Heritage means something that is handed over from the past as a tradition and includes buildings, artefacts, structures, open and excavated areas, natural features and precincts that are of historic, aesthetic, architectural or cultural significance. As the heritage structures are old, challenge before conservationist, architects and engineers is to conserve them in original conditions, structurally safe simultaneously providing services and facilities which might not have been originally designed but required now due to change of use of such structures keeping their original character intact. These may include essential services like fire safety measures, water supply, sewerage, lighting, air-conditioning, toilets, offices and accessibility provisions.

Heritage buildings may be world heritage buildings declared by UNESCO, monumental buildings of Archaeological Survey of India (ASI), or heritage structures declared by state governments/local bodies. India has 40 world heritage sites inscribed by UNESCO, legally protected pursuant to the Law of War under the Geneva Convention. Considerable care and protection are required of these sites as these are largely visited by international tourists carrying country’s reputation.

Further, there are more than 3650 ancient monuments and archaeological sites of national importance belonging to different periods, ranging from the pre-historic period to the colonial period scattered all over the country maintained by ASI.

Then there are heritage structures declared by central and state governments based on their own criteria for heritage structures. These are generally classified as Grade I, Grade IIA and B and Grade III in descending order of importance. Grade I comprises buildings and precincts of national or historic importance, embodying excellence in architectural style, design, technology and material usage and/or aesthetics associated with a great historic event, personality, movement or institution and Grade II (A and B) of regional or local importance possessing special architectural or aesthetic merit, or cultural or historical significance though of a lower scale than Grade I. Grade-III heritage structures are of town importance that evoke architectural, aesthetic, or sociological interest through not as much as of Grade I or II. Grade I heritage structures richly deserve careful preservation, Grade II intelligent conservation and Grade III also intelligent conservation though on a lesser scale than Grade II with special protection to unique features and attributes. Thus, intelligent conservation is the primary requirement for any conservation irrespective of the classification.

No intervention is permitted in Grade I structures either on exterior or interior of the heritage building or natural features unless it is necessary in the interest of strengthening and prolonging the life of the buildings/or precincts or any part or features thereof. For this purpose, absolutely essential and minimum changes are allowed in conformity with the original. In Grade II A, internal changes and adaptive re-use may by allowed ensuring the conservation of all special aspects for which it is included in Grade II. In Grade II B, in addition to Grade II A, extension or additional building in the same plot or compound could in certain circumstances, be allowed provided that the extension/additional building is in harmony with the existing heritage building or precincts especially in terms of height and façade. In Grade III, external and internal changes and adaptive reuse is generally allowed. Development permission for changes can be given on the advice of the Heritage Conservation Committee so that new buildings are taken considering the heritage character of the precincts.

Restoration And Reconstruction
According to Burra Charter, conservation includes the processes of retention or reintroduction of a use, retention of associations, meaning; maintenance, preservation, restoration, reconstruction, adaptation and interpretation and commonly includes a combination of more than one of these. Conservation may also include retention of the related places and related objects that make cultural significance of a place. While preservation protects fabric without obscuring evidence of its construction and use, restoration and reconstruction should continue to reveal culturally significant aspects of the place. Preservation process is applied where the evidence of the fabric is of such significance that it should not be altered while restoration is appropriate only if there is sufficient evidence of an earlier state of the fabric. Reconstruction is appropriate only where a place is complete through damage or alteration, and only where there is sufficient evidence to reproduce an earlier state of fabric or as a part of a use/practice that retains the cultural significance of the place. New work such as additions or other changes to the place may be acceptable where it respects and does not distort or obscure the cultural significance of the place through consideration of its siting, bulk, form, scale, character, colour, texture and material.

Indian Institute of Adavanced Studies, Shimla

Conservation Practice
The studies related to physical, documentary, oral and other evidence, drawings, skills available and disciplines related to work have to be undertaken before planning of conservation. Individuals, groups and associations connected to the work/place as well as those involved in its management should be involved to contribute and participate in understanding cultural significance of the place. The impact of proposed changes, including incremental changes, should be assessed with reference to the heritage and modifications required to retain cultural significance without disturbance of significant fabric for study. Records associated with the conservation of a place should be preserved. New decisions should respect and have minimal impact on the cultural significance of the place. Thus, conservation practice should include understanding the place, assessing cultural significance, identification of all factors and issues, development of policy, preparation of management plan and its implementation. Finally, the results are monitored and plan reviewed.

Case Studies
Case Study 1: Indian Institute of Advanced Studies (IIAS): Some Architectural Features
The building that houses the institute was originally built as a home for Lord Dufferin, Viceroy of India from 1884–88 and was called the Viceregal Lodge. It housed all the subsequent viceroys and governor generals of India. It occupied observatory hill, the second highest point in Shimla. The hill was levelled and flattened for the construction. Light blue lime stones and grey sandstones were used and transported to the hill by mules.

The building is designed in Indo-Gothic style by Henry Irwin of the then Public Works Department, now considered as CPWD. The building was provided with electricity and sophisticated fire fighting mechanism through wax-tipped water ducts which is functional even today. The British brought some of their latest technologies to Shimla while building it. Many historic decisions have been taken in the building during the Indian independence movement such as ‘Simla Conference’ held in 1945 and the decision to carve out Pakistan and East Pakistan from India taken in 1947. Some of the important architectural features of this building are:

Switches in brass. Fire fighting mechanism through wax-tipped water ducts which is functional even today. Well lit corridor at entrance with glass roof, beautifully carved balustrades, arches and panelling. Prisms in ceiling with one row fixed and one hanging. This system transfers maximum light inside falling from any angle. The 19th century clock needs winding once in 2 days, displays correct time and displays actual position of moon. Drier room using the technology for drying clothes. Hot air comes up through perforated floor having hot water pipes running below and dries clothes spread in the room. Rectangular rain water pipes (RWP) in harmony with stone shape and colour. One damaged pipe was got specially fabricated for replacement to maintain the heritage character. Two other pipes also are matched with colour, retain the beauty near entrance due to no leakage problems even today. The fire place with decorative wooden panelling all around and decorative brass work in front and decorative tables on both sides demarcate the heritage character. Rich wooden panelling, false ceiling, brass lights and chandelier denoting the heritage character. Interior of the library with decorative ceiling work and merging arched wall with finely detailed wooden railing demarcates heritage character. The main entrance with terrazzo flooring and the heritage lights in ceiling at entrance and hanging down in corridor. Stone jambs add to the strong heritage character of the building. A bell made of eight metals, presented by the king of Nepal was available for display upto April 2010. The lawn in front of the lodge is above a rain water harvesting tank designed and constructed during 19th century. It was the first electrified building in Shimla. This electricity was produced by the steam engines brought from Britain.

The splendid heritage features have to be preserved. CPWD is planning to carry out the conservation in a phased manner preserving the original character and past glory of the building.

Case Study 2: Gortan Castle Building, Shimla
The building was commenced during November, 1901 and completed during May, 1904. The original design of this building was conceptualized by Sir Swinton Jacob with the expenditure of Rs 13,42,901. Presently, it houses the office of Pr. Accountant General (A and E) and Accountant General (Audit) of Himachal Pradesh. It has many special features which resemble to the Neo-gothic and Rajasthani style, some of which are summarized as under:

1. Load bearing stone masonry walls in lime mortar of varying thickness.
2. Use of composite masonry stone walls with coursed rubble on exposed face and random rubble on hidden face.
3. Roofing in high pitched Nainital pattern iron sheets.
4. Roof canopies.
5. Flag post and spires on roof.
6. Stone ornamental balconies, jharokhas, railings, brackets, facia, cornices, coping, soffits and jambs.
7. Wooden floors on steel girder in attics.
8. Glass roof in central staircase.
9. Wooden spiral service staircases.
10. Wooden ornamental brackets and eve boards.
11. Cast iron railings with combination of wood work on central staircase.
12. Wooden ornamental trusses on central staircase.
13. Jack-arch intermediate floors.
14. Central courtyards.

The brackets are made in a single piece of sand stone. The ornamental carvings are provided on these brackets further enhancing the aesthetics of the structure. Balconies are having ornamental stone work in Rajsthani style on jalies, balusters, handrails and brackets. Ornamental copings and stone ornamental arches/mehrabs at number of locations enhance the aesthetics of the structure. Stone ornamental facia on doors, windows and many other locations on the structure such as central courtyard, external façade of corridor wall etc., stone ornamental cornice and sand stone chimneys add to the heritage look of the structure.

Gorton Castle Building, Shimla

Major fire broke out on 27-28th January, 2014 causing massive damage to the building. Top two floors including roof had been totally gutted in fire causing severe damages including structural damages to most of the components of the building. All iron trusses, roof sheets, iron girders, steel tables, furniture and various other steel items were totally melted and distorted. The table glass and other glass items also melted. Stone masonry walls expanded and disintegrated. Stones got fragmented and are getting chipped off easily showing sign of very low residual strength. All wooden items such as wooden trusses, wooden boarding under roof sheets, wooden plank floors and false ceiling were totally gutted in fire.

Re-establishing the office functioning from the left out portion of the building was the main concern. Temporary electric supply was provided to the ground floor first after taking necessary safety precautions. 80% of the left out portion was made re-operational within next 3 months. In order to make ground and first floor re-operational, repairs to doors and windows, painting, distempering, removal of hazardous materials, making of water supply and sewerage lines re-functional and many other connected activities were carried out including temporary roofing on the entire structure to make it safe against any further damage.

This building is listed as heritage building by Himachal Pradesh Government and Ministry of Housing and Urban Affairs (MoHUA), Government of India, New Delhi. To take care of all such heritage buildings falling under ambit of MoHUA, a dedicated heritage cell under MoUHA is functioning. After inspection of the building and considering retrofitting measures suggested by CBRI Roorkee and original drawings of the British architects prepared during original construction, the followings recommendations are made to retain the heritage character of the existing building:

• Walls to be reconstructed using matching stones with lime/cement mortar.
• Similar type of floor finishes as already existing i.e. mosaic/cement concrete flooring.
• Minimum amount of false ceiling to be used. Items for false ceiling proposed are calcium silicate/aluminium perforated/aluminium strips.
• All the doors to be built as per original design and specifications.
• Door/window frames and shutters, staircase roofing and railing, eve boards, jalis, cornices and arches are to be provided matching to the existing ones.
• All toilets with modern fixtures and tiles.
• Energy efficient lighting and central heating system to be redesigned and provided.
• Fire place to be restored, but not to be used.
• Open court yards to be maintained. However all round chhajjas added in court yard are to be redesigned to merge with the building features.
• Chimney’s feature to be retained for aesthetics but vent pipes to be sealed.
• All external walls to have exposed stone finish as existing.
• No lift available in the existing building. Machine less/capsule type lifts with features matching to building features may be provided.
• Provisions to be made for barrier free norms, rain water harvesting and DGUs.
• All services may be redesigned keeping visual aesthetics in mind. A well documented comprehensive scheme to be developed for the entire building.
• All unauthorized constructions are to be removed. Original building external envelope to be reclaimed. A clear 6 m wide access all round the building for fire tenders should be provided.
• Various features such as cornices, balconies, jharokhas, eves board, jams, sills, soffits etc to be redeveloped in harmony with the existing ones.
• Appropriate seismic retrofit measures to be provided as per recommendations of CBRI, Roorkee.
• Additional fire protection measures to be adopted during restoration of the building as per prevailing norms.

In addition to above, there is a requirement to cover the open courtyards and also to cover the open corridors on top floor/terrace level. It is very challenging as the top floor has many floor levels and many attic floors. Providing roofing to take care of rain and snow disposal by covering the entire building having flooring at various levels is a herculean task. The courtyards which were previously used for rain and snow disposal are also to be covered. It has been decided to prepare a study model with roofing in transparent material to see all the floor levels below and also to study the rain and snow disposal. It has also been decided by the heritage cell to remove all shabby looking chhajjas, grills and glasses provided for protection from rain after covering the courtyard. The existing toilets of the building are being re-planned and reconstructed at the same location.

Case Study 3: Western Court, New Delhi
Western Court at Janpath, New Delhi was built during pre-independence period as a hostel for legislative councillors of Imperial Delhi. It is a Grade II heritage Building. As per bye-laws, internal changes by and large may be allowed in Grade II A heritage building subject to strict scrutiny. In Grade II B, in addition to above, extension or additional building in the same plot or compound could in certain circumstances be allowed provided that extension or additional building is in harmony with the existing heritage building(s) or precincts especially in terms of height and façade. Accordingly, design of new constructed 4-storied Annexe building reflects the built form of the existing building in terms of linear planning, symmetry, number of storeys and classical character of the building and green ambience .

LOP, Western Court Existing Building	LOP, Western Court with New Annexe Building

Within height of 3 floors of existing building, new construction has been worked out for 4 floors. Two basements are provided which cater to parking requirements of existing and new constructed Annexe building. Earlier only existing surface parking could serve the purpose. Other features are as given below:

• Plot area : 31,408.45 Sqm.
• Permissible ground coverage : 9422.53 Sqm.(30%)
• Total ground coverage (Existing + proposed) : 5891.51 Sqm (18.75%)
• Total built up area (Existing + proposed) : 19036.41Sqm (60.6%)
• Permissible FAR : 120
• Proposed FAR : 60.6
• Parking required : 381 ECS (2 ECS/100 Sqm.)
• Parking provided : 381 ECS (Open: 195, Basement: 186)

Main character of the existing building has been retained in newly constructed building as shown in some of the photographs. The upper row is showing features of the existing and lower row of the proposed building.

Western Court Existing Building	New Annexe Building

The main architectural character of the building has been retained in the newly constructed building. Due to ground area used for construction of new building, two basements are provided to cater the parking requirement of both the buildings. The entrance porch not provided in existing building has been provided in the new building using the architectural features of the existing building. Hard area at entrance of existing building has been replaced by beautiful landscaped garden enhancing the beauty of the area. Arched opening with keystone has been retained but proportions of rectangular openings have been changed, also horizontal member has been added reducing the grandeur character in the new building. Balustrades have been replaced with modern glass lowering the heritage character. Although, there is change in joints between column and slab and column and beam but the architectural character is not altered. Change in door detail does not change the exterior character of the building. Although detailing of projection at roof level differs from the original existing detail and would have looked much better if followed in new building but does not lower the main architectural character. Solar PV system provided on rooftop of existing as well as new building is the need of the hour and is not visible.

Conclusions

• Location of Heritage building increases the pride of residents in their community and provides good ambience.
• Conservation, restoration and reconstruction measures are interconnected, so that, according to the circumstances, they may be carried out one after the other or simultaneously.
• Use of inappropriate methods cause great deal of unintentional aesthetical and technical damage.
• Written and photographic records of restoration work can prove to be very useful for future reference.
• Whenever a building is listed as heritage building, changes carried out prior to listing, which don’t match to the heritage character of building must be reversed for retaining its heritage character.
• Good technology used in heritage buildings may be reconsidered for providing in modern buildings including use of prisms as used in IIAS Shimla and exposed pipes without leakage even after more than 100 years of their use.
• Architectural features and their effect must be studied carefully and appropriately so that essence of heritage character is not lost.

References

1. The Burra Charter (2013). The Australia ICOMOS Charter for Places of Cultural Significance, Australia. Incorporated International Council on Monuments and Sites.
2. Batra, Usha (2018). Role of Architecture in Heritage Conservation and Restoration, Preliminary publication of Indian Buildings Congress, 26(1).
3. Soni, K M (2018). Preservation of Heritage Structures”. Preliminary publication of Indian Buildings Congress,
   26(1), 7-18.
4. Soni, K.M. & Batra, Usha (2020). Global and Local Retrofitting of Buildings, CE&CR, 33(10),
   46-51.
5. DPR of Gorton Castle building at Shimla by CPWD
6. DUAC report of western court building, Janpath, new Delhi
7. https://en.wikipedia.org/wiki/Indian_Institute_of_Advanced_Study

Anchoring is very useful technique for repair and rehabilitation as it provides adequate strength and helps in minimising generation of construction and demolition waste. Anchoring is also used in new works in many applications for fixing structural and non structural members.

Anchoring is used in seismic retrofitting of masonry structures in which seismic belts are installed with the existing substrate to provide unity to the structure and also in RCC structures to achieve unity of additional section to be provided with the existing one. Thus, anchors have very important role in repair and retrofitting of civil engineering structures.

Types Of Anchors
There are two types of anchors as

• Mechanical anchors
• Chemical or adhesive or resin anchors

Three basic principles are applied in the anchoring system i.e. friction, keying and bonding. Sometimes a combination of these may also be used. Mechanical anchoring system is based on friction or keying principle while chemical anchoring is based on bonding principle.

Mechanical anchors are categorised as expansion and non-expansion anchors. Those expansion anchors expanding by tightening are called torque expanding anchors such as stud anchors, shield anchors and sleeve anchors suitable for use in cracked and non-cracked concrete depending on the anchor type. Anchors expanded by the displacement of an expander plug are known as deformation controlled anchors, commonly used in overhead applications for the suspension of threaded rods to support mechanical and electrical systems.

