The Application of Air-Coupled Impact-Echo Method to Detect Concrete Structure
The damage that is induced by corrosion is one of the most serious problems affecting the service life for reinforced concrete structures. Corrosion-induced delamination causes the concrete bridge deck deterioration . Recently, structural failures in Europe have attracted concern about post-tensioned concrete structures; voids within grouted tendon ducts, due to insufficient grout filling, significantly accelerate the corrosion of the embedded steel tendons . Nondestructive evaluation (NDE) techniques that can detect, locate, and characterize delamination and duct void defects in concrete draw great interests to agencies of infrastructure management . Air-coupled impact-echo is one of the successful applications for nondestructive evaluation of concrete. And this final report will describe the air-coupled impact-echo method and discuss how it applies to a concrete structure.
- Description of Impact-echo Method
First, let us briefly start with what is the impact-echo method. The impact-echo method is a technique used for detecting cracks inside structures. This method is based on monitoring the surface motion due to a short-duration mechanical impact. And it overcomes many barriers associated with flaw detection. We first tap an object with a hammer and the integrity of the structural member can be assessed based on whether the result is a high-pitched sound or a low-frequency sound. The method is subjective because it depends on the experience of the operator, and it is only limited to detecting near surface defects.
In the nondestructive testing of metals, the ultrasonic pulse-echo technique is well known to be a reliable method for locating cracks or other internal defects. An electromechanical transducer is used to produce a short pulse of stress waves which can propagate into the inspected object. Reflection of the stress pulse occurs at boundaries divided materials with different densities and different kinds of elastic properties. The transducer that also acts as a receiver will receive the reflected wave. The received signal is displayed on an oscilloscope, and the time of travel of the pulse is measured electronically. With the known speed of the ultrasonic stress wave, we can determine the distance to the reflecting interface .
2.1 Basic relationship
When a disturbance (stress or displacement) is suddenly applied at a point on the surface of a solid, such as through an impact, the disturbance can propagates through the solid as one of three different types of stress waves: a P-wave (In SE 263, it is called a longitudinal wave), an S-wave (In SE 263, it is called a shear wave), and an R-wave (In SE 263, it is called a Rayleigh or surface wave). As shown in Figure 1, the P-wave and S-wave propagate into the solid along spherical wave fronts. The P-wave is associated with the normal stress propagation and the S-wave is related to shear stress. Besides, the R-wave can travel away from the disturbance along the surface of object.
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Figure 2 demonstrates the results of a finite-element analysis of an impact response concerning a plate . This figure is a plot of the nodal displacements in the finite element mesh. In this analysis, the S-wave arrives at the bottom of the plate and the P-wave reflection is around halfway up the plate.
In an isotropic, infinite and elastic solid, the P-wave speed, Cp, is associated with Young’s modulus of elasticity, Poisson’s ratio and the density.
The S-wave, Cs, is below:
where G is the shear modulus of elasticity.
The ratio of S-wave speed to P-wave speed is below:
Generally, with a Poisson’s ratio of 0.2 for concrete, ratio equals 0.61. The ratio of the velocity of R-wave, Cr, to the S-wave speed is given by the following approximate formula:
2.2 Impact-echo Method
The greatest success in the real-world application of stress wave methods for flaw detection in concrete is using mechanical impact to produce the stress pulse. The impact generates a high energy pulse that could penetrate deep into the concrete. The first productive applications of impact methods occurred in geotechnical engineering to evaluate the integrity of concrete piles and caissons. The technique is known as the sonic-echo or seismic-echo methodology [ACI 228.2R]. The impact response of skinny concrete members is more complicated than the one of long slender members. The research by , however, led to the development of the impact-echo method, which has proven to be and well known as a powerful technique for flaw detection in relatively skinny concrete structures.
Figure 3 is a schematic diagram of impact-echo testing on a concrete plate with an air void below the surface. The impact on the surface produces P-waves and S-waves that travel into the plate and R-waves that travel away from the impact point. The P-waves and S-waves are reflected due to the internal defects or external boundaries. When the reflected waves return to the surface, they will cause displacements which are measured by a receiving transducer. If the transducer is placed nearby the impact point, P-wave echoes will dominate the response . The right-hand side of Figure 3 shows the pattern of surface displacements that will occur. The large downward displacement at the start of the waveform is caused by the R-wave. The series of repetitive downward displacements of lower amplitude is because of the arrival of the P-wave when it experiences multiple reflections between the surface of concrete and the internal void.
2.3 Frequency Analysis
In the initial work resulting in the impact-echo method, time domain analysis is used to measure the time from the beginning of the impact to the arrival of the P-wave echo . While this was doable, the whole process was time-consuming and required skills to determine the time of P-wave arrival properly. A key development resulting in the success of the impact-echo method was the utilization of frequency analysis rather than time domain analysis of the waveforms .
