JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABADB. Tech IV Year II SemesterExaminations, April - 2014REHABILITATION AND RETROFITTING OF STRUCTURES(Civil Engineering)1. Describe the causes of distress in structures and explain the methods of prevention of distress in structures. (15M)There are lot many causes in generating distress in a structure, some of the causes are: 1. Accidental Loadings 2. Chemical Reactions a. Acid attack b. Aggressive-water attack c. Alkali-carbonate rock reaction d. Alkali-silica reaction e. Miscellaneous chemical attack f. Sulfate attack 3. Construction Errors 4. Corrosion of Embedded Metals 5. Design Errors 6. Inadequate structural design 7. Poor design details 8. Erosion 9. Abrasion 10. Cavitation 11. Freezing and Thawing 12. Settlement and Movement 13. Shrinkage 14. Plastic 15. Drying 16. Temperature Changes a. Internally generated b. Externally generated 17. Fire 18. Weathering The process of explaining of these causes can be done in three steps 1. Mechanism 2. Symptoms 3. Prevention Coming to the explanation of causes, 1. Accidental loadings: a. Mechanism: Accidental loadings may be characterized as short-duration, one-time events such as the impact of a barge against a lock wall or an earthquake. b. Symptoms: Visual examination will usually show spalling or cracking of concrete which has been subjected to accidental loadings. c. Prevention: Accidental loadings by their very nature cannot be prevented. Minimizing the effects of some occurrences by following proper design procedures (an example is the design for earthquakes) 2. Chemical reactions: Acid attack: a. Mechanism: The deterioration of concrete by acids is primarily the result of a reaction between the acid and the products of the hydration of cement. b. Symptoms: Visual examination will show disintegration of the concrete evidenced by loss of cement paste and aggregate from the matrix c. Prevention: A dense concrete with a low water-cement ratio (w/c) may provide an acceptable degree of protection against a mild acid attack. Aggressive water attack: a. Mechanism: Soft or aggressive waters will leach calcium from cement paste or aggregates b. Symptoms: Visual examination will show concrete surfaces that are very rough in areas where the paste has been leached. c. Prevention: The aggressive nature of water at the site of a structure can be determined before construction or during a major rehabilitation. Additionally, the waterquality evaluation at many structures can be expanded to monitor the aggressiveness of water at the structure Alkali-carbonate rock reaction:a. Mechanism:Certain carbonate rock aggregates have been reactive in concrete. Mechanism is available in codes. b. Symptoms: Visual examination of those reactions that are serious enough to disrupt the concrete in a structure will generally show map or pattern cracking and a general appearance which indicates that the concrete is swelling c. Prevention: The best prevention is to avoid using aggregates that are or suspected of being reactive. Alkali-silica reaction:a. Mechanism; Some aggregates containing silica that is soluble in highly alkaline solutions may react to form a solid non-expansive calcium-alkali-silica complex or an alkali-silica complex which can imbibe considerable amounts of water and then expand, disrupting the concrete. b. Symptoms: Visual examination of those concrete structures that are affected will generally show map or pattern cracking and a general appearance that indicates that the concrete is swelling. c. Prevention: In general, the best prevention is to avoid using aggregates that are known or suspected to be reactive or to use a cement containing less than 0.60 percent alkalis. Miscellaneous chemical attack:a. Mechanism:Concrete is seldom attacked by solid dry chemicals. Also, for maximum effect, the chemical solution needs to be circulated in contact with the concrete. Concrete subjected to aggressive solutions under positive differential pressure is particularly vulnerable. b . Visual examination of concrete which has been subjected to chemical attack will usually show surface disintegration and spalling and the opening of joints and cracks. c. Prevention: Typically, dense concretes with low w/c (maximum w/c = 0.40) provide the greatest resistance. The best known method of providing long-term resistance is to provide a suitable coating. Sulphate attack: a. Mechanism: The sulfate reacts with free calcium hydroxide which is liberated during the hydration of the cement to form calcium sulfate (gypsum). Next, the gypsum combines with hydrated calcium aluminate to form calcium sulfoaluminate (ettringite). Both of these reactions result in an increase in volume. b. Symptoms : Visual examination will show map and pattern cracking as well as a general disintegration of the concrete. Laboratory analysis can verify the occurrence of the reactions described. c. Prevention: Use of a dense, high-quality