A new Method for Retrofitting Reinforced Concrete Columns - Muhammad Hadi

A new Method for Retrofitting Reinforced Concrete Columns

School of Civil, Mining & Environmental EngineeringUniversity of Wollongong

M.N.S. Hadi, T.M. Pham, and X. Lei

Presentation Outline

Introduction & Research Significance

Experimental Program

Test Results

Discussions & Conclusions

Introduction

FRP has been widely used to strengthen existing structures;

For concrete columns, FRP is used to wrap the columns thereby provides confining pressure;

Experimental studies confirmed that FRP confinement increases the ultimate load and ductility by a significant amount;

Introduction

For square columns, the confinement efficiency is lower compared with circular columns;

A common solution is to round the sharp corners, but limited improvement was found.

Research Significance

Circularisation – A new technique is proposed and examined to increase the confinement efficiency for FRP-confined square columns;

Eccentric loading was incorporated to investigate the combined load – bending moment behaviour.

Research Significance

Circularisation – Ideas:

Four pieces of segmental circular concrete covers were bond onto the surface of the original column to make the cross-section from a square to a circular.

Original column

Concrete covers

Experimental Program

An experimental program was carried out at the High Bay Laboratory at the University of Wollongong.

Features:• 12 square reinforced columns;• Half of the specimens were

circularised;• Confined with CFRP or steel

straps;• Loading conditions:

concentric, 15 mm & 25 mm eccentric.

Design of Specimens

150

150

800

Longitudinal steelN12 (500 MPa)

TiesR6@120 (250 MPa)

Ready-mixed Concretef’c=32 MPa

Design of Specimens

Carbon Fabric 34GM/SM 75 mm

Band-IT © C306 (19.1 mm in width)

Buckles C356

Test Configuration

150

150

800

Reference group

Group RF:Square sectionRound cornersFRP wrapped

Group CF:Circular sectionFRP wrapped

Group CS: Circular sectionSteel straps wrapped

3 layers of FRP

Test Configuration

Cross-section of Specimens (Units in mm)

Preparation of Specimens

• Two sets of formworks were used, one for columns and one for segmental circular concrete covers;

• Foams were used to generate required shape.

Formworks (a) Columns (b) Concrete covers(a) (b)

Circularisation Process

Remove segmental circular concrete covers from formwork

Remove foams attached on the covers

Bond the segmental circular covers onto the original columns with epoxy resin

Circularisation Process

Concrete covers

Columns bonded with segmental circular covers

Columns wrapped with CFRP

Construction of External Confinement

FRP

Wet layup method

3 layers continuous

confinement

100 mm overlap for each rings

Steel Straps

Special tool was used to tighten the

straps

Each band was fixed by a

buckle

Each ring had 30 mm clear

spacing

Preliminary Tests

Materials Strength

(MPa)

Concrete 27

N12 deformed bars 568

R6 plain bars 478

CFRP 1674

Steel Straps 598

Loading System

Loading System

(a) Overview

(b) Overhang edge

(c) Loading head

• Eccentricity was created by

placing the overhang edge

into the gauges on the

loading heads;

• Loading was displacement

controlled

• Loading rate was 0.3

mm/min

Test Results – Group N

Specimen Yield Load(kN)

Corres. Axial Disp.

(mm)

Ultimate Load(kN)

Corres. Axial Disp.

(mm)

Corres. Lateral Disp. (mm)

N-0 707 1.31 506 1.83 -

N-15 563 1.24 492 2.02 3.35

N-25 414 0.96 363 1.27 3.11

Summary of the tested columns, Group N

Test Results – Group N

Load - displacement diagrams, Group N

Test Results – Group RF

Summary of the tested columns, Group RF

Specimen Yield Load(kN)

Corres. Axial Disp.

(mm)

Ultimate Load(kN)

Corres. Axial Disp.

(mm)

Corres. Lateral Disp. (mm)

RF-0 796 1.83 1485 24.5 -

RF-15 635 1.56 583 8.12 34.64

RF-25 515 1.68 477 7.94 33.86

Test Results – Group RF

Load - displacement diagrams, Group RF

Test Results – Group CF

Summary of the tested columns, Group CF

Specimen Yield Load(kN)

Corres. Axial Disp.

(mm)

Ultimate Load(kN)

Corres. Axial Disp.

(mm)

Corres. Lateral Disp. (mm)

CF-0 1478 2.00 2471 13.96 -

CF-15 1202 2.02 1266 9.73 38.97

CF-25 1011 1.69 995 8.63 39.66

Test Results – Group CF

Load - displacement diagrams, Group CF

Test Results – Group CS

Summary of the tested columns, Group CS

Specimen Yield Load(kN)

Corres. Axial Disp.

(mm)

Ultimate Load(kN)

Corres. Axial Disp.

(mm)

Corres. Lateral Disp. (mm)

CS-0 1060 1.85 948 3.48 -

CS-15 905 1.05 779 3.39 7.3

CS-25 766 1.40 655 2.80 8.2

Test Results – Group CS

Load - displacement diagrams, Group CS

Increase of Column capacity

Nominal average axial stress – strain diagrams

Increase of Column ductility

Ductility of the tested columns

Discussions and Conclusions

• Circularisation dramatically increases the efficiency of FRP confinement. By circularising square concrete columns, the efficiency of FRP confinement can be maximised;

• Eccentricity dramatically reduced the ultimate load of all specimens, regardless of the technique incorporated;

• Most significantly, test results showed that the proposed circularisation technique is a viable and efficient method for strengthening square RC columns.

Thank you