ABSTRACTThis paper focuses on experimental results from two scanning impact

echo systems on the internal condition of posttensioned ducts. The firstsystem uses an impact echo head attached to an X/Y scanner and the sec-ond system is a rolling impact echo scanning system. The experimentaltests were performed by two different research agencies and comparisons ofthe blind interpretation and the actual design conditions of the postten-sioned ducts and slab are included herein. Background of the impact echotechnique and its implementation with a rolling scanning transducer arediscussed in the paper. The impact echo technique is generally used to ei-ther determine the internal condition of concrete structures or to measurethe thickness of concrete members. The rolling transducer in the impactecho scanner expedites the test process by allowing for rapid, near continu-ous testing. The results from the rolling impact echo scanning system arepresented in a three dimensional fashion to provide better interpretation ofthe internal conditions of the ducts.

In these studies, the impact echo results from both research agenciesshow good agreement in correctly identifying grouting discontinuities intendon ducts. Discontinuities of grout in bridge ducts are located based onan indirect indication of a void due to an apparent impact echo increase inbridge wall/slab thickness that actually reflects the lower resonant echo fre-quency due to the decreased stiffness associated with the duct void. No di-rect reflection from the duct with grouting discontinuities was observed inthese experiments.Keywords: concrete, grout, honeycomb, impact echo, impact echo scanning,nondestructive testing, posttensioned bridge ducts, voids, void detection.

INTRODUCTIONThis paper discusses the experimental results of impact echo

tests by the Bundesanstalt für Materialforschung und –prüfung(BAM) X/Y scanner and a rolling impact echo scanning system interms of their ability to detect and image discontinuities in postten-sioned ducts of a mockup slab and bridge (Figure 1). Comparisonsof the blind interpretation and the actual design conditions of theposttensioned ducts and slab are presented. The impact echo re-sults are presented in a three dimensional fashion using thicknesssurface plots to provide better visualization and interpretation ofthe internal conditions of the slab. The impact echo tests were per-formed twice with the two different scanning systems (by two dif-ferent research agencies): the first with a traditional point by pointimpact echo head mounted on an automated measurement framesimilar to an X/Y plotter (the X/Y scanner) and a second with a

portable rolling scanning system (the impact echo scanner). Resultsfrom both test systems are presented.

The impact echo technique has traditionally been used on apoint by point basis to either determine the internal condition ofconcrete structures or measure the thickness of structures. The au-tomation of point by point testing with an X/Y scanner approachthat quickly raises, moves, lowers and performs an impact echo testis discussed first. The use of a rolling transducer, the impact echoscanner, further expedites the impact echo test process by allowingfor rapid, near continuous testing and true “scanning” capabilitiesto test concrete structures even faster than the X/Y scanner system.

This paper also includes results of impact echo scanning of theinternal conditions of ducts from a posttensioned bridge. Coringwas performed on the tested duct to confirm the accuracy of the im-pact echo scanning results.

BACKGROUND OF THE IMPACT ECHO TECHNIQUEThe impact echo test involves impacting a concrete structure

with a small impactor and identifying the reflected wave energywith a displacement transducer. The resonant echoes in the dis-placement responses are usually not apparent in the time domain,but are more easily identified in the frequency domain. Conse-quently, amplitude spectra of the displacement responses are calcu-lated by performing a fast Fourier transform analysis to determinethe resonant echo peaks. The relationship among the depth fre-quency peak f, the compressional wave velocity VP and the echodepth D is expressed in the following equation:

(1)

Here, β is a factor which varies based on geometry and is equal to0.96 for a slab/wall shape (Sansalone and Streett, 1997).

DV

f=

β p

2

64 Materials Evaluation/January 2005

Impact Echo Scanning for DiscontinuityDetection and Imaging in PosttensionedConcrete Bridges and Other Structures

by Y. Tinkey,* L.D. Olson† and H. Wiggenhauser‡

Submitted November 2004

* Olson Engineering, Inc., 12401 W. 49th Ave., Wheat Ridge, CO 80033-1927; (303) 423-1212; fax (303) 423-6071; e-mail <yajai@olsonengineering.com>.

