LATEST ACHIEVEMENT IN TECHNOLOGY AND RESEARCH
OF RETROFITTING CONCRETE STRUCTURES
Ueda T, Hokkaido University, Japan
Wu Z, Ibaraki University, Japan
Kanakubo T, Tsukuba University, Japan

Abstract
Major retrofitting methods for concrete structures, such as external bonding,jacketing (or wrapping) and overlaying, are characterized with interface (or bonding)between an existing concrete surface and the retrofitting material. The mechanism ofretrofitted members is essentially the same in the sense that it depends much on theinterface constitutive model. Therefore, those methods can be dealt with in the samemanner that is to provide an interface model for predicting the member behavior.
In this paper, which summarizes the report of JCI TC-014A “Committee on
Retrofitting Technology for Concrete Structures”, latest information on the interface
mechanical properties and the interface models for external bonding and overlaying
methods with various retrofitting materials is presented. Some numerical calculations,which disclose the interface mechanism, are also introduced.
Behaviors of the retrofitted members are introduced in relation to the interface
mechanical property. In addition some new technologies in external bonding andjacketing are presented briefly. Lastly a new direction for the development ofretrofitting material and method to achieve an optimum desired performance of theretrofitted member behavior is presented for discussion.
Keywords: retrofit, interface, bond, external bonding, jacketing, overlaying
1. Introduction
External bonding, jacketing (or wrapping) and overlaying are considered major retrofitting methods forconcrete structures. Retrofitting materials used in these methods include reinforced concrete,prestressed concrete, other concrete materials, steel plate, FRP sheet, FRP plate and FRP grid. Bond
properties between these materials and the existing structures and their mechanical properties
significantly influence the mechanical properties of retrofitted structures. Despite numerous studies in
the past, methods to accurately evaluate the mechanical properties such as ultimate strength,
deformation and stiffness of retrofitted members have yet been developed. This is because the stresstransferred mechanism namely slip and debonding at the bonding interface has not been fully clarified
and also because there are various retrofitting materials with various mechanical properties. JCI set
up a Technical Committee on Retrofitting Technology for Concrete Structures in 2001 to clarify the stresstransfer mechanism at bonding interface and the mechanical properties of the retrofitted members. At
the end of its 2-year term, the Committee is to (i) provide useful information needed to develop a morerational retrofitting design, and (ii) propose optimum retrofitting materials and retrofitting design methods.
The next chapter summarizes the Committee Report [1].

2. Outline of Committee Report
The Committee Report consists of 6 chapters. Chapter 1 is the introduction. Chapter 2 describes
general classification of retrofitting methods and the retrofitting methods discussed in this committee
report. Chapter 3 clarifies microscopic stress transfer mechanism at bonding interface, while Chapter
4 presents methods to predict numerically macroscopic bond properties based on the microscopic
mechanism clarified in Chapter 3. Chapter 5 describes properties of retrofitted members, relating themto the bond properties. Based on the findings in Chapters 3 to 5, Chapter 6 presents the optimum
microscopic bond properties and retrofitting material properties for macroscopic bond properties andmember properties after retrofitting. The following sections, 2.1 to 2.4 briefly present the contents ofChapters 3 to 6.
2.1 Local Bond Properties in Various Retrofitting Methods
It is common among various retrofitting methods that retrofitting materials share forces acting on aconcrete member, which is to be retrofitted. It is important to clarify the mechanism of how theretrofitting materials share the forces to accurately evaluate the retrofitting effects and develop rational
retrofitting design methods.
As shown in Fig.1 researches on retrofitting technology in the past can be classified into those for“element level” and for “member level”. In the element level research, experiments were conductedwith specimens prepared in such a way that mechanical behavior at bonding interface between
retrofitting materials and concrete could be observed. In the member level research, properties inflexural and shear of retrofitted members were investigated using specimens with a configuration similar
to the real ones. Simulations of retrofitted members were conducted by using the finite element
methods or the like.
In the Committee Report retrofitting methods are classified into overlaying, FRP sheet externalbonding, steel plate external bonding, FRP plate/grid external bonding and FRP spraying. The
element level research on each method is summarized with respect to the following 3 points and latest
data are presented:
1) Retrofitting materials at bonding interface
2) Local bond properties at interface and investigation of the factors
3) Problems remaining to be solved
The materials for each retrofitting method are listed, and the properties of bonding interface between
these materials are examined separately as “macroscopic properties” and “microscopic properties” (seeFig.1). The former is a maximum load obtained by pullout bond tests and an average bond strengththat is the maximum load divided by bonding area, while the latter is the local bond properties at a point,e.g. relationship between local bond stress τ and slip s and interfacial fracture energy G f . Since
these local bond properties can be directly applied to the simulation of member level behaviors bynumerical analysis such as finite element analysis, extensive researches are currently going on. In
order to investigate uniquely the local bond properties regardless retrofitting materials, the following two
points are considered; (i) classification of bond property according to direction of forces acting on
interface, and (ii) factors for the bond property.