Non expansion anchors include undercut and self tapping screw anchors. Undercut anchors are installed mainly by mechanical interlock provided by an undercut in the concrete.

Expansion anchors are generally unsuitable for use in weaker base materials such as brickwork and stonework as they are likely to crack or crush the brickwork/stonework during expansion of the anchors. Chemical anchors are suitable in such cases.

Chemical anchors are of capsule and injection resin type. The size of the stud or steel members in chemical anchoring is to be selected as per the design requirements. They are used with resins and mortars for the bond into drilled holes in concrete or masonry. The capsules contain a predetermined quantity of resin for predetermined drilled hole. Chemical anchoring is obtained through pre-packed injection resin systems containing two chemicals mixed in the nozzle.

Cleaning of hole is extremely essential in chemical anchoring as chemical anchors work on the principle of bondage between base material and chemicals though few producers claim to have developed the anchors that do not require cleaning of the hole. Chemical anchoring system also requires adequate curing time before they are loaded while mechanical anchors can be loaded almost immediately.

Applications Of Mechanical And Chemical Anchors
Mechanical anchoring system is used in various applications for light, medium and high loadings. These are also suitable in steel and RCC structures. Heavy duty anchors are used for heavy loadings both in non cracked and cracked concrete.

Medium and light duty anchors are used for RCC in facades, curtain walls, ceilings, angles, tracks, and electric installations etc.

Chemical anchors are suitable for cracked and un-cracked concrete, brickwork, block work, stone work etc. Thus, chemical anchors are used except for very heavy loading requirements.

Repair, Rehabilitation And Seismic Retrofitting
Structural repairs, seismic retrofitting and repair of non structural members are to be carried out in sequence. In case non structural members are repaired first, they superficially block the visibility of the repair required in the structural members. Anchoring should be utilised for strengthening of structural members.

For repair and rehabilitation of distressed structures, selection of method and materials depends upon the type, conditions and importance of the structure and availability of the resources. For rehabilitation and seismic retrofitting, the following methods are normally adopted:

• Plate bonding
• Jacketing
• Fibre wrapping
• GI wire mesh in strengthening of slab
• GI wire mesh belts in masonry structures

Judicious decision is to be taken on the method of rehabilitation or replacement of structural members. For example, fibre wrapping may be costly compared to jacketing but is quick, adding negligible thickness to the member and not requiring strengthening of the foundation. Plate bonding and jacketing are carried out in columns while fibre wrapping can also be taken up in other structural members. In case, jacketing is adopted, it has to be anchored to existing members and foundation to be checked for additional loads and strengthened, if required. Fibre wrapping is done using adhesives but in plate bonding or jacketing, anchors can be used.

Anchoring is also useful in rehabilitation of slab. Sometimes, “I” beams or similar arrangements are provided to support distressed slab and wire mesh is inserted above them. If wire mesh is not anchored to the slab, it has no unity to the slab and does not serve the purpose of strengthening or rehabilitation. For anchoring wire mesh in the slab, anchors with washers made of MS flats of sizes larger than aperture of the mesh can be used. Similar arrangement can be provided in RCC structures made from sections and anchored on RCC members for getting unity action of additional reinforcement with the existing member.

Mechanical anchors can be used in case of RCC and chemical anchors for masonry structures.

Case Studies
Two bungalows having old brick masonry were seismically retrofitted in Delhi using chemical anchors. Horizontal and vertical seismic belts with GI wire mesh as per IS 13935 were provided with vertical reinforcement in inner corners.

Fig. 1: Stud, GI Wire Mesh and MS Washer

Fig. 2: Anchoring

The seismic belts were anchored with the brickwork using chemical anchoring system. Stainless steel studs (Fig. 1) of size 8mm diameter having resistance against direct pull of 110 kN were used at a spacing of 600mm in a staggered way in mesh reinforcement (Fig. 2). A single washer of MS of size larger than the opening/aperture of the mesh was used on the wire mesh for anchoring. Chemical used was HY 50/310 in a ratio of 2:1 having resin and hardener.

In anchoring process, a hole was drilled using drill machine and thereafter the hole cleaned by blowing air from the pump. The chemical was then inserted in the hole through a foil pack by inserting into holder and screwed on mixer. The cartridge was then put into the dispenser containing resin and the hardener, mix of which comes out in a fix proportion. First two-trigger pull mix was thrown out for proper mixing and then chemical grout injected into the hole and stainless steel stud inserted. After self curing time, the anchor attains the strength. A view of the anchors is shown in Fig. 2/3. In the inside corner reinforcement, a prefabricated MS piece was used having two holes, one used for inserting into the bar which can be adjusted at any position and other end used for inserting anchor as shown in Fig. 3. Anchoring on wire mesh was done in staggered (zig zag) way.

Fig. 3: Anchoring at Interior Corner

A building repaired, rehabilitated and seismically retrofitted using chemical anchoring is shown in Fig. 4.

   Fig. 4: A Retrofitted Building

Other situations where anchors/fasteners can be used include fixing of grills over concrete blocks/RCC blocks/stonework/brickwork, fixing of railing in steps and parapet, fixing of members like chhajja, projections etc. Sometimes fixing new members is required in existing structural members like rope anchoring in existing buildings, fixing projections for cantilever porches, awnings, trusses etc where anchoring is very useful. In a similar situation, anchoring system was used to install the trusses. There is a need of IS code for specifications and design anchoring system to ensure the quality.

Conclusion
Anchoring has very important role in repair, rehabilitation and seismic retrofitting as it provides unity action required between existing structural member and new section to be added. Anchoring can also be used in applications where new members are to be added in existing structures with quality and safety.

Mechanical anchors are used in RCC structures while chemical anchors in load bearing structures. There is a need of IS code for design and installation of anchoring system.

References

1. IS 13935:2009. Seismic Evaluation, Repair and Strengthening of Masonry Buildings – Guidelines.
2. Hilti Anchoring Fastening Technology Manual, 2012. Hilti Corporation Schaan.
3. https://www.fastenerandfixing.com/construction-fixings/chemical-versus-mechanical-anchors-the-pros-and-cons/
4. http://www.engineersjournal.ie/2014/10/28/selecting-specifying-effective-anchor-type/
5. https://www.nbmcw.com/68-products/fastners-accessories/29039-hiltis-fasteners-for-facades-and-cladding.html
6. Soni, K M (2015). “Rehabilitation and Retrofitting of Buildings”. Civil Engineering & Construction Review, Vol. 28 No. 5, May issue, pp 46-51.
7. Soni, KM and Batra, Usha (2020). Global and Local Retrofitting of Buildings, CE&CR, 339(10), Oct issue.

	Dr. S. K. Manjrekar   Managing Director   Sunanda Speciality Coatings Pvt. Ltd.   Honorary Member – American Concrete Institute			Dr. R. S. Manjrekar  Director  Sunanda Speciality Coatings Pvt. Ltd.

Reinforced concrete has been a material of choice and is the second most consumed material per capita in the world after water. The Indian construction industry is set to rise from a value of US$ 428.1 billion today to US$ 563.4 billion in 2020(R¹). Exponential growth in Indian concrete construction over past 40 years has concurrently created very sizeable need as well as market for repairs related activities.

Size Of Repair Industry In India
In India repair industry is not organized. Hence the exact numbers on annual cost to owners/public funding for repair, protection and strengthening are not available. Today India is placing new concrete to the tune of approximately 1.75 – 2 billion m3/per annum which of course needs to be protected. As against if we look at already placed concrete in past 50 years, it would be 55 – 60 billion m3 which now needs much protection.

Essentially deterioration of concrete takes place due to environmental factors, damage caused to structures due to basic defects in the concrete structures and change of use which can take place subsequently. Almost all the concretes are made as per the structural requirement of each structure and most of its concrete is typically specified. These concretes have to use local material from multiple sources having different quality which can be marginal. Sometimes the mix designs also are not standard and at times one has to work at neck breaking speed for producing the output which results in accelerated construction processes but may sacrifice quality. Above factors lead to malfunctioning and early distress signs in a structure and have led to rise of repair industry which is likely to assume a form of a parallel industry to new construction industry. In India this market/industry though of a large size and spread over all the nooks and corners of the subcontinent, is not organized. Yet, the magnitude can be realistically worked out by interpolation as well as extrapolation. When interpolated with regards to the distress and the health of inventory over past 40 years, the annual cost to owners for repair, protection and strengthening could be estimated between US$ 40 to 45 billion.

Indian Repair Industry Scene And Why Has This Deterioration Happened?

1. India is second largest manufacturer of cement. Hence total inventory of various structures is also large, major part of which is needing repairs after obvious distress signals followed by their health assessment.
2. India has transitioned itself from 15 MPa to 60 MPa in the span of 50 years as a general trend, though till two decades ago prevailing strength was 15-25 MPa. This means inventory of lower grade structures exist as a legacy of the past.
3. Low concrete cover, low w/c, site mixing, associated with chloride attack with advent of traffic jamming number of vehicles and continuous industrialization carbonation have worsened the scene.
4. India is hot weather country and has a long coast of 7000 kms with high humidity and tropical climate. 35000 sq. kms coastal area is under constant attack of airborne chlorides.

Fig. 1: R¹ – www.concareplus.com/technology.html
R² – www.indianmirror.com

Current Practice And Status Of Repair In India

• To remove deteriorated loose concrete
• Expose corroded reinforcement
• Semblance of cleaning and removal of corrosion products on heavily corroded steel
• Use of chemical, rust removers to remove corrosion products on corroded steel rebars
• Use of bonding agents
• Use of specialty polymer mortars as per global industry norms/specifications
• Use of protective coatings

The exponential growth of repair industry and unsatisfactory performance in the past 40 years has resulted in highlighting several shortcomings and need for improvements in:

• Materials
• Design practices
• Installation procedures
• Contracting processes
• QA/QC procedures
• Education and allied several aspects

Despite challenges the growth in repair activity is continuous, because common man/client notices the manifestation of distress in the form of cracks, delamination, failure and even sudden collapse and seeks urgent attention to allay his fears about safety. Collapses at regular intervals along with life loss brings lots of visibility, hence alarm. A large inventory of the concrete in India is 10 years plus old. Most of this concrete was site mixed and without many controls. Naturally, it is more vulnerable to carbonation, chloride attack, loss of alkalinity and attack of other aggressive chemicals etc.

Longevity Of Repairs
The short life cycle of repairs is raising questions on the knowledge and awareness about the technical competencies of related personnel and agencies. Rebuilding of older structures is not the general norm. Hence structures are typically repaired and often re-repaired.

Engineering students as well as engineers are not taught concrete or the science of steel corrosion with a special emphasis on materials. Repair is not a subject taught in engineering schools. As a result many areas remain ‘grey’ due to lack of formal training, education, learning, ‘State of the Art’ procedures and hands on training etc. This results in short life cycle of the repairs.

Special repair materials like Polymers, Epoxies, Protective Coatings, Nano Materials etc. also are not part of academics of civil engineering students/engineers. (Probably not in other parts of the world as well).

Performance of Repairs
One of the largest inventory of concrete structures are with the U.S. Army Corps of Engineers and their experience is given in a Pie chart as referenced.

FIg. 2: Performance of RCC Structures Owned by US Army Corps of Engineers

That means even in an advanced country like USA, barely 50% of the repairs perform satisfactorily and remaining repairs fall into fair, poor, failed category due to the problems in design, installati-on, materials and other parameters.

                            Fig. 3: R³ – REMR – CS -2 Report

What could be the success rate in the repairs in India and why? Possible extrapolation of repairs in India is as under.

Whole Life Cost and Performance of Repairs
The analysis of re-repairs pattern shown in Fig. 4 by Tuutti is very applicable to repairs in India.

            Fig. 4: R⁴ – Tuutti Kyosti, (1982) CBI Research Report 4:82, 304 p

However unorganized, repairs is a big industry in India and merits judicious attention to control the colossal loss to national wealth by avoiding frequent failures. Repair operations are extremely sensitive and important. Various steps involved in repair projects are dependent on the personal knowledge level of the specifier. More often the specifications are copied from one specifier/job to other. As a result, though, repairs intended to extend the service life, structures often seem to fail prematurely due to the improper strategy of repairs and lack of defined responsibility criterion.

Trained personnel are required but often semi-skilled persons replace trained ones due to which project suffers – but realization after the failure and the loss is irreversible. Some engineers though small in number charge only nominal fees which is not the accepted norm of repair industry. However, gullible people are victims to this and succumb by awarding the projects. These are particularly small sized projects. Similarly there are mushrooming repair chemical suppliers and they seem to indirectly advise the small client on using wrong chemicals in uneducated fashion. Hence a strategy is presented to increase the accountability of stake holders of repairs industry through understanding individual and collective responsibilities.

There are several operations needed in any repairs job depending on the type and extent of damage which are the stake holders mentioned above. Some of the stake holders are from following trades.

• Membranes, Sealants, Coatings
• Waterproofing
• Surface Preparation – Hydro/Shot Blasting/Chipping
• General Concrete Repairs
• Grouting/Crack Injection
• Foundation Underpinning
• Formwork
• Steel Placement
• Guniting/Shotcrete
• Post-Tensioning
• Underwater Repairs
• Cathodic Protection
• Industrial Floor Repairs

Hence comes the need of Specialist Contractors with their responsibility defined. Simultaneously the issue of the responsibilities of all concerned e.g. designers, contractors and government agencies comes in the picture. Individual repair projects are smaller in value as compared to new construction project. Hence, it will be difficult for government authorities to give the permissions and completion certificates for all these projects due to real time enormity of sheer numbers for the machinery to handle. However, it is imperative to make sure that consulting practicing engineer is specially trained and certified for repairs technology and practices as this is totally different science of civil engineering which deals with material properties and behaviour of the structure as a result of deterioration.

                     Fig. 5: Metaphoric Representation of Construction Industry

Special training courses/certification exams/eligibility criteria must be worked out. This is not difficult task as abundant reference material is available locally and globally. Otherwise in the absence of such unified procedure every qualified civil engineer will have to wear several caps like NDT expert, material specialist, corrosion experts etc. However, this is possible when the engineers are so qualified in multiple specialties. More often the story becomes like an Elephant and Six Blinds.

Required Action To Improve Performance Of Repairs In India
In India, due to unorganized nature of the industry and outdated methods of execution the performance would be still more marginal and can be illustrated as shown in fig. 5.

Individual repair contracts are smaller in value as compared to new projects and hence multi level supervision is difficult and uneconomical. Hence the final responsibility comes on the client’s appointed consulting engineer and the contractor. However, often times the entire (360°) idea about analytical, diagnostic and QC parameters for the contract both in prescriptive and performance format are not known to either the consultant or the contractor. This is more because of multifaceted complexity of the subject as well as ignorance.

The logical remedy can be achieved only by increasing awareness and defining responsibilities of all the stake holders in the repair industry.

There are several beneficiaries of repairs industry

• Engineers
• Testing companies
• Architects
• Educators
• Contractors
• Researchers
• Equipment suppliers
• Lawyers etc. and most importantly owners
• Material manufacturers

Most of the concerned stake holders in the industry are in dire need of upgrading their skills, State of the Art knowledge. At the same time individual and interdependent responsibilities of the stake holders also should be well defined so as to improve service life, reduce costs and reduce conflicts. Mere fixing responsibilities is not enough, but enlightenment is imperative.

Based on the actions above the repair Industry in India will hopefully improve to 20% success as under:

                    Fig. 6: Target Performance for Repair Industry in India

At various levels persisting attitudes of following age old procedures, equipments and materials is seen prevalent in India and needs to immediately change towards “State of the Art” approach. Appointment of specialty engineer which is by definition “Licenced Design Professional” shall be retained by a contractor as well as owner.

A devoted group having representatives of Contractors, Engineers, Material Manufacturers, Researchers, Educators, Owners, Material Scientists and Industry associations needs to be formed to resolve various problems and seek out the solutions. Looking at the size of the industry, a Nodal Federal Agency should also participate in the process to offer credibility and authority to the recommendations of the group. This would be a faster and far reaching approach than leaving the improvement issue for repairs industry to resolve. It must be made a time bound initiative in the interest of all stake holders. The draft of the code will be peer reviewed and then sent for wider circulation throughout the industry for suggestions.

This Task Group shall do an important job of developing a ‘VISION’ which will change periodically as this is ever-growing dynamic industry.

Various improvements would make qualitative difference and also bring about ‘total responsibility concept’ and will reduce –

• Mistakes in repair methods and choice of materials
• Poor Performance
• Poor Workmanship

The task group must look into finding better repairs methodologies that reduce costs by delaying or avoiding re-repairs and enhance service life. This vision accompanied by the goals will help the industry, client, research institutes, federal departments like roads, railways etc. and all the stake holders.

