CHAPTER 1 INTRODUCTION
Civil engineering sees advances in major measures in modern times. However, it has become imperative to retain or restore structures presentl y believed to pose a suitable s uitable threat to habitation especially in seismically active areas. It is with this regards the field of retrofitting or post construction strengthening of structural members came to be. The reasons for retrofitting vary from repair of structures damaged by a natural disasters or earthquake to the repair of deterioration in historical buildings. Retrofitting is in itself is broadly divided into global and local retrofitting strategies. Global strategies are that which aim to stiffen the building by providing additional lateral load resisting elements so that it has reduced lateral deformation. Local strategies on the other hand deal with the local strengthening of individual structural members as and when they are found to be deficient in dealing with expected loadings. Confinement techniques mainly fall into the category of local retrofitting. Of the many prevalent confinement techniques, Fibre reinforced composites have found a predominant place in the field today because of their high strength to weight ratio, relative ease of implementation and space saving characteristics. This paper deals with the application of Carbon Fibre Reinforced Polymer sheets to improve the performance characteristics of columns in a structure. Columns are chosen as they are the primary vertical load transfer members in a vertical structure and also because they are most prone to damage due to variety of issues ranging from damage (manmade or natural) to deterioration due to exposure exposure and aging.
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CHAPTER 2 LITERATURE REVIEW
Advanced composite materials are generally used in the industrial fields like aerospace, marine and automotive industries due to their excellent engineering properties such as high strength, high stiffness, high durability, low density, high fatigue endurance, low thermal coefficient, corrosion resistance and good strength-toweight the civil infrastructure, civil engineers and the construction industry have realized for many years the use of potential benefits of the composites as materials for strengthening or retrofitting of the structures.
The use of Fibre reinforced polymer (FRP) composites that are externally applied for strengthening reinforced concrete structures such as beams, slabs and columns has been done experimentally b y many researchers and has been applied in construction. c onstruction. A column is one of the essential elements in civil engineering structures that transmits loads from the upper levels to the lower levels and then to the soil through the foundations. During their service life, columns can undergo deterioration caused by environmental effects or fatigue of its constituent materials thus leading to the reduction retrofitting or strengthening can be taken as an alternative way to maintain the columns. Retrofitting or strengthening reinforced concrete columns by using FRP composites is preferred than by using other materials like steel due to its high strength-to-weight ratio and high corrosion resistance. Several investigations on FRP retrofitted or strengthened concrete columns have been undertaken for many years. Core confinement of RC columns provided by transverse reinforcement has been extensively studied and specified in Codes, although some procedures are still under debate. It has been indicated, for instance, that the response of cylinders subjected to equivalent levels of pressure depends on how that lateral pressure is transmitted, and not on its magnitude alone
The strengthening of existing RC columns using steel or FRP jacketing is based on the well-established fact that the lateral confinement of concrete can substantially enhance its axial compressive strength and ductility (Paulay,1991). Since the pioneering experimental work , many studies have been conducted on compressive
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CHAPTER 2 LITERATURE REVIEW
Advanced composite materials are generally used in the industrial fields like aerospace, marine and automotive industries due to their excellent engineering properties such as high strength, high stiffness, high durability, low density, high fatigue endurance, low thermal coefficient, corrosion resistance and good strength-toweight the civil infrastructure, civil engineers and the construction industry have realized for many years the use of potential benefits of the composites as materials for strengthening or retrofitting of the structures.
The use of Fibre reinforced polymer (FRP) composites that are externally applied for strengthening reinforced concrete structures such as beams, slabs and columns has been done experimentally b y many researchers and has been applied in construction. c onstruction. A column is one of the essential elements in civil engineering structures that transmits loads from the upper levels to the lower levels and then to the soil through the foundations. During their service life, columns can undergo deterioration caused by environmental effects or fatigue of its constituent materials thus leading to the reduction retrofitting or strengthening can be taken as an alternative way to maintain the columns. Retrofitting or strengthening reinforced concrete columns by using FRP composites is preferred than by using other materials like steel due to its high strength-to-weight ratio and high corrosion resistance. Several investigations on FRP retrofitted or strengthened concrete columns have been undertaken for many years. Core confinement of RC columns provided by transverse reinforcement has been extensively studied and specified in Codes, although some procedures are still under debate. It has been indicated, for instance, that the response of cylinders subjected to equivalent levels of pressure depends on how that lateral pressure is transmitted, and not on its magnitude alone
The strengthening of existing RC columns using steel or FRP jacketing is based on the well-established fact that the lateral confinement of concrete can substantially enhance its axial compressive strength and ductility (Paulay,1991). Since the pioneering experimental work , many studies have been conducted on compressive
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strength and stress-strain behavior of FRP. These studies have shown that FRPconfined concrete behaves differently from steel-confined concrete, so design recommendations for steel-confined concrete columns cannot be applied to FRPconfined columns. Several models have been proposed to estimate the confined compression strength and the corresponding strain and some of these models result from the adjustment to FRP of the reinforced concrete model(Mander et al., 1998). Others, like Samaan et al.(1998), al.(1998), assume a stress_strain relationship of bi-linear type for the concrete confined with FRP under monotonic actions
Toutanji et al. al. (2006) presented an incremental model. The author considers that throughout the loading the lateral strain is equal to the strain present in the FRP composite and at rupture the FRP reaches its tensile strength.
Saadatmanesh and Ehsani(1994) found that the strength and ductility of bridge concrete columns can be significantly increased by wrapping FRP straps around the columns due to the confinement of concrete and prevention of the buckling of longitudinal reinforcement bars. The confinement effectiveness of various influence parameters, such as concrete compressive strength, thickness and spacing of FRP straps and type of FRP, were studied. A stress-strain model for concrete confined by FRP was suggested and used to predict the compressive strength and strain. With the confinement of FRP, a desirable ductile flexural failure mode rather than a brittle shear failure mode can be achieved in seismic strengthening for concrete columns. In their subsequent study on the strengthening method of bridge concrete columns prefailed in a severe earthquake using wrapped FRP sheets, Saadatmanesh and Ehsani (1994) also found the enhancement of concrete compressive strength and strain. Purba and Mufti(1999) studied seismic retrofitting of RC columns with prefabricated FRP products. The seismic performance of RC columns was also found to be improved by increased ductility with the confinement by FRP composites or sheets. Xiao et al .(1999) .(1999) proposed proposed an analytical model, model, which takes into consideration the bond-slip deterioration of lap-spliced longitudinal bars, was developed for seismic assessment and retrofit design. Although the confinement effect of FRP sheets or jackets were studied and some useful stress-strain relation models were suggested in previous research for seismic
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strengthening of RC columns with FRP, the determi nation of the amount of CFRP for design purpose dose not appear to have been properly resolved by existing research. The effect of confinement on properties of concrete in compression was studied on concrete cylinders subjected to lateral fluid pressure and later on cylinders confined by spiral steel reinforcement (Seible et al.,1997). al.,1997). After that, extensive experimental and theoretical works on the behavior of reinforcing steel-confined concrete members have been conducted . However, it has been shown by Mirmiran and Shahawy(1998) that the behavior of FRP-confined concrete differs from that of reinforcing steelconfined concrete and that the use of models developed for reinforcing steel confinement may result in overestimating the strength of FRP- confined columns resulting in an unsafe design. Also, it was determined experimentally that as the number of FRP layers increase, the FRP tube surrounding the member increases the confinement.
