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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

50 stress-strain curve for concrete

Strain

Finite Element Modeling Material

(contd.) Concrete model

Tensile behavior

Gf = 90 J/m2

(contd.) Reinforcing steel

Grade 60 steel fy = 60,000 Psi εy = 0.0021 E = 29 x 106 Psi ν = 0.3 Elastic- perfectly

plastic

(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.)

(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.)

(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)

(contd.) CFRP (contd.)

Orthotropic material

E1 E2 E3 Nu12 Nu13

G12 G13 G23

8.2 x 106 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

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

(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

(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

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

(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

NCHRP H1 0.52

H2 0.36

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