Rehabilitation of Buildings and Bridges

Rehabilitation of Buildings and Bridges

Name: K.E.Ramesh Kumar

E-mail: cit_ramesh@vsnl.net

ABSTRACT

The paper emphasize on Rehabilitation of Buildings and Bridges. The purpose of the paper is to highlight the methods of repair and rehabilitation to be undertaken for structures with defects and deficiencies that necessitate rehabilitation. The paper outlines the issues and trends in the sphere of repair and rehabilitation of structures and gives a compelling description of the latest developments, which could shape the future along with its potential for further innovation. Repair and Rehabilitation methods currently used are reviewed on the basis of present knowledge and the merit of a holistic system approach, which takes into account not only the individual processes and phenomena but also most importantly their interaction. This paper focuses on visible symptoms of the problem rather than on visible and invisible problems as well as the possible causes behind them. This paper focuses about the repair materials and the techniques used since the use of appropriate repair materials and techniques is essential for the satisfactory performance of the repaired structure. This paper presents an analysis of concrete illnesses, curing treatments, and problems leading to unsatisfactory performance of repaired concrete structures. This paper describes the types of surface preparations that are commonly adopted in Indian conditions and their limitations. An attempt has been made in this paper to discuss the properties and types of grouts, the application techniques. The paper highlights the problem of corrosion of reinforcing steel in concrete structures and attempts to provide the measures available in design to mitigate the effects of corrosion. The various types of coatings available and the precautions to be taken in the selection of coating systems in view of these limitations are also discussed. This presents a review on the use of silica fume to control damaging alkali-silica reaction in concrete, with particular emphasis on the development of a new standard practice. In particular, the latest information on important technical findings pertaining to hot-dip galvanizing is discussed. This highlights the importance of epoxy resins and systems in the construction/civil engineering applications such as repairing of cracks, patching and grouting of concrete, industrial flooring, structural adhesives, anti-corrosive linings, etc. This also discusses how electrochemical repairs of reinforced concrete structures are proving to be highly effective in terms of durability, life cycle costing and the ability to extend concrete protection beyond the boundaries of localized patch repairs. Traditional measures are not covered in detail and the newer technologies and materials are featured. The paper concludes with a typical session of the expert system, which is a system for diagnosing causes and repairs of defects in reinforced concrete structures.

INTRODUCTION

Structural rehabilitation work has come to the forefront of industry practice in response to the aging of buildings and civil infrastructure worldwide. The general decrease in the new construction activity forced the concrete repair and rehabilitation. Current repair and protection techniques and practices used in the rehabilitation of deteriorated structures have been derived largely from methods used in new construction. An effective retrofit solution is one that is durable and relatively easy to design and evaluate, while minimizing the need for specialized labour and service disruption and lowering total costs, even when requiring more expensive material.

The tasks of the structural engineers are not only designing and building new structures. Increasingly, they are called up to consider the maintenance, repair and rehabilitation of existing structures, both bridges and buildings. The Rehabilitation of structures include the following

Inspection methods, assessment, monitoring, maintenance of structures

Concrete durability, fatigue issues in bridges, seismic strengthening issues, laboratory studies, dynamic testing and analysis

Seismic strengthening of bridges and buildings

General repairs for bridges and buildings

The rehabilitation methods involve the attachment of new materials to existing structures or applying protective coatings to the structures. Research in rehabilitation includes the prevention of corrosion of steel which is the most important structural member used in the construction.

Research in design, behavior, and analysis of reinforced building and bridge construction includes studies in materials, components, and complete structural systems. Materials studies have included normal- and high-strength concretes; effects of materials aging and materials deterioration on component properties; bond between concrete and steel reinforcement, and use of headed reinforcement.

