The impact-echo method is a technique for flaw detection in concrete based on stress wave propagation. A team of researchers at the National Institute of Standards and Technology (formerly the National Bureau of Standards) initiated a study in 1983 that developed the rudimentary basis for this method. Subsequently, research carried out at Cornell University, under the direction of Dr. Mary Sansalone, has refined the theoretical basis of the method, extended its applications to a broad spectrum of problems and lead to the development of a field test system. Studies have shown that the impact-echo method is effective for locating voids, honeycombing, delaminations, depth of surface opening cracks, and measuring member thicknesses. Details of these studies may be found in the listed publications. This write up provides the basic ideas of the technique.
The principle of the impact-echo technique is illustrated in Fig.1. A transient stress pulse is introduced into a test object by mechanical impact on the surface. The stress pulse propagates into the object along spherical wavefronts as P- and S-waves (Fig. 1(a)). In addition, a surface wave (R-wave) travels along the surface away from the impact point. The P- and S- stress waves are reflected by internal interfaces or external boundaries. The arrival of these reflected waves at the surface where the impact was generated produces displacements which are measured by a receiving transducer (Fig.1 (b)). If the receiver is placed close to the impact point, the displacement waveform is dominated by the displacements caused by P-wave arrivals. The displacement waveform can be used to determine the travel time, t, from the initiation of the pulse to the arrival of the first P-wave reflection. If the P-wave speed, Cp, in the test object is known, the distance, T, to the reflecting interface can be determined.
An impact-echo test system is comprised of three components:
A commercial impact-echo test system was developed in 1990 at Cornell University. It includes a rugged laptop style computer that functions as the data acquisition system. Special software has been written to set up the data acquisition parameters and perform the data analysis. The system also includes a handheld unit that houses the receiving transducer and a series of different-sized impactors. A suitable system can also be assembled from off-the-shelf components. The waveform analyzer (or data acquisition card) must have a high sampling frequency (500 kHz as a minimum), and the receiving transducer should preferably be a broadband displacement transducer. Accelerometers have been used but they must not have resonant frequencies in the range of those measured during impact-echo testing and additional signal processing is required.
The force-time history of the impact may be approximated as a half-sine curve, and the duration of the impact is the contact time (Fig. 1(b)). The contact time is an important variable because it determines the size of the defect which can be detected by impact-echo testing. As the contact time decreases, smaller defects can be detected. However, as contact time decreases the penetrating ability of the stress waves also decreases. Thus the selection of the impact source is a critical aspect of a successful impact-echo test system. For the development work performed at NIST, contact times on the order of 30 to 60 microseconds were obtained by dropping steel spheres of different diameters. In later work at NIST, a commercially available impactor was adapted for use as the impact source. The components of the impactor are shown in Fig. 2(a). The spherically-tipped mass is propelled by the spring-loaded device. A hardened steel ball is attached to the end of the device and contacts the test surface. The ball serves to increase the repeatability of the input pulse and shorten the contact time of the impact. Typically the impactor produces impacts with contact times ranging from 30 to 50 microseconds, depending on the characteristics of the concrete at the impact point. The commercially available impact-echo test system uses steel spheres on spring rods for the impact source. The user selects the size of the impactor based on the depth and size of flaw that is to be detected.
The receiving transducer must be capable of accurately measuring surface displacement. A conically-tipped transducer, developed at NIST as a reference standard for calibrating acoustic emission transducers, is used in current work. A special housing was built to hold the transducer so that it could be used on vertical surfaces (Fig. 2(b)). A thin lead strip is used to provide acoustic coupling between the transducer and the test surface. The same conical transducer is incorporated in the commercial test system.
A waveform analyzer, or computer with high-speed digital data acquisition hardware, is used to capture the transient output of the displacement transducer, store the digitized waveforms, and perform signal analysis. A suitable waveform analyzer, or data acquisition card, should have a sampling frequency of at least 500 kHz.
During the initial development of the impact-echo technique, interpretation of the recorded waveforms was performed in the time domain. This required establishing the time of impact initiation and the arrival time of the first P-wave echo. While this was feasible, it was found to be time-consuming. An alternative approach is frequency analysis of the displacement waveforms.
The principle of frequency analysis is illustrated in Fig. 3, which shows a solid plate of thickness T subjected to an impact-echo test.
