Study on Bond Degradation of Rebars in Cracked
Concrete due to Bar Corrosion

SYLL AMADOU SAKHIR(Master's Program in Engineering Mechanics and Energy )Advised by Kanakubo Toshiyuki

Submitted to the Graduate School of
Systems and Information Engineeringin Partial Fulfillment of the Requirements
for the Degree of Master of Engineering
at theUniversity of Tsukuba

March 2020

ABSTRACTThis study aims to quantify the bond strength of reinforcing bars in cracked concrete. It
focuses on the more fundamental effect of the cracking itself (in absence of corrosion products).
To achieve this aim, the simulation of the cracking of concrete due to corrosion of reinforcing
bars by using an aluminum pipe embedded in concrete and filled with an expansion agent was
proposed as a novel method. With the increase of the crack width over elapsed time from thefilling of the expansion agent, a target crack width was obtained. The splitting cracks play afundamental role in reducing the bond, therefore two cases were set: “Single-split type” when
the splitting cracks are parallel or along the rebar and “Side split-type” when the splitting cracks
are perpendicular to the rebar.
Following to tension test of the ribbed aluminum pipe, pull-out test was conducted on 28
specimens divided into two categories as described above. In this study, the induced crack width
was taken as the main variable to find its influence on bond degradation.
These experiments confirmed that the expansion agent filled pipe is a promising method thatallows focusing on the cracking itself. The tensile test also shows that the strain in the axis, top
and between the ribs tended to increase over the time after the filling of the expansion agent. In
addition, at the beginning of the loading, with the increasing of the axial strain, the yieldstrength and modulus of elasticity of the pipe filled with an expansion agent also increase.
However, the elongation at failure tended to decrease. The tensile strength was not affected.
In pull-out test, all specimens experienced failure due to splitting. A group of specimens
failed by newly generated splitting crack despite existing induced crack and other specimens
failed by the opening of induced crack that the pull-out strength load reduces exponentially with
the increase of crack width and this demonstrates that the surface crack width can potentiallybe a good indicator to evaluate the bond strength degradation. The research has also shown thatthe decrease of the pull-out strength is more severe in 18MPa specimens than in 30 MPa
specimens. In addition, the deterioration of the bond due to the induced cracks was more severe
in a “Side-split type” than in “Single split-type”.
Keywords: Concrete crack width, Rebar corrosion, Crack width, Expansion agent, Bond test,
Deterioration of bond strength1

Table of Contents
Study on Bond Degradation of Rebars in Cracked Concrete due to Bar Corrosion ..
鉄筋腐食によりひび割れが生じたコンクリート中の鉄筋の付着劣化に関する研究
ABSTRACT ..................................................................................................................... 1
List of Figures .................................................................................................................. 4
List of Tables ................................................................................................................... 5
Chapter 1 Introduction ........................................................................................................ 6
1.1.

Background ............................................................................................................ 6

1.2.

Objectives of this study .......................................................................................... 8

1.3.

Methodology of this study ..................................................................................... 8

1.4.

Outline of the thesis ............................................................................................. 11

Chapter 2 Tensile properties of ribbed aluminum pipe filled with an expansion agent
12
2.1.

Introduction .......................................................................................................... 122.2. Background of crack simulation by aluminum pipe filled with an expansion
agent 12
2.3.

Experiment outline............................................................................................... 13

2.3.1. Specimen Overview ........................................................................................... 13
2.3.2. Loading and Measurement ............................................................................... 15
2.4.

Experiment results ............................................................................................... 16

2.4.1. Strain before loading due to the expansion agent .......................................... 16
2.4.2. Tensile test results ............................................................................................. 18
2.5.

Conclusions ........................................................................................................... 21

Chapter 3 Bond Degradation of Rebars in Cracked Concrete due to BAR: Single
Splitting Case ...................................................................................................................... 22
3.1.

Introduction .......................................................................................................... 22

3.2.

Experiment outline............................................................................................... 22

3.2.1. Aluminum pipe with ribs .................................................................................. 22
3.2.2. Pull-out specimen .............................................................................................. 22
3.2.3. Crack simulation by EAFP............................................................................... 24
3.2.4. Loading and measurement ............................................................................... 25
3.3.

Experiment results ............................................................................................... 27

3.3.1. Crack simulation by EAFP............................................................................... 27
3.3.2. Results of pull-out test....................................................................................... 30
3.3.2.1. Failure mode ............................................................................................... 30
3.3.2.2. Pull-out load and slip.................................................................................. 31
3.4.

Bond strength degradation.................................................................................. 332

3.4.1. Comparison of the results with the fib model code 2010 ............................... 33
3.4.2. Bond degradation and surface crack width relationship .............................. 35
3.5.

Conclusions ........................................................................................................... 37

Chapter 4 Bond Degradation of Rebars in Cracked Concrete due to Rebar corrosion:
Side-Splitting Case.............................................................................................................. 38
4.1.

Introduction .......................................................................................................... 38

4.2.

Experiment outline............................................................................................... 38

4.2.1. Pull-out specimen and materials ...................................................................... 38
4.2.1. Crack simulation by EAFP............................................................................... 41
4.2.2. Loading and measurement ............................................................................... 42
4.3.

Experiment results ............................................................................................... 44

4.3.1. Crack simulation by EAFP............................................................................... 44
4.3.1. Results of pull-out test....................................................................................... 46
4.3.1.1. Failure mode ............................................................................................... 46
4.3.1.1. Pull-out load and slip.................................................................................. 49
4.3.1.2. Crack opening versus slip .......................................................................... 52
4.4.

Bond strength degradation.................................................................................. 53

4.4.1. Comparison of the results with the fib model code 2010 and JCI 1998 ....... 53
4.4.1. Bond degradation and surface crack width relationship .............................. 54
4.5.

Conclusions ........................................................................................................... 56

Chapter 5 Conclusions ....................................................................................................... 57
DEDICACES ................................................................................................................. 58
REFERENCES .............................................................................................................. 59