In recent decades it was proved that crucial factors in identifying the level of strengthening of compressed members are: the number of FRP reinforcement layers, the shape of cross-section and the type of composite sheet . Trapko and Musial( 2011) observed that the most beneficial results of strengthening strengthening were observed observed in case of of cylindrical specimens confined with CFRP sheet for several times. Less spectacular effects were received for quadrangular cross-sections – square and rectangular. In both cases the level of strengthening depends on the radius of corner rounding. However, in practice it is difficult to round corners with high radius in strengthening column with quadrangular cross-section. Very often it is possible only to chamfer concrete cover in the corner upto 10 to 20mm.
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CHAPTER 3 SEISMIC RETROFITTING 3.1 RETROFITTING
Retrofitting strategy refers to options of increasing the strength, stiffness and/or ductility of a structural member. The combination of strength and ductility for a building will provide the proper balance for seismic retrofitting in a building. Several retrofit strategies may be selected scheme of a building The goals of seismic retrofitting can be summarized as follows(IS 13935:1993) a. Increasing the lateral strength and stiffness of the building. b. Increasing the ductility and enhancing the energy dissipation capability c. Giving unity to the structure d. Eliminating sources that produce concentration of stresses e. Enhancement of redundancy in the number of lateral load resisting elements f.
Each retrofit strategy should consistently achieve the performance objective
To decide the amount of retrofit, a performance based objective can be adopted (ATC 40 1996). The performance-based approach identifies a target building performance level under an anticipated earthquake level. For retrofit of the buildings an optimal
safety objective can be selected. Under this objective the dual
requirement of design basis earthquake and structural stability under maximum credible earthquake is aimed at. In a complete retrofit programme, (Basu,2002) elaborates that the following steps are necessary: 3.1.1 Seismic evaluation
The seismic evaluation identifies the deficiencies of a building. The evaluation involves
visual
inspection,
nondestructive
testing,
examination
of
as-built
information and structural analysis, The guidelines for seismic evaluation are drawn from ACMA(2001) guidelines for performance evaluation under earthquake actions 3.1.2 Decision to retrofit
Based on the extent of deficiency of the building, the economic viability, the expected durability of the upgraded structure and the availabilit y of the materials, a decision is taken on whether to repair, retrofit or demolish the building.
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3.1.3 Selection and design of the retrofit scheme
The selection and design of the retrofit scheme influences the decision to retrofit. It is crucial to have good knowledge of individual retrofitting strategies in this aspect. 3.1.4 Verification of the Retrofit scheme
Structural analysis is necessary to justify the selected retrofit scheme. Critical parameters that need to be studied after retrofitting include the alteration of the load path, redistribution of the member forces and changes in the failure modes after retrofitting. 3.1.5. Construction Methodology
The effective ness of the retrofit strategy greatly depends upon the quality of construction. Proper execution of the prescribed retrofitting method is of paramount importance 3.1.6 Performance monitoring
Monitoring the performance of the retrofitted building is essential to detect any defect or unresolved deficiency. Retrofitting a building to withstand seismic forces can be through two distinct methodologies. Either, there can be procedures adopted to increase the stiffness of a building so much so that it is virtually unaffected by the design earthquake. An alternative approach is to increase the ductility of the structural members in the building. This concept has been demonstrated by Sugano(1990) as shown in the Figure 3.1
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Figure 3.1 Basic Concept of Seismic Retrofit It is clear from the figure that in order for an existing building to achieve the required seismic capacity, it needs to have a dramatic increase in its strength. This can prove to be extremely difficult and expensive as it calls for the structural members to be designed to take up unnaturally high loads which in turn increase the dimensions of the members. Such retrofit may not even be possible due to space constraints in the building and also because of the fact that such kind of strengthening may call for intrusions into established members thereby inadvertently weakening it. The Second approach is to increase the ductility of members in the building manifold so that it finally reached to the required seismic capacity. But this approach leaves us with a structure which may undergo severe deformations and unstable behavior under loading conditions. Sugano(1990) emphasizes that a combination of Strengthening and ductility enhancing procedures is the ideal way forward to increase the seismic capacity of the structure. As it is necessary to retrofit only constructions vulnerable to the design earthquake, a vulnerability evaluation is obviously needed before attempting any seismic retrofitting. Oliveto & Marletta(2005) provided a definition of seismic resistance and the corresponding vulnerability of a construction to the design earthquake is also defined. As has been seen, the design earthquake is specified by means of a design spectrum which depends on the energy dissipation capacity through the structure behaviour factor.
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Assuming that the structure behaviour factor for the structure being considered can be evaluated, the design spectrum can be drawn. An example of such a spectrum is shown in Figure 3.2 .
Figure 3.2 Comparison between Seismic Resistance and Seismic Demand
a. If a structure exhibits seismic resistance larger than that required by the design earthquake, it obviously possesses an over-resistance and therefore is not vulnerable. This is the case shown by the longer ordinate in Figure 1.2. A structure with the resistance specified by such an ordinate is capable of withstanding an earthquake with an anchoring acceleration larger than that associated with the design earthquake.
b. Instead if the seismic resistance of the structure corresponds to the shorter ordinate in Figure 3.2, it is obvious that the resistance capacity is smaller than the demand that the earthquake places on it and the structure is vulnerable to the design earthquake. In this second case the structure can only withstand an earthquake with an anchoring acceleration smaller than the design one.
It is, therefore, necessary to retrofit the structure to allow for the satisfaction of the design inequality: Capacity ≥ Demand
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The design inequality above must be satisfied not only in terms of strength or resistance, but also in terms of stiffness. The stiffness capacity of the building must not be less than the stiffness demanded of it by the earthquake. If it were not so, displacements would be too large, especially inter-story drifts, and damage could result to non-structural components. The stiffness control is usually performed indirectly by checking the inter-story drifts. 3.2 LEVELS OF RETROFITTING
Based on the importance and utility of a building, there are four basic levels of seismic retrofitting.
