Engineering Structures 124 (2016) 29–35

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Engineering Structures
journal homepage: www.elsevier.com/locate/engstruct

Strengthening system using post-tension tendon with an internalanchorage of concrete members
Tatsuhiko Mimoto a,b, Takuya Sakaki a, Takafumi Mihara b, Isamu Yoshitake a,⇑ab

Department of Civil and Environmental Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan
Kyokuto Kowa Corporation, Hikarimachi 2-6-31, Higashi-ku, Hiroshima, Hiroshima 732-0052, Japana r t i c l e

i n f oArticle history:
Received 7 January 2016
Revised 4 June 2016
Accepted 6 June 2016

Keywords:
Internal anchorage
Strengthening
Prestressing tendon
Pull-out testa b s t r a c t
The present study aims to develop a strengthening system using prestressing tendons embedded into aninternal anchorage of concrete member. The internal anchorage, a wedge-shaped hole, is made by using a
special drilling machine. The tip of the hole is filled with high strength mortar to anchor firmly the prestressing tendon. The strengthening system is acceptable even in relatively narrow workspaces, and also
applicable for joints between existing and additional concrete members. The experimental study conducted a pull-out test of the prestressing tendon to examine the load-bearing capacity. A finite element
simulation was also performed to confirm the bonding and unbonding effects of the prestressing tendon.
The test result confirms adequate load-bearing capacity of the anchorage for the prestressing tendon. In
addition, the FE simulation shows that the strengthening system can provide a constant prestress even if
the prestressing tendon is not bound to the mortar in the anchorage.
Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction
Numerous civil infrastructures, including bridges, have been
constructed over the past few decades in Japan. Civil engineers
are tasked with maintaining and appropriately retrofitting thesestructures to sustain modern infrastructure. Various strengthening
materials and methods have been developed and applied to concrete members. One of the more effective and reliable strengthening methods for concrete is an application of the prestressing
system.
Conventional prestressing systems using metallic bars havebeen employed to strengthen existing concrete structures. In addition, fiber-reinforced polymer (FRP) laminates have been used forprestressing systems in recent decades. The strengthening system
has been investigated for structural applications and has been discussed in previous studies. Kim et al. [1] examined the effects ofprestressing levels of carbon fiber-reinforced polymer (CFRP) laminate. They also performed a three-dimensional finite element (FE)simulation to quantify the flexural behavior of the strengthened
beam. Based on these investigations, they recommended a prestress level of 20–30% of the ultimate strain of the CFRP laminate
to strengthen concrete beams. Michels et al. [2] developed ananchorage system for prestressing CFRP strips to strengthen concrete members. This system has some advantages, including faster⇑ Corresponding author.
E-mail address: yositake@yamaguchi-u.ac.jp (I. Yoshitake).
http://dx.doi.org/10.1016/j.engstruct.2016.06.003
0141-0296/Ó 2016 Elsevier Ltd. All rights reserved.

installation and higher durability than conventional techniques.
Schmidt et al. [3] summarized a state-of-the-art mechanicalanchorage system for FRP tendons and discussed investigations
using several anchorages such as spikes, wedges and clamping.
Several directions for future research were also proposed. You
et al. [4] studied a prestressed CFRP system using a new anchorage.
According to their study, the prestressed CFRP system improved
first-cracking and steel-yielding strengths by 133–235% and 77–
186%, respectively. Wang et al. [5] focused on a prestressing tendon of basalt fiber reinforced polymer (BFRP), and investigated
the structural performance of RC beam strengthened externallywith this BFRP prestressing tendon. The paper presented appropriate prestressing level based on creep rupture limit. Faria et al. [6]studied flat slabs strengthened by a new post-tensioning systemwith anchorage using an epoxy adhesive. Their experimental
investigation shows that deflection and cracks of the strengthened
slab were reduced, and the punching shear strength was increased
as compared with unstrengthened slabs. In addition, Faria et al. [7]reported the post-punching behavior of flat slabs strengthened
with the post-tensioning system. Koppitz et al. [8] also examinedpunching shear behaviors of RC slabs strengthened with nonlaminated prestressed CFRP straps. Their experimental investigation developed and used a modified mechanical anchoring system
of CFRP straps for the strengthened slabs.
Several post-tension anchor systems were also investigatedusing pull-out tests or FE modeling. Kim et al. [9] conducted a large
number of experimental tests for post-installed anchorage

