Bond in Concrete 2012 – Bond in New Materials and under Severe Conditions
J. W. Cairns, G. Metelli and G. A.Plizzari (eds)2012 Publisher creations, ISBN: 978 - 88 - 907078 - 3 - 4

Study on size effect in bond splitting behavior of ECC
K. Asano & T. Kanakubo

University of Tsukuba, Tsukuba, Japan

ABSTRACT: This paper describes the test results of the pullout test in order to obtain the bond behavior between Engineered Cementitious Composites (ECC) and steel reinforcing bar. There is a possibility that thesize of cover thickness of ECC around reinforcing bar affects the orientation of fiber, so bridging performanceof fiber is influenced by specimen size. To evaluate the size effect, the main parameters of specimens are reinforcing bar diameter and cover thickness. The shape of specimen is a similarity shape based on reinforcing
bar diameter. From the test results, bond strength tends to increase with cover thickness. Moreover, bondstrength decreases with increasing reinforcing bar diameter. The length of internal crack is evaluated by theassumption of the “elastoplastic stage” in which the perimetric stress around the reinforcing bar is expressedby the summation of “partially cracked elastic stage” and “plastic stage”. The length of internal crack is affected by cover thickness rather than the size of specimen.
1 INTRODUCTION
High Performance Fiber-Reinforced Cementitious
Composites (HPFRCC), which show a strain hardening branch and multiple cracking under uniaxial tensile stress, have been focused by lots of researchers
because of its unique mechanical performance. Engineered Cementitious Composites (ECC) exhibit amaximum tensile strain of several percent owing tothe synergetic effect of high-performance fiber andspecifically designed mortar matrix (Li, V. C. 1993).
Unprecedented high-performance structural members can be expected when ECC is applied to seismic components (Kanda, T. 2006).
It has been cleared that cementitious materials
such as concrete show scale effect on their mechanical properties due to size of aggregates, existence of
air void, and so on. In addition, it is considered thatthe fiber in fiber-reinforced cementitious composite
causes scale effect which is mainly influenced by fiber orientation. For example, the small size ofspecimens such as plate type shows higher tensile
strength and deformation capacity because of twodimensional fiber orientation (Kanakubo, T. 2006).
If we use some test pieces to check the mechanicalproperties of ECC, it is necessary to have information about the relationships between properties obtained by test pieces and those in actual structures.
This paper describes the test results of the pullout
test to obtain the local bond behavior between ECC
and steel reinforcing bar. There is a possibility thatthe size of cover thickness of ECC around reinforcing bar affects the orientation of fiber as shown inFig.1, so bridging performance of fiber at the splitting crack is influenced by specimen size. To evaluate the size effect, similar specimens using several
diameters of reinforcing bars with same ratio ofcover thickness to bar diameter are tested. The test
results are mainly discussed in bond strength by stress of ECC.
Slit
Reinforcing bar

Crack
FiberFig.1 Fiber orientation according to size difference

2 EMPLOYED MATERIALS
Table.1 shows the characteristics of PVA (PolyvinylAlcohol) fiber used in this study. The binders are ordinary Portland cement and fly ash. Fine aggregate
is silica sand. The volume fraction of PVA fiber is855

2.0%. Specimen is cast continuously to avoid thediscontinuity of fiber. Table.2 shows the characteristics of ECC used in this study. Tensile strength andultimate strain is calculated by 4-point bending test
according to JCI-S-003-2007 (JCI. 2007).
Fig.2 shows bending moment and curvature
curves obtained from the experiment. The deflection
hardening behavior in which the load increases after
first cracking can be recognized from the results
shown in Fig.2.
Table.3 shows the characteristics of reinforcing
bar. The data of D22 was not obtained by data error.
In the experiment of this study, the reinforcing bar
does not yield.