3.1.1. Public Safety Retrofitting - In this type of seismic retrofitting, a structure is
reinforced so that people should not be killed in an earthquake, although they may be injured. In a large earthquake, the structure itself may become unsafe and need to be destroyed and rebuilt. For structures which are not very valuable, this type of seismic retrofitting is a reasonable option, if the company does not want to rebuild the structure altogether.
3.1.2. Structure survivability -The next level of seismic retrofitting is designed to
ensure that the structure will endure the earthquake, although it may need significant repairs.
3.1.3. Primary structure undamaged retrofitting – This is a type of seismic
retrofitting in which the majority of the damage to a structure as a result of an earthquake should be cosmetic.
3.1.4. Structure unaffected - This is the highest level of seismic retrofitting, chosen
for buildings of high economic, social, or cultural value. Some of these terms are a bit misleading, as no structure can be made entirely safe.
A major concern for companies that handle seismic retrofitting is historic buildings. It is important to preserve historic buildings with seismic retrofitting, but it is also important to ensure that the integrity of the building is not compromised. This take
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extensive work and cooperation with companies which specialize in restoration of historic buildings.A number of measures, both internal and external, are undertaken as part of seismic retrofitting as is shown by Figure 3.3
Figure 3.3 Key Idea of Seismic Retrofitting
Sugano(1990) outlines that retrofitting encompasses a broad range of techniques that help extend the life of a structure. Hence, any such attempt can be considered as a restoration work for the building
3.3 SEISMIC RETROFIT STRATEGIES
Retrofit strategies for building are classified into Global and local retrofit strategies. 3.3.1 Global Strategies
Global retrofit strategies aim to retrofit the building by providing additional lateral load resisting elements so that it has reduced lateral deformation. Global strategies are primarily employed to address Global deficiencies which address the building as a whole . Global deficiencies are primarily Plan irregularities such as torsional irregularity due to plan asymmetry, out of plane offset for columns along perimeter,
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etc and Vertical Irregularities such as mass irregularity, in plane discontinuity for perimeter columns, stiffness irregularity creating soft storeys etc. Global Retrofitting can be done by the following strategies Structural Stiffening - The Stiffness of the structure can be enhanced by the addition of lateral load bearing members which also have high energy dissipation capability. Addition of infill walls, Shear walls etc work towards this end. Additionally, bracing systems can also be implemented for providing stiffness of the structure by distributing the building loads better throughout the building. Reduction of Irregularities-Torsional irregularities can be corrected by the addition of frames or shear walls. Eccentric masses can be relocated. Seismic joints can be created to transform an irregular building into multiple regular structures. Discontinuous components such as columns and walls can also be extended beyond the zone of discontinuity Energy dissipation devices and base isolation- Active and passive devise and control systems can be used for energy dissipation and base isolation devices which control the seismic loads imposed on the structural members of the building.
3.3.2 Local Retrofit Strategies
Local Retrofit strategies include local strengthening of beams, columns, slabs, beamcolumn joints or slab to column joints, walls and foundations. This strategy is economical when only a few structural members are found to be deficient. The main local retrofit strategies applied for different structural members are Columns- Column strengthening can be done by Concrete Jacketing, Steel Jacketing, Prestressed Wire wrapping or by Fibre Reinforced polymer wrapping Beams – Beams strengthening can be achieved by Addition of concrete, Steel plating, FRP Wrapping and External prestressing. Beam-Column Joints- Strengthening can be achieved by Concrete jacketing, Concrete fillet, Steel Jacketing and FRP Jacketing. Choice of strategy for a particular retrofitting job depends upon the condition of the structure, importance of the structure and also economic considerations. IS 13935:1993 states that “Retrofitting an existing inadequate building may involve as much as 4 to 5 times the initial extra expenditure required on seismic resisting
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features. Thus, even if global strategies seem to be the safe solution, economical factors may be in favour of a combination of local retrofitting strategies which provide the same benefits. Moreover, Local retrofitting strategies can also be applied specifically to partially deteriorated or damaged members without affecting the whole structure. 3.4 SEISMIC RETROFITTING FOR COLUMNS
Until the last quarter of the twentieth century seismic loading was not generally taken into account in the design of reinforced concrete buildings or, when considered, the resulting reinforcement detailing might not be satisfactory by the standards of the current structural codes. For this reason, significant damage can occur in old buildings, even with the occurrence of moderate seismic loads. In most cases, columns represent the most vulnerable elements since their failure leads to the collapse of the structure. Some of the major structural deficiencies observed in columns are: a. Inadequate Shear capacity b. Lack of confinement of column core c. Inadequate capacity of corner columns under biaxial seismic loads d. Existence of short and stiff columns due to infill walls of partial height. Since the last few years, conventional materials used to strengthen reinforced concrete columns are being replaced with carbon and glass fibre reinforced polymers (CFRP or GFRP). The advantages of these composite materials are the strength/weight and stiffness/weight high ratios, as well as the high resistance to environmental actions, lightness, durability and ease of application. The confining of columns with external FRP composite jacket is the equivalent for applying steel spiral rebars as internal transverse reinforcement. The confining causes three-axial state of stress in cross-section of compressed members and reduces the increase of transverse strains as well. Fundamental difference between members confined with internal steel spiral and external FRP is that in case of composite reinforcement the whole crosssection is strengthened and composite material works elastically till failure, whereas in case of the steel spiral reinforcement the confinement concerns only the core of cross-section.
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CHAPTER 4 FIBRE REINFORCED COMPOSITES FOR RETROFITTING
4.1 GENERAL
A Composite is a combination of two or more materials, which differ in form or composition on a macroscale. The constituents retain their identities, i.e., they do not dissolve or merge into each other, although they act together. Normally, the components can be physically identified and exhibit an interface between each other. Fibre Reinforced Polymers (FRP) are composites that consist of high performance fibers embedded in polymer matrices. Some of the Fibres utilized in FRP are Carbon, Glass & Aramid. 4.2 GEOMETRY AND ARRANGEMENT
When fibres are composed in layers, they can exhibit one of the following types of arrangements as shown by Figure 4.1
Figure 4.1 Arrangement of Fibres in Polymer matrix
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Usually, laminates of the required orientation of FRP are glued together to form a composite sheet which has the required desirable properties. Figure 4.2 shows two multi layer composites, one with unidirectional orientation for all layers and the other with quasi isotropic fibre orientations. Unidirectional orientation of fibres prove advantageous when the sheet is used in such a way that it only has to resist tensile loads along the direction of the fibre (Figure 4.3). This being the case when the CFRP sheets are wrapped around columns for confinement. However, when they are wrapped in regions which are likely to experience stresses in more than one direction, it is desired to have quasi isotropic sheets which is essentially a combination of fibre sheets in different directions glued together. This is particularly required when the sheets are applied at beam-column joints, slab-column joints etc where in addition to the confinement action of the sheet, it may also need to develop sufficient resistance against shearing stresses developed.