3

List of Figures
Fig. 1.1 Mechanical interaction between concrete and deformed steel bar ........................ 6
Fig. 1.2 Sequences and consequences of corrosion on reinforcements[2] ........................... 7
Fig. 1.3 Case of single-splitting ................................................................................................ 9
Fig. 1.4 Case of side-splitting .................................................................................................. 9
Fig. 1.5 Schematic illustration of the methodology followed in the present study ........... 10
Fig. 2.1 Concrete cracking with expansion agent ................................................................ 12
Fig. 2.2 Specimen for cracking confirmation ....................................................................... 13
Fig. 2.3 Crack width over time .............................................................................................. 13
Fig. 2.4 Scheme of aluminum pipe with rib ......................................................................... 14
Fig. 2.5 Picture of aluminum pipe with rib .......................................................................... 14
Fig. 2.6 Position of strain gauges ........................................................................................... 15
Fig. 2.7 Loading setup ............................................................................................................ 16
Fig. 2.8 Strain and time relationship after filling the expansion agent ............................. 17
Fig. 2.9 Failure of the specimen ............................................................................................ 19
Fig. 2.10 Stress-strain relationship ....................................................................................... 20
Fig. 3.1 Aluminum pipe with ribs ......................................................................................... 22
Fig. 3.2 Specimen detail ......................................................................................................... 23
Fig. 3.3 Picture of the mold.................................................................................................... 23
Fig. 3.4 Filling of expansion agent ........................................................................................ 24
Fig. 3.5 Loading and measurement setup ............................................................................ 25
Fig. 3.6 Teflon sheet................................................................................................................ 26
Fig. 3.7 Picture of specimen at loading ................................................................................. 26
Fig. 3.8 Cracking of concrete after filling the expansion agent.......................................... 28
Fig. 3.9 Distribution of crack width ...................................................................................... 29
Fig. 3.10 Splitting with crack opening and newly generated crack ................................... 30
Fig. 3.11 Splitting with crack opening only .......................................................................... 30
Fig. 3.12 Load-slippage relationship ..................................................................................... 32
Fig. 3.13 Maximum pull-out load vs crack width ................................................................ 33
Fig. 3.14 Result in comparison with fib Model Code 2010 ................................................. 34
Fig. 3.15 Result and prediction model .................................................................................. 35
Fig. 4.1 Specimen details ........................................................................................................ 39
Fig. 4.2 Specimen mold .......................................................................................................... 39
Fig. 4.3 Filling of the expansion agent .................................................................................. 41
Fig. 4.4 Loading and measurement setup ............................................................................ 42
Fig. 4.5 Picture of specimen at loading ................................................................................. 43
Fig. 4.6 Teflon sheet................................................................................................................ 43
Fig. 4.7 Cracking of concrete after filling the expansion agent.......................................... 44
Fig. 4.8 Distribution of crack width ...................................................................................... 45
Fig. 4.9 Failure mode.............................................................................................................. 47
Fig. 4.10 Damage inside the bond part ................................................................................. 48
Fig. 4.11 Pull-out load versus slip at free-end for 18 MPa specimens .............................. 50
Fig. 4.12 Pull-out load versus slip at free-end for 30 MPa specimens .............................. 51
Fig. 4.13 Maximum pull-out load versus crack width before loading ............................... 52
Fig. 4.14 Crack opening versus slip ...................................................................................... 52
Fig. 4.15 Comparison with fib model 2010 and JCI 1998 ................................................... 53
Fig. 4.16 Result and prediction model .................................................................................. 54
Fig. 4.17 Proposed evaluation formula from test result...................................................... 55
4

List of Tables
Table 2.1 Dimension of aluminum pipe with rib ................................................................. 14
Table 2.2 List of specimens .................................................................................................... 14
Table 2.3 Strain before loading ............................................................................................. 16
Table 2.4 Tensile test results.................................................................................................. 19
Table 3.1 Concrete proportion of used concrete ................................................................. 23
Table 3.2 Concrete mechanical properties ........................................................................... 24
Table 3.3 Specimen list........................................................................................................... 24
Table 3.4 Crack level .............................................................................................................. 29
Table 3.5 Maximum crack width .......................................................................................... 29
Table 3.6 Test result list ......................................................................................................... 31
Table 3.7 fib model code 2010 ............................................................................................... 34
Table 3.8 Regression analysis result ..................................................................................... 35
Table 3.9 Comparison of the calculated and experimental values .................................... 36
Table 4.1 Specimen list........................................................................................................... 40
Table 4.2 Concrete mix proportion ...................................................................................... 40
Table 4.3 Concrete mechanical properties ........................................................................... 40
Table 4.4 Mechanical properties of reinforcement ............................................................. 40
Table 4.5 Maximum crack width .......................................................................................... 46
Table 4.6 Test result list ......................................................................................................... 48
Table 4.7 Regression analysis result ..................................................................................... 54

5

Chapter 1 Introduction
1.1.
BackgroundTo guarantee the composite action of reinforced concrete members, the bond at the steelconcrete interface is an important property of reinforced concrete. It allows the forces to betransferred from the reinforcement to the surrounding concrete in a structure. The scientists who
have contributed to the knowledge of the many aspects of bonding agree that the bond stressincludes three components: chemical adhesion, friction, and mechanical interlock (see Fig. 1.1)
[1]. When the rebar is in tension, the concrete crushing and splitting occur due to mainly themechanical interlock. It is considered that the bond between reinforcement and concrete has alarge influence on the structural behavior of RC members. Therefore, to get satisfactory
performance of RC structures, it is necessary to assure adequate bond.
However, the deterioration of RC structures due to the corrosion of rebar is now becoming
a demanding problem.

Fig. 1.1 Mechanical interaction between concrete and deformed steel bar
In recent decades, lots of RC structures are threatened by corrosion worldwide, causing huge
direct and indirect costs annually. As shown in Fig. 1.2 [2], bond deterioration can negatively
affect the structural properties of RC structures. This was confirmed in many studies. To better
estimate the mechanical properties of corroded RC structures through theoretical analysis or
numerical modeling, it is essential to fully understand how corrosion affects bond.
Many studies investigated the degradation of bond strength due to the corrosion ofreinforcement which causes cracks along bars. Relevant bond tests were carried out by many
researchers, these studies have developed numerical models which are in good agreement withtest results [3] [4] [5].
However, even if these current models can give a good prediction, there are still some
limitations when they are put into practice. One of the most significant current discussions isthat these models mostly use the steel section loss or mass loss as a variable, which is very
difficult to measure in existing structures that are under services conditions. Regarding thecorrosion of rebars in reinforced concrete, the surface crack width provides the clearest visual
manifestation. It is a key parameter to solve this problem by directly correlating the bonddeterioration of the surface crack width because it can be easily measured. A rough correlation
between bond strength reduction and surface crack width is proposed in the fib Model Code2010 [6]. On another hand, most studies use accelerated electrical corrosion techniques. The
combination of the effect of cracking and the formation of rust around the bar can lead todifficulties in analyzing the processes at a fundamental level. Also, when concrete cracks
6

without corrosion, those results could be difficult to use.
Very little is currently known about the potential correlations between the bond and thesurface crack width. However, further investigations with this respect should be performed togive more experimental evidence.

Fig. 1.2 Sequences and consequences of corrosion on reinforcements[2]

7

1.2.
Objectives of this study
The major aim of this study is to increase the knowledge of the bond behavior in reinforced
concrete structures with cracks due to corrosion, as well as investigating the possible links
between visual inspection data (crack width) and bond degradation. To achieve this goal, there
are some specific objectives which relate to the different stages of the research project:• Testing the tensile properties of ribbed aluminum pipe filled with an expansion agent• Investigating the bond degradation with different crack width along the reinforcing
bars.
• Investigating the bond behavior with different crack width occurring on thesurrounding concrete

1.3.

Methodology of this study

The scientific approach of this study has been primarily based on experiments andcorresponding observations. The simulation of the cracking of concrete due to corrosion ofreinforcing bars by using an aluminum pipe embedded in concrete and filled with an expansion
agent was used as a novel method to simulate reinforcing bars in which volume expands due tocorrosion. Throughout this thesis, the abbreviation ‘EAFP’ is used to refer to the expansion
agent filled pipe. With the increase of the crack width over elapsed time from filling of
expansion agent, a target crack width can easily be obtained.
First, for grasping the mechanical properties of EAFP, tensile tests of the aluminum pipes
with ribs are carried out. In addition, two series of experiments are carried out to study the effectof induced crack width on bond degradation. The one end pull-out test is conducted with a shortbond length designed to enable the focus on local bond behavior. The splitting cracks plays afundamental role of reducing the bond, therefore two cases are set: “Single-split type” when the
induced cracks are along the rebar (Fig. 1.3) and “Side split-type” when the induced cracks arelocated on the surroundings of concrete (Fig. 1.4). The simulated cracks on specimens aremeasured before the tests. During the test, the pull-out load, slippage at the free end of the rebar
and the opening of the width of the induced crack are measured. These data are compared tothose in the existing literature. Regression analysis is also conducted to propose a simple modelfor evaluating the bond degradation with crack width as the main variable. Fig. 1.5 shows aschematic illustration of the methodology followed in this study.

8

Fig. 1.3 Case of single-splitting

Fig. 1.4 Case of side-splitting

9Formulating the research question
How the cracking of concrete due to corrosion can affect the bond in
reinforced concrete structures?