The P-wave generated by the impact propagates back and forth between the top and bottom surfaces of the plate. Each time the P-wave arrives at the top surface it produces a characteristic displacement. Thus the waveform is periodic, and the period, t, is equal to the travel path, 2T, divided by the P-wave speed. Since frequency is the inverse of the period, the frequency, fp, of the characteristic displacement pattern is:
fp = Cp/2T Eq. (1)
Thus, if the frequency of an experimental waveform can be determined, the thickness of the plate (or distance to a reflecting interface) can be calculated:
Note that Eq. (2) is an approximation that is suitable for most applications in plate-like structures. When using the method to measure plate thickness, a correction factor is needed. For prismatic members, the value of the correction factor depends on the geometry of the member. Consult the listed references for additional information. In practice, the frequency content of the recorded waveforms is obtained using the fast Fourier transform (FFT) technique to obtain the amplitude spectrum. Appendix A gives additional background information on digital frequency analysis, and examples of its application may be found in the listed publications.
Figure 4 illustrates how frequency analysis is used in impact-echo testing. In Fig. 4(a) an impact- echo test was performed over a solid portion of a 0.5-m thick concrete slab. In the amplitude spectrum there is a frequency peak at 3.42 kHz. This frequency corresponds to multiple reflections between the bottom and top surfaces of the slab. Using Eq. (1) and solving for Cp, the P-wave speed is calculated to be 3410 m/s. Figure 4(b) shows the amplitude spectrum obtained from a test over a portion of the slab containing a disk-shaped void. The peak at 7.32 kHz results from multiple reflections between the top of the plate and the void. Using Eq. (2), the calculated depth of the void is 3410/(2 * 7320) = 0.23 m, which compares favorably with the known distance of 0.25 m.
Appendix A explains that the resolution in the amplitude spectrum, that is, the frequency difference between adjacent points, is equal to the sampling frequency divided by the number of points in the waveform record. This imposes a limit on the resolution of the depth calculated according to Eq. (2). Because depth and frequency are inversely related, it can be shown that for a fixed resolution in the frequency domain, the resolution of the calculated depth improves as the frequency increases, that is, as depth decreases.
In 1998, ASTM adopted a standard test method on using the impact-echo method to measure the thickness of concrete members (ASTM C 1383, Standard Test Method for Measuring the P-wave Speed and Thickness of Concrete Plates Using the Impact-Echo Method). The method involves two procedures. Procedure A is to determine the P-wave speed in the concrete by measuring the travel time between two surface receivers separated by a known distance. Procedure B is to measure the thickness of the member by measuring the thickness frequency using the impact-echo procedure. The method is applicable to plate-like structures in which the smallest lateral dimension is at least six time the thickness of the member. The standard includes procedures for estimating the systematic errors associated with the thickness measurement due to the digital nature of the data. In the background research leading to the standard, researchers at Cornell University found that Eq. (2) has to be modified by a factor of 0.96 to correctly estimate the thickness based on the measured P-wave speed from Procedure A and the measured frequency from Procedure B.
An impact on the top surface of an infinite plate results in multiple reflections of stress waves between the top and bottom surfaces. The multiple reflections give a periodic character to the displacement response at points close to the impact point. In finite solids containing flaws, multiple reflections occur between a variety of interfaces and free boundaries. As a result, time domain waveforms become complex and difficult to interpret. However, if the waveforms are transformed into the frequency domain, multiple reflections from each interface become dominant peaks in the amplitude spectrum -- at frequency values corresponding to the frequency of arrival of reflections from each interface. These frequencies can be used to calculate the location of the interface at each test point. It has been found that, for impact-echo testing, data interpretation is much simpler and quicker in the frequency domain than in the time domain.
The transformation from the time to the frequency domain is based on the idea that any waveform can be represented as a sum of sine curves, each with a particular amplitude, frequency, and phase shift. This transformation is carried out using the principles of the Fourier transform. As an example, Fig. A1(a) shows the digital time domain waveform, g(t), given by the function:
g(t) = sin (2 (pi) 20 t) + sin (2 (pi) 40 t) + sin (2 (pi) 60 t) Eq. (A.1)
where t = time, s.
This function is composed of three sine curves of different amplitudes having frequencies of 20, 40, and 60 Hz.
The digital sample in Fig. A1(a) is made up of discrete points. The time interval between points is 0.001 seconds; this is equivalent to a sampling frequency of 1000 Hz.
The objective of frequency analysis is to determine the dominant frequency components in the digital waveform. This is most easily accomplished by using the fast Fourier transform (FFT) technique. The FFT results can be used to construct the amplitude spectrum, which gives the amplitudes of the various frequency components in the waveform. The amplitude spectrum obtained by the FFT contains half as many points as the time domain waveform, and the maximum frequency in the spectrum is one-half the sampling rate, which for this example is 500 Hz. Figure A1(b) shows the initial portion of the computed amplitude spectrum; the peaks occur at 20, 40, and 60 Hz. Each of the peaks corresponds to one of the component sine curves in Eq. (A.1).