This subdivided vision statement can have further sub issues and some of them can be as

1. Repairs industry must be made a fully organized sector by forming a federation or trade association.
2. Indian repairs Industry should have outreach beyond civil engineering to establish mechanisms for inter-organizational and inter-disciplinary cooperation to create state of the art technology as well as its dissemination.
3. Indian Concrete Repairs Institute (ICRI) will be formed on a national level which will have affiliations to other such global institution. This will facilitate Technology Transfer.
4. Develop and implement the methodology to hasten documents creation and dissemination within industry stake holders.
5. Create a repairs/rehabilitation code to enhance the evaluation, design, materials, field and inspection practices which raise the level of performance of repairs and protection systems.
6. Establish clear responsibilities and authorities for all participants. This should provide the local government officials/authorities a guideline to issue licenses to concerned stake holders.
7. Develop concurrently performance-based guide specifications for specific and generic repairs designs.
8. This will instill the confidence in customer’s minds and also will bring a system to the approach of the repairs industry.
9. Improve repair materials design and performance
   –  to eliminate cracking
   –  to carry structural loads
   –  to define properties of set and cured finished repairs
10. Develop environmental and worker friendly repairs methods, equipments and materials that will greatly reduce the adverse effects on workers, the public and the earth’s ecosystem.
11. Develop a means for predicting repairs system performance to help ensure the use of proper materials, design details and installation methods based upon predictive models validated by experience.
12. Develop and implement a strategic research plan for the repairs industry with University, Industry and Government (UIG) partnership.
13. Create the conducive environments to increase the number of material, engineering and construction related professionals interested to upskill in repairs and protection practice. This will support the growing need of trained and qualified personnel for evaluation of design, new materials and construction practices related to repairs.
14. Develop selection processes, contractual agreements, procurement methods and relationship arrangements (partnering) that will greatly reduce conflicts, rework, claims and lawsuits resulting from disagreements among contractors, general contractors, engineers and owners.
15. Develop client education programs that will promote awareness of the effects of deterioration and the means to reduce the risks while protecting their investments.
16. Develop improved means and methods for accurate and thorough condition assessment.
17. Develop specific repairs system needs for expanded use, efficiency and failure reductions.
18. Train and assimilate unorganized sector in the main stream by knowledge dissemination and inclusion in trade association. It would be a national program executed all over the country. Skilling is a large initiative taken by Federal Government of India with a special ministry.
19. Evolve specifications and standards for the performance criteria of repairs, matching with international standards in collaboration with Bureau of Indian Standards (BIS).
20. Members of the Industry should engage in continuous innovation, based on the conditions of Indian subcontinent as well as training the personnel/applicators on regular basis in a structured manner.

The strategy will keep evolving as it is just the beginning of making an incredibly large business more structured and responsible.

The vision 2025 will ascertain the improvement in the repairs performance by 2025 as under –

                                Fig. 7: Vision 2025 for Performance of Repairs in India

Important Note
This article is based on studies and personal experience of last three and half decades of the Indian scenario of repairs and exhaustive referencing done from the literature.

However, the conclusions and recommendations are entirely personal and based on self-experience on national and international projects. It means there can be another view point as well and which together would improve the performances of the repairs.

References

1. R¹ : http://www.concareplus.com/technology.htm
2. R² : www.indianmirror.com
3. R³: REMR – CS -2 Report
4. R⁴: Tuutti Kyosti, (1982) CBI Research Report 4:82, 304 p

Dr K M Soni
Chief Engineer,
CPWD, West Zone I, Mumbai

Many structures have been constructed in the country without any structural or proper structural design. In addition to this, there are many structures constructed with poor quality, in terms of materials, workmanship and design. Such structures show distress after a few years of construction. Rehabilitation and retrofitting becomes a necessity in such structures, though a costly affair. In case structures are not rehabilitated in time, deterioration of such structures accelerates at a faster rate. Since a large number of such structures exist in the country particularly in semi urban and urban areas, this is high time that rehabilitation is carried out on large scale, both in government and private sectors. A brief on rehabilitation and retrofitting of such structures is given in the following.   

Causes Of Deterioration Of Concrete
RCC is not considered an impervious material and therefore penetration of water and/or aggressive chemicals during the service life of structures are considered primary reasons of deterioration of concrete. Penetration of water or chemicals leads to carbonation, chloride ingress, leaching, sulphate attack, alkali silica reaction etc., and causes deterioration of the concrete and corrosion in the reinforcement. But one must understand that main reason of penetration of water or chemicals is porosity of the concrete. Therefore, it is to be examined whether sufficient precautions can be taken to avoid penetration of water.

Water penetration in the concrete is stopped by taking various measures. No concrete surface is left exposed until it is rich in mix. Roof is covered with water proofing materials as it attracts maximum quantity of water. External surfaces of concrete are plastered and then a water proofing coating is applied on the plastered surface like water proofing paints. Internal surfaces attract less water hence internal concrete surfaces are plastered or rendered and then a protective coating of paints is applied over the same. Thus, penetration of water or chemicals is not feasible as the surface becomes almost impervious with all such measures. Therefore, penetration of water or chemicals is not a direct cause of deterioration but it is the failure of the concrete or plaster or protective coating which allows penetration of water or chemicals. Thus, when construction is not carried out as per the design and specifications, it helps in penetration of water or chemicals in the concrete. Therefore, deterioration of concrete is due to poor quality of design, construction or maintenance. Poor quality gets reflected in the form of seepage, leakage, corrosion, porosity, carbonation of concrete surface etc. 

Design of a structure is based on the knowledge available in terms of books, codes and ultimately designer. Normally, codes are prepared once a technology and design are proved theoretically, in the lab and also through mock up structures or prototype. Sometimes, it is observed that design philosophy changes due to unknown factors, which come in the knowledge afterwards. Therefore, codes are revised. In such cases, retrofitting of structures is required.

Once the design is finalized, execution has to be done as per the design and properties of the materials considered in the design. These are called the specifications. Thus specifications of materials are to be selected according to the design requirements and quality has to be ensured during the construction. Design issued by the structural designers is for foundation, slab, beams, and columns. Therefore, in RCC frame structures, other members are constructed according to the specifications or general practices. Suppose design of a lintel, chajja or fins is not given then at site, the same may be cast according to the specifications or general practice or even as per the will of the site in-charge. In such a case, adequate bearing may be provided in a lintel or may not be; brick work may be as per the specifications or may not be. In case these are not provided as per design requirements or specifications, they become highly vulnerable to deterioration. Similarly, in load bearing structure, in case design and detailing of bands, and corner reinforcement are not given, the same may not be provided as per the design requirements. Therefore, these members start deterioration at an early stage and even may become source of deterioration for other structural members. Poor quality construction requires frequent repair and after some time rehabilitation.

Water ingress into external RCC members is not easy. In roof, over the RCC slab, water proofing is done which does not allow water ingress. In columns and beams, over the RCC surface plaster and paint do not allow water ingress. Therefore, all RCC surfaces have protection layers. Still if water ingress is noticed in RCC members, it means that all protection layers have failed. Even after failure of protection layers, steel is protected with concrete cover and if water ingress causing corrosion in the reinforcement is noticed, it means, concrete cover has also failed. Thus, corrosion in the reinforcement means:

1. Quality of paint is poor
2. Quality of plaster is poor
3. Quality of concrete cover is poor and/or
4. Quality of RCC is poor

Quality of materials used in plaster and RCC include quality of cement, reinforcement, sand, aggregate, water and admixtures. Quality of workmanship is also equally important. Then, there is quality of procedure i.e., compaction, curing, and temperature during its placement etc. Thus, when quality is being discussed, it includes all of them.  A quality structure is durable and requires normal maintenance during its service life but a poor quality structure needs heavy and frequent maintenance. Repair is to be carried out more often which is very difficult and costly affair, that too, normally carried out by small contractors and thus in such structures, rehabilitation of members becomes necessary. During earthquakes, such structures may show large distress due to repetitive dynamic loadings and thus may require seismic retrofitting also.

If quality of RCC is not poor but quality of protection layers is poor or protection layers are not provided, RCC members may be attacked by environmental factors. For example, if RCC members are not painted and surfaces remain wet and dry for considerable period, water ingress may deteriorate RCC. Thus timely maintenance and its quality are important during the service life of a structure to prevent deterioration of concrete.

Structures also get deteriorate due to aging as aging reduces various strengths and resistance to environment. Hence, aging structures require repair and rehabilitation.

Therefore, causes of deterioration of RCC can be summarised as:

1. Inadequate design
2. Poor quality
3. Inadequate maintenance 
4. Aging of structures

Structures which are attacked by more number of causes as mentioned above deteriorate fast.

Decision On Repair And Rehabilitation
Cost of distressed structures depends upon the quality of the design, construction, maintenance, and aging of structures. A structure having design deficiencies requires retrofitting as per latest codes. As per the guidelines for repair, restoration, condition assessment and seismic strengthening of masonry building “As a thumb rule, if the cost of repair and seismic strengthening is less than about 30% of the reconstruction cost, retrofitting is adopted”. This cost may be only 5 – 6% of cost of reproduction of a building if only seismic members are to be provided but repair and rehabilitation is also to be carried out, it may lead to a very high cost. In case of poor quality construction, rate of distress is very high and cost of repair, rehabilitation and retrofitting is also very high. Also, such structures cannot be brought to the required quality level. For example, if the joints of brickwork have not been filled up properly, any rehabilitation work such as grouting may not fill up all the hollow joints.

Sometimes, it becomes economical to go for reconstruction but due to some other considerations such as non availability of buildings for the users, short life of structures, and litigation etc., repair, rehabilitation and retrofitting are carried out. Even after repair and rehabilitation, such structures need continuous maintenance and may not last for originally designed life.

In the structures where quality has been taken care of in the design and construction but not during maintenance period or maintenance is not carried out as per the frequency, repair and rehabilitation may be required due to distress in the structures but cost of repair and rehabilitation is comparatively low in such cases. In case of aging of structures, cost is also not very high though it depends upon the elapsed and remaining life but in case such structures have been constructed with poor quality or maintained poorly, cost becomes very high.

Structural Evaluation
Structural evaluation is carried out through condition survey and non destructive testing. Condition survey is carried out at four stages through preliminary inspection, planning, detailed visual inspection and field and laboratory testing. Non destructive testing tests include tests for insitu compressive strength like rebound hammer test, ultrasonic pulse velocity test, Windsor probe test, pull out test, core tests and load tests. Tests for chemical attack include carbonation tests, chloride test and sulphate test. Corrosion potential assessment can be made from cover meter/Profo meter, half cell method, and resistivity meter. Normally a series of tests or a combination of tests or all tests are carried out based on condition assessment and importance of the structure. Though interpretation of tests is important and to be carried out by the experts, testing procedure is equally or more important as incorrect testing procedure may provide incorrect results. Problem sometimes is that field testing is not normally carried out by the experts themselves. Sometimes results are so erratic that one may require retesting. Results also vary with the equipments used, calibration conditions of the equipments and site conditions and accessibility of the members. Hence, there are numerous factors on which correctness of indirect tests depends and thus one has to be very cautious while doing such tests and interpreting results based on them. 

Materials For Repair And Rehabilitation

Selection of materials for repair and rehabilitation depends upon many factors such as:

1. Technical requirements
2. Cost
3. Availability
4. Expert’s advice
5. Importance of the structure
6. Balance life of the structure
7. Toxicity
8. Aesthetic requirements

Technical requirements of materials used for repair may include their shrinkage properties, compatibility to base materials, setting and hardening properties, workability, bond strength, thermal expansion properties, mechanical strength, curing requirements, permeability, and durability. Most of the materials used for repair are cements, admixtures, polymer modified mortars/concrete, aggregates, polymers, epoxies, resins, grouts, plasticizers, steel in the form of reinforcement, sections, nuts/bolts, wire mesh, metallic sheets, glass fibre sheets, carbon fibre sheets, geo-synthetics in the same form or in modified form. 

Rehabilitation And Retrofitting Methods

Repair and rehabilitation methods are to be planned carefully and to be followed in the required sequence. Structural repairs are to be carried out first and thereafter retrofitting works for seismic requirements and then repair of non structural members. Finally, repair and rehabilitation of architectural components should be taken up. Such a sequence is followed as repair of non structural members initially may cover up structural cracks or members requiring structural strengthening.

Weak protective surfaces and materials like concrete, plaster, water proofing materials, and corroded steel are to be removed first. Repair is thereafter to be carried out by ensuring design requirements, compatibility of materials and also other factors mentioned earlier. Repair should also be compatible to design requirements. For example, if a lintel is to be repaired, it should be compatible to seismic retrofitting, if being carried out. For repair of all distressed structures, no standard method and materials may be listed but depend upon structure to structure.

During rehabilitation of distressed structures or retrofitting, following methods may be carried out:

1. Sand blasting to remove rust
2. Binding/adding additional reinforcement
3. Binding of wire mesh
4. Welding
5. Anchoring to the existing members through shear keys or anchors
6. Shotcreting
7. Plate bonding
8. Jacketing
9. Fibre wrapping
10. Underpinning

One should take decision judiciously on the method of rehabilitation or replacement of structural members. For example, a slab will always cost more in rehabilitation compared to replacement. Slab may require sand blasting to remove rust of the reinforcement, anti rusting coat, additional reinforcement, binding coat, welding, anchoring or shear key and shotcreting. Shotcreting itself is costlier than new slab, hence rehabilitation becomes very costly. Thus, it should be examined whether slab replacement is feasible. In case of roof, other factors like water supply system, water proofing etc are also to be considered. Sometimes, repair and rehabilitation is to be carried out in a roof slab, without disturbing water supply system to other residents, hence replacement is not feasible even if it economical. Many times, heavy “I” beams are provided to support distressed slab and wire mesh is inserted above the “I”beams (Fig. 1). Such system provides a feeling of unsafe structure. Also, there becomes a large distance between I beams, and wire mesh itself may sag after a few years. Hence, it is recommended that angle sections should be provided at the ends and T sections in between in shorter direction (Fig. 2) and wire mesh provided on top. Thereafter shotcreting can be done. Small sections at closer intervals do not allow wire mesh to sag due to small spans between beams and in future, it becomes easy to repair small portion if needed. Small portions of slabs may be rehabilitated by tying additional reinforcement but tying should be proper; else welding should be preferred with the existing reinforcement. In case wire mesh is needed, it may also be anchored in the existing slab by providing washers made of MS flats in case welding is not carried out. Mere binding additional reinforcement to the existing reinforcement with binding wire does not serve the purpose for long as binding wire gets corroded after sometime and thereafter there is no monolithic action between existing reinforcement and additional reinforcement.

Thus, main procedure to any rehabilitation work is removal of loose materials and rusting, providing protective layer, bonding coat between old and new surfaces, providing steel members/welding/anchoring/shear keys to ensure monolithic action between old and new reinforcement and concrete work. Two surfaces may be joined through welding, anchoring, providing shear key, adhesives or bonding coat according to the surfaces and materials used in rehabilitation work.

Retrofitting is also done through plate bonding, and fibre wrap techniques. Materials used in plate may vary according to the requirements such as metallic, glass fibre or carbon fibre. Fibre wrappers are wrapped around the structural members after repair and rehabilitation of distressed members and may be of different materials such as carbon fibres, glass fibres etc.

Figure 3: Rehabilitation Through Jacketing in a Column

Masonry structures are rehabilitated and retrofitted with similar materials but with different techniques. Horizontal seismic belts are provided in lieu of plinth band, lintel band and roof band. In case of RCC slab, roof band is not required. Vertical belts are provided at corners and around openings. Vertical reinforcement bar is provided at inside corners. These bands are essential for seismic retrofitting or strengthening. For monolithic action segmental arches are to be connected. Main precaution to be taken is to ensure that these belts do not get detached during shaking due to seismic forces hence anchoring of belts plays an important role. Anchoring can be done with mechanical anchors or chemical anchors. Mechanical anchors may damage brickwork and thus chemical anchors (Fig. 4) are most suitable in brickwork though mechanical anchors can be provided in strong concrete.Materials for belts may be Galvanised wire mesh or fibre sheets. Fibre sheets are costly but they do not require micro concreting/thick mortar for its covering. Also micro concrete/thick mortar over wire mesh forms bands (Fig. 5) on the surface while fibre sheets have small thickness and can be fixed with high quality adhesives.

Conclusions
There are large number of un-engineered and engineered structures constructed with poor quality of design, materials and workmanship. Some structures are not even maintained with proper quality. All such structures are highly prone to distress and require frequent repair and rehabilitation. Such structures cannot be demolished due to high cost involved in reconstruction and non availability of assets. Thus, there is a need to develop simple and user friendly guidelines for municipalities, individuals, engineers, architects and contractors. A brief on repair and rehabilitation methods of structures including causes of distress are discussed in the paper. 