The current research interests also include design, behavior, and analysis of reinforced concrete buildings and bridge construction with an emphasis on earthquake engineering applications; seismic evaluation and rehabilitation of existing construction; and performance-based earthquake engineering. REHABILITATION OF STRUCTURES ( Buildings and Bridges )

Structure rehabilitating or retrofitting is a process whereby an existing structure is enhanced to increase the probability that the structure will survive for a long period of time and also against earthquake forces. This can be accomplished through the addition of new structural elements, the strengthening of existing structural elements, and/or the addition of base isolators.

Structural repair of reinforced concrete members incorporates numerous repair techniques depending on the structure's type of reinforcement, extent of deterioration, type of distress and logistical access points. Different types of reinforcement require various demolition and surface preparation techniques. Typically, structural deterioration of reinforced concrete members can occur as surface scaling, spalling, cracking, corrosion of reinforcing steel, weathering, post-tension losses, deflection beam shortening, volume shrinkage and strength reduction. Moisture, chlorides, carbonation, and chemical attack induce these; freeze thaw disintegration, and sulfate attack, erosion and alkali aggregate reaction.

The rehabilitation measures includes epoxy mortar, epoxy bonding coat, epoxy grout, polymer based bonding slurry and mortar, jacketing of columns, shotcreting, epoxy grouting, cement grouting accordingly to the type of distress. The members load bearing capacity, structural shape and location greatly influence material placement techniques and material selection.

Rehabilitation of bridges is confined to both superstructure and substructure. The distress generally observed is appearance of cracks, honeycombing, and spalling of concrete and corrosion in reinforcement/metallic bearings in simply supported and balanced cantilever bridges with articulations. The rehabilitation measures include application of cement/polymer modified mortar, guniting, shotcreting, cement/epoxy grouting, epoxy protective/bonding coating, repairing of articulations by lifting suspended spans or without lifting suspended spans by transferring their load by prestressing cables to adjacent spans, replacement of bearings by lifting superstructure, repair/replacement of expansion joints and strengthening of bridges by external prestressing. Deficient bridges are to be rehabilitated according to their needs. Bridges must be rehabilitated to the same criteria as new bridges. The techniques to achieve earthquake resistant design includes; adding base isolators, wrapping columns, strengthening footings, adding hinge restrainers, and increasing the width of supports at abutments so that the superstructure will not fall off the support.

In rehabilitation process good/sound concrete sharing the load should not be removed for any reason, as is being done today. The second principle of restoration of structurally distressed RC members is to restore the building portion by portion.

Strengthening physically-deteriorated concrete structuresBuildings and other concrete structures suffer deterioration due to a variety of reasons. These may vary from corrosion of rebars to deficient workmanship at site during construction.

The first principle should therefore be that the treatment should be limited to locations where the problem (e.g. spalling of cover concrete due to corrosion of rebars) occurs. The efficiency of various techniques like guniting, jacketing, etc. in restoration can be considerably enhanced. It is practically impossible to de-stress the RC members to be restored before treatment and reload them back after the restoration - as restoration has to be taken up in as is condition, and when the building is occupied.

CONCRETE REPAIRS

Concrete repair has moved from a relatively unknown phenomenon to a position of international importance. Characteristics of the repair product may now be stipulated; these may include, for example, strengths, bond and permeability characteristics. Equally as important is the limitation of drying shrinkage. Excessive shrinkage can lead to the negation of other specified properties.

Surface preparation and Interfacial bond for application of patch repairs, sealers and coatings in concrete repair

The main purpose of surface preparation is to provide maximum coating adhesion and to increase the surface area by increasing the roughness of the surface. Indeed, in order to achieve a proper rehabilitation job, it is important to have proper surface preparation. The durability of the bond in repair systems is a lasting interfacial coexistence of repair and existing phases. Achieving an adequate lasting bond between repair materials and existing concrete is a critical requirement for durable concrete repair. Good surface preparation using proper concrete removal methods and workmanship is the key element in a long-lasting concrete repair technique.

