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Regain in Strength of Reinforced Concrete Columns

with Shape Modification and GFRP wraps

Akash K rupeshkumar Chauhana

a M Tech (Computer Aided Structural Engineering)International Institute of Information Technology Hyderabad,

Hyderabad- 500 032Andhra Pradesh, India.

akash.chauhan@students.iiit.ac.in, +91 9542837050

Abstract

The objective of this paper is to compare and discuss effectiveness of different

strengthening methods used to improve the performance of FRP wrapped rectangular

columns. Fiber Reinforced Polymer (FRP) lamina are being used in structural strengthening

applications due to advantages such as lightweight, high strength and ease of application.

Column is an important component of any structure. The strengthening is carried out to

increase compressive strength and ductility of the column. The confinement due to FRP

enhances both the ultimate compressive strength and the ultimate compressive strain of the

concrete. This process is significantly more efficient with circular rather than square or

rectangular columns. This research work desires to improve the confinement effectiveness ofFRP composites for square and rectangular columns by changing the cross-sectional shape.

This shape alteration is done using different materials such as normal concrete, micro

concrete. The paper presents results obtain with modifying shape by micro concrete and

GFRP.

Keywords: Concrete; Column; Strengthening, J acketing, FRP, GFRP

1.1 INTRODUCTIONStrengthening or retrofitting of existing reinforced concrete (RC) structures is

required for a variety of reasons. Sometimes it may be change in use causing higherloads, or deterioration due to factors like environmental factors, or for withstanding

lateral loads.[Fig 1] Column is an important component of any structure. Thestrengthening is carried out to increase compressive strength and ductility of thecolumn. The confinement due to FRP enhances both the ultimate compressive

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strength and the ultimate compressive strain of the concrete. The three mostcommonly used materials for column jacketing are concrete (with reinforcing barsor fibre-reinforced), steel, or fibre-reinforced polymers. Kunio Fukuyama et al. [1].The jacket material is selected depending on the column cross sectional shape andthe column deficiency needing correction. At first concrete and steel jackets wereused widely, later FRP jackets are being used.

FIG 1A Repaired Jetty using FRP[www.fyfeco.com]

Fig 1B Typical damaged column

1.2

FRP COMPOSITES FOR STRENGTHENING OF RCSTRUCTURAL ELEMENTS

Wrapping of FRP sheets around beams, and columns has become acommon strengthening technique to increase the ductility and load carrying capacityof existing structural. C. Arya et.al. [1]. The most common fibre materials are E-glass (EFRP), Glass (GFRP) or Carbon (CFRP). The main advantages of FRPmaterials are (1) High specific strength and Stiffness (2) High corrosion resistance: -Composites are not prone to rust. (3) Non Magnetic Properties: (4) Lowermaintenance cost: - Due to inherent properties like resistance to corrosion,environment & chemical solvents, composites need lesser maintenance cost. (5)They can easily be applied to existing elements and can conform to any structuralshape

Strengthening of column using FRP- Jacketing on RC column with FRPprimarily improves column performance, as it provides lateral confining pressure tothe column. This confining pressure places the concrete in a triaxial state of stress,altering the load-deformation characteristics of the concrete as shown in Fig. 2. High

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levels of confining pressure enable concrete to sustain both greater axial loads andgreater ultimate axial strain by changing the failure mode from cleavage of theconcrete to the crushing of its cement paste. FRP jackets can apply confiningpressure either actively or passively. In the active retrofit scheme the fibers aretensioned either as they are wrapped around the column or by pressure injectinggrout or epoxy between the jacket and the column. In the passive scheme, theconfining pressure is a result of the reaction of the jacket against the lateral dilationof the column cross section as it is loaded axially.

1.3 STRENGTHENING OF RECTANGULAR COLUMNSThe confinement due to FRP is significantly more efficient with circular ratherthan square or rectangular columns: with the latter, the confinement action is mostly

concentrated at the corners. Poor confinement may be due to low FRP jacketstiffness. Another reason for this is that FRP composite jackets resist axial loads bymembrane action, and are more effective for circular sections as opposed to squareor rectangular column sections with corners and long flat sides; stress concentrationsat the corners and inefficient confinement at the flat sides cause loss of membraneaction of the FRP composite and reduction of confinement.

The improvement in strength can be achieved by rounding the corners ofrectangular sections with effectiveness increasing with rounding radius, until acertain threshold is reached. Because of the presence of steel ties, rounding of thecorner radius in existing square/rectangular columns is limited.

Fig.2 Stress-strain diagram of confined unconfined concrete,L. Lam and J.G. Teng [3]

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Fig 3 : Effect of Corner Radius :

The confinement effectiveness of FRP composites for square and rectangular

columns can be increased by changing the cross-sectional shape. Patil et.al. [3] This

shape alteration was done using micro concrete. Now this shape alteration will be

done using different materials such as normal concrete, micro concrete, cement

based polymer and epoxy based polymer. This study will focus on the use of

different materials and techniques also discuss their merits and demerits.