 References

1. Rai, D C, Draft code with commentary on seismic evaluation and strengthening of existing buildings, IIT Kanpur.
2. Handbook on repair and rehabilitation of RCC buildings, Central Public Works Department, New Delhi.
3. Guidelines for repair, restoration, condition assessment and seismic strengthening of masonry buildings, Arya, A S.
4. Soni, K.M.& Khatri, N.K.(2006). “Retrofitting of a Brick Masonry Bungalow in Lutyen’s zone”. New Building Materials & Construction World, Vol. 12, Issue 4 pp140-148.

Mayank Rawal
Director Technical,
Asian Laboratories

Corrosion of steel embedded in concrete is one of the major causes of degradation of concrete structures. In the marine environment, the steel bar protected by passive layer is easily attacked by the chlorides which can penetrate through the concrete cover. When the concentration of chlorides on the surface of the steel exceeds the chloride threshold level, the passive layer is locally broken down and pitting corrosion initiates.

Concrete is very sensitive material if proper precaution are not taken during its placement then structures deteriorate due to a numbers of processes leading to cracking & spalling of the concrete. Corrosion of the Reinforcement is an important reason for the concrete failure.

What is Corrosion of Steel

The corrosion of structural steel is an electrochemical process that reduces the simultaneous presence of moistures & Oxygen. Essentially the iron in the steel is oxidized to produce rust, which occupies approximately six times the volume of the original material.

Reinforced concrete with high W/c Ratio or with weak cover, prone to deteriorate due to penetration of moisture inside the concrete. The moist concrete increases the corrosion potential by changing ferrous ion to ferric ion metal.

(Dark) 3Fe ++ + 4 OH → Fe +++ (Reddish Colour)
Ferrous ion                      Ferric
Ferric metal is more voluminous & hence it cracks the concrete on expansions.

Corrosion in Steel Nails

Bipolar Treated Nails

To reduce corrosion of steel reinforcement following methods are practiced:

1. Cover of concrete should be as per IS: 456-2000.
2. W/c Ratio of concrete should be as per I.S : 456.-2000
3. Coating of reinforcement with inhibitor solution as per Patent Nos._109784/67
4. Use of Bipolar Concrete Inhibitor CEMWET BPCP-900 is recommended
5. Use of epoxy coated bars.

Use of coating on reinforcement bars is sometime not done properly or the coating wears out while bending the bars and during the handling of reinforcements, As such corrosion protection is not fully achieved. New development in corrosion protection is achieved by incorporating in the concrete bipolar chemicals. These bipolar chemicals are alkaline in nature free from nitrites & chromates and gives passivity to the reinforcement and a passive film is covered on the reinforcement due to the vapour phase transmission.

Rusted Iron Bar

These chemical can be applied over the normal RCC structure also where corrosion protection is required to protect the reinforcement for longer period. These migrated vapours forms a unimolecular protective layer on the steel thereby inhibiting corrosion.

Treated Iron Bar

Major application of bipolar concrete penetrating corrosion inhibiting admixture is in Building facades, RCC Structures exposed to corrosive environment, sewerage systems. RCC in or near marine environments.
Advantages of using CEMWET BPCP-900 due to its high penetrating power resulting in total protection of rebar. It’s effective even in presence of high amount of chloride. It is eco-friendly & free from chromates & nitrites. It has simultaneous cathodic & anodic protection of steel.

The Electrical Corrosion

Once corrosion is initiated by chloride attack and/or carbonation, an electrochemical corrosion cell is created. Rust formation occurs at the anode as the steel reinforcing bar is ultimately converted to iron oxides. Since the volume of this rust is several times greater than the steel it replaces, expansive forces build up within the concrete, resulting in cracking and spalling.

Testing of Bipolar Corrosion Inhibitor for Vapour Diffusion in Lab

Cathode Process O2 + 2H2O + 4e¯→ 4OH¯ Anode Process Fe → Fe++ + 2e¯ Technical details of CEMWET BPCP-900 are as under: Appearance                    :          Light brown coloured liquid pH                                   :          9.5minimum Specific Gravity              :          1.04 at 27°C      The dosage of BPCP -900 is 2 to 3 kg Per Cubic meter of concrete.The product is added to the batch water during mixing of cement. Sand & aggregates. It may be added to ready-mix trial at the concrete plant or at a job site. The other admixtures should be added separately.

For Fighting Corrosion in Hardened Concrete

BPCP-900 is impregnated in the concrete surface on spraying or by brush or roller application on the concrete. This water based organic compound migrates through the hardened pore structure via diffusion. Upon contact with reinforced steel it from a monomolecular protective layer which reduces corrosion dramatically for concrete repair & overlays.
After damaged concrete has removed and apply CEMWET BPCP-900 over the entire substrate prior to placing the overlay. This will be for added protection.
After isolated repairs have been made apply inhibitor coatings. As CEMWET BPCP-900 migrates. It protects the reinforcing steel & helps prevent additional cracking & spalling in the future.
Accidental method to check the efficiency of Corrosion Inhibitor in Laboratory.

1. Modified accidental Corrosion Test method based on Japanese Standard JISZ1535
2. Immersion test method for 720 hrs to record the weight loss.
3. Polarization Test by Tafel Polarization with 3.5% NaCI for 20 days.

Product CEMWET BPCP-900 has passed all the above tests and successfully used in concrete.

Supradip Das
Consultant

With the increase in the construction activities in the last two decades mainly in the building sector, it has been observed that many of the structures are already showing signs of distress. In some cases repair measures become necessary even within a span of 5 to 10 years of completion of structure. This may happen due to host of factors like bad quality concreting materials poor workmanship, lack of maintenance, atmospheric effects, abuses, accidents or natural calamities. Thus retrofitting of a concrete structure becomes necessary to extend its life to ensure durability of the structure. The rehabilitation envisages restoration of structural system as close as possible to the original position. The distressed structure needs to be brought in line, level and to required strength so that it can be put into service without endangering its safety and utility.

Looking to the magnitude and complexity of the work involved in the restoration process, the requirement of various technology & materials is going to take a giant leap in years to come. The concept of retrofitting & rehabilitation has also been accepted as one of the important discipline of the construction industry.

The article presents one such case study of a clusters of buildings in Delhi where concrete had deteriorated due to poor concreting practice, quality and lack of maintenance. It also gave details of systematic repairing solutions.

Broad Features

Based on the study of critically affected buildings, corrosion of reinforcement has been the prime cause for deteriorations of RCC member. The corrosion occurred mainly due to leakage/overflow of water from service water tanks and leakages from roof penetrating into porous concrete has been the common cause for corrosion in all the buildings. Severe distress was also due to lack of concreting knowledge, ignorance about resultant effects & its maintenance.

Most of the residential buildings in that area are less than fifteen years of age & showed variable distress. The areas of distress in all the buildings are of same nature. Concrete at several places especially on the columns, mumty holding the water tank, staircase & roof are the main areas where the concrete clearly showed aging, severe honeycombing, poor compaction, corrosion resulting in spalling of cover concrete.

Fig.1: Distress Water Tanks, Balcony Roof & Roof

In some of the roofs, the cover was not adequate to protect from the reinforcement. Exterior Column-beam junctions of critically effected building developed vertical cracks on almost all the faces. Also column of balcony has the vertical cracks where the concrete showing aging, honeycombing, poor compaction, corrosion of reinforcement resulting in spalling of cover concrete. (Figure 1 & 2).

Fig.2: Photographs of Critically Affected Buildings

In some places especially on the mumty columns, the corrosion has been very heavy due to the ingress of water from the over flowing water tanks. The diameter of the bar has been reduced to less than 50% of its original diameter. On the staircase, the concrete was at the threshold of getting corroded. Cracks have also started developing on the slopped portion.

Chemical Kinetics
Majority of the distress happen to be related to severe corrosion to the reinforcement. The corrosion reaction i.e. localised breakdown of the passive film (cathode) on steel, which mainly starts from the ingress of moisture, penetration of chloride & carbon dioxide through porus concrete.

Corrosion of steel in concrete is an electrochemical process, which takes place as a result of the formation of a corrosion cell. (Figure 3 )

Fig.3: Corrosion Cell

Mechanism of corrosion (Figure 4) shows the anodic reactions where metallic iron (Fe) transformed to rust (Fe₂O₃). The rust formation on the surface of reinforcement is accompanied by an increase in volume, may be as large as 6 times the volume of Fe. Sound concrete with pH between 11.5 & 13.5 is an ideal environment for protection of steel but as stated above alkalinity can be lost due to the formation of alkali carbonates, sulphates & chlorides & also due to anodic reaction takes place in the presence of moisture. The Anodic reaction can be explained from the following chemical reaction:

3Fe + 4H₂O Õ Fe₃O₄ + 8H⁺ + 8e-

2Fe + 3H₂O Õ Fe₂O₃ + 6H⁺ + 6e

Fig.4: Mechanism of Corrosion

In other words, if the carbonated front penetrates sufficiently deeply into the concrete to intersect with the concrete reinforcement interface, protection is lost and, since both oxygen and moisture are available, the steel is likely to corrode. The extent of the advance of the carbonation front depends, to a considerable extent, on the porosity and permeability of the concrete and on the conditions of the exposure. In the case of carbonation, atmospheric carbon dioxide (CO₂) reacts with pore water alkali according to the generalized reaction,

Ca(OH)₂ + CO₂ Õ CaCO₃ + H₂O

It consumes alkalinity and reduces pore water pH to the 8–9 range, where steel is no longer passive.

Fig.5 (a): Extent of Corrosion	Fig.5 (b): Addition of Steel

Approach to Repairing Method

Since repair & rehabilitation of a structure expects to set right the damaged areas, it is therefore obvious that repairs need assured inputs of right material, quality workmanship & last but the least the proper system or specification to be adopted in a phased manner. In this case, a complete retrofitting plan of constructional defects was chalked out based on the investigation. Main treatments were required for strengthening of the structure, arrest of the corrosion, cover protection & consolidation of the concrete. Since no adequate treatment was carried out for the water tightness during the construction & its maintenance afterwards, special emphasis was given on the consolidation of the concrete. Among various techniques & newer material available for restoration of distressed concrete structures, it was proposed to use cementitious repairing material for cover & low viscous epoxy as grouting material for consolidation purely because of the structural and thermal compatibility and similar physical properties to those of the parent concrete.

Since each material has its own characteristics, it requires very careful study for selecting right material for a particular application. While selecting a system one should be careful to evaluate that whether it meets all the requirements such as placing problem, strength development & durability. For any repair & rehabilitation job, it is always better to take the manufacturer into confidence on the various aspect of the material & its suitability for that particular type of job. Based on the investigation & type of building, a detailed system for treatments were chalked out. The system included type of material, tools to be used, work schedule & rehabilitation design. The repair was carried out under strict supervision of quality control experts.

Material Used for Retrofitting

During the rectification process, large number of materials staring from corrosion inhibitor to micro-concrete were used at different stages. The selection of material does contribute to the performance of the repairing system in a long term basis.
It has been observed in many cases, the compatibility of various chemicals with cement depends on the physico-mechanical behavior of the system & plays an important role in the performance. In this case, the following products were used.

1. Rust converter: it converts the rust on the reinforcement to a protective chemical barrier & dissolve the ferric oxide.
2. Corrosion Inhibitor: it slows down the corrosion reaction in the concrete & help in regain its alkalinity by creating a passive film
3. Low viscous epoxy: Low viscous epoxy used for densify the concrete, consolidate the structure by filling up the voids, fissures & capillaries.
4. Zinc rich primer: Two component system used as an inhibitive epoxy primer used in coating of reinforcement.
5. Bonding agent: Two component epoxy used as bonding agent between old & new concrete
6. Micro concrete: High Performance high strength self-levelling concrete used for strengthening by jacketing various places to minimize the porosity & enhancing the density.
7. Protective Coating: Polyurethane based clear coating. This coating protects the concrete from aggressive attack such as ingress of moisture.
8. Acrylic Polymer: Single Component Acrylic polymer for cement based waterproofing system for providing effective & economical cementitious acrylic plaster.

Pre-treatment Guidelines

Following pre-treatment guidelines were chalked out prior to the commencement of treatment. This was done to ensure the safety of the structure & also to make proper working conditions.

1. a) Erection of scaffolding – This shall be done keeping in mind the locations where repairs are to be carried. Adequate precautions are necessary while erecting the scaffolding to facilitate proper working conditions.
2. b) Repair to column – The repair shall be undertaken from bottom and then progressed upwards. Column length between floors shall be considered for repair only after column below are repaired.
3. c) Repair to beams, slab and stair case waist slab- after completing column repairs, works of beams and slabs are to be undertaken.
4. d) Before undertaking the repair work, appropriate propping must be provided preferably using steel pipes. The supporting structure should such that it should transfer the load coming on the member under consideration. Locations of props should be such that the proper force flow maintained. This aspect is important to ensure safety during the course of repairs. The slabs and beams transferring the load to the particular column shall be supported before the repair to that column is undertaken.

Methodology of Treatment

Surface Preparation of Concrete
Reinforcement was exposed by removing all loose and honeycombed concrete. All spallings were removed. The concrete was chipped off to a minimum depth of 25mm behind the reinforcement. This was required to be done to treat the corroded reinforcement. Concrete surface & reinforcement were washed with water jet to remove all soluble & insoluble salt such as chlorides present as indicated in the report.

Surface Preparation of Reinforcement
Removal of the rust from reinforcement completely by rubbing with wire brush or emery paper. Proper cleaning of the existing exposed reinforcement with rust converter. This de-rusting compound starts reacting within few minutes after application and converts the rust into a non-metallic complex compound of black colour. It forms non-porous, impermeable and moisture repelling coating of good mechanical stability. It shall be applied liberally by brush till such time it penetrates through the rust layer. Though the reaction starts within few minutes, it generally takes 6 hours to complete the same. However, next activity should start only after 24 hours.

Provision of Additional Reinforcement
In case of severe corrosion (diameter of reinforcement bar is reduced substantially say more than 20%), the affected bars were anchored with equivalent additional reinforcement by welding. This was done in consultation with structural Engineer at site. (Figure: 5b)

Application of Corrosion Inhibitor
The concrete was porus. Due to the ingress of acidic water, the pH value of the concrete slowly got reduced. In many areas the pH was observed to be below 10 when concrete tends to be acidic & accelerates the process of corrosion (refer Figure 4). This was taken care by injecting corrosion inhibitor through non reversible packers into the concrete. With this process, pH of the concrete tends to increase to safer level of 12.5 & 13. The process involved drilling of holes at a grid pattern on the concrete & non reversible packers were fixed using fast setting cementitious compound. In case of very poor concrete, the nozzles were fixed after the strengthening of the structure. (Figure 6)

Fig.6: Injection of Corrosion Inhibitor Through Non Reversible Pump

Treatment of Reinforcement
New & old reinforcement were treated with zinc rich epoxy. This anticorrosive epoxy coating acts as a barrier and also stops propagation. The coating was applied in a manner such that the dry film thickness was around 60micron.

Pressure Grouting
For consolidation of weak concrete & strengthening the, pressure grouting to be carried out through the non-returnable nozzles of 6 to 12 mm dia. at a spacing of 400-500 mm c/c in zigzag pattern by drilling operation. During drilling & fixing of nozzles, precaution to be taken that damage to the concrete should be minimal. Minimum Depths of nozzle should be 40 to 50 mm. Firstly corrosion inhibitor was injected into the concrete. Once the treatment is over, subsequently low viscous epoxy was injected in to the concrete at a pressure 2 to 3 kg/cm². Once the grouting was over, the nozzles were removed. Pressure grouting operation is to be carried out before polymer cement mortar plaster (cover concrete) and plaster.

Fig.8: Fixing of Nozzles After Filling Up with Micro-Concrete

Provision of Bonding Agent
Prior to the application of Cover mortar plaster/micro concrete, the existing surface was treated with a coat of epoxy based bonding agent to bond old to new concrete. Bonding agent was applied on the surface of old concrete after removing all loose concrete & some hacking was done on it. This epoxy bond coat also acts as a barrier preventing entry of chlorides and other harmful agents.

Jacketing & Strengthening of Columns Using Micro Concrete
In case of severe damages 50 mm or thickness or as per site requirement, micro-concrete to be used for jacketing the columns. Micro-concrete, cementitious non shrink free flow high strength micro concrete with ultimate compressive strength of 650kg/cm² was used. In some cases, it was substituted with maximum of 25% of 4.75 mm downgraded aggregates. The proportion of the mix in many areas varied between 10 to 25% based on the site conditions. This was done as per the manufacturer’s instructions in the product data sheet. For bulk pouring, mechanical mixer too be used or can be mixed manually for smaller requirement. Water / Powder ratio to be adjusted between 0.14 & 0.18 depending on the site condition. Physical properties of Micro-concrete used is given in Table 1.