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T. Mimoto et al. / Engineering Structures 124 (2016) 29–35

systems. The pull-out strength of the anchorage system was foundto be affected by three parameters: torque ratio, embedment depth
and diameter of embedded bars. The shear strength, however, wasunaffected by torque ratio. They also carried out three-dimensional
FE modeling of the anchor systems using ABAQUS to confirm theexperimental test results. Yang et al. [10] analyzed pull-out testsof the anchor from the anchor–mortar–concrete anchorage system.
Two different boundary conditions were used. The study presented
variations in shear stresses along the thickness of the mortar layer
and interfacial shear stress along the embedment length. MartíVargas et al. [11–13] investigated the transmission and anchor
length of pre-tensioned prestressed concrete members. Test specimens were made with seven-wire prestressing steel strands of
13 mm diameter. They investigated the influence of concrete
strength on transmission and anchorage lengths, and analyzed
the bond behavior after bond failure. In addition, they developed
an analytical model to predict strand slips. Obata et al. [14]reported on the structural behavior of a bond-type anchorage
and found that the stress cone strength is proportional to the bonddepth raised to the power of 1.5. Akisanya and Ivanović [15] carried out pull-out tests to examine debonding effects on the loadbearing capacity of the model anchor and discussed the interaction
between the toughness of interface crack development and consequent reduction in load-bearing capacity. Yilmaz et al. [16] discussed tensile behavior of post-installed chemical anchors
embedded in low-strength concrete blocks. They recommended
that a free-edge distance and embedment depth of at least 15
times the anchor diameter should be chosen for an economicaland efficient anchor design.
Reinforcing materials such as FRP rods and steel bar/cable are
generally arranged outside of existing concrete members. Adequate workspace is often required for this strengthening work.
New and old concrete joint members are often constructed as reinforced concrete structures, so steel corrosion may occur at thejoint. Conventional strengthening systems are influenced by environmental conditions.

Existing concrete

To mitigate the environmental impact and workspace issues,
the authors have developed a new strengthening method for existing concrete members where a post-tension prestressing tendon isembedded into a wedge-shaped internal anchorage. Fig. 1(a) and (b) shows a schematic of the developed system and a typical application of the strengthening system, respectively. The
machinery used in the strengthening work is compact and it can
locally strengthen concrete members by embedding prestressing
tendons at various points. Hence, the system can be used in relatively narrow work spaces such as the foundations shown in
Fig. 1(b). In addition, FRP tendons are applicable as well as a conventional prestressing tendon when ring-nut or similar are
attached to the FRP tendon. A previous study [17] reported a
push-out test using full-scale concrete specimens embedding thewedge-shaped anchorage. The fundamental test was conducted
to evaluate the anchor performance and determine the dimensionsof the anchorage for a pull-out test using full-scale concrete specimens. To examine the load-bearing capacity of the embedded
anchorage, a pull-out test of prestressing tendons was also performed using full-scale concrete specimens. In addition, a finite
element simulation was performed to examine the bonding andunbonding effects of the prestressing tendon in the anchorage. This
paper presents experimental and numerical investigations of thedeveloped strengthening system.
2. Anchorage system
Fig. 2 shows the strengthening process of the post-installedanchorage system. The tip of the hole in the existing concrete isenlarged using a special drilling device as shown in Fig. 3. The
anchorage that is filled with mortar acts as an internal wedgeshaped hole to resist the tensile force. The strengthening system
is implemented as follows:
Step 1: Core-drill into existing concrete and enlarge the hole tip(wedge-shaped anchorage).
Step 2: Install a prestressing tendon and grout a filler (highstrength mortar) into the wedge-shaped hole.
[Step 1] Concrete core drilling for internal anchorage (wedge-shaped hole)

