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The use of surface scanning waves to detect surface-opening cracks in concrete
The use. of surface scanning waves to detect surface-opening cracks in concrete R.L. Smith The use of ultrasonic surface waves to locate surface-opening cracks in concrete is described. The system works in both transmit-receive and reflection modes. Laboratory results are presented to demonstrate the effectiveness of the system, and the advantages and possible applications of the technique are discussed.
Keywords: ultrasonic
testing, surface waves, concrete, cracks
The use of ultrasonics for non-destructive testing of concrete has in general been limited to measurement of low frequency velocities for assessment of structural uniformity and correlation with compressive strength datall,21. Also very low frequency echo techniques have been used for gauging thick sections of concrete (say greater than 0.5 m). Standard ultrasonic pulse-echo techniques are extremely limited in their use for concrete testing because of the highly attenuating nature of most concrete, which has large aggregate particles, paste porosity and large air voids. A technique for ultrasonically detecting surface-opening cracks in concrete structures is presented in this paper. The problem of high attenuation is overcome by the use of ultrasonic waves propagating a few wavelengths only along the surface of the concrete. The system described here is an immersion type which would seem to be applicable to liquid-filled concrete tanks, underwater and offshore structures, but could also be developed for use on dry structures.
Ultrasonic pulser
Transmitter/ k receiver ~ /
I
,,-'¢;" .Oc
Surface wave propagation Surface waves, often known as 'Rayleigh waves', can be considered as consisting of two elastic waves, one of which is longitudinal and the other transverse, which propagate along the surface with the same velocity but which attenuate with depth according to different exponential laws. The resultant motion is an elliptical particle displacement with amplitude falling with depth of penetration, being about 20% of its surface value at a depth of one wavelength. These waves strictly only propagate at a surface bounded by a vacuum but in the present case a liquid-solid interface is a good approximation. The most important difference is that as the surface wave propagates along the solid-liquid interface it continually radiates a longitudinal wave at the "critical angle' into the liquid. This critical angle 0c is given approximately by
Oc = sin-X
VL(liquid) VT(solid)
Amiiiiiilr
er
Oscilloscope
Amp- x recorder
~
(1)
x-
y position control
C-scan (x-y) water bath Fig. 1 Schematicof the ultrasonicscanningsystem
0308-9126/84/050273-03 $3.00 © 1984 Butterworth Et Co (Publishers) Ltd NDT INTERNATIONAL. VOL 17. NO 5. OCTOBER 1984
273
where Vt is the longitudinal wave velocity in the liquid (~1.5 x 103 m s-1 for water) and lit the transverse wave velocity in the solid (*2.3 x 103 m s-1 for concrete). Thus 0 c is about 40 ° for a concrete/water interface. In the present inspection this mode conversion phenomenon is used to both initiate and receive the surface
F ......
1:3.! E t3
o31
J
wave.
L,__
Equipment
A
Figure 1 is a schematic of the immersion scanning system used. A 0.5 MHz, 25 m m diameter compression wave transducer is used as a transmitter and is inclined to the surface to be scanned at the critical angle 0 c to induce the propagation of surface waves. The transducer is shock excited by a high-power ultrasonic driver producing about a 1000 V pulse. This transducer also operates in the pulseecho mode (ie receives reflected surface waves). A second matched transducer faces the transmitter, also inclined at the critical angle, to receive the mode-converted surface wave (ie a compression wave). The two transducers are mounted in a conventional scanning frame. The distance between the two transducers is variable and is adjusted for optimum signal amplitude at the receiving transducer while ensuring that the surface wave propagates over an appreciable length of the concrete sample. This distance is usually of the order of a few centimetres, the wavelength of the surface waves being of the order of 4 to ~ mm. If a crack interrupts the path of the surface wave then the signal will be 'lost' at the receiver and if the crack is suitably orientated (ie perpendicular to the direction of propagation of the surface wave) an echo will be received at the transmitter. The signal analysis is, at present, carried out by a conventional analogue linear gate and pulse height
A
a
B
[
150ram
~.]
2 5 0 mm
Steel bar
~
\ \ \ \
N \ \ \ \
/ /xz"~
Extent of C-scan
/
b
250
mm
~
Fig. 2 Concrete test blocks: a -- block 1, [50 mm thick; b -- block 2, 90 mm thick
274
Distance along block I
Eo "5
A
Distonce along block I
B
b Fig. 3 Amplitude-distance scans across concrete block 1: a - - pulsereceive mode; b - - pulse-echo mode
discriminator and the signal amplitude recorded on an
x-y recorder of a 'Mufax' recorder with four levels of quantization for C-scan presentation.