Figure 4.2 Combination of FRP Laminates
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Figure 4.3 Effect of Fibre Orientation in Unidirectional Composites
4.3 PROPERTIES
Fibres being composed of different primary materials shows deviations in their properties also. A comparison of major properties between Carbon, Glass and Aramid fibres are drawn up in Table 4.1 in order to ascertain the most suitable type of fibre reinforced polymer for a particular job. Table 4.1 Comparison of properties of Fibre Reinforced Polymer composites Type of Fibre Chemical Attack Thermal Expansion Heat Resistance Impact Tolerance Creep Rupture and Fatigue Electrical Conductivity
Glass Damaged in Alkaline media Similar to Concrete upto 1000 C High Low Resistance
Carbon Good Resistance
Aramid Not Tolerant
Unaffected upto 650 C Low High Resistance
Unaffected Upto 200 C High Low Resistance
Insulator
High
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Insulator
Each type of Fibre composite has its own domain of application. However, it can be seen that Carbon fibre is superior to glass and aramid fibres on account of its resistance to chemical attack, excellent tensile strength. A competition of the Uniaxial tensile behaviour of Carbon Fibre and Glass fibre sheets clearly indicate that Carbon is capable of withstanding a higher tensile load in addition to its lesser straining properties with glass fibre composites for the same load as shown in Figure 4.4
Figure 4.4 Comparision of Uniaxial Tensile Behaviour of FRP Sheets Fibre sheets or filaments (uni- or bi-directional) are used to obtain a polymer laminate. The sheets are available as raw fibres or pre-impregnated (with curing on site). Carbon fibre sheets are normally uni-directional. Often layers are superposed for bi-directional strengthening.
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CHAPTER 5 CARBON FIBRE REINFORCED POLYMER COMPOSITES Carbon Fibre Reinforced Polymer sheets are unidirectional Fibre Reinforced composites used in structural strengthening on basis of their excellent Strength to weight ratio. Usually, many layers of these sheets are oriented in different directions and combined together to form a composite capable of taking loads in all directions. The strengthening of RC columns with wrapped CFRP sheets to improve seismic performance is one of the major applications of this type of retrofitting. The wrapped CFRP sheet around the plastic hinge region of RC columns provides not only enough shear strength which results in a ductile flexure failure mode in accordance with the concept of strong shear and weak flexure, but also confinement of concrete in the plastic hinge region to increase the ductility of the column.
Figure 5.1 Carbon Fibre Reinforced Polymer Sheets
This strengthening technology is being used for bending, shearing and compressed members. The confining of columns with external FRP composite jacket is the equivalent for applying steel spiral rebar as internal transverse reinforcement. The confining causes three-axial state of stress in cross-section of compressed members and reduces the increase of transverse strains as well. Fundamental difference between members confined with internal steel spir al and external FRP jacket is that in case of composite reinforcement the whole cross-section is strengthened and composite material works elastically till failure, whereas in case of the steel spiral reinforcement the confinement concerns only the core of cross-section. Moreover,
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when stress in steel reaches a yield strength strains increase under constant level of load. In case of quadrangular members the investigations of strengthening technologies with longitudinal CFRP reinforcement has been started. On the basis of the researches of Trapko and Musial(2011) it was proved that strengthening with longitudinal CFRP ,
strip segments causes the increase of column rigidity. It is revealed with the limitation of longitudinal strains in strengthened elements in relation to control elements (without strengthening). The velocity of strains increase depends on the longitudinal strengthening intensity, whereas it does not depend on the way of constructing of transverse strengthening. The influence of transverse strengthening in the form of CFRP bands (arranged similar as steel stirrups) and in the form of continuous confinement with CFRP sheet was analysed. It was found that strengthening only with CFRP strips is disadvantageous. The fibres of strip are directed parallel to the axis of the column. It causes debonding of composite and concrete. The longitudinal external strengthening should be supported with transverse reinforcement to prevent from delamination of composite. Carbon Fibre Reinforced Polymer Composites have excellent Strength parameters which make them excellently suited for providing as tensile reinforcement. Their low weight and ease of application to structural members make them an ideal choice as external reinforcement for structures in need of retrofitting. Table 5.1 explores the strength properties of both CFRP Sheets and CFRP strips. Table 5.1 Strength Properties of CFRP Materials
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The flexible nature of CFRP sheets give it precedence for use as external reinforcement especially in requirements where the structural member needs to be wrapped. This is typically with respect to columns, tie beams, footings etc. CFRP strips on the other hand may be used when the whole member cannot be wrapped but instead need additional tension reinforcement on one or more faces. This is typically in the case of reinforcing portions of slabs, reinforcing tunnel soffits etc. CFRP strips are also employed as Near Surface mounted reinforcement where they are partially embedded into the concrete surface and given concrete cover so that the can provide the desired requirements of external reinforcement without being exposed to climatic conditions, wear and tear, vandalism etc. 5.1 APPLICATION OF CFRP SHEETS TO COLUMNS
The affixing of Carbon Fibre Reinforced Polymer sheets to columns is more preferred than other retrofitting methods due to its ease of handling and controlled quality during manufacture. Though it is not as labour intensive as other retrofitting activities, skilled labour is a prerequisite as the improper placing of the sheets with inadequate bonding to the structural member will prove to be ineffective. The procedure for the application of CFRP sheets to columns is as follows: 5.1.1 Preparation of the base
The Surface to receive the sheets should be made clean, dry, firm rough and free of grease and contaminants. Light to medium scarification of the surface can be done to improve the bonding between the surface and the CFRP sheets. Figure 5.2 elaborated the various processes used for effective cleaning and hacking of the surface.
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Figure 5.2 Base Preparations for CFRP Application 5.1.2 Priming with Epoxy base
The Cleaned surface is primed with Epoxy base so as to provide bonding between the CFRP sheet and the structural member. Figure 5.3 shows the application of Epoxy based binding resin for CFRP Sheets.
Figure 5.3 Application of Epoxy Primer
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5.1.3 Placing of CFRP Sheet
The CFRP sheets are then wrapped around the concerned structural member. Partial wrapping or full wrapping may be provided as per the requirement. Whenever full wrapping is involved, it may be carried out in a helicoidal manner. Partial wrapping involves the wrapping of CFRP in bands with a certain clear distance between each band. Figure 5.4 shows the helicoidal wrapping of CFRP sheets around a column.