Fig.1 Bond property in element and member levels

(1) Classification of bond property according to force direction
Interface bond types are classified as shown in
Fig.2 according to the Adhesion Handbook
complied by The Adhesion Society of Japan [2].
The first type is “shear bond” in which forces act
on the interface in parallel. The second type is“cleavage bond” in which forces act at an end of
the interface to open it, and the last type is“tensile bond” created by forces acting
uniformly in the direction normal to interface.
Mouth opening type or Mode I in fracture
mechanics corresponds with shear bond, while
in-plane shear type or Mode II with T-type
cleavage bond.
The Committee Report
Fig.2 Classification of bond property [2]summarizes the state-of-the-art for each ofthese three bond types.
(2) Factors for bond property
As shown in Fig.3 mechanical and environmental
factors for the bond property are listed for eachretrofitting method.
The mechanical factors
include mechanical properties of retrofitting
materials and concrete, other factors for failure
criteria, adhesive forces at bonding interface andWBL (Weak Boundary Layer). The mechanicalproperties of retrofitting materials and concrete areelastic modulus/stiffness of retrofitting materials
and concrete strength. Loading rate, member size
and their combined effects are the other factors for
failure criteria. There are two types of adhesive
forces; macro and micro adhesive forces. It isconsidered that the combined adhesive forces act
on the concrete-retrofitting material interface. The
macro adhesive forces are called anchor effect
Fig.3 Factors for bond property
which is created by roughness of the concretesurface. WBL, which is among the mechanicalfactors, are laitance and defect at concrete surface caused by poor construction. The environmental
factors are related to environmental conditions. A retrofitting method, which is suitable to achieve thebest bond property under effects of the mechanical factors, may not be the best for the environmental
factors. The environmental factors are related to physical and chemical properties of retrofitting
materials.
(3) Summary of each retrofitting method
Below are the local bond property and the remaining problems to be solved for each retrofitting
method:Overlaying method
Previous studies on interface bond are mostly with qualitative approach, which present concrete surface
roughness, wet and dry condition, and direction of joint as the factors. As one of the studies withquantitative approach, the study on peeling in shear at bonding interface of PCM (Polymer Cement
Mortar) overlaying at member bottom is introduced. It is expected that a more accurate design foroverlaying method can be developed by conducting more studies with quantitative approach.
FRP sheet external bonding method
Extensive researches after the Great Hanshin-Awaji Earthquake in 1995 enhance the reliability andsafety of external bonding method with FRP sheet significantly despite the fact that it is a rather new
method. Factors investigated in the previous studies with element level experiments are shown.
Proposed formula for average bond strength and local bond stress and slip relationship are compared.
The remaining problems are clarification of (i) thermal stress mechanism, (ii) deterioration of bonding
interface due to blockage of water seepage, and (iii) fracture and durability of interface with newadhesives.
Steel plate external bonding method
Bonding width, length, concrete strength, cyclic loading, and concrete surface condition are explained
as the factors. The remaining problems are clarification of interfacial fracture energy and effects ofanchor which are used for positioning of the steel plate. Researches with quantitative approach arenecessary to develop a rational design method.
FRP plate/grid external bonding method and FRP spraying method
There are not many studies on local bond property in FRP plate/grid external bonding method and FRPspraying method since they were introduced rather recently. There is an experimental study on macrobond property with single-plane or double-plane shear tests, in which micro bond property is examinedindirectly through the macro bond property. It is necessary to quantify the local bond property basedon further studies and studies on FRP sheet.
2.2 Simulation of stress transfer in element level by using local bond property
Possibility of predicting macro bond property through simulation of element level behavior by numericalanalysis with micro bond property as input data is investigated in the following two cases:(1) FRP sheet external bonding method
The shear bond and cleavage bond areinvestigated. Figure 4 shows different local bondstress and slip relationships of concrete surfaces
with different surface treatments, which wereobtained by double shear planes tests. It isshown that the sheet strain distribution and the
maximum pullout force can be predictedaccurately.
(2) Steel plate external bonding method
Stress analysis with consideration of shear
deformation of adhesive layer and numericalanalysis with FEM are conducted. The stressanalysis can predict measured bond stresses
reasonably except those at the plate end.
Underestimation of the stresses at the plate end isconsidered due to the fact that the assumption of
uniformly distributed stress in a cross section is
not true near the plate end.

