failure process in shear bonding strength tests between...

Failure Process in Shear Bonding Strength Tests between Existing Concrete and Repairing Material by Acoustic Emission Technique

Kentaro Ohno1, So Kurohara1, Kimitaka Uji1 and Atsushi Ueno1 1 Dept. of Civil and Environmental Engineering, Tokyo Metropolitan University, Tokyo, Japan

ABSTRACT: The reasonable and simple method is required to evaluate bonding strength on interface of existing concrete and repairing material. In this study, four types of shear strength tests are carried out in a laboratory as a fundamental study on evaluation of the shear bonding strength. In addition, interfaces on substrate concretes are prepared in three different roughnesses. Since fracture behavior of specimen is different by test methods and the interface roughness, results of the shear bonding strength in these tests vary widely. In order to investigate fracture process in these tests, acoustic emission (AE) method is applied to each test. Also, the center line average roughness expresses the interface roughness. As a result, suitable and brief test method is suggested based on the number of failed specimens with highly rough interface and results of the AE-SiGMA analysis which demonstrate failure process and failure mode.

1 INTRODUCTION

Since existing concrete structures suffer from sever conditions due to corrosion of reinforcing bar, earthquakes and cyclic load, repairing of existing concrete structures has become significant matter. A tight bonding between existing concrete and repairing material is necessary for a sufficient repairing of the concrete structure. Therefore, a reasonable standard method is required to evaluate the shear bonding strength. It is known that the shear bonding strength depends on the test method, surface roughness and soundness of existing concrete (Momayez et al (2005)).

In this study, four types of shear bonding strength tests of bi-surface shear test, direct shear test, punching shear test (Uji et al (2000)) and slant shear test are carried out as a fundamental study on evaluation of the shear bonding strength. In addition, interfaces on substrate concretes are prepared in three different roughnesses. Since failure processes in these tests might be different, acoustic emission (AE) method is applied to these tests to investigate generation of micro cracks. Failure process of concrete member can be investigated by applying SiGMA (simplified Green’s functions for moment tensor analysis) procedure (Ohtsu (1991)). Result of each shear bonding strength is compared each other, and failure process in each test is discussed based on results of the SiGMA analysis.

2 EXPERIMENTAL PROCEDURE

Figure 1 shows four types of shear bonding strength tests and dimensions of specimens in every test. Bi-surface shear test (BSST) and direct shear test (DST) can estimate the shear bonding

strength under flexural stress working state. On the other hand, punching shear test (PST) and slant shear test (SST) bring the shear bonding strength under compressive stress working state. Table 1 indicates the mixture proportion of substrate concrete. Table 2 shows mechanical properties of hardened substrate concrete and repairing material. Premixed type repairing material, which does not include polymer and fiber, is applied to the experiment. Compressive strength of repairing material is higher than that of substrate concrete, and elastic modulus of repairing material is similar to that of substrate concrete. Substrate concrete is cured in 14 days, and then repairing material is applied on the substrate concrete surface. Each test is carried out after substrate concrete with repairing mortar is cured in water until 28 days. Figure 2 shows three different roughnesses of substrate concrete surfaces in the experiment. The set retarder sheet is attached to the surface of substrate concrete to make rough surface after hardening. Cement paste or mortar on the interface of substrate concrete is removed by wire brush in next day of casting substrate concrete. Then, the interface of substrate concrete is prepared in three roughnesses such as without treatment (Figure 2(a)), medium in rough (Figure 2(b)) and highly roughness (Figure 2(c)). Five specimens are prepared for each roughness on one shear bonding strength test. In order to quantify the roughness of the interface, it is traced by laser displacement transducer. The laser displacement transducer has 0.05 mm resolution capability for height and sampling interval is set to 0.0234 mm to slipping direction.

In this study, AE method is applied to investigate failure process in each test method. AE signals are detected by six AE sensors (150 kHz resonance frequency), and they are recorded by SAMOS AE system (PAC). The threshold level is 40dB. Detected AE signals are amplified with 40 dB gain by a pre-amplifier and 20 dB gain by main amplifier in the SAMOS. AE waveforms are recorded at 1MHz sampling frequency.