The principle of frequency analysis is shown in Figure 4. The P-wave generated by the impact experiences multiple reflections between the surface of concrete and the reflecting interface. The P-wave arrives at the test surface for each time, which causes a characteristic displacement. Therefore, the waveform contains a pattern that based on the round-trip travel distance of the P-waves. If the receiver is near the impact point, the round-trip travel distance is double distance of surface and interface. As shown in Figure 4, the time interval between successive arrivals of the various reflected P-wave is the distance of wave travel divided by the wave speed. And the frequency (f) of the P-waves arrival is the inverse the time interval and has the approximate relationship below:
Cpp = the P-wave speed travels the thickness of the plate,
T = the depth of the reflecting interface.
2.4 Amplitude spectrum
In frequency analysis of impact echo method, the goal is to determine the dominant frequencies in the waveform. Here, we need to use the Fourier transform technique to transform the waveform into the frequency domain . The transformation produces an amplitude spectrum that shows the amplitudes of the various frequencies included in the waveform. Take plate-like structures as an example, usually, the thickness frequency is the dominant peak in the spectrum.
The peak frequency value in the amplitude spectrum after Fourier transform is used to determine the depth of the reflecting interface by expressing equation as below:
Figure 5 shows the use of frequency analysis of impact-echo tests.
Figure 5(a) shows an example from a test of the amplitude spectrum over a solid portion of a 0.5 m thick concrete slab. The peak frequency is at 3.4 kHz, which with respect to multiple P-wave reflections between the top and bottom surfaces of the slab. By using the equation above, the P-wave speed in the slab we got is 3420 m/s.
Figure 5(b) shows the amplitude spectrum over a portion of the slab containing a disk-shaped void  . The peak frequency at 7.3 kHz induced by multiple reflections between the top part of the slab and the void. The calculated depth of the void is 3420/(2 x 7300) = 0.23 m, which compares with the known distance of 0.25 m.
- Air-Coupled Sensing System
3.1 Air-Coupled Sensor
A measurement microphone manufactured by PCB Inc. was used in the air-coupled impact echo tests. It has a small size, 6.3 mm diameter, broad frequency range (4-80 kHz at ±2 dB), and high sensitivity 4 mV/Pa. This sensor is well compatible for impact echo scanning because it can detect a broad range of frequencies and the small size is able to improve spatial resolution in scanning tests .
A special enclosure was used to support the microphone. And it provides sound insulation to block ambient noise and direct acoustic waves.
Figure 6 shows a drawing of the microphone and the insulation enclosure. The enclosure wall has an inner layer of rubber, a cylinder made in aluminum, and an outer foam layer. The foam and aluminum work at the same time to absorb and reflect most ambient noise and direct acoustic waves, in the contrast, the inner rubber layer absorbs the leaky waves from the concrete surface and inhibit the formation of resonances inside the cylindrical enclosure cavity. The microphone is inbuilt into the enclosure by crossing a hole. The microphone height is able to be easily adjusted. Experimental studies were carried out to work on the sound insulating efficiency of the enclosure. The tests were implemented on two same concrete slabs that were placed near each other, but fully isolated. The impact was applied to one slab and at the same time, the sensor monitored that same slab at a fixed spacing configuration to measure the total response impact echo plus direct acoustic wave .
As shown in Figure 7, with respect to the total response amplitude, this design of enclosure reduced the amplitude of direct acoustic waves by 40% from 50 to 10%. Figure 7a shows the signals in the situation of without insulation, the direct acoustic wave/total response amplitude ratio is 1:2. In contrast, in Figure 7b, the ratio is 1:10. Meanwhile, ambient noise amplitudes were significantly dropped .
3.2 Air-Coupled Sensing
As far as the problem of slow testing rate of mechanical wave methods, one solution for it is the application of contact-less sensing. By erasing the contact between sensors and concrete surfaces, the possibility of an automated scanning system can be considerable. The air-coupled acoustic sensors can be used for contact-less mechanical wave detection in solids. However, the intrinsic rough surface of the concrete has some limitations in laser application. Despite the huge four orders of magnitude acoustic impedance mismatch between solids and air, air-coupled ultrasonic sensing has experienced rapid development in recent decades, especially for guided wave detection in metals . However, the inhomogeneous concrete limits the practical application of fully air-coupled contact-less excitation and detection ultrasonic methods . The wave energy transmission is obviously increased when contact is used with an air-coupled receiving sensor. Although this system is not fully contact-less, elimination of surface coupling for the receiving sensor reduces testing time and air-coupled sensors are prone to show improved signal consistency over contact sensors . Efforts to use air-coupled acoustic sensors to inspect concrete date back to 1973, when the Texas Transportation Institute in College Station, Tex. developed an automated delamination detection device called Delamatec . The essential components of Delamatec are automated tappers, a strip chart recorder, and acoustic receivers. When applied over sound defect-free concrete, the obtained time domain signal is very close to zero; the signal becomes irregular whenever delamination. However, due to poor accuracy, the application of the Delamatec has been limited.