Fig.4 Local bond property

2.3 Member property in each retrofitting method
Latest research development on FRP external bonding, steel plate external bonding and concrete
overlaying methods are presented with emphasis on clarification of influences of micro bond properties
on the member properties after retrofitting.

Normalized average stress(1) FRP sheet external bonding method
Tension stiffness of member retrofitted with
FRP sheet
response of RC
In the past, tension stiffness of member
1
retrofitted with FRP sheet was not thoroughly
response of RCC
studied the tension stiffness effects and crack
spacing prediction are therefore investigated.
Figure 5 illustrates the tension stiffness curves
lower amount of CFS(or average concrete stress curves), which are
0.5
obtained by numerical analysis, of reinforced
preconcrete with and without FRP sheet (RC andyielding
RCC). After yielding of steel reinforcement thegreater amount of CFS
tension stiffness increases as the FRP sheet
0(CFS) amount decreases. The crack spacing
2000
4000
6000
8000
formula presented is a modified version of
Average strain
CEB-FIP formula. In the formula the local
stress and slip relationship is considered.
Flexural property of member retrofitted with
Fig.5 Tension stiffness
FRP sheet
A finite element analytical method for members retrofitted with FRP sheet in flexure and its results arepresented. Based on the analytical results a simplified design method for flexural retrofitting by FRP
sheet and design examples is investigated.
Evaluation of shear retrofitting effects by FRP sheet
The same reinforced concrete member with FRP sheet as shear reinforcement is analyzed using threemethods; with the modified compression field theory, the finite element method and a macro model. In
the analyses where the local bond stress and slip relationship at sheet-concrete interface is applied, theultimate shear capacities differ between the cases with and without the bond. Interfacial fractureenergy affects the shear behavior significantly.
Recent development of retrofitting methods with continuous fiber (or FRP)
Debonding of externally bonded sheet prevents members from developing full structural performances.
New methods to overcome this shortcoming are being developed. One of them is prestressed FRPConcrete structures

Adhesive (epoxy)Lw? tf : Prestress? tf

1. Pretension of FRP sheets? tf

FRP sheets

D

Lw

Lw

D

Lw
t

b
FRP sheets? tf

jt = jw

lw

lw
FRP sheets

jc = jf

2. Bonding and curing

jt = jw = jf
jc

be=α・be≧t

be-?

3. Release and cutting of FRP
sheets endslwc■Column only■lw■ Column with wing walls■

σc

Stress distribution ina structural section

Fig.7 Modeling of column with wing walls

Fig.6 External bonding with prestressed FRP sheet

Fig.9 Out-plane peeling of steel plate
A-TypeFig.8 Near Surface Mounted FRP

150

170

D13*2

tp

200
sheet/plate external bonding method (see Fig.6). Like
prestressing steel in prestressed concrete, thehp
prestressing utilizes the material strength more
efficiently. Enhancement of the yield load as well as
D22*2
crack width control can be easily achieved.
510
460
Retrofitting of column with wing walls by FRP sheet
was recently proposed, however its shear resistingmechanism has yet been clarified.
A prediction
method of the shear capacity with consideration of
sheet delamination is introduced. In this method acolumn with wing walls is replaced by an equivalent
rectangular column (see Fig.7).
Carbon FRP plates have been used as externallybonded flexural reinforcement for beams in Japan
recently. Its anchorage and fatigue properties are970
introduced for practical design.
1200
Near surface mounted FRP, which is embedded in
1300
groove near the surface and bonded by adhesive resin,is being developed in USA and Switzerland (see Fig.8)
Fig.10 Shear strengthening by steel plate[3]. NSM FRP has a much better bond strength thansheet, so that debonding failure can be controlled fully.
The surface treatment before execution, which creates grooves, is necessary for part of the surface,
while sheet external bonding requires pre-treatment of the entire surface. The stress transfer
mechanism, however, is still being investigated, after which the rational design method will be
developed.
Researches to improve the bond property of sheet-concrete interface are being conducted. The
methods for improvement are (i) increase surface roughness by sandblasting and water jet, (ii)application of soft adhesive resin. There are some practical applications with those methods.

