is necessary to diagnose the root cause of cracks. If it is ascertained that the cracks in concrete has 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.).

(ii) Determine degree of cracks, spalling of concrete cover and corrosion of steel for each member. Following table gives the classification of crack with crack width:

Crack Width	Classification of crack
Upto 1mm	Thin cracks
1 to 2 mm	Medium cracks
More than 2mm	Wide cracks

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

(ii) Determine the extent of damage to the reinforcement bars.

(iii) Investigation about failure of previous repairs if any.

2. Repair of concrete cracks:

(i) Materials:

Following materials are generally used for repairing of cracks and rehabilitation of RCC structures.

(a) 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: Guiniting / shotcrete as replacement of concrete or cover concrete.
• Concrete with or without plasticizers as replacement of existing concrete.

(b) Polymer modified concrete (PMC)

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

(c) 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.

(d) Ferro-cement concrete: Ferro-cement is a composite material of reinforcement (GI woven wire mesh) and cement sand mortar modified with polymers or other chemicals. Ferro-cement concrete is used to replace cover concrete due to rusting.

(e) Selection of material: Selection of materials depends upon test data

2. Concrete repair methods:

• In case corrosion of steel has not started but carbonation of concrete has started and cracks are thin, coating of polymer or epoxy resins or polymer modified mortars prevent / retard entry of moisture, CO2 and O2 etc. such coatings prevent concrete and prevent corrosions for a period of 10 to 15 years.
• If corrosion has started, following process is adopted:

(i) Remove weak concrete and expose reinforcement all around.

(ii) Clean the rust of steel by wire brushes or sand blasting

(iii) Apply rust removers and rust preventers.

(iv) Provide reinforcement to supplement rusted steel if required with anchorage i.e. shear connectors.

(v) Apply tack coat (bonding coat to provide bond between old concrete and new concrete) of polymer or epoxy based bonding material.

(vi) 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.

(vii) Injection of cement slurry or polymer modified slurry or epoxy to fill up pores or internal cracks or honey combing.

(viii) Apply suitable protective coating.

• 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.

(i) Provide the required supporting system to the structure.

(ii) Remove weak concrete.

(iii) Clean the surface and clean the rust of steel.

(iv) Apply rust removers and rust preventers.

(v) Provide additional steel all around the section.

(vi) Provide required formwork.

(vii) Provide polymer based bonding coat between old and new concrete.

(viii) Place the concrete of required thickness and grade and workability admixed with plasticizers.

• Chajjas or other thin members should be completely replaced or repaired with ferro-cement concrete.