nonlinear finite element analysis of concrete columns confined by fibre

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by  New Alamin

NONLINEAR FINITE ELEMENT ANALYSIS OF CONCRETE COLUMNS
CONFINED BY FIBRE- REINFORCED POLYMERS

SABREENA NASRIN
STUDENT NO: 1009042348

MASTER OF SCIENCE IN CIVIL ENGINEERING (STRUCTURAL)

DEPARTMENT OF CIVIL ENGINEERING
BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY
DHAKA-1000, BANGLADESH
JULY, 2013
NONLINEAR FINITE ELEMENT ANALYSIS OF CONCRETE COLUMNS
CONFINED BY FIBRE- REINFORCED POLYMERS

SUBMITTED BY
SABREENA NASRIN
STUDENT NO: 1009042348

A Thesis submitted in Partial Fulfillment of the Requirements for the Degree
of Master of Science in Civil Engineering (Structural)

DEPARTMENT OF CIVIL ENGINEERING
BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY
DHAKA-1000, BANGLADESH
JULY, 2013
DEDICATED
TO
My Beloved Parents

Late Dr. Nazrul Islam Miah
Mrs. Meherunnesa Khan
Engr. Humayun Kabir
&
Mrs. Nurunnahar Begum

CERTIFICATE OF APPROVAL
The thesis titled “Nonlinear Finite Element Analysis of Concrete Columns Confined by
Fibre-Reinforced Polymers” submitted by Sabreena Nasrin, Student Number 1009042348F,
Session: October, 2009 has been accepted as satisfactory in partial fulfillment of the requirement
for the degree of Master of Science in Civil Engineering (Structural) on 29th July, 2013.

BOARD OF EXAMINERS

1. Dr. Mahbuba Begum Chairman
Associate Professor (Supervisor)
Department of Civil Engineering
BUET, Dhaka-1000

2. Dr. Md. Mujibur Rahman Member
Professor and Head (Ex-Officio)
Department of Civil Engineering
BUET, Dhaka-1000

3. Dr. Sk. Sekender Ali Member
Professor
Department of Civil Engineering
BUET, Dhaka-1000

4. Dr. A. M. M. Taufiqul Anwar Member
Professor
Department of Civil Engineering
BUET, Dhaka-1000

5. Dr. Md. Mozammel Hoque Member
Associate Professor (External)
Department of Civil Engineering
DUET, Gazipur
CHAPTER 1
INTRODUCTION