Ring nut

Wedge-shaped holeHigh strength mortar (Filler)

Prestressing tendon

Existing concrete[Step 2] Install of a prestressing tendon
Fill high strength mortar into the wedge-shaped holePrestressing tendon

Internal anchorage(Wedge-shaped hole)(a) Internal anchorage system

Filler[Step 3] Placing of new concrete (option)
Provide tensile force (pull) to the prestressing tendon

Work space

TensionNew concreteStrengthening

Strengthening[Step 4] Grouting
Caisson

Grout

Internal anchorage
for PC tendon(b) Typical application-concrete foundationsFig. 1. Schematics of the developed strengthening system.

Fig. 2. Strengthening system process using a prestressing tendon fixed in theinternal anchorage.

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T. Mimoto et al. / Engineering Structures 124 (2016) 29–35

Special drilling device

A

A
I. Insertion of the special drilling device

Table 1
Materials.

A-A cross section
Segment for enlarging

B

Concrete materials

Type

Density

Water (W)
Cement (C)Fine aggregate (S)Coarse aggregate (G)Admixture (AD)Ordinary Portland cement
Crashed sand
Crashed stone
Superplasticizer

1.00 g/cm3
3.16 g/cm3
2.64 g/cm3
2.68 g/cm3
1.04 g/cm3

Materials

Properties

Reinforcing bar

JIS G-3112; Nominal diameter: 13 mm (D13)and 10 mm (D10)
Yield strength: 345 MPa; Young’s modulus:
206 GPaJIS G-3109; Nominal diameter: 23 mm; Type
B-1
Yield strength: 1080 MPa; Young’s modulus:
202 GPaJIS G-3101; Nominal diameter: 38 mm;
Yield strength: 235 MPa; Young’s modulus:
206 GPaJIS G-3101; Dimensions:
120  120  25 mm
Yield strength: 235 MPa; Young’s modulus:
206 GPa
High-strength mortar (see Tables 2 and 3)

Prestressing tendon

II. Enlargement of the holeC

B-B cross section

B

Wedge-shaped hole

Ring nut

Bearing plate with a hole

C-C cross section

C
III. Removal of the device after drilling(a) Drilling process

Segment for enlarging
Head

Shaft

(b) Special drilling device

Filling material

dard tendon. In addition, reinforcing bars (13 mm in diameter)
were mainly used in reinforced concrete (RC) specimens.
Table 2 summarizes the mixture proportions of the concreteand high strength mortar. The fresh concrete had a slump valueof 12 cm (JIS A 1101) [18], and the fresh mortar had a flow valueof 18.5–28.5 cm (JIS R 5201) [19]. Note is that the parentheses
show the Japanese standard tests. The primary material of mortar
is a commercial pre-mixed type used in sleeve joints of rebars. The
high strength mortar was used to achieve sufficient strength,
workability, non-shrinking and non-bleeding performance.
Conventional compression, splitting tension, Young’s modulus
and Poisson’s ratio tests were conducted to determine the mechanical properties needed for a FE simulation. The Young’s modulus isdetermined from the compression test, and the modulus is defined
as a secant modulus under one-third of the maximum stress. Three
concrete cylinders 100 mm in diameter  200 mm in height wereused in each test. Table 3 gives the mechanical properties of theconcrete and the high strength mortar.

Fig. 3. Wedge-shaped hole in concrete.

3.2. Test specimen
Step 3: Place new concrete if necessary. Provide tensile force(pull the prestressing tendon with a hydraulic jack).
Step 4: Grout mortar into the hole.
Mimoto et al. [17] conducted a fundamental test for the internalanchorage system. They reported that concrete member of
400  400 mm or greater and a specified cylinder strength of
24 MPa or higher (in compression) are preferable to decrease thepossibility of splitting cracks at the internal anchorage. It is notedthat the strengthening system is rarely applicable for thin structures having a lot of reinforcement such as flat slabs and walls.