† Olson Engineering, Inc., 12401 W. 49th Ave., Wheat Ridge, CO 80033-1927;(303) 423-1212; fax (303) 423-6071; e-mail <ldolson@olsonengineering.com>.

‡ BAM Bundesanstalt für Materialforschung und –prüfung, Unter denEichen 87, Berlin D-12205, Germany; 49 (0) 30 81041440; fax 49 (0) 3081041447; e-mail <herbert.wiggenhauser@bam.de>.

Figure 1 — The X/Y scanner system with ultrasonic and IE-1 impactecho (inset) devices mounted on a pneumatic lift system.

07_49_70_TPs_22pgs 12/14/04 12:39 PM Page 64

General Description of the Mockup Slab In 2002, a large concrete slab was designed and constructed at the

BAM main campus in Berlin, Germany. Practical experience in non-destructive testing (NDT) for more than ten years and pressing re-search topics defined the construction. The concrete slab covers anarea of 10 by 4 m (32.8 by 13.1 ft) with a principal thickness of 300 mm(1 ft). The large dimensions of the specimen are necessary to mini-mize boundary effects on the measured signals and to establish welldefined discontinuities without interference between them.

The concrete slab is partitioned in two sections with differenttesting problems. One section contains tendon ducts with varyingdiameters and grouting discontinuities and different amounts ofposttension wire strand cables. The other section provides areaswith varying thickness and voids. Auxiliary elements, like ther-moelements, water inlet and reinforcement mats, are implemented.In addition, 10 m (32.8 ft) long ducts with 300 mm (1 ft) spacingbelow the bottom of the slab in the subsurface allow future radiog-raphy for detailed reference testing.

The large concrete slab section with metal tendon ducts has di-mensions of 4 by 5 m (13.1 ft by 16.4 ft) and contains 11 tendonducts with well defined grouting discontinuities (Figure 2). Themetal ducts were chosen and positioned to represent typical testingsituations as they are encountered in structures. Because of the ex-ceedingly difficult testing problem, test situations were createdwithout introducing crossing ducts. Also, only one layer of rein-forcement (diameter 8 mm [0.3 in.], spacing 150 mm [5.9 in.]) waspositioned below the surface, with a concrete cover of 50 mm (2 in.).

Tendons with the following properties were built in:■ diameter of 35, 40, 80, 100 and 120 mm (1.4, 1.6, 3.2, 3.9 and4.7 in.)■ concrete cover of 70, 80, 100, 110, 115, 140, 170 and 190 mm (2.8,3.2, 3.9, 4.3, 4.5, 5.5, 6.7 and 7.5 in.) and one sloped duct 50 to160 mm (2 to 6.3 in.) deep■ the size of each of the grouting discontinuities is at least 200 mm(8 in.) in length and represents either a fully ungrouted section(void) or a half filled duct (the exact position of the discontinuities isnot revealed to the public in order for others to be able to performfuture blind tests)

■ varying number and position of wire strand cables (individualdiameters of 15.2 mm [0.6 in.]) in the tendon ducts.

X/Y SCANNER FOR IMPACT ECHO TESTS A multifunctional scanner frame to perform automated radar,

ultrasonic and impact echo testing has been developed and imple-mented at the large concrete slab facility at the government researchlaboratory BAM. The full size of the 10 by 4 by 0.3 m (32.8 by 13.1by 1 ft) thick large concrete slab specimen is covered with the X/Yscanner, although in principle the scanner is independent of thelarge concrete slab and can be mounted on flat surfaces in horizon-tal and overhead arrangements.

The scanner system has a very high positioning resolution forthe sensors. Positioning and repositioning accuracy of the sen-sors is better than 1 mm (0.04 in.) over the entire area. Sensorsthat require direct coupling to the surface, such as ultrasonicprobes or impact echo sensors, are mounted on a holder at-tached to a pneumatic lift system. The sensors can such bepressed to the surface with preset force (up to 100 N [22.5 lb]).The system allows automated pointwise scanning in regulargrids, which can be defined using the software required for thescanner system.

NDT equipment is either integrated into the scanner throughTCP/IP sockets (which require software changes in the equipmentsoftware) or through electrical trigger signals, which are generatedby the control box of the scanner axes.

