The document summarizes research on structurally retrofitting reinforced concrete circular columns using carbon fiber reinforced polymer (CFRP). Finite element modeling was used to simulate column tests and compare results to design guidelines. The modeling captured material behavior accurately. Guidelines could be improved by better defining key parameters like confining pressure. Further research is needed on inclined wrapping schedules and other factors' effects.

1. STRUCTURAL RETROFIT OF
REINFORCED CONCRETE
CIRCULAR COLUMNS
USING CFRP
By Ahmed Haider Mohiuddin

2. Biographical Information
 Graduated with Masters degree in Civil Engineer with specialization in
Structures and Applied Mechanics
Obtained Bachelor of Technology degree from Jawaharlal Nehru
Technological University- Hyderabad in Civil Engineering
 Successfully passed Fundamentals of Engineering exam and is working on
attaining Professional Engineer license

3. OUTLINE
Introduction
Research Background
Experimental Program
Finite Element Modeling
Results
Conclusion
Further Research

4. Introduction
 Estimation of the axial load capacity of the specimen through design guidelines
 ACI.2R-08 and NCHRP design guidelines
 Identification of parameters controlling the design
 Experimental test
 Test setup for uniaxial compression along with the arrangement of strain
measurement

5. Introduction
 Finite Element Modeling
 Development of appropriate model to compare with the guidelines
 Applicable material models adopted
 Techniques to simulate the real behavior
 Comparison between guidelines and model
 Appropriate comparison
 Performance of the equations

6. Research Background
 The columns were ready to
test. 4 specimen were
considered in this study
 Peak loads were estimated
 For model, the material
characteristics were of very
high importance
 Similar case studies were
examined

7. Research Background
 Main characteristics
 Height of column: 48 in
 Diameter of column: 10 in
 Average compressive strength of concrete: 6236 Psi
 Longitudinal reinforcement : 6 #5 bars
 Transverse reinforcement : # 3 ties
 FRP
 Carbon Fabric Sikawrap Hex 117C
 Epoxy resin Sikadur Hex 300
 ACI mix design
 Slump: Max 4 in, min 1 in
 Max Aggregate size: 0.75 in
 Typical air entrapped: 2%
 W/C ratio: 0.40

8. Experimental Testing
 Setup
 Actuator and load cell for load
application
 Strain gauges
 Procedure
 Specimen centered
 Load applied in 44.4 KN (10 Kip) steps
 Load controlled
 StrainSmart program

9. Experimental Testing
 Limitations
 Safe operating capacity of 2670 KN
(600 Kip)

10. Experimental Testing
 Specimen response
 No crack or warping in FRP wraps
 Slow increase in strain values
 No decrease in load values with increase in
strain

11. Finite Element Modeling
 Material
 Concrete Model
 Concrete damage plasticity model: linear elastic- plastic
both in compression and tension. Models both
compressive crushing and tensile cracking failures

12. Finite Element Modeling
 Material (contd.)
 Concrete model (contd.)
 f’c=6236 Psi
 Compressive behavior
 Model given in Obaidat, 2011
 Elastic upto 0.4f’c
 Plastic strain hardening branch
ascending between 0.4f’c and f’c
 Plastic softening decreasing branch
0
5
10
15
20
25
30
35
40
45
50
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035
stressinMPa
Strain
stress-strain curve for concrete

13. Finite Element Modeling
 Material (contd.)
 Concrete model
 Tensile behavior
 Ec = 4700 fc
′
= 30820 Mpa
 fct = 0.33 𝑓𝑐
′
= 2.16 Mpa
 Gf = 90 J/m2

14. Finite Element Modeling
 Material (contd.)
 Reinforcing steel
 Grade 60 steel
 fy = 60,000 Psi
 εy = 0.0021
 E = 29 x 106 Psi
 ν = 0.3
 Elastic- perfectly plastic

15. Finite Element Modeling
 Material (contd.)
 CFRP
Storage Conditions Store dry at 40 – 350 C (400 – 950 F)
Color Black
Primary Fiber Direction 00 (unidirectional)
Weight per Square Yard 300 g/m2 (9.0 oz.)

16. Finite Element Modeling
 Material (contd.)
Cured laminate properties Design Values
Tensile Strength 724 MPa (1.05 x 105 Psi)
Modulus of Elasticity 1.0%
Thickness 0.51 mm (0.02 in)
Elongation at break 1.0%
Strength per inch width 9.3 KN (2100 lb/layer)
 CFRP (contd.)

17. Finite Element Modeling
 Material (contd.)
 CFRP (contd.)
Fiber Properties Design Values
Tensile Strength 3,793 MPa (550,000 Psi)
Tensile Modulus 234,000 MPa (34 x 106 Psi)
Elongation 1.5%
Density 1.8 gm/cc (0.065 lb/in3)

18. Finite Element Modeling
 Material (contd.)
 CFRP (contd.)
 Orthotropic material
E1 E2 E3 Nu12 Nu13 Nu23 G12 G13 G23
8.2 x 106 Psi 5.96 x 105 Psi 5.96 x 105 Psi 0.36 0.36 0.36 2.13 x 105 Psi 2.13 x 105 Psi 2.13 x 105 Psi

19. Finite Element Modeling
Interactions and boundary conditions
 Steel and Concrete
 Embedded
 Concrete and CFRP
 Tie Constraint
 Concrete
 Bottom end pinned
 Mesh element size : 0.7 in
 For tie constraint mesh size critical

