Application Of MSR Nondestructive Testing Technique To Monitor The ...

M. Momayez et.al., HydroVision Conference 2002

Application Of MSR Nondestructive Testing Technique To Monitor The Condition Of Concrete Dams By Moe Momayez, Applied Wave Technologies, Inc., Montreal, Canada Ferri Hassani, Subsurface Sensing Laboratory, McGill University, Montreal, Canada K.Saleh, Concrete Group, IREQ, Hydro-Quebec, Varennes, Canada and P. Guevremont, , Subsurface Sensing Laboratory, McGill University, Montreal, Canada ABSTRACT For the past seven years, the Subsurface Sensing Laboratory at McGill University, HydroQuebec and Applied Wave Technologies, Inc. have been involved in a project to assess the concrete condition in hydroelectric dams. This paper presents the results of a field campaign to test the MSR (Miniature Seismic Reflection) technique on a 20 m long buttress. This improved Impact-Echo technique is based on detection of multiple P and S wave reflections on the surface of concrete. Knowing the P and S wave velocities in the section of concrete under investigation, one can use this information to look for the presence of flaws in concrete and determine the integrity of the material in terms of its elastic properties. The main advantages of this method are repeatability and high degree of accuracy, which help in locating flaws as well as determining the modulus of elasticity and Poisson’s ratio. A large number of tests were conducted at the base of the buttress and at a distance of 14 m from the ground for the purpose of calibration and mapping of anomalies. The average P wave velocity was found to be 3750 m/sec. Two major anomalies and the plane of construction joints were successfully mapped. The results show that this technique can be used as a viable tool to monitor the condition of concrete in large civil engineering structures. Introduction The use of nondestructive evaluation (NDE) techniques has increased substantially in the field of civil engineering since the 1980’s due mainly to the need to monitor and maintain existing infrastructures. Recent technological advances have made these techniques quick and useful to users. Areas of interest include among others, crack detection and material property evaluation in concrete structures especially in dams. This paper is the result of an on-going collaboration between the Subsurface Sensing Laboratory at McGill University, Hydro-Quebec and Applied Wave Technologies, Inc. to use NDE methods to detect and map internal cracks in concrete dams as well as assess the quality of concrete in terms of its mechanical properties such as strength, dynamic modulus of elasticity and Poisson’s ratio. It presents the results of a campaign to map anomalies in a buttress at the MANIC 5 multi-arch hydroelectric dam in Quebec. Background When the surface of a material is submitted to a force such as an impact, internal compression (P) waves and shear (S) waves are generated. On the surface, Raleigh (R) waves are produced. The P and S waves travel along hemispherical wavefronts inside a medium. P waves are approximately 1.6 times faster than S waves. Figure 1 illustrates this principle. Wave reflections are caused by a sudden change in the acoustic impedance of the medium. The angle of incidence, radiation pattern, elastic material properties, and acoustic properties of the second medium all contribute to the amplitude of the reflected waveform received at the surface of the object being tested. The reflected waves are detected by piezoelectric 1


transducers that respond to vertical and shear movements on the surface of the material. The reflected P and S waves create a cyclic waveform at the surface (vertical and tangential, respectfully). The signals are captured by a data acquisition system and displayed in the time domain. The time domain signals are then converted separately to the frequency domain by applying a Fast Fourier Transform (FFT) algorithm. In the resulting frequency spectrums (one for the P wave and one for the S wave), the dominant peek reflection frequencies can be determined.

Impact source

S

P

Amplitude

FFT

T

Dynamic

Crack

Cp T=

2f

E, ν, Shear, Bulk

Frequencies for P and S waves Figure 1: Schematic of the MSR system. The frequency peaks are the result of multiple P and S wave reflections between the surface and a boundary. The periods of P and S wave arrivals at the surface of a medium (∆tp and ∆ts) are related to the distance the waves travel (twice the thickness between the interfaces), 2T, and their respective velocities Cp and Cs :