Fig. 4(a) and (b) shows the pull-out test, and Fig. 4(c) presents
the dimensions of the test specimen. Three specimens of
400  400  1300 mm were prepared to investigate the behaviorof the experimental members tensioned with a prestressing tendon. Fig. 4(d) shows the prestressing tendon used in the study.
The tendon was installed to a wedge-shaped hole in the concrete
according to the procedure described above (Fig. 2). Dimensionsof the wedge-shaped hole are 66–42 mm diameter  95 mm long.
Fig. 4(c) shows the arrangement of strain gages. Wire strain
gages (120 X) were placed on the concrete surfaces to examine
Table 2
Mixture proportions.

3. Methodology
3.1. Materials
Table 1 gives the concrete and reinforcing materials used in thisexperimental investigation. Prestressing tendons of high yieldstrength (1080 MPa) were used to examine the ultimate loadbearing capacity of the internal anchorage with embedded a stan-

w/
cma

W

C

Concrete

0.53

b

0.12

175 kg/
m3
3 kg/
mix

330 kg/
887 kg/m3
m3
25 kg/mix

Mortarab

Water-cementitious material ratio.
Premix mortar (filler).

S

G

AD

Air

978 kg/m3
N/A

1.16 kg/
m3
N/A

4.5%
N/A

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T. Mimoto et al. / Engineering Structures 124 (2016) 29–35

bers strengthened with the post-tensioning prestress system. The
prestressing tendon was pulled out axially to determine the performance of the anchorage system and to investigate the behavior of aconcrete member subjected to a longitudinal compression force.
The pull-out test was conducted at a mortar-age of 28 days or later.
Table 4 shows the load step in the pull-out test. High-strength prestressing tendons (SBPR930/1080: Type B-1) were used to confirm
the anchorage effect under ultimate loading. According to Japaneseindustrial standards (JIS G 3109) [21], the yield strength (Py) andultimate load-bearing capacity are 386.4 and 448.7 kN, respectively. In the pull-out test (Fig. 4(a)), the ‘‘slip” of a prestressing
tendon is determined by subtracting the LVDT measurement (D)from the measurement (C). In addition, the ‘‘elongation” of a tendon is obtained by subtracting the LVDT measurement (C) fromthe measurement (A).

Table 3Mechanical properties of the materials used.
Concrete

MortarCompressive strength (MPa)41.4, 42.8, 42.9

90.1, 96.3, 94.7

Splitting tensile strength (MPa)Av. 42.4
2.6, 2.8, 2.6

Av. 93.7
4.5, 4.7, 3.8

Young’s modulus (GPa)Av. 2.7
31.1, 30.8, 34.1

Av. 4.3
35.9, 38.9, 38.7

Poisson’s ratio

Av. 32.0
0.19, 0.19, 0.20

Av. 37.8
0.21, 0.24, 0.23

Av. 0.19

Av. 0.23

the strain (stress) distribution. Embedded strain gages (350 X) [20]were also set into the concrete members to confirm the inner
stress. Linear variable differential transformers (LVDTs) wereplaced on the top and bottom surfaces of the concrete to monitor
the deformation. In addition, a center-hole type compression loadcell (capacity: 1 MN) was used to measure the force of the prestressing tendon.
Fig. 4(e) and (f) shows cross sectional views including thearrangement of the rebars.

Table 4
Load steps for pull-out test.

3.3. Pull-out test
The pull-out test was conducted using the prismatic specimen
to examine the fundamental behavior of full-scale concrete mem-

InitialDesign load
Allowable load for design
Allowable load for prestressing
Load at yielding
Ultimate tensile load
Maximum load

Steel plate150x150x25: Enbedded Strain gage
: Strain gage
Bearing plate
120x120x25

Load cell(C)(D)

Prestressing tendon
23mm dia.
42

66(unit: mm)(unit: mm)(a) Schematics of pull-out test

100

865

500
Wire-strain
gages

200

6x50=
135 300

Specimen
400x400x1300

100

1070

Specimen

1300

Enbeddedstrain gage

LVDT

570

Center-hole jack(B)

45 100Center-hole jack(500 kN): LVDT

Load

0
269.2
314.1
347.8
386.4
448.7
See Table 6

1300

(A)

Load (kN)00.60 Pu
0.70 Pu
0.90 PyPy
Pu
Pmax

85 145Center-holeload cell

Load step

400

Ring nut
38mm dia.

