Retrofitting of reinforced concrete beams using composite laminates
ABSTRACT This paper presents the results of an experimental study to investigate the behaviour of structurally damaged full-scale reinforced concrete beams retrofitted with CFRP laminates in shear or in flexure. The main variables considered were the internal reinforcement ratio, position of retrofitting and the length of CFRP. The experimental results, generally, indicate that beams retrofitted in shear and flexure by using CFRP laminates are structurally efficient and are restored to stiffness and strength values nearly equal to or greater than those of the control beams. It was found that the efficiency of the strengthening technique by CFRP in flexure varied depending on the length. The main failure mode in the experimental work was plate debonding in retrofitted beams.
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ABSTRACT: This paper presents test results at failure of 16 reinforced concrete (RC) continuous beams with different arrangements of internal steel bars and external carbon fibre reinforced polymer (CFRP) laminates. All test specimens had the same geometrical dimensions and were classified into three groups according to the amount of internal steel reinforcement. Each group included one unstrengthened control beam designed to fail in flexure. Different parameters including the length, thickness, position and form of the CFRP laminates were investigated. Three failure modes of beams with external CFRP laminates were observed, namely laminate rupture, laminate separation and peeling failure of the concrete cover attached to the laminate. The ductility of all strengthened beams was reduced compared with that of the respective unstrengthened control beam.Simplified methods for estimating the flexural load capacity and the interface shear stresses between the adhesive and concrete at failure of beams tested are presented. Comparisons between results from experiments and those obtained from the simplified methods show that most beams were close to achieving their full flexural capacity and the longitudinal elastic shear stresses at the adhesive/concrete interface calculated at beam failure conformed to the limiting value recommended in the Concrete Society Technical Report 55.Cement and Concrete Composites. 01/2004;
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ABSTRACT: This paper investigates the flexural behaviour of reinforced concrete beams strengthened using Carbon Fibre Reinforced Polymers (CFRP) sheets. The effect of reinforcing bar ratio ρ on the flexural strength of the strengthened beams is examined. Twelve concrete beam specimens with dimensions of 150 mm width, 200 mm height, and 2000 mm length were manufactured and tested. Beam sections with three different reinforcing ratios, ρ, were used as longitudinal tensile reinforcement in specimens. Nine specimens were strengthened in flexure by CFRP sheets. The other three specimens were considered as control specimens. The width, length and number of layers of CFRP sheets varied in different specimens. The flexural strength and stiffness of the strengthened beams increased compared to the control specimens. From the results of this study, it is concluded that the design guidelines of ACI 440.2R-02 and ISIS Canada overestimate the effect of CFRP sheets in increasing the flexural strength of beams with small ρ values compared to the maximum value, ρmax, specified in these two guidelines. With the increase in the ρ value in beams, the ratios of test load to the load calculated using ACI 440 and ISIS Canada increased. Therefore, the equations proposed by the two design guidelines are more appropriate for beams with large ρ values. In the strengthened specimens with the large reinforcing bar ratio, close to the maximum code value of ρmax, failure occurred with adequate ductility.Engineering Structures. 01/2007;
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ABSTRACT: This paper presents a joint experimental–analytical investigation aimed at studying the brittle failure modes of RC members strengthened in flexure by FRP plates. Both midspan and plate end failure modes are studied. The finite element analyses are based on nonlinear fracture mechanics. The model considered the actual crack pattern observed in the tests by using a smeared and an interface crack model. This paper shows how concrete cracking, adhesive behavior, plate length, width and stiffness affect the failure mechanisms. The numerical and experimental results show that debonding and concrete cover splitting failure modes occur always by crack propagation inside the concrete.Composites Part B: Engineering. 01/2007;
OF REINFORCED CONCRETE BEAMS
USING CARBON FIBRE REINFORCED
YASMEEN TALEB OBAIDAT
Copyright © 2010 by Structural Mechanics, LTH, Sweden.
Printed by Wallin & Dalholm Digital AB, Lund, Sweden, May, 2010 (Pl).
For information, address:
Division of Structural Mechanics, LTH, Lund University, Box 118, SE-221 00 Lund, Sweden.