Repair to Columns

The first step in repairs was undertaken by repairing the column starting from the ground floor. Repair to columns was undertaken one by one. The most distressed column in the structure be taken first. The repair process shall be as under:

1. Removal of all plaster over the distressed column.
2. Remove cover and concrete up to 25 mm clear from the main reinforcement bar using electric cutter/pneumatic chipper or manually to completely expose the reinforcement.
3. It has to be ensured that during the removal of cover, there should not be any damage to the concrete element.
4. Quantification of deterioration/distress
5. Treatment of columns to be taken up as per the suggestions given below

(a) In case the distress at the location is restricted to single face no jacketing will be required. Repair be carried out as given in sketch

(b) If the distress is more than on face of a column and reinforcement is corroded less than 20 % no new reinforcement is needed only Jacketing is required as per sketch ”A” shown below. If the corrosion is more than 20 % the treatment to be done as per sketch “B”

A – Column Section : When Corrosion Is Less than 20%

(c) If Corrosion is More than 20%

Fig.7: Jacketing of Columns with Micro-Concrete

c) Cover / Finished Plaster
20mm thick cement mortar plaster admixed with acrylic polymer provided on the micro-
concrete for finishing.

d) Curing
All the treated areas like columns to be cured using curing compound & other areas to be cured with conventional systems.

e) Protective Coating
After the curing, the treated surface are to be provided with a polyurethane based clear coating. Dry film thickness of around 100micron. This coating will protect the concrete from aggressive attack & will make the repaired surface durable for a considerable period of time.

Fig.8: : P-61 Building before Treatment

Fig.9 : P-61 Building During & After the Treatment

Conclusion

The repair rehabilitation measure suggested in the report was initially executed in one of the building in Station Workshop, Delhi. It was been periodically inspected since last two years & on the basis of the performance, The authorities had specially included the above methodology in their BOQs for other structures. It is always required to study behaviour of the restored structure and in the present case also the same assessment pattern was undertaken to check whether the affected structural elements were properly repaired or not. The performance of the rehabilitation measure was satisfactory. Some of the photographs are shown below.

Acknowledgment

The author wishes to acknowledge the contribution of project authority with their inputs while making the specification & also implementing the methodology for retrofitting of severely affected building under his zone. 

References

1. Das Supradip & et al, ‘Repair and Rehabilitation of Concrete Structure: A Case Study”, CE&CR, Mar’05
2. IS 516: 1959, Method of test for strength of concrete.
3. IS 456: 2000, Plain and reinforced concrete – Code of Practice.
4. Das Supradip, “Repair and rehabilitation of concrete structures a case study.”: New Building Materials & Construction World,: pp 150 – 157. ( Feb’2007)
5. Sivagnanam B. “ Damage Assessment and rehabilitation of concrete structures : Three case studies.”: Indian Concrete Journal ( Dec’2002 )
6. Das Banabir & Rajendra Kumar (Dec’2003): Civil Engineering & Construction Review, “Rehabilitation of Leaky Basement-A case study.
7. Das Supradip: Rectification & Consolidation of water retaining structure: case study
8. Das Supradip, “ Repair & rehabilitation of structures- causes of distress & its solutions”, NBM&CW, pp 118 – 136 ( Feb’2014 )
9. Handbook on Repairs & Rehabilitation of RCC Buildings – Central Public Works Department 2011

Sumeet Agarwal,
Head-Recovery,
Sustainable Environment and Ecological Development Society, New Delhi

Buildings, with poor quality of construction or in various states of disrepair or dilapidation, are a common sight in cities. Many of these buildings, that serve as living, working and learning spaces, in addition to being eyesores, are highly vulnerable to disasters as evidenced by incidents of building collapse accompanying disasters in India and other countries. The extensive damage to buildings in the earthquake in Gujarat in 2001(3) and in Haiti in 2010(2) illustrate consequences of neglect of building construction, maintenance and repair issues. Considering the high disaster vulnerability of India and high population densities in cities, issues of substandard building stock need to be prioritized to avoid a catastrophe. Based on damage need for repair can be structural, non-structural or both.

Non- structural Damage

Repair of non-structural componentsis generally taken up after structural repairs have been carried out. Such repairs generally help to restore architectural features of the building and make it possible to develop a more congenial living/working environment and tore-use the spaces but they do not enhance the structural strength of the building and may, in fact, conceal the defects of the building making it difficult for engineers to address and remove the defects at a later date. Non-structural repair activities generally include the following:

• Repair to electrical fittings and wiring
• Repair of doors, windows and glass panes
• Patching up of cracks and fallen plaster
• Repair of plumbing and gas pipes
• Re-building of non-structural elements like boundary walls and chimneys.
• Replacement or rearranging of roof tiles.
• Re-laying of flooring.
• Redecoration- white washing, painting etc.

Structural Damage

Extensive structural damage can be addressed through repair or reconstruction such damage can be avoided through timely repair or retrofitting. The decision of strengthening or retrofitting v/s reconstruction needs to be made after careful consideration of the advantages and disadvantages. Replacement or reconstruction of damaged or unsafe buildings by reconstruction is generally avoided for the following reasons:

• Higher cost than that of strengthening or retrofitting
• Preservation of historical architecture
• Maintaining functional, social and cultural environment.

As a thumb rule if the cost of repair and strengthening is less than about 50 percent of the reconstruction cost, retrofitting is adopted. It may be possible to retrofit a structure in shorter time as compared to reconstruction and would have a less disruptive effect on the occupants of the building. However, incorporating provisions of relevant codes during construction of a building works out much cheaper than retrofitting a structurally inadequate buildings.

Strengthening of Buildings

Strengthening generally does not limit itself to increasing the strength of deteriorated or damaged structural members, but also considers behavior of the entire structure. It may also be taken up specifically to improve the resistance of the buildings to disasters, like earthquakes. The seismic resistance of existing buildings, which do not meet seismic strength requirements of earthquake codes, in force, due to original structural inadequacies and material degradation due to time or alterations carried out during use over the years can be upgraded to the level of present day codes. Strengthening procedures aim at the following:

• Increasing the lateral strength in one or both directions by reinforcement, increasing wall areas or increasing number of walls and columns.
• Improving unity of the structure by providing better connections between its resisting elements so that forces generated by vibration of a building can be transmitted to members, which can resist them. Connection between roof and wall and intersecting walls are examples of elements whose connections need to be strengthened.
• Elimination of features that are a source of weakness to the structure or lead to concentration of stresses in some members. Abrupt changes in stiffness from one floor to the other, large openings in walls without peripheral reinforcement, asymmetrical plan distribution of load resisting members, abrupt changes in stiffness from one floor to the other are all examples of defect of this kind.
• Avoiding likelihood of brittle modes of failure by proper reinforcement and connection of resisting members.
• Since signs of deterioration may not be visible due to cosmetic maintenance work, assessments and tests of the structural members would be necessary to reveal the nature and extent of deterioration. A detailed damage assessment would help to determine:
• If the structural condition of the building is amendable for repair, whether continued occupation of the building is permitted, if whole or part of the structure requires demolition.
• Need for detailed damage assessment of structural components ( mapping of crack pattern, distress location, crushed concrete, reinforcement bending/ yielding, etc.). Non-destructive techniques could be employed to determine the residual strength of the members
• Temporary supporting arrangement of the distressed members so that they do not undergo further distress due to gravity loads(1).
• Based on the assessment of the building and available cost and time the appropriate strengthening measures can be decided.

Assessment

Various ways of conducting assessments of structures are as under:

• Non Destructive tests (NDT)
• Visual inspection of building
• Study of relevant drawings and documents
• Discussion with personnel involved in design and construction of the building

Brief details of each of the methods are given below:

Non-destructive Tests
Non Destructive tests can accurately determine the retrofitting and repair techniques required for various structural members in a structure. Different types of NDT methods are:

• Rebound Hammer Test
• Ultrasonic Pulse Velocity test
• Core extraction
• Pull out test
• Carbonation test

Visual Inspection of Buildings

Rebound Hammer

Detailed visual inspection of buildings can provide adequate information about structural distress of the building and the following is to be seen during the visual inspection of a building: Colour, rust stains and water stains. Pattern and width of cracks on structural members. Spalling of concrete Sinking of structural members Additions and extensions to the building

Study of Relevant Drawings and Documents

Study of drawings and documents of a building can provide a wealth of information on a building. It would help to determine the additions/ alterations to the building and design omissions or inadequacies if any. The drawings also help to determine the structural system in areas of a building made inaccessible by furniture, machines or fixtures and may also provide information about codes used in the structural design, helping to determine if the building conforms to current codes or not.

Discussion with Personnel and Study of Documents

• Study of soil test reports.
• Study of the structural and architectural drawings of the structure.
• Study of recommended grade of concrete and building material specifications.
• Results of tests such as core tests, brick tests etc.
• Issues faced during construction.

Case Study: Assessment for Retrofitting of an Office Building in New Delhi

The author was part of a team that conducted an assessment of a large office complex in New Delhi. The details of the office building cannot be mentioned for contractual reasons but the assessment process, findings and recommendations will be described. The description seeks to demonstrate the application of assessment processes described above, the kind of structural deterioration that a building can undergo and the possible strengthening interventions that can be proposed in such cases.

The assessment team decided on the following methodology for the work:

• Conduct a visual inspection of the building to detect visual signs of distress.
• Carry out a condition survey of the building by conducting Non Destructive Tests on the structure.
• Recreate as-built structural drawings of the building.
• Carry out soil testing to ascertain strength and condition of foundation strata.
• Carry out structural analysis of the building to ascertain structural integrity of the building, existing grade of concrete and existing steel reinforcement.
• Recommend remedial and strengthening measures to ensure satisfactory performance of the building

The Complex
The office complex, consisted of five multi storied buildings constructed between 1962 and 1984.The building was highly vulnerable to earthquakes since the area, where it was located, has been marked as a high seismic hazard zone by Seismic Hazard Micro zonation map, published by Earthquake Risk Evaluation Centre, Indian Meteorological Department. One of the reasons for designating the area as a high hazard zone was due to liquefaction potential of the soil.

Visual Inspection of the Buildings

A visual inspection of the buildings was first carried out. Available drawings were also studied and relevant personnel were interviewed for structural issues faced. The inspection and study revealed the following information on the buildings: Building 1 (G+5 floors): Constructed in 1962. Building 2 (G+1 floor): Constructed in 1962 Building 3 (G+4 floors): Constructed in 1971-73 Building 4 (G+4 floors): Constructed in 1982-84 Building 5 (G+3 floors): Built for residential use and modified extensively between 2004 and 2007.

Issues
The following main issues were revealed during a visual inspection of the complex:

• A bridge connecting building 1 and building 3 had been added and subsequently a fire escape staircase was also added. No structural information is available about these elements although they have a bearing on the structural performance of the building.
• Incidents of plaster and concrete pieces falling from shading louvres and overhangs had been reported and surface cracks were noticed on louvers and overhangs of the buildings.
• The maintenance personnel informed that the roof slab of building 3 was 75mm thick.
• The personnel also informed that a number of walls between columns had been removed and replaced with large glass windows for aesthetic purposes.
• Intermediate walls between rooms and corridor have been removed to convert them into halls.
• Roof spaces of buildings were found to have been used for installation of solar panels mounted on concrete stubs bolted to reinforcement of the roof slab. The panels have not only added to the dead load on the building but also added a factor of wind load exerted on the large panels bolted with a single hook.
• An additional steel frame has been constructed on the roof space of building 4 to extend the roof area and almost hangs from the roof and the rear wall of the building. The frame was meant to create additional space for the AHU as the roof space is occupied with solar panels.
• Large deep cracks were noticed on the bottom of beams and slabs of the buildings. A discussion with the maintenance staff revealed that large lumps of concrete had also fallen from the beam, which at the time of inspection was concealed under a false ceiling. An additional pillar also had to be provided to support the cracking beam.
• An additional floor was added to building 5 with independent external columns and beams.

Inferences from Visual Inspection

• Modifications to the building and additional live loads are significant in view of the fact that change in Seismic zone in Delhi in 2002 shall alter the factor of safety applied in the building design originally. Since the upgrading of the zone calls for a higher design factor under building code provisions for safety of structure it needs to be analysed against changed loads.
• Structural integrity of the building connections of smaller built additions like the fire escape staircase that have been inserted in the existing structure are a cause for concern.
• Changes made to the structure of the building like removal of walls between columns and replacement with glass windows would have an impact in the event of seismic activity since the columns have been deprived of the bracing provided by the intermediate walls.
• Distress that has developed in various structural elements of the complex over the years needs to be addressed.

The visual inspection of the building was followed by the following tests.

Soil Test
Field penetration tests and liquefaction studies were carried out and the findings are:

• Though liquefaction is not anticipated in the foundation soil during an earthquake, the factor of safety against liquefaction is found to reduce as the ground is inundated with water and the soil is completely saturated. Hence proper drainage of the premises needs to be ensured so that the ground does not get flooded.
• The value of standard penetration resistance blow count is less than 15 in the top 6 metre of ground and below 6 metre its value increases. Hence, it would be advisable to improve ground strength at shallow depth by grouting.

Non-destructive Tests
The following non-destructive tests that were carried out.

• Rebound Hammer Test as per IS: 13311 (Part-2)-1992 for determining the compressive strength of concrete.
• Ultrasonic Pulse Velocity Test as per IS: 13311 (Part-1)-1992 for ascertaining the quality of concrete.
• Drilling out Concrete Cores, preparing the test specimens and testing the cores as per IS: 516-1959 for determination of compressive strength of concrete.
• Carbonation Test for determining the depth of carbonation of concrete.
• Corrosion Potential Assessment by conducting half-cell potential test as per ASTM: C876 – 1991 for assessing the risk of corrosion of reinforcement.
• Rebar Locator Test for establishing the existing depth of concrete cover over reinforcement.
• Chemical Tests on concrete in the laboratory for determining the following parameters:
• Chloride content as per IS : 14959 (Part 2) – 2001
• Sulphate Content as per relevant B.S.
• pH value.

The test results from one of the buildings has been described below for illustrative purposes. The NDT results from other buildings were found to be the similar and have not been described to avoid repetition.

Building 1

Rebound Hammer Test
The test results reveal that the concrete in various structural elements of the building at different floor levels falls in M 10 & M 15 grade categories. However, at first floor level the grade of concrete in all structural elements falls in M 15 grade. Since 10 Nos. test results out of total 19 Nos. fall in M10 to M15 grade, keeping safety considerations in view, for structural analysis for seismic assessment of Main Building, the overall grade of concrete for RCC works can be considered as M12 grade.

Ultrasonic Pulse Velocity Test
Reviewing the test data, it is seen that the ultrasonic pulse velocity investigations carried out on foundation footing of column & various structural elements at different floor levels of the Main Building indicate that overall quality of concrete can be considered to fall in ‘Doubtful’ to ‘Medium’ category reflecting unsatisfactory workmanship with presence of likely internal deficiencies.

Concrete Strength
As per IS:456-2000, the concrete in the member represented by a core test shall be considered acceptable if the average equivalent cube strength of the cores is equal to at least 85 % of the cube strength of the grade of concrete specified for the corresponding age and no individual core has a strength less than 75 %. The concrete is seen to fall in M 10 to M15 grade.

Carbonation Test
The carbonation test was conducted on foundation footing of column & various RCC elements at all floor levels, & the depth of carbonation has been found to range from 20 mm to 80 mm. The tests have revealed that the carbonation has penetrated up to or beyond the reinforcement bars at most of the locations.

Half-cell Potential
The results of the half-cell potential survey indicate that the likelihood of corrosion of reinforcement at present in various structural members at different floor levels is varying from less than 10 % to more than 90%.

Cover Meter Test
There is no deficiency in depth of cover recorded as compared to the requirement of minimum concrete cover as per IS: 456-2000.

pH value, sulphate and chloride analysis
As per Clause 8.2.5.2 of IS:456-2000, for RCC containing embedded metal, max. total acid soluble chloride content expressed as Kg/m³ of concrete should not be more than 0.60. As such, all the test results in respect of chloride content are within permissible limit.
As mentioned earlier above tests were conducted on other buildings, including building 2 but have not been included to avoid repetition. However, considering the state of structural elements, test results and height of the buildings strengthening measures were proposed for buildings 1 & 2, which are given above.

Strengthening Measures Proposed

The following measures are presented as sample.

Building 3

• The columns would be required to be strengthened heavily with new reinforcement and concrete jacketing with micro-concrete. Similarly, the beams would require strengthening using FRP laminates/ concrete jacketing. These would further increase the size and weight of the structural members and therefore, the foundations would be further overburdened requiring their sizes to be increased and strengthening.
• For earthquake resistance, the foundations would require to be thickened and tied with beams along with strengthening and enlargement.
• The strength of concrete is found to be very poor for all the structural members, which would require pressure grouting using low viscosity epoxy grouts.
• The building aesthetics may not be maintained because of resizing of columns and beams.

Building 4

•  The columns would require strengthening with new reinforcement and concrete jacketing with micro-concrete. Similarly, the beams would require strengthening using FRP laminates/ concrete jacketing. These would increase the size and weight of the structural members and therefore, the foundations would be required to be enlarged.
• For earthquake resistance, the foundations would require to be tied with beams along with strengthening and enlargement.
• The strength of concrete is found to be very poor for all the structural members and the same would need to be improved adopting pressure grouting using low viscosity epoxy grouts.

