behaviour of rc beams wrapped with gfrp-i

7/30/2019 Behaviour of RC beams wrapped with GFRP-I

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Study of Flexural Behavior of

Reinforced Concrete Beam with andwithout GFRP overlays

A

Report

submitted in the partial fulfillment of

Semester Projectoffered by the Department of Civil Engineering,

National Institute of Technology, Durgapur

Rajdip NayekRiya Dutta

Somak Ghosh

Department of Civil Engineering,

National Institute of Technology, Durgapur

Durgapur-713209 (India)

Under the supervision of Prof. Soumen Saha


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ABSTRACT

Reinforced concrete elements such as beams and columns may be strengthened in flexure

through the use of Glass Fiber Reinforced Polymer (FRP) composites epoxy -bonded to theirtension zones, with the direction of fibers parallel to that of high tensile stresses. In the lasttwo decades, several seismic retrofitting techniques for both concrete and masonrystructures have been developed and practiced and fibre-reinforced polymer (FRP) materialhas been increasingly used owing to its high strength/stiffness to mass ratio and easyapplication. In this study both analytical and experimental methods are being used topredict the deflection and crack pattern of rectangular reinforced concrete beamsstrengthened by FRP composites applied at the bottom of the beams. The validity of thefinite element model would be verified by comparing with the results of the experiments.


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Table of Contents

Chapter 1: Introduction to Fibre Reinforced Polymer (FRP)1.1 Introduction 5

1.1.1 Process Definition1.1.2 Material requirement

1.2 Advantages and Limitations 61.2.1 Failure Modes

1.3 Types of Fibre Reinforced Polymer 61.3.1 Glass Fibre

1.3.2 Carbon Fibre1.4 Application 7

1.4.1 Structural Application of FRP1.4.2 Design Consideration

1.5 Disposal and Recycling 8

Chapter 2: Literature Review 9

2.1 Introduction 9

2.2 Strengthening of RC Structures with FRP 9

2.2.1 Flexural Strengthening

2.2.2 Shear Strengthening

2.2.3 Confinement

2.2.4 Masonry

2.2.5 Timbers and Metals

2.3 Special Strengthening Techniques 12

2.4 Codal Provisions and Standards 13

2.4.1 United States

2.4.2 Europe

2.5 Provisions to be included in the Codes and Standards 142.6 Concluding Remarks 14

Chapter 3: Finite Element Modelling of RC Beam 15

3.1 Finite Element Modelling of RC Beam 15

3.1.1 Cross sectional view of the RC Beam

3.1.2 Finite Element Modelling

3.1.3 ABAQUS Coding in Input File

3.1.4 Material Behaviour

3.1.4.1 Concrete3.1.4.2 Smeared Crack Model


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3.1.4.3 Tension Stiffening

3.1.4.4 Cracked Shear Retention

3.1.4.5 Failure Ratio

3.1.4.6 Behaviour of Reinforcing Steel

3.1.5 Interaction3.1.6 Loading

3.2 Results and Conclusions 25

Chapter 4: Mixed Design 28

Chapter 5: Moment Capacity of the Beam 31

Tasks to be Performed 32References 33


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Chapter 1Introduction to Fibre-Reinforced Polymer (FRP)

1.1 Introduction

Fibre-reinforced polymer (FRP) is a composite material made of a polymer matrixreinforced with fibres. The fibres are usually fibreglass, carbon or aramid, while thepolymer is usually an epoxy, vinylester or polyester thermosetting plastic. FRPs arecommonly used in the aerospace, automotive, marine, and construction industries.

1.1.1 Process DefinitionA polymer is generally manufactured by polycondensation. When combined with

various agents to enhance or in any way alter the material properties of polymers the resultis referred to as a plastic. Composite plastics refer to those types of plastics that result frombonding two or more homogeneous materials with different material properties to derive afinal product with certain desired material and mechanical properties.

Fibre reinforced plastics are a category of composite plastics that specifically usefibrous materials to mechanically enhance the strength and elasticity of plastics. The originalplastic material without fibre reinforcement is known as the matrix. The matrix is a toughbut relatively weak plastic that is reinforced by stronger stiffer reinforcing filaments orfibres. The extent that strength and elasticity are enhanced in a fibre reinforced plasticdepends on the mechanical properties of the fibre and matrix, their volume relative to one

another, and the fibre length and orientation within the matrix. Reinforcement of the matrixoccurs by definition when the FRP material exhibits increased strength or elasticity relativeto the strength and elasticity of the matrix alone.

1.1.2 Material RequirementThe matrix must also meet certain requirements in order to first be suitable for the

FRP process and ensure a successful reinforcement of it. The matrix must be able to properlysaturate, and bond with the fibres within a suitable curing period. The matrix shouldpreferably bond chemically with the fibre reinforcement for maximum adhesion. The matrixmust also completely envelope the fibres to protect them from cuts and notches that wouldreduce their strength, and to transfer forces to the fibres. The fibres must also be kept

separate from each other so that if failure occurs it is localized as much as possible, and iffailure occurs the matrix must also debond from the fibre for similar reasons.

Finally the matrix should be of a plastic that remains chemically and physicallystable during and after reinforcement and moulding processes. To be suitable forreinforcement material fibre additives must increase the tensile strength and modulus ofelasticity of the matrix and meet the following conditions; fibres must exceed critical fibrecontent; the strength and rigidity of fibres itself must exceed the strength and rigidity of thematrix alone; and there must be optimum bonding between fibres and matrix.

1.2 Advantages and Limitations
http://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Carbon_(fiber)http://en.wikipedia.org/wiki/Aramidhttp://en.wikipedia.org/wiki/Epoxyhttp://en.wikipedia.org/wiki/Vinylesterhttp://en.wikipedia.org/wiki/Polyesterhttp://en.wikipedia.org/wiki/Thermosetting_plastichttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Plastichttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Elastic_modulushttp://en.wikipedia.org/wiki/Elastic_modulushttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Plastichttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Thermosetting_plastichttp://en.wikipedia.org/wiki/Polyesterhttp://en.wikipedia.org/wiki/Vinylesterhttp://en.wikipedia.org/wiki/Epoxyhttp://en.wikipedia.org/wiki/Aramidhttp://en.wikipedia.org/wiki/Carbon_(fiber)http://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Composite_material


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FRP allows the alignment of the glass fibres of thermoplastics to suit specific designprograms. Specifying the orientation of reinforcing fibres can increase the strength andresistance to deformation of the polymer. Glass reinforced polymers are strongest and mostresistive to deforming forces when the polymers fibres are parallel to the force being

exerted, and are weakest when the fibres are perpendicular. Thus this ability is at once bothan advantage and a limitation depending on the context of use. Weak spots of perpendicularfibres can be used for natural hinges and connections, but can also lead to material failurewhen production processes fail to properly orient the fibres parallel to expected forces.