GROUTING PROCESS

Grouting is the process of placing a material into cavities in concrete or masonry structures for the purpose of embedding reinforcing bars, increasing the load bearing capacity of a structure, restoring the monolithic nature of a structural member, filling voids around precast connections and steel base plates, providing fire stops, stopping leakages, placing adhesives and soil stabilization. Primary grouting materials and their common uses are:

PRIVATECHEMICALControl SeepageShut-off SeepageSoil StabilizationCEMENTITIOUSMass PlacementArchitectural (non staining)Structural (high-strength)Caustic EnvironmentsHigh Temperatures

EPOXYSeal CracksBolt AnchoringBase Plate LevelingAcidic EnvironmentsPOLYURETHANESBuilding Envelope InsulationAcoustical Sealant

POLYESTERSBolt AnchoringSILICONESSmoke SealsFire Stops

Grouting is one of the age-old tools, which is becoming more and more useful with the progress in material science. A large spectrum of new materials - organic polymers as well as inorganic chemicals - is being used in structural and non-structural repair of concrete structures.

Methods of application normally used include: hand pumps, piston pumps, single and plural component pumps, gravity and dry packing placement, micro capsules and single component pressurized cartons. In practice, grouts must be placed at pressures consistent with good engineering practice and at rates, which make use of grout materials economical and successful.

Grouting is a procedure in which grout replaces the natural fluids or materials in the formation of a void. The specific mechanical properties of a grout are important factors in the selection of a grout for a specific job. These include mechanical permanence, penetrability and strength. Similarly, the chemical properties include chemical permanence, gel time control, sensitivity and toxicity and the economic factors of availability and cost.

Used for soil stabilization - Characterized by low mixed viscosities, controllable/variable gel times, ability to handle dilution (set up in moving water), low tensile and compressive strengths. Two components (dual pumps) usually used, although not required for urethanes only.

"iso" component must be added to main ingredient, cannot tolerate as much moving water as gels and hydrophilic foams, much higher tensile and compressive strengths, little expansion or elongation. One or two component pumps used.

GUNITING PROCESS

Guniting is an effective technique, which has been extensively used in the rehabilitation of structurally distressed RC members. There have been cases of heavy rusting of the mesh in the form of powder or in the form of a sheet coming out. In the present practices the cover concrete is removed on a mass scale, and even good/sound concrete is removed forcibly by chisel and sledge hammer, has to make way for a more focused treatment of the affected part of the structure.

It is not known if the restored portion of the RC member shares the load as it did prior to the treatment. De-stressing before restoration is possible only in the case of overhead tanks which can be restored when the tanks are empty.

The guniting technique suffers from other drawbacks like dust and noise nuisance. Though the guniting technique is now being increasingly replaced by other methods, the following points need to be kept in mind for better results:

Coating of existing as well as new bars by zinc rich epoxy primer to guard against corrosion.

Mesh reinforcement is not advised. In the present-day practice of guniting, mesh reinforcement is invariably placed further. This is not coated with any protective coat against corrosion and many times the mesh reinforcement provided is already rusted before guniting is taken up.

JACKETING PROCESS

In this method the entire height of the column section is increased and a cage of additional main reinforcement bars with shear stirrups is provided right from the foundation as per the requirement of additional load, etc. However, there are many instances where the column section is increased with additional reinforcement bars only on one face, and that too starting from the floor slab level of a particular floor and only up to the height of deterioration of the column. This additional section with additional reinforcement helps by reducing the floor space of the room with obstacles. This is commonly referred to as jacketing.

Application of Epoxy Resins to strengthen the Structural member with external reinforcementThe prolonging service life and enhancing the load-carrying capacity of reinforced concrete (RC) structures calls for new approaches in addition to the well-established techniques. The recent development in the field of structural adhesives, particularly the ones that are based on epoxy resins, has led to the development of new technique of strengthening RC structural by adhesively-bonded external plates. In these methods of strengthening, an epoxy adhesive (normally consisting of two components - a resin and a hardener) is used to bond steel plates to overstressed regions of RC members. Normally, the steel plates are located in the tension zone of concrete to enhance the flexural capacity. The plates can also be placed in the compression and shear regions for enhancing the axial and shear-capacities of the RC structural elements. As the adhesive provides a continuous shear connection between the RC member and the external plates, a concrete-adhesive-steel composite structural member is developed to cater for the additional live load effects on the structures.