Water Cement Fine aggregate Coarse aggregate(mm) (mm) (%) w/c W C S G

20 80 4.5 0.58 174 300 821 985

unit content (kg/m3)Maximum

aggregate sizeSlump

Aircontent

Watercement ratio

Table 1. Mixture proportion of substrate concrete

Figure 1. Test specimens and methods. (a) BSST

Repairing material

substrate concrete

400

250 150100

100

100

(b) DST

Substrate concrete

Repairing material

110 100100

10087

210

100

(c) PST

230

150

100

Repairing material

substrate concrete

30

150

Carbon fiber sheet

(d) SST

substrate concrete

Repairing material

100100

400

100

60°

unit(mm)

Table 2. Mechanical properties of hardened substrate concrete and repairing material BSST* DST** PST*** SST****

Compressive strength (MPa) 37.7 37.7 36.5 41.4Tensile strength (MPa) 3.12 2.98 3.18 3.25Elastic modulus (GPa) 27.5 26.6 26.7 27.7

Compressive strength (MPa) 62.6 62.7 68.7 66.3Tensile strength (MPa) 3.39 5.05 3.63 4.09Elastic modulus (GPa) 27.9 30.0 32.0 30.7

* : Bi-surface shear test, ** : Direct shear test, *** : Punching shear test, **** : Slant shear test

Substrateconcrete

Repairingmaterial

Figure 2. Surface textures of substrate concretes.(b)Medium (a) Smooth (c) High

3 RESULTS AND DISCUSSIONS

3.1 Relation between roughness index and the shear bonding strength

The shear bonding strength significantly depends on friction and aggregate interlock factors even if same shear bonding strength test method is carried out. Therefore, quantitative roughness index are required to evaluate interface of substrate concrete. In this study, interface in substrate concrete is measured by laser displacement transducer. Figure 3 shows the results of interfaces in substrate concretes with different surface roughnesses. It is found that maximum roughness difference between smooth interface and highly rough interface is within about 2.0 mm. As for the roughness index, a standard method has not been adopted for evaluation of roughness on interface in concrete. In order to quantify the roughness of substrate concrete, center line average roughness is applied to roughness index. The center line average roughness is calculated by following equation,

dxxfL

Have )(1

(1)

where, Have is center line average roughness (mm), f(x) is height from center line (mm), L is the number of sampling data as shown in Figure 4.

Table 3 shows results of the center line average roughness for all specimens. It is found that roughness differences in substrate concretes are quantitatively represented by the center line average roughness. Therefore, it is suggested that the center line average roughness can be adopted as roughness index for interface of substrate concrete.

The relation between the roughness index and the shear bonding strength for all specimens is given in Figure 5. The surface roughness index correlates with the shear bonding strength as shown in Figure 5, the shear bonding strength increases in all test methods with increases of the roughness index. The results of the BSST are similar to those of the DST because both methods

f(x)

f(x)

Figure 4. The center line average roughness.

Roughness(mm)Specimen S-1 S-2 S-3 S-4 S-5 M-1 M-2 M-3 M-4 M-5 H-1 H-2 H-3 H-4 H-5

BBST 0.10 0.10 0.08 0.10 0.10 0.34 0.42 0.38 0.43 0.54 0.85 0.56 0.69 0.75 0.90DST 0.04 0.04 0.06 0.07 0.04 0.29 0.31 0.48 0.52 0.31 0.83 0.94 0.70 0.97 0.80PST 0.05 0.06 0.06 0.08 0.07 0.34 0.32 0.39 0.34 0.44 0.37 0.66 0.72 0.64 0.76SST 0.16 0.24 0.21 0.09 0.13 0.49 0.41 0.47 0.61 0.47 0.58 0.75 0.66 0.62 0.87

Average

Medium High

0.41 0.730.09

SmoothTable 3. Calculation results of the center line average roughness

R² = 0.878

R² = 0.722

R² = 0.696

R² = 0.741

0

5

10

15

20

0.00 0.20 0.40 0.60 0.80 1.00

She

ar b

ondi

ng s

tren

gth

(N/m

m2 )

Roughness index (mm)

BSSTDSTPSTSST

Figure 5. The relation between the roughness

index and the shear bonding strength.