Figure 5.4 Placing of CFRP Sheets 5.1.4 Applying protective coating
Protective coating may be applied on the CFRP coating to protect it from exposure to climatic conditions, moisture etc. Figure 5.5 shows the protective coating applied to the CFRP sheets after being placed in position
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Figure 5.5 Protective Coating applied to CFRP sheets
5.2 ADVANTAGES AND DISADVANTAGES OF CFRP SHEETS
Carbon Fibre Reinforced Polymers are regarded as excellent retrofitting materials to structural members because of their many favourable parameters. a. The High Strength to weight ratio ensures that thins layers of Sheets can be applied onto member which provide the necessary strengthening required without compromising on the internal spaces of the structure. b. Being flexible, CFRP sheets can also be applied across curved surfaces as and how it is present on site. c. Unlike other stiffening materials, it need not be prefabricated and precut to irregular or curved shaped to suit site conditions. d.
Another important aspect of CFRP sheets is that it can be applied in lesser time to structural member as compared to conventional retrofitting method like jacketing etc. This is also enabled with minimum restrictions to the normal operation of the structure
However it does also possess some undesired characteristics a. Though Carbon Fibre Reinforced polymer composites have excellent ultimate load bearing characteristics, they also undergo brittle failure after these loads are exceeded. Brittle failure is characterised by sudden collapse and hence,
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retrofitting with CFRP materials must be done after ensuring that in no case will the loads imposed on the structure will exceed the ultimate loads. b. In cases where the CFRP sheets need to be stressed, it is observed that anchorage to the structural member is often difficult to achieve and is also prone to slippage and debonding of binding resin. c. The major factor however is the high cost of Carbon Fibre materials. It is imperative to justify the huge cost to retrofitting by taking into consideration the factors such as the importance of the building, predicted effectiveness of retrofit etc.
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CHAPTER 6 EXPERIMENTAL CASE STUDY 6.1 INTRODUCTION
A case study was conducted by Colomb et al (2007) to study the effect of lateral forces on short columns with different FRP configurations Eight short columns were tested; their longitudinal reinforcement was higher than the Euro code 8 upper limit whereas transverse reinforcement was insufficient, in order to ensure shear failure. Seven were continuously or discontinuously reinforced by CFRP or GFRP. They were tested under a constant compression load combined with a horizontal quasi-static cyclic load. It was therefore possible to evaluate the efficiency of such reinforcement by measuring the gain in terms of load and ductility. Shear Failure is brittle and non-dissipative. It is characterized by diagonal cracks in the concrete as soon as the tensile stress of the concrete is reached, after which the transverse reinforcement yields followed by the buckling of the longitudinal rebar, at which stage the concrete is completely crushed (Seible et al , 1997). Care must be taken to avoid this failure mode (Teng et al ., 2002). Shear failure appears when the transverse reinforcement is insufficient. It is frequent in short columns (Figure. 6.1). Short columns can have a structural origin (e.g. underground, with openings of low height, under-floor spaces, etc.). Short columns can also result from structural modifications: the original columns being shortened by the installation of barges or partial masonry, symmetrically or not.
Figure 6.1 Shear Rupture in column
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Seismic reinforcement strategy of a structure generally has three aims: – Enhancement of load-bearing capacity. – Enhancement of strength and ductility. – Increase in ductility. The choice of strategy depends on the category of the structure, but seismic reinforcement of the reinforced concrete posts always consists of increasing the ductility, for the more ductile the behaviour, the more it dissipates the energy induced by the seismic load, preventing non-dissipative (brittle) failure, which civil engineers generally try to avoid (Paulay, 1991). The choice of strengthening to increase the ductility rather than the load-bearing capacity is explained by the fact that this involves an increase in seismic load and a reduction in post-elastic deformation. This reinforcement is necessary when there is insufficient longitudinal reinforcement due to design error or rebar corrosion. Studies by Ilki et al . (2004) on the effect of external bonded FRP reinforcement on the shear behaviour of strengthened short columns have already been done. The experimental work done enhance the fact that the continuous FRP reinforcement column wrapping allows to increase the ultimate displacement and the ultimate strength. Taking all the results into account, there still exist a need to increase the data base of CFRP strengthened short column loaded in shear. It is important to understand how the composite sheet wrapping enhances the ductility. The aim of this study is to evaluate the increase in strength and in ductility resulting from the use of FRP depending on the FRP reinforcement ratio, the FRP reinforcement position (continuous or partial) and the material properties (Young’s modulus, ultimate strength). The columns fabricated for the study have a square section of 200 mm x 200 mm2, which is a 2/3 scale of real short columns in buildings. These specimen sizes were selected as being representative of reality. The selected slenderness is equal to three (H/L = 3), which finally gives columns of 200 mm x 200 mm x 600 mm. A slenderness of 3 is the upper limit of the short column notion. Eight 16 mm steel rebars are used for the longitudinal reinforcement and three 6 mm frames separated by 200 mm for transverse reinforcement. In order to reproduce the boundary conditions as accurately as possible, the columns were embedded at their two ends in two
25
reinforced concrete blocks of 600 x 600 x 300 mm3 to avoid failure in the embedding. Figure 6.2 shows the specimen dimensions. Figure 6.3 describes the reinforcement.
Figure 6.2 Dimensions of Column specimen
Figure 6.3 Details of column reinforcement
The set of eight columns may be described as follows (one was not reinforced (SC-1) and will be used as a reference). For the seven remaining columns, the parameters taken into account were the thickness of the FRP reinforcement (numbers of layers) and its configuration. The second column was reinforced by carbon layers of 100 mm
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in width, 100 mm apart (SC-PW-2C 1). The third and fourth columns were reinforced on the entire height by, respectively, two and three layers of carbon FRP (columns SC- FW-2C and SC-FW-3C). In order to optimise the reinforcement, the remaining columns were partially wrapped, and the number of layers varies along the height. SC-PW- 3C 1 was reinforced by carbon strips of 100 mm in width and 100 mm apart; it had three layers near the embedding and two at middle height. The same reinforcement ratio was used for SC-PW-3C 2, with 50 mm bands every 50 mm. Two 150 mm wide strips of three carbon layers were glued on specimen SC-PW-3C 3. Glass fibre was used for the last column. Strength equivalence of 3 glass fibre layers for 1 Carbon fibre layer was adopted. The eight columns prescribed for study can be summarized as follows: a) SC1 – Not reinforced and taken for reference b) SC-PW-2C 1 -Reinforced by carbon layers of 100 mm in width, 100 mm apart c) SC- FW-2C - Reinforced on the entire height by two layers of carbon FRP d) SC- FW-3C - Reinforced on the entire height by three layers of carbon FRP e) SC-PW- 3C 1 - Reinforced by carbon strips of 100 mm in width and 100 mm apart; it had three layers near the embedding and two at middle height f) SC-PW-3C 2 - Reinforced by carbon strips with 50 mm bands every 50 mm. it had three layers near the embedding and two at middle height g) SC-PW-3C 3- Reinforced by two 150 mm wide strips of three carbon layers h) SC-PW-9G - Reinforced by Glass fibre strips of 100 mm in width and 100 mm apart with three layers near the embedding and two at middle height. 9 Layers of Glass fibre was given to achieve equivalence with 3 layers of Carbon fibre
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Figure 6.4 Columns for experimental study The premature failure of the FRP reinforcement may be due to irregularities in the concrete; this phenomenon is more pronounced for square sections. Even when the angles are rounded, the stress is still concentrated at the angles, so two layers of FRP is a minimum (Mirmiran et al., 1998). The angles were rounded to avoid FRP cracks due to local failure and stress concentration. The columns and their footing were cast at the same time in order to ensure their perfect structural integrity. The concrete mix is given in Table 6.1 Table 6.1 Concrete Mix for Columns Material Cement Water Coarse Aggregate Fine Aggregate
Batch Weight (kg/m3) 350 190 950 880
The concrete compression tests were carried out on five 16 x 32 cm cores. It was therefore possible to determine the compressive strength of the concrete fc=31.5MPa± 1.5 MPa.
