Ultrasonic bridge inspection using 3D-SAFT

M. Krause, H. WiggenhauserFederal Institute for Materials Research and Testing (BAM), Division VII.3, Berlin,Germany

W. MiillerFraunhofer Institute for Non-Destructive Testing, Saarbriicken, Germany

J. Kostka, K. J. LangenbergUniversity of Kassel, Dept. of Electrical Engineering, Kassel, Germany

Abstract

Advances in the experimental equipment and data analysis have shown, that ultrasonicpulse echo technique can be used in bridge inspection to detect different types of defects0 injection faults in tendon ducts, because they lead to a loss of basic protection of the

tendon steel and may result in corrosion damage0 compaction faults or honeycombing, because the reduce the concrete strength and

therefore may influence the static stability.To overcome the problems of low signal-to-noise ration and bad coupling conditions

of the ultrasound to the rough concrete surface a combination of piezoceramic transducerand receiving laser vibrometer was used to scan a 2D aperture. The ultrasonic RF echoesreceived were digitized and stored in a PC for 3DSAFT (Synthetic Aperture FocusingTechnique) reconstruction, which performs a focusing of the ultrasound into the materialleading to an improved signal-to-noise ration and a 3D-image of the internal structure ofthe concrete.

The EFIT-code (Elastodynamic Finite Integration Technique) was used to model theultrasonic wave propagation in the concrete and the interaction with tendon ducts ofgood quality and the case of injection faults. These simulations showed, that the behav-iour of the interaction differs for both situations leading to an evaluation scheme to detectinjection faults. Compaction defects are detected by an increased attenuation of the ul-trasound leading to a suppression of the echoes of e. g. the tendon duct.

In cooperation with the Federal Highway Research Institute (BASt) test measure-ments on a specimen containing artificial defects were carried out, which confirmed theprediction of the EFIT modelling.Keywords: injection faults, modelling ultrasonic wave propagation (EFIT), prestressedconcrete, reconstruction calculation (3D=SAFT), scanning laser vibrometer, tendon -ducts, ultrasonic testing

1 Introduction

The structural integrity of post-tensioned concrete structures is primarily determinedby the condition of internal ducts and tendons. Grouting faults in tendon ducts must berevealed as the tendons are susceptible to corrosion when they are not completely cov-ered by mortar. Compaction faults around the tendon ducts are frequently other causesof damage and failure.

Today the regular inspections of engineering structures are mainly visual assess-ments. This implies that any damage is in general only identified when deterioration be-comes visible. There is a demand for non-destructive testing methods in this area in orderto establish the condition of structures and to identify faults before they become visiblethrough deterioration effects. It will help to reduce costs and to extend the servicelifespan, when regions to be maintained can be pinpointed early.

Non-destructive test methods such as radiography and impact-echo methods have sofar been applied for identifying such problems. Radiographic techniques may only be ap-plied when the component to be examined is accessible from two sides and moreover,these methods have their own attendant problems in connection with radiation protectionmeasures. The second method has been widely applied, but its performance is judgeddifferently.

We report on experiments using an ultrasonic echo system. To overcome the knowndifficulties encountered with ultrasonic testing of concrete due to the inherent propertiesof the material, an enormous progress can be stated in the last years [l] [2][3][4]. For themethod described in the present paper, ultrasonic signals in a so-called synthetic apertureare processed to obtain three-dimensional images and the ultrasonic backscatter and re-flections from the area below the synthetic aperture are calculated. We used a bistaticmeasurement principle, i.e. separate ultrasonic transmitters and receivers were em-ployed. The results were visualized and may be superimposed with the construction planor the results of other imaging measurements.

In order to predict, what is possible to measure in concrete structures by use of ultra-sonic echo methods, simulation calculations by the Elastodynamic Finite IntegrationTechnique (EFIT) have been applied. The propagation of elastic waves in concrete wassimulated in dependence of the concrete admixture and the presence of defects.

2 Experimental Set-up

Significant amounts of data of several thousand positions are required to reconstructmeaningful images. BAM has developed a special test arrangement with a laser vibrom-eter as the ultrasonic receiver in order to automate data recording and to reduce the prob-lems of positioning and coupling the transducer [5]. The ultrasonic transmitter ispiezoelectric with a centre frequency of about 100 kHz.

The laser vibrometer registers all surface vibrations within a wide frequency rangeof 0.1 mHz to 1 MHz, i.e. the vibrometer measures all relative motions between the vi-brometer and the surface to be measured (in the direction of the laserbeam). Undesiredvibrations may easily suppressed by frequency filtering and time averagingtechniques [ 61.

-

If a large structure is to be examined, several surfaces areas with different transmitterpositions must be scanned. The scanned areas should overlap to produce optimum re-sults. The position of the tendon duct should be known to install the transmitter most ef-fectively, this can be ascertained by the radar method [4][7][8].

Figure 1 (left): Result o f 3D-SAFTreconstruction showing a tendon duct,for details of the specimen, see Figure 7.The experiments were carried out fromthe back side of the specimen as shownin Figure 7.Upper part: top view (C-scan) of theprojection from a chosen depth rangeindicated by horizontal lines in the B-scan.Lower part: side view (B-scan) showingthe y-axis vs. depth. Reflections fromthe near and the lower surface of theduct are seen at ca. 230 mm and 320

.,:,:,.,. . . ..C,.,... :.:.>.,. ‘,~:: ~ . ...: :.:,:..‘.,.,..:.i:.....~......,.,....~,~,~.::. . . . . . . . . :’. . . . . ..,.. )‘)‘........t . . . . . . . . . . . .,. . . . . . .,..,._,.a :y.:.::...>.~.$$$.:>,.~:.:.~,:. :: y.: >: >>:: ::,:.,:

,., . ,. :::.

:4:::3. ..v.: ““..,,,,:.,.,..,,,,,,,, .,.:.$:~::;;;;I;; ;:;>;g:;;, ; ; ; ;,; :‘:‘:‘:111:,‘. :, ~, . ,t

ijjT :... j .r:.......::

:. :

. ..a 3x:1:~i:~:.‘i;:Id:‘,.,.,. . .._.....

::>i,;:.:+: .,.,, >,~,,:::.,.... . . ..r.‘............:.:;:;i ::.:::: :::‘. .::: .

::. :. ‘.~.~~~:~~‘;~~

,.. : ~, : .::::::::y$::::::::::::::,::::

colourscale logarithmic 0,8 dB steps

mm, respectively.

Figure 2: Principal sketch for theprojection planes of the results of a 3D-SAFT reconstruction.

3 Evaluation methods and simulation of acoustic wave propagation

To get a rough idea of the geometry inside the material examined The data are eval-uated by a time-of-flight-corrected technique [ 51.

Three-dimensional reconstruction images are obtained by the Synthetic Aperture Fo-cusing Technique (SAFT). High-frequency echoes (HF-A-scans) from the interior of thecomponent are registered, digitized, and superimposed in a special way. In principle thiscorresponds an averaging of high-frequency signals. A 3D-SAFT reconstruction methodwas applied for our inspections. After rectifying and filtering the three-dimensional dis-tribution of reflections and backscatter from the interior of the tested object will be ob-tained [9][ lo].

The 3D-SAFT results are visualized as two dimensional projections from the three-dimensional reconstructions of the scattered ultrasonic waves. Figure 1 shows the recon-structed image of a tendon duct. An interpretation will be given below. In the upper partof the figure the top view of the projection from a chosen depth range of the interior ofthe tendon duct is shown (C-