Table.2 Characteristics of ECC
Fiber
Ultimate Compressive
Tensilevolumestrengthstrain*strength*fraction
(MPa)
(%)
(MPa)
(%)
2.0
4.24
1.25
47.9
* By 4-point bending test (JCI-S-003-2007)
Table.3 Characteristics of reinforcing bar
Yield
Tensile strength
Type of barstrength
(MPa)
(MPa)
D10378533
D13369543D16442665
D22*: value of D16
Bending moment（kN・m）

14db

Elastic modulus
(GPa)
40.6

14db

Table.1 Characteristics of PVA fiber
Length Diameter Tensile strength
(mm)
(mm)
(MPa)
12.0
0.04
1690

Elasticmodulus(GPa)

Load direction

Castdirection

1.5db 4db 1.5db

800mm

Fig.3 Shape of specimen

16.2
Table.4 Specimen list
Name ofspecimen

Elasticmodulus
(GPa)
192
193
190
190*

D10-05D10-10D10-15D10-20D13-05D13-10D13-15D13-20D16-05D16-10D16-15D16-20D22-05D22-10D22-15D22-20-1

0.1 0.2 0.3 0.4Curvature（1/m）

Teflon sheet

CdbSlit2016, 22 mm) and cover thickness (C). Cover thickness is adjusted by the size of slit.
Three specimens are tested for each parameter,and total number is 48. Specimen list is shown in
Table.4. To investigate the size effect of bond behavior between ECC and reinforcing bar, size ofspecimen is proportionate to the diameter of reinforcing bar. Sectional size of specimen is set tosquare by 14 times of reinforcing bar diameter, andbond length is 4 times of the bar diameter. The otherparameter is cover thickness, which is set to 0.5 –2.0 times of reinforcing bar diameter. At the bothends of reinforcing bar of 1.5 times of the bar diameter, the bar is covered by Teflon sheet to insulate thebond to ECC.

1~3
1~3
1~3
1~3
1~3
1~3
1~3
1~3
1~3
1~3
1~3
1~3
1~3
1~3
1~3
1~3Diameterdb (mm)Sectionalsize(mm)10140×140

13182×182

16224×224

22

308×308

C/db
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0

Coverthickness
C (mm)
5101520
6.5
13
19.5268162432
112233440.5

4 LOADING AND MEASUREMENTS

Fig.2 Bending moment and Curvature curves

3 SPECIMENS
The shape of specimen is shown in Fig.3. The slits
are set in the two side of specimen to cause splitting
cracks around reinforcing bar. The parameters ofspecimens are reinforcing bar diameter (db=10, 13,Fig.4 shows the method of loading. The loading is
conducted by monotonic pullout test. Teflon sheet isset between specimen and support plate to not restrict lateral displacement of ECC block. Measurement items are pullout load and free end slippage.
(Refer to the Fig.4 for the meaning of "load end" and"free end".)856

Load end
Teflon sheet

D10
D13
D16
D2210Plaster

Free end

C/db=0.5

Support plate

5

Load direction

0Fig.4 Method of pullout test

24

6

Load end slippage

Load end
Free end(D22 C/db=1.5)
Fig.5 Splitting crack after loading

C/db=1.5
D10
D13
D16
D22105204

Load end slippage

6s(mm)

C/db=1.0
D10
D13
D16
D22105204

6

Load end slippageC/db=2.0D10D13D16D22108

155024Load end slippage

6s(mm)

8

Bond strength τmax (MPa)

15

10105
C/db=0.5
C/db=1.0
C/db=1.5
C/db=2.008s(mm)15Fig.6 Bond stress and load end slippage curves

Bond strength τmax (MPa)Fig.5 shows the examples of sketch of specimens after loading. In case of the small cover thickness,
splitting cracks were recognized between reinforcing
bar and the slit at the load end.
The reinforcing bar diameter increases, the tendency that the splitting cracks took place throughboth slits is observed. The observed crack of load
end is more remarkable than free end.
Fig.6 shows the relationship between bond stressand load end slippage (τ-s relationship). Bond stressis the average value of bonding surface area. Theload end slippage is calculated by free end slippage
adding elongation of reinforcing bar under the uniform bond stress assumption. After the maximum
bond stress, the bond stress decreases gradually
without sudden failure. The slope of decrement ofbond stress becomes larger as the maximum bondstress increases.
Fig.7 shows the relationship between bondstrength (τmax) and reinforcing bar diameter. In allcases, bond strength decreases with increasing reinforcing bar diameter. Fig.8 shows the relationshipbetween the bond strength and cover thickness.
Bond strength tends to increase with cover thickness. The size of reinforcing bar seems to not so effect on the increasing ratio.