CORROSION OF STEEL IN RC STRUCTURES

Concrete provides a thick and dense barrier and also creates and inhibitive environment that helps in preventing reinforcement corrosion. But sometimes concrete fails and hence the structures should be provided with economic and long-term protection against corrosion problems. Corrosion of steel in concrete occurs when the passive state of the steel is altered. When the building is constructed and poured with "new" concrete, the pH of the concrete was approximately 12-13. The high pH offers a protective environment for the passive steel. However, due to a variety of factors, concrete structures deteriorate. The two mechanisms that can change the passive state: Chlorides (salt contaminates) and Carbonation.

Salt Contaminates: The chloride salt produces a continual layer of moisture on the steel. Once the salt comes in contact with the steel, corrosion begins.

Carbonation: The second mechanism which causes corrosion of steel and the deterioration of the concrete is the chemical reaction of carbon dioxide with the concrete to produce carbonates. The reaction of carbon dioxide with alkaline hydroxides in the cement lowers the pH of the concrete environment around the steel, and the corrosion process is initiated.

Once the reinforcing steel begins to corrode, layers of corrosion products (rust) form on the surface of the steel, causing a tremendous amount of pressure within the concrete. The pressure eventually exceeds the tensile strength of concrete and the surfaces of the concrete rupture, producing spalls on the surfaces and major cracks. When spalling of concrete occurs, corrosion activity has advanced to an extreme level and a major concrete rehabilitation project is just around the corner.

PROTECTION AGAINST CORROSION

The technologies to protect against corrosion include cathodic protection (CP) for concrete and metallic structures, realkalisation and chloride extraction, anodic protection for vessels in very high or very low pH environments and on-line corrosion monitoring. Patching practices, membranes & sealers, painting, galvanizing and other coating systems can also prevent steel corrosion. Tremendous expense is involved in rehabilitating reinforcing steel in concrete.Fixing of chloride for corrosion protection

A method to reduce chloride in concrete has been attempted by using aluminium oxide as a chemical admixture. The conclusion is that aluminium oxide may effectively reduce the free chloride content in concrete and thereby reduce the risk of reinforcement corrosion.

PROTECTIVE COATINGS FOR CORROSION PROTECTIONSteel can often be protected adequately by the application of suitable coatings. Coatings to concrete provide protection against reinforcement corrosion, chemical attack, and water and salt intrusion. The traces of impurities can significantly reduce the rate of corrosion and can be added in low concentration to the surrounding medium. Paint is the most common coating used to slow the rate of atmospheric corrosion. Many other materials, such as plastics, ceramics, rubbers, and even electroplated metals, can be used as protective coatings. The corrosion resistance of a metal can be greatly increased by the proper choice of alloys.

A suitable protective coating system will go a long way in enhancing the durability of such structures under aggressive exposures. Among the various coating systems, two systems namely, fusion-bonded epoxy coating and hot-dip galvanizing have found wide use.

A user-friendly method based on polymer-cement-inhibitor coating on steel which can give a good impermeable membrane as well as effective passivating environment for steel. Test results indicate that such treatment could be useful for both stressed as well as normal steel.

A significant development occurred in the coatings to manufacture powder coatings. These powders, based on thermosetting epoxy, polyester or acrylic technology, are electro statically sprayed. For general industrial metal finishing they offer advantages in film toughness as well as eliminating solvent emission.

CATHODIC PROTECTION FOR CORROSION PROTECTION

Cathodic protection is an electrochemical method of preventing corrosion and is being installed on various condominiums and other concrete structures for corrosion protection. This reduces future rehabilitation resulting from rebar corrosion. It has been used to prevent corrosion for more than 70 years.