Department of Construction Sciences
ISRN LUTVDG/TVSM--10/3070--SE (1-76)
OF REINFORCED CONCRETE BEAMS
USING CARBON FIBRE REINFORCED
YASMEEN TALEB OBAIDAT
Denna sida skall vara tom!
This thesis details experimental work and finite element simulations of reinforced concrete
beams retrofitted with carbon fibre reinforced polymer (CFRP). The objectives of this study
were to investigate the behaviour of retrofitted beams experimentally, develop a finite
element model describing the beams, verifying the finite element model against the
experimental results and finally investigating the influence of different parameters on the
behaviour of the retrofitted beams.
The experimental tests were performed to investigate the behaviour of beams designed in
such a way that either flexural or shear failure will be expected. The beams were loaded in
four-point bending until cracks developed. The beams were then unloaded and retrofitted
with CFRP. Finally the beams were loaded until failure. The ABAQUS program was used to
develop finite element models for simulation of the behaviour of beams. The concrete was
modelled using a plastic damage model and two models, a perfect bond model and a cohesive
model, were evaluated for the concrete-CFRP interface. From the analyses the load-
deflection relationships until failure, failure modes and crack patterns were obtained and
compared to the experimental results. The FEM results agreed well with the experiments
when using the cohesive model regarding failure mode and load capacity while the perfect
bond model was not able to represent the debonding failure mode. The results showed that
when the length of CFRP increases the load capacity of the beam increases both for shear and
flexural retrofitting. FEM results also showed that the width and stiffness of CFRP affect the
failure mode of retrofitted beams. The maximum load increases with increased width.
Increased CFRP stiffness increases the maximum load only up to a certain value of the
stiffness, and thereafter it decreases the maximum load.
The financial support provided by the Erasmus Mundus External Cooperation Window Lot 3
is greatly acknowledged.
My most grateful appreciation goes to Professor Ola Dahlblom for his knowledgeable
insight and motivating words.
I also feel so lucky and blessed to have Dr. Susanne Heyden as my co-advisor. To me, she
is a role model for living and working.
A special thanks to Dr. Kent Persson for his assistance in using the finite element software
(ABAQUS). I would also like to thank everyone from Structural Mechanics.
Finally, I would especially like to thank my parents, brothers, sisters and close friends for
their love, vote of confidence and support throughout this time. I would also like to share this
moment of happiness with my father and mother.
Yasmeen Taleb Obaidat
Lund in May 2010
1.2 Aim and Scope...............................................................................
Retrofitting of Reinforced Concrete Beams
2.1 FRP Material...................................................................................
2.2 Application in Retrofitting..............................................................
3.1 Experimental Work.........................................................................
3.2 Modelling Work..............................................................................
Summary of the Papers
Conclusions and Future Work
5.2 Future Work....................................................................................
Retrofitting of Reinforced Concrete Beams using Composite
The Effect of CFRP and CFRP/Concrete Interface Models when
Modelling Retrofitted RC Beams with FEM.
Nonlinear FE Modelling of Shear Behaviour in RC Beam Retrofitted
FEM Study on the Effect of CFRP Stiffness and Width on Retrofitted
Reinforced Concrete Beam Behaviour.
Reinforced concrete structures often have to face modification and improvement of their
performance during their service life. The main contributing factors are change in their use,
new design standards, deterioration due to corrosion in the steel caused by exposure to an
aggressive environment and accident events such as earthquakes.
In such circumstances there are two possible solutions: replacement or retrofitting. Full
structure replacement might have determinate disadvantages such as high costs for material
and labour, a stronger environmental impact and inconvenience due to interruption of the
function of the structure e.g. traffic problems. When possible, it is often better to repair or
upgrade the structure by retrofitting.
In the last decade, the development of strong epoxy glue has led to a technique which has
great potential in the field of upgrading structures. Basically the technique involves gluing
steel plates or fibre reinforced polymer (FRP) plates to the surface of the concrete. The plates
then act compositely with the concrete and help to carry the loads.
FRP can be convenient compared to steel for a number of reasons. These materials have
higher ultimate strength and lower density than steel. The installation is easier and temporary
support until the adhesive gains its strength is not required due to the low weight. They can be
formed on site into complicated shapes and can also be easily cut to length on site.
This work is a study of the behaviour of concrete beams retrofitted with carbon FRP
(CFRP), using experiments and finite element modelling.