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6.2 LOADING DEVICE
The loading device (Figure 6.5) permits the application of a horizontal displacement to the top of the column. A constant compressive load is applied to the top of the column, representing the loads of the higher floors of buildings and the service loads. The horizontal loading must form a double curve; this involves identical moment at the embedding (foot and head) and a constant shear stress on the whole height of the column. It is applied to the middle of the column through an L-shaped steel framework. The horizontal part of the "L" is fixed on the column, with threaded rods passing through the holes in the upper column footing. The load applied to the column must produce only horizontal displacement of the top of the column. The rotation of the foundation must be limited. To do this, a pantograph is used. This double parallelogram allows only horizontal or vertical translations and prevents rotation.
Figure 6.5 Loading Device The axial load is applied via prestressed rods ( Fpeg = 443 kN, Fprg = 547 kN). The column's head is mobile (in translation only), so the jack should be able to move in
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order to maintain a constant load. Despite being prestressed, the threaded rods retain a rotational capacity. 6.3 LOADING DIAGRAM
The horizontal loading is controlled in displacement: three push-pull cycles are carried out each time in order to stabilise the hysteresis loops. Relative displacement (drift) noted R corresponds to the ratio displacement at the column head D (imposed) and height H.
Figure 6.6 Load Diagram for testing The drift values are R = 0.2%; 0.4%; 0.6%; 0.8%; 1% and the displacements imposed after 1% are multiples of 1 (Fig. 6.6). The drift values were chosen because most failures of short columns found in the literature were obtained for a drift ranging between 0.5% and 1%, whereas for CFRP-strengthened columns they appeared for values of over 4%. For R < 1%, closer cycles were used so as to have enough data for the unstrengthened column. 6.4 INSTRUMENTATION
The horizontal cyclic loading was applied quasi-statically, by a hydraulic jack with a capacity of 500 kN in compression and 170 kN in tension. A load cell with a tensile capacity of 500 kN in compression was placed between the L-shape and the jack. Vertical load was measured by a 1000 kN load cell. Horizontal displacement at the top of the column was measured by an LVDT. Three other LVDT were placed on the column, and the one on the column base measured rotation. Steel and composite
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strains were measured with strain gauges (Fig. 6.5) bonded in various places. The acquisition frequency is 1 Hz. 6.5 RESULTS 6.5.1 Load and failure mode
Ultimate load is the first parameter of comparison. It allows the direct quantification of the effects of FRP reinforcement. The second parameter is failure mode. These results are presented in Table 6.2. The partially reinforced columns have a lower resistance than the fully confined ones. The fully reinforced columns present bending failure, while for the partially reinforced ones it is mainly shearing failure. For a same cracking pattern the ultimate loads are almost identical, at all rates of reinforcement (Fig. 6.8). The ultimate load is doubled for the columns SC-PW-2C and 3C. The load variation between these two specimens is less than 1.5%, which corresponds to the measurement uncertainty. The load increase of the columns reinforced by bands varies from 65% to 70% for the bands of 100 and 150 mm. The column with bands spaced at 50 mm (SC- PW-3C 2) seem to have the least effective configuration, with a load-increase limited to 55% (relative to SC 1), although it was reinforced at the same rate of FRP as SC-PW-3C 1. Distributed cracking between bands occurred. This column is thus more susceptible to damage. Table 6.2 Summary of Ultimate loads and Associated failure Mode Column Name
Ultimate load (kN)
Failure mode
SC 1
128.30
Shear
SC-PW-2C
217.90
Shear
SC-FW-2C
256.60
Bending
SC-FW-3C
260.10
Bending
SC-PW-3C 1
211.56
Shear
SC-PW-3C 2
199.11
Shear
SC-PW-3C 3
218.66
Shear
SC-PW-9G
223.47
Shear
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Figure 6.7 Description of the three behaviour phases The load-displacement diagram of column SC-PW-2C shows three stages (Figure 6.7): a first, elastic stage, where the two curves (SC-1 and SC-PW-2C) are perfectly superimposed: the composite does not change column stiffness before the columns crack. After the cracking, the curve slope inflects. This stage corresponds to column damage. The degradation of the mechanical properties is due to the cracking of the concrete and to the rebars yielding. Increased displacement involves an increasingly diffuse cracking pattern. The reinforcements yield gradually. The third stage is a yielding stage which results from the total yielding of the reinforcement: the rupture of column SC-PW-2C occurred after the composite failure following stress concentration at the angles (which did not occur with t he reinforced columns). The column reinforced by glass fibre presents a load increase of close to 75%. The reinforcement design and its bonding configuration are strictly identical to the column SC-PW-3C 1. To facilitate the analysis of the various reinforcement configurations, the loaddisplacement curves of the partially reinforced columns and of the non-reinforced columns were plotted together (Figure 6.7). Then the same curve was plotted taking into consideration the reinforcement material (Figure 6.10) and the bonding configuration (continuous or partial reinforcement) (Fig. 6.9); for these two parameters, the unstrengthened reinforced concrete column SC 1 and the partially reinforced column
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SC-PW-3C 1 were used. The latter column was chosen because it was strictly identical to column SC-PW-9G in terms of resistance and reinforcement configuration. 6.5.2 Influence of the width and spacing of the bands
A clear modification of column behaviour appears. The unstrengthened column SC-1 has elastic behaviour until failure. The break is brittle and does not allow the column to support any more loading. The reinforcement by bands of column SC-PW-2C (smallest quantity of bonded reinforcement), confers a greater strength and deformation capacity (respectively, +70% and +455%). So this reinforcement strategy is based on increased resistance and ductility. SC-PW-3C-2 showed a relatively high deformation before failure perhaps due to formation of micro cracking between the bands of reinforcement. Each band being only 50mm thick provided more localised cracking and thus enabled higher displacements. SC-PW-3C-1 and SC-PW-3C-3 exhibited behaviour which permitted the increase in axial loads but did not cause excessively high deformation due the fact that the regions bonded by the FRP had provided more stiffness at the region of confinement. Whatever the width and the spacing of the bands, the overall behaviour of the strengthened columns clearly indicated
a
significant
increase
in
strength
as
well
Figure 6.8 Influence of width and spacing of CFRP bands
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as
ductility.