concrete with a low water-cement ratio; Use of either a Type V or a Type II cement, depending upon the anticipated severity of the exposure 3. Construction errors: a. Mechanism: Failure to follow specified procedures and good practice or outright carelessness may lead to a number of conditions that may be grouped together as construction errors. b. Symptoms: Visible cracks will form, spalling will occur etc., c. Prevention: Taking proper care in using codes and having proper guidance to the labour.4.Corrosion of embedded metals:a. Mechanism: High alkalinity and electrical resistivity of the concrete. The high alkalinity of the concrete pore solution can be reduced over a long period of time by carbonation. b. Symptoms: Visual examination will typically reveal rust staining of the concrete. This staining will be followed by cracking. Cracks produced by corrosion generally run in straight, parallel lines at uniform intervals corresponding to the spacing of the reinforcement. c. Prevention : Use of concrete with low permeability; Use of properly proportioned concrete having a low w/c; Use of as low a concrete slump as practical; Use of good workmanship in placing the concrete; curing the concrete properly; 5.Design errors:a. Mechanism: The failure mechanism is simple-- the concrete is exposed to greater stress than it is capable of carrying or it sustains greater strain than its strain capacity. b. Symptoms: Errors in design resulting in excessively high compressive stresses will result in spalling. Similarly, high torsion or shear stresses may also result in spalling or cracking. c. Prevention: Careful review of all design calculations. 6.Abrasion:a. Mechanism: Abrasion-erosion damage is caused by the action of debris rolling and grinding against a concrete surface. b. Symptoms: Concrete surfaces abraded by water- borne debris are generally smooth and may contain localized depressions. c. Prevention : While designing, selecting the materials, and while on operations proper maintenance should be taken. 7.Cavitation:a. Mechanism: When the water flow is fast enough (greater than 12.2 m/sec) and where there is surface irregularity in the concrete, cavitation damage may occur. b. Symptoms: Concrete that has been damaged will be severely pitted and extremely rough. c. Prevention: While hydraulic design, selecting conventional materials, other cavitation materials, and while construction practices proper care should be taken. 8. Freezing and thawing: a. Mechanism: As the temperature of a critically saturated concrete is lowered during cold weather, the freezable water held in the capillary pores of the cement paste and aggregates expands upon freezing. b. Symptoms: Visual examination of concrete damaged by freezing and thawing may reveal symptoms ranging from surface scaling to extensive disintegration. c. Prevention: Designing the structure to minimize the exposure to moisture. For example, providing positive drainage rather than flat surfaces whenever possible.Using a concrete with a low w/c.Selecting suitable materials, particularly aggregates that perform well in properly proportioned concrete.9.Settlement and movement:a. Mechanism: The differential movement increases, concrete members can be expected to be subjected to an overstressed condition. Ultimately, the members will crack or spall. b. Symptoms: Visual examination of structures undergoing settlement or movement will usually reveal cracking or spalling or faulty alignment of structural members. c. Prevention: Prevention of settlements and movements is possible with proper design of footings. 10. Shrinkage: a. Mechanism: During the period between placing and setting, most concrete will exhibit bleeding to some degree. Bleeding is the appearance of moisture on the surface of the concrete; it is caused by the settling of the heavier components of the mixture. b. Symptoms: Cracking caused by plastic shrinkage will be seen within a few hours of concrete placement. Typically, the cracks are isolated rather than patterned. These cracks are generally wide and shallow. c. Prevention: Determination of whether the weather conditions on the day of the placement are conducive to plastic shrinkage cracking is necessary. 11. Temperature changes: Internally generated: a. Mechanism: The hydration of Portland cement is an exothermic chemical reaction. In large volume placements, significant amounts of heat may be generated and the temperature of the concrete may be raised by more than 38 C (100 F) over the concrete temperature at placement. b. Symptoms: Cracking resulting from internal restraint will be relatively shallow and isolated. c. Prevention: Using as low a cement con- tent as possible; Using a low-heat cement or combination of cement and pozzolans; placing the concrete at the minimum practical temperature; Externally generated: a. Mechanism: The temperature change leading to the concrete volume change is caused by external factors.