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More recently air-coupled sensing for surface waves in concrete structures was proposed by Zhu and Popovics in 2001. The test results, which were proved by comprehensive theoretical analyses , demonstrated that directional microphones are very sensitive to leaky surface waves propagating along with concrete. Leaky surface waves exist at the boundary between a solid and arise from the propagating mechanical surface waves in the solid: The resulting wave motion at a point on the surface of the solid produce acoustic waves that leak into the surrounding fluid . Subsequent studies by Zhu  have shown that air-coupled sensors can replace contact sensors in most surface wave measurement tests. Further, air-coupled surface wave sensing can also be applied to locate surface cracks in concrete slabs .
Impact-echo is well known to be effective for defect detection in concrete. However, the application of air-coupled sensing for impact-echo is more challenging than for surface waves. Air-coupled impact-echo scanning tests were implemented over two concrete slabs that contain embedded artificial defects. Test results, shown as images, identify locations and areal size of most of the defects.
- Testing Setup
Figure 8 is shown is the testing setup of air-coupled impact-echo. The configuration is very similar to conventional impact-echo, and the only difference is there is no contact between the sensor and the test surface. The sensor is located nearby the impact location and the distance between the sensor axial projection point on the surface and the impact point x is less than 40% of the slab thickness. In air-coupled leaky surface wave detection , direct acoustic waves do not break the time signal as the acoustic waves can be isolated by increasing the source-receiver spacing x. Moreover, the leaky surface wave pulse is isolated and extracted by applying a Hanning window to the time signal. However, in the impact-echo testing scheme, the sensor is installed nearby the impact location. Because of the relatively small source-receiver spacing in the impact-echo test setup, the impact source induced much acoustic noise in the received signals, which cannot be isolated and eliminated in the time domain.
- Concrete Specimens Containing Artificial Defects
Two steel-reinforced concrete slabs were cast. The slabs are nominally 0.25 m thick with 1.5m by 2.0 m lateral dimensions. The 28-day compressive strength of the concrete is 42.3 MPa. The p-wave velocity of the mature concrete is 4,100–4,200 m/s. This results in a nominal full thickness impact-echo frequency of 7.8–8.0 kHz through previous researches .
Slab 1 contains two continuous embedded ducts. Each duct has three sections: fully grouted, half grouted, and un-grouted. The voids of the half-grouted and un-grouted regions are simulated with foam inserts. The diameters of both ducts are 70 mm, and the centerlines of the which are 125 mm below the surface. The same specimen also contains a surface-opening notch with linearly increasing depth. The plan view and cross-section of the slab source-receiver are shown in Figure 9 .
Slab 2 contains artificial delamination and voids of various sizes and depths. The plan view and cross-section of the slab is shown in Figure 10. Because the loading capacity of the slab is dramatically reduced by artificial defects, the slabs are reinforced in two dimensions and at two layers. The top layer of steel bars is supported by 5 steel chairs. The concrete cover thickness is 60 mm. Metal wire mesh 150mm by 150 mm was placed above each rebar layer. Artificial delamination was simulated by embedding 6 double-layer plastic sheets. Three double-layer sheets are located 60 mm below the surface sheets and three 200 mm below the top surface of bottom sheets. The actual depths of the sheets were measured in the slab form before casting concrete and are shown in Table 1 .
- Air-Coupled Impact-Echo for Delamination Detection
6.1 Point Test Result
Individual air-coupled impact-echo tests were implemented over solid no defect and all defect regions on Slab 2. The goal here is to demonstrate that contact and air-coupled impact-echo results are effectively equivalent. The nine defects are labeled 1 to 9 from the left top corner to the right bottom corner Figure 10. The sensor was installed over the center of each defect where defect regions were tested .
The results from air-coupled impact-echo are shown in Table 1. The test locations are grouped based on the type of defects. For instance, Test Locations 1, 3, and 6 are over shallow delamination about 55 mm below the top surface. There is a defect at Point 0, where the frequency 7.8 kHz was obtained .
Flexural mode frequency measurement is more compatible and gets less influenced by ambient noise than that of the impact-echo resonance mode. The flexural mode frequency measured by the air-coupled sensor without the sound insulation enclosure agrees with the consequence measured by a standard contact sensor. The emitted impact sound direct acoustic waves frequency corresponds to the flexural mode frequency. This theory explains why conventional sounding methods .