Results Results of the examination of two concrete test blocks are presented here. The first of these (Figure 2a) was a rectangular block with machined surfaces and a machined slot at each end. Figure 3 shows the amplitudedistance recorder traces obtained by scanning along the length of the block. The horizontal axis is the distance along the block and the vertical axis is the received signal amplitude; Figure 3a is from using the pulse-receiver mode and Figure 3b is from the pulse-echo mode. The complementary nature of the two signals is immediately apparent. In the pulse-receive mode the slots are indicated by a lack of signal and in the pulse-echo mode the slot is indicated by a signal being present. The varying amplitude in Figure 3a is due to variations in the microstructure (and hence could also be used as a monitor of the consistency of the microstructure as demonstrated by Volkweinl31) but these effects can be averaged out by using the C-scan mode as presented for the second block. The second block was a piece of cast concrete containing a steel reinforcing bar. This block had been fatigued to produce surface breaking cracks (Figure 2b). The block was scanned in the C-scan mode over the area indicated in Figure 2b and the data recorded on the Mufax recorder. The results are presented in Figure 4 and the crack system is clearly defined. The cracks are effectively magnified in width (but not appreciably in length) due to the length of the surface wave path on the concrete. The C-scan indicates that the system in pulse-receive mode is not very sensitive to crack orientation as cracks both perpendicular (the best orientation for pulse-echo detection) and parallel to the surface wave propagation direction are clearly shown.
NDT INTERNATIONAL. OCTOBER 1984
to detect surface-opening cracks in concrete. The technique of using surface waves has a n u m b e r of advantages. N o r m a l bulk compressive or shear waves are highly attenuated by the coarse microstructure of most concrete. By using low frequencies and limiting the path length of propagation in the material to a few centimetres, large amplitude signals can still be received while the whole surface is scanned. As the surface wave has a transverse c o m p o n e n t the technique is still sensitive to liquid-filled cracks. Recent experiments have also shown that the surface wave technique will still operate even if the concrete is coated with other material, such as paint, resin or marine fouling, provided that the coating is less than one wavelength thick.
\
This work has demonstrated the potential of using scanning surface waves to automatically inspect large areas of concrete for surface-breaking cracks which may be o f significance to the integrity of the structure.
Author The author is in the N D T Centre, A E R E Harwell, Didcot, Oxon O X l l 0RA, UK,
References \ Fig. 4
B
Pulse-receive C-scan of concrete block 2
Conclusions It has been demonstrated that surface waves can be used
1 Thomsett, H.N. "The practical use of ultrasonic velocity measurements in the assessment of concrete quality" Mag of Concrete Research 32 (1980) pp 1-16 2 Reynolds, W.N. 'Measuring concrete quality nondestructively" BritJNDT26 No 1 (1984)pp ll-14 3 Volkwein, A. "Non-destructive testing of natural stone by ultrasonic attenuation measurements" Materialpr~fung 2,4 No 4 (1982) pp 119-124
Paper received 14 May 1984
NDT INTERNATIONAL. OCTOBER 1984
275
Keywords: ultrasonic
testing, surface waves, concrete, cracks
The use of ultrasonics for non-destructive testing of concrete has in general been limited to measurement of low frequency velocities for assessment of structural uniformity and correlation with compressive strength datall,21. Also very low frequency echo techniques have been used for gauging thick sections of concrete (say greater than 0.5 m). Standard ultrasonic pulse-echo techniques are extremely limited in their use for concrete testing because of the highly attenuating nature of most concrete, which has large aggregate particles, paste porosity and large air voids. A technique for ultrasonically detecting surface-opening cracks in concrete structures is presented in this paper. The problem of high attenuation is overcome by the use of ultrasonic waves propagating a few wavelengths only along the surface of the concrete. The system described here is an immersion type which would seem to be applicable to liquid-filled concrete tanks, underwater and offshore structures, but could also be developed for use on dry structures.
Ultrasonic pulser
Transmitter/ k receiver ~ /
I
,,-'¢;" .Oc
Surface wave propagation Surface waves, often known as 'Rayleigh waves', can be considered as consisting of two elastic waves, one of which is longitudinal and the other transverse, which propagate along the surface with the same velocity but which attenuate with depth according to different exponential laws. The resultant motion is an elliptical particle displacement with amplitude falling with depth of penetration, being about 20% of its surface value at a depth of one wavelength. These waves strictly only propagate at a surface bounded by a vacuum but in the present case a liquid-solid interface is a good approximation. The most important difference is that as the surface wave propagates along the solid-liquid interface it continually radiates a longitudinal wave at the "critical angle' into the liquid. This critical angle 0c is given approximately by
Oc = sin-X
VL(liquid) VT(solid)
Amiiiiiilr
er
Oscilloscope
Amp- x recorder
~
(1)
x-
y position control
C-scan (x-y) water bath Fig. 1 Schematicof the ultrasonicscanningsystem
0308-9126/84/050273-03 $3.00 © 1984 Butterworth Et Co (Publishers) Ltd NDT INTERNATIONAL. VOL 17. NO 5. OCTOBER 1984
273
where Vt is the longitudinal wave velocity in the liquid (~1.5 x 103 m s-1 for water) and lit the transverse wave velocity in the solid (*2.3 x 103 m s-1 for concrete). Thus 0 c is about 40 ° for a concrete/water interface. In the present inspection this mode conversion phenomenon is used to both initiate and receive the surface
F ......
