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 1 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. 2 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. 3 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 4 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 5 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) 6
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