When forces are exerted perpendicular to the orientation of fibres the strength andelasticity of the polymer is less than the matrix alone. In cast resin components made of glassreinforced polymers such as UP and EP, the orientation of fibres can be oriented in two-dimensional and three-dimensional weaves. This means that when forces are possiblyperpendicular to one orientation, they are parallel to another orientation; this eliminates thepotential for weak spots in the polymer.

1.2.1 Failure ModesStructural failure can occur in FRP material when:

Tensile forces stretch the matrix more than the fibres, causing the material to shear atthe interface between matrix and fibres.

Tensile forces near the end of the fibres exceed the tolerances of the matrix,separating the fibres from the matrix.

Tensile forces can also exceed the tolerances of the fibres causing the fibresthemselves to fracture leading to material failure.

1.3 Types of Fibre Reinforced Polymer

There are mainly two types of fibre reinforced polymer namely Glass fibre materialand Carbon fibre material.

1.3.1 Glass FibreFRP plastics use textile glass fibres; textile fibres are different from other forms of

glass fibres used for insulating applications. Textile glass fibres begin as varyingcombinations of SiO2, Al2O3, B2O3, CaO, or MgO in powder form. These mixtures are thenheated through a direct melt process to temperatures around 1300 degrees Celsius, afterwhich dies are used to extrude filaments of glass fibre in diameter ranging from 9 to 17 m.These filaments are then wound into larger threads and spun onto bobbins fortransportation and further processing. Glass fibre is by far the most popular means toreinforce plastic and thus enjoys a wealth of production processes, some of which areapplicable to aramid and carbon fibres as well owing to their shared fibrous qualities.

1.3.2 Carbon FibreCarbon fibres are created when polyacrylonitrile fibres (PAN), Pitch resins, or Rayon

are carbonized (through oxidation and thermal pyrolysis) at high temperatures. Throughfurther processes of graphitizing or stretching the fibres strength or elasticity can beenhanced respectively. Carbon fibres are manufactured in diameters analogous to glassfibres with diameters ranging from 9 to 17 m. These fibres wound into larger threads fortransportation and further production processes. Further production processes include


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weaving or braiding into carbon fabrics, cloths and mats analogous to those described forglass that can then be used in actual reinforcement processes.

1.4 Application

Fibre-reinforced plastics are best suited for any design program that demands weightsavings, precision engineering, finite tolerances, and the simplification of parts in bothproduction and operation. A moulded polymer artefact is cheaper, faster, and easier tomanufacture than cast aluminium or steel artefact, and maintains similar and sometimesbetter tolerances and material strengths.

1.4.1 Structural application of FRP

FRP can be applied to strengthen the beams, columns and slabs in buildings. It ispossible to increase strength of these structural members even after these have been severelydamaged due to loading conditions.

For strengthening beams, two techniques are adopted. First one is to paste FRP platesto the bottom (generally the tension face) of a beam. This increases the strength ofbeam, deflection capacity of beam and stiffness (load required to make unitdeflection). Alternatively, FRP strips can be pasted in 'U' shape around the sides andbottom of a beam, resulting in higher shear resistance.

Columns in building can be wrapped with FRP for achieving higher strength. This iscalled wrapping of columns. The technique works by restraining the lateralexpansion of the column.

Slabs may be strengthened by pasting FRP strips at their bottom (tension face). Thiswill result in better performance, since the tensile resistance of slabs is supplemented

by the tensile strength of FRP. In the case of beams and slabs, the effectiveness of FRP strengthening depends on the

performance of the resin chosen for bonding.

1.4.2 Design ConsiderationFRP is used in designs that require a measure of strength or modulus of elasticity

that non-reinforced plastic and other material choices are either ill suited for mechanically oreconomically. This means that the primary design consideration for using FRP is to ensurethat the material is used economically and in a manner that takes advantage of its structuralenhancements specifically. This is however not always the case, the orientation of fibres alsocreates a material weakness perpendicular to the fibres. Thus the use of fibre reinforcement

and their orientation affects the strength, rigidity, and elasticity of a final form and hence theoperation of the final product itself.

Orienting the direction of fibres either, unidirectional, 2-dimensionally, or 3-dimensionally during production affects the degree of strength, flexibility, and elasticity ofthe final product. Fibres oriented in the direction of forces display greater resistance todistortion from these forces and vice versa, thus areas of a product that must withstandforces will be reinforced with fibres in the same direction, and areas that require flexibility,such as natural hinges, will use fibres in a perpendicular direction to forces.

Using more dimensions avoids this either or scenario and creates objects that seek to

avoid any specific weak points due to the unidirectional orientation of fibres. The propertiesof strength, flexibility and elasticity can also be magnified or diminished through the


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geometric shape and design of the final product. These include such design considerationsuch as ensuring proper wall thickness and creating multifunctional geometric shapes thatcan be moulding as single pieces, creating shapes that have more material and structuralintegrity by reducing joints, connections, and hardware.

1.5 Disposal and Recycling concerns

As a subset of plastic FRP plastics are liable to a number of the issues and concerns inplastic waste disposal and recycling. Plastics pose a particular challenge in recyclingprocesses because they are derived from polymers and monomers that often cannot beseparated and returned to their virgin states, for this reason not all plastics can be recycledfor re-use, in fact some estimates claim only 20% to 30% of plastics can be material recycledat all. Fibre reinforced plastics and their matrices share these disposal and environmentalconcerns.

