Nondestructive Evaluation of Deterioration around Rebar Based … · 2009. 4. 23. · In this paper, a new nondestructive evaluation (NDE) method of delamination around reinforcement

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

Nondestructive Evaluation of Deterioration around Rebar Based on Elastic Waves Generated by Electromagnetic Force

Kotaro MUNAKATA1, Toshiro KAMADA2, Shinya UCHIDA3,

Hirofumi MAE3, Hiroyuki MINEZAWA3

1 Osaka University, Osaka, Japan, [email protected] Osaka University, Osaka, Japan, [email protected] Osaka University, Osaka, Japan

Abstract In this paper, a new nondestructive evaluation (NDE) method of delamination around

reinforcement bars in concrete is described. This electromagnetic pulse (EMP) based method can evaluate delamination around rebar. It is confirmed that this EMP method is effective for evaluating delamination due to the corrosion of RC members.

Résumé

Ce document décrit une nouvelle méthode d’évaluation non destructrice (NDE) du délaminage des environs des barres de renforcement du béton armé. Cette méthode, fondée sur les impulsions électromagnétiques (EMP), permet d’évaluer le délaminage autour de barres de renforcement. Il confirme que la méthode EMP est efficace pour mesurer la délamination due à la corrosion des poutres en béton armé.

Keywords Concrete, Non-destructive evaluation, Electromagnetic force, Corrosion, Delamination

1 Introduction

1.1 Purpose of the research This research aimed to develop a new NDE method of delamination around reinforcement

bars in concrete by using the EMP method. First, the principle of this method is explained: how to use the electromagnetic pulse force which causes the vibration of reinforcement bars inside the concrete and how to use the generated elastic waves. The application of the EMP method [1] to reinforced concrete beam test specimens whose rebar have been corroded through electricity is also described.

1.2 Background of the research In civil structures, the corrosion of reinforcement bars in concrete structures is a serious

problem that can cause cracks or delamination in the concrete. Delamination between reinforcement bars and concrete reduces the load carrying capacity of RC members[2] , so it is important to inspect for deterioration and to maintain civil structures. Various nondestructive evaluation methods such as X-ray methods and impact echo methods can be used, but delamination in reinforced concrete structures has not been analyzed in detail.

1.3 Scope of the research This paper explains the EMP method which can evaluate delamination around rebar in

concrete structures nondestructively, how to evaluate the delamination, and how to apply the method to real structures. It does not consider other nondestructive evaluation techniques such as impact echo and the X-ray method, nor describe the established NDE method for civil

mailto:[email protected]

mailto:[email protected]


structures.

2 Method and materials

2.1 Electromagnetic pulse (EMP) method A magnetic coil is used to vibrate rebar inside concrete, as shown in Fig. 1. When

pulse-electricity is passed through the magnetic coil, a momentary magnetic field is generated around the coil. The rebar within the magnetic field is acted on by electromagnetic force, and so the reinforcing bar inside the concrete is forced momentarily and vibrated by electromagnetic pulse (EMP) force. The vibration that then propagates through the concrete as elastic waves includes information about the status around the rebar. If corrosion or delamination exists, the propagation of elastic waves through concrete will be interrupted. Thus, the elastic waves detected by the sensors on the surface of the concrete include information on whether the rebar is normal or has deteriorated. Based on this information, nondestructive evaluation of the deterioration (range, thickness and length of delamination, etc.) is possible. The maximum amplitude of the elastic waves is used as evaluation parameters.

a) Plane view

Coil ［mm］

410

Concrete 205 205

AE Sensor 20 180

Rebar

b) Cross section

［mm］

Concrete

180

90

90

180

20

Coil

AE Sensor

Figure 1. Test piece of RC beam in experimental2.2 Experimental program In the experiment, two types of test pieces of reinforced concrete (RC) were prepared.

Experiment series 1 aimed to confirm the mechanism of the EMP method, and to confirm the influence of the diameter of rebar, length of delamination and thickness of delamination. Experiment series 2, on the other hand, aimed to develop the EMP method in test specimens corroded by electricity.