Review of publishedexperimental studies

Literature study(state-of-art)

Identification ofknowledge gaps

PHASE 1PHASE 2

Keywords
objectives






Tensilebehavior ofribbed EAFP
Expansion
agent
Aluminum
pipe with ribs
Filling ofspecimens
Induced cracks

One end Pull-out experiments

Design ofexperiment

Simulation of concrete cracking due corrosion
rebarsDefinition of objectives

Keywords
objectives






Pull-out load
Slippage
InducedcrackspeningLoadingstagesBondbehavior

Evaluation of experimental studies
Analytical models
Data analysis
Comparison withprevious research

PHASE 3

Regression analysis

Interpretation of
results

New findings

Fig. 1.5 Schematic illustration of the methodology followed in the present study

10

1.4.

Outline of the thesis

The thesis consists of 5 chapters. In Chapter 1, the background, objectives, and methodologyof the study are provided. In order to grasp the mechanical properties of ribbed aluminum pipes
filled with an expansion agent when the pipe replaces reinforcing bars, the tensile tests of thosepipes are carried out, therefore Chapter 2 presents the tensile properties of ribbed aluminum
pipe filled with an expansion agent. Chapter 3 present the pull-out study of the Single-split
specimen when the splitting cracks are parallel or along the rebar. Chapter 4 outlines the pullout experience in case of “Side split”, in other words when the splitting cracks areperpendicular to the rebar. Conclusions and suggestions for future research are placed in
Chapter 5.

11

Chapter 2 Tensile properties of ribbed aluminum pipe
filled with an expansion agent
2.1.
Introduction
The possibility to simulate the cracking of concrete due to corrosion of reinforcing bars in
the laboratory in a relatively short time by using EAFP was demonstrated. In this chapter, inorder to grasp the mechanical properties of EAFP when the pipe replaces reinforcing bars,
tensile tests of those pipes are carried out.
2.2.
Background of crack simulation by aluminum pipe filled with an
expansion agent
Many studies have been conducted about the effects of reinforcing bar corrosion on the
structural performance of RC members using experiments that simulate cracks due to corrosion
expansion of reinforcing bars by an electrolytic corrosion test or by slits in the previous research,
but the conformity to real cracks is unclear. At the Kanakubo lab, in the University of Tsukuba,
an aluminum pipe embedded in concrete and filled with an expansion agent has been proposed
as a method to simulate reinforcing bars whose volume expands due to corrosion.
An expansion agent is mainly used for the destruction of rocks and RC structures. In powder
form, it expands when humified. Due to this expansion, cracks are generated in the concrete(see Fig. 2.1).
Fig.2.2 shows the specimen used in order to confirm the possibility of crack simulation [7].
An aluminum pipe with 18 mm as outer diameter and 1 mm thickness was embedded in theconcrete. The compressive strength of the concrete was 25.3 MPa and the ratio of the water toexpansion agent was 30%. The specimen was placed as the axial direction of the pipe was set
vertically, and an expansion agent was filled from the top of the pipe
As can be seen in Fig.2.3, the possibility to simulate the cracking of concrete due to corrosionof reinforcing bars in the laboratory in a relatively short time was confirm. Also, it was foundthat the crack width can be easily targeted due to the fact that cracks width increases over time.

Fig. 2.1 Concrete cracking with expansion agent

12Fig. 2.2 Specimen for cracking confirmation

Crack Width(mm)0.3

18-15-3-Top
18-15-3-Center
18-15-3-Bottom

0.2
0.1
0
0

50

100
Time(h)Fig. 2.3 Crack width over time

2.3.

Experiment outline
2.3.1.Specimen Overview
According to JIS G 3112 (Steel bars for concrete reinforcement), the ribs were machined on
an aluminum pipe. In order to simulate a deformed bar D19 with an aluminum pipe with anouter diameter of 21.7mm and a thickness of 2.5mm was used. The details of the aluminum
pipe with ribs are shown in Fig.2.4 and Table 2.1. Table 2.2 summarizes the list of specimens.
The test was conducted on specimen filled with an expansion agent or not. For specimens
without expansion agent, three with ribs and three without ribs were set. 5 specimens with ribs
were filled with an expansion agent and a target strain before loading was taken as the main
parameter.

13

Fig. 2.4 Scheme of aluminum pipe with rib

Fig. 2.5 Picture of aluminum pipe with rib
Table 2.1 Dimension of aluminum pipe with rib
Pipe diameter(d)
(mm)
21.70

Height of ribs (h)
(mm)
1.50

Interval between ribs(l)
(mm)
13.40

h/l

h/d

l/d

0.11

0.07

0.61

Table 2.2 List of specimens
Specimen

Ribs

Expansion
Agent

Target strain
beforeloading－

Number of specimens

P19-NRNF

No

No

P19-RNF

Yes

－

3

0.3%

1

P19-RF2

0.5%

1

P19-RF3

0.8%

1

P19-RF4

1.0%

1

P19-RF5

1.2%

1

P19-RF1

Yes

14

3

2.3.2.Loading and Measurement
As shown in Fig.2.6, strain gauges were fixed, and the time course of axial strain after
expansion agent filling and circumferential strain on and between ribs were measured. The
variable factor was the target axial strain at the start of the tensile test, with a range of 0.3% to
1.2%.
Monotonic tensile loading was carried out using a universal testing machine of 500 kN
capacity. Fig.2.7 shows the loading and measurement method for tension tests. To prevent the
deformation of the grasped part by the chuck, round steel bars having a length of 80 mm wereinserted into the specimen ends. In addition to the load, the axial and circumferential strain were
measured.

With ribs

Without rib
Fig. 2.6 Position of strain gauges

15

Fig. 2.7 Loading setup

2.4.

Experiment results
2.4.1.Strain before loading due to the expansion agent
Table 2.3 summarizes the strain due to the effect of the expansion agent before the loading.
For the specimen P19-RF5, due to the peeling of gauge during measurement after filling the
expansion agent, the circumferential strain was not measured. Fig.2.8 shows the progression ofeach strain after filling the expansion agent. The temperatures in the figure were obtained fromthe Japan Meteorological Agency located in Tsukuba city. The axial strain and circumferential
strain increased both over time, and the inter-rib circumferential strain was larger. The speed of
the reaction sharply increases between 4 to 10 hours after filling the expansion agent and tends
to slow down. 4 hours after filling the expansion agent, the strain evolution is greatly affectedby the temperature. This phenomenon is remarkably observed when the temperature exceeded
15 ° C.
Table 2.3 Strain before loading
Specimen Axial strain (%)

P19-RF1
P19-RF2
P19-RF3
P19-RF4
P19-RF5

0.297
0.521
0.799
0.892
1.23

Circumferential strain on top of
the rib (%)
0.088
0.328
1.04
1.21
－*

Inter-ribs
Circumferential strain
(%)
0.664
1.13
1.93
2.54
－*

* Due to the peeling of gauge during measurement after filling the expansion agent, thesestrains were not measured.