One method of cathodic protection utilizes thin layers of zinc anode that are installed on the floor or ceiling of a balcony and connected to the steel rebar. When two dissimilar metals, zinc anode (1.1 volts) and steel rebar (.5 volts) are connected, the metal with the higher voltage (zinc) will generate a small electrical current from the zinc to the steel rebar. Through a series of electrochemical reactions at the surface of the steel, corrosion activity will be arrested. The system stops the corrosion process and eliminates deterioration of steel rebar and subsequent damage to concrete surfaces.

This electrochemical method of corrosion prevention is becoming a standard part of preventive maintenance for condominiums and can be applied at the time of new construction, before concrete damage has occurred. It can also be applied on old buildings or after a restoration project has been completed to stop future corrosion. Applying a cathodic protection system can reduce the overall cost of present and future concrete restoration projects.

MATERIALS USED IN REPAIRS

The polymer modified cement/concrete mortars; epoxy resins, high performance cement, polymer based materials and FRP materials are best suited for repair and rehabilitation jobs. The use of advanced composite materials using fibre-reinforced composites is the latest technique in the emerging market of structural rehabilitation industry. The materials used for patching and other filling purposes should propagate much slower, thereby extending the lifetime. The materials should possess high degree of extensibility.Polymer modified concrete/cement mortar

The use of polymer-modified concretes for major rehabilitation works is gaining wider acceptance in the Indian subcontinent. Despite increased use of polymers in concrete, there are no standards and specifications, particularly in India, to guide the specifiier, client and contractor. This is based on first hand experiences of repair and restoration works of high rise buildings, bridges, marine installations and bomb-blast affected structures.

Fiber-Reinforced Plastics

The Fiber-reinforced plastic composites are now being considered for many possible applications in infrastructure repairing and retrofitting. Bonding fiber-reinforced composites on steel or concrete structures should increase their service life and fortify them against earthquakes or other natural hazards. The materials that are used for cracks are applied over it like a patch, using high strength epoxy adhesive.

Epoxy resins

The epoxy resins are widely used in the repairing of cracks, patching and grouting of concrete, industrial flooring, structural adhesives, anti-corrosive linings, etc. Various types of resins, hardeners and modified epoxy systems are commonly used in structures. Polymer-based materialsPolymer-based materials are being widely used in the building industry in various forms such as coatings, membranes, adhesives, sealants, etc because of their high durability. These materials possess predictable performance, the designer must have a good knowledge and understanding of the properties of polymers, how the materials will interact with the environment in-service, and a clear description of maintenance procedures and intervals.

High Performance Cement

High performance cement is the cement along with new complex admixture. Intergrinding the complex admixture with clinker, gypsum, and selected mineral admixtures allows to rise the strength and durability of the cement. Strength increasing is the demand for engineering of cement with high volume of mineral admixtures or utilizing of industrial by-products and waste in the cement composition. This phenomenon allows to use a required amount of mineral admixtures like granulated blast furnace slag (35- 50%) in the composition to increase chemical and thermal resistance.

High performance cement based mortars possess low permeability, high resistance to chemical attack, thermal resistance, and excellent freezing and thawing resistance. Very low permeability of HP cement systems provides high resistance to chemical attack. HP cement system possesses excellent freezing and thawing resistance. The production and application of HP cement and concrete significantly increases the lifetime of structures and reduces cost of repairing and rehabilitation works due to high durability.

Fibre reinforced polymer tubes for pile/column

Fibre reinforced polymer (FRP) can be used for bridges to prevent corrosion. The FRP tube filled with concrete seems to be a good alternate to address this problem. The FRP tube can be engineered to provide sufficient confinement to filled concrete and to increase the capacity of the section in shear and compressive strength and also provide increased resistance to earthquake forces.