(2) Steel plate external bonding method
Retrofitted member in flexure
A typical failure mode is out-plane peeling at plate end in the case of a short bonded length for steel
plate (see Fig.9). The out-plane peeling is different from the in-plane peeling which often happens in
the case of sheet and typical for externally bonded materials, such as steel and FRP plates, whose
out-plane flexural stiffness is rather large. Deterioration of bond due to flexural cracking increases theAnalysis PS-1 Exp.

Displacement

Load
Load

Load

Analysis①
Analysis①

Analysis②
PS-2 Exp.

Displacement

Fig.11 Example of FE analytical resultsbond stresses near the plate end and leads to this failure.
Retrofitted member in shear
A study on shear reinforcement by steel plates externally bonded on both sides of beam (see Fig.10)
discloses that shear failure is caused by out-plane deformation of the steel plates and that the steel
plate height affects the shear reinforcement efficiency [4].
Performance evaluation of retrofitted member by FEM
Non-linear FE analysis is conducted for two cases; (i) no peeling of steel plate and (ii) consideration of
peeling of steel plate, the latter of which can predict experimental results with a good accuracy (seeFig.11).
(3) Concrete overlaying method
Retrofitting methods by spraying polymer cement mortar (PCM) and highly ductile mortar at the bottom
surface of slabs are being developed for shrinkage crack control.
Interface bonding property between the overlaid concrete and the existing member concrete has yet
been clarified. Some numerical analyses, in which bi-linear or cut-off types of bond stress and slip
curves are applied for bond link element, are introduced. It is expected that further data collection of
the bond property can improve the accuracy of the numerical analysis.
2.4 Optimization of retrofitting material and its effect(1) Micro bond property to optimize macro bond property
Macro bond properties such as the maximumsipullout force and corresponding slip depend on
S i-1
micro bond properties.
The macro bondproperties are calculated with different values ofτb,ipf
Pf − ∆ Pf
interfacial fracture energy and sheet stiffness(product of elastic modulus and thickness), which is
Adhesiv e
Reinforcina factor to determine the value of the fracturegenergy. Figure 12 illustrates a model of thematerial
analysis.
Relationships between interfacial
Pm +ΔPm
Pm
fracture energy and maximum pullout force andbetween interfacial fracture energy and slip at theConcr etemaximum force shown in Fig.13 indicate that theδm,imacro bond strength (or pullout force) increasesdxwith the interfacial fracture energy. In some cases
effective bond length increases with fracture energy.
Fig.12 Model for analysis
Therefore, bond length should be longer than theeffective bond length. It can be seen that the slip
at the maximum pullout force is in proportion to the root square of the fracture energy. This is because
the local bond stress and slip relationship are assumed to be similar as shown in Fig.14. With thisassumption the effective bond length does not change as the fracture energy changes. Figure 15
shows the relationships between sheet stiffness and maximum pullout force and between sheet stiffness
and slip at the maximum force under the condition of constant interfacial fracture energy. The macrobond strength increases with the sheet stiffness, however the slip at the maximum force does notchange.
(2) Micro bond property and mechanical property of retrofitting material to optimize mechanicalproperty of member
Analysis on shear capacity of retrofitted member, which is similar to the one in (1), is introduced [5].
Table 1 shows the calculation condition, while Fig.16 indicates the relationships between interfacial
fracture energy and the shear force carried by sheet for different sheet thickness t f . In the case of0.5
Slip at max. load (mm)Maximum load (kN)

20
15
P max = 13.4 G f0.5

10
5

0

0.5
1
1.5
Fracture energy (N/mm)

0.4
0.3
0.2
0.1

2

0

(a) Maximum pullout force(a) Maximum pullout force

Slip at max. load (mm)

0.5

80

0.3

s l,max = 0.335

Maximum load (kN)