16

Fig. 2.8 Strain and time relationship after filling the expansion agent

17

2.4.2.Tensile test results
The tensile test results are shown in Table 2.4. Fig.2.9 shows examples of the fracture of thespecimen after loading.
The tensile stress was calculated dividing the load by the cross-sectional area of thealuminum pipe. For the specimen without ribs, the cross-section is 150.8 mm2 and 55.6 mm2for one with ribs.
Fig.2.10 shows the stress-strain relationship of the loaded specimen. The yield strength of
specimens without expansion agent (P19-NRNF and P19-RN) was taken when the stressbecomes almost constant just after the elastic region. The yield strength of the other test
specimens was obtained by 0.2% offset strength from the axial strain.
The modulus of elasticity was calculated by the least-squares method in a section that can be
regarded as the elastic zone in the stress-strain curve. Similarly, the absolute value of Poisson’s
ratio was calculated using the inter-ribs strain. It was obtained by dividing the elastic modulus
by the lateral elastic modulus. Two gauges points were marked at the specimen before loading.
The percentage of elongation after failure was determined by dividing the deformation distanceby the initial distance before loading.
According to the result, there is no evidence that the presence of ribs and the filling of an
expansion agent has an influence on the tensile strength. For all specimens, the tensile strengthis from 210 to 220 MPa.
On another hand, for the specimen without an expansion agent, no significant differences inyield strength and elastic modulus are found between the specimens with ribs and ones without
ribs.
At the beginning of the loading, with the increasing of the axial strain, the yield strength andmodulus of elasticity of the pipe filled with an expansion agent increase too. However, theelongation at failure tended to decrease. The tensile strength was not affected.
It can be seen that the presence of the ribs tends to decrease the Poisson’s ratio. A possible
explanation for this might be that the occurrence of restraint in the circumferential direction at
the ribs decreases the transversal elastic modulus between ribs.

18

Table 2.4 Tensile test results
TensileMax load
Specimenstrength
(kN)
(MPa)

Yield
strength(MPa)

Young Poisson’s ratio(%)
modulus(GPa)

Elongation(%)

P19-NRNF

33.02

219

96.7

74.4

0.341

－

P19-RNF

12.00

216

102

78.2

0.247

8.5

P19-RF1

11.78

212

97.5

64.0

0.239

7.6

P19-RF2

11.67

210

121

63.6

0.226

7.0

P19-RF3

11.56

208

146

84.0

－

5.9

P19-RF4

11.84

213

158

70.2

0.162

5.3

P19-RF5

12.34

222

170

95.7

－

3.9

Fig. 2.9 Failure of the specimen

19

STRESS (MPa)200

P19-RF1
P19-RF2
P19-RF3
P19-RF4
P19-RF5

100

0

1

2

3

STRAIN (%)
STRESS (MPa)

200

100
P19-RNF-1
P19-RNF-2
P19-RNF-3
0

1
2
STRAIN (%)

3

STRESS (MPa)

200

100
P19-NRNF-1
P19-NRNF-2
P19-NRNF-3
0

1

2

STRAIN (%)
Fig. 2.10 Stress-strain relationship

20

3

2.5.
Conclusions
The aim of this chapter was to examine the mechanical properties of ribbed aluminum pipes
filled with an expansion agent when the pipe replaces reinforcing bars. The experimentconfirmed that: The strain in the axis, top and between the ribs tended to increase over the time after thefilling of the expansion agent.
 There is no evidence that the presence of ribs and the filling of an expansion agent has
an influence on the tensile strength.
 The presence of the ribs tends to decrease the Poisson’s ratio At the beginning of the loading, with the increasing of the axial strain, the yield strengthand modulus of elasticity of the pipe filled with an expansion agent increase too.
However, the elongation at failure tended to decrease. The tensile strength is notaffected.

21

Chapter 3 Bond Degradation of Rebars in Cracked
Concrete due to Rebar Corrosion: Single Splitting
Case
3.1.
Introduction
In this chapter, to develop a simple formula for the bond strength of a corroded bar withsurface crack width as a variable, the pull-out test is conducted on a concrete block with anembedded bar following different corrosion crack width simulated by EAFP. The test is carriedout on a single split type specimen as described previously. One advantage of using thissimulation method is that it allows focussing on the effect of the cracking while ignoring thesection loss.
A rough correlation between bond strength reduction and surface crack width is suggestedby the fib Model Code 2010. In addition to that Japan Concrete Institute in the concrete structure
rehabilitation research committee report 1998 gave an evaluation formula of the bonddegradation as a function of corrosion, also an overview of the current research indicates thatexisting studies have achieved a primary knowledge regarding the potential correlation between
bond and the surface crack width. However, there still a research gap in this respect.
3.2.

Experiment outline
3.2.1.Aluminum pipe with ribs
Fig. 3.1 shows an overview of a processed aluminum pipe with ribs set according to JIS G
3112. An aluminum pipe with 21.7 mm as outer diameter and 2.5mm thickness was used toimitate the D19 rebar. The fundamental properties of the aluminum pipe and the expansion
agent filled pipe have been reported in Chapter 2.

Fig. 3.1 Aluminum pipe with ribs
3.2.2.Pull-out specimen
The pull-out specimen was designed as shown in Fig.3.2. The dimensions of the specimen
were 260×260×82mm and the aluminum pipe with ribs was embedded at 38mm from thespecimen side. The framework is shown in Fig. 3.3. The bond length of 51.6mm was chosen to
avoid the rupture of the pipe by tensile force. Moreover, an unbonded part was set to avoid cone
failure of concrete. The M6 coupler was fixed 100×100 mm at the up left position to set a LDVT
for measuring the slip of pipe. A π-type displacement transducer was placed on the concrete
cover to measure the opening of crack during loading. Table 3.1 shows the mix proportion ofused concrete and Table 3.2 shows the mechanical properties of concrete obtained from theconcrete cylinder test on the day of filling of expansion agent. Table 3.3 shows the list of
specimens. The test variable was set on the level of crack width induced by expansion agent
22

filled pipe before pullout loading. A total of 12 specimens were tested.

Fig. 3.2 Specimen detailFig. 3.3 Picture of the mold
Table 3.1 Concrete proportion of used concrete
Water-cement
ratio (%)
W/C

Cement(kg/m3)Water(kg/m3)Air

Fine
Aggregate(kg/m3)CoarseAggregate(kg/m3)

82.5

242

200

4.5 %

937

827

23

Water
Reducing
Agent(kg/m3)
2.42

Table 3.2 Concrete mechanical properties
Compressive strength(MPa)

Young’s modulus(GPa)

Splitting tensile strength
(MPa)

17.8

22.8

1.78

Table 3.3 Specimen list
Specimen
name
S-L1

Cover
Thickness

Expectedcrack level
Level 1

Number of
specimens

S-L2
S-L3

38mm

Level 2
Level312

3.2.3.Crack simulation by EAFP
The ratio of the water to an expansion agent was set to 30%. The specimen was placed as theaxial direction of the pipe was set vertically, and an expansion agent was filled from the top of
the pipe as shown in Fig.3.4. With the increase of the crack width over elapsed time after fillingof expansion agent, a target crack width was easily obtained by using the time after filling as aparameter.
Filling of expansion agent

Fig. 3.4 Filling of expansion agent

24

3.2.4.Loading and measurement
The loading was carried out at Kanakubo lab in the University of Tsukuba. Fig. 3.5 shows
the general set-up for the pull-out test. The specimen was placed on the Teflon sheet and theloading plate on which the hole with the same diameter corresponding to concrete cover inorder to not restrict the lateral deformation of concrete. This detail can be seen from Fig 3.6 andFig.3.7 The pipe was subjected to monotonic pull-out loading at a speed of 0.5mm/min with a
Universal Testing Machine. The measurement items are pull-out load, crack opening andslippage of the pipe at the free end.

Fig. 3.5 Loading and measurement setup

25

Fig. 3.6 Teflon sheet

Fig. 3.7 Picture of specimen at loading

26

3.3.

Experiment results
3.3.1.Crack simulation by EAFP
Fig. 3.8 shows an example of crack patterns after filling the expansion agent. The expansion
agent reaction was heavily influenced by the ambient temperature, so to control the width ofthe crack, specimens were placed in variable temperature conditions. Also, because cracks
continued to grow after filling the expansion agent, crack width was measured only whengrowth had stopped fully.
Cracks were categorized according to their width, as shown in Table 3.4. The maximum crack
width in every specimen is summarized in Table 3.5. To better observe the distribution of
cracking among the specimens, the maximum cracks widths are plotted in Fig 3.9.