2.2.1 Experiment series 1 (Confirmation of mechanism of EMP method) (1) Test pieces In this section, delamination is considered to be air space in order to facilitate

phenomenon. The dimensions of the test piece were 180×180×410 mm (as shown in Fig. 1) and the depth from the surface to rebar was 50 mm. The situation of experiment series 1 is outlined in Tables 1 and 2. In total, 10 sample concrete beams were made. Every rebar of “delamination” in Table 1 was completely wrapped by poly-hydrochloric vinyl in order to simulate delamination around the rebar. Different diameters of rebar (φ16, φ25 and φ32) were used, and delamination was simulated for different lengths (308 mm and 410 mm) and different thicknesses (0.1 mm and 3 mm). Normal plane rebar were used in every sample. The compositions of materials are shown in Table 3. In the experiment, high-early-strength cement was used.


(2) Measurement of EMP method A magnetic coil was placed on the side of the RC beam without contact. The center of the

coil and the test piece were matched. Sensors were placed on the surface of the test piece. (The outline of the experiment is shown in Fig. 1.) Pulse-electricity was generated by a 622 V power supply (maximum voltage) and electromagnetic pulse force occurred. Elastic waves generated by the electromagnetic force were detected on the surface of the RC beam and recorded by a wave collecting device. In the experiment, sensors sensitive at around 60 kHz were used. The sampling frequency was 2 MHz.

Table 1. Case of experimental series 1-1 Diameter

State of test pieces 16mm 25mm 32mm

Normal ○ ○ ○ Depth: 50mm

Delamination ○ ○ ○

※ Length of delamination: 3mm ※ Thickness of delamination: 410mm

Table 2. Case of experimental series 1-2 Length of delamination

0mm 0.1mm 3mm

410mm ○ ○ ○ Thickness of delamination 308mm - - ○

※ Diameter: φ16mm, depth of rebar: 50mm

Table 3. Mixture propotion of concrete Unit wait (kg/m3) W/C

(%) s/a (%) W C S G

Water reducing agent

(g) 50 41.0 184 368 694 1002 1472

2.2.2 Experiment series 2 (Application for corroded RC beams) (1) Test pieces In order to develop the EMP method to corroded RC beams, test specimens corroded by

electricity were made. The details of the RC beams are shown in Fig. 2, with dimensions of 240×360×3000 mm. In the main rebar, D22 (SD345) was used and the depth was 38 mm. Each RC beam had two main rebar in both the upper and lower sides. The depth from the side edge to the rebar was 39 mm. In hoop rebar, R13 (SR235) was used, and 14 hoop rebar were placed in every 200 mm. The depth of hoop rebar was 25 mm from the upper and lower sides, and was 26 mm from the side edges. To prevent corrosion of the main rebar placed in the upper side, the connection part between the main rebar and the hoop rebar was insulated by friction tape. In total, three RC beams were made. In the experiment, high-early-strength cement was used. The composition of materials is shown in Table 4. The rebar was corroded electrically, with three patterns for the amount of corrosion: total electric current of 6.14 A·day (No. 1), 61.40 A·day (No. 2) and 170.54 A·day (No. 3).

(2) Measurement of EMP method The same method and devices as in section 2.2.1 were used. The coil was placed on the

L-side on the bottom. The measured points are shown in Fig. 3 (○: No. 1 to 5). As shown in Fig. 4, the center of the coil (line-b) and the sensor (line-a) were harmonized.


L-side

R-side

1

i

W/C (%) (62.5 4

［unit：mm］

Figure 2. Test piece of RC beam in experimental series

3 Results and dis

3.1 Series 1 Figure 5 shows an exa

shown in Fig. 5) is usedmaximum amplitude of thefirst figure are the ratio of “

The relationship betwe6. For every diameter, the delamination ones. This ddelamination.

The maximum amplituIn this figure, the maximumis considered that the thpropagation of elastic wavlength of delamination and

Table.4 Composition of concrete Unit waite (kg/m3) s/a

%) W C S G Water reducing agent 5.7 162 260 842 1026 2.60

532 4

i

Figure 3. Measured points

Bottom of test piece

Sensor

Concrete

Magnetic coil

ba

20 ［unit：mm］

Figure 4. Measurement by EMP method

cussion

mple of the waveforms. In this study, the maximum amplitude (as as an evaluation parameter. Figures 7 to 9 show the average wave received on the surface of the concrete. The numbers in the normal” and “delamination”.

en maximum amplitude and diameter of the rebar is shown in Fig. maximum amplitude is clearly bigger in normal test pieces than in ifference in maximum amplitude is considered to be evidence of

de and thickness of delamination are correlated as shown in Fig. 7. amplitude decreased with increasing thickness of delamination. It

ickness of delamination or the amount of voids prevents the es through concrete. Figure 8 shows the relationship between the the maximum amplitude. In this figure, the maximum amplitude


0.0

0.1

0.2

0.3

0.4

0.5

0.6

Max

imum

am

plitu

de (V

)

0 0.1Thickness of delam

also decreased as the length of delamination increased. This may have happened for the same reason as in Fig. 5, that the voids prevented the propagation of elastic waves.