In addition to these concerns, the fact that the fibres themselves are difficult to

remove from the matrix and preserve for re-use means FRP amplify these challenges. FRPare inherently difficult to separate into base a material that is into fibre and matrix, and thematrix into separate usable plastic, polymers, and monomers. These are all concerns forenvironmentally informed design today, but plastics often offer savings in energy andeconomic savings in comparison to other materials, also with the advent of new moreenvironmentally friendly matrices such as bioplastics and uv-degradable plastics, FRP willsimilarly gain environmental sensitivity.


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Chapter 2Literature Review

2.1 Introduction

In last few years, rapid advancement in construction material technology haveenabled us to achieve impressive gains in safety, economy and functionality of structuresbuilt to serve the common needs of the society. Through such gains the health and standardof living of individuals are improved.

Fibre reinforced has been used worldwide for construction and retrofitting purposes.Some spheres of civil engineering that has wide developed the use of FRP are bridgeconstruction, highway construction, building construction and used for retrofitting andrepair work of structures.

2.2 Strengthening of RC Structures with FRP

Composites have gained widespread use as strengthening materials for RCstructures in applications where conventional strengthening techniques may be problematic.For instance, one of the most popular techniques for upgrading RC elements hastraditionally involved the use of externally epoxy-bonded steel plates. This technique issimple, cost-effective, and efficient, but it suffers from the following: deterioration of thebond at the steel-concrete interface caused by the corrosion of steel; difficulty inmanipulating the heavy steel plates at the construction site; need for scaffolding; and limiteddelivery lengths of steel plates in the case of flexural strengthening of long elements. As analternative, the steel plates can be replaced by FRP strips or sheets.

Another common strengthening technique involves the construction of RC, shotcrete,or steel jackets. Jacketing is quite effective as far as strength, stiffness, and ductility areconcerned, but it increases the cross-sectional dimensions and dead loads of the structure, islabour intensive, obstructs occupancy, and provides RC elements with a potentiallyundesirable stiffness increase. Alternatively, FRP sheets may be wrapped around RCelements, resulting in considerable increases in strength and ductility without an excessivestiffness change. Furthermore, FRP wrapping may be tailored to meet specific structuralrequirements by adjusting the placement of fibres in various directions.

2.2.1 Flexural Strengthening

Flexural strengthening of RC elements using composites may be provided by epoxybonding the materials to portions of the elements in tension, with fibres parallel to theprincipal stress direction. Well-established strengthening procedures for RC structures maybe followed, provided that special attention is paid to issues related to the linear-elasticnature of FRP materials and the bond between the concrete and FRP.

Central to the analysis and design of FRP-strengthened RC elements is theidentification of all of the possible failure modes. These include the following modes:

Steel yielding followed by FRP fracture. Steel yielding followed by concrete compressive crushing while the FRP is intact


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Concrete compressive crushing. FRP peel-off at the termination or cut-off point, due to shear failure of the concrete. FRP peel-off initiating far from the ends, due to inclined shear cracks in the concrete. FRP peel-off at the termination point or at a flexural crack due to high tensile stresses

in the adhesive.

Debonding at the FRP-concrete interface in areas of concrete surface unevenness ordue to faulty bonding.

2.2.2 Shear Strengthening

Shear strengthening of RC elements may be provided by epoxy bonding FRPmaterials with fibres as parallel as practically possible to the principal tensile stresses.Depending on accessibility, strengthening can be provided either by partial or by fullwrapping of the element.

The effectiveness of the external FRP reinforcement and its contribution to the shearcapacity of RC elements depend on the mode of failure, which may occur either by peeling-off through the concrete near the concrete-FRP interface or by FRP tensile fracture at a stressthat may be lower than the FRP tensile strength e.g., because of stress concentrations atrounded corners or at debonded areas. Whether peeling-off or fracture will occur firstdepends on the bond conditions, the available anchorage length and the type of attachmentat the FRP termination point full wrapping versus partial wrapping, with or withoutmechanical anchors, the axial rigidity of the FRP, and the strength of the concrete. In manycases, the actual mechanism is a combination of peeling-off at certain areas and fracture atothers. In light of the above, the load carried by FRP at the ultimate limit state in shear of theRC element is extremely difficult to quantify based on rigorous analysis.

According to a simplified method of calculation of shear force in external FRPreinforcement, the FRP material is assumed to carry only normal stresses in the principalFRP material direction. It is also assumed that, in the ultimate limit state in shear, the FRPdevelops an effective strain (in the principal material direction) , which is generally lessthan the tensile failure strain. For RC elements with a rectangular cross section, theaforementioned effective strain decreases as the axial rigidity of the FRP (that is, the productof the FRP shear reinforcement ratio times its elastic modulus in the principal materialdirection) increases and as the concrete tensile strength decreases (Triantafillou andAntonopoulos 2000). Moreover, keeping in mind that large values of correspond toconsiderable opening of diagonal cracks, to the extent that the contribution of concrete

shear-resisting mechanisms is reduced by degraded aggregate interlock, should be limitedto a value on the order of 0.004 0.005 for the case of fibres perpendicular to the longitudinalaxis of the RC element.

In the case of elements with a circular cross section e.g., wrapped circular columns!,experimental and analytical studies have demonstrated that FRP jackets with fibres in thecircumferential direction significantly increase both the strength and the ductility in thepresence of monotonic or cyclic lateral loads. The contribution of FRP to shear resistance insuch cases may be estimated by taking approximately equal to 0.004 (Priestley and Seible1995).


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2.2.3 Confinement

The enhancement of confinement in structurally deficient RC columns in seismicallyactive regions of the world has proven to beone of the most significant early applications ofFRP materials in infrastructure applications. Proper confinement increases the rotational

capacity (and hence the ductility) in plastic hinge regions and prevents debonding of theinternal reinforcement in lap splices. Confinement may be beneficial in nonseismic zonestoo,where, for instance, survivability of explosive attacks is required (Crawford et al. 1997) orthe axial load capacity of a column must be increased due to higher vertical loads (e.g.,increased traffic ona bridge). In any case, confinement may be provided by wrappingRCcolumns with FRP materials (prefabricated jackets or in situ cured sheets), in which theprincipal fibre direction is circumferential.