The building aesthetics may not be maintained because of resizing of columns and beams.

Building 5

The building arrangement appears to be non-engineered and the seismic resistivity of the structure is doubtful.The extent of strengthening requirement for all the Buildings would be extremely high and would require almost strengthening from foundation to columns, beams & slabs, viz., strengthening of each structural member.
It is obvious from the above description of the building that it is highly vulnerable to earthquakes due to the following factors

• Ageing
• Unplanned modifications
• Non- conformity to present day codes

The building would sustain heavy damage during a severe earthquake as strength of concrete is less than acceptable limit with medium to doubtful quality. Corrosion of reinforcement will also continue to increase as depth of carbonation is quite high. In addition to the deterioration the building has been constructed under old codes and needs to be upgraded to conform to present day codes and revised seismic zoning. In order to improve the resilience of the building. Structural members need to be treated in multiple ways which include removal of loose concrete, protection of existing reinforcement steel from corrosion, jacketing with concrete/steel/ carbon fibre reinforced polymer or a combination of these. The appropriate treatment would need to be decided on the basis of cost and structural design. The example also illustrates the extent of deterioration that can occur in a building if left unaddressed. The treatment to the structural members would not only be expensive but would also affect the functioning of occupants of the building. Thus timely assessment, repair and treatment of buildings is essential.

References

1. National Building Code of India
2. https://www.newscientist.com/article/dn18406-why-the-haiti-quake-killed-so-many/
3. https://architexturez.net/doc/az-cf-21244
4. http://pwd.delhigovt.nic.in/pims/right_to_info/handbook.pdf
5. https://www.ripublication.com/ijcer_spl/ijcerv5n4spl_05.pdf
6. https://www.nicee.org/iaee/E_Chapter9.pdf
7. IS 13935, 2009: Seismic evaluation, repair and strengthening of masonry buildings- Guidelines

Every structure has itself life after which the members of structure require maintenance for their deterioration and distress in the form of cracking, corrosion etc. The maintenance consists of extensive Rehabilitation and Retrofitting by means of conventional technique such as (RCC jacketing), steel plating or modern technique like Fibre Reinforced Polymer (FRP).
This paper gives a brief case study of structures treated with combination of all sustainable technique for maximum durability of structure. The above case study was a turnkey project where design, manufacturing and execution are done by the DGC team.

Introduction

In today’s growing economy, infrastructure construction is mainly depending upon Reinforced Concrete as its major construction material. Constantly advancing technologies has made a big impact in the engineering world with its new and emerging technologies, which has brought in new safety rules and requirements. Most of the structures built in remote past by traditional methods have suffered the consequences of extreme loading events over a long period of time. Retrofitting is an approach based on recent technological developments and scientific knowledge, whereby modern construction methods and materials are applied to the repair and strengthening of historical structures. Fibre Reinforced Polymer (FRP) composites are gaining wide applications in retrofitting of Reinforced cement Concrete (RCC) structures due to its inherent advantages over the conventional methods of strengthening. FRP is used widely in strengthening of structures facing deficiencies due to usage, design and change in loading.  Fibre Reinforced Polymer is used widely mainly due to its ease and fast way of application and as it does not affect the functionality of the existing structure during application. Use of Fibre Reinforced Polymer (FRP) and other advanced conventional technologies of Rehabilitation of heritage structures.

Saptashrungi temple is located in Nanduri, Kalwantaluka, a small village near Nashik in India. Devotees visit this place in large numbers every day. The temple is also known popularly as one of the “three and half Shakti Peethas” of Maharashtra.  The present architect of this temple structure is “Studio Architectonic” and design consultant is “Jagdish Deshmukh Consultants”. It is a very old structure built in 1700 AD. The temple has undergone renovations recently also with creations of many facilities. The facilities created at the shrine consist of about 500 steps (474 is also mentioned cut into the rock slopes of the hill, from above the road point, leading to the temple entrance, a community hall, a gallery for devotees to form queues and have orderly darshan of the goddess.  The steps were built by Umabai Dabhade in 1710 AD. A portico like structure, an addition made to the main shrine of the goddess is attributed to the Satara Commander-in-Chief and the plain structure at the beginning of the last century. Subsequent additions were made by the Chief of Vinchur. Based on the data given by the Client the temple needs to be strengthened/ retrofitted due to severe cracks and spilling of plaster observed in the beams and columns of the structure. Also the results obtained from the core test reports indicate that the strength of concrete is beyond the permissible grade required for such a heritage structure. Hence, DGC Engineering Pvt. Ltd. proposed to conduct a site visit to study the same and propose a solution.

Investigation

The Non-destructive testing of the structure was carried out by Advanced Diagnostic Laboratory some of the relevant points of the report are as under:-

1. a) The Ultrasonic Pulse Velocity and Schmidt Rebound Hammer Test Report indicate that the columns of ground floor are badly damaged.
2. b) The testing of concrete reveals that grade of concrete is doubtful.
3. c) The Columns of Ground floor show large distress.
4. d) The grading of concrete is poor as confirm by the Rebound Hammer test.
5. e) The chloride test report it is clearly shown that the values are higher than the limiting value, it confirm that corrosion has taken place in the reinforcement.

Site Visit for Visual Observations

		Damage observed in internal beams due to peeling of plaster			Severe cracks and spalling of cover with corroded reinforcement exposure observed in beams	Spalling of plaster observed in columns at core cutting locations
		Severe spalling of cover and corroded reinforcement exposure observed in beams			Severe cracks observed at beam bottom

		Cracks observed along the length of columns			Severe cracks observed along the length of column and along the beam column junctions	Severe map cracking observed at beam bottom
		Severe cracks along the length of beams seen			Pop out observed in beam along with severe cracks along the length	spalling of cover and corroded reinforcement exposure observed in columns

Poor quality concrete observed in the roof beams

Surface crazing observed in columns

Surface crazing due to alkali aggregate reaction observed in beams

Structural Modelling

1. a) Structural analysis of the building frame has been performed by creating model in STAAD and STRUDS software. Geometry of the model is developed based on the available RCC drawing forwarded by the client. Refer figure 1 and 2 for STRUDS and STAAD model of the building.
2. b) The concrete grade is considered as M10 for the complete structure. However it is to be noted that the non-destructive testing report of the ground floor columns suggest that the pulse velocity test and rebound hammer test result reveal poor quality of concrete i.e grade even lesser than M10. But in the software used for analysis minimum concrete grade taken is M10 and hence considered. It implies that the compressive strength as obtained from analysis will be higher than actual present in the column at site.
3. c) Reinforcement grade for main flexure reinforcement and shear stirrups is considered to be Fe 415.
4. d) Nominal cover to reinforcing steel is considered as 40mm for column and 25mm for beams.
5. e) All columns considered as fixed at base.
6. f) Main basic loads applied on the structure are dead loads, imposed loads, wind and seismic loads
7. g) It is assumed that concrete and reinforcing steel have
   not deteriorated and do not have corrosion or loss of strength

Structural Treatment View

Surface preparation of column

Application of Anti-corrosive coating

Drilling for shear connector

Additional new reinforcement

Recommendations

• The main structural elements i.e. beams and columns at all the levels which are showing severe cracks needs to be strengthened by providing jacketing after initial surface treatment
• Columns which are badly damaged (20 percent) needs to retrofitted/ strengthened with concrete jacketing with micro concrete of 75 to 100 mm thickness followed by carbon fibre wrapping and rest of the columns which shows damage needs to be fibre wrapped after surface treatment to increase the strength and prevent damage in future.
• Beams which show damage needs to be carbon fibre wrapped after proper surface treatment.

Site Visit for Visual Observations

Peeling of plaster observed at certain locations of columns

Epoxy putty application on beam

Carbon fibre wrapping on beam (430 GSM)

Fixing of carbon fibre anchor.

Strengthened beam after fibre wrapping

Conclusion

It is a challenging experience to Retrofit Saptashringi temple (Shakti peeth in India) 700 steps high on steep mountain. All Tuesday, Friday and all other auspicious day the temple was over crowded. So without disturbing pilgrim the entire retrofitting scheme was implemented.
At present, columns and beams were strengthened for future life; the slab on top floor needs to be addressed in second phase.

Samir Surlaker
Director,
Indian Institute of Talent Development

Concrete is the most versatile man-made construction material of our times on account of its flow into the most complicated forms while fresh and its strength and durability characteristics when set. Concrete construction is economical considering the longevity of the structures.

Concrete Bridges are designed to be inherently durable and usually require a minimum of repair and maintenance. However, there are occasions when damage in defects requires remedial treatment to be carried out. Before carrying out any remedial measures on a concrete bridge, it is most important to identify the basic causes of deterioration. This paper is a general introduction to assist the reader in identifying likely causes of defects or deterioration, selection of correct material systems and application methods. An RCC bridge is composed of primarily three components
(Figure 1):

Fig.1 : Bridge Components, Substructure, Superstructure and Deck

The Substructure: The substructure consists of all parts that support the super structure, like Abutments or end-bents, Piers or interior bents, Foundation / Footings, Piles and Bearings. The Superstructure: The component of a bridge, which supports the deck or riding surface of the bridge. It includes, Bridge deck, Structural members like girders, slabs, etc. and Parapets, handrails, sidewalk, lighting and drainage features. The Deck / Carriageway: The component of a bridge, which is driven upon, including shoulders. Some decks are asphalt while others are constructed as reinforced concrete slabs. Average Daily Traffic determines which surface is used.

The basic causes of deterioration originate due to factors occurring before or during construction or factors occurring post construction. It is very important that the root cause of damage to the bridge be evaluated prior to commencing repair operations. These causes are shown in Figures 2 & 3.

Fig.2 : Causes of Deterioration Before and  During Construction

Fig.3 : Causes of Deterioration occurring  Post Construction

Other faults which can require repair include: insufficient cover to steel, honeycombing or voids in the concrete, blemishes such as conspicuous or excessive blow-holes which are not structurally significant but are aesthetically displeasing.

EN 1504 Guidelines on Assessment of RCC Structures [Including Bridges]

EN 1504 is the new European Standard for the protection and repair of reinforced concrete. There are ten parts to the standard covering test methods for material properties and specification for the key repair materials, including coatings, mortars, bonding agents and injection materials. It also includes general principles for repair work and a standard for site application of products and systems. The code suggests the following steps for assessing the structure:

1. Present Condition: Condition Assessment of the Bridge. Visual evaluation of the members, in case of structural damage, evaluation of the load carrying capacity of the bridge.
2. The design approach: What was the design intent of the bridge? Is the bridge performing to design standards? Is it capable of performing through its design life?
3. Exposure and environmental conditions the structure is exposed to: Evaluation of physical, mechanical and chemical loading the structure would be exposed to during its design life.
4. Assessing the conditions and studying construction records: Evaluate the construction and maintenance records if any on the bridge structure.
5. Usage Conditions and history: What was the bridge designed to be used as? Is it serving the purpose? What would be the purpose of the rehabilitation – restoring load carrying capacity or upgrading of the capacity of the bridge?
6. Intended future use: What would be the use of the rehabilitated bridge?

Once the evaluations are made, the following courses of action can be taken:

• Do nothing for a certain time
• Re-analyse the structural capacity; possibly down grade the function
• Prevention or reduction of further deterioration
• Improvement, strengthening of concrete structure
• Reconstruction of concrete structure
• Demolition

Principles of Repair in Accordance with EN 1504

When a bridge shows signs of distress or deterioration, the following steps should be taken in principle. EN 1504 recommends the steps for the entire procedure for repairs from assessment to maintenance. A guidance note (No. 4) from the Concrete Society UK recommends:

1. Assessment of damage to the Bridge [substructure, superstructure or carriageway]
2. Choose Options of Repair
3. Choose the appropriate repair Principles based on EN 1504: These principles address the methods for repair of both concrete and methods for mitigating corrosion of the reinforcement, based on evaluated damage.
4. Choose the appropriate methods and materials of repair in accordance with the principles mentioned in Section 3 above
5. Specify on-going requirements such as maintenance of records of repair, maintenance schedules and periodic evaluation of residual life of the structure

Following this a thorough QA system needs to be established to ensure the durability of repairs.

Basic Steps of Repair

When a structure shows signs of distress or deterioration, the following steps should be taken in principle.  The steps are as under:

1. Preliminary Investigation, detailed Investigation
2. Diagnosis
3. Laying out specifications for repairs
4. Selection of Materials
5. Surface Preparations
6. Actual Repairs
7. Periodical maintenance
8. Maintenance of Reports etc. for future repairs

Preliminary Investigation and Detailed Investigation

Fig.4 : Compact Kit for Testing / Diagnosis of Deteriorated Concrete (Courtesy: MC-Bauchemie, Germany)

The first step is to conduct visual inspection of the damaged structure and to inspect the records. Further surveys to be conducted after preliminary investigation are delamination surveys, crack surveys, compressive strength tests etc. Preliminary investigation and diagnosis are carried out to ascertain the nature of damage and the extent of distress to establish feasibility of repairs. Destructive as well as non-destructive methods are at our disposal for determining the extent of distress. Figure 4 shows a compact kit for evaluation of damage Unless the cause of distress is established, the remedial measures shall have no meaning. It is the cause that is to be rectified rather than the surface appearance of the damaged structure.

Diagnosis

Diagnosis is actually interpretation of the results obtained from the investigations. The interpretation requires sound knowledge and experience in this field and should essentially be done by qualified engineers.

Laying out Specifications for Repair

Since the field of repairs and maintenance is a specialized one, it is very important that proper specifications are laid out for carrying out the remedial measures. The specifications should include:

1. 1. Materials for repairs
2. Calculations for extra reinforcement for structural repairs
3. Materials for injecting the Cracks
4. Guideline for surface preparations
5. Steps for Repairs
6. Precaution to be taken while using the materials as well as the curing procedures etc.
7. Supervision and quality control at site
8. Scope of work and quantities

Only proper specifications can lead to proper repairs with minimum clashes between the owner and the contractor. Methods of measurement should be unambiguous. A qualified bridge engineer, the executing contractor, the material supplier and the owner of the structure should draw up the specification of repairs together. This would ensure that the design, material, application and contractual needs of the project are completely anticipated and the repair to the bridge structure is durable.

Repair Methods and Materials

Injections For Repairs
Cracks occur in the concrete despite the fact that quality is controlled. Cracks indicated image or distress in the structure. But, not all cracks are a sign of structural failure. Cracks have to be repaired for two reasons viz. for structural or for durability purposes. The selection of material for injection requires thorough understanding of the properties of the material and functions that such a repair has to perform. In all the cases, it is imperative that the cause of crack is properly determined otherwise the selection of material willgo wrong.Injections is the first step in repair program.
The repair of cracks alone cannot guarantee the structural stability or durability of concrete and therefore, if necessary should be complimented with other treatments as per the established practices of civil engineering. Under all circumstances it is advisable to trust these types of jobs to experienced contractors having the knowledge of materials as well as experience in the use of specialized equipment. After completion of diagnosis and selection of materials for injection, the work of injection passes through following stages:

1. Preparation of the crack
2. Location of points for injection
3. Surface sealing of cracks
4. Injection of resin
5. Removal of packers and plugging
6. Removal of sealing material
7. Final surface treatment after injection resin/grout hardens

Guideline for Material Selection
Prior to selection of a material and method to remediate a crack or void, the characteristics of the defect need to be clearly assessed. The properties to be assessed include:

1. Need for Crack Filling: Enhance load transfer or stop water through crack
2. Depth of Crack: Surface or deep cracks
3. Crack Width
4. Crack Movement: Moving or non-moving
5. Condition of the Crack: Dry or damp / actively water bearing

The pattern of the cracks decides the reason for cracking which in turn reflects on the selection of base material. Width of the crack has direct bearing on the viscosity of the material required. It depends on the movements in the crack, which reflects the type of material required whether it should act as structural injection or just an elastic seal. Structural injection should be able to transfer stresses from one crack face to the other. The moisture in the crack calls for a water compatible system of injection.

Materials for Injections: Properties
All cracks are different. They vary depending on the construction material, cause, location and environment. One single system is not able to achieve durable and reliable results. Various solutions based on different materials, which are tailored to specific application needs, are now available to users. Table 1 shows selection of materials with respect to job and site conditions. Table 2 shows salient properties of injection materials.

