This document summarizes a study on a new method of retrofitting reinforced concrete columns called "circularisation". The study involved wrapping originally square concrete columns with circular concrete covers to change their cross-section to circular. Twelve square columns were tested under concentric and eccentric axial loads, with half undergoing circularisation first. Test results showed that circularisation dramatically increased the efficiency of FRP confinement, maximizing its strengthening effects. While eccentric loading reduced column capacity for all specimens, circularisation proved an effective technique for strengthening square reinforced concrete columns.

1. A new Method for Retrofitting Reinforced Concrete
Columns
School of Civil, Mining & Environmental Engineering
University of Wollongong
M.N.S. Hadi, T.M. Pham, and X. Lei

2. Presentation Outline
Introduction & Research Significance
Experimental Program
Test Results
Discussions & Conclusions

3. 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;

4. 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.

5. 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.

6. 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

7. 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.

8. Design of Specimens
150
800
Longitudinal steel
N12 (500 MPa)
Ties
R6@120 (250 MPa)
Ready-mixed
Concrete
f’c=32 MPa

9. Design of Specimens
Carbon Fabric
34GM/SM 75 mm
Band-IT © C306
(19.1 mm in width)
Buckles C356

10. Test Configuration
150
800
Reference group
Group RF:
Square section
Round corners
FRP wrapped
Group CF:
Circular section
FRP wrapped
Group CS:
Circular section
Steel straps wrapped
3 layers of FRP

11. Test Configuration
Cross-section of Specimens (Units in mm)

12. 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)

13. 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

14. Circularisation Process
Concrete covers
Columns bonded
with segmental
circular covers
Columns wrapped
with CFRP

15. 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

16. Preliminary Tests
Materials Strength
(MPa)
Concrete 27
N12 deformed bars 568
R6 plain bars 478
CFRP 1674
Steel Straps 598

17. 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

18. 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

19. Test Results – Group N
Load - displacement diagrams, Group N

20. 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

21. Test Results – Group RF
Load - displacement diagrams, Group RF

22. 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

23. Test Results – Group CF
Load - displacement diagrams, Group CF

24. 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

25. Test Results – Group CS
Load - displacement diagrams, Group CS

26. Increase of Column capacity
Nominal average axial stress – strain diagrams

27. Increase of Column ductility
Ductility of the tested columns

28. 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.

29. Thank you