1.1 General
In recent years, considerable attention has been focused on the use of fibre-reinforced
polymer (FRP) composite materials for structural rehabilitation and strengthening purpose.
Highly aggressive environmental conditions have a significant effect on the durability and
structural integrity of steel reinforced concrete piles, piers and columns. Corrosion of steel
rods is a potential cause for the structural damage of these reinforced concrete columns.
Dealing with the problem of steel reinforcement corrosion has usually meant improving the
quality of the concrete itself, but this approach has had only limited success. A traditional
way of repair of damaged concrete columns is wrapping a sheet of steel around the column.
While the strength of repaired columns can be increased for a short-term, the steel wrapping
suffers from the same problem as the steel rebar, corrosion and poor durability. It also suffers
from labor-intensive construction problem due to its weight.
In a new approach, FRPs are now being used as alternatives for steel wrappings in repair,
rehabilitation and strengthening of reinforced concrete columns. If correctly applied, the use
of FRP composites for strengthening reinforced concrete (RC) structures can result in
significant enhancements to durability, and decreased maintenance costs, as well as in
improved serviceability, ultimate strength, and ductility. Moreover, the FRP composites can
generally be applied while the structure is in use, with negligible changes in the member
dimensions. Other advantages include high strength and stiffness-to-weight ratios, a high
degree of chemical inertness, controllable thermal expansion, damping characteristics, and
electromagnetic neutrality. In addition to repair, FRP confined concrete columns have been
developed in new construction and rebuilding of concrete piers/piles in engineering
structures.
Extensive experimental studies have been conducted by several research groups on the
behavior of confined concrete columns (Benmokrane and Rahman, 1998; Saadatmanesh and
Ehsani, 1998; Meir and Betti 1997; El-Badry 1996). However, most of these studies are
confined to circular shaped columns. Experimental studies related to rectangular and square
columns are limited (Bousias et.al. 2004). Despite of the availability of a large amount of
experimental data for predicting the behavior of FRP confined concrete circular columns, a
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complete 3-D finite element model for understanding the influence of geometric shapes,
aspect ratios and FRP stiffness is somewhat lacking. As a contribution to fill this need an
attempt has been taken to develop a complete 3-D finite element model to investigate the
effect of aspect ratios, corner radius and thickness of FRP wrap on the behavior of FRP
wrapped concrete columns. This study also aims to evaluate the effect of FRP-concrete
interface on the behavior of FRP confined concrete.
1.2 Objectives of the Study
The objectives of the study are
1 To perform a nonlinear 3D finite element analysis on concrete columns of different
shapes confined with FRP wrap.
2 To validate the numerical model with respect to the experimental database available in
the literature.
3 To study the effect of selected parameters such as aspect ratio (a/b), the corner radius (R)
and the thickness of FRP wrap (t) on the strength and ductility of FRP confined concrete
f
columns under concentric axial loading only.
1.3 Scope
The numerical simulation of concentrically loaded FRP confined concrete column has been
performed using ABAQUS, a finite element software package. A 3D finite element model
incorporating the nonlinear material behavior of concrete has been developed. The interface
between concrete and FRP has been modeled using contact pair algorithm in ABAQUS. A
perfect bond and a cohesion based surface interaction model have been assumed to define the
contact behavior of the concrete-FRP interface. The nonlinear load displacement response up
to failure of the confined columns has been traced using Riks solution strategy.
The performance of the developed model has been studied by simulating test columns
confined with FRP available in the published literature. These columns had various geometric
shapes as well as various FRP configurations. Finally the effect of the selected parameters
like cross-section shape factor, corner radius and the thickness of the FRP wrap on the
strength and ductility of FRP confined concrete columns have been investigated.

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1.4 Organization of the Study
The thesis has been organized in six chapters. Chapter 1 includes the background of the
work along with the objectives and scope of current study. A brief review on the available
literatures regarding the characteristics and available types of composites as well as different
rehabilitation schemed for various structural components has been reported in chapter 2.
Moreover, this chapter presents various analytical models proposed by different research
groups for predicting the behavior of concrete rectangular and square columns confined with
Fibre reinforced polymers
.
Chapter 3 includes the properties of reference columns and the characteristics of the finite
element. The performance of the FE model has been studied in chapter 4 by comparing the
numerically obtained graphs with available experimental graphs.

Chapter 5 incorporates the parametric study which includes the effects of aspect ratio, corner
sharpness and confinement effectiveness of FRP-strengthened concrete columns. Finally, the
summary and conclusions of the work along with the recommendations for future research
have been included in chapter 6.