Figure 3. Measurement results of interfaces in substrate concretes. Horizontal distance (mm)

20 40 60 80

SmoothMiddleHigh

0-3.0

0.01.0

2.0

3.0

-2.0

-1.0

Hei

ght o

f sur

face

(mm

)

are carried out under the flexural moment. On the other hand, shear bonding strengths of the PST and the SST are higher than those of the BSST and the DST. This is because these tests are carried out under the compressive loading, and slip motion on interface is restrained by carbon fiber sheet in the PST and loading plates in the SST.

3.2 Failure Process in Shear Bonding Strength Tests

3.2.1 Bi-surface Shear Test

The shear bonding strength in the BSST is summarized in Table 4. Specimens which have smooth interface and medium in rough interface failed at the interface. Straight failure planes are observed for smooth interface specimens and some coarse aggregate in substrate concrete are removed by repairing material in highly rough interface specimens (H-1 and H-4 specimens). By applying SiGMA analysis to obtained AE signals, fracture process zone is visually found as shown in Figure 6. It is confirmed that shear bonding strength tests are successfully carried out for specimens, which have smooth interface and medium in rough interface. However, only two specimens with highly rough interface failed at bonding surface, the others failed at bending span in the BSST. Figure 7 shows the relation between applied load and the number of AE events identified by the SiGMA analysis along with time in H-1 specimen. In stage1, micro-cracks which are classified as shear-mode localize at bottom of interface as shown in Figure 8(a). When the applied load decreases in stage 2, a lot of tensile cracks are generated at around bending span. After decreasing the applied load, failure plane suddenly occurs at interface in stage 3. From results of the SiGMA analysis in H-1 specimen, tensile cracks mainly occur under flexural stress, while shear cracks intensively generate under shear stress. It is difficult to generate bonding failure for specimens with highly rough interface

0

20

40

60

80

0

200

400

600

800

0 250 500 750 1000

Load (kN

)

The

num

ber o

f AE

eve

nts

Time (s)

SHEAR

MIXED-MODE

TENSILE

Stage1 Stage2 Stage3

Figure 7. The relation between the applied

load and AE events in H-1 specimen.

0.00

0.02

0.04

0.06

0.08

0.10

-0.25 -0.15 -0.05 0.05 0.1

y(m

)

x(m)

Hei

ght(

m)

0.10

0.08

0.06

0.04

0.02

0.00

Figure 6. Results of the SiGMA analysis in M-2

specimen.

TENSILE MIXED-MODE SHEAR

Figure 8. Results of the SiGMA analysis in H-1

specimen.

0.00

0.02

0.04

0.06

0.08

0.10

-0.25 -0.15 -0.05 0.05 0.1

y(m

)

x(m)

Hei

ght(

m)

0.10

0.08

0.06

0.04

0.02

0.00

(b) Stage 2

0.00

0.02

0.04

0.06

0.08

0.10

-0.25 -0.15 -0.05 0.05 0.1

y(m

)

x(m)

Hei

ght(

m)

0.10

0.08

0.06

0.04

0.02

0.00

(a) Stage 1

0.00

0.02

0.04

0.06

0.08

0.10

-0.25 -0.15 -0.05 0.05 0.1

y(m

)

x(m)

Hei

ght(

m)

0.10

0.08

0.06

0.04

0.02

0.00

(c) Stage 3

S-1 0.10 0.85S-2 0.10 0.25S-3 0.08 0.23S-4 0.10 0.51S-5 0.10 0.79M-1 0.34 2.03M-2 0.42 1.78M-3 0.38 1.44M-4 0.43 1.81M-5 0.54 1.34H-1 0.85 3.38H-2 0.56 -H-3 0.69 -H-4 0.75 3.42H-5 0.90 -

surfaceroughness

(mm)

bondingstrength(MPa)

0.80 3.40

0.10 0.53

0.42 1.68

BBST

Specimenbondingstrength(MPa)

surfaceroughness

(mm)

average

Table 4. Results of the BSST

in the BSST. In addition, it is realized that failure mode would change from bonding failure to flexural failure with increasing roughness index.