1:3.! E t3
o31
J
wave.
L,__
Equipment
A
Figure 1 is a schematic of the immersion scanning system used. A 0.5 MHz, 25 m m diameter compression wave transducer is used as a transmitter and is inclined to the surface to be scanned at the critical angle 0 c to induce the propagation of surface waves. The transducer is shock excited by a high-power ultrasonic driver producing about a 1000 V pulse. This transducer also operates in the pulseecho mode (ie receives reflected surface waves). A second matched transducer faces the transmitter, also inclined at the critical angle, to receive the mode-converted surface wave (ie a compression wave). The two transducers are mounted in a conventional scanning frame. The distance between the two transducers is variable and is adjusted for optimum signal amplitude at the receiving transducer while ensuring that the surface wave propagates over an appreciable length of the concrete sample. This distance is usually of the order of a few centimetres, the wavelength of the surface waves being of the order of 4 to ~ mm. If a crack interrupts the path of the surface wave then the signal will be 'lost' at the receiver and if the crack is suitably orientated (ie perpendicular to the direction of propagation of the surface wave) an echo will be received at the transmitter. The signal analysis is, at present, carried out by a conventional analogue linear gate and pulse height
A
a
B
[
150ram
~.]
2 5 0 mm
Steel bar
~
\ \ \ \
N \ \ \ \
/ /xz"~
Extent of C-scan
/
b
250
mm
~
Fig. 2 Concrete test blocks: a -- block 1, [50 mm thick; b -- block 2, 90 mm thick
274
Distance along block I
Eo "5
A
Distonce along block I
B
b Fig. 3 Amplitude-distance scans across concrete block 1: a - - pulsereceive mode; b - - pulse-echo mode
discriminator and the signal amplitude recorded on an
x-y recorder of a 'Mufax' recorder with four levels of quantization for C-scan presentation.
Results Results of the examination of two concrete test blocks are presented here. The first of these (Figure 2a) was a rectangular block with machined surfaces and a machined slot at each end. Figure 3 shows the amplitudedistance recorder traces obtained by scanning along the length of the block. The horizontal axis is the distance along the block and the vertical axis is the received signal amplitude; Figure 3a is from using the pulse-receiver mode and Figure 3b is from the pulse-echo mode. The complementary nature of the two signals is immediately apparent. In the pulse-receive mode the slots are indicated by a lack of signal and in the pulse-echo mode the slot is indicated by a signal being present. The varying amplitude in Figure 3a is due to variations in the microstructure (and hence could also be used as a monitor of the consistency of the microstructure as demonstrated by Volkweinl31) but these effects can be averaged out by using the C-scan mode as presented for the second block. The second block was a piece of cast concrete containing a steel reinforcing bar. This block had been fatigued to produce surface breaking cracks (Figure 2b). The block was scanned in the C-scan mode over the area indicated in Figure 2b and the data recorded on the Mufax recorder. The results are presented in Figure 4 and the crack system is clearly defined. The cracks are effectively magnified in width (but not appreciably in length) due to the length of the surface wave path on the concrete. The C-scan indicates that the system in pulse-receive mode is not very sensitive to crack orientation as cracks both perpendicular (the best orientation for pulse-echo detection) and parallel to the surface wave propagation direction are clearly shown.
NDT INTERNATIONAL. OCTOBER 1984
to detect surface-opening cracks in concrete. The technique of using surface waves has a n u m b e r of advantages. N o r m a l bulk compressive or shear waves are highly attenuated by the coarse microstructure of most concrete. By using low frequencies and limiting the path length of propagation in the material to a few centimetres, large amplitude signals can still be received while the whole surface is scanned. As the surface wave has a transverse c o m p o n e n t the technique is still sensitive to liquid-filled cracks. Recent experiments have also shown that the surface wave technique will still operate even if the concrete is coated with other material, such as paint, resin or marine fouling, provided that the coating is less than one wavelength thick.
\
This work has demonstrated the potential of using scanning surface waves to automatically inspect large areas of concrete for surface-breaking cracks which may be o f significance to the integrity of the structure.
Author The author is in the N D T Centre, A E R E Harwell, Didcot, Oxon O X l l 0RA, UK,
References \ Fig. 4
B
Pulse-receive C-scan of concrete block 2
Conclusions It has been demonstrated that surface waves can be used
1 Thomsett, H.N. "The practical use of ultrasonic velocity measurements in the assessment of concrete quality" Mag of Concrete Research 32 (1980) pp 1-16 2 Reynolds, W.N. 'Measuring concrete quality nondestructively" BritJNDT26 No 1 (1984)pp ll-14 3 Volkwein, A. "Non-destructive testing of natural stone by ultrasonic attenuation measurements" Materialpr~fung 2,4 No 4 (1982) pp 119-124
Paper received 14 May 1984
NDT INTERNATIONAL. OCTOBER 1984
275