1.4 Experimental ProgramTo test the applicability of proposed alteration of cross section method, an

experimental programme has been carried out.

Stage 1 Casting of Prisms: In this stage total of 2 prisms of M20 grade werecast. Ready mix concrete was used to get the desired strength. The columns had size150x150 and the height of all prisms was kept 300 mm. Special steel moulds wereused for casting prisms. The prisms were cured for 28 days.

Stage 2 Alteration of cross section: The next stage was to alter the crosssection of prisms. Special steel moulds having oval shape were used in which prismswere kept and micro concrete was poured around the prisms. Next day moulds wereremoved and these modified prisms were allowed to cure for 8 days.

Stage 3 Wrapping of specimens: In this stage the cross section modifiedprisms were wrapped with FRP. Wrapping of FRP laminates to concrete surface is adelicate job and needs special attention. Prisms were wrapped following standardprocedure indicated below:

1. Surface Preparation: The surface of the prisms was cleaned using wire brush.

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2. Primer Application: The primer part A and part B were mixed in equalproportions and thoroughly mixed for 2-3 minutes using a wooden stick. The coat ofPrimer was applied to the prism surface by brush. The primer prepares the surface ofthe concrete for the application of the FRP sheets and has low viscosity.

3. Now Epoxy part A and part B were mixed in equal proportions andthoroughly mixed for 2-3 minutes using a wooden stick. The coat of epoxy wasapplied to the prism surface by paint brush. The surface is now ready for installationof FRP sheets.

4. Now the FRP Sheet was wrapped around the prisms surface. The sheet waspressed against the surface of prism to see that there are no air bubbles developed.And overlap of 150 mm ensured that no splitting occurs at the end.

5. A second hand coat of epoxy was given on this wrapped FRP to saturate itfully. A second application is necessary to ensure good penetration of the saturantaround the fibres.

6. In case of prisms having second wrap epoxy coat was given on first layer andwrapping was continued for second layer. This second layer was the then coatedwith epoxy to saturate it further. After the required layers of the sheet were installed,the FRP wrapped prisms were cured for 10 days. These prisms are now ready fortesting.

Stage 4 Testing of prisms: The cross section altered, wrapped specimens weretested in UTM and Compression testing machine. Due to limitations of UTM(Capacity 100 tonnes) the specimens were tested in compression testing machine.

The observations are shown in Table 1.

TABLE 1. STRENGTH OF SPECIMENSSr.No

SpecimenDetails

Strength ofUnconfined Concrete(MPa)

Strength ofNormallyWrappedSpecimen(MPa)

Strength of CrossSection Altered andWrapped Specimen(MPa)

Single wrapSinglewrap

Doublewrap

1Squarespecimen

20 31 63 73

2Rectangularspecimen

20 38 73 78

3Squarespecimen

25 41 79 82

4Rectangularspecimen

25 49 85 90

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1.5 CONCLUSIONS1. The observations show that by changing the shape of rectangular columns to

elliptical, substantial regain in strength of FRP wrapped columns can beachieved.

2. Corner rounding is a well-accepted procedure that is normally used whenretrofitting rectangular reinforced concrete columns to avoid stressconcentration where FRP laminates are bent. No rounding of corners isrequired in this method as the shape is modified in this experiment. This is anadvantage as rounding is cumbersome process creating noise pollution.

3. It is also observed that strength and deformation is affected by theseparameters. Strength highly increases with increase in grade of concrete.Strength increases and with increasing number of GFRP wraps.

1.6 ACKNOWLEDGEMENTI am very grateful to Sri. Y S Patil of SVNIT, Surat for encouraging and

discussing fruitfully on the subject. My special thanks are also to ShripadConstruction to support us. I also thanks to Sri. Nirav B Umravia, CKPCET, Surat.

REFRENCES:

[1] Kunio Fukuyama, Yasuo Higashibata, Y asuyoshi Miyauchi ,Studies on Repair

and Strengthening Methods of Damaged Reinforced Concrete Columns

,Cement & Concrete Composites,22, (2000),81-88.

[2] Yousef A. Al-Salloum (2007). Influence of edge sharpness on the strength of

square concrete columns FRP composite laminates, Composites: Part B, 38,640650.

[3] Y.S. Patil1, H.S. Patil and J.A. Desai,(2011), Analysis of Performance of Glass

Fibre Reinforced Polymer Wrapped Columns - A Parametric Study,

International Journal of Applied Engineering Research, Volume 6, Number 19,

2251-2266.

[4] Pendhari Sandeep S. Kant, Tarun, Desai, Yogesh M. Application of Polymer

Composites in Civil Construction: A General Review. Composite Structures,

(84),(2008), 114124.

[5] Lam L., Teng J.G. (2003). Design-Oriented StressStrain Model for FRP-

Confined Concrete.Construction and Building Materials(17): 471489.

[6] C. Arya , J.L. Clarke, E.A. Kay , P.D. ORegan, TR 55: Design Guidance for

Strengthening Concrete Structures using Fibre Composite Materials: A

Review, Engineering Structures, 24 ,(2002), 889900.