Durability of Concrete Repairs

One of the primary factors assuring durability of concrete repair is its resistance to cracking. The control of cracking depends upon the amount of strain in tension before cracking occurs, that is, on the tensile strain capacity of the system. The main thing is the understanding of the concept of tensile strain capacity on extensibility of the repair phase. Crack prevention or its significant reduction can be achieved by providing materials with a high degree of extensibility as a response to restrained volume changes for protective repairs.

Strengthening chemically-deteriorated concrete structures

The structural integrity of chemically deteriorated reinforced concrete beams is restored by repairing one set of beams by epoxide resin latex and another by polymer-based latex system. It is interesting to observe an increase in the load-carrying capacity and rigidity of the beams after repair and rehabilitation work of the structure.

Fiber-Reinforced Polymer Bridge Decks Fiber-reinforced polymers or FRP's are robust materials that are highly resistant to corrosive action, have a high strength to weight ratio and are well suited for assembly line production into modular components that can be rapidly erected. However, FRP material costs are significantly greater than traditional concrete and steel materials. Therefore, cost savings due to either reduced weight, increased speed of construction or lower maintenance and increased life expectancy must offset this higher cost to make sensible use of FRP materials.

Because of the severe environmental conditioning that bridge decks are subject to and the fact that they account for a major percentage of a bridge structures dead load, they are the most suitable bridge application for FRP materials. An 8-inch deep FRP deck weighs approximately 20-lbs./sq. ft. as compared to 100-lbs./sq. ft. for a concrete deck of the same depth. In addition, FRP decks can be constructed faster than conventional cast-in-place decks that take more time due to formwork construction, rebar placement and concrete curing.

Overview of Typical Deck Systems

The majority of the FRP deck systems on the market today utilize glass-reinforcing fibers set in a polyester or vinyl ester resin matrix. Other FRP material systems that utilize carbon or aramid fibers and epoxy resins offer superior structural performance characteristics but are cost prohibitive for use in bridge deck systems.

The typical deck systems on the market today consist of two principal types: pultruded tubes that are bonded together with adhesive and honeycomb or sandwich core systems that are hand laid-up or utilize vacuum assisted resin transfer molding techniques.

The overlay system can consist of a conventional latex concrete, micro-silica concrete or high-density concrete; however, these types of overlays do not have comparable stiffness, tensile strength and compressive strength properties as compared to FRP deck systems. Thin polymer modified concrete and epoxy overlays are better suited for FRP deck applications.

Hot-applied asphalt has been used as an overlay for FRP decks; however, the temperature of the asphalt typically exceeds the glass transition temperature of the resin. FRP materials begin to lose their rigidity as they approach the glass transition temperature of the polymer and start to exhibit a viscoelastic type behavior. The corresponding effect on the behavior and performance of the deck should be analyzed and tested prior to the use of a hot-applied asphalt overlay.

FRP decks that are supported by beams require a haunch or fillet between the beam and deck to provide adequate tolerance to accommodate geometric imperfections introduced during fabrication or erection of the beams. Either a conventional non-shrink grout or polymer-modified grout can be used to form the haunch. Regardless of whether the bridge is designed for composite or non-composite action under superimposed loads, the deck must be connected to the beams with a nominal number of connectors in order to provide adequate confinement of the haunch. Otherwise, the haunch will break apart over time as the deck rotates over the beam line and separates from the haunch as the deck is subject to unsymmetrical live loading.

The method of connecting FRP decks to beams is one topic. Welded shear studs contained within grout-filled pockets have been used successfully for connecting FRP decks to steel beams to achieve full composite action. The key to this approach is to ensure adequate strength and confinement of the grout in order to develop the required connection capacity. The advantage to this approach is that it utilizes conventional technology; however, the trade-off is the increased fabrication cost associated with cutting holes in the deck and forming the pocket at each connection location.