b. Symptoms: Visual examination will show regularly spaced cracking in the case of restrained contraction. Similarly, spalling at expansion joints will be seen in the case of restrained expansion. c. Prevention: The best prevention is obviously to make provision for the use of contraction and expansion joints

2. Explain in detail the different types of damage of structures. (15M)Types of damages in structures: 1. Damage to short columns 2. Damage to columns for elevated water tanks 3. Deficiencies in structural layout 4. Deficiencies in placement of utility pipes, insufficient concrete cover in structural elements, and finishing falling 5. Pounding In detail: 1. Damage to short columns: This type of damage was extensive at schools and public buildings. Insufficient gaps between columns and non-structural elements caused large shear forces to be induced on the short columns. The damage was the worse for the insufficient transverse reinforcement and for the rusty rebars causing spalling of cover concrete.2. Damage to columns for elevated water tanks: Mostly of elevated water tanks undergo tension and comphressive stress act on one time because of the wind lpressure apart from these sesmic waves make structure to collapse3. Deficiencies in structural layout: The current code encourages the use of stiff frames in both longitudinal and transversal directions of a building. It is, however, a customary of long standing to provide them only in one direction, and it does not seem to be done away with ease 4. Deficiencies in placement of utility pipes, insufficient concrete cover in structural elements, and finishing falling 5. Pounding: An insufficient joint opening between two buildings caused pounding. Failure mechanism and its control of a structural system: The designability of structure systems means that the failure mechanism of the structure system under unexpected disasters can be designed. The control of the failure mechanism is to make the structure system fail under earthquake following a desirable failure mechanism. Once the failure mechanism can be controlled, the collapse behavior under severe earthquakes can be predicted and proper measures can be taken to enhance the seismic capacity of the structural systems. Because failure mechanisms are the ultimate states of structural systems, the demands of load-bearing capacity, stiffness, deformability, energy- dissipating capacity and safety margin are also determined by the failure mechanisms. For building structures, the seismic failure mechanisms have two types, the local and the global failure mechanism, as shown in Fig. The following structure types are vulnerable to local failure: frame-supported structure or soft story structure, block structure (masonry structure). In such structures, local damage will induce severe damage or even collapse of the whole structure system. For such structures, the robustness can only be improved by increasing the safety margin of local key elementsLocal failure mechanism and its controlIt should be noted that therere two kinds of local failure mechanisms. The first is the soft story failure mechanism, such as the structures with pier columns For the story failure mechanism in Fig.(a), the failure is centralized in the soft story while upper stories remain undamaged with little faculties, thus the damage degree may considered not too severe. Actually, the soft story failure mechanism is something like the seismic isolation structures, where a low-stiffness layer is arranged to centralize the earthquake energy input. The difference is that special-designed isolators instead of the soft story columns are used. The isolators are special structure elements that can sustain large deformation and remain elastic with little damage, while the soft story columns often suffer from severe damage under large deformation. The local failure mechanism in Fig.(b), which is vulnerable under earthquakes, is another special topic relating to the progressive collapse.Global failure mechanismThe ideal failure sequence for the structural systems with global failure mechanisms is: redundant elements, secondary elements, common elements, important elements and at last the key elements. In such structures, important and key elements are always connected with many redundant, secondary or common elements, which should fail prior to the important and key ones The structural damages progress gradually without sudden collapse in case of the failure of some local elements.