Typical measurement times for point scanning are 400 s for 100points in the case of impact echo or ultrasound. Radar scanning istypically done with a speed of 4 m/min (13.1 ft/min). Experiencehas proven that due to this automated data acquisition system,high quality data independent of any operator influence is collectedand a high reproducibility of the measurements is guaranteed.

X/Y Scanner Impact Echo Testing of Tendon Duct GroutingDiscontinuities

Extensive impact echo testing was performed on the large con-crete slab to validate the performance and reliability of impact echolocalization of grouting discontinuities in tendon ducts (Wiggen-hauser, 2003). Automated testing in a 50 mm by 50 mm (2 by 2 in.)grid was carried out utilizing the X/Y scanner system. In addition,each tendon duct was scanned separately exactly along its position.

The impact echo system was slightly modified to allow interfac-ing with the X/Y scanner software and hardware through electricaltriggers. In house software was used for data collection and dataanalysis. Typically, at each position the average of three impactspreceeded by one preshot was recorded. The preshot was intro-duced to account for any dust or “soft” spots on the concrete sur-face and consequentially ignored.

Data analysis and visualization was achieved using the in housesoftware. Raw data were first filtered in the time domain (after off-set removal a band pass selected data in the range of 0 to 3 ms afterthe impact). A subsequent fast Fourier transform transformed thedata into the frequency domain, where larger power amplitudeswere assigned to geometrical properties of the specimen.

Impact echograms were generated by combining measurementsalong a line or over an entire area. Grayscale images of the poweramplitudes over position and frequency/depth provide the impactecho results as an image.

Interpretation of Impact Echo DataLocalization of grouting discontinuities through impact echo in

the large concrete slab is based on an indirect indication. A directsignal from the discontinuity, for example an impact echo frequen-cy indication which corresponds to the depth of the discontinuity(given by the formula d = v/2f, where v = the velocity of sound andf = frequency), was never observed with BAM equipment. The onlyindication for the presence of tendon ducts is the apparent minorincrease in slab thickness in the presence of a tendon duct. Groutingdiscontinuities do cause a further increase of the apparent slabthickness. This is in accordance with the interpretation of the im-pact echo signal as a resonance effect, rather than a reflection of alocalized acoustical wave. In Figure 3, the average of all impact

Materials Evaluation/January 2005 65

Figure 2 — Components of the concrete slab: (a) tendon ducts withdifferent diameters before casting; (b) duct with groutingdiscontinuities before placement.

(a)

(b)

07_49_70_TPs_22pgs 12/14/04 12:39 PM Page 65

echo data in the direction of the tendon ducts is shown. Dark colorsindicate high frequency/position values. The position of the ductsis indicated by the letters at the top margin of the image. The fre-quency shift towards lower frequencies in the presence of the void-ed ducts is clearly shown. Also, the magnitude of this frequencyshift largely depends on the diameter of the duct and, to a lesser de-gree, also on the concrete cover (A = 35 mm [1.4 in.]; B = 120 mm[4.7 in.]; C = 100 mm [3.9 in.]; E and G = 80 mm [3.2 in.]; K = 60 mm[2.4 in.]; H, I and J = 40 mm [1.6 in.] diameter). The apparent thick-ness of the slab as measured by impact echo is not constant; it seemsthinnest near the ducts with a 40 mm (1.6 in.) diameter. The reasonfor this remains unknown.

X/Y Impact Echo Scanning of Grouting DiscontinuitiesExtensive impact echo scanning tests were performed on the

large concrete slab to validate the performance and reliability of theimpact echo method to locate grouting discontinuities in tendonducts. The scanning was carried out in a line fashion for an intervalof 50 mm (2 in.) using the impact echo scanning system. The designthickness of the slab is 300 mm (11.8 in.) and at the time of initialtesting, the sound velocity v was determined to be 4050 m/s(13300 ft/s) from the average value from approximately 8000 pointmeasurements for the average thickness echo frequency of 6.75 kHz ±2%. Results from a few of the tendon duct tests are discussed below.

Tendon Duct BThis is by far the least difficult test problem presented by any of

the tendon ducts. The diameter of the duct is 120 mm (4.7 in.) andthe cover is 70 mm (2.8 in.), which is less than the diameter. Thethickness frequency of the slab increased to 6.1 kHz at grouted po-sitions, indicating an apparent increase of thickness of 10% to332 mm (13.1 in.).