Cost Effective Applications

The cost of FRP deck systems is approximately two to three times that of conventional cast-in-place reinforced concrete decks. This high initial cost must be offset by either a savings associated with a reduction in life cycle cost or a savings associated with the reduction in dead load. Thus, the two primary market applications for FRP decks are replacement of deteriorated concrete decks on high volume roadways and rehabilitation of weight sensitive structures. The benefit of FRP decks is that they result in a system that can be rapidly erected and offer-enhanced durability that significantly reduces the need for future rehabilitation.

The second major area is related to weight sensitive structures including cable-stayed, suspension, arch, moveable and truss bridges. However, the most promising markets seem to be replacement of open grating decks on moveable bridges and replacement of open steel grating or concrete decks on truss bridges. FRP decks offer the advantage of a closed deck system that protects the floor system of the bridge thereby increasing the overall durability of the structure. In addition, there are a significant number of existing truss bridges with concrete decks that are deficient with respect to live load capacity. FRP decks are approximately 20 percent of the weight of a concrete deck. By replacing an existing concrete deck with an FRP deck, the live load carrying capacity of the bridge can be increased without requiring significant rehabilitation or replacement of the main structural members.

FRP deck systems offer the benefit of a lightweight decking system that can be rapidly erected and provides excellent long-term durability. FRP decks systems are available today as a viable alternative to traditional decks. Nonetheless, further research, development and validation of FRP deck systems is necessary in order to further optimize and standardize these decks systems so that they gain widespread acceptance in the industry. Standard design and construction specifications are necessary to properly design and build FRP decks. FRP's have long-term durability characteristics relating to fatigue, freeze-thaw, creep and moisture.

FRP decks will likely never be competitive with conventional deck systems based upon first cost alone. Our industry needs to make a fundamental change in our approach to quantifying costs and making decisions regarding the use of specific structure types and components. The appreciable advantage of the speed of construction and durability attributes the FRP decks and reduces the future rehabilitation works much.

balanced cantilever bridges

A number of bridges using balanced cantilever superstructure with suspended spans have been constructed in different parts of the country. However in view of inadequate detailing of reinforcement and lack of proper precautions during concreting of the crucial articulation portion, many of these bridges experienced problems in service and needs rehabilitation works.

Quality assurance tests for corrosion resistance of steel reinforcement of bridgesFor enhancement of durability of reinforced concrete (RC) structures in marine environment like bridges, different protective systems, namely coating to steel/concrete surface, corrosion resistant rebars, addition of inhibitor admixtures, etc are adopted. The efficiency of each system should be ensured by conducting appropriate quality assurance tests.Seismic strengthening of existing concrete structures

A large number of reinforced concrete (RC) structures in India are deficient for seismic design. The reasons for this state include: modifications in seismic zone map, upgrading in design seismic force, advancement in engineering knowledge. The earthquake resistant design of existing structures includes the addition of non-structural elements or equipments in order to resist the lateral forces acting on the structure.The basic approach underlying more advanced techniques for earthquake resistance is not to strengthen the building, but to reduce the earthquake-generated forces acting upon it. Among the most important advanced techniques of earthquake resistant design and construction are:

Installation of Isolators

Installation of Energy Dissipation Devices

Rehabilitation Measures to be taken before Construction

Long term Durability with Selection of sand

Quartz sand does not contain silt and gives high strength along with polymer mortar. Good quality river sand with permissible silt content is available almost at 1/3rd the rate of quality quartz sand. Both river sand and quartz sand have similar properties. The strengths of the RC members, which are taken up for restoration, are designed for M-15 or M-20 and strength of the polymer mortar gives good strength even when river sand is used. This should be a point to consider especially since quartz sand costs almost three times as much as good river sand.