3. Explain the mechanism of corrosion of reinforcement and also describe the various steps to prevent corrosion. (15M)1) Crevice Corrosion mechanismCrevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas). Oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal. Oxygen content of liquid at the mouth of the crevice which is exposed to the air is greater, so a local cell develops in which the anode, or area being attacked, is the surface in contact with the oxygen-depleted liquid.2) Pitting Corrosion mechanismTheories of passivity fall into two general categories, one based on adsorption and the other on presence of a thin oxide film. Pitting in the former case arises as detrimental or activator species, such as Cl-, compete with O2 or OH at specific surface sites. By the oxide film theory, detrimental species become incorporated into the passive film, leading to its local dissolution or to development of conductive paths. Once initiated, pits propagate auto-catalytically according to the generalized reaction,M+n + nH2O + nCl- M(OH)n + nHCresulting in acidification of the active region and corrosion at an accelerated rate (M+n and M are the ionic and metallic forms of the corroding metal).PREVENTION METHODS1) Keep concrete always dry, so that there is no H2O to form rust. Also aggressive agents cannot easily diffuse into dry concrete. If concrete is always wet, then there is no oxygen to form rust.2) A polymeric coating is applied to the concrete member to keep out aggressive agents. A polymeric coating is applied to the reinforcing bars to protect them from moisture and aggressive agents. The embedded epoxy-coating on steel bars provide a certain degree of protection to the steel bars and, thereby, delay the initiation of corrosion. These coatings permit movement of moisture to the steel surface but restrict oxygen penetration such that a necessary reactant at cathodic sites is excluded.3) Stainless steel or cladded stainless steel is used in lieu of conventional black bars.4) FLY ASH : Using a Fly Ash concrete with very low permeability, which will delay the arrival of carbonation and chlorides at the level of the steel reinforcement. Fly Ash is a finely divided silica rich powder that, in itself, gives no benefit when added to a concrete mixture, unless it can react with the calcium hydroxide formed in the first few days of hydration. Together they form a calcium silica hydrate (CSH) compound that over time effectively reduces concrete diffusivity to oxygen, carbon dioxide, water and chloride ions.5) A portion of the chloride ions diffusing through the concrete can be sequestered in the concrete by combining them with the tricalcium aluminate to form a calcium chloro-aluminate (Friedels salt). It can have a significant effect in reducing the amount of available chlorides thereby reducing corrosion.6) Electrochemical injection of the organic base corrosion inhibitors, ethanolamine and guanidine, into carbonated concrete.7) The rougher the steel surface, the better it adheres to concrete. oxidation treatment (by water immersion and ozone exposure) of rebar increases the bond strength between steel and cement paste to a value higher than that attained by clean rebars. In addition, surface deformations on the rebar (such as ribs) enhance the bond due to mechanical interlocking between rebar and concrete.8) As the cement content of the concrete increases (for a fixed amount of chloride in the concrete), more chloride reacts to form solid phases, so reducing the amount in solution (and the risk of corrosion), and as the physical properties improve, the extent of carbonation declines, so preventing further liberation of chloride from the solid phase.9) Electrochemical Chloride Extraction (ECE) is a relatively new technology for which long-term service data are limited. This method employs a temporary anode that is operated at current density4.Explain the following: a) Phenomenon of Desication(7M)The phenomena of desication takes place in clayey soils which increases the hydrallic conductivity of soil due to the loss of water content present in the soil. it makes the clayey soils to shrink. The moisture developed in the soil gels reduced which eventually leads to increase in shrinkage strain and correspondingly cracks were formed. . The soil should be compacted to achieve a hydraulic conductivity in range of 1 x 10-9 or less, By compacting the soil with optimum moisture content and max dry density. it results in formation of lower old ratio and hence low shrinkage strain can be attained after the point of saturation takes place The zones containing dry unit weight and water content yield low hydraulic conductivity, which is acceptable. Hence, It is called as the line of optimums and on other hand, if they are present on the wet side, defined line of optimum. The volume increases with increase in water content. Greater change is observed between both the wet and dry sides of line of optimum and increase of water content takes place on wet side. and decrement towards dry side. To reduce the hydraulic conductivity and shrinkage strain, the method involved is to increase the compaction effort or compaction energy. By using this method, sometimes. it leads to increase in construction costs. As compaction effon increases. the increase in dry density talc. place by decreasing of water content. For higher water content soils. die increase in compaction energy does not reduce the shrinkage strain and hydraulic conductivity, The study behaviour of many materials gets involved here to reduce the shrinkage present in soils. When soil gets mixed with fiber. it reduces the shrinkage strain up to 40% mostly, and the fiber increases the hydraulic conductivity up to 50 times the hydraulic conductivity of soil.

b) Fire rating of structures.(8M)Fire rating of structures is based on Fire loads and fire resistance: Fire load on steel structures:Examples of fire load in various structures:Type of steel structure Kg wood / m2School 15

