Date post: | 09-Feb-2017 |
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STRUCTURAL RETROFIT OF REINFORCED
CONCRETE CIRCULAR COLUMNS USING CFRP
By Ahmed Haider Mohiuddin
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
OUTLINE
IntroductionResearch BackgroundExperimental ProgramFinite Element ModelingResultsConclusion Further Research
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
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
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
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
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
Experimental Testing Limitations
Safe operating capacity of 2670 KN (600 Kip)
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
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
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 0.005 0.01 0.015 0.02 0.025 0.03 0.0350
5
10
15
20
25
30
35
40
45
50 stress-strain curve for concrete
Strain
stre
ss in
MPa
Finite Element Modeling Material
(contd.) Concrete model
Tensile behavior
Gf = 90 J/m2
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
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.)
Finite Element Modeling Material
(contd.)
Cured laminate properties Design ValuesTensile 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.)
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)
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
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
Finite Element ModelingColumn 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
Finite Element ModelingColumn 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
Finite Element ModelingColumn 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
Finite Element Modeling Column bonded by 1 layer FRP
(Contd.) Composite manager
Material orientation Thickness of the laminate
Material Orientation
Layup editor
Finite Element ModelingColumn bonded by 1 layer
FRP Material orientation
Finite Element ModelingColumn with second layer and
inclined layer FRP Composite layup editor
Thickness of laminate Orientation
Results
Specime
n
D
(mm)
H
(m)
Ag
(cm2)
ρf
(%)
ρl
(%)
fy
(MPa)
f’c
(MPa)
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
Results Design guideline
ACI: as mentioned by Rocca Silvia NCHRP: , f’cc and f’co can be calculated fro the peak load values. f’l is th stress in
CFRP at its rupture
Results
Guideline Specimen f’cc
(MPa)
εccu
(mm/mm)
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
Results
Guideline Specimen
ACIH1 0.64 0.33H2 0.47 0.21O1 NA NA
NCHRPH1 0.58
NAH2 0.38O1 NA
Comparison between values from guidelines and FEA
Results
Guidelines Specimen
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.)
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
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