Assessing these properties will help us in selecting the correct material and in turn determine the success of the crack repair. Internationally and in India at the moment, the following types of materials are most preferably used for crack injections:

• Epoxy resin (EP)
• Polyurethane (PUR)
• Cement slurry (CF)
• Micro fine cement suspension (CMF)

In addition to the substances above injection gels are also used in Europe for the injection of structural components. Injection gels (hydro-gels) are aqueous systems based on special acrylate or polyurethane resins. The filling of cracks and voids corresponds to the following application principles:

• Protection against the penetration of substances into the concrete structural components
• Strengthening concrete structural components

Fig.6 : Selection of Injection / Impregnation Materials Based on Crack Width

In Brief, Table 3 below gives an idea of the type of Injection materials available and the conditions these materials can be used under. Impregnation can only be used to fill cracks in near the surface areas of horizontal or slightly tilted surfaces from above. Injection is able to fill cracks and voids going deep into the structural component. The filling of crack and voids requires minimal crack widths at the concrete surface and minimal dimensions of the voids, depending on the method of filling and the chosen filling material. Selection of materials based on crack width is shown in Figure 6.

Injection: Components
Injection measures comprise of the Mixing device, Injection device, Packers (filler plugs), possibly an injection hose and Insulation, if necessary. The application expertise for these materials is as important as the materials themselves.
In Conclusion, modern injection technology coupled with proper equipment can solve almost all types of rehabilitation problems thereby providing economical solution in comparison to demolition and reconstruction of structures. The specifications should be very clear and unambiguous. The specifications should at least cover points like material, viscosity, techniques to be adopted, the equipment to be employed, and type of nozzles and spacing, pressure to be applied etc. The repair of cracks is a part of repairs of damaged structures and cannot replace other remedial measures adopted for rehabilitation.

Surface Preparation

Concrete: The existing base has to be firm, stable and free from oils, impurities of all kind including form oil residues and cement laitance. The loose and damaged concrete should be removed using light hammers and chisels. Care should be taken not to remove sound concrete to provide a good base to the repair material. Only mechanical means (rotary wire brushes, grinding, chipping, grit blasting and breakers) or hydraulic methods (use of water jets 10 to 250 MPa) should be used to remove concrete. Using hydraulic is the best means, as it reduces impact of damage to the substrate concrete.

Fig.7 : Complete Concrete Repair System (Courtesy: MC-Bauchemie, Germany)

Reinforcement: Sand blasting or wire brushing is an effective water-free method to clean reinforcement. In this case suitable wire-brushes were used to remove the loose material, dust and laitance. Use of rust removers and water is not recommended to protect the reinforcement from further corrosion, as it may reduce the pH of the surrounding substrate and promote further corrosion. The reinforcement shall be prepared till bare metal is seen. Additional reinforcement if needed should be incorporated at this time by anchoring new steel; lapping or welding. Figure 7 shows the complete sequence of repair.

The Actual Concrete Repair System and Practical Considerations

Protection of Exposed Reinforcement
All exposed reinforcement cleaned to bare metal should be protected immediately after preparation by using suitable corrosion inhibiting active or barrier coatings. These are proprietary materials to be used to provide corrosion resistance to the cleaned reinforcement, prior to application of the polymer modified mortar / concrete system. The materials that can be used to provide corrosion protection include:

Active coatings for Reinforcement: These are coatings, which contain OPC or electrochemically active pigments, which may function as inhibitors or which may provide localized cathodic protection. Typical product that can be used is a one-component polymer modified mineral based corrosion protection coat.

Barrier Coatings: These are coatings, which isolate the reinforcement from pore water in the surrounding cementitious matrix. Typical product that can be used is a two component Epoxy Resin based Zinc Rich Primer and Coating Material for use in repairs subject to aggressive environmental and chemical attacks.

Bonding Coats: These are also proprietary materials used for bonding of fresh concrete to hardened concrete using adhesive bonded joints where it forms a part of the structure and is required to act compositely. The materials that can be used to provide bonding include:

1. A one-component polymer modified mineral based corrosion protection and bonding coat, which can be used for most repair applications
2. A two component Epoxy Resin based solvent free, universal bonding agent and coating for use in repairs subject to aggressive environmental and chemical attacks.

Polymer Concrete / Mortar (PCC) Repair

Small areas and patches less than 100 mm thick are usually repaired with hand / trowel or spray applied polymer modified or epoxy modified repair mortars. Some of these products are proprietary. On the other hand, these mortars can also be site batched using polymer additives. In most cases a pre-bagged manufactured ready-to-use polymer modified mortar is preferred. These manufactured products can also be available in special grades allowing thicknesses more than 40 mm to be applied in a single operation. PCC Types include:

1. Site Batched Polymer Modified Mortar: This is a mixture of OPC, well graded, clean, washed, quartz sand, Styrene Acrylic or SBR based polymer additive and water as required for consistency.
2. Prebaked / Manufactured Polymer Modified Mortar: The premixed / manufactured or site batched PCC for Structural Repairs to be used should have the following properties:

• Air Content: ≤ 3%
• Compressive Strength: ≥ 45 MPa
• Bond Strength to Concrete: ≥ 2 MPa
• Chloride Ion Content: < 0.05%
• Capillary Absorption: ≤ 0.5 kg/m2.h0.5
• Adhesive Bond Strength to Concrete: ≥ 2 MPa

These mortars are most suitable for repairs when the section to be replaced ranges from a depth of 5 mm to 50 mm. In case of depths more than 100 mm need to be placed, additional layers maybe needed. Follow manufacturer’s recommendations for mixing, placement, compaction and curing of the repair mortar. In case of large structural elements, a ready to use non-shrink micro concrete can be used. Cure the exposed surfaces for 7 days or more, preferably using acrylic based curing compounds (so as not to hinder bond in subsequent applications).

Fine Filling: To achieve a visually uniform surface and to provide additional preventive protection the repaired concrete surface should be fine-filled. This is done with a fine polymer modified, concrete cosmetic or fine sand and a mixing liquid composed of water and or the polymer component.

Concrete Surface Protection, Carbonation inhibitor, Coloured finishes
On completion of the work described above, the entire concrete surface must be provided with a protective coating. Such surface treatments perform several duties at the one time. Firstly, all the concrete is protected from further stress due to aggressive pollutants in the air and from progressive carbonation. Unfortunately most specifications for Repairs do not specify Anti-Carbonation Coatings for full area thereby jeopardizing the repair system. The concept of Repair is to stop or retard the corrosion process already initiated in RCC Structures. To stop further entry of CO₂, SO₂, moisture and chlorides in few cases the entire surface has to be coated with a system to maintain the status quo of corrosion in the structures. Selection of coating depends on exposure conditions.
These coating systems must have to a high CO₂ resistance if they are to be effective in protecting against carbonation and, on the other hand, they must not have a negative effect on the buildings water vapor diffusion rate. These materials should also be water resistant, crack bridging, UV resistant and breathable. The selection of protective coatings depends on user requirements and exposure conditions. In case of structures in contact with chlorides, it may be required that the coatings are resistant to chloride ion penetration as well.

Other Techniques for Strengthening

Repair involves replacing or correcting deteriorated, damaged or faulty materials components or elements of a structure. In rehabilitation and retrofitting, we not only restore the concrete to take loads for present service conditions but we can also enhance load carrying capacities while maintaining the durability of structures. Following methods are adopted:

1. Section enlargement / Jacketing
2. Post tensioning
3. Externally bonded reinforcement

In addition to this there are several specialized techniques used like shear strengthening, use of shear collars, confinement strengthening, span shortening techniques etc.

Section Enlargement: Jacketing

This is one of the oldest techniques used to increase the cross section of element tying additional reinforcement, which would lead enhancing to its load bearing capacity. The main thing is to bond the new material to the old element fully so that perfect load transfer and monolithic behaviour is guaranteed. This is achieved by means of through surface preparation, bonding agents as well as shear and other connections. The materials used can be: Conventional concrete/mortar, Cementitious grouts, Polymer modified cement mortars, Micro-concretes or Shotcrete.

Post Tensioning

Normally this technique is used where the damage is due to undesirable or excessive deformation. The technique of tensioning is same as through tendons but since it is done externally better inspection is possible. The flexural capacity of these structural members is increased. This requires a shear transfer mechanism and end bearing as an assembly.

Externally Bonded Reinforcement

Working principle of this system is the plate-bonding technique. When the steel plate is bonded to concrete with epoxy adhesive the structure behaves like composite structure. Two systems are commonly used:

1. Steel Plate Bonding   2. GFRP/CFRP Bonding.

Steel plate Bonding: It is an efficient method of increasing flexural capacity in beams when applied to the tension side, which transfers the load to the epoxy which in turn transfers tensile loads from concrete to steel. Its case of constructability was reason for its major success. Disadvantage was cutting of steel plates to suit the geometry, its weight and problem of corrosion.

Fibre Reinforced Plastic Bonding: This is an extension of the steel plate bonding technique with tremendous advantage of light weight, ease of cutting and mould ability to suit any element and high chemicals resistance. Two systems commonly used are based on Glass Fibre reinforced plastic (GFRP), and Carbon Fibre reinforced plastic (CFRP). The rapid acceptance of this material is due to serviceability and ease of application without disturbing the structure. Wrapping of columns provides passive confinement with increase of ductility and strength. Shear strengths are also increased. Wrapping gives excellent protection against explosions. FRP plates can be bonded with epoxies to increase flexural strengths. The limitation is the use of epoxy, which can change its characteristics during thermal variation and fire.

Preventive / Periodical Maintenance

Premature failure of building components in a structure is not uncommon. To stem this, preventive maintenance is being explored today. Preventive maintenance is a part of asset management system and is required for important structures such as bridges. It seeks to forestall the deteriorating process of bridge components. Minor repairs and replacements are categorized under preventive maintenance activity. The preventive maintenance program precludes major repair work or replacement of substantial part of the structure.

Maintenance of Records

Maintenance of proper reports about the damages and subsequent repairs can offer a lot of information. This is crucial for future diagnosis of subsequent deteriorations. The records about the types of materials used at previous repairs along with other relevant technical data will clearly show the suitability for future usage as well as the durability of such materials. Documentation of this kind will go a long way to ascertain the quality of repairs, which would provide pointers for future from the past failures.

Conclusion

Rehabilitation of bridges is a key aspect of the bridge management system. In this case it is imperative that the owners and specifies understand the extent of the damages and the correct solutions to be used in rehabilitating the bridge. Once the process is understood, the correct systems and methodologies can be used. A combination of various systems can address any rehabilitation scenario. International codes such as EN 1504 can be referred to as they provide excellent step-by-step guidelines to address various defects.

References

1. EN 1504: Parts 1 To 10
2. IRC SP: 40
3. North Cadsawan, Weathering The Storm, Concrete Repair Digest, June/July 1993
4. C. Kreijger, InhomogeneityIn Concrete, Protection Of Concrete, E. &F. N. Spon
5. Grant T. Halvorse, Concrete Cover, Concrete Construction, June 1993
6. C. Hewlett, Methods Of Protecting Concrete, International Conference, Scotland, Sept 1990.
7. L. Leening, Surface Treatment For The Protection Of Concrete, Ove Aurp &Partners, UK
8. Dafstb , Richtlinie Fur Schutz Und Instandsetzung Von Betonbauteilen, August 1990
9. Peter Pullar-Strecker – Corrosion Damaged Concrete – Assessment &Repair.
10. Peter H. Emmons, A.M.Vaysburd &Jay Thomas: Strengthening Concrete Structures – Advanced Composites
11. Emmons, Peter H. Concrete Repair And Maintenance – Problem Analysis, Repair Strategy Techniques.
12. Tracy, Robert G And Fling Russel S., Rehabilitation Strategies, Repair And Rehabilitation Of Concrete Structures.
13. Warner, James, Selecting Repair Materials, Concrete Repair And Restoration.
14. Patch Repair Of Reinforced Concrete, Technical Report No.38, Concrete Society, London.
15. Polymers In Concrete, Technical Report No.39, Concrete Society, London.
16. American Concrete Institute, Concrete Repair Manual,Vol.1 & 2, Third Edition, Published By ACI &ICRI
17. Peter H, Emmons, Gajanan M. Sabins – Concrete Repair &Maintenance, Galgoton Publication Delhi.
18. M Vaysburd, B.Bissannette Durability Of Concrete Repair & Research: Some Random Thoughts, Journal Of 3r’s Vol. 1, Jan – Mar 10
19. Surface Preparation And Coating Of Concrete Published By SSPC.

Murari Ratnam Former Director Central Soll and Materials Reseach Satiton, Ministri of  Water Resource, Govt of India , New Dehli

Introduction

The construction activity in India has been increasing geometrically since independence. During this period, we have created substantial infrastructural assets in terms of buildings, bridges, sport stadiums, dams, etc., which are lifelines for the civilized society. Presently, there are about 5,100 large dams in the country, one-third of which are concrete and masonry dams. Apart from the large dams, India has innumerable major and minor structures, which have been created through huge investments.
Several large dams in India are more than 100 years old and require constant maintenance. It is therefore essential to maintain them in functional condition. As numerous dams are in a state of distress, a systematic approach is required to deal with such problems. The identification of the causes of deterioration and consequent repair/rehabilitation strategy at optimum cost is essential. The current paper thus, focuses on issues pertaining to repair and rehabilitation several large dams in India.

Repair, Rehabilitation Or Retrofit?

It is important to distinguish between repair, rehabilitation and retrofit of a structure. All three terms refer to modifications carried out on a structure, but in different contexts; while retrofit and rehabilitation may involve addition of new structural elements or a change in the structural system, repair is restricted to the built-in system.

Repair: Repair refers to the action that improves the functionality of components in a structure that have been rendered defective, deteriorated or damaged due to some cause. The purpose of repair is to rectify the observed defects and bring the structure to its original architectural shape and intended purpose. This action is generally non-structural in nature; it does not guarantee structural safety. In fact, a repaired structure may be deceptive in a manner that it will appear to be in a good state and give the occupants a false sense of safety.

Rehabilitation: Rehabilitation refers to structural interventions that improve the strength of the components in a structure that are either deteriorated or damaged; the rehabilitation process is intended to regain the original strength of these structural members. For example, in the event of a fire in a structure, rehabilitation work is undertaken to replace or strengthen the damaged structural members. Such intervention provides only the original strength of the structure and is appropriate if the original strength provides an adequate level of safety. Further, the term restoration is sometimes interchangeably used with rehabilitation.

Retrofit: Retrofit specifically aims to enhance the structural capacities (strength, stiffness, ductility, stability and integrity) of a structure to mitigate the effect of a future earthquake. The term seismic retrofit is used in the specific context of enhancing the resistance of a vulnerable structure to earthquakes. Sometimes, the terms ‘seismic rehabilitation’, ‘seismic upgrading’ and ‘seismic strengthening’ are used in lieu of ‘seismic retrofit’. It is not necessary for the structure to be deteriorated or damaged for retrofitting to be performed.

Causative Factors

Concrete dams could be deteriorated due to one or more of following causes and the distress may manifest in terms of cracks, seepage, leakage, pit formation, surface abrasion, etc. The causes are discussed below in detail.

Design/Construction Deficiencies

This includes inadequate design criteria adoption of incorrect or unsafe parameters based on deficient investigation, inadequate investigation leading to erroneous assumptions, under estimation of design floods and seismic potential. Several old dams were rehabilitated by strengthening, providing additional spillway capacity, widening from d/s side, providing u/s impermeable barrier, etc. for correcting different kinds of design/construction deficiencies.

Erosion

Erosion of concrete occurs by three forces namely, cavitation, abrasion, or chemical action.

Cavitation Erosion: Cavitation is the formation of bubbles or cavities in a liquid. In hydraulic structures, the liquid is water, and the cavities are filled with water vapour and air. The cavities form where the local pressure drops to a value that causes the water to vaporize at the prevailing fluid temperature. Moreover, the concrete surface irregularities trigger the formation of these cavities; the pressure drop caused by these irregularities is generally abrupt and is caused by local high velocities and curved streamlines. The cavitation bubbles grow and travel with the flowing water to an area where the pressure field causes collapse; the cavitation damage can begin at that point leading to pitting of concrete.
A few examples of dams that experienced cavitation erosion: In USA, Dworshak dam, Glen canyon dam, Lower monumental dam, Lucky peak dam, Yellowtail afterbay dam, Keenleyside dam and Hoover dam; in Iran, Karun dam; in Venezuela, Guri dam; in Brazil, Jupia dam; in Pakistan, Tarbela dam.

Fig. 1: Cavitation Erosion at the Glen Canyon Dam

Abrasion Erosion: Damage results from the abrasive effects of waterborne silt, sand, gravel, rocks, ice, and other debris impinging on a concrete surface during operation of a hydraulic structure. Further, the hardness of the particle, quantity of particles impinging on concrete surface, quality of surface and velocity of water affect the extent of damage. Abrasion Erosion is readily recognized by the smooth and worn-appearing concrete surface. Spillway aprons, stilling basins, sluiceways, drainage conduits or culverts, and tunnel linings are the most susceptible to abrasion erosion. Few examples of abrasion of dams: In USA, Espinosa irrigation diversion dam, Kinzua dam, Nolin lake dam, Pamona dam, Red rock dam and Kentucky dam; in Pakistan, Tarbela dam; in Bangladesh, Karnafuli dam; in Brazil, Ilha Solteira dam; in India, Maneri dam and Ichari dam.