20. Finite Element Modeling
Column bonded by FRP
 Concrete section
 3D deformable homogenous solid element
 Element type: Tetrahedral element C3D4
Abaqus naming convention
4-node tetrahedral element C3D4
Concrete section: Isometric view

21. Finite Element Modeling
Column bonded by FRP (Contd.)
 Steel Reinforcement
 3D wires with truss section
 Element type: Truss element T3D2
 Deformation compatibility
Abaqus naming convention
2-node truss element T3D2
Steel Reinforcement: Isometric view

22. Finite Element Modeling
Column bonded by FRP (Contd.)
 CFRP
 Shell element.
 Element type: Composite
 Conventional shell element
 Deformation compatibility
 Composite layup
CFRP: Isometric view
4 node shell element S4R

23. Finite Element Modeling
 Column bonded by 1 layer FRP (Contd.)
 Composite manager
 Material orientation
 Thickness of the laminate
Material Orientation
Layup editor

24. Finite Element Modeling
Column bonded by 1 layer FRP
 Material orientation

25. Finite Element Modeling
Column with second layer and inclined layer FRP
 Composite layup editor
 Thickness of laminate
 Orientation

26. Results
Specimen D
(mm)
H
(m)
Ag
(cm2)
ρf
(%)
ρl
(%)
fy
(MPa)
f’c
(MPa)
𝑓𝑐𝑐
′
𝑓𝑐𝑜
′
𝜀 𝑐𝑐𝑢
𝜀 𝑐𝑢
H1 254 1.22 506.8 0.8 2.36 414 43 2.194 6.26
H2 254 1.22 506.8 1.6 2.36 414 43 3.55 14.56
O1 254 1.22 506.8 0.8 2.36 414 43 1.80 5.1
 Values from FEA
Notation for specimen is of form letter (H/O) defines orientation of plies, number (1/2) defines number of
plies

27. Results
 Design guideline
 ACI: 𝑓𝑐𝑐
′
= 𝑓𝑐
′
2.25 1 + 7.9
𝑓𝑙
′
𝑓𝑐
′ − 2
𝑓 𝑙
𝑓𝑐
′ − 1.25 , 𝑓𝑙 =
2𝐸 𝑓 𝑛𝑡 𝑓 𝜀 𝑓𝑒
𝐷
as mentioned by Rocca Silvia
 NCHRP: 𝑓𝑐𝑐
′
= 𝑓𝑐
′
1 +
2𝑓 𝑙
𝑓𝑐
′ , 𝑓𝑙 = 𝜑 𝑓𝑟𝑝
2𝑁 𝑓𝑟𝑝
𝐷
≤
𝑓𝑐
′
2
1
𝑘 𝑒 𝜑
− 1
 f’cc and f’co can be calculated fro the peak load values. f’l is th stress in CFRP at its rupture

28. Results
Guideline Specimen f’cc
(MPa)
𝑓𝑐𝑐
′
𝑓𝑐𝑜
′
εccu
(mm/mm)
𝜀 𝑐𝑐𝑢
𝜀 𝑐𝑢
ACI H1 56.9 1.42 0.0062 2.06
H2 68.2 1.70 0.0092 3.07
O1 NA NA NA NA
NCHRP H1 51.3 1.28 NA NA
H2 54.6 1.36 NA NA
O1 NA NA NA NA
 Values from design guidelines

29. Results
Guideline Specimen 𝑓𝑐𝑐
′
𝑓𝑐𝑜
′
𝑇ℎ𝑒𝑜
𝑓𝑐𝑐
′
𝑓𝑐𝑜
′
𝐹𝐸𝐴
𝜀 𝑐𝑐𝑢
𝜀 𝑐𝑢 𝑇ℎ𝑒𝑜
𝜀 𝑐𝑐𝑢
𝜀 𝑐𝑢 𝐹𝐸𝐴
ACI
H1 0.64 0.33
H2 0.47 0.21
O1 NA NA
NCHRP
H1 0.58
NAH2 0.38
O1 NA
 Comparison between values from guidelines and FEA

30. Results
Guidelines Specimen 𝑃𝑡ℎ𝑒𝑜
𝑃𝐹𝐸𝐴
ACI H1 0.57
H2 0.43
O1 NA
NCHRP H1 0.52
H2 0.36
O1 NA
 Comparison between values from guidelines and FEA (contd.)

31. Conclusions
 Finite element model was compared with the results from the work of Rocca, Silvia; Galati, Nestore;
Nanni, Antonio
 Finite element model proved to be adequate means of simulating the real material behavior and
comparing the results with design guidelines
 Mode of failure for the specimen was tensile rupture of the CFRP
 Experimental tests could not be conclusive because of the limitations of guideline equations and test
setup
 Accuracy of the design guidelines can be increased if some key parameters like confining pressure
(f’l) are properly defined
 Thickness of laminate and orientation of fibers has great effect on results

32. Further Research
 The effect of inclined wrapping schedule
 Concentrating strain measurements along the perimeter of FRP jackets
 Longitudinal bar instability due to the concrete dilation
 Contribution of lateral reinforcement in the confinement
 Crack propagation detection
 Confirming the assumptions made in the material models so that they can be used to
understand the controlling factors of design