Load end
Free end(D10 C/db=1.5)

Bond stress τ (MPa)

5 TEST RESULTS

8s(mm)

1515

Bond stress τ (MPa)

Bond stress τ (MPa)15Bond stress τ (MPa)

LVDT36912 15 18 21 24

Diameter of reinforcing bar

Fig.7 τmax-db relation

db (mm)

50D10D13D16D220.5

1C/db

1.5

2Fig.8 τmax-C/db relation

6 SIZE EFFECT ON BOND STRENGTH
It is known that the size effect of HPFRCC is expressed by the power of the highly-stressed volume(Yamada, K. 2001). In this paper, the evaluation ofsize effect for bond strength is conducted quantitatively by the result of pullout test.
The highly-stressed volume in pullout test is defined as the volume of a cylindrical column asshown in Fig.9.
Fig.10 shows the relation between the normalizedbond strength and the normalized highly-stressedvolume. The bond strength is standardized by bondstrength of D10 specimen. Similarly, the highlystressed volume is standardized by volume of D10
specimen. As for specimen of each ratio of cover
thickness to diameter of reinforcing bar, the normalized bond strength decreases with increasing ofnormalized highly-stressed volume. However, theunited evaluation to all specimens is difficult. It maybe considered that other indexes which represent
stress condition of ECC around the reinforcing bar
will be required.

857

Slitmethod and Formula (2) is obtained. As shown inFig.14, this result leads that the length of internalcrack is not sensitive to the size of specimen.

Reinforcing bar

Cdb

ru

ri σσθ

σθr
4db

Normalized bond strength

Fig.9 Definition of highly-stressed volume

C/db=0.5
C/db=1.0
C/db=1.5
C/db=2.01σθ

rpr

ru

468rprrurp(c) Plastic stage(b) Elastic stage

2σθσθ0.500r
Internal crackrpruri(a) Partially cracked elastic stage

Reinforcing barrirur(d) Elastoplastic stage

Fig.11 The state of the circumference stress of reinforcing bar

10 12Normalized highly-stressed volume(1)ri : Plastic zone length around reinforcing bar (mm)ru : ru = d b / 2 + C
rp : rp = db / 2

σ t : Tensile strength of ECC

d b : Diameter of reinforcing bar
C : Cover thickness
α : Angle between splitting force to axial direction =34degree
2015ri (mm)In the case of ordinary reinforced concrete, Tepfers
modeled the stress state in the circumference of a reinforcing bar in a hollow cylinder as shown in
Fig.11, and calculated bond splitting strength(Tepfers, R. 1982). The perimetric stress has been
assumed in three cases; (a) partially cracked elastic
stage, (b) elastic stage and (c) plastic stage. Tepfers
concluded that the bond splitting strength ranges between partially cracked elastic stage and plastic
stage in case of ordinary concrete.
In the case of ECC, it is considered that the tensile stress after cracking takes place due to bridging
effect of fiber. So, the perimetric stress in the circumference of the reinforcing bar in ECC is assumed as (d) elastoplastic stage. The bond strength(τmax) is expressed by formula (1) as the summationof partially cracked elastic stage and plastic stage.
The value of ri represents the length of internalcracks.
In this study, the value of ri is confirmed by reverse calculation from experimental bond strength
using Formula (1). The value of cotα is assumed as
the same value with ordinary concrete (Sakai, T.,1999). It the value of ri exceeds ru, ri is fixed as ru.
Fig.12 shows the relationship between the ri anddiameter of reinforcing bar. As for specimen of eachratio of cover thickness to diameter of reinforcing
bar, ri increases with increasing of diameter of reinforcing bar. However, the tendency is not remarkable.
Fig.13 shows the relationship between the ri andcover thickness. As for all specimens, the ri increases with increasing of cover thickness. The line
shown in the figure is calculated by the least-squarer (r 2 − r 2 ) ⎫⎪τ max 2 ⎧⎪= ⎨(ri − rp ) + i 2u 2i ⎬ cot α
σ t d b ⎪⎩ru + ri ⎪⎭10C/db=0.5
C/db=1.0
C/db=1.5
C/db=2.0