In the FFT technique, the frequency interval, deltaf, in the spectrum is equal to the sampling frequency divided by the number of points in the waveform. For this example, there are 256 points in the complete time domain waveform, and the frequency interval is equal to 1000 Hz divided by 256, or 3.9 Hz. Since the frequency interval is proportional to the sampling frequency, a slower sampling rate enhances resolution in the frequency domain. However, slower sampling rates leads to longer record lengths which can result in complex spectra due to reflections from side boundaries of the test object. Experience has shown that a sampling frequency of 500 kHz with 1024 points per record is desirable in most applications.
Sansalone, M. , Lin, J.M., Streett, W.B., "Determining the Depth of Surface-Opening Cracks Using Impact-Generated Stress Waves and Time-of-Flight Techniques," ACI Materials Journal, Vol. 95, No. 2, March-April 1998, pp. 168-177.
Jaeger, B.J., Sansalone, M.J., and Poston, R.W., "Using Impact-Echo to Assess Tendon Ducts," Concrete International, Vol. 19, No.2, February 1997, pp. 42-46.
Lin, J. M., and Sansalone, M., "A Procedure for Determining P-wave Speed in Concrete for Use in Impact-Echo Testing Using a Rayleigh Wave Speed Measurement Technique," Innovations in Nondestructive Testing, ACI SP-168, S. Pessiki and L. Olson, eds., American Concrete Institute, Farmington Hills, MI, 1997, pp.137-165.
Poston, R. and Sansalone, M, "Detecting Cracks in Beams and Columns of a Post-Tensioned Parking Garage Using the Impact-Echo Method," Innovations in Nondestructive Testing, ACI SP-168, S. Pessiki and L. Olson, eds., American Concrete Institute, Farmington Hills, MI, 1997, pp. 199-219
Sansalone, M. , Lin, J.M., Streett, W.B., "A Procedure for Determining P-Wave Speed in concrete for Use in Impact-Echo Testing Using a P-Wave Speed Measurement Technique," ACI Materials Journal, Vol. 94, No. 6, November-December 1997, pp. 531-539.
Sansalone, M, "Impact-Echo: The Complete Story," ACI Structural Journal, Vo. 94, No. 6, November-December, 1997, pp. 777-786.
Sansalone, M., and Streett, W. B., Impact-Echo: Nondestructive Testing of Concrete and Masonry, Bullbrier Press, 1997.
Sansalone, M.; Lin, J. M.; and Streett, W. B., "A Procedure for Determining Concrete Pavement Thickness Using P-Wave Speed Measurements and the Impact-Echo Method," Innovations in Nondestructive Testing, ACI SP-168, S. Pessiki and L. Olson, eds., American Concrete Institute, Farmington Hills, MI, 1997, pp. 167-184.
Williams, T. J.; Sansalone, M.; Streett, W. B.; Poston, R.; and Whitlock, R., "Nondestructive Evaluation of Masonry Structures Using the Impact-Echo Method," TMS Journal, The Masonry Society, Vol. 15, No. 1, June 1997, pp. 47-57.
Jaeger, B.J., Sansalone, M.J., and Poston, R.W., 1996, "Detecting Voids in Grouted Tendon Ducts of Post-Tensioned Concrete Structures Using the Impact-Echo Method," ACI Structural Journal, Vol. 93, No. 4, July-August, pp. 462-472.
Lin, J.M. and Sansalone, M.J., 1996, "Impact-Echo Studies of Interfacial Bond Quality in Concrete: Part I - Effects of Unbonded Fraction of Area," ACI Materials Journal, Vol. 93, No. 3, May-June, pp. 223-232.
Lin, J.M. and Sansalone, M.J., 1996, "Impact-Echo Studies of Interfacial Bond Quality in Concrete: Part II - Effects of Bond Tensile Strength," ACI Materials Journal, Vol. 93, No. 4, July-August, pp. 318- 326.
Cheng, C. and Sansalone, M., "Determining the Minimum Crack Width that can be Detected Using the Impact-Echo Method, Part 1: Experimental Study," Materials and Structures, Vol. 28, No. 176, March 1995, pp. 74-82.
Cheng, C. and Sansalone, M., "Determining the Minimum Crack Width that can be Detected Using the Impact-Echo Method, Part 2: Numerical Fracture Analyses," Materials and Structures, Vol. 28, No. 177, April 1995, pp. 125-132.