∆t p =

2T 2T and ∆t s = Cp Cs

Knowing that period and frequency have an inverse relationship, the frequencies of the P and S wave reflections can be written as:

fp =

Cp 2T

and f s =

Cs 2T

The P and S wave velocities Cp and Cs in an elastic medium can therefore be determined by the following equations:

C p = 2T × f p and C s = 2T × f s The P wave velocity (Cp) and the S wave velocity (Cs) are dependent on the material properties of the tested medium namely, dynamic Young’s Modulus (E), Poisson’s ratio (ν), and the material density (ρ). In 1970, Timoshenko and Goodier related these properties to the velocities 2


of P and S waves in a homogeneous, linear elastic, and isotropic solid. The velocity equations are given below:

Cp =

E (1 −ν ) E and Cs = 2ρ (1 − ν ) ρ (1 + ν )(1 − 2ν )

The use of long wavelength, low frequency seismic waves allows the users to evaluate concrete as a homogeneous medium. This occurs as long as the wavelength is longer than the largest aggregate diameter in the concrete. When narrow beams of short wavelengths are used, wave dispersion and scattering occur inside the concrete and may render these types of beams inappropriate for defect location in large concrete structures. The Subsurface Sensing Laboratory at McGill University has developed a system based on the principles described above called Miniature Seismic Reflection (MSR). It is capable of measuring thickness, detecting flaws and delaminations and determining the dynamic mechanical properties of rock and concrete. Field Experiments Concrete dams suffer from many forms of deterioration such as alkali-aggregate reaction, surface delaminations, reinforcement corrosion and internal cracking. The development of cracks in these hydraulic structures remains an important preoccupation for their owners. Thermal, physical, and hydrostatic cycles contribute to the propagation of internal cracks, which to some extent weaken dams. These internal cracks often have irregular profiles and inclinations. For crack detection and location, the MSR’s vertical displacement P wave transducer is used. This paper presents the results of the investigation campaign conducted on buttress 8 of the Manic 5 dam. The picture below shows the buttress 8 where all data were collected from the west side and on the face at the height of the footbridge. To calibrate the system and to measure an average P wave velocity, various locations on the west side of the buttress were considered and tested. One such location was the area at the same elevation as the footbridge. Here, the thickness of concrete was 19.2 m and the P wave velocity was 3750 m/s, which is indicative of a good quality concrete. It is important to note that wave attenuation over such a large distance is great. However, because P and S wave velocity measurements using the MSR system are extremely accurate, that is there is no variation from one test to another, the P wave reflection frequency was found to be 97,66 Hz.

Picture of the ladder and the footbridge on buttress 8.

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Tests On The West Side – Base; Description Two scan lines were setup near the base of the buttress in order to verify the results in this section. Figure 2 shows all the details. The first point is situated downstream on the lower line. Point number 2 is located on the left of point 1 at a distance of 50 cm. On the upper line, point number 3 is situated at a distance of 50 cm on top of point number 2. The horizontal grid spacing for the remaining points is 100 cm. The total number of points is 28 over a distance of 13.5 m. The vertical distance between point number 26 and the ground is approximately 50 cm. The two horizontal lines follow a construction joint, which starts near the ladder. Points 24, 26, 27 and 28 are located in plot DE. A sampling frequency of 50kHz was used and a total of 4096 points were collected. This corresponds to a total signal length of 81.92 msec and a frequency resolution of 12.2 Hz. A 400 g spherical tip hammer was used to generate the stress waves with a contact time between 100 and 200 msec, thus providing all the P wave reflection frequencies required for this project. Results and Discussion Figure 3 presents graphically the results of these tests. The analysis of the data showed 3 major reflections. One must mention that the cracks in the buttress are relatively small and are usually filled with calcite deposits. Because of this, a portion of the incident wave travels through the crack, and therefore the P wave reflection frequency from these anomalies has lower amplitude. Table 1 provides a summary of the data analysis. It is important to note that the anomaly situated at a depth of 3.0 m (between 2.19 m and 3.75 m) is present at every point suggesting that it may be due to an irregular or extremely thin crack or a construction joint. The analysis revealed the presence of a second anomaly at a depth of 5.0 m (between 4,65 and 5,91 m). This anomaly is not present in plot DE (point 24 to 28) because the frequency corresponding to P wave reflections from a depth of 5.0 m was absent in the analyzed data. It was concluded that this crack is present only in plot CD. A third anomaly was detected at a depth of 11.82 to 13.96 m. Few points (points 6, 8, 13 and 27) show a discrepancy along the survey lines; however, the results on average are very consistent. One must take into account the fact that an open crack or a void can mask any feature that may present behind it. Such may be the case at points 6 and 8 (or other points) where the calculated depth is 10 m. At point 24, the survey picked up the entire thickness of the buttress. Figure 4 shows the location of each interface in plan view. Tests On The West Side – Ladder; Description This series of tests was conducted to follow the progression of anomalies in the vertical direction all the way to the height of the footbridge along the access ladder. Figure 5 shows the location of the stations along the scan line. The first point is directly above point number 25 at a distance of 0.5 m. The distance between the subsequent points is 1 m except in the case of points 33 and 34. Here, the distance was 1.2 m because of the presence of a construction joint. In order to maintain the same spacing between the remaining stations as before, the distance between points 34 and 35 was set at 0.8 m. The last station is point number 43.