There are three intended voids in the duct: 250 mm (9.8 in.) and650 mm (25.6 in.) fully ungrouted and 650 mm (25.6 in.) half un-grouted. Both grouting discontinuities very clearly show up as alarge frequency shift to 5.2 kHz full. The apparent thickness at thesediscontinuities is 390 mm (15.4 in.), a very large shift of 30% relativeto the design slab thickness of 300 mm (11.8 in.). However, there arefrequency shifts at positions where no discontinuities have inten-tionally been placed. Also, the half filled tendon discontinuity doesnot result in a distinct frequency shift. Radiographic tests are need-ed to verify the correct realization of the discontinuity design.

At positions without grouting discontinuities, an additional fre-quency indication is observed at less than twice the frequency of theslab thickness signal (10.8 kHz). As can be seen in Figure 3, this sig-nal is not directly above the ducts, but at the edges of the ducts.

Tendon Duct IIn this case, the concrete cover of 190 mm (7.5 in.) far exceeds the

40 mm (1.6 in.) diameter of the tendon duct and the testing problemrepresents one of the more difficult testing scenarios. Test condi-tions in this case were the same as in the previous case. The mea-surement line along the duct shows the frequency signal of the slabthickness at 6.9 ± 0.1 kHz (corresponding to an apparent thicknessof 290 ± 50 mm [11.4 ± 2 in.]). Over the intended discontinuities, thefrequency shifts very slightly to 6.7 kHz, which is due to the duct.However, the signal interpretation as to the duct void/grout condi-tions would not be possible without prior knowledge of the discon-tinuity location for this small diameter, deep duct.

ResultsThe other tendon ducts in the large concrete slab lie somewhere

between the two cases described above. A systematic analysis re-veals that the frequency shift at the presence of a discontinuityshifts systematically with the ratio of duct diameter to concretecover. Further research is needed to quantify this relation.

The large concrete slab was designed to overcome the problemof boundary effects in impact echo problems of specimens with alimited size. However, even the large dimensions of the large con-crete slab do not suppress the effect of surface waves reflected at theedges of the specimen. This experimental observation is a verystrong indication that point measurements at structures must becarefully checked against effects due to such boundary effects, evenif their distance to any edge is as large as 3 to 5 m (118 to 197 in.).

IMPACT ECHO SCANNER TEST RESULTS The impact echo rolling scanner was first conceived by Olson

and researched and developed as a part of a US Bureau of Recla-mation prestressed concrete cylinder pipe integrity research project(Sack and Olson, 1995). This technique is based on the impact echotechnique (Sansalone and Streett, 1997; ASTM, 2004). In general, thepurpose of the impact echo test is usually to either locate delamina-tions, honeycombing or cracks parallel to the surface or to measure

66 Materials Evaluation/January 2005

Figure 3 — Averaged B-scans across all ducts. The darker colorindicates high intensity at the given frequency and X position. Ductpositions are indicated by letters at the top margin.

Figure 4 — Impact echo scanner unit and the point by point impactecho unit that was used in the X/Y scanner in Figure 1: (a) top view;(b) bottom and lateral view.

(a)

(b)

07_49_70_TPs_22pgs 12/14/04 12:39 PM Page 66

the thickness of the structures (concrete beams, floors or walls). Toexpedite the impact echo testing process, an impact echo rollingscanning device has been developed with a rolling transducer as-sembly incorporating multiple transducers, which is attached un-derneath the test unit. When the test unit is rolled across the testingsurface, an optocoupler on the central wheel keeps track of the dis-tance. This unit is calibrated to send an impact at intervals of nomi-nally 25 mm (1 in.). If the concrete surface is smooth, a couplingagent between the rolling transducer and test specimen is not re-quired. However, if the concrete surface is rough, water can be usedas a coupling material. A comparison of the impact echo scannerand the point by point impact echo unit is shown in Figure 4. Typi-cal scanning time for a line of 4 m (157 in.), approximately 150points, is 60 s.