Lin, J.M., and Sansalone, M., "The Impact-Echo Response of Hollow Cylindrical Concrete Structures Surrounded by Soil or Rock, Part 1 - Numerical Studies," ASTM Geotechnical Testing Journal, Vol. 17, No. 2, June 1994, pp. 207-219.
Lin, J.M., and Sansalone, M., "The Impact-Echo Response of Hollow Cylindrical Concrete Structures Surrounded by Soil or Rock, Part 2 - Field Studies," ASTM Geotechnical Testing Journal, Vol. 17, No. 2, June 1994, pp. 220-226.
Cheng, C. and Sansalone, M. "The Impact-Echo Response of Concrete Plates Containing Delaminations: Numerical, Experimental, and Field Studies," Materials and Structures, Vol. 26, No. 159, June 1993, pp. 274-285.
Cheng, C., and Sansalone, M., "Effects on Impact-Echo Signals Caused by Steel Reinforcing Bars and Voids Around Bars," ACI Materials Journal, Vol. 90, No. 5, Sept-Oct 1993, pp. 421-434.
Lin, J.M., and Sansalone, M., "The Transverse Elastic Impact Response of Thick Hollow Cylinders," Journal of Nondestructive Evaluation, Volume 12, No. 2, 1993, pp. 139-149.
Lin, Y. and Sansalone, M., "Detecting Flaws in Concrete Beams and Columns Using the Impact-Echo Method," ACI Materials Journal, Vol. 89, No. 4, July-August 1992, pp. 394-405..
Lin, Y., and Sansalone, M., "Transient Response of Thick Circular and Square Bars Subjected to Transverse Elastic Impact," Journal of the Acoustical Society of America, Vol. 91, No. 2., February 1992, pp. 885-893.
Lin, Y., and Sansalone, M., "Transient Response of Thick Rectangular Bars Subjected to Transverse Elastic Impact," Journal of the Acoustical Society of America, Vol. 91, No. 5, May 1992, pp. 2674- 2685.
Pratt, D. and Sansalone, M., "Impact-Echo Signal Interpretation Using Artificial Intelligence," ACI Materials Journal, Vol. 89, No. 2, March-April 1992, pp. 178-187.
Sansalone, M., and Poston, R., "Detecting Cracks in the Beams and Columns of a Post-Tensioned Parking Garage Structure Using the Impact-Echo Method," Proceedings, Conference on Nondestructive Evaluation of Civil Structures and Materials, Boulder, CO, May 1992, pp. 129-137.
Carino, N.J., and Sansalone, M., "Detecting Voids in Metal Tendon Ducts Using the Impact-Echo Method," ACI Materials Journal, Vol. 89, No. 3, May-June 1992, pp. 296-303.
Lin, Y., Sansalone, M., and Carino, N.J., "Finite Element Studies of the Impact-Echo Response of Plates Containing Thin Layers and Voids," Journal of Nondestructive Evaluation, Vol. 9, No. 1, 1990, pp. 27-47.
Sansalone, M., Lin, Y., Pratt, D., and Cheng, C., "Advancements and New Applications in Impact-Echo Testing," Proceedings, International Conference on Evaluation and Rehabilitation of Concrete Structures and Innovations in Design, Hong Kong, December 1991, ACI SP-128, pp. 135-150 .
Lin, Y., Sansalone, M., and Carino, N.J., "Impact-Echo Response of Concrete Shafts," ASTM Geotechnical Testing Journal, Vol. 14, No. 2, June 1991, pp. 121-137.
Sansalone, M., Lin, Y., and Carino, N.J., "Impact-Echo Response of Plates Containing Thin Layers and Voids," Proceedings, Review of Progress in Quantitative NDE, eds. D. Thompson and D. Chimenti, Vol. 9B, 1990, pp. 1935-1942.
Carino, N.J. and Sansalone, M., "Impact-Echo: A New Method for Inspecting Construction Materials," Proceedings, Conference on NDT&E for Manufacturing and Construction, Aug. 1988, Urbana, IL., H.L.M. dos Reis, Ed., Hemisphere Publishing Corp., 1990, pp. 209-223.
Sansalone, M., and Carino, N.J., "Finite Element Studies of the Impact-Echo Response of Layered Plates Containing Flaws," in International Advances in Nondestructive Testing, 15th ed., W.G. McGonnagle, Ed., Gordon & Breach Science Publishers, New York, 1990, pp. 313-336.