6.5.3. Influence of the reinforcement type (continuous or discontinuous)
Column SC-FW-2C (reinforced with two continuous carbon sheets) presents a nearly identical behaviour to SC-PW-3C 1. It too presents three stages. However, the elastic stage continues to a load of 21.6 kN (an increase of 68%). After reaching the level of loading at which column SC-1 failed, the slope of the curve did not change: the initial stiffness is not affected by the FRP. The cracking of the concrete and its expansion are blocked by the composite. The prolongation of the first stage is made to the detriment of the second, and concrete damage is limited. During this second stage we observed multi-cracking of the columns between the bands in SC-PW-3C 1. The third stage was reached at loading levels higher than for column SC-PW-3C 1; this is explained by the fact that the CFRP confining pressure improves steel – concrete adherence. Because of the limitation of the tensile bars’ slip, this is a better use of the reinforcements (yielding). Though the straining for the partially strengthened and fully strengthened columns are more or less close, as seen in Figure 6.9, there is an observed improvement in the Ultimate load carrying capacity of the fully reinforced column
Figure 6.9 Influence of Reinforcement type-Continuous and Discontinuous At the end of the test, all columns reinforced by bands presented longitudinal cracks. These cracks occurred along the longitudinal rebars; the degradation of the steel – concrete interface is then obvious. Another phenomenon explaining the increase in
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resistance is that the concrete is well confined when the column is fully reinforced. The confined concrete can reach higher levels of resistance and deformation. The resistance increase of the confined concrete was evaluated at 15%. For the entirely reinforced column, energy dissipation is mainly performed by two plastic hinges in the embeddings, due to the yielding of the longitudinal reinforcements; this phenomenon was confirmed by the steel strain gauges. The behaviour of column SCFW-3C is nearly identical to that of column SC-FW-2C. 6.5.4. Influence of reinforcement material (carbon or glass)
The three stages of behaviour are identical to the previous case. The Young modulus for glass reinforcement is lower than that for carbon and column SC-PW-9G is slightly more flexible than the carbon equivalent column SC-PW-3C 1. So the yield stress changed, although ultimate resistance was equivalent (Figure 6.10). The inference obtained is that we can obtain the same desired strength and ductility as offered by carbon fibres provided we consider the strength equivalent between the different fibres. This is important in design as though the cost of the same area of glass fibre is significantly lesser; there arises a need to use more layers of glass fibres which may finally turn out to be more expensive than the overall cost of using the carbon fibre reinforced sheets.
Figure 6.10 Influence of Reinforcement material (Carbon and Glass)
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6.5.5 Ductility
European regulations PS - 92 or EC 8 are followed to give the structure a certain freedom of displacement. This capacity of deformation must occur in the plastic range. A principle for reinforcement would thus be to allow the greatest possible ultimate displacement. Another means of evaluating this capacity is by calculating the ductility coefficient. Ductility refers to the capacity of a part to yield without breaking. Rupture occurs when a defect (crack or cavity) induced by the yield strains becomes critical and is propagated. Ductility is thus the aptitude of a structure to deform without breaking. If it does this well, it is known as ductile, if not it is known as brittle. The index of ductility is calculated in the following way:
Table 6.3 Index of Ductility Column
Elastic Displacement(mm)
Ultimate Displacement(mm)
µ
SC 1
5.148
5.148
1.000
SC-PW-2C 1
7.826
28.389
3.628
SC-FW-2C
9.238
20.159
2.182
SC-FW-3C
3.758
17.173
4.570
SC-PW-3C 1
8.963
17.226
1.922
SC-PW-3C 2
18.017
44.090
2.447
SC-PW-3C 3
13.513
15.282
1.131
SC-PW- 9G
13.593
15.640
1.151
Table 6.3 calculates the coefficient of ductility µ for comparison between the columns. Unstrengthened concrete column SC 1 presents a coefficient of ductility µ equal to 1, a sign of brittle failure. The ductility of the CFRP reinforced columns, on the other hand, avoids the problem of brittle failure. The ductility coefficient of columns SC-PW-3C 3 and SC-PW-9G is equal to µ =1.15, so their failure is also regarded as brittle. Other columns too present ductile behaviour ( µ >2). The most ductile column was one reinforced on its entire height by 3 layers of carbon sheet SCFW-3C; its ductility is quite relative because it does not increase with a rise in the bending deformation of the column but by considerable rotation at the embeddings.
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The reinforcement caused a significant increase in displacement (1.5-3.5 times). Ductility indices of 5 or 6, which occur in civil engineering, were not reached. Reinforcing short columns against seismic forces is, thus, complex. Regarding the possible strategy of reinforcement it would be necessary to choose a mixed strategy (ductility + load) since the gain in load is real while the gain in ductility is limited. 6.5.6 Stiffness Evolution
With the increase in displacement and in the number of cycles, the hysteretic loops tend to be inclined due to a reduction in stiffness. This characteristic permits a quantification of the damage. Indeed, with the imposed boundary conditions, the stiffness can be evaluated by the following expression:
Where E and I are respectively the elastic modulus and the inertia of the column and L is its length. At the time of damage, the cracking of the concrete reduces the structural integrity: this corresponds to a reduction in the inertia of the section. The variation in stiffness corresponds to the modification of the product of E x I. Stiffness was calculated for the three cycles with the same drift, and an average was taken. Stiffness is always greater in the first cycle than the others. This is explained by the fact that the first cycle damages the materials (by cracking of the concrete, steel yielding, and failure of the steel-concrete adherence). After this drift-specific damage, the column properties stabilise. Fig. 6.11 illustrates the progressive reduction in stiffness which renders a logarithmic curve. It is particularly marked for a 0.8% drift.
Figure 6.11 Evolution of stiffness
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This first stage corresponds to the behaviour of uncracked concrete columns. The load on the composite increases as the concrete degrades. This produces the second curve, in which degradation seems linear relative to drift. Beyond R = 0.8%, the average degradation of the stiffness is 300 daN/mm for every 1% of additional drift,
6.5 INFERENCES
FRP reinforcement completely changed the failure mode of the columns. For the two entirely wrapped columns brittle shear failure changed to ductile bending failure, while in the strip-reinforced column failure was due to shear-bending. The strategy of FRP reinforcement in this study involves the increase of both resistance and ductility. Reinforcement by strips provides a more advantageous dissipative behaviour than the fully wrapped columns. This is due to the ductility gained through the following two mechanisms:
Damage to the concrete by cracking between the FRP str ips.