In circular columns, an FRP wrap effectively curtails the lateral expansion of concreteshortly after the unconfined strength is reached. It then reverses the direction of thevolumetric response, and the concrete responds through large and stable volume contraction

(this is not the case with steel confinement jackets, where yielding is associated withunstable volumetric expansion). As a result, the stress-strain response of FRP-confinedconcrete is characterized by a distinct bilinear response with a sharp softening at a stresslevel near the strength of unconfined concrete. After this softening point, the tangentstiffness stabilizes at a nearly constant value. The ultimate state is characterized by tensilefailure of the wrap. At failure, the tensile stress in the FRP wrap is generally less than theuniaxial tensile strength of the FRP material due to triaxial stresses and variations in thequality of installation that could lead to unequal load sharing among fibres, misalignment,and damaged fibres at sharp corners and local protrusions.

From the arguments discussed above, it is realized that reliable models for FRP-

confined concrete must account for a number of parameters, including: The circumferential stiffness of the FRP The continuous effect of the restraint provided by the FRP on the dilation tendency

of the concrete. The composite action of the FRP-concrete column and the FRP-concrete interaction,

based on micromechanics.As a simplified approach, one may assume a maximum compressive axial strain in the

concrete at the ultimate limit state of approximately 0.01 and a fixed Poissons ratio ofapproximately 0.5 to determine the confinement effect.

Confinement of RC columns is less effective if the cross section is rectangular. In this

case, the confining stress is transmitted to the concrete at the four corners of the cross sectionand increases with the corner radius. The confinement model in this case must account for areduced volume of fully confined concrete (Mirmiran et al. 1998).

2.2.4 Masonry

Recent years have seen proposals and practical applications thatuse composites asalternative strengthening materials for masonrystructures, including those of considerablehistorical importance. The general approach is to epoxy-bond FRP strips to the surface ofmasonry in locations and directions dictated by the principal tensile stress field (Schwegler1994).


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In terms of design, masonry strengthened with FRP strips or sheets may be treated inthe same manner as RC, following the procedures of modern design codes (Triantafillou1998). The analysis of simple cases has led to the following conclusions:

When out-of-plane bending dominates (e.g., as in the case of upper levels of masonrybuildings), horizontally applied FRP may offer a considerable strength increase.

In the (rather rare) case of in-plane bending, the amount and distribution ofreinforcement are of high importance; high reinforcing ratios placed near the highlystressed zones give a significant strength increase.

The achievement of full in-plane flexural strength depends on the proper anchorageof the strips at their ends, in the sense that short anchorage lengths and the absenceof clamping at the strips curtailment positions may result in premature failuresthrough peeling-off beneath the adhesive (as in the case of RC).

The in-plane shear capacity of masonry walls strengthened with FRP may be quitehigh, too, especially in the case of low axial loads.

FRP composites can also be applied as confinement reinforcement to masonry using

unbonded strips that are colour-matched with the underlying masonry structure, and can beremoved if necessary at a later time (Triantafillou and Fardis 1993). Recent applicationsinclude strengthening of vaults in old masonry buildings either from below, usingtransparent glass FRP fabrics, or from above, using epoxy-bonded FRP sheets in a grid likepattern (Borri et al. 2000).

2.2.5 Timbers and Metals

The high potential of FRP strips or sheets to increase the strength(flexural or shear),stiffness, and ductility of timber beams and columns has been demonstrated in variousresearch studies andquite a few field applications (Plevris and Triantafillou 1992).Moreover,

FRP wrapping has been used as an effective means of enhancing the durability of timberelements (Qiao et al. 1998).

Research and development related to FRP combined with metals used inconstruction (e.g., steel, cast iron, and wrought iron) have started relatively recently(Karbhari and Shulley 1995). High-stiffness sheets (such as carbon) may enhance themechanical properties of metallic elements while offering certain other advantages, such asthe low weight of bonded material, the easy applicability, and the ability to effectively coverareas with high bolt or rivet congestion.

2.3 Special Strengthening Techniques

A number of special techniques related to the application of composites as externallybonded reinforcement are mentioned as below:

Prestressed strips: Prestressing of composite strips prior to the bonding procedureresults in a more economical use of materials (Triantafillou et al. 1992), but requiresspecial clamping devices.

Automated wrapping and curing: Wrapping of columns (or other vertical elements,such as chimneys) with flexible FRP sheets is possible today by using automatedmachinery. The machinery can also apply heat and vacuum to assist curing.

Fusion-bonded pin-loaded straps: A number of nonlaminated thermoplastic FRPlayers that may move relative to each other when loaded are applied in a single,


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continuous, thin tape that is fusion-bonded (welded) to itself for anchorage(Winistoerfer and Mottram 1997).

Placement inside slits: FRP strips or even rods may be bonded into slits, which arecut into the concrete or into masonry mortar joints (Blaschko and Zilch 1999; Tinazzi etal. 2000).

Prefabricated shapes: Prefabricated angles or shells may be externally bonded tostructures. Mechanically fastened FRP strips: Specially designed, precured FRP strips can be

rapidly attached to concrete beams for flexural strengthening using powder-actuatedfasteners (Lamanna et al. 2001).

2.4 Codal Provisions and Standards

Due to the importance of controlling risk in matters of public safety, standards andcodes for FRP materials used in civil structures have been in development since the 1980s.

FRP materialswarrant separate treatment in standards and codes on account oftheir lowermodulus and ductility in comparison with conventional materials such as metals. Withoutstandards and codes, it isunlikely that FRP materials could make inroads beyond limitedresearch and demonstration projects. Standardized test methods and material identificationschemes minimize uncertainty in theperformance and specification of FRP materials.

Codes allow structures containing FRP materials to be designed, built, and operatedwith safety and confidence. This section describes the standard and code developmentactivities in the United States, and Europe. The main accomplishments of these activities, todate, pertain to the use of FRP materials for the reinforcement of new structures and for therepair and retrofit of existing structures.