0.4
0.5
Gf

0.2
0.1
0

0.5
1
1.5
Fracture energy (N/mm)

60

20

2

0

200
400
600
800
1000
Stiffness of reinforcing material (kN/mm)(b) Slip at maximum pullout force

Fig.13 Relationships between interfacial fractureenergy and macro bond properties

Fig.15 Relationship between FRP stiffness andmacro bond properties

10
Local bond stress (MPa)P max = 2.52 ( t×E)0.475

40(b) Slip at maximum pullout force

Table 1 Condition for calculation

8

G f = 2.045N/mm

350
Whole depth（mm）
300
Effective depth (mm）Compressive failure strain (µ)
2500
Sheet thickness tf （mm）
0.05〜0.6
Elastic modulus of the sheet (GPa)240
Tensile strength of the sheet (MPa)3000
Shear stress tc (MPa)0.94〜60
Slip δc（mm）
0.2

6
4
2
0.045
0

200
400
600
800
1000
Stiffness of reinforcing material (kN/mm)

0.1

0.2
0.3
Local slip (mm)

0.4

0.5

tf =0.05(mm)tf =0.30

3. Concluding Remarks
In the Committee Report external bonding,tf =0.10
tf =0.45

tf =0.15
tf =0.60
Compression failure

1200

The shear force, Vcf

small thickness (or stiffness) the shear capacity
decreases as the fracture energy increases
because sheet fracture happens before sheet
debonding and this fracture happens sooner withlarger fracture energy. In the case of large
stiffness the shear capacity increases as thefracture energy increases. Consequently thereis the optimum combination of fracture energy
and sheet amount (thickness or stiffness) toachieve the high shear capacity.

(kN)

Fig.14 Local bond stress and slip relationship

900

Sheet ruptured

600
300
0

Sheet ruptured
prior delaminating
2

4

6

8

The fracture energy (N/mm)

Fig.16 Relationship between interfacial fractureenergy and shear force carried by FRP sheet

jacketing and overlaying methods, which have been treated separately, are investigated in the same
concept with respect to their interfacial bonding property. The classification according to the types ofpeeling phenomena (or bond property) is applied instead of types of retrofitting methods. It shows thatthe macro bond property and the retrofitted member property can be simulated by numerical analysis
with modeling of micro bond properties. As a result, it presents the micro bond properties andmechanical properties of retrofitting materials to optimize macro bond properties and member properties
after retrofitting.
It is hoped that the findings presented in the Committee Report will help improve the current
retrofitting technology and develop rational verification methods for the performances of retrofitted
structures. The current report focuses on upgrading of mechanical properties by retrofitting, upgradingof other properties such as durability will be dealt with in our future work.
The Committee Report is also available in English. The report is compiled with papers presented
at the JCI International Symposium [1].
Acknowledgments
The authors are grateful to the committee members, especially the two WG Coordinators, Dr
KAMIHARAKO Akihisa and Dr MITSUI Masakazu and Mr INOUE Kazuhisa of JCI who served as
secretary to the Committee.
References[1] Technical Committee on Retrofitting Technology for Concrete Structures (JCI TC-014A):
International Symposium on Latest Achievement of Technology and Research on Retrofitting
Concrete Structures, Proceedings and Technical Report of JCI Technical Committee, Japan
Concrete Institute (JCI), JCI-C59E, July 2003.
[2] The Adhesion Society of Japan: The Adhesion Handbook, Third Edition, Nihon Kogyo
Shimbun-sha, 1996 (in Japanese).
[3] Rizkalla, S. and Hassan, T.: Various FRP Strengthening Techniques for Retrofitting Concrete
Structures, FRP Composites in Civil Engineering, Vol.2, pp.1033-1040, Dec. 2001.
[4] Adhikary, B. B. and Mutsuyoshi, H.: Nonlinear FEM Model and Design Formula for Externally
Bonded Steel Plates for Shear Enhancement of RC Beams，Proceedings of the Japan Concrete
Institute，Vol.23，No.1，2001.
[5] Shimbo, T., Shimomura, T., Maruyama, K. and Kamiharako, A.: Sensitivity Analysis on Shear
Capacity of Continuous Fiber Reinforced Concrete, Proceedings of the Japan Concrete Institute,Vol.22, No.3, pp.313-318, 2000 (in Japanese).