Debonding of FRP EBR: from testing to design

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Chapter 1 
Introduction 
 
1.1 General 
 
Reinforced concrete (RC) is widely used as a conventional material for building in civil 
and industrial engineering construction since it was invented in 19
th 
century. Nowadays, 
the unsatisfactory behavior of existing structures is becoming one of the major problems 
faced by structural engineers. Reasons for inadequate behavior include previous errors in 
design, inferior materials, corrosion due to environmental attack, bad construction 
practices, damage due to exceptional loads. In addiction load that for what they were 
originally designed for. Depending from the circumstances, there are two possible 
solutions: demolish and rebuilt or strength the structure. 
There is a large variety of material and techniques that are available for repair of 
damaged concrete building and structures. However, the traditional strengthening and 
repair methods present a great number of practical problems. In addition, most traditional 
techniques are intrusive, time consuming and expensive. 
In recent years the development of synthetic and epoxy adhesives combined with the 
using of Fiber Reinforced Concrete (FRP) has led to a very effective technique for 
repairing or strengthening concrete structures.  
There has been an increase in the use of fiber reinforced polymer to repair and strengthen 
structural members such as slabs, beams and columns. FRP uses to increase flexural 
strength of slabs, flexural strength and shear strength of beams and shear strength and the 
confinement of columns. Another application is to enhance continuity between precast 
members at joint. The principal reasons are to use FRP: the structural members that need 
to be strengthened by FRP are subjected to more applied load, for example, increasing 
the vehicle loads on bridges, changing the use of buildings from residential to 
commercial and accidental events such as earthquakes. Therefore it is necessary to add 
the reinforcement to the weaker members. Moreover, the occurrence rate of earthquake 
has increase over the past years, structures and building suffer from lack of proper 
maintenance and also the effects of natural disasters. Hence, FRP sheets are used to
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improve the capacity of the structural members. It is preferred to repair the deteriorated 
structures rather than rebuilding in terms of economic and environment. In particularity, 
strengthened technique is simple, quick and effective. Although FRP plays an efficient 
role to strengthen the reinforced concrete members, premature failures can occur before 
the composite section reaches its ultimate capacity. Therefore, a considerable number of 
parametric studies have been conducted in order to understand the failure behavior of the 
composite of the composite sections and predict the ultimate load.   
A fiber-reinforced polymer (FRP) composite is a combination of fibers with matrix 
material. The fibers is responsible to bring the strength to the composite material. 
Whereas resins bond the fibers together, transfer the loads from FRP plate to the 
reinforced concrete member and protect the fibers from the environment. Fibers can be 
formed in three effective ways. 
- Unidirectional fibers give higher strength in the parallel direction of fibers. 
- Fibers are produced in a weave or mat shape. This approach brings the strength in 
several directions, although the strength is decreased compared to unidirectional 
fibers. 
- Chopped fibers, shot lengths of fibers are distributed randomly. Therefore, the 
strength is equal for all directions 
Fiber reinforced polymer is used as composite material to strengthen the structural 
members in shear and flexure as shown. This material has different properties which 
depend on type of fibers (carbon, glass, aramid or basalt), resin matrix (either 
thermosetting or thermoplastic). Orientation of the fibers and process of manufacturing. 
Therefore, the characteristics of FRP can be controlled to meet the design requirements. 
The choice of manufacturing process is based on the type of matrix and fibers. The most 
common methods of manufacturing are hand lay up (wet lay-up) and pultrusion. 
 
An important issue in the strengthening of concrete structures using FRP composites is to 
design against various debonding failure modes, some of which were first studied for 
concrete beams bonded with a steel plate, including: (a) cover separation; (b) plate end 
interfacial debonding; (c) intermediate (flexural or flexural-shear) crack (IC) induced 
interfacial debonding and (d) critical diagonal crack (CDC) induced interfacial 
debonding.
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The bond strength between FRP and concrete is a key factor controlling debonding 
failures of various forms in FRP-strengthened structures. As a result, extensive research 
on this topic has been carried out, in addition to earlier work concerned with steel plates 
bonded to concrete which provided a useful initial basis. The existing work has included 
experimental studies conducted using single shear tests, e.g. (Yao et Al., 2005; Ceroni 
and Pecce, 2010), double shear tests, e.g. (Pellegrino et Al., 2008) and modified bending 
tests, e.g. (Pellegrino et Al., 2008; Miller and Nanni, 1999; Gartner et. Al., 2010), 
theoretical studies using fracture mechanics analysis (Fib code, CNR guidelines), and the 
development of empirical models (Jansze 1997; Teng and Yao 2007; Oehlers et al. 
2004). 
 