2.4.1 United States

The United States has had a long and continuous interest in fibre basedreinforcement for concrete structures. Accelerated development and research activities onthe use of these materials started in the 1980s through the initiatives and vision of theNational Science Foundation and the Federal Highway Administration, who supportedresearch at different universities and research institutions. Committee 440 recently producedtwo documents approved by the Technical Activities Committee for publication in the year2001. The documents are Guide for the design and construction of concrete reinforced withFRP bars (ACI Committee 440 2001) and Guide for the design and construction ofexternally bonded FRP systems for strengthening concrete structures. The committee iscurrently working on the following documents: Stay-in place structural FRP forms;Durability of FRP forconcrete structures; and Guide for the design and construction ofconcrete members prestressed with FRP.

2.4.2 Europe

Research on the use of FRP began in Europe in the 1960s.The program wasaimed atdeveloping FRP reinforcement for concrete, and included partners from the UnitedKingdom, Switzerland, France, Norway, and The Netherlands. Task Group 9.3 is dividedinto subgroups on material testing and characterization, RC, prestressed concrete, externally

bonded reinforcement, and marketing and applications. The task group consists of membersrepresenting most European universities, research institutes, and companies involved with


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FRP reinforcements for concrete. Membership includes representatives from Canada, Japan,and the United States. The task group has completed the development of an FIB bulletin ondesign guidelines for externally bonded FRP reinforcement for reinforced concretestructures.

In the United Kingdom, the Institution of Structural Engineers has published an

interim guide on the design of RC structures with FRP reinforcement (Institution 1999).Prestressing and externally bonded reinforcements are not addressed in the guide.

2.5 Provisions to be included in the Codes and Standards

From a technical standpoint, the need for specialized standards and codes for FRPmaterials arises from their substantially different mechanical and physical properties incomparison with conventional construction materials. As the preceding discussion pointsout, the development of standards and codes for the use ofFRP reinforcement with concretestructures is ongoing and is expected to continue in the next several years. Much of this

activity is motivated by immediate, obvious needs for improved, economical materials forthe repair and retrofit of structures that areobsolete, degraded, or located in seismic zones.

In other cases such as new construction, where the need for new materials is not always clear from a short-term economic standpoint, standardsand codes will facilitate theuse of FRP materials so that additional long-term experience can be assured. This experiencemayeventually lead to the realization of promised life-cycle cost benefits of FRP materialsby designers and owners of structures. As per the previous studies, FRP shapes and bridgedecks suffer from the least amount of development of standards and codes. Future researchefforts on standards and codesshould therefore be increasingly concentrated in these areas.

2.6 Concluding Remarks

The use of advanced composites as external reinforcement of concrete and otherstructures has progressed well in the past decadein selective applications where their costdisadvantage is outweighed by a number of benefits. There are clear indications thatthe FRPstrengthening technique will increasingly continue to be the preferred choice for manyrepair and rehabilitation projects involving buildings, bridges, historic monuments, andother structures.


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Chapter 3Finite Element Modelling of RC Beam

3.1 Description of the Model

3.1.1 Cross Sectional View of the RC Beam

Figure 1 shows the loading and the boundary conditions. The beam is1200mm long having a depth of 150mm and width of 120mm. Because of symmetry, onlyone quadrant of the beam needs to be modelled. The axes of symmetry are taken as thevertical section through the point of loading and vertical section through the beam.

Figure 1

3.1.2 Finite Element Modelling

Finite Element Modelling of the above mentioned RC beam is carried out inABAQUS version 6.7so as to match the experimental results with the results obtained frommodelling. For modelling, 8-noded solid brick elements (C3D8R) have been used. The

reinforcement is modelled using 2 noded truss elements.

Figure 2: Finite element mesh of RC Beam


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3.1.3 ABAQUS Coding in Input File

*HEADINGREINFORCED CONCRETE BEAM ANALYSISUnits : Length - mm, Force N.

***NODE****TOP EDGE**1 , 0.0, 150.0, 0.010 , 600.0, 150.0, 0.0**** REINFORCEMENT LEVEL**21 , 0.0, 25.0, 0.0

30 , 600.0, 25.0, 0.0**** BOTTOM EDGE**31 , 0.0, 0.0, 0.040 , 600.0, 0.0, 0.0****STEEL NODES**101 , 0.0, 25.0, 40.0110 , 600.0, 25.0, 40.0

//The above data lines define the nodes at the boundary of the front plane (PLANE 1).

***NGEN, NSET = STEELNODE1101, 110, 1

//This creates a node set called STEELNODE 1 with all the nodes that form the reinforcement.

**

*NGEN, NSET = TOP1,10,1*NGEN, NSET = REINFLEVEL21,30,1*NGEN, NSET = BOTTOM31,40,1*NFILLTOP, REINFLEVEL, 2, 10*NSET, NSET = PLANE1, GENERATE1,40,1

//The above data lines generate all the nodes in the front plane. These nodes are then groupedtogether into a set called PLANE 1.


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*NCOPY, CHANGE NUMBER=40, OLD SET=PLANE1, REFLECT=MIRROR, NEWSET=PLANE20.0, 0.0, -25.0, 600.0, 0.0,-25.00.0, 150.0, -25.0

//The above commands result in the nodes in the front plane being copied to the back plane(PLANE 2).PLANE 2 nodes are formed by reflecting the nodes in PLANE 1. The reflection surface is the verticalplane midway between the two planes. The second line defines the x, y and z coordinates of points aand b (say). The next line defines the coordinate of point c (say). Together the lines a -b and a-cform the reflection plane.

The CHANGE NUMBER= 40 option copies node 1 to node 41 i.e. the node numbers areincreased by a value of 40.

ELEMENTS

***NSET, NSET = CONCSTEEL61, 63, 64, 65, 66, 67, 68, 69, 70

//This defines a set called CONCSTEEL which consist of all concrete nodes common which arealso common to the reinforcement

*ELEMENT, TYPE = C3D8R1, 11, 12, 52, 59, 7, 2, 42, 41*ELGEN, ELSET = CONC1, 9, 1, 1, 3, 10, 10

//The above statements define the concrete element and generate all the concrete elements. 8-noded brick element with reduced integration points (C3D8R) has been used. All the generatedelements of concrete are identified by the set name CONC.

*ELEMENT, TYPE = T3D231,101,102*ELGEN, ELSET = STEEL31,9,1,1

//The element used to model the reinforcement is 2-noded truss element (T3D2). All the

reinforcement elements are generated and grouped together in a set called STEEL.