(c) Dimensions of test specimen(b) Pull-out test

55 290 55

Rebar
(D13)

25

Rebar
(D13)

10x125=1250
1300

Rebar
(D13)
Drilling hole
(unit: mm)(d) Prestressing tendon(e) Rebars arrangement (top view)Fig. 4. Detail of specimen for pull-out test.

25

55 290 55
Rebar
(D10)

Rebar
(D13)

30 340 30
400

400
30 340 30

100

Rebar
(D10)

(f) Rebars arrangement (side view)33

T. Mimoto et al. / Engineering Structures 124 (2016) 29–35

3.4. FE simulations

Table 5
Mechanical properties for FE simulation.

Numerical simulation using a linear FE program [22] was performed to examine the anchorage effect of a tendon under adesigned prestressing force. Fig. 5 shows an FE model of the pullout test. A half-dimensional model is used in the FE simulation.
The FE simulation uses different bond conditions for the PC bar(Types A and B). Type A is a full-bond model in which the prestressing tendon interface is glued firmly to the mortar filler. The
model addresses the idealized condition for anchorage of the tendon. Type B is a model that includes an unbonded interface shownin Fig. 5. The model is used to investigate the mechanical resistanceof the ring nut without mortar bonding. Mechanical properties
used in the simulation are given in Table 5. The concrete and fillingmaterial properties were determined according to the test resultsin Table 3.

4. Results and discussion
4.1. Load-bearing capacity
Table 6 gives the maximum load-bearing capacity of the prestressing tendon in the pull-out test. All test specimens achieved
the ultimate tensile force (448.7 kN) required in JIS G 3109 [21].
Concrete cracking did not occur at ultimate load. The loadbearing capacity is sufficient using the internal anchorage up to
the ultimate limit state. This result confirms that the prestressing
tendon is firmly restricted by the filling material in the wedge
anchor.

Materials

Young’s modulus (GPa)Poisson’s ratio

Concrete
Prestressing tendon
Filling material (mortar)

32
200
37.8

0.19
0.30
0.23

Table 6
Maximum loads in pull-out test.
Pmax

No. 1

No. 2

No. 3

Maximum load (kN)

489.6

490.9

490.5

4.2. Deformation
Fig. 6 shows load–slip responses (dash lines) and elongation
behaviors (solid lines) of the prestressing tendon. A slight slip
was observed at a load of 300 kN or higher. The (maximum) slip
reached 0.24 mm at 0.90 Py. This observation confirms that thenegligible slip of the tendon hardly affects the internal anchorage
under the allowable prestressing force. The dashed line indicatesthe calculation value estimated using the Young’s modulus. The
test results confirmed the elastic behavior under a 400 kN load.
The prestressing tendon indicated plastic behavior after the load
at yielding (Py). Plastic behavior after yielding is not required forthe actual strengthening system because the load is significantly
higher than the design load (269.2 kN). The most important observation is that the tendon can be anchored without slip deformation
until the design load. This result implies that the developed
strengthening method can be appropriately designed using theelastic behavior of the prestressing tendon.

Tensile force
4.3. Strains

Prestressing
tendonWedge-shaped anchorage

Fig. 7 shows the load–axial strain responses of the prestressing
tendon (specimen No. 1). A dashed line indicates elastic strain byusing the Young’s modulus of the tendon. The strain at 135 mm
indicates a linear behavior at a load of approximately 250 kN.
The linear strain behavior is almost equal to the elastic responseof the tendon. Except for the strain at 135 mm, strains were lower
than the elastic strain of the tendon.
Fig. 8 shows the axial strains measured in test specimens subjected to a design load of 269.2 kN. In addition, strain distributions
estimated from the FE simulation are presented on the graph forcomparison. Strains in the load–transition zone of the FE simulation agree relatively well with the experimental data. The FE sim-

600

Bond or
Unbond

Slip

Calc.