Hospital 20

Hotel 25

Office 35

Departmental store 35

Textile mill show room >200

The term fire load in a compartment of a structure is the maximum heat that can be theoretically generated by the combustible items and contents of the structure. The fire load could be measured as the weight of the combustible material multiplied by the calorific value per unit weight. Fire load is conveniently expressed in terms of the floor space as MJ/m2 or Mcal/m2. More often it would be expressed in terms of equivalent quantity of wood and expressed as Kg wood / m2 (1 Kg wood = 18MJ). The fire ratings of steel structures are expressed in units of time , 1, 2, 3 and 4 hours etc. The specified time neither represents the time duration of the real fire nor the time required for the occupants to escape. The time parameters are basically a convenient way of comparative grading of buildings with respect to fire safety. Basically they represent the endurance of structural steel elements under standard laboratory conditions. The rate of heating of the unprotected steel is obviously quite high as compared to the fire-protected steel. We shall see in the following sections that these two types of fire behavior of steel structure give rise to two different philosophies of fire design. The time equivalence of fire resistance for steel structures or The fire rating could be calculated as Teq (Minutes) = CWQfWhere Qf is the fire load MJ/m2 which is dependent on the amount of combustible material,W is the ventilation factor relating to the area and height and width of doors and windows andC is a coefficient related to the thermal properties of the walls, floors and ceiling. As an illustration, the W value for a building with large openings could be chosen as 1.5 and for highly insulating materials C value could be chosen as 0.09.

5.a) Explain the methods of testing of structures for the assessment of damage. -------------------------------------------------------------------------------------------------Parameter Test / Methods-------------------------------------------------------------------------------------------------- Concrete Compressive Strength Rebound Hammer,Windsor Probe,Ultrasonic Pulse Velocity,Core,CapoPull-outCombined Methods Flexural StrengthBreak-off Direct Tensile Strength Pull-off Concrete Quality, Homogeneity Ultrasonic Pulse Velocity,Pulse Echo,Endoscopy,Gamma Ray Radiography Damage Fire / Blast Rebound HammerUltrasonic Pulse Velocity Cracks- Water Tanks / PavementsUltrasonic Pulse Velocity,Acoustic Crack DetectorDye Penetration TestX-Ray RadiographyGamma-Ray RadiographyCrack Scope Steel Location, Cover, Size Re Bar Locator, Bar Size CorrosionHalf-Cell PotentialResistivityCarbonationChloride Content ConditionEndoscope / Borescope Integrity & Performance TappingPulse-EchoAcoustic EmissionRaderLoad Test-------------------------------------------------------------------------------------------------

b) Describe the various NDT methods.Non-destructive testing (NDT) methods are techniques used to obtain information about the properties or internal condition of an object without damaging the object. Non-destructive testing is a descriptive term used for the examination of materials and components in such way that allows materials to be examined without changing or destroying their usefulness. NDT is a quality assurance management tool which can give impressive results when used correctly. It requires an understanding of the various methods available, their capabilities and limitations, knowledge of the relevant standards and specifications for performing the tests. NDT techniques can be used to monitor the integrity of the item or structure throughout its design life. The various Non destructive / partial destructive tests are as belowGroup - I A: Non Destructive Tests for Concrete Surface Hardness Tests Rebound Hammer Test Ultrasonic Pulse Velocity TestGroup - I B: Partially Destructive Tests for Concrete Penetration Resistance Test (Windsor Probe) Pull-out Test Pull-off Test Break-off Test Core CuttingGroup - II: Other Properties of Fresh / Hardened Concrete Chemical Tests Cement Content & Aggregate / Cement Ratio Sulphate Determination Test Chloride Determination Test Alkalinity Test Carbonation Test Absorption & Permeability Tests Crack Monitor Moisture Measurement Abrasion Resistance Test Fresh Concrete Tests For W/C Ratio And Compressive StrengthGroup - III: Reinforcement location, size and corrosion Rebar Locator & bar sizer Corrosion mapping Half-cell Potentiometer Resistivity meterGroup - IV : Miscellaneous Test Radiographic Test X- Ray Cobalt Gamma ray

SCHMIDTS REBOUND HAMMER TESTOBJECTS The rebound hammer method could be used for : Assessing the compressive strength of concrete with the help of suitable co-relations between rebound index and compressive strength Assessing the uniformity of the concrete Assessing the quality of concrete in relation to the standard requirements Assessing the quality of one element of concrete in relation to another.(1)Principle of test: The test is based on the principle that the rebound of an elastic mass depends on the hardness of the surface upon which it impinges. When the plunger of the rebound hammer pressed against the surface of the concrete, the spring controlled mass rebounds and the extent of such rebound depend upon the surface hardness of concrete. The surface hardness and therefore the rebound is taken to be relation to the compressive strength of concrete. The rebound is read off along a graduated scale and is designated as the rebound number or rebound index.