5010

1520

Diameter of reinforcing bar

Fig.12 ri-db relation

20db (mm)ri=0.124C+7.67

15
ri (mm)Fig.10 Normalized bond strength-normalized highly-stressedvolume relation

10500

Fig.13 ri -C relation858

ri = 0.124C + 7.671020

db=10(mm)db=13(mm)db=16(mm)db=22(mm)30

Cover thickness

40C (mm)50(2)ru riruσθri σσACKNOWLEDGEMENTSσθThe ECC was provided by Techno Material Co. andOkumura Corporation.

θrr

REFERENCES

Small size specimen

Large size specimen

Fig.14 Example of comparison of a stress state

The length of internal crack is seemed to be affectedby cover thickness, and it is suggested that stresstransfer in the reinforcing bar circumference is determined by cover thickness.
Fig.15 shows the relationship between the experimental value and the calculated value of bondstrength. The experimental values are averaged forthree same specimens. The calculated value is obtained by Formulas (1) and (2). The calculated values well correspond to the experimental values.

Experimental value (MPa)

20
15
10
C/db=0.5
C/db=1.0
C/db=1.5
C/db=2.0

500

5

1015

Calculated value (MPa)20

Fig.15 Experimental value-Calculated value relation of bondstrength

7 CONCLUSIONS
1. In case of the small cover thickness, splitting
cracks were recognized between reinforcing bar
and the slit.
2. The size effect is clearly recognized on the bondstrength by the ratio of cover thickness to diameter of reinforcing bars.
3. From the result of evaluation of bond strength
using highly-stressed volume, the normalizedbond strength decreases with increasing of normalized highly-stressed volume.
4. The length of internal crack is evaluated by theassumption of the elastoplastic stage in which
the perimetric stress is expressed by the summation of partially cracked elastic stage and plasticstage. The length of internal crack is affected bycover thickness rather than the size of specimen.

859

Japan Concrete Institute, 2007: Method of test for bending
moment-curvature curve of fiber-reinforced cementitious
composites JCI-S-003-2007
Kanakubo, T., 2006: Tensile Characteristics Evaluation
Method for Ductile Fiber-Reinforced Cementitious Composites, Japan Concrete Institute, Journal of Advanced
Concrete Technology, Vol.4, No.1, pp.3-17
Kanda, T., Tomoe, S., Nagai, S., Maruta, M., Kanakubo, T.
and Shimizu, K., 2006 : Full Scale Processing Investigation
for ECC Pre-cast Structural Element, Journal of Asian Architecture and Building Engineering, Architectural Instituteof Japan, Vol.5, No.2, pp.333-340
Li, V. C., 1993 : From Micromechanics to Structural Engineering, The Design of Cementitious Composites for Civil
Engineering Applications, Journal of Structural Mechanics
and Earthquake Engineering, JSCE, Vol.10, No.2, pp.37-48
Sakai, T., Kanakubo, T., Yonemaru, K., Fukuyama, H., 1999:
Bond Splitting Behavior of Continuous Fiber Reinforced
Concrete Members, Fiber Reinforced Polymer for Reinforced Concrete Structures, ACI SP-188, pp. 1131-1144
Tepfers R.: Lapped Tensile Reinforcement Splices, ASCE,
Journal of Structural Division, Vol.108, No.ST1, pp.283301, 1982.1
Yamada, K., Mihashi, H., Itagaki, N. and Ishiyama, S., 2001:
Stochastic Study on Size Effect in Tensile Strength ofShort-Fiber Reinforced Cementitious Composites, Journalof Struct Constr. Engng., Architectural Institute of Japan,
No.540, pp7-12 (in Japanese)