Erosion By Chemical Attack: Aggressive water includes soft water, water containing sulphates, chlorides, acids, etc.; the concrete may be affected by any one or a combination of these factors. Soft Water: Because of high dissolving capacity, soft water is known to leach lime from the binding material over a period, resulting in increase in seepage, drainage flows, uplift and a decrease in loss of strength. Some of the examples of dams affected by soft water are: In India, Talkalake dam, Hidkal dam, Nagarjuna Sagar dam, Sree Ramsagar dam; in Italy, Arno dam; in Egypt, Aswan dam; in Australia, Avon dam; in Spain, Caspe dam. Sulphates: Sulphates are commonly found in industrial outflows, acid mine discharge and water in contact with sulphide bearing mineral in earth. The sulphates in water react with the free lime in set concrete, leading to the following sequential reactions:

Fig. 2: Cavitation Damage to the Concrete Wall of Arizona Spillway at the Hoover Dam

• Conversion of Ca(OH)₂ to CaSO₄
• Crystallization of CaSO₄ and consequent increase in volume
• Conversion of hydrated tri calcium aluminates and ferrites in set concrete to hydrated calcium sulpho-aluminates and sulpho-ferrites with consequent increase in volume and disruption
• Decomposition of hydrated calcium silicate causing loss of strength.

Some examples of dams affected by sulphates in India include: Pandoh Dam, Nathpa Jhakri H.E. and Myntdu H.E.
Seepage/Leakage: Seepage may be caused through contraction joint, improper lift joints and cracks. Uncontrolled seepage may further cause leaching, piping and build-up of internal pressure, which may lead to sliding or overturning.
Prominent examples of adverse effects of seepage/leakage on concrete/rubble concrete dams are: In Maharashtra, Koyna dam; in Andhra Pradesh, Nagarjuna Sagar; in Kerala, Periyar dam, and the affected masonry dams are: In Rajasthan, Parbati dam; in Madhya Pradesh, Tigra dam, Bargi dam and Barna dam.
Chlorides: The chief cause of damage by chloride is the corrosion (the reaction of a metal with its environment) of steel in the reinforced concrete, leading to cracking and spalling of the concrete cover. In electrochemical corrosion, metals take part in chemical reaction in solution under the influence of self-induced current. Electrical potential develops when iron/metal is in contact with an electrolyte; the electric current passes due to this potential but the corrosive reaction takes place when oxygen is available at the cathode.

Anodic reaction: Fe —> Fe⁺⁺ + 2 e^–

Cathodic reaction: O₂ + 2H₂O + 4 e > 4(OH)^–

It is believed that a passivating film of Fe(OH)₂ or lime rich iron oxide complex is formed by the reaction of Fe(OH)₂ with highly alkaline pore water (pH 12.5). Even in presence of oxygen, the film protects the metal against corrosion so long as high pH value is maintained. However, presence of chloride ions in pore fluid accelerates corrosion in concrete in several ways, which are:

• They increase conductivity and hence, the activity of the corrosion cell.
• They can reduce the alkalinity of the pore fluid to a level, which allows break down of protective film and hence, allows corrosion to proceed.

On hydration of cement, 90% of the chloride reacts with aluminates and ferrites to give solid tricalcium chloroaluminate and tetra calcium chloroferrite. Thus, only 10% of the chloride, which remains in the pore fluid of concrete and chloride present in aggregate interstices is responsible for corrosion.

Nature Fury
It includes, earthquake induced motion of the impounded water, deformability of foundation rock and interaction of motion of water, foundation rock and dam body. Dams with a height of 100 m or more show some amount of induced seismicity. Induced seismicity may be caused by superimposed water weight, reduction of frictional resistance in underlying rock due to pore pressure and decline in rock strength due to chemical alteration.

Alkali Aggregate Reaction
Alkali-aggregate reaction is a chemical reaction between certain type of aggregates and hydroxyl ions (OH^–) associated with alkalis (Sodium Oxide and Potassium Oxide) in the cement. Usually, the alkalis originate from the Portland cement, but they may also appear from other ingredients in the concrete or from the environment. Under some conditions, the reaction may result in damaging the expansion and cracking of the concrete. Concrete deterioration caused by alkali-aggregate reaction is generally slow, but progressive; map like cracking due to alkali aggregate reaction generally becomes visible when concrete is 5 to 10 years old. The cracks facilitate the entry of de-icing salt solutions that may cause corrosion of the reinforcing steel, thereby accelerating deterioration and weakening the structure.

This alkali-aggregate reaction has two forms:

• Alkali-silica reaction (ASR): Alkali reaction with amorphous silica.
• Alkali-carbonate reaction (ACR): Alkali reaction with dolomitic carbonates. Damage to concrete from this reaction will normally only occur when all the following are present together:
• A high moisture level, within the concrete.
• Cement with high alkali, content, or another source of alkali.
• Aggregate containing an alkali reactive constituent.
• Two classic examples are Hirakud dam and Rihand dam in India

Non-Structural Cracking

Plastic Shrinkage: When the rate of evaporation of bleed water is faster than the bleeding rate, it gives rise to possibility of an effect called ‘plastic shrinkage’. The removal of water by evaporation or suction leads to a reduction in the bulk volume of the concrete and if the concrete cannot follow this change in volume fast enough, such as by settlement, it results in cracks. Plastic shrinkage cracking is most common on flat slabs placed during hot or windy weather which are not protected almost immediately after laying. A particularly severe form of plastic shrinkage cracking occurs when concrete is placed on a high suction substrate.

Drying Shrinkage: Evaporation of moisture at a rate higher than the rate at which water rises to the top is a definite cause of such shrinkage. The environment, RH, wind velocity, temperature of concrete and environment contribute to this drying out.

Thermal Cracking: The cracking occurs in the interior when the temperature rises and, on the surface, when the temperature decreases. The peak temperature rise of concrete is estimated to be around 12^°C per 100 kg of cement content, in cubic metres of concrete. A maximum temperature difference of 20^°C is recommended to limit the potential thermal cracking. It is therefore advisable to keep this gradient inside-outside as low as possible; this can be achieved by preventing heat loss from the surface.

Fig. 3: Alkali Aggregate Reaction (AAR) in Concrete

Other Causative Factors
Poor quality control during construction and fire damage (less reported) also contribute to the deterioration of concrete.

Evaluation Of Structures
The aim of performing evaluation of concrete is to identify and define the area of distress. The objectives are:

• To identify causes of distress and their sources. Systematic documentation of all observations is essential, which will greatly facilitate the diagnosis and assessment of dams;
• To assess the extent of distress occurred due to corrosion, earthquake or any other reason;
• To estimate the residual strength of the structure;
• To assess the extent of requirement of repair and/or rehabilitation. Available space and accessibility will determine the selection of the repair method and repair strategy. Also, prioritization of repairs and their sequencing are important components for deciding the repair strategy.

In-situ tests, sizeable portion of which can be non-destructive, can be performed on concrete at the site itself. The few tests performed at site can be destructive to a limited extent. Furthermore, destructive tests on concrete cores are performed only in the laboratory. The destructive and non-destructive tests provide valuable information.

Non-Destructive Tests

• Surface Hardness
• Windsor Probe test
• Pull-out test
• Pull off method
• Ultrasonic Pulse Velocity
• Core test
• Ground Penetrating Radar
• Detection of Reinforcement in Reinforced Concrete Structures
• Half Cell measurements

Destructive Tests

• Core recovery
• Compressive, pulse wave velocity and other non-invasive tests
• Carbonation test
• pH value of concrete
• Chloride ion content
• Mineralogical tests
• Any other chemical tests as per requirement

Repair Materials/Techniques
Various repair materials have been adopted in the past to rehabilitate concrete structures. The suitability of a material depends on the type and extent of repair, cause of damage, type of structure, economy and availability of materials, etc. Some of the promising repair materials/techniques are mentioned below briefly.

Grouting
Cement grouting is suitable for cracks more than 0.25 mm wide. For cracks finer that 0.25 mm, superfine cement grouting or chemical grouting is suitable. Polyurethane, epoxy and other polymeric compounds are used for grouting cracked concrete.

Fig. 4: Cement Grout

High Strength Concrete (HSC) Concrete with strength higher than 60 MPa can be readily prepared with the use of modern-day chemical and mineral admixtures. The HSC can be used for repairing existing concrete of equal or less strength. Further, it has good bond strength with parent concrete, improved resistance against abrasion/cavitation erosion, and durability. Roller Compacted Concrete (RCC) The concrete produced from conventional concrete ingredients with stiff consistency which can support the load of rollers and can be effectively vibrated by conventional soil compaction equipment, is termed as RCC.  The RCC can be utilized for mass construction and rehabilitation of existing dams with a high speed due to the specific placement method. Further, as the RCC has a dry consistency, low cement content is required to achieve the given strength.

Moreover, in this method, large volume of cement can be replaced with mineral admixture like fly ash. The saving in cement and utilization of waste disposal (fly ash) therefore makes RCC environment friendly and economical.Apart from new constructions, RCC is also suitable for enhancement of spillway capacity, design flood, and construction of impermeable upstream barrier in existing masonry/earth dams. It has been effectively used for enhancing reservoir capacity, improving safety of existing dams against earthquake, overtopping, etc.

Fibre Reinforced Concrete/Shotcrete
Fibre-reinforced shotcrete, as per the report of ACI Committee 506-1R, is defined as mortar or concrete containing discontinuous discrete fibres that are pneumatically projected on to a surface at a high velocity. The fibres for shotcrete can be made of steel, glass, synthetic and natural materials. In shotcrete, for structural and non-
structural purposes, steel fibre is preferred. Recently, synthetic fibres have also been effectively used for underground applications. Further, when the fibres are made of steel, the material is called steel fibre reinforced shotcrete (SFRS). SFRS is thus essentially a conventional shotcrete to which steel fibres are added. The shotcrete may also contain pozzolana and other admixtures used with the conventional concrete.
The addition of fibres in shotcrete matrix mainly improves its toughness, impact and fatigue resistance. In dams, SFRS is effective against abrasion/cavitation erosion and impact resistance. It offers a reduced rate of damage by arresting crack propagation. However, SFRS is not so effective in low velocity water flows carrying small size pebbles.

Sprayed Concrete
The Indian Standard 9012-1978 defines sprayed concrete/shotcrete as mortar or concrete conveyed through a hose and projected on to a surface at high velocity. The sprayed concrete is highly suitable for application in restricted areas, as no form work is required in this process and it can be applied in any profile. Also, it can be used on u/s face of masonry dam to reduce seepage.

Further, it is most suitable when delicate embedded parts are required to be protected from getting misaligned due to vibration, for example, second stage concreting in gate grooves in hydraulic structures.

Other Materials/Techniques

• Epoxy mortar/concrete
• Polymer impregnated concrete
• Polyurethane coating/neoprene paint on spillway to guard against cavitation
• Steel cladding for erosion protection
• Cable anchors to anchor dam body with foundation to counter development of tensile strength
• Geomembrane on u/s face to control seepage in RCC dams/masonry dams and protection against ASR/acids
• Chemical Rust removers for corroded reinforcement
• Passivators for reinforcement protection
• Surface coatings for protection of RCC
• Steel/concrete jacketing
• Raking and Pointing

Fig. 6: Self-Compacting Concrete

Case Studies Of Dam Repairs  Rihand Dam Project, Uttar Pradesh It was constructed during the period 1954-62 and is across the river Rihand, Pipri village, district Sonebhadra, Uttar Pradesh, India. It is a 91.96 m high concrete gravity dam, which includes 61 blocks, 13 spillway blocks,and 6 intake and powerhouse blocks. A powerhouse having 6 units of generation capacity: 6×50 MW = 300 MW. Rocks at the dam site mainly consist of granite, gneiss, and minor bands of phyllite, schist and quartzite.

Within a decade of commissioning of the project, the following manifestations of structure’s distress started appearing:
In some of the machines, clearance between moving and stationary parts had gone beyond permissible limits and there was frequent tripping.
Also, problems were encountered in moving draft tube crane, sealing of intake gates, operation of stop log gates, spillway and passenger lift.

• Map like cracks were witnessed at various locations.
• High level technical experts committee set up in 1985 for safety of the dam observed that the development of cracks in the concrete were solely due to the onset of alkali-aggregate reaction (AAR). The remedial measures suggested by the committee were implemented. The measures were:
• Closing of emergency passenger lift located in shaft of the block 34 at RL 830 ft
• Rehabilitated of the penstock gallery columns showing distress by epoxy grouting and steel jacketing.
• Treatment of cracks using epoxy
• The treatment of the cracks is being followed by regular monitoring of cracks using 2D and 3D crack monitors.

Fig. 7: Cracks after being Treated with Epoxy. Cracking Pattern Typical Of AAR.

Kadamparai Project, Tamil Nadu The Kadamparai Project, commissioned in 1983, is in the Annamalai Hills of the Coimbatore district, Tamil Nadu. It is a composite structure consisting of a central stone masonry gravity dam with earthen embankment sections. Further, the masonry dam is 67 m high and 478 m long, with a central spillway, a scour vent tower and one inspection gallery. It is used as a fore bay reservoir to the Kadamparai Pumped Storage Hydro Electric Project. The dam started facing the problem of excessive seepage through the drainage gallery. Consequently, several repairs were carried out between 1990 and 2000, such as packing and pointing at selective locations on the upstream face, vertical drilling and grouting from the crest at close intervals, and underwater treatment of the leaking areas by chemicals and cement. The repair work at first reduced the leakage from 4,200 to 800 l/min, but eventually proved to be unsuccessful and the leakage increased again, reaching 11,800 l/min.

After a detailed study, it was decided to dewater the dam first. Thus, the lake water was stored in Lower Aliyar project and the u/s face and the inside portion of the dam was fully treated by grouting and standard measure. Thereafter, the dam was carpeted on the u/s face with geomembranes as per details given below:

• First Layer – Geotextile 1 cm thick (to provide mobility to seepage water, if any)
• Second Layer – 3-4 mm thick impervious geo composite (geomembrane + geotextile)

Moreover, seepage measuring sensors were placed at appropriate locations during the carpeting exercise. The seepage condition before repairs was 38,000 l/min, whereas post the repair, seepage in the entire dam body mass was 60 l/min. The dam has been functioning without any hindrance since 2005. The total cost of the project was Rs. 12 Crores with a warranty of 10 years.

Fig. 8: Heavy Seepage Condition in the Drainage Gallery before Repair Dewatering

Fig. 9: First Layer of Geotextile followed by Geocomposite (geomembrane with a layer of geotextile)

Fig. 10: Complete Carpeting with Geomembrane (impervious barrier)

Fig. 11: Reservoir Filling after Carpeting

Fig. 12: Condition of Drainage Gallery after Reservoir Filling

Conclusion

The responsibility of undertaking the repair and rehabilitation of a structure lies with the owner. Therefore, it becomes incumbent upon the owner to assess the repair/rehabilitation needs using modern tools. Once done, cost-effective solutions are to be applied diligently to enable the structure to serve its intended purpose and lower the risk of failure considerably.

References

1. Aboutaha, R. S., & Jirsa, J. O. (1996). Steel jackets for seismic strengthening of concrete columns. In Proceedings of the 11th World Conference on Earthquake Engineering.
2. CSMRS Report on testing of Epoxy Concrete Mixes for Singur Dam, AP
3. Ghobarah, A., Aziz, T. S., & Biddah, A. (1996). Rehabilitation of reinforced concrete beam-column joints. In Eleventh world conference on earthquake engineering, Mexico.
4. Guerrero, J. J., Gomez, B., & Gonzalez, O. M. (1996). Jacketing of reinforced concrete members. In World conference on earthquake engineering. Elsevier Science Ltd.
5. Metcalf, M., Dolen, T.P., & Hendricks, P.A. (1992). Santa Cruz Dam Modification. Proceedings of Conference sponsored by ASCE, Roller Compacted Concrete III, Edited by K.D. Hansen and F.G.Mclean.
6. ACI MNL-3(16) Guide to the Code for Assessment, Repair, and Rehabilitation of Existing Concrete Buildings.
7. ACI 207.6R-17: Report on the Erosion of Concrete in Hydraulic Structures.
8. ACI 364.1R-19: Guide for Assessment of Concrete Structures Before Rehabilitation.
9. ICOLD Bulletin – 165 Selection of Materials For Concrete In Dams
10. ICOLD Bulletin 135, 2010 Geomembrane sealing systems for Dams.
11. ICOLD Bulletin 119, 2000Rehabilitation of dams and appurtenant works – State of the art and case histories. State of the art and case histories
12. ICOLD Bulletin 107, 1997 Concrete dams – Control and treatment of cracks.
13. Control and treatment of cracks
14. ICOLD Bulletin 79, 1991 Alkali-aggregate reaction in concrete dams – Review and recommendations. Review and recommendation
15. ICOLD Bulletin 71, 1989 Exposure of dam concrete to special aggressive waters-Guidelines