Yielding of the reinforcements in all column sections.
For the columns which were fully wrapped in FRP, ductility was increased, mainly due to transfer to the embeddings, creating a hinge by advanced yielding of the longitudinal reinforcements. The FRP reinforcement allowed rotation in the embedding sections, without buckling of the compressed reinforcements, although they greatly exceeded their elastic limit. So it seemed that using a different thickness of reinforcement would be advantageous. Composite material reinforcement endowed the short columns with ductile behaviour, although the columns did not contain the necessary transversal reinforcement ratio. Care must be taken not to oversize the FRP reinforcement, as this results in a transfer of effort to the nodes. Finally, the strategy of reinforcement must be total and non-local. Though all configurations of FRP composite sheet wrapping provided strength and ductility gains which increased the seismic resistance, additional studies need to be done to ascertain whether a full wrapping of the column is necessary, the type of FRP composite to be chosen and also the configuration of wrapping to be followed.
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CHAPTER 7 CONCLUSIONS
The essential prerequisite before Seismic retrofitting is the determination of the scope of works required. A decision regarding this is made taking into account the importance of the building, the seismic retrofitting strategy required and costs for the procedure. Given that most buildings require only certain structural members to be retrofitted, it is imperative to employ a local retrofitting strategy which is also convenient to execute without disrupting the normal operations of the building. Retrofitting with Carbon Fibre Reinforced Polymer composites has proved to be an ideal solution with these factors in mind. Studies conducted on the CFRP composites have also yielded positive results for this method of strengthening structural members, especially columns The ductility of RC columns can be substantially improved by strengthening using wrapped CFRP sheets due to the confinement from CFRP especially under the influence of strong lateral loads such as seismic loads. There is also a considerable improvement in terms of strength reinforced concrete columns are strengthened with a CFRP composite by wrapping. The following conclusions were drawn from the parameters studied: Resistance to lateral loads CFRP wrapping around columns has significantly increased the Ultimate load carried by the column before failure. Full wrapping has even enabled the column to carry almost double the load of an unstrengthened column though its deformations may exceed acceptable limits. However, it has been instrumental in changing the mode of failure from brittle shear failure to bending failure in fully wrapped columns. This is key considering that the structure will not collapse without undergoing large deformations, which may help save a lot of lives especially when exposed to seismic loading. Though the partially strengthened columns still showed shear failure, these happened at much higher loads and hence can be viewed as an improvement of the seismic resistance as compared to an unstrengthened column.
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Type of FRP used for confinement The choice of type of FRP reinforcement has to be considering a variety of factors such as exposure, fire resistance, cost as well as the key parameters of strength and ductility. Testing shows that that Glass Fibre based composites can be used in place of Carbon fibre based composites provided the strength equivalence is taken into consideration. Number of CFRP layers The number of FRP layers has a clear correlation with the enhancement of strength and ductility of the column by better confinement of the column core. Testing of the FRP strengthened columns clearly indicate that the specimens wrapped with three layers of FRP are superior to those wrapped with two la yers of FRP. Configuration of FRP wrapping Different configuration of FRP wrapping were tried and tested. Though all the different configurations gave an enhancement in strength and ductility, there was no marked difference between the banded configurations with FRP to prefer one over the other. It was observed that FRP confinement throughout the column was instrumental for better strength and ductility than partial FRP wrapping. Ductility Improvement Carbon Fibre Reinforced Polymer strengthening of columns was successful in increasing the ductility of the columns. The effect was marginal in case of partially wrapped columns but was pronounced in the case of fully wrapped columns. Ductility was seen to improve 2-3 times for the columns taking the failure mode away from a brittle shear failure. However, it can still not be deemed enough to fully counter a powerful seismic loading. Hence, it is advisable to improve the ductility only up to a certain point. The key strategy is to bring about an enhancement in both the strength and ductility parameters for effective seismic resistance of the structural member. The study concludes by effectively pointing out the increase in seismic resistance of the structural member by application of FRP composite sheets. But it is imperative conduct a large range of studies and evaluation to understand the most effective
40
method to implement this method considering the part rendered by each retrofitting procedure to help increase the seismic resistance of the building.
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REFERENCES 1. ATC 40(1996) Seismic Evaluation and Retrofit of Concrete Buildings , Volume 1&2. 2. Basu P.C.(2002), Seismic Upgradation of Buildings: An Overview , The Indian Concrete Journal, the Associated Cement Companies Ltd., August, pp 461-475. 3. Colomb F. , H. Tobbi , E. Ferrier and P. Hamelin (2007) Seismic Retrofit of
Reinforced Concrete Short Columns by CFRP Materials , Elsevier science Composite Structures 82 (2008 ) 475 – 487
.
4. Ilki A, Peker O, Karamuk E, Kumbasar N . (2004) External Confinement of Low Strength Brittle Reinforced Concrete Short Columns, Proceedings of International Symposium on Confined Concrete, Changsa, Chine, 12-14 June 2004. 5. IS 13935 :1993 , Indian Standard for Repair and Seismic Strengthening of Buildings- Guidelines, Bureau of Indian Standards, New Delhi 6. Mander J. B. , M. J. N. Preistley and R. Park (1988) Theoretical stress – strain model for Confined Concrete. J Struct Eng 1988;114:1804 – 26 7. Mirmiran A. , M. Shahawy, M Samaan, and H. Echary (1998) Effect of column parameters on FRP confined concrete. J Compos Constr, ASCE 1998;2(4):175-85. 8. Oliveto G. and M. Marletta (2005), Seismic Retrofitting of Reinforced Concrete Buildings using Traditional and Innovative Techniques. , ISET Journal of Earthquake Technology, Paper No. 454, Vol. 42, No. 2-3,, pp. 21-46 9. Paulay T. (1991) Seismic design strategies for ductile reinforced concrete structural wall. In: Proceedings of international conference on buildings with load bearing concrete walls in seismic zones, Paris; 1991. p. 234-44. 10. Purba B. K . and A. A. Mufti (1999) Investigation of the Behavior of Circular Concrete Columns reinforced with Carbon Fiber Reinforced Polymer (CFRP) Jackets. Can J Civ Eng 1999;26(5):590-6. 11. Saadatmanesh H. and M. R. Ehsani (1994), Strength and Ductility of Concrete Columns Externally Reinforced with Fiber Composite Straps. ACI Struct J 1994;91(4):434-47.
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