3.2.2 Direct Shear Test

In the DST test, except for H-2 specimen, all specimens failed at interface as shown in Table 5. However, it is clearly found that cracks concentrate at bottom of the substrate concretes with medium in rough and highly rough interfaces as given in Figure 9. Since a lot of shear cracks generate in this area, shear stress might works mechanically at the bottom of the substrate concrete. In H-2 specimen as shown in Figure 9(c), since maximum flexural moment generates at around center of the bending span, flexural failure occurs. In this case, a small amount of misalignment between loading position and interface might introduce bending crack. Therefore, shear stress does not work effectively at interface between substrate concrete and repairing material. Figure 10 shows the relation between the number of AE events and length of specimen. Dominant movement of micro-cracks on failure plane is shear crack in M-2 and H-1 specimens, while tensile cracks mainly generate at around bending span in H-1 specimen. On the other hand, a lot of shear cracks generate at center of the bending span in H-2 specimen. This phenomenon is explained that tensile crack firstly generate at upper of the bending span, and then a lot of shear cracks follow as shown in Figure 11.

S-1 0.04 0.60 M-1 0.29 1.79 H-1 0.83 2.52S-2 0.04 0.39 M-2 0.31 2.38 H-2 0.94 -S-3 0.06 0.61 M-3 0.48 1.86 H-3 0.70 3.04S-4 0.07 0.34 M-4 0.52 1.79 H-4 0.97 2.75S-5 0.04 0.62 M-5 0.31 2.76 H-5 0.80 2.77

surfaceroughness

(mm)Specimen


surfaceroughness

(mm)


Specimensurface

roughness(mm)


surfaceroughness

(mm)


Specimensurface

roughness(mm)


average

2.110.38

DST

0.85 2.77

surfaceroughness

(mm)


average average

0.05 0.51

Table 5. Results of the DST

Figure 10. The number of AE events for

specimen length.

(a) M-2 specimen

0

20

40

60

80

0 0.05 0.1 0.15 0.2

The

num

ber o

f AE

eve

nts

Length(m)

TENSILEMIXED-MODESHEAR

Interface RightLeft

(b) H-1 specimen

0

50

100

150

200

0 0.05 0.1 0.15 0.2

The

num

ber o

f AE

eve

nts

Length(m)


Interface RightLeft

(c) H-2 specimen

0

100

200

300

400

500

0 0.05 0.1 0.15 0.2

The

num

ber o

f AE

eve

nts

Length(m)


Interface RightLeft

TENSILE MIXED-MODE SHEAR

Figure 9. Results of the SiGMA analysis

in the DST.

0.00

0.02

0.04

0.06

0.08

0.10

0 0.05 0.1 0.15 0.2

y(m

)

x(m)

Hei

ght(

m)

(b) H-1 specimen

0.00

0.02

0.04

0.06

0.08

0.10

0 0.05 0.1 0.15 0.2

y(m

)

x(m)

Hei

ght(

m)

(a) M-2 specimen

0.00

0.02

0.04

0.06

0.08

0.10

0 0.05 0.1 0.15 0.2

y(m

)

x(m)

Hei

ght(

m)

(c) H-2 specimen

3.2.3 Punching Shear Test

The shear bonding strength in the PST as shown in Table 6 is higher than those of the BSST and the DST because the PST is carried out under the compressive loading. Similar failure mode is obtained both specimens with medium in rough and highly rough interfaces as shown in Figure 12. A lot of micro-cracks are localized at specific area. In specimens with medium in rough and highly rough interfaces, repairing material and substrate concrete suddenly split off after the applied load reached maximum load as illustrated in Figure 13. The failure process is estimated that failure area is not all interfaces but a specific area of interface in the PST. Therefore, it is difficult to estimate failure mode whether shear failure on interface or splitting of repairing material and substrate concrete. Since the orientation of the interface is significant on the failure mode and the shear bonding strength, it requires that interface between substrate concrete and repairing material is accurately-parallel to loading direction in the PST.

3.2.4 Slant Shear Test

The shear bonding strength in the SST becomes much higher than other test methods as summarized in Table 7. AE sources are concentrated at interface in specimen with medium in rough interface as shown in Figure 13(a). It is found that crack distribution widely expands comparing shear bonding strength tests under the flexural stress. In specimens with highly rough interface, since the applied load reached maximum load of substrate concrete, a true shear bonding strength cannot obtained, and a few AE sources are observed as illustrated in Figure 13 (b). As increasing roughness index of the interface, it is required high interface angle from horizontal axis in Figure 1 (Austin et al., 1999). With increasing roughness index, failure mode would change from bonding failure to compressive failure of repairing material or substrate concrete.

