1

Strength Gaining of RC Columns

Using FRP Retrofit Technique

Akash Krupeshkumar Chauhan a, Nirav B. Umravia b

a M Tech (Computer Aided Structural Engineering)

International Institute of Information Technology Hyderabad, Hyderabad- 500 032

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

b Assistant Professor,

Civil Engg. Dept., C K Pithawala College of Engineering & Technology, Gujarat Technological University, Surat, India.

nirav_dr@indiatimes.com , +91 9998511458

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 of

FRP 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, cement based polymer and epoxy based polymer. The paper presents results of one

experiment and explains ongoing programme.

Keywords: Concrete; Column; Strengthening, Jacketing, FRP

2

1.1 INTRODUCTION

Strengthening or retrofitting of existing reinforced concrete (RC) structures is required for

a variety of reasons. Sometimes it may be change in use causing higher loads, or deterioration

due to factors like environmental factors, or for withstanding lateral loads.[Fig 1] 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. The

three most commonly used materials for column jacketing are concrete (with reinforcing bars

or fibre-reinforced), steel, or fibre-reinforced polymers. Kunio Fukuyama et al. [1]. The

jacket material is selected depending on the column cross sectional shape and the column

deficiency needing correction. At first concrete and steel jackets were used widely, later FRP

jackets are being used.

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

Fig 1B Typical damaged column

Fig 1C Shear failure of a reinforced concrete column in the 2001 Bhuj earthquake

3

Fig 1D. The column damaged during blast and after FRP jacketing. [www.wai.com]

1.2 FRP COMPOSITES FOR STRENGTHENING OF RC STRUCTURAL ELEMENTS

Wrapping of FRP sheets around beams, and columns has become a common strengthening

technique to increase the ductility and load carrying capacity of 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 FRP materials are (1) High specific strength and Stiffness

(2) High corrosion resistance: - Composites are not prone to rust. (3) Non Magnetic

Properties: (4) Lower maintenance 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 structural shape

Strengthening of column using FRP- Jacketing on RC column with FRP primarily

improves column performance, as it provides lateral confining pressure to the 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 levels of confining

pressure enable concrete to sustain both greater axial loads and greater ultimate axial strain by

changing the failure mode from cleavage of the concrete to the crushing of its cement paste.

FRP jackets can apply confining pressure either actively or passively. In the active retrofit

scheme the fibers are tensioned either as they are wrapped around the column or by pressure

injecting grout or epoxy between the jacket and the column. In the passive scheme, the

4

confining pressure is a result of the reaction of the jacket against the lateral dilation of the

column cross section as it is loaded axially.

1.3 STRENGTHENING OF RECTANGULAR COLUMNS

The confinement due to FRP is significantly more efficient with circular rather than square

or rectangular columns: with the latter, the confinement action is mostly concentrated at the

corners. Poor confinement may be due to low FRP jacket stiffness. Another reason for this is

that FRP composite jackets resist axial loads by membrane action, and are more effective for

circular sections as opposed to square or rectangular column sections with corners and long

flat sides; stress concentrations at the corners and inefficient confinement at the flat sides

cause loss of membrane action of the FRP composite and reduction of confinement.

Fig 3. Strengthening Of Circular Columns V/S Rectangular Columns, (Yousef A. Al-Salloum

[2])

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

5

The improvement in strength can be achieved by rounding the corners of rectangular

sections with effectiveness increasing with rounding radius, until a certain threshold is

reached. Because of the presence of steel ties, rounding of the corner radius in existing

square/rectangular columns is limited.

Fig 4 : 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 Program (Part 1)

To test the applicability of proposed alteration of cross section method, an experimental

programme will be carried out.

Stage 1 Casting of Prisms: In this stage total of 2 prisms of M20 grade were cast. Ready

mix concrete was used to get the desired strength. The columns had size 150x150 and the

height of all prisms was kept 300 mm. Special steel moulds were used for casting prisms. The

prisms were cured for 28 days.

6

Stage 2 Alteration of cross section: The next stage was to alter the cross section of

prisms. Special steel moulds having oval shape were used in which prisms were kept and

micro concrete was poured around the prisms. Next day moulds were removed and these

modified prisms were allowed to cure for 8 days.

Stage 3 Wrapping of specimens: In this stage the cross section modified prisms were

wrapped with FRP. Wrapping of FRP laminates to concrete surface is a delicate job and needs

special attention. Prisms were wrapped following standard procedure indicated below:

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

2. Primer Application: The primer part A and part B were mixed in equal proportions and

thoroughly mixed for 2-3 minutes using a wooden stick. The coat of Primer was applied to the

prism surface by brush. The primer prepares the surface of the 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 and thoroughly mixed

for 2-3 minutes using a wooden stick. The coat of epoxy was applied to the prism surface by

paint brush. The surface is now ready for installation of FRP sheets.

4. Now the FRP Sheet was wrapped around the prisms surface. The sheet was pressed

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 it fully. A

second application is necessary to ensure good penetration of the saturant around the fibres.

6. In case of prisms having second wrap epoxy coat was given on first layer and wrapping

was continued for second layer. This second layer was the then coated with 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 for testing.

Stage 4 Testing of prisms: The cross section altered, wrapped specimens were tested in

UTM and Compression testing machine. Due to limitations of UTM (Capacity 100 tonnes) the

specimens were tested in compression testing machine.

7

The observations are as detailed below.

TABLE 1. STRENGTH OF SPECIMENS Sr. No

Specimen Details

Strength of Unconfined Concrete (MPa)

Strength of Normally Wrapped Specimen (MPa)

Strength of Cross Section Altered and Wrapped Specimen (MPa)

Single wrap Single wrap

Double wrap

1 Square specimen 20 31 63 73

2 Rectangular specimen 20 38 73 78

1.5 EXPERIMENTAL PROGRAM (part 2)

In this programme it is proposed to use different materials like normal concrete, micro

concrete, cement based polymer and epoxy based polymer for changing the shape.

Following experimental program has been planned.

Concrete Grade: M25

Size of Rectangular Specimen: 100 X 150 X 300

1.6 CONCLUSIONS

The strength of FRP wrapped rectangular columns can be increased by changing the

shape to elliptical.

Use of different materials for shape change plays a key role to gain strength.

Various parameters can be verified and quantified with more number of specimens.

Corner rounding is a well-accepted procedure that is normally used when retrofitting

rectangular reinforced concrete columns to avoid stress concentration where FRP

laminates are bent. No rounding of corners is required in this method as the shape is

modified in this experiment. This is an advantage as rounding is cumbersome process

creating noise pollution.

8

1.7 ACKNOWLEDGEMENT

I am very grateful to Shree Y S Patil of SVNIT, Surat for encouraging and discussing

fruitfully on the subject. My special thanks are also to Shripad Construction to support us.

REFRENCES: [1] Kunio Fukuyama, Yasuo Higashibata, Yasuyoshi 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, 640–650.

[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), 114–124.

[5] Lam L., Teng J.G. (2003). Design-Oriented Stress–Strain Model for FRP-Confined

Concrete. Construction and Building Materials (17): 471–489.

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

Strengthening Concrete Structures using Fibre Composite Materials: A Review”,

Engineering Structures, 24 ,(2002), 889–900.