Long term Durability with selection of concrete

High performance concrete of grade M 75, using silica fume, is being used for the purpose of long durability. Silica fume is not only used as a part replacement or addition to cement in a concrete mix but also to enhance the performance characteristics of concrete. Silica fume concrete is now being used in the construction of hydraulic structures like bridges. The use of silica fume in combination with potentially-reactive aggregates and determines the minimum level of silica fume on the basis of the degree of aggregate reactivity, the alkali content of the cement, and the nature of the structure. This approach is supported by data from expansion tests on concrete in the laboratory and under field conditions, and by the results of studies on the effect of silica fume on the composition of the pore solution. By the use of this type of concrete the Alkali silica reaction in concrete can be prevented.

Corrosion protection with hot-dip galvanizing

Hot-dip galvanized steel has been effectively used. The value of galvanizing stems from the relative corrosion resistance of zinc, which under most service conditions is considerably better than iron and steel. In addition to forming a physical barrier against corrosion, zinc, applied as a galvanized coating, cathodically protects exposed steel. Furthermore, galvanizing for protection of iron and steel is favored because of its low cost, the ease of application, and the extended maintenance-free service that it provides. Galvanizing primary component is zinc. The fundamental steps in the galvanizing process are:

Soil & grease removal - A hot alkaline solution removes dirt, oil, grease, shop oil, and soluble markings. Pickling - Dilute solutions of either hydrochloric or sulfuric acid remove surface rust and mill scale to provide a chemically clean metallic surface.

Fluxing - Steel is immersed in liquid flux (usually a zinc ammonium chloride solution) to remove oxides and to prevent oxidation prior to dipping into the molten zinc bath. In the dry galvanizing process, the item is separately dipped in a liquid flux bath, removed, allowed to dry, and then galvanized. In the wet galvanizing process, the flux floats atop the molten zinc and the item passes through the flux immediately prior to galvanizing.

Galvanizing - The article is immersed in a bath of molten zinc at between 815-850 F (435-455 o C). During galvanizing, the zinc metallurgical bonds to the steel, creating a series of highly abrasion-resistant zinc-iron alloy layers, commonly topped by a layer of impact-resistant pure zinc.

Finishing - After the steel is withdrawn from the galvanizing bath, excess zinc is removed by draining, vibrating or - for small items - centrifuging. The galvanized item is then air-cooled or quenched in liquid.

Galvanizing is used throughout various markets to provide steel with unmatched protection from the ravages of corrosion. A wide range of steel products from nails to highway guardrail to the Brooklyn Bridges suspension wires to NASAs launch pad sound suppression system benefit from galvanizing superior corrosion prevention properties. Galvanizing delivers incredible value in terms of protecting our infrastructure. Less steel is consumed and fewer raw materials are needed because galvanizing makes bridges, roads, buildings, etc., last longer. Additionally, because galvanized steel requires no maintenance for decades, the rehabilitation against corrosion of steel is insufficient.

CONCLUSION

The severely deteriorated reinforced concrete frame structures continue to live for years disproving apprehensions of imminent mishap. If carefully nursed through proper rehabilitation techniques based on the fundamental principles elucidated earlier, many a building can be restored to a state of health and vitality. The materials used for repair and rehabilitation purposes should have high degree of extensibility and should possess long-term durability. As of today the rehabilitation technique by composite materials with fibre-reinforced composites addresses the problems of restoration admirably and is perhaps the best suited. The proper method of rehabilitation for different structures should be adopted. The rehabilitation method should be performed without de-stressing the structure. The performance of the repaired or rehabilitated structure is more important such that it should unnecessitate the future rehabilitation for the fore coming years. The need for rehabilitation is due to the improper design of the structures during construction. Just constructing the structures with long-term durable materials can eliminate the rehabilitation process. Steel for reinforcement in structures should be provided with proper protection against corrosion. The structures to be built in future should be installed with corrosion resistant reinforcement bars. Various methods for corrosion prevention are there and the right one for the purpose can be chosen. The modifications in seismic zone map, upgrading in design seismic force and advancement in engineering knowledge has proved that every place in India is predicted to suffer earthquake forces. Hence the existing structures and the structures to be built in future has to seismically designed to resist the lateral forces acting on the structure from the ground motion.