Fig.1 : Basic Features of Rebound Hammer

Working of rebound hammer: A schematic cut way view of schmidt rebound hammer is shown in fig. 1. The hammer weight about 1.8 kg., is suitable for use both in a laboratory and in the field. When the plunger of rebound hammer is pressed against the surface of concrete, a spring controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete.The rebound distance is measured on a graduated scale and is designated as rebound number. Basically, the rebound distance depends on the value of kinetic energy in the hammer, prior to impact with the shoulder of the plunger and how much of that energy is absorbed during impact. The energy absorbed by the concrete depends on the stress-strain relationship of concrete. Thus, a low strength low stiffness concrete will absorb more energy than high strength concrete and will give a lower rebound number.

Fig.2 : Schematic Cross Section of Rebound Hammer & Principle of Operation

6. Explain the methods of repairs in concrete structures and under water structures? (15M)Repairs in the underwater structures are done by identifying the type of repair that is required. Some of the techniques for concrete repair are 1. surface spalling repair 2. large scale repair 3. preplaced aggregate concrete 4. injection 5. guinite(shortcrete)

Where a large surface area is to be repaired or a column or beam is to be encased, then the use of gunite may be the best solution. The dry mix process where sand and cement are passed through the delivery hose and mixed with water at the nozzle is generally used. Although gunite cannot be applied under water, the use of additives to promote very rapid setting can enable the method to be used in the splash/tidal zone. Products are available (e.g. Sigunite from Sika Inertol) that can produce an initial set within 30 s and a final set within 1 min. The successful application of gunite is very dependent on the skill and experience of the nozzleman in adjusting the water supply and the pressure and ensuring uniformity of thickness. With careful application concrete strengths of 3OMPa can readily be achieved with good bonding to the parent concrete and high abrasion resistance. The thickness of the gunite should generally be limited to a maximum of 50 mm, although second layers can be applied if an increased thickness is required. Shortcrete Procedure :cementitious binder and aggregates thoroughly mixed (central mixing, transit mixer, volumetric proportioning mixer or dry bagged premix) Water Added if necessary to bring shotcrete to Earth Dry consistency 3 to 6% moisture content. Mix added to shotcrete delivery equipment or gun. Compressed air conveys the shotcrete from the gun down the hose. Water introduced under pressure through a water ring at the nozzle. Shotcrete jetted from a nozzle at high speed onto a surface with the force of the impacting jet compacting the material. All ingredients including mixing water thoroughly mixed.The mortar or concrete is added to the chamber of the delivery equipment. The mix is either pumped or pneumatically conveyed down the hose by compressed air. High-pressure air is added at the nozzle to jet the shotcrete at high velocity onto the impacting surface.

7. Define the term Retrofitting. Explain the methods of retrofitting of various structural components. (15M)

RETROFITING is the modification to existing structures. The retrofitting techniques are also applicable for natural hazards. Retrofitting is predominantly concerned with structural improvements to reduce unwanted hazards. Performance can be greatly enhanced through proper initial design or subsequent modifications.METHODS OF RETROFITTING: Generally the strengthening for a structure requires when the strength of the system got reduced or else if the forces acting on the system increases. By increasing the strength of the system by methods like 1. Concrete Jacketing, 2. Steel Jacketing, 3. FRP Wrapping.

Concrete Jacketing Involves addition of a thick layer of Reinforced Concrete (RC) in the form of a jacket, using longitudinal reinforcement and transverse ties. Additional concrete and reinforcement contribute to strength increase. Minimum allowable thickness of jacket = 100 mm. The sizes of the sections are increased and the free available usable space becomes less. Huge dead mass is added. The stiffness of the system is highly increased. Requires adequate dowelling to the existing column. Longitudinal bars need to be anchored to the foundation and should be continuous through the slab. Requires drilling of holes in existing column, slab, beams and footings. Increase in size, weight and stiffness of the column. Placement of ties in beam column joints is not practically feasible. The speed of implementation is slow.