0

10

20

30

40

50

60

0

200

400

600

800

1000

0 300 600 900 1200 1500 1800

Load(kN

)

The

num

ber o

f AE

eve

nts

Time (s)


TENSILE

SHEAR

MIXED-MODE

Figure 11. AE event generateion behavior in H-1

specimen under the DST.

Figure 13. Splitting crack in H-5 specimen.

S-1 0.05 1.83 M-1 0.34 (6.80) H-1 0.37 (7.46)S-2 0.06 1.75 M-2 0.32 (6.29) H-2 0.66 (7.12)S-3 0.06 1.61 M-3 0.39 (6.53) H-3 0.72 (7.59)S-4 0.08 1.51 M-4 0.34 5.34 H-4 0.64 (5.44)S-5 0.07 1.53 M-5 0.44 5.31 H-5 0.76 (7.14)

() represents the split failure of substrate concrete or/and repairing material

PST

Specimensurface

roughness(mm)


average

Specimensurface

roughness(mm)


averagesurface

roughness(mm)


0.36 5.33 0.63

Specimensurface

roughness(mm)


averagesurface

roughness(mm)


surfaceroughness

(mm)


(6.95)0.07 1.65

Table 6. Results of the PST

Figure 12. Results of the SiGMA analysis

in the PST.

(a) M-1 specimen (b) H-5 specimen

4 CONCLUSIONS

In order to estimate shear bonding strength between substrate concrete and repairing material, four types of shear tests are carried out. Results can be summarized as follow;

(1) Surface roughness in substrate concrete is quantitatively estimated by applying the center line average roughness to detected waveform by laser displacement transducer. It is confirmed that the shear bonding strength correlate with the center line average roughness for all test methods.

(2) The shear bonding strength in the BSST is similar to that of the DST, while higher shear bonding strength in the PST and the SST are obtained. These differences are concluded that the BSST and the DST are carried out under the flexural moment, compressive stress effects for the PST and the SST.

(3) Failure mode is investigated by applying AE-SiGMA analysis for all test methods. As a result, the number of failure specimens with highly rough interface is different by test methods. It is difficult to fail at interface of specimens with highly rough interface in the PST and the SST under the compressive loading.

(4) The reasonable and easy shear bonding strength test method is not conducted under the compressive stress but a procedure of which displacement of interface is not restrained such as the BSST and the DST. However, it requires a technique to reduce flexural moment in the BSST and the DST.

ACKNOWLEDGMENT

This work was supported by KAKENHI (22360173) of JSPS.

REFERENCES Austin, S., Robins, P. and Pan, Y. (1999) “Shear bond testing of concrete repairs”, Cement and Concrete

Research, 29: 1067-1076 Uji, K., Satoh, K. and Kobayashi, A. (2007) “Effects of Retrofitting Method using CFRP Grid on The

Shear Behavior of Existing Concrete Members”, Proceedings of FRPRCS-8, (in CD-ROM) Momayez, A., Ehsani, M.R., Ramezanianpour, A.A. and Rajaie, H. (2005) “Comparison of Methods for

Evaluating Bond Strength between Concrete Substrate and Repair Materials”, Cement and Concrete Research, 35: 748-757

Ohtsu, M. (1991) “Simplified Moment Tensor Analysis and Unified Decomposition of AE Source”, Journal of Geophysical Research, 96(B4): 6211-6221

S-1 0.16 5.36S-2 0.24 3.51S-3 0.21 5.61S-4 0.09 7.49S-5 0.13 4.56M-1 0.49 -M-2 0.41 17.18M-3 0.47 15.35M-4 0.61 15.92M-5 0.47 13.18H-1 0.58 -H-2 0.75 -H-3 0.66 -H-4 0.62 -H-5 0.87 -

Specimen

SSTaverage


surfaceroughness

(mm)


0.70 -

surfaceroughness

(mm)

0.16 5.30

0.49 15.41

Table 7. Results of the SST

Figure 13. Results of the SiGMA analysis in the SST.

0

0.1

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0.3

0.4

-0.0500.05

y(m

)

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0.4

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ght(

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(a) M-5 specimen

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ght(

m)

(b) H-1 specimen

failure process in shear bonding strength tests between...

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