Retrofitting RCC Grid by Jacketing

Retrofitting of RCC structural members is carried out to regain the strength of deteriorated structural concrete elements and to prevent further distress in concrete. This case study gives a quick and easy reference on the effective retrofitting and rehabiltation technique of a structurally damaged RCC beam open grid, by way of Jacketing.
E S Jayakumar (M.Tech – Structural & MIE, FIAStruct E.), Superintending Engineer, BSNL Civil Wing

RCC structural elements are subjected to deterioration and corrosion due to various reasons such as exposure to aggressive marine environment, thermal cracking, moisture ingress, freezing, thawing, corrosion of reinforcement (induced by chlorides as well as carbonation), sulphate attack, and so on. Poor construction practices, inadequate concrete cover to reinforcement, use of inferior materials, faulty formwork, wrong mix proportioning, improper placing of concrete, lack of proper supervision, insufficient compaction, lack of proper curing, etc. also add to the corrosion of concrete elements such as slabs, beams, columns, and fins.

The retrofitting of the partially damaged RCC base grid of a roof top communication structure is taken as a case-study in this article. The RCC grid structure comprised of stub columns with beams grid situated over it on the terrace of a framed structure building. Due to the highly corrosive marine environment and other reasons, the concrete was seen damaged to such an extent that a few reinforcement bars and the stirrups of beams had got exposed after spalling of the cover took place. Figure 1 shows one of the beams of the grid that has been subjected to extensive damage. The reinforcement bars were also seen corroded to some extent.

Process of Retrofitting: Since this was an open grid structure without any slab, the treatment all around the grid was possible. It was decided to use the jacketing technique to restore the entire RCC grid. Jacketing is the process of fastening durable materials over damaged concrete to a definite thickness. The materials used for jacketing, in general, are concrete, ferrocement and Carbon Fibre Reinforced Polymer (CFRP).

Jacketing restores the structural strength of structural concrete to good extent, protects the reinforcement from exposure to harmful elements and aggressive environment and improves the appearance of the original concrete surface. We can add additional reinforcements also as per requirement, which will be confined within the compatibly chosen thickness of the jacketing. As a rule, if more than 15% of the reinforcement is corroded, we have to incorporate additional reinforcement bars, which can be welded to the existing bars, after treating them. The activities involved in the rehabilitation can be broadly summarised as follows:

• Provide the required supporting system to the structure
• Remove weak concrete
• Clean the surface and clean the rust on steel by applying rust removers and rust preventers
• Provide additional steel all around the section
• Provide anti-corrosive coating
• Provide required formwork
• Provide a polymer-based bonding coat between old and new concrete
• Place micro concrete of the required thickness
• Providing Required Support: In case the concrete and reinforcement bars are heavily damaged and the concrete around the reinforcement bars is to be removed, then it is advisable to prop the structural element for safety. In our case, this was not found necessary.
• Remove weak concrete: All the weakened, deteriorated, delaminated or unnecessary portions of concrete / cement plaster were removed prior to the repair. The impact exerted on the structure during this operation should be mild, so as not to cause excessive damage or even collapse.
• Clean surface and remove rust of steel: A single component liquid that removes rust and prevents further rusting of steel reinforcement/steel structures by providing a passivating coat was used. Substrate should be free from oil, dirt, and grease. Remove cement skin, loose particles etc. on the concrete surface. Cavities, pin holes should be levelled. For heavily rusted surface, first clean mechanically by wire brushing, sand blasting etc., depending on the extent of corrosion. Rust remover can be applied with brush, cotton waste swab, or spray gun on the affected surface which should be dry before application.
• Provide additional steel all around the section: As discussed earlier, if more than 15% of the reinforcement is rusted, the loss may be compensated by providing additional steel by welding or tying to the existing bars or by U-clamps. But, in the present case, the extra longitudinal reinforcement and stirrups could be tied to the beams as shown in Fig 2.


Figure 2: Additional Reinforcement & Anticorrosive Coating


• Provide anti-corrosive coating: A two component, low solvent, zinc rich epoxy resin-based protective coating and anti-corrosive primer for steel was used. The two parts must be thoroughly mixed in the prescribed ratio, using a low-speed electric stirrer for about 3 to 5 minutes until smooth and even consistency is achieved. The substrate must be clean, dry and free of all contaminants such as rust, dirt, oil, grease, coatings and surface treatments, etc. The mixed material is to be applied to the substrate by an appropriate brush. The prepared surface is shown in Fig 2.
• Provide required formwork: The formwork necessary for jacketing was provided, keeping in mind the thickness of the layer of micro-concrete being applied. In this case, the soffit of the beams was first treated, followed by the two sides after an interval. The formwork was provided according to this sequence.
• Provide a polymer-based bonding coat between old and new concrete: SBR polymer modified cementitious bonding coat was used. SBR (Styrene Butadiene Rubber) is a synthetic rubber emulsion which when added to cement slurry / cement mortar / concrete / grout provides good adhesion and water resistance. Bonding coat with SBR shows extremely good bonding to bases like concrete, stone, brick etc. It comes in the form of a milky liquid, and is fully soluble in water, and is to be added directly to the gauging water of slurry / mortar / concrete / grout. The bonding coat is to be prepared by mixing SBR, water, and cement in the ratio 1:4:8, by volume. (The CPWD DSR recommends the following composition: SBR Polymer @10% of cement weight, modified cementitious bond coat @ 2.2 kg cement per sqm of surface area).