500

Concrete
Load (kN)

400

Elongation
1.00 Pu (448.7 kN)
0.90 Py (347.8 kN)
No.1
No.2
No.3
No.1
No.2
No.3
Calculation (Elastic)

300
200
100
0
0

5

10

15

20

Displacement (mm)Fig. 5. FE simulation model (1/2 model).

Fig. 6. Slip distance and elongation of prestressing tendon.

25

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T. Mimoto et al. / Engineering Structures 124 (2016) 29–35

600

Calculation (Elastic)
500

15 mm

55 mm

Prestressing
tendonLoad (kN)

400
300

5. Conclusions

135 mm

200

95 mm

100

15 55
Ring nut

0
0

2000

4000

6000

95

135

unit: mm
8000

10000

Strain (x10-6)
Fig. 7. Load–strain responses of prestressing tendon (specimen No. 1).

Distance from top end (mm)0200FEA (Type A)

400

FEA (Type B)

600

Experiments

800

45o

1000
1200
60

40

20

0

-20

-40

-60

-80

Fig. 8. Distribution of axial strain and FE simulation (269.2 kN).

ulation indicates a stable strain from 300 to 900 mm. The result
confirms a constant prestress applied to the concrete. Both distributions of axial strains vary in the area below 800 mm becauseof the different source and friction surface on the prestressing load.
The filling material and ring nut contribute to the load transfer.
However, each effect is different in the type A or B model. Of interest is that the strain distributions are equal for a constant prestress.
The strengthening system can provide a constant concrete prestress even if the prestressing tendon is unbonded to the anchor
as given in the simulation for Type B.
Fig. 9 shows the horizontal strain distributions for the 269.2 kN
load. The graph indicates the distributions of experimental andDistance from top end (mm)0200FEA (Type A)
FEA (Type B)

600

Experiments
800

45o

1000
1200
60

40

20

0

-20

-40

-60

-80

The foci of this study were the load-bearing capacity of internalanchorage for the strengthening system, and the structural
responses of the strengthened member. In this study, a pull-out
test using a full-size specimen and a FE simulation were conducted. The conclusions of this investigation are summarized asfollows:
 A new strengthening method is proposed for existing concrete
members. In the strengthening system, a prestressing tendon
can be anchored firmly in an internal wedge hole filled withhigh-strength mortar.
 Experimental results confirmed that the load-bearing capacityof the internal anchorage was sufficient to comply with the ultimate load defined by the Japanese criterion (JIS G 3109).
 The developed strengthening method can be designed using theelastic behavior of the prestressing tendon, as the tendon wasanchored without slip deformation until the design load.
 The slip was 0.24 mm at 0.90 Py in the pull-out test. Negligible
slip resulted with the tendon subjected to an allowable load forstrengthening.
 The FE simulation confirmed that the developed system canprovide a constant prestress even if the prestressing tendon is
not bound to the mortar in the anchorage.

-100

Strain (x10-6)400

analytical results. Both distributions of horizontal strains vary
below 600 mm because of the different boundary conditions. The
comparative result confirms that the FE simulation (Type A) agrees
well with the experimental results.

-100

Strain (x10-6)
Fig. 9. Distribution of horizontal strain and FE simulation (269.2 kN).

While fundamental structural performance was confirmed in
the study, further investigations are warranted before any practicalapplications. In particular, extensive experimental and analyticalstudies on the properties of the anchorage system and existing
concrete members etc., in addition to research on the environmental conditions necessary for the strengthening work, are highly
desirable.

Acknowledgments
The authors thank to Dr. Kawakane (Kyokuto Kowa Co.), and aformer graduate student Mr. Harada (Chuden Engineering Consultants Co.) for their assistance.

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