In an impact echo scanning line, the resolution of the scanning isabout 28 mm (1.09 in.) between impact points. Data analysis and vi-sualization was achieved using impact echo scanning software de-veloped by Tinkey for the National Cooperative Highway ResearchProgram Innovations Deserving Exploratory Analysis (NCHRP-IDEA) grant for stress wave scanning of posttensioned bridges.Raw data in the frequency domain were first filtered using a butter-worth filter with a bandpass range of 2 to 20 kHz. Due to somerolling noise generated by the impact echo scanner, a bandstop fil-ter was also used to remove the undesired rolling noise frequency.Automatic and manual picks of dominant frequency were per-formed on each data spectrum and an impact echo thickness wascalculated based on the selected dominant frequency. A three di-mensional plot of the condition of the large concrete slab was gen-erated by combining all the calculated impact echo thicknessesfrom each scanning line. The three dimensional results can be pre-sented in either color or grayscale.

Results from the Portable Impact Echo Rolling ScannerAs mentioned earlier, the design thickness of the slab is 300 mm

(12 in.) and the sound velocity during impact echo scanner testswas determined to be 4270 m/s (14 000 ft/s). A dominant thicknessfrequency for the slab was 6.9 kHz. Changes in slab thickness wereclearly detected by impact echo scanning tests and well visualizedby the three dimensional display of impact echo scanning results(Figure 5). In this portion of the slab, the thickness varies in a stepfashion from 177 to 300 mm (7 to 12 in.).

Tendon Duct C The interpretation of the internal conditions of the ducts was

more complicated. However, visualization from three dimensionalsurface plots (combined impact echo thickness plots) helped withinterpretation of the data. Figure 6 presents the impact echo scan-ning results indicating duct locations inside the slab, with raisedareas indicating full to half area duct void conditions. The exact lo-cation of the voids presented herein remain unpublished so thatothers can perform blind tests on the large concrete slab specimen.

Test results from duct C, which has a diameter of 100 mm(3.9 in.) and a cover of 80 mm (3.2 in.), are presented in an expand-ed view in Figure 7. The results are interpreted to indicate that onesection of the duct has a partial grout discontinuity and two sectionsof the duct are empty or void. The dominant frequency of the fullygrouted duct is approximately 6.8 kHz, resulting in an apparent

Materials Evaluation/January 2005 67

Figure 5 — Impact echo scanning results showing changes in large concrete slab thickness: (a) three dimensional view; (b) profile view.

(a) (b)

Figure 6 — Impact echo scanner results indicating duct locations inslab (voided areas are indicated by higher peaks).

Figure 7 — Three dimensional apparent thickness plot for duct C.

07_49_70_TPs_22pgs 12/14/04 12:39 PM Page 67

impact echo thickness of 310 mm (12.2 in.). The dominant frequencyshifted to approximately 5.27 kHz for an empty duct, which corre-sponds to an apparent impact echo thickness of 404 mm (15.9 in.).This is a relatively large thickness shift of over 30% compared to thenominal thickness of the slab. A partial grouting discontinuity wasinterpreted from double peak features in the frequency spectrumand a few data points in this area that show apparent higher impactecho thickness. The partial discontinuity zone did not produce aslarge of a thickness increase as can be seen in Figure 7.

Acomparison of this blind interpretation of the impact echo scan-ner results and the large concrete slab design discontinuity locationsis presented in Figure 8. The interpretation of the impact echo scan-ner results, however, shows a downshift in frequencies, indicatingempty or problematic areas at duct locations where no discontinu-ities were intentionally placed. Radiographic tests are needed to veri-fy the correct realization of the design and are planned by BAM in thefuture as similar unplanned discontinuities were identified in theX/Y scanner point by point impact echo testing.

IMPACT ECHO SCANNER TEST RESULTS FROM APOSTTENSIONED BRIDGE

This section discusses results from impact echo scanning on acable stayed posttensioned bridge by the first two authors. The typ-ical thickness of the wall section was 300 mm (12 in.) with four steelducts inside. The sound velocity used to calculate the impact echothickness was determined to be 3960 m/s (13 000 ft/s). The domi-nant impact echo thickness frequency of a 300 mm (12 in.) bridgewall was found to be 6.5 kHz between the ducts. The ducts werefirst located with ground penetrating radar. Impact echo scanningwas used to test the internal grout condition of four ducts in a boxgirder, which were also destructively investigated. The impact echoscans were performed in such a way as to cross all four ducts verti-cally and along each duct laterally but the results from only the ver-tical scan are presented herein.