The bonding coat is applied on the repair surface by a stiff nylon bristle brush. The surface should be thoroughly cleaned by brushes and by blowing air from hand operated pump, prior to applying the bonding coat. A single coat of bonding agent is to be applied to obtain a thin layer. When the bond coat is still fresh and sticky, the mortar or concrete is applied, approximately at one hour interval. During application, the mixture of bonding coat needs to be continuously stirred to prevent the cement particles from settling. Prepared material must be used within 20–30 minutes depending upon temperature, humidity, etc.

• Place the micro concrete of required thickness: Retrofitting is carried out using ready-to-use factory designed pourable, non-shrink, repair micro-concrete with selected cement, aggregate, and other chemicals. Micro-concrete is a dry, ready-mix cementitious composition formulated with additives, which imparts controlled expansion characteristics in the plastic state along with reduced demand of water.

The repair concrete should have low shrinkage value. If it shrinks more than the substrate, then it results in development of shrinkage stresses and strains that can lead to delamination due to de-bonding. Shrinkage cracks can also be developed at the interface. The advantages of using ready-made micro-concrete are:

• High fluidity enables placement without vibration
• Expansion system compensates for shrinkage settlement in plastic stage
• Easy to mix and apply
• High flow characteristics and rapid strength development
• Good bonding with old concrete

Micro concrete is to be added to water and mixed till a pourable consistency is obtained with recommended water–powder ratio as per manufacturer’s specifications (around 0.15 litres of water per kg of powder). Put around 80 to 90% of required water in the mixing drum, followed by the micro concrete, and then add the balance water and mix thoroughly. This is then poured into the gap of specified thickness between old concrete and formwork. In the present case, average 75mm (3”) thick retrofit was chosen.


Alternatively, SBR Polymer modified concrete can be used for jacketing, which is more cost–effective. Concrete, having the specified minimum characteristic strength, preferably not less than M25, with ordinary Portland cement, coarse sand and graded stone aggregate of 10mm maximum size in proportion as per design criteria, with specified average thickness all-round existing core of RCC member, may be used for the retrofit.

Proper compaction is to be ensured for the concrete poured. SBR of approved make must be added @ minimum 2% by wt. of cement used. If the thickness of retrofit required is less, the repair surface can be plastered with SBR polymer modified cement mortar in proportion of 1:4 (1 cement: 4 graded coarse sand with polymer added @ minimum 2% by wt. of cement used), as per CPWD DSR.

Finally, the retrofit is to be cured properly. In places where continuous water curing is difficult, curing compounds of proven efficacy may be used.

A look at other materials used for rehabilitation:

• Ferrocement: Ferrocement membrane used for rahabilitaion of moderately distressed RCC elements is very effective due to its crack arrest mechanism, high tensile strength, fire resistance, water-proofing quality, and toughness. Ferrocement is composed of cement mortar reinforced with closely spaced layers of continuous and smaller size diameter hot dipped galvanized wiremesh section in minimum 2 layers. Hexagonal chicken mesh in 2 or 3 layers is also used frequently. The wiremesh is connected to the repair surface by using U-clamps or epoxy bonding system. The cement mortar may be admixed with plasticizers and polymers for sealing pores. The treated surface is then adequately cured.
• Carbon Fibre Reinforced Polymer (CFRP) Wraps: The confinement of concrete columns and beams by wrapping with Carbon Fibre Reinforced Plastic (CFRP) sheets is an efficient technique for structural strengthening, and is globally popular. CFRP is a type of composite material that consists of carbon fibre and polymer. The carbon fibre provides the strength and stiffness while the polymer acts as cohesive matrix to protect and hold the fibres together. The principal advantages of this technique are:
• Alkali resistance.
• Corrosion resistance due to which it is widely used for corrosion control and rehabilitation of reinforced concrete structures.
• Low thermal conductivity.
• High strength to weight ratio which eliminates requirement of heavy construction equipment and supporting structures.
• Short curing time.
• CFRP has high ultimate strain.
• High fatigue resistance so no degradation and less frequent maintenance.
• CFRP is bad conductor of electricity and is non-magnetic.
• Due to its lightweight, prefabricated components in CFRP can be easily transported.

Thus, CFRP Wrap provides remedies for many problems associated with the deterioration of infrastructure such as bridges and buildings. The listed merits notwithstanding, the drawback of CFRP Wrap application in India include a dearth of genuine applicators and the higher cost of application compared to Jacketing and Ferrocement techniques.

References

• CPWD Handbook on Repair and Rehabilitation of RCC Buildings
• Varghese P.C., Maintenance, Repair & Rehabilitation & Minor Works of Buildings, Prentice Hall of India
• Modi P.I & Patel C.N., Repair and Rehabilitation of Concrete Structures, Prentice Hall of India
• Bhattacharjee J., Concrete Structures: Repair, Rehabilitation and retrofitting, CBS Publishers & Distributors
• Vidivelli B., Rehabilitation of Concrete Structures, Standard Publishers Distributors
• Vidivelli B., Advanced Concrete Technology and Ferrocement, Standard Publishers Distributors
• CPWD Delhi Schedule of Rates (2018), Volume-II