27

(a) Top view(b) Bottom view(c) Side viewFig. 3.8 Cracking of concrete after filling the expansion agent

28

Table 3.4 Crack level

Table 3.5 Maximum crack width

Level

Crack width
range

SpecimennameMaximum crack
width (mm)Level 1

≤ 0.5mm

S-1-L1

0.15

Level 20.5mm to 1.
0mm

S-2-L1

0.1

Level 3

> 1. 0mm

S-3-L1

0.2

S-4-L1

0.35

S-2-L2

0.5

S-1-L2

1.0

S-1-L3

1.5

S-2-L33.0

S-3-L32.0

S-4-L3

1.8

S-5-L32.0

S-6-L3

2.6

Specimen nameexplanation

Cracked specimen

Crack width (mm)32
10Specimen
Fig. 3.9 Distribution of crack width29

3.3.2.Results of pull-out test
3.3.2.1.Failure mode
All specimens experienced failure due to splitting. A group of specimens failed by newly
generated splitting crack despite existing longitudinal crack due to corrosion as shown in
Fig.3.10 and other specimens failed by widening of existing crack induced by the expansion
agent as can be seen in Fig.3.11. The results of pull-out tests are summarized in Table 3.6.

Before loading

After loading
Fig. 3.10 Splitting with crack opening and newly generated crack

Before loading

After loading
Fig. 3.11 Splitting with crack opening only30

Table 3.6 Test result list
Specimen
nameBiggestcrack width(mm)

S-1-L1At Maximum
Load
(kN)

Slippage(mm)

Remarks

0.15

7.99

0.088

New crack

S-2-L1

0.1

8.39

0.146

Crack opening

S-3-L1

0.2

6.96

0.218

New crack

S-4-L1

0.35

5.79

0.144

New crack

S-2-L2

0.5

6.14

0.198

New crack

S-1-L2

1.0

6.99

0.890

New crack

S-1-L3

1.5

4.11

0.262

Crack opening

S-2-L3

3.0

2.12

0.804

Crack opening

S-3-L3

2.0

3.77

0.572

Crack opening

S-4-L3

1.8

4.69

0.658

Crack opening

S-5-L3

2.0

3.92

0.768

Crack opening

S-6-L3

2.6

2.30

1.380

Crack opening

3.3.2.2. Pull-out load and slip
The pull-out load versus slippage relationships are shown in Fig. 3.12. In the case of Level 3
cracks, the load – slippage relationship tends not to results in a severe decrease of load after
maximum pull out force. The maximum pull-out load versus crack width relationship is shownin Fig. 3.13. It can be seen that the maximum load decreases as the crack width increases. As
expected, there is a significant correlation between residual bond strength and induced crack
width.
To better observe the degradation due to the induced crack, the maximum pull-out load isnormalized by calculated pull-out splitting strength as reported in the previous study [8] as a
non-cracked specimen using the following equation:

τb,max = 0.601⋅σt ⋅(ru/db) ⋅ cotα
where, τb,max : bond splitting strength, σt: splitting tensile strength of concrete, ru:C+db/2,
db: diameter of the pipe (19mm), C: the thickness of cover concrete, α: the angle between thelongitudinal axis and splitting force (=34 degree).

31

S-1-L1(0.15mm)
S-2-L1(0.10mm)
Calculated value

Load (kN)

Load (kN)

10

5

5

0

S-3-L1(0.20mm)
S-4-L1(0.35mm)
Calculated value

10

0

1
2
Free-end slip(mm)

3

0

0

1
2
Free-end slip(mm)3Level 110

S-1-L2(0.5mm)
S-2-L2(1.0)
Calculated value
New Crack

Load (kN)

8
6
4
2
0

0

1
2
Free-end slip (mm)3

Level 2

S-1-L3(1.5mm)
S-4-L3(1.8mm)
S-5-L3(2.0mm)
Calculated value

10

8

4

Load (kN)

Load (kN)

8

2

2

6

0

S-2-L3(3.0mm)
S-3-L3(2.0mm)
S-6-L3(2.6mm)
Calculated value

10

6
4

0

1
2
Free-end slip(mm)

3

0

0

1
2
Free-end slip(mm)Level 3Fig. 3.12 Load-slippage relationship32

3

Maximum load (kN)

10
8
6
4
2
0

1

2

Crack width (mm)3Fig. 3.13 Maximum pull-out load vs crack width

3.4.

Bond strength degradation

3.4.1.Comparison of the results with the fib model code 2010
In the fib Model Code 2010[6], the reduction in bond strength depending on the surface crack
width was introduced. For a certain range of surface crack width, the possible variation of thepull-out load degradation can be predicted as shown in Table 3.7. The first development of
cracks at corrosion penetrations (i.e. reductions in rebar radius) of between 0.015mm and
0.040mm was reported. Also, a relationship between crack width and corrosion penetration as
corrosion progressed was derived.
In this study, the proposed relationship is compared with the test results in Fig. 3.14. The
prediction with Model Code 2010 is in good agreement with Level 1 (crack width ≤ 0.5mm)
and for the specimen with 2.6mm and 3mm as crack width as an extrapolation. However, forLevel 2 (crack width 0.5mm to 1.0mm), the fib Model Code 2010 gives underestimation. Itshould be noted that the authors of the fib model code 2010 assume that the residual strengthof concrete structures is also affected by cross-section loss of steel. In this study, only on theinfluence of the induced crack in the concrete cover is focused on. Only the bond degradationof level 2 specimens was higher than the prediction from the fib Model Code. This finding was
unexpected and suggests that when the crack width is between 0.5mm to 1 mm, the influenceof the bar profile change becomes greater. Interestingly, Yang et al (2019) [9] found that
corrosion crack in concrete is concluded to be a more dominant factor than the corroded rebar
shape and rust accumulation in bond deterioration mechanism.

33

Table 3.7 fib model code 2010
Residual capacity %Corrosion
penetration(mm)

Equivalentsurface crack
(mm)

0.05
0.1
0.25
0.05
0.10
0.25

Confinement

0.2-0.4
0.4-0.8
1.0-2.0
0.2-0.4
0.4-0.8
1.0-2.0

No links

Links

1

Bar type
Ribbed

Plain

50-70
40-50
25-40
95-100
70-80
60-75

70-90
50-60
30-40
95-10095-10090-10Present study
Model Code 2010: upper limit
Model Code 2010: lower limit

Normalized load

0.8
0.6
0.4
0.2
0

1

2

Crack width (mm)3Fig. 3.14 Result in comparison with fib Model Code 201034

3.4.2.Bond degradation and surface crack width relationshipAs mentioned in the literature review, the existing models related to bond deterioration focus
on the relationship between bond strength and mass loss, and they are mostly formulated aslinear, exponential or logarithmic equations. The present study proposes a simple formula forthe bond strength of corroded bar with surface crack width as a variable and thus contributes toaccurately assess the effects of deterioration.
As can be seen from the results of regression analysis in Table 3.8, an exponential equation
provides the best fit (higher R2). Therefore, the following equation can be used for theprediction of the maximum pull-out load:

P(Wcr) = P0 ⋅ e-0.515Wcr
where P(Wcr): Pull-out strength in cracked concrete; P0: Pull-out strength of specimen
without crack; Wcr: Crack width.
Fig 3.15 shows the experimental result and the prediction formula comparison. The
calculated and experimental results are compared in Table 3.9. The ratios of the experimental
to calculated values are close to one, which indicates that the concrete crack width canpotentially be a good indicator to characterize bond strength degradation.
Table 3.8 Regression analysis result
Analysis