Sansalone, M., and Carino, N.J., "Stress Wave Propagation Methods," in Handbook on Nondestructive Testing of Concrete, V.M. Malhotra and N.J. Carino, eds., CRC Press, Inc., 1991, pp. 275-304.
Carino, N.J. and Sansalone, M., "Flaw Detection in Concrete Using the Impact-Echo Method," Proceedings, NATO Conference on Bridge Evaluation, Repair and Rehabilitation, A.S. Nowak, Ed., Kluwer Academic Publishers, Dordrecht, Netherlands, 1990, pp. 101-118.
Sansalone, M., and Carino, N.J., "Detecting Delaminations in Reinforced Concrete Slabs with and without Asphalt Concrete Overlays Using the Impact-Echo Method," ACI Materials Journal, March/April, 1989, pp. 175-184.
Sansalone, M., and Carino, N. J., "Impact Echo: Detecting Honeycombing, the Depth of Surface-opening Cracks, and Ungrouted Ducts," Concrete International, April, 1988, pp. 38-46.
Sansalone, M., and Carino, N.J., "Laboratory and Field Study of the Impact-Echo Method for Flaw Detection in Concrete," in Nondestructive Testing of Concrete, ACI SP-112 of the American Concrete Institute, 1988, pp. 1-20.
Sansalone, M., and Carino, N.J., "The Transient Impact Response of Thick Circular Plates," National Bureau of Standards Journal of Research, Nov./Dec., 1987, pp. 355-367.
Sansalone, M., and Carino, N.J., "The Transient Impact Response of Plates Containing Disk-shaped Flaws," National Bureau of Standards Journal of Research, Nov./Dec., 1987, pp. 369-381.
Sansalone, M., Carino, N.J., and Hsu, N.N., "A Finite Element Study of Transient Wave Propagation in Plates," National Bureau of Standards Journal of Research, July/Aug., 1987, pp. 267-278.
Sansalone, M., Carino, N.J., and Hsu, N.N., "A Finite Element Study of the Interaction of Transient Stress Waves with Planar Flaws," National Bureau of Standards Journal of Research, July/Aug., 1987, pp. 279-290.
Carino, N.J., Sansalone, M., and Hsu, N.N., "Flaw Detection in Concrete by Frequency Spectrum Analysis of Impact-Echo Waveforms," International Advances in Nondestructive Testing, 12th Edition, W.J. McGonnagle, Ed., Gordon & Breach Science Publishers, New York, 1986, pp. 117-146.
Carino, N.J., Sansalone, M., and Hsu, N.N., "A Point Source - Point Receiver Technique for Flaw Detection in Concrete," Journal of the American Concrete Institute, Vol. 83, No. 2, April, 1986, pp. 199-208.
Sansalone, M., Carino, N.J., and Hsu, N.N., "Flaw Detection in Concrete and Heterogeneous Materials Using Transient Stress Waves," Journal of Acoustic Emission, Vol. 5, No. 3, July-Sept., 1986, pp. S24-S27.
Pessiki, S.P. and Carino, N. J., "Measurement of the Setting Time and Strength of Concrete by the Impact-echo Method," NBSIR 87-3575, National Bureau of Standards, July 1987, 121 pp.
Sansalone, M., Carino, N.J., and Hsu, N.N., "Finite Element Studies of Transient Wave Propagation," Proceedings, Review of Progress in Quantitative Nondestructive Evaluation, D. O. Thompson and D. E. Chimenti, Editors, Plenum Press, New York, Vol 6A, 1987, pp. 125-134.
Pessiki, S.P. and Carino, N.J., "Setting Time and Strength of Concrete Using the Impact-echo Method," ACI Materials Journal, Vol. 85, No. 5, Sept-Oct 1988, pp. 389-399.
Sansalone, M., and Carino, N.J., "Impact-Echo: A Method for Flaw Detection in Concrete Using Transient Stress Waves," NBSIR 86-3452, National Bureau of Standards, Gaithersburg, Maryland, Sept., 1986, 222 pp. order from NTIS, PB#87-104444/AS
Carino, N.J., and Sansalone, M., "Pulse-echo Method for Flaw Detection in Concrete," Technical Note 1199, National Bureau of Standards, July, 1984.
Carino, N.J., "Laboratory Study of Flaw Detection in Concrete by the Pulse-echo Method," in In Situ/Nondestructive Testing of Concrete, V.M. Malhotra, Editor, ACI SP-82, American Concrete Institute, 1984, pp. 557-579.
Contact: Nicholas J. Carino E-mail: ncarino@nist.gov