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Retrofitting Reinforced Concrete
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.
Retrofitting Reinforced Concrete
- 1. A new Methodfor 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 beenwidely 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 experimentalprogram 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 Longitudinalsteel N12 (500 MPa) Ties R6@120 (250 MPa) Ready-mixed Concrete f’c=32 MPa
- 9. Design of Specimens CarbonFabric 34GM/SM 75 mm Band-IT © C306 (19.1 mm in width) Buckles C356
- 10. Test Configuration 150 800 Reference group GroupRF: 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 ofSpecimens (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 circularconcrete 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 Columnsbonded with segmental circular covers Columns wrapped with CFRP
- 15. Construction of ExternalConfinement 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) Concrete27 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 Columncapacity Nominal average axial stress – strain diagrams
- 27. Increase of Columnductility 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.
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