*ELSET, ELSET = LOADING SURFACE, INTERNAL, GENERATE1, 9, 1*SURFACE, TYPE=ELEMENT, NAME= LOADING SURFACE, INTERNALLOADING SURFACE S1, S1

// This forms a surface for a set of elements on which load shall be applied in loading step.


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3.1.4 Material Behaviour

3.1.4.1 Concrete

The concrete is modelled using the concrete model available in ABAQUS.

The Youngs Modulus (initial tangent modulus) as per IS 456:2000 = 5000fck= 25000N/mm2Poissons ratio = 0.2Characteristic compressive strength, fck = 25 N/mm2Ultimate Tensile strength, fcr= 0.7fck= 0.725 = 3.5 N/mm2

The uniaxial stress-strain curve of concrete has been taken from Reinforced ConcreteStructures by R. Park and T. Paulay (Figure 2.1, Page 12).However for the implementation of the curve in the model, an idealized curve has been

assumed as shown below:

Figure 3: Idealized curve of concrete (Park & Paulay, 1974)

0 = 2fck/Ec = 2*25/25000 =0.002c = 0.0038

The curve used for modelling purpose has been shown below

Initially an elastic behavior is assumed upto a stress equal to 0.6f ck and thereafter theslope changes at 0.6 fck from where the strain hardening portion begins for concrete incompression. Again a linear curve is assumed upto maximum compressive stress f ck whichoccurs at a strain of 0.002. Thereafter the curve falls linearly upto 0.85fck indicating the strainsoftening portion and at a strain of 0.0038, failure of concrete occurs by crushing.


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Figure 4The data lines for ABAQUS input for concrete material are as follows:

*MATERIAL, NAME = CONC*ELASTIC25000, 0.2

// The *CONCRETE keyword is followed by the absolute value of the compressive stress andthe absolute value of the plastic strain.

*CONCRETE

15.0, 0.025.0, 0.00221.25, 0.0038

3.1.4.2 Smeared Crack Model

The smeared crack model represents the discontinuous macro crack brittlebehaviour. In this approach individual macro cracks are not tracked, rather the presence ofcracks enters into the calculations by the way the cracks affect the stress and materialstiffness associated with each material calculation point. For simplicity, the term crack isused to mean a direction in which cracking has been detected at the material calculation

point in question. With this modelling approach, the concrete behaviour is consideredindependently of the rebar.

Effects associated with the rebar/concrete interface, such as bond slip and dowelaction, are modelled approximately by introducing some tension stiffening into theconcrete modelling to simulate load transfer across cracks through the rebar.

The model is intended as a model of concrete behaviour for relatively monotonicloadings under fairly low confining pressures (less than four to five times the magnitude ofthe largest stress that can be carried by the concrete in uniaxial compression).

The concrete model is a smeared crack model in the sense that it does not trackindividual macro cracks. Constitutive calculations are performed independently at each


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integration point of the finite element model. The presence of cracks enters into thesecalculations by the way in which the cracks affect the stress and material stiffness associatedwith the integration point.

Figure 5: (a) Discrete Crack(b) Smeared Crack

3.1.4.3 Tension stiffening

The post-failure behaviour for direct straining across cracks is modelled with tensionstiffening, which allows you to define the strain-softening behaviour for cracked concrete.This behaviour also allows for the effects of the reinforcement interaction with concrete to be

simulated in a simple manner. The tension stiffening effect must be estimated; it depends onsuch factors as the density of reinforcement, the quality of the bond between the rebar andthe concrete, the relative size of the concrete aggregate compared to the rebar diameter, andthe mesh.

3.1.4.4 Cracked shear retention

As the concrete cracks, its shear stiffness is diminished. This effect is defined byspecifying the reduction in the shear modulus as a function of the opening strain across thecrack. This reduced shear modulus will also have an effect when the normal stress across acrack becomes compressive. The new shear stiffness will be degraded by the presence of the

crack. There is an option in ABAQUS for defining the reduction of the shear modulusassociated with crack surfaces in a concrete model as a function of the tensile strain acrossthe crack.


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Figure 6: Failure surfaces of concrete

3.1.4.5 Failure Ratio

It is used to define the shape of the failure surface. As seen in the above Figure.It canbe specified as the absolute value of the ratio of the uniaxial tensile stress at failure to theultimate uniaxial compressive stress.

Failure ratio= 3.5/25 = 0.14

The ABAQUS command lines for incorporating smear cracking model are:

*FAILURE RATIO1.16, 0.14*TENSION STIFFENING1.0, 0.00.0, 0.0018*SHEAR RETENTION1.0, 1000.0

3.1.4.6 Behaviour of Reinforcing Steel


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The properties of reinforcing steel, unlike concrete, are generally not dependent onenvironmental conditions or time. In this study we will use Fe 415 rebars, having yield stressof 415 N/mm2

Figure 7: Stress-strain curve for Fe415

The steel stress-strain relation exhibits an initial linear elastic portion, a yield plateau,a strain hardening range in which stress again increases with strain and, finally, a range in

which the stress drops off until fracture occurs. The extent of the yield plateau is a functionof the tensile strength of steel.

In this model reinforcing steel is modelled as a linear, perfectly plastic material, asshown. In this study the reinforcing steel is modelled as a linear elastic, linear strainhardening material with yield stress y, as shown in Figure below. The reasons for thisapproximation are: (1) the computational convenience of the model; (2) the behaviour of RCmembers is greatly affected by the yielding of reinforcing steel when the structure issubjected to monotonic bending moments. Yielding is accompanied by a sudden increase inthe deformation of the member. In this case an elastic-perfectly plastic model is used so thatnumerical convergence may be achieved.


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Figure 8: Idealized stress-strain curve for Fe 415 steel

The assumption of a linear strain hardening behaviour immediately after yielding of thereinforcement does not adversely affect the accuracy of the results as has been reported byBashur and Darwin, 1998.