1.2 Aim and Scope
The overall aim of the present study is to investigate and improve the understanding of the
behaviour of reinforced concrete beams retrofitted with CFRP. Experimental tests were
performed to investigate the behaviour of beams designed in such a way that either flexural or
shear failure will be expected. The beams were loaded in four-point bending until cracks
developed. The beams were then unloaded and retrofitted with CFRP. Finally the beams were
loaded until failure. The ABAQUS program was used to develop finite element models for
simulation of the behaviour of beams. From the analyses the load-deflection relationships
until failure, failure modes and crack patterns were obtained and compared to the
experimental results. The models were then used to study how different parameters affect
retrofitted beam behaviour and investigate how CFRP should be applied in order to get
maximum increase of load capacity.
2 Retrofitting of Reinforced Concrete Beams
2.1 FRP Material
Fibre reinforced polymer (FRP) composites consist of high strength fibres embedded in a
matrix of polymer resin as shown in Figure 1.
Figure 1: A schematic diagram showing a typical unidirectional FRP plate.
Fibres typically used in FRP are glass, carbon and aramid. Typical values for properties of
the fibres are given in Table 1. These fibres are all linear elastic up to failure, with no
significant yielding compared to steel. The primary functions of the matrix in a composite are
to transfer stress between the fibres, to provide a barrier against the environment and to
protect the surface of the fibres from mechanical abrasion. Typical properties for epoxy are
given in Table 1.
The mechanical properties of composites are dependent on the fibre properties, matrix
properties, fibre-matrix bond properties, fibre amount and fibre orientation. A composite with
all fibres in one direction is designated as unidirectional. If the fibres are woven, or oriented
in many directions, the composite is bi- or multidirectional. Since it is mainly the fibres that
provide stiffness and strength composites are often anisotropic with high stiffness in the fibre
direction(s). In strengthening applications, unidirectional composites are predominantly used,
Figure 1. The approximate stiffness and strength of a unidirectional CFRP with a 65% volume
fraction of carbon fibre is given in Table 1. As a comparison the corresponding properties for
steel are also given.
Adhesives are used to attach the composites to other surfaces such as concrete. The most
common adhesives are acrylics, epoxies and urethanes. Epoxies provide high bond strength
with high temperature resistance, whereas acrylics provide moderate temperature resistance
with good strength and rapid curing. Several considerations are involved in applying
adhesives effectively. Careful surface preparation such as removing the cement paste,
grinding the surface by using a disc sander, removing the dust generated by surface grinding
using an air blower and carful curing are critical to bond performance.
Table 1. Typical strength and stiffness values for materials used in retrofitting, .
Material Tensile strength
2.2 Application in Retrofitting
For structural applications, FRP is mainly used in two areas. The first area involves the use of
FRP bars instead of steel reinforcing bars or pre-stressing strands in concrete structures. The
other application, which is the focus of this thesis, is to strengthen structurally deficient
structural members with external application of FRP.
Retrofitting with adhesive bonded FRP has been established around the world as an
effective method applicable to many types of concrete structural elements such as columns,
beams, slabs and walls. As an example, a highway RC bridge slab in China was retrofitted
using CFRP as shown in Figure 2(a) and a column in India was retrofitted using glass FRP
wrapping as shown in Figure 2(b), .
FRP plates can be bonded to reinforced concrete structural elements using various
techniques such as external bonding, wrapping and near surface mounting. Retrofitting with
externally bonded FRP has been shown to be applicable to many types of RC structural
elements. FRP plates or sheets may be glued to the tension side of a structural member to
provide flexural strength or glued to the web side of a beam to provide shear strength. FRP
sheets can also be wrapped around a beam to provide shear strength and be wrapped around a
column to provide confinement and thus increase the strength and ductility. Near surface
mounting consists of sawing a longitudinal groove in a concrete member, applying a bonding
material in the groove and inserting an FRP bar or strip.
Modulus of elasticity
Modulus of elasticity to
density ratio (Mm2/s2)
(a) Flexural strengthening of a highway RC bridge slab in China.
(b) Seismic retrofit of supporting columns for a cryogenic tank in Gujarat, India.
Figure 2. Examples of use of FRP in existing structures, .