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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