Equations

R2

Linear

-0.306Wcr+1

0.66

Logarithmic

0.175ln(Wcr) + 0.4916

0.80Exponential

e-0.515 Wcr

0.91

1

Present study
Prediction model

Normalized load

0.8
0.6
0.4
0.2
0

1

2

Crack width (mm)Fig. 3.15 Result and prediction model

35

3

Table 3.9 Comparison of the calculated and experimental values
Specimen Biggest Experimental Calculated Exp /Cal
name
crack
maximum
maximum
width
load
load
(mm)
(kN)
(kN)
S-1-L1
0.15
7.99
8.93
0.89
S-2-L1
0.1
8.39
9.16
0.91
S-3-L1
0.2
6.96
8.70
0.80
S-4-L1
0.35
5.80
8.06
0.72
S-2-L2
0.5
6.14
6.72
0.91
S-1-L2
1.0
6.99
7.46
0.94
S-1-L3
1.5
4.11
3.81
1.07
S-2-L3
3.0
2.12
2.05
1.03
S-3-L3
2.0
3.77
3.44
1.09
S-4-L3
1.8
4.69
5.20
0.90
S-5-L3
2.0
3.92
3.44
1.13
S-6-L3
2.6
2.30
2.52
0.90

36

3.5.

Conclusions

In this chapter, the pull-out test is conducted on 12 concretes block with an embedded bar
following different corrosion crack width simulated by EAFP. The test is carried out on a single
split type.
The prediction with Model Code 2010 is in good agreement with Level 1 (crack width ≤0.5mm) and for the specimen with 2.6mm and 3mm as crack width as an extrapolation.
There was a significant exponential correlation between pull-out strength and surface crack
width. Also, a simple formula to predict bond degradation is proposed by using the surface
crack width as the main variable.

37

Chapter 4 Bond Degradation of Rebars in Cracked
Concrete due to Rebar Corrosion: Side-Splitting Case
4.1.

Introduction

In this chapter, a concrete cracked by EAFP in bond test specimens has been designed to failwith side splitting of the cover. Those specimens are subjected to a pull-out test and a
relationship between the maximum pull-out load of bar and surface crack width as a variable isdiscussed.
4.2.
Experiment outline
4.2.1.Pull-out specimen and materials
The pull-out specimen was designed as shown in Fig.4.1. The dimensions of the specimen
were 260×170×170mm and the D19 rebar is embedded at 47.5mm (2.5db) from the specimen
cover side. The framework is shown in Fig. 4.2. A short bond length of 4 times the diameter ofthe rebar equal to 76 mm was chosen to focus on the local bond. To simulate the cracking of
the surrounding concrete of the rebar, 2 aluminum pipes with a diameter of 22mm and a
thickness of 1mm were set to 50mm from the rebar. Moreover, an unbonded part was set to
avoid cone failure of concrete. The M6 coupler was fixed 100×50 mm at the upright position
to set a LDVT for measuring the slip of the free end of the bar. Two π-type displacementtransducers were placed on the top and two others on the east and west side of the specimen tomeasure the opening of crack during loading. Table 4.1 shows the list of the specimens. Table
4.2 shows the mix proportion of used concrete and Table 4.3 shows the mechanical propertiesof concrete obtained from the concrete cylinder test on the day of filling of expansion agent.
The reinforcement is a deformed bar of a nominal diameter of 19 mm. Its mechanical properties
are presented in Table 4.4.

38

Fig. 4.1 Specimen details

Fig. 4.2 Specimen mold39

Table 4.1 Specimen list
SeriesSpecimennameConcrete target
strength

Inducedcrack

Number of
specimens

No

2

Level 1 - 3

6

No

2

Level 1 - 3

6

S.18.NC

I

18MPa

S.18.C.

S.30.NC

II30MPa

S.30. C

Table 4.2 Concrete mix proportion
Concretetargetstrength
18MPa
30MPa

Unit weigth (kg/m3)

W/C
(%)

C

W

S

G

Ad

78.5
56.0

245
321

192
180

942
846

852
918

2.45
3.21

Table 4.3 Concrete mechanical properties
Concretetarget strength

Compressive
strength(MPa)

Young modulus(GPa)

Splitting strength
(MPa)

18MPa

21.8

19.0

2.26

30MPa

31.7

21.6

2.59

Table 4.4 Mechanical properties of reinforcement
Nominaldiameter

Tensile strength(MPa)

Yield strength(MPa)

Young modulus
(GPa)

19

535

366

193

40

4.2.1.Crack simulation by EAFP
The ratio of the water to the expansion agent was set to 30%. The specimen was placed as
the axial direction of the pipes was set vertically, and an expansion agent was filled from the
top of the pipes as shown in Fig.4 .3.

Fig. 4.3 Filling of the expansion agent

41

4.2.2.Loading and measurement
The loading was carried out at Kanakubo lab in the University of Tsukuba. Fig. 4.4 shows
the general set-up for the pull-out test. The specimen was placed on the Teflon sheet and theloading plate on which the hole with the same diameter corresponding to concrete cover inorder to not restrict the lateral deformation of concrete. This detail can be seen from Fig 4.5 andFig.4.6. The D19 rebar was subjected to monotonic pull-out loading at a speed of 0.5mm/minwith a Universal Testing Machine. The measurement items are pull-out load, crack opening andslippage of the D19 rebar at the free end.

Fig. 4.4 Loading and measurement setup

42

Fig. 4.5 Picture of specimen at loading

Fig. 4.6 Teflon sheet

43

4.3.

Experiment results
4.3.1.Crack simulation by EAFP
Fig. 4.7 shows an example of crack patterns after filling the expansion agent. The expansion
agent reaction was heavily influenced by the ambient temperature, so to control the width ofthe crack, specimens were placed in variable temperature conditions. Also, because cracks
continued to grow after filling the expansion agent, crack width was measured only when thetarget crack width was attained.
The maximum crack width on the top side (bottom side at the filling) in every specimen issummarized in Table 4.5. To better observe the distribution of cracking among the specimens,
the maximum cracks widths was are plotted in Fig 4.8.

Top
de

West
side

Fig. 4.7 Cracking of concrete after filling the expansion agent

44

Crack width (mm)18MPa specimen
30MPa specimen

2

10Specimen
Fig. 4.8 Distribution of crack width45

Table 4.5 Maximum crack width
Specimen
nameCrack width (mm)Top side

East side

West side

S.18.NC.1

000

S.18.NC.2

000

S.18.C.0.08

0.08

0.40

0.40

S.18.C.0.2

0.20

0.40

0.40

S.18.C.0.6

0.60

1.20

1.20

S.18.C.0.65

0.65

1.00

1.20

S.18.C.0.75

0.75

1.30

1.20

S.18.C.0.85

0.85

1.00

1.00

S.30.NC.1

000

S.30.NC.2

000

S.30.C.0.15

0.15

0.35

0.35

S.30.C.0.25

0.25

0.80

0.70

S.30.C.0.35

0.35

0.80

0.85

S.30.C.0.50

0.50

1.00

1.00

S.30.C.0.60

0.60

1.30

1.20

S.30.C.0.85

0.85

1.40

1.30

4.3.1.Results of pull-out test
4.3.1.1.Failure mode
All specimens experienced failure due to splitting. The specimens without crack failed bysingle splitting due to the limited cover thickness. In the crack induced specimens, most of themfailed by side-splitting due to the opening of pre-existent cracks. In side-splitting failure, somespecimens presented newly side crack (named N1 with one new crack, named N2 with two new
cracks). All the failure modes are summarized in Fig 4.9. Depending on the failure mode,
concrete between the ribs was totally damaged or without any damage. This can be seen in Fig
4.10. The results of pull-out tests are shown in Table 4.6.