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Plot CD

Plot DE Construction Joint

C8 WEST

1,5 m

1,0 m

5 3

25

28

0,5 m 4 2 1 13,5 m

Figure 2: Scanning grid at the base of the buttress. PLOT CD

Downstream Face

PLOT DE 3,27 28

3,41 27

3,75 25

2,19 23

3,84 21

--19

3,20 17

3,74 15

3,14 13

--11

3,27 9

--7

26 ---

24 ---

22 ---

20 3,14

18 ---

16 ---

14 3,01

12 ---

10 ---

8 ---

6 ---

--- 3,07 5 3

4 2 1 3,41 --- ---

(a) PLOT CD

Downstream Face

PLOT DE 5,29 28

--27

--25

4,65 23

--21

4,80 19

4,52 17

5,91 15

4,65 13

5,12 11

5,30 9

4,96 7

4,965,12 5 3

26 ---

24 ---

22 5,12

20 4,96

18 4,96

16 4,96

14 ---

12 5,12

10 4,66

8 ---

6 4,80

4 2 1 , 512 5,12 4,96

--5

(b) PLOT CD PLOT DE 12,80 12,80 28

10,24 27

--25

11,82 23

--21

12,80 19

15,36 17

--15

10,24 13

12,80 11

12,80 9

12,80 7

26 12,80

24 19,20

22 13,96

20 13,96

18 11,82

16 11,82

14 10,97

12 13,96

10 13,96

8 17,06

6 17,06

3

Downstream Face

4 2 1 , 13,96 12,80 12,80

(c) Figure 3: Impact-Echo results at the base of the buttress from the west side: (a) first interface, (b) second interface, (c) third interface. 5


Table 1: Detected Depths At The Base Of Buttress 8 On The West Side. Test Point Interface 1 (m) Interface 2 (m) Interface 3 (m) 1 — 5,12 12,80 2 — 4,96 12,80 3 3,07 5,12 12,80 4 3,41 5,12 13,96 5 4,96 — —♦ 6 — 4,80 17,06 7 — 4,96 12,80 8 — — 17,06 9 3,27 5,30 12,80 10 — 4,65 13,96 11 — 5,12 12,80 12 — 5,12 13,96 13 3,14 4,65 10,24 14 3,01 — 10,97 15 3,74 5,91 — 16 — 4,96 11,82 17 3,20 4,52 15,36 18 — 4,96 11,82 19 — 4,80 12,80 20 3,14 4,96 13,96 21 3,84 — — 22 — 5,12 13,96 23 2,19 4,65 11,82 24 — — 19,20* 25 3,75 — — 26 — — 12,80 27 3,41 — 10,24 28 3,27 5,29 12,80 * Thickness of buttress ♦ Frequency obtained from the signal does not correspond to the depth of interface