1.2 Background of the problem 
 
In RC elements strengthened in bending debonding of the external FRP reinforcement 
can occur at various locations, which are named “premature failures” due to occur before 
reaching the ultimate capacity.. Surface cracks and irregularities could represent weak 
points for bond behavior, but in many cases debonding happens at the end of the external 
reinforcement, where a high concentration of normal and shear stresses occurs, with 
subsequent propagation along the beam and detachment of the concrete cover (Chen and 
Teng 2001; fib 2001). The focus of this study is on concrete cover separation. This mode 
of failure is also called concrete rip off, local shear failure or concrete delamination. 
Concrete rip off failure takes place owing to concentrating high shear and normal stresses 
at the termination of FRP plate. 
When diagonal shear cracks intersect the external reinforcement near the plate end and 
lacking internal steel stirrups, debonding initiates due to shear and normal interfacial 
stresses on the side of the diagonal crack. The crack propagates toward the plate end  
(Jansze 1997; Teng and Yao 2007; Oehlers et al. 2004) and failure happens generally in 
the concrete adjacent to the adhesive-concrete interface.  
Several theoretical formulations have been proposed to prevent end debonding (Chen and 
Teng 2001; fib 2001, CNR 2004) most of them calibrated on experimental results of 
bond tests. However, the calibration have been obtained used several test methods and
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indication has yet been provided to find out the test method that better represent the 
debonding strength.  
The literature contains a large amount of research on characterization of bond, bond 
development length, and bond fracture mechanics. Furthermore, many researchers are 
attempting to define the behavior of bond using these varied approaches with the added 
complication of bond durability. One drawback of much of the work is that the specimen 
configuration, material properties, test procedures, and data interpretation vary among 
the researchers. 
While the research has intrinsic value, an accepted standard test method for predict the 
debonding strength that could be used as a benchmark might make individual research 
results more widely comparable. Such a test method might also be more readily accepted 
and consistently used by university and commercial laboratories than methods currently 
in the literature that require specialized equipment or elaborate analysis techniques. 
 
1.3 Aims and Objectives 
 
FRP material plays an important role in improving shear and flexural capacity of the 
structural members. Premature debonding failures is predicted using several models: 
theoretical studies using fracture mechanics analysis and the development of empirical 
models. A current study is carried out to understand the reliability of debonding models. 
Another aim is to find out a standard test method to predict a debonding strength as well 
as to a better calibration of the fracture mechanic models. 
The aims of this dissertation were archived by collecting experimental data from the 
published literature and using 3-D non linear finite element method to model 
strengthened beam. 
The following objectives are archived based on analytical study and statistical analysis 
for experimental data. 
- Understanding the mechanism of the debonding failure for the strengthened 
beams. 
- Reviewing existing models which were proposed to predict load carrying 
capacity, both shear models and models based on fracture mechanic.
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- Assessing the performance of the existing bond models by comparing predicted 
failure load/stress with experimental failure load/stress. 
- Proposing recommendations to increase the accuracy of prediction. 
- Understanding the behavior of test methods to predict bond law between concrete 
and reinforcement 
- Assessing the performance of the test methods by comparing predicted failure 
load/stress using “fracture energy models” with experimental failure load/stress. 
- Proposing recommendation to increase the accuracy of test methods for the 
fracture energy models 
The following objective are achieved by using ABAQUS software. 
- Parametric study of test methods, modeled from the literature. Investigating the 
load and boundary condition the direction and trend of the stresses. 
- Stress investigation using three kinds of test methods with same geometrics and 
materials properties. 
- Proposing recommendation to carry out which test methods assess a better 
prediction of debonding strength, as well as may be used to calibrated the fracture 
energy models. 
 
1.4 Layout of the dissertation 
 
This dissertation consists of five main chapters which aim to archive the objectives 
mentioned above. 
In Chapter 1, introduction clarifies the importance of use of FRP material, advantages 
and disadvantage of FRP material and the problem that occurs in the strengthened RC 
beams. Finally, the aims and objectives of this dissertation are detailed. 
In Chapter 2, the focus is to define externally bonded reinforcement, investigate the 
properties of test methods and  investigate failure modes which occur in the strengthened 
RC beams. The study also involves understanding bond behavior and shear stress-slip 
relationship. Also the study of models based on fracture energy . Moreover, reviewing 
the existing shear strength models and design code.
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In Chapter 3, Experimental data of RC beams EBR were collected from the published 
literature in order to calculate and to compare the failure load and statistical 
characteristics such as average ratio of experimental load/stress to predict plate end 
debonding, standard deviation, coefficient of variation and coefficient of correlation for 
each model with varying the application technique. These statistical results are discussed 
to assess the performance of existing rip off models. 
Experimental data of test methods were collected in order to compared with predicted 
failure stress in the FRP using the “facture energy models”. As the in the RC beams data 
statistical procedure is proposed and discussed. 
In Chapter 4, ABAQUS software was used to model the tests methods. In the first 
analysis a parametic study was cariied out. The specimen was modeled using the 
effective material and geometric properties. In the second analysis the test methods were 
modeled with the same geometric and material properties. The test modeled were a single 
test, a double test and a bending test. An investigation of the stress and load was 
conducted. 
In Chapter 5, conclusions of this dissertation are draw with recommendations for future 
research.