46

BEFOREAFTERSINGLE SPLITTING
BEFORE

AFTER

SIDE SPLITTINGAFTERBEFORE

SIDE SPLITTING N1

BEFORE

AFTER

SIDE SPLITTING N2
Fig. 4.9 Failure mode

47

SIDE SPLITTINGSINGLE SPLITTING

Fig. 4.10 Damage inside the bond part
Table 4.6 Test result list
At Maximum load
Specimen name

Crack width(mm)

Load (kN)

Slip (mm)S.18.NC.1
S.18.NC.2
S.18.C.0.08
S.18.C.0.2
S.18.C.0.6
S.18.C.0.65
S.18.C.0.75
S.18.C.0.85
S.30.NC.1
S.30.NC.2
S.30.C.0.15
S.30.C.0.25
S.30.C.0.35
S.30.C.0.50
S.30.C.0.60
S.30.C.0.85

0
0
0.08
0.20
0.60
0.65
0.75
0.85
0
0
0.15
0.25
0.35
0.50
0.60
0.85

50.51
46.36
31.36
22.87
17.22
25.10
4.19
4.29
42.97
61.73
49.46
31.25
26.86
33.20
19.77
12.53

0.444
0.962
0.508
0.352
0.832
0.121
1.102
0.676
0.592
0.166
0.348
0.308
0.468
0.61
0.488
0.724

48

Remarks
Single split
Single split
Side split N1
Side split N2
Side split O
Side split N1
Side split O
Side split O
Single split
Single split
Single split
Single split
Side split O
Side split O
Side split N1
Side split O

4.3.1.1.Pull-out load and slip
During the pull-out tests, the slippage of the D19 rebar is measured so the pull-out load versus
the slip relationship can be plotted in Fig. 4.11 for 18 MPa and in Fig.4.12 for 30MPa. A plotof the pull-out load – slippage relationship of a typical uncracked and cracked specimen showsthat the slope of the initial ascending branch seems to not change with the present number,indicating a no decrease in stiffness. As the pull-out load increase, the slip seems to increase inan almost linear way. As soon as the bond strength is reached however, a steeper descending
curve is measured for uncracked specimens when compared to cracked ones. Hence a moresudden bond degradation is noted due to the occurrence of new cracks or the opening of theinduced cracks.
The maximum pull-out load versus crack width before loading relationship is shown in Fig.
4.13. It can be seen that the maximum load decreases as the crack width increases. As expected,
there is a significant correlation between residual bond strength and induced crack width.
The decrease of the pull-out load is more severe in 18MPa specimens than in 30 MPa
specimens. A possible explanation for this might be that the 30MPa concrete has a stiffer
response than the 18MPa concrete in both compression and tension therefore the crack opening
may be delayed. Another possible explanation for this is that the confinement effect developedby the same concrete cover is perhaps more effective in 30MPa concrete.

49

S.18.NC.1
S.18.NC.2
S.18.C.0.08

LOAD (kN)

50
40
30
20

60

0

60

20

0

5

0

60

LOAD (kN)

30
20

1
2
3
4
FREE-END SLIP (mm)5S.18.NC.1
S.18.NC.2
S.18.C.0.65

50

40

40
30
20
10

10

0

0

1
2
3
4
FREE-END SLIP (mm)5S.18.NC.1
S.18.NC.2
S.18.C.0.75

50
40
30

1
2
3
4
FREE-END SLIP (mm)5S.18.NC.1
S.18.NC.2
S.18.C.0.85

50
40
30

20

20

10
0

0

60

60
LOAD (kN)

30

LOAD (kN)

LOAD (kN)

1
2
3
4
FREE-END SLIP (mm)S.18.NC.1
S.18.NC.2
S.18.C.0.6

50

0

40

10

10
0

S.18.NC.1
S.18.NC.2
S.18.C.0.2

50

LOAD (kN)

60

10
0

1
2
3
4
FREE-END SLIP (mm)

0

5

0

1
2
3
4
FREE-END SLIP (mm)Fig. 4.11 Pull-out load versus slip at free-end for 18 MPa specimens

50

5S.30.NC.1
S.30.NC.2
S.30.C.0.15

40

40

20

20

0

1
2
FREE-END SLIP (mm)03S.30.NC.1
S.30.NC.2
S.30.C.0.35

LOAD (kN)60

3S.30.NC.1
S.30.NC.2
S.30.C.0.50

60

20

20

0

1
2
FREE-END SLIP (mm)

0

40

1
2
FREE-END SLIP (mm)3S.30.NC.1
S.30.NC.2
S.30.C.0.85

60
LOAD (kN)

LOAD (kN)03S.30.NC.1
S.30.NC.2
S.30.C.0.60

60

40

20

0

1
2
FREE-END SLIP (mm)

40

40

0

0

LOAD (kN)0S.30.NC.1
S.30.NC.2
S.30.C.0.25

60
LOAD (kN)

LOAD (kN)

60

20

0

1
2
FREE-END SLIP (mm)

3

0

0

1
2
FREE-END SLIP (mm)3Fig. 4.12 Pull-out load versus slip at free-end for 30 MPa specimens

51

60
Maximum Load (kN)

18MPa Specimens
30MPa Specimens

40

20

0

0.2

0.4
0.6
0.8
Crack Width (mm)1Fig. 4.13 Maximum pull-out load versus crack width before loading
4.3.1.2.Crack opening versus slip
The Crack opening versus slip is plotted in Fig.4.14. The crack opening is measured bydisplacement transducers without including crack width before loading. In the case of plural
cracks, crack opening shows the summation of the width of cracks that occurred in the gauge
length of displacement transducers. With increasing of slip, the opening seems to start earlier
and develop faster in case of side-splitting without new crack. In the case of an apparition of
new cracks, the opening of the crack is fastest and reaches the highest value compared to otherspecimens. However, in some specimens, no evidence was found.
3

1

0

0

1
2
Free-end slip (mm)Crack opening (mm)Crack opening (mm)2S.30.NC.1
S.30.NC.2
S.30.C.0.15
S.30.C.0.25
S.30.C.0.35
S.30.C.0.50
S.30.C.0.60
S.30.C.0.85

3

S.18.NC.1
S.18.NC.2
S.18.C.0.08
S.18.C.0.2
S.18.C.0.6
S.18.C.0.65
S.18.C.0.75
S.18.C.0.85

2

1

0

3

0

1
2
Free-end slip (mm)Fig. 4.14 Crack opening versus slip

52

3

4.4.

Bond strength degradation
4.4.1.Comparison of the results with the fib model code 2010 and JCI 1998
The proposed relationship in the fib Model Code is compared with the test results in Fig.
4.15. The prediction with Model Code 2010 is in good agreement with S.30.C.0.25 andS.30.C.0.30. However, for other specimens, our results are overestimated or underestimatedfor 18 MPa specimens. It should be noted again that the authors of the fib model code 2010
assume that the residual strength of concrete structures is also affected by cross-section loss ofsteel.
In addition to that, Japan Concrete Institute in the concrete structure rehabilitation researchcommittee reports 1998 gave an evaluation formula of the bond degradation as a function of
corrosion.
τcor = exp(-3.551×Wcr) ⋅ τnon-cor
The prediction with Japan Concrete Institute 1998 is in good agreement with S.18.C.0.08,
S.18.C.0.2, S.18.C.0.75 and S.18.C.0.85.However, our results are overestimated for 30 MPa
specimens.