0 5000 10000 15000

5

0

5

Time (μs)

Am

plitu

de(V

) On the rebar On the concrete

0 5000 10000 15000

2

0

2

Time (μs)A

mpl

itude

(V)

Figure 5. Waveform

M

axim

um a

mpl

itude

(V)

×0.59 ×0.11

×0.44

0.0 0.1 0.2 0.3 0.4 0.5

φ 16 φ 25 φ 32

“Normal” “Delamination”

Diameter of rebar (mm)

Figure 7. Relationship beamplitude and thickness of

3.2 Series 2 Figure 9 shows the aver

surface of the concrete. Thecorroded RC) to former amdeterioration progressed. Fideterioration, but in No. 2, 2were no cracks on the surmaximum amplitude was carebar. Therefore, in No. 2, 2method.

Also, in Fig. 9 (b) and and the difference between uThe surface condition of test

Figure 6. Relationship between maximumamplitude and diameter of rebar

3ination (mm)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Max

imum

am

plitu

de (V

)

308 410Length of delamination (mm)

tween maximum delamination

Figure 8. Relationship between maximum amplitude and length of delamination

age ratio of maximum amplitude of the wave received on the ratio of maximum amplitude is the ratio of latter amplitude (in plitude (in normal RC). The maximum amplitude decreased as gure 10 (a) shows two results which did not change by the 0 and 26, the ratio of the maximum amplitude decreased. There

face of the concrete, so it is considered that the decrease of used by the cracks and delamination between the concrete and 0 and 26, it is possible to detect the deterioration by the EMP

(c), the maximum amplitude decreased as corrosion progressed, n-corroded test piece and corroded one is bigger than in Fig. 9 (a). pieces A, B and C is shown in Fig. 10. This figure suggests that


the decrease of the maximum amplitude in Fig. 10 (b) and (c) is caused by delamination and cracks.

1

1

4 Conclusions

1

R-side

R-side

L-side

L-side

L-side

R-side

(a) Botto

(b) Bottom o

(c) Bottom

(b) Test piece B

(a) Test piece A

2 8 14 20 260.0

0.2

0.4

0.6

0.8

1.0

1.2

1 2 3 4 5 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Rat

io o

f

Max

imum

ampl

itude

1 2 3 4 5 Rat

io o

f

Max

imum

ampl

itude

2 8 14 20 260.0

0.2

0.4

0.6

0.8

1.0

1.2

(c) Test piece C 1 2 3 4 5 R

atio

of

M

axim

umam

plitu

de

1. A new non-desconcrete was pro

2. The maximum aalong with the de

3. The EMP methodelamination due

References 1. Takanabe, M., Hashim

the evaluation of ReiVol.52, No.11, pp.628

2. C. Q. Li., J. J. ZhengCaused by Steel”, pp.591-600.

Figure 9. Ratio of maximum amplitude

2 3 4 5

2 3 4 5

2 43 5

×：Place of AE sensor

m of test piece A（Total amount of electricity 6.14A·day）

f test piece B（Total amount of electricity 61.40A·day）

of test piece C（Total amount of electricity 170.54A·day）

trucposmpgre

d d to

otnfo-63., WJou

Figure 10. Condition on the surface

tive method of evaluating delamination around rebar within ed. litude of elastic waves received at the concrete surface changed e of corrosion. escribed in this paper could be an effective method of evaluating the corrosion of RC members.

o, M. (2003) “Suggestion of Electromagnetic Pulse Method for rced concrete”, Japanese Society of Non-Destructive Inspection, 2, 2003, (in Japanese) . Lawanwisut., R. E. Melchers. (2007) “Concrete Delamination

rnal of Materials in Civil Engineering, ASCE, July 2007,

Documents

Nondestructive Evaluation of Deterioration around Rebar Based … · 2009. 4. 23. · In this paper, a new nondestructive evaluation (NDE) method of delamination around reinforcement