Results and Discussion Table 2 presents the results obtained from this phase of the investigation. A total of 21 points were tested. It can be seen that a number of anomalies are present in this section of the buttress. However, the interface at a depth of 3.0 m could not be identified as conclusively as in the case of the tests conducted at the base of the buttress. Since this interface was not detected consistently, it was assumed that the detected anomaly is caused not by a crack but rather by a construction joint. In general, experience has shown that a crack reveals itself in a continuous manner in the analyses of the data. The construction plans show that two interfaces converge at a depth of 3.5 m around points 48 and 49. Due to their position and proximity to the downstream surface of the buttress, it is possible that a lot of dispersion occurs in the reflection signals making it difficult to obtain a clear frequency peak. The analysis of the data shows the presence of two more anomalies. The first of the two interfaces is located at a depth of 4.65 m to 6.12 m while the second interface was found to be at a depth of 11.82 m to 13.96m. At point 42, which is located near the footbridge, the entire thickness of the buttress was detected. It can therefore be assumed that the velocity obtained during the calibration tests is valid in this section of the buttress as well. 6


Crest of the dam

C8 West

C8 East

Footbridge 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29

3,0 m

5,0 m

12,0 m

13,0 m

Figure 4: Location anomalies on the buttress face.

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46

43

44 45 49

42

48 47

41 40 39 13,5 m

Plot DE

38 37 36

Plot CD

35 34 33 32 31 30 29

1,0 m 0,5 m

Figure 5: Location of scan line near the ladder on the west side of the buttress. Tests On The Downstream Face – Footbridge; Description The third series of tests were conducted on the footbridge to map the debonding of the construction joint between plot CD and DE. Figure 6 shows the location of the stations along the two scan lines, the lower line being at the same elevation as the base of the footbridge. The horizontal and vertical separation between each point is 1 m. Because some areas near the edges of the buttress were replaced, it was difficult to place the stations at the desired location. Such is the case for points 50 and 51 located at 0.31 m from the west edge, while points 88 and 89 are at a distance of 0.31 m from the east edge. Points 86 and 87 are 1.2 m from the east edge of the buttress. Results and Discussion The objective of these tests was to identify any opening in the construction joints that might have occurred over time. Due to the thickness of the buttress between the upstream and downstream face and the presence of many horizontal construction joints, it was expected to get many spurious peaks in the frequency spectrums. To remedy this situation and to detect the correct peak in the spectrum the following assumption was made. Since the location of the construction joints were known and since the P wave velocity in the concrete was found to be consistent with the calibrated value, the corresponding peak in the spectrum can easily be calculated. If the desired peak is not present or very weak, it means that the wave has been able to pass through the joint indicating no debonding. However, if we get a strong reflection anywhere else, including at a location where a construction joint is expected, it is indicative of a major opening or flaw in the concrete.

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West

East

0,31 m 1,00 m

1,00 m

0,31 m 1,20 m

1,00 m

50 52

54

56

58

60

62

64

66

68

70

72

74

76

78

80

82

84

86

88

51 53

55

57

59

61

63

65

67

69

71

73

75

77

79

81

83

85

87

89

Ladder Figure 6: Location of test points on the downstream face of the buttress at the footbridge level. Table 2: Detected depths on the west side of the buttress near the access ladder. Test Point Interface 1 (m) Interface 2 (m) Interface 3 (m) 29 3,94 4,65 17,06 30 — 12,80 —♦ 31 3,41 5,12 11,82 32 — — 12,80 33 — 6,14 13,96 34 2,65 — 11,82 35 — 5,30 11,82 36 3,34 5,30 13,96 37 — 4,80 15,36 38 — 4,80 — 39 — 5,30 — 40 — 5,12 13,96 41 — 4,96 11,82 42 — 4,66 19,20* and 12,80 43 — 5,49 12,80 44 3,57 — 12,80 45 — 5,91 — 46 — 5,12 12,80 47 — 4,80 12,80 48 2,56 and 3,49 5,91 — 49 2,4 and 3,07 5,12 — * Thickness of buttress ♦ Frequency obtained from the signal does not correspond to the depth of interface