18MPa specimens
30MPa specimens
Model Code 2010: upper limit
Model Code 2010: lower limit
JCI 1998

Normalized load

1
0.8
0.6
0.4
0.2
0

0.2

0.4

0.6

Crack width (mm)0.8

Fig. 4.15 Comparison with fib model 2010 and JCI 1998

53

1

4.4.1.Bond degradation and surface crack width relationshipA simple formula for the bond strength with surface crack width as a variable is studied
similarly as Chap.3.
As can be seen from the results of regression analysis in Table 4.7, an exponential equation
provides the best fit (higher R2). Therefore, the following equation can be used for theprediction of the maximum pull-out load:

P(Wcr) = P0 ⋅ e-0.-2.06Wcr

where P(Wcr): Pull-out strength in cracked concrete; P0: Pull-out strength of specimen
without crack; Wcr: Crack width.
Fig 4.16 shows the experimental result and the prediction model comparison. The ratios ofthe experimental to calculated values are close to one, which indicates that the concrete crack
width can potentially be a good indicator to characterize bond strength degradation.
In Chap 3, a formula expressing the deterioration of the bond obtained with single splitting
type specimens has been proposed. In Fig.4.16, that formula is compared to the one obtained inthis chapter. It can be seen that the deterioration of the bond due to the induced cracks is more
severe in a side-splitting specimen than in single-splitting specimen. This result may be
explained by the fact that the number and position of induced crack can heavily affect thedeterioration of the bond.
Table 4.7 Regression analysis result
Analysis

Equations

R2

Linear

-0.1.03Wcr+1

0.71

Logarithmic-0.24(Wcr) + 0.23

0.62

Exponential

e-2.06 Wcr

0.82

1

Present study 18MPa
Present study 30MPa
Prediction model

Normalized load

0.8
0.6
-2.06Wcr

e

0.4
0.2
0

0.2

0.4

0.6

0.8

Crack width (mm)Fig. 4.16 Result and prediction model

54

1

Side-splitting
Single-splitting

1
Normalized load

0.8
0.6
0.4
0.2
0

1

2

Crack width (mm)Fig. 4.17 Proposed evaluation formula from test result

55

3

4.5.

Conclusions

In this chapter, the pull-out test is conducted on 16 concrete blocks with embedded D19 rebar
following different corrosion crack width simulated by 2 EAFPs. The test is carried out on aside split type specimen. In summary, these results show that there was a significant exponential
correlation between pull-out strength and surface crack width. Also, a simple formula to predictbond degradation is proposed by using the surface crack width as the main variable.
The prediction with Model Code 2010 is in good agreement only with S.30.C.0.25 andS.30.C.0.30.
The prediction with Japan Concrete Institute 1998 is in good agreement with S.18.C.0.08,
S.18.C.0.2, S.18.C.0.75 and S.18.C.0.85.However, our results are overestimated for 30 MPa
specimens.
A comparison of the two results (single splitting and side splitting) reveals that thedeterioration of the bond due to the induced cracks was more severe in a side-splitting specimen
than in single-splitting specimen.

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Chapter 5 Conclusions
The present study deals with the relationship between the bond strength of corroded steel
bars and the surface crack width. A total of 28 specimens were subjected to pull-out test. On thebasis of the experimental evidence and discussions presented in this thesis, the following
remarks can be drawn:
1) The expansion agent filled pipe is a promising method that allows focussing in thecracking itself. The tensile test shows that the strain in the axis, top and between the ribs
tended to increase over the time after the filling of the expansion agent. In addition, at the
beginning of the loading, with the increasing of the axial strain, the yield strength andmodulus of elasticity of the pipe filled with an expansion agent also increase. However,
the elongation at failure tended to decrease. The tensile strength was not affected.
2) The specimens were categorized in: “Single-split type” when the induced cracks are along
the rebar and “Side split-type” when the induced cracks are located on the surroundings
and perpendicular to the rebar. All specimens experienced failure due to splitting. A groupof specimens failed by newly generated splitting crack despite existing induced crack andother specimens failed by the opening of the induced crack.
3) There was a significant correlation between residual bond strength and induced crack
width. This demonstrates that the surface crack width can potentially be a good indicator
to evaluate the bond strength degradation.
4) The decrease of the pull-out load is more severe in 18MPa specimens than in 30 MPa
specimens.
5) The deterioration of the bond due to the induced cracks was more severe in a “Side-splittype” than in “Single split-type”.
6) Empirical models, which relate the bond deterioration with the longitudinal cracks andthe lateral cracks have been proposed. However, more research is necessary to investigate
the influence of other involved parameters (e.g. cover/diameter ratio, confinement, bar
profile) to develop a predictive model for general applicability.

57

DEDICACES
The work presented in this master thesis represents two years of study and research at the
University of Tsukuba.
The present work was carried out between April 2018 and March 2020 in the Kanakubo lab.
This was possible thanks to the scholarship received from the Japanese Government
Scholarship, funded by the Ministry of Education, Culture, Sports, Science and Technology(MEXT). I am very appreciative for giving me this opportunity.
First of all, I would like to express my most sincere gratitude to my supervisor, Prof.
Kanakubo Toshiyuki. Throughout my journey in Japan, he has shared his vast knowledge andexperience, providing valuable discussion, advice and insights yet he has always given me thefreedom to conduct my own research, allowing me to evolve. Given his level of commitment
to my research, his understanding and his thorough review of my work, I cannot think of anyone
better to have as supervisor. Also, a big thanks to his wife, Kanakubo Noriko, for being avaluable advisor during my life in Japan.
I want to convey my appreciation to my vice-supervisor, Prof. Yasojima Akira and Prof.
Shoji Gaku, for showing their interest in my work and taking the time to get involved and share
their valuable thoughts, comments and also having their door always open to share their vast
knowledge.
I would also like to thank all my colleagues, former and present, at Kanakubo laboratory forcreating such a nice working environment. Special thanks go to Aburano Togo and Kojima
Atsushi for their help in executing all the experimental work.
Finally, I want to direct my special thanks to my lovely family, my wife Aida, who has
been very understanding and patient during my study. Their love and affection have been
indispensable in this journey and none of this would have been possible without theirunconditional support. For all that and much more, thank you.

58

REFERENCES[1] R. Tefers, Cracking of concrete cover along rebars, Mag. Concr. Res. 31(106) (1976) 3-12
[2] fib Bulletin No. 10. (Bond of reinforcement in concrete 2000)[3] Rodriguez et al.: Corrosion of reinforcing bars and service life of reinforced concrete
structures: corrosion and bond deterioration. Int. Conf. on Concrete across Borders, Odense,
Denmark, Vol. II, pp 315-326, 1994
[4] Auyeung et al: Bond-behavior of corroded reinforcement bars. ACI Structural Journal
97(2):214-220,2000[5] Al-Sulaimani et al.: Influence of corrosion and cracking on bond behavior and strength ofreinforced concrete. Proceedings American Concrete Institute, Vol 87, No.2, Mar-Apr.
pp220-231 ,1990[6] fib Model Code 2010, Vol.1. International Federation for Structural Concrete, Lausanne,
Switzerland.
[7] SYLL, A. S.，Kawamura, Y.，Kanakubo, T.: Simulation of Concrete Cracks due to Bar
Corrosion by Aluminum Pipe Filled with An Expansion Agent, Summaries of Technical Papersof Annual Meeting, Architectural Institute of Japan, Structure IV, pp.55-56, 2018.9[8] Yasojima, A., Kanakubo, T., Bond Splitting Strength of RC Members Based on Local Bond
Stress and Slip Behavior, 11th International Conference on Fracture, No.4486, 2005.
[9] Yang, Y., Nakamura, H., Miura, T., Yamamoto, Y., Effect of corrosion‐induced crack andcorroded rebar shape on bond behavior, Proceedings of the Japan Concrete Institute, Vol. 40,
No. 1, pp.927-932, 2019.

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