Steel Jacketing Encasing the column with steel plates and filling the gap with a non-shrink grout. Provides passive confinement to core concrete. Its resistance in axial and hoop direction can neither be uncoupled nor optimized. Its high youngs modulus causes the steel to take a large portion of the axial load resulting sometimes in premature buckling of the steel. General thickness of grout = 25 mm. Rectangular steel jackets on rectangular columns are not generally recommended and a use of an elliptical jacket is solicited. Since steel jacket is vulnerable to corrosion and impact with floating materials, it is not used for columns in river, lake and seas.

FRP for strengthening It provides a highly effective confinement to columns. The original size, shape and weight of the members is unaltered (unlike any other jacketing), thus not attracting higher seismic forces. Due to the fact that the original shape and size of the members is practically unaltered, this method is particularly useful for strengthening historic and artistic masonry structures. Due to the orthotropy built in by fiber orientation, the wraps essentially provide only confinement without interfering with the axial load which is taken completely by concrete column as against steel jacketing, where the jacket takes most of the axial load and becomes susceptible to buckling. No drilling of holes is required as against concrete and steel jacketing. The FRPs have extremely good corrosion resistance, which makes them highly suitable for marine and coastal environments. FRP wraps prevent further deterioration of concrete and inside reinforcement. As the wraps are available in long rolls, construction joints can be easily avoided. Ease of installation, which is similar to putting up wall papers, makes the use of FRP sheets a very cost-effective and efficient alternative in the strengthening of existing buildings. Provides minimal disturbance to existing structure and generally the strengthening work can be performed with normal functioning of structure.8. Explain the following:

a) Necessity of health monitoring of structures

Structural health monitoring (SHM) is a process aimed at providing accurate and timely information about the condition and performance of a Structure. It can be either short term (eg. repairs efficacy) or a long term (monitoring parameters continuously or periodically) process. A need for SHM arises with the fact that properties of both concrete and steel depend on large number of factors which are often hard to predict in practice. The representative parameters selected for health monitoring of a structure in general can be of mechanical, physical and chemical in nature.

The Need of SHM is to monitor the in-situ behavior of a structure accurately and efficiently, to assess its performance under various service loads, to detect damage or deterioration, and to determine the health or condition of the structure. The SHM system should be able to provide, on demand, reliable information pertaining to the safety and integrity of a structure. The information can then be incorporated into bridge maintenance and management strategies, and improved design guidelines.

b) Building Instrumentation.

INSTRUMENTATION: To measure deflection bending and shear in the girders the different types of sensors and techniques were used which are presented below. 1. Embedded strain gauges 2. Surface strain gauges 3. Linear Potentiometers for deflection measurements 4. Water level measurement for deflection measurements.

1. Embedded strain gauges Special strain gauges were put inside the girder beam area before casting which got embedded in the concrete after casting. For this TML make model PMFL- 60T- 3LT was used. A special Casing was designed for making sure that while pouring the concrete the gauge is safe and does not get damaged and at the same time it should get fully in contact with the concrete for sensing the strains developed. 2. Surface strain gauges: Surface Strain Gauges were also installed to measure same parameters as the embedded type strain gauges. These strain gauges were installed to cross check the measured strains by embedded gauges. These were also helpful in measuring strains in case the embedded strain gauge has undergone damage and the readings are unacceptable. The surface strain gauges for shear were installed at side surface just over the embedded Surface strain gauges for tension were installed at the bottom surface at middle position just over embedded strain gauge of girder beam. 3. Linear Potentiometers for deflection measurements When the temporary supports are released for the beam. Stresses are developed in the beam it deflects due to its self-weight. Deflection Measurements were to be done after each stage of construction is completed. Two linear potentiometers were installed for each girder beam. One at the middle of beam and other at L/3 distance from one end, L being the length of the girder beam. 4. Water level measurement for deflection measurements. Sometimes the readings by linear potentiometer is not reliable as it requires a stable support with respect to the ground frame which is not possible at higher floors. Due to this limitations water level in a tube which directly uses gravity can be used. one end of tube is kept in the middle of girder and the other end is fixed to the main column which is fixed. When the girder deflects in middle it changes the water level in the tube by the amount beam has deflected and hence gives us the proper deflection. This is one of the oldest andreliable method of finding difference in level between two points