The above assumption is best illustrated in the case of points 51, 54, 69, 72 and 81 where the reflection peak was almost invisible. Figure 7 presents a typical spectrum for point 54. It was concluded that the joint at this location is completely closed. On the other hand, the analysis shows the presence of the construction joint, labeled D, at a depth varying from 8.5 to 10.2 m. These results are obtained assuming a constant velocity of 3750 m/sec. However, many researchers have reported that major changes in P wave velocities occur within the same body of concrete. At present, it is difficult to measure these variations on a continuous basis from one point to another. Assuming that the depth of the joint does not change laterally, the analysis of the data provides a P wave velocity varying between 3347 m/s and 4240 m/s which well within the range of P wave velocities commonly encountered in concrete.

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Voltage Units

Figure 7: Frequency response of the wave captured at point 54. Figure 9 is a three-dimensional view of the joint in Plot CD. This representation is based on a P wave velocity of 3750 m/s. It shows the location the scanning grid. It shows a good visual correlation between the actual position of the grid and the results of the MSR Impact-Echo tests.

PLOT CD

Joint D PLOT DE

Figure 9: 3D representation of joint D in the buttress. Conclusion This paper presented an application of the MSR Impact-Echo technique on large civil engineering structures. A typical MSR Impact-Echo system can measure a thickness in the order of one to two meters. The system developed by the Subsurface Sensing Laboratory is the first prototype to measure concrete thickness in excess of 15 m. The system was used on a section of a buttress dam to evaluate the condition of concrete, to detect joints as well as to assess its overall performance and capacity. The tests were conducted in a section of the dam where the concrete was known to be in a good condition. Three vertical joints in the lower section of the buttress and one horizontal joint at an elevation of 8.5 m were successfully delineated.

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Acknowledgement The authors would like to thank Hydro-Québec for its financial support of this project. Special thanks are due to Mr. G. Verzeni, Director of Dam Safety, Mr. P. Nguyen, Head of Dams and Civil Structures, Mr. A. Massad, Head of Civil Structures and Materials and the staff of HydroQuébec’s offices in the Manicouagan region for their collaboration during the field test. References Hassani, F.P., Guevremont, P., Momayez, M., Saleh, K. and Tremblay, S., 2001. “A New Method for Testing Concrete in Dams”. Hydro-Review Magazine, Vol. XX, No.1, March 2001, pp.54-64. Momayez, M., Hassani, Guevremont, P. and O’Donnell, Denis, 2001. “Evaluation of Shotcrete Rock Support Systems in Underground Mines by a New Non-Intrusive Technique”. MineSpace 2001, April 29th-May 2nd, Québec, Canada. Momayez, M., Guevremont P. and Hassani F.P., 1999. “Nondestructive Shotcrete Thickness Measurement in Underground Mines”. 9th ISRM International Congress, August 25-28, paper CAN 13, pp 1297-1302., Paris, France. Sadri, A., Hassani, F.P. and Momayez, M., 1998. “Miniature Seismic Reflection (MSR) System for Evaluation of Physical Properties of Shaft and Tunnel Linings”. Journal of Pure and Applied Geophysics, Vol. 150, No. 3-4, pp. 677-691. Timoshenko, S.P., and Goodier, J.N., 1970. “Theory of Elasticity”. McGraw-Hill, New York, 3rd edition. Authors Moe Momayez, PhD, P.Geo, is president of Applied Wave Technologies, Inc., a company that specializes in the design and manufacturing of nondestructive testing instruments for monitoring of civil and mining infrastructures. Ferri Hassani, PhD, C.Eng. is the Webster Chair Professor of Rock Mechanics and Director of Mining Program at McGill University. His areas of research are Rock Mechanics, Geosensing and backfill design. Kaveh Saleh, PhD, P.Eng. is the supervisor of Hydro-Quebec research projects in civil engineering. He is involved in monitoring, diagnostics and repair of hydro-electric concrete structures. Philippe Guevremont, M.Eng., P.Eng. is a PhD candidate in the Department of Mining, Metals and Materials Engineering at McGill University.

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