In ABAQUS, the steel reinforcement is treated as an equivalent uniaxial material smearedthroughout the element section. Following data lines represent the input parameters of

reinforcing steel:

*MATERIAL, NAME = STEEL*ELASTIC2.0E5, 0.2*PLASTIC415.0 0.0432.0, 0.0050441.0, 0.0055450.0, 0.0060

*SOLID SECTION, MATERIAL = CONC, ELSET = CONC*BEAM SECTION, MATERIAL = STEEL, ELSET = STEEL112

3.1.5 Interaction

For modelling in ABAQUS an embedded steel model is used. It is used to specify anelement or a group of elements that lie embedded in a group of host elements whoseresponse will be used to constrain the translational degrees of freedom of the embeddednodes. The reinforcing steel is considered as an axial member. Such a model implies perfect

bond between concrete and steel.


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The ABAQUS command line:

*EMBEDDED ELEMENT, HOST ELSET = CONCSTEEL

// This command allows selection of the host elements i.e. CONC for embedding the rebar elementsthat is STEEL.

Boundary Conditions

Figure 8: Boundary conditions

Since we have modelled for only a quadrant of the beam so the boundary conditionsat one end will be hinged and other is vertical roller with rotation restrained.

The ABAQUS command lines:

** BOUNDARY CONDITIONS***NSET, NSET=Y-SYMM10, 20, 30, 40, 50, 60, 70, 80, 110***NSET, NSET=HINGE

32, 72**** NAME: ROLLER TYPE: DISPLACEMENT/ROTATION*BOUNDARYY-SYMM, 1**** NAME: HINGE TYPE: DISPLACEMENT/ROTATION*BOUNDARYHINGE, 1


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3.1.6 Loading

Loading given are following:a) A uniform dead load of 135 N to carry its self weight. This was applied as pressure

load. So the pressure was 1350/(60060)= 0.00375 N/mm2 .

b) A concentrated load of 50000 N was applied at 360 mm from left end of beam.Static, RIKS was used to counter the possibility of unstable behaviour due to non-linearity.

The ABAQUS command lines:

*STEP, INC=200*STATIC, RIKS.02, 1.00, .02, 1.0, 10, 2, 20.0*CLOAD10, 2, -25000.0

50, 2, -25000.0*DSLOADLOADING SURFACEP0.0375

3.2 Results and Conclusions

To get the output from ABAQUS following command lines were written to input file:

*NODE PRINT, NSET=Y-SYMM, FREQUENCY=1U*NODE FILE, NSET=Y-SYMM, FREQUENCY=1U*NODE PRINT, NSET=HINGERF*NODE FILE, NSET=HINGERF*EL PRINTS*EL FILE, FREQUENCY=50

SE*END STEP


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The figure below shows the value of xx

Plastic Strain along xaxis of Figure


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Displacement along Y-axis

Plot of Load vs midspan deflection

0

5

10

15

20

25

30

35

40

45

0 5 10 15

Load

(in

kN)

Deflection (in mm)

"Load vs midspan

deflection"


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Conclusion

The model does not give satisfactory results as from the stress contour, it is found that thebottom portion of beam shows lesser stress than that in the mid depth portion of the beam inthe midspan.

It is probable that the beam can take more load. However from the load deflection response,it can be interpreted that cracking initiates at about a load of about 15 kN. Upto a load of15kN reinforced concrete shows elastic behavior. Thereafter concrete cracking starts andpropagates. This can be understood from the change of slope of the curve at 15kN load.However the point from which steel yielding starts cannot be clearly interpreted.

Further improvement in the model is required to get proper results and experiments needsto be conducted to verify the results.


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Chapter 4Mixed Design

4.1 Material to be used for experimental purpose

Grade of Concrete: M25Grade of Steel: Fe 415

STIPULATION FOR PROPORTIONING

GRADE OF CONCRETE: M25

TYPE OF CEMENT : ORDINARY PORTLAND CEMENT MAXIMUM NOMINAL SIZE OF AGGREGATE : 12.5 mm MINIMUM CEMENT CONTENT : 300 kg/m3 (table 5, IS 456 : 2000) MAX. FREE WATER CEMENT RATIO : 0.50 (table 5, IS 456 : 2000) WORKABILITY : 100 mm SLUMP EXPOSURE CONDITIONS : MODERATE METHOD OF CONCRETE PLACING : HAND PLACING DEGREE OF SUPERVISION : GOOD TYPE OF AGGREGATE : CRUSHED ANGULAR AGGREGATES GRADED

(10mm, 12.5 mm)

MAXIMUM CEMENT CONTENT : 450 kg/m3 (cl 8.2.4.2, IS 456 : 2000) CHEMICAL ADMIXTURES : NONE

DATA:

CEMENT USED : OPC SPECIFIC GRAVTIY OF CEMENT : 3.15 ADMIXTURES : NONE SPECIFIC GRAVITY OF COARSE AGGREGATES : 2.74 SPECIFIC GRAVITY OF FINE AGGREGATES : 2.62 WATER ABSORPTION OF C.A: 0.5% BY MASS OF AGG. WATER ABSORPTION OF F.A: 1.0% BY MASS OF AGG. SURFACE MOISTURE : SATURATED SURFACE DRY AGGRGATES SIEVE ANALYSIS : Conforming to table 2, IS 383 : 1970 FINE AGGREGATES : GRADING ZONE II

TARGET MEAN STRENGTH

fck = fck + 1.65s

fck = target mean strength of concrete.fck = characteristic strength of concrete in Mpa (for M25 concrete, 25 Mpa)


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s = standard deviation = 4 (table 1, IS 10262 : 2009)fck = 25 + 1.65 x 4 = 31.6 Mpa.

SELECTION OF WATER/CEMENT RATIO

With reference to table 5 of IS 456 : 2000 we have maximum free water /cement ratioas per :1) Moderate exposure conditions,2) M25 grade of concrete,

Equal to 0.5.By experience, this ratio is reduced to 0.44.

CEMENT CONTENT, WATER CONTENT

Now, maximum cement content = 450 kg/m3 and minimum cement content = 300kg/m3 .Nominal maximum size of aggregate = 12.5 mm.By table 2, maximum water content per cubic metre of concrete for 12.5 mm maximumnominal size of aggregate = 191.5 kg.Now, slump = 100 mm.Excess slump over 50 mm = 100 50 = 50 mm.Crushed angular aggregates are being used, so no need to modify the water content, asper cl 4.2 IS 10262 : 2009.