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CHAPTER 2
LITERATURE REVIEW

2.1 General
Recent evaluation of civil engineering infrastructure has demonstrated that most of it will
need major repairs in the near future. The strength and stability of these structural members,
bridges, water retaining structures, sewerage treatment plants, wharfs, etc. are provided by
concrete. Therefore it is very important to protect concrete and any deterioration or damage
to concrete must be repaired promptly in order not to compromise the integrity of structures
built with concrete. Concrete rehabilitation particularly in critical infrastructures is as
important as any other maintenance activity and must be carried out in a timely manner.
Repairs performed at early stage would save extremely expensive remediation that may
become necessary at latter stages. Concrete can be deteriorated for many reasons such as-
(cid:131) Accidental Loadings
(cid:131) Chemical Reactions
(cid:131) Construction Errors
(cid:131) Corrosion of Embedded Metals
(cid:131) Design Errors
(cid:131) Abrasion and Cavitations
(cid:131) Freezing and Thawing
(cid:131) Settlement and Movement
(cid:131) Shrinkage
(cid:131) Temperature Changes
(cid:131) Weathering etc.
The strengthening and retrofitting of existing concrete structures to resist higher design loads,
correct deterioration-related damage or increased ductility has traditionally been
accomplished using conventional materials and construction techniques. Externally bonded
steel plates, steel or concrete jackets and external post tensioning are just some of the many
techniques available. However, to repair and extend the life of damaged structures externally
bonded fibre reinforced polymers (FRP) have been proved to be the most effective alternative
to the conventional ones. Despite a high material cost, some advantages like high strength to
weight ratio, high corrosion resistance, easy handling and installation processes are
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establishing them as the most convenient option over the traditional strengthening materials
for rehabilitation of corroded RC structures, seismic damaged structures and so on. (Nasrin et
al., 2010). The composition and the type of this new composite material are presented in this
chapter. The material’s mechanical behavior is also included here. This chapter mainly
focuses on the repairing techniques by FRP laminates for shear and flexural strengthening of
corroded RC structures, strengthening of concrete beam-column joints and strengthening of
rectangular concrete columns in accordance with the numerical and experimental
investigations. The behavior of FRP confined concrete columns along with the design
guidelines are also reported in the literatures.
2.2 Fibre-Reinforced Polymers
Fibre-reinforced polymer (FRP) composites consist of continuous carbon (C), glass (G) or
aramid (A) fibres bonded together in a matrix of epoxy, vinylester or polyester. The fibres are
the basic load carrying component in FRP whereas the plastic, the matrix material, transfers
shear. FRP products commonly used for structural rehabilitation can take the form of strips,
sheets and laminates as shown in Figure 2.1.

(a) (b)
Figure 2.1 FRP products for structural rehabilitation, (a) FRP strips and (b) FRP sheets
(Rizkalla et al. 2003).
Use of FRP has now become a common alternative over steel to repair, retrofit and strengthen
buildings and bridges. FRP materials may offer a number of advantages over steel plates
which include,
1. High specific stiffness (E/ρ).
2. High specific strength (σ /ρ)
ult
3. High corrosion resistance
4. Ease of handling and installation
Moreover, its resistance to high temperature and extreme mechanical and environmental
conditions has made it a material of choice for seismic rehabilitation. Some of the
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disadvantages of using FRP materials include their high cost, low impact resistance and high
electric conductivity.
2.3 Properties and Behavior of FRP
2.3.1 Tensile Behavior
The tensile strength and stiffness of FRP material is dependent on several factors. As the
fibres of FRP are the main load-carrying constituents, so the type of fibres, the orientation of
fibres and the quantity of fibres govern the tensile behavior mostly. When this FRP is loaded
under direct tension it does not exhibit any plastic behavior (yielding) before rupture. Most of
the time, FRP shows a linearly elastic stress-strain relationship until failure. Table 2.1 present
the tensile properties of commercially available FRP system.
Table 2.1 The tensile properties of some of the commercially available FRP systems
Fibre type Elastic modulus Ultimate Strength Rupture
strain, min

103 ksi GPa ksi MPa %
Carbon

General Purpose 32-34 220-240 300-550 2050-3790 1.2
High Strength 32-34 220-240 550-700 3790-4820 1.4
Ultra- High Strength 32-34 220-240 700-900 4820-6200 1.5
High modulus 50-75 340-520 250-450 1720-3100 0.5
Ultra- High modulus 75-100 520-690 200-350 1380-2400 0.2
Glass

E-glass 10-10.5 69-72 270-390 1860-2680 4.5
S-glass 12.5-13 86-90 500-700 3440-4140 5.4
Aramid
General Purpose 10-12 69-83 500-600 3440-4140 2.5
High performance 16-18 110-124 500-600 3440-4140 1.6
(Italian National Research Council, 2004)

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