The impact echo scanner results showed that the dominant fre-quency of the fully grouted duct is approximately 6.0 kHz, resulting

68 Materials Evaluation/January 2005

Figure 8 — Comparison of the blind interpretation of the impact echo scanner results and the actual design discontinuity locations for duct C.

Figure 9 — Comparison of the blind interpretation of the impact echo scanner results and the actual conditions of four ducts inside a posttensionedbridge.

07_49_70_TPs_22pgs 12/14/04 12:39 PM Page 68

in an apparent impact echo thickness of 330 mm (13 in.). The fre-quency shifted to 4.6 to 5.4 kHz for an empty duct and resulted inan apparent impact echo thickness of 365 to 430 mm (14.4 to16.9 in.). The thickness/frequency shift is between 21 to 43% com-pared to the nominal thickness of the wall. Destructive coring testsand further testing using a video borescope were performed to ob-serve the actual condition of all four ducts after impact echo scan-ner tests. A comparison of the blind interpretation of the impactecho scanner results from these four ducts and the actual conditionof the ducts (from coring and video borescope) are presented in Fig-ure 9. The video borescope image of the actual condition of duct 1(with void) is presented in Figure 10. The video borescope showedthat the first two ducts were empty, the third duct was fully grout-ed and the fourth duct was mostly grouted, with a little water.

Review of Figure 9 shows that the results from the impact echoscanner can be used effectively to identify internal conditions of thegrouted ducts in the fully grouted and empty condition. However,

the impact echo scanning tests were not yet able to identify themostly grouted but water filled condition of duct 4. This is likely be-cause compressional waves can travel through water, resulting inconditions similar to those of the fully grouted ducts. More experi-mental research is planned with an impactor with a shorter contacttime (generating higher frequency) during the NCHRP research.

SUMMARYBoth impact echo tests (with the X/Y scanner automatic mea-

surement frame and the portable rolling impact echo scanner sys-tem) showed good agreement in identifing grouting discontinuitiesin tendon ducts that were located using impact echo scanning. Inthe impact echo testing by the authors to date, the clearest indica-tion of the presence of grouting discontinuities is the apparent in-crease in slab thickness. No direct reflection from the duct withgrouting discontinuities was observed in these experiments. This isbecause of the larger wavelength generated by the impactor insidethe impact echo scanner. It is also more difficult to interpret the im-pact echo scanner results correctly when the ratio of concrete coverand duct diameter becomes larger. Further research is needed toquantity this relation. In addition, the impact echo scanning resultsfrom a cable stayed bridge showed good agreement with furthertesting using a video borescope. However, the impact echo scan-ning was not able to identify voids if the duct was mostly groutedand partially (or completely) filled with water. Further experimen-tal research is planned to address the sensitivity of the impact echoscanner for grout/void condition assessment in posttensionedducts during the current NCHRP research.

REFERENCESASTM International, C1383-04: Test Method for Measuring the P-Wave Speed

and the Thickness of Concrete Plates Using the Impact-echo Method, West Con-shohocken, Pennsylvania, ASTM International, 2004.

Sack, D. and L.D. Olson, “Impact-echo Scanning of Concrete Slabs andPipes: Advances in Concrete Technology,” Proceedings of the 2nd CAN-MET/ACI International Symposium, Las Vegas, Nevada, 1995, pp. 683-692.

Sansalone, M.J. and W.B. Streett, Impact-echo Nondestructive Evaluation of Con-crete and Masonry, Ithaca, New York, Bullbrier Press, 1997.

Wiggenhauser, H., “Duct Inspection Using Scanning Impact-echo,” Interna-tional Symposium on Non-Destructive Testing in Civil Engineering(NDT-CE), CD ROM Proceedings BB 85-CD, V101, Berlin, Germany, 16-19 September 2003.

Materials Evaluation/January 2005 69

Figure 10 — Voids inside duct 1 (see Figure 9) as seen by a videoborescope.

07_49_70_TPs_22pgs 12/14/04 12:39 PM Page 69