Because the signals over shallow delamination are dominated by flexural vibration, the depth of delamination cannot be inferred directly from the dominant peak frequency. Based on the conventional impact-echo equation, the impact-echo mode frequency should be large for shallow delamination. Therefore, broader frequency contents must be investigated if depth information is to be acquired. Figure 11a and b show impact-echo signals acquired from the air-coupled sensor over a shallow delamination Defect 8. In the frequency spectrum, a peak at 33.0 kHz is seen in addition to that at 2.7 kHz, which corresponds to the flexural mode.
For exciting high frequencies, a very small ball (5 mm diameter impactor) need to be used since the maximum exciting frequency is inversely related to increasing ball size . The 33.0 kHz peak frequency, however, is hard to detect with a standard contact impact-echo sensor even when a small impactor is used, as seen in Figure 11c and Figure 11d. The contact sensor detects vertical out-of-plane displacement on the concrete surface. The air-coupled sensor measures air pressure, which is equivalent to the out-of-plane velocity on the surface . Displacement sensors are less sensitive to higher frequency content than velocity sensors. Velocity responses can represent satisfactory sensitivity across a broader and higher range of frequency.
6.2 Two-Dimensional Imaging
A two-dimensional 2D scanning test was implemented over the entire area of Slab 2 with 200 cm by 150 cm. The measurement of grid spacing is x = y = 10 cm for both directions; therefore, totally, 261 signals were acquired. None of the data were collected along the slab edges. The property of the contact-less sensor allowed enough scanning of the specimen, with a testing time of approximately 10 seconds per point. The testing efficiency would be improved in the future if an array of sensors were employed. Data were collected along parallel to the scan lines. A 2D matrix composed of the frequency of the amplitude spectrum of a signal at the highest amplitude peak frequency at each testing location is used for image construction. Figure 12 shows the 2D scan contour image of Slab 2. The image was created using the “contour” plotting function for the contour lines in MATLAB. In the color image, warm colors represent high frequencies, while cold colors represent low frequencies. The designed defect locations and areal size are demonstrated on the image with solid lines .
Most of the defects are identified in the image. The large and shallow delamination and voids agree well with the actual areal size. For the small defects, the image shows frequencies that are lower than the normal full-thickness frequency. This indicates the possible presence of small defects. Although the size and depth of the small defects can’t be precisely determined, they can still be different from the surrounding solid regions. Hot spots high frequency is observed over Defect 2 and 7, which indicates the existence of deep delamination. The peak frequency will shift to a lower frequency as the test point is located over the edges of the defect.
In addition to the nine designed defects, regions labeled “A” and “B” in Figure 12 show warmer colors from the surrounding solid regions. These regions show a higher frequency than the full-thickness frequency at 7.8 kHz even though there are no intended defects in these regions .
6.3 Refined Scans
Refined 2D scans were conducted over Defects 1 shallow delamination, 7, and 9 deep delaminations by using a 5 cm scan spacing. The imaged regions for Defects 1 and 7 are 40 cm by 40 cm of squares and for Defect 9 a 25 cm by 20 cm of the rectangle. The scan images are shown in Figure 13 below. The refined scans provide an improved definition of defect size, especially for the shallow delamination such as Defect 1. In Figure 13a, a clear boundary is observed between the delaminated and solid regions. The lowest flexural mode frequency is acquired over most of the central region over the defect, and the frequency increases when the test point moves to the nearby edges of the defect. In contrast, Figure 13b represents two-dimensional contour images of Defect 7 in slab 2, and Figure 13c represents two-dimensional contour images of Defect 9 in slab 2 .
Based on the testing and simulation we carried out above, we can get the conclusion below:
- Air-coupled sensing provides a method for the effective evaluation of concrete structures through imaging. Multiple point data that are presented together in one image can provide more diagnostic information than the same data which are evaluated individually .
- The amplitude spectra of individual air-coupled impact-echo signals are equivalent to those acquired from traditional contact impact-echo sensors. The same analysis and interpretation procedures can be applied to both cases .
- Air-coupled sensors with broad frequency response can offer much more information about the dynamic response of concrete slabs that include shallow delamination when both the impact-echo and flexural vibration responses from the delamination are sensed .
- In 2D impact-echo scan images, flexural resonances from shallow delamination permit precise definition of defect areal size. The depth of shallow delamination can’t be determined from the flexural resonance frequency directly. However, in contrast, the depth of deep defects can be determined directly from the impact-echo resonance frequency as measured above the center of the defect .
- Finer scan point spacing improves the ability to define defect areal size in the created 2D image. To define it in the image precisely, scan point spacing should be less than half of the areal size of a defect. A 2 cm scan spacing is adequate to define all defects in a concrete structure .
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