So, increase in water content = 6%.Water content = 191.5 kg x 1.06 = 203 kg/m3 of concrete.Now, cement content = 203/0.44 = 461.34 kg/m3 of concrete.However maximum cement content cannot exceed 450 kg/m3 of concrete. So, cementcontent is limited to 450 kg/m3 of concrete.

ESTIMATION OF COARSE AND FINE AGGREGATES

Volume of Coarse aggregates per unit volume of aggregate for zone II of fine aggregate(table 3, IS 456 : 2000) = 0.5 (maximum nominal size of aggregate = 12.5 mm)

w/c ratio : 0.44, so volume of C.A. per unit volume of aggregate = 0.5 + 0.012 = 0.512(The volume of coarse aggregates to the volume of aggregates is increased by 0.01 as thewater cement ratio decreases by 0.05)The concrete is not pump-able, so no need to modify the coarse aggregate portion.Volume of Fine aggregates per unit volume of aggregate is 0.49.

AGGREGATE QUANTITY

Volume of water = 203/1000 = 0.203 m3 (per m3 of concrete)Volume of concrete = 450/3150 = 0.143 m3 (per m3 of concrete)Volume of aggregates = 1 0.143 0.203 = 0.654 m3 (per m3 of concrete)

Volume of Coarse aggregates = 0.654 x 0.51 = 0.334 m3

Volume of Fine aggregates = 0.654 x 0.49 = 0.320 m3.


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Weight of coarse aggregates = 0.334 x 2.74 x 1000 = 915.16 kg/ m3 of concrete.Weight of fine aggregates = 0.320 x 2.62 x 1000 = 838.4 kg/ m3 of concrete.

WATER ABSORPTION

C.A.: 915.16 x 0.5% = 4.58 kg.

F.A.: 838.40 x 1.0% = 8.40 kg.

C.A. used = 910.6 kg.

F.A. used = 830 kg.WATER content = 203 + 4.58 + 8.40 = 216 kg.

RATIO OF WATER : CEMENT : CA : FA

Water : Cement : CA : FA = 216 : 450 : 910.6 : 830

The final ratio is = 0.48 : 1 : 2.024 : 1.844


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Chapter 5Moment Capacity of the RC beam

DIMENSIONS:

Breadth (b) = 120 mm.Depth (D) = 150 mm.Length of Beam = 1200 mm.

MOMENT DUE TO SELF-WEIGHT:

Self-weight of beam = 0.12 x 0.15 x 25 kN/m = 0.45 kN/mMoment due to self-weight = (0.45 x 1.22)/8 kN-m = 0.081 kN-m

MOMENT CAPACITYOF BEAM:

Now effective depth = 150 mm 25 mm 6 mm = 119 mm.For M25, Fe 415 grade steel:xu/d = 700/ (1100 + 0.87 fy)fy = 415Mpa, fck =25Mpa, xu = depth of neutral axis from the top concrete fibre, d = effectivedepth.So, neutral axis is located at a depth of 0.48d = 0.48 x 119 mm = 57.12 mm.Assuming balanced section:Moment capacity Mcap = 0.36 fck b xu (d 0.416xu)d= 119 mm

fck = 25 Mpab = 120 mmxu = 57.12 mmMcap = 5.8752 kN-m.


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Task to be Performed

Fifteen beams of the above mentioned dimension are to be casted using the designmix and suitable diameter of reinforcements.

Glass Fibre Reinforced Polymer lamina will be used to strengthen the beams byattaching them to the tension face in different lengths and proportions.

Three of the beams will be tested without FRP, as control beams and the rest withGFRP.

All the beams are to be tested under two point loading in Universal Testing Machineto find out the failure load

To study the flexural behaviour, deflection and crack pattern of the beams. After obtaining the experimental results they are to be compared with the FEM

results and conclusions are to be drawn as per the observations.


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References

Three dimensional nonlinear finite element modeling of reinforced concrete structures (1993)A.F. Ashour and C.T. Morley

Modeling of Shear-Critical Reinforced Concrete Structures Repaired with Fiber-ReinforcedPolymer Composites(2008) Sang-Woo Kim and Frank J. Vecchio

SHEAR AND FLEXURAL STRENGTHENING OF RIC BEAMS WITH CARBON FIBERSHEETS By Tom Norris,t Hamid Saadatmanesh/ Member, ASCE, and Mohammad R.Ehsani/ Member, ASCE

Davis, D., and Porter, M. L. (1999). Glass fiber reinforced polymer dowel bars fortransverse pavement joints. Proc., 4th Int. Symposium, Fiber Reinforced PolymerReinforcement for Reinforced Concrete Structures, C. W. Dolan, S. H. Rizkalla, and A.Nanni, eds., SP-188, American Concrete Institute, Farmington Hills, Mich., 297304.

Deitz, D. H., Harik, I. E., and Gesund, H. ~1999!. One-way slabs reinforced with glass fiberreinforced polymer reinforcing bars. Proc., 4th Int. Symposium, Fiber Reinforced PolymerReinforcement for Reinforced Concrete Structures, C. W. Dolan, S. H. Rizkalla, and A.Nanni, eds., SP-188, American Concrete Institute, Farmington Hills, Mich., 279286.

Lees, J. M., and Burgoyne, C. J. (1999). Design guidelines for concrete beams prestressedwith partially-bonded fiber reinforced plastic tendons. Proc., 4th Int. Symposium, FiberReinforced Polymer Reinforcement for Reinforced Concrete Structures, C. W. Dolan, S. H.Rizkalla, and A. Nanni, eds., SP-188, American Concrete Institute, Farmington Hills, Mich.,807 816.

A GENERAL METHOD FOR DEFLECTIONS EVALUATION OF FIBERREINFORCED POLYMER (FRP) REINFORCED CONCRETE MEMBERS MariaAntonietta Aiello and Luciano Ombres, University of Lecce, Lecce, Italy

Reinforced concrete structures by R. Park & T. Paulay Finite element analysis of reinforcedconcrete structures under monotonic loads (1990) Hyo-Gyoung Kwak & Filip C. Filippou

ABAQUS Documentation v6.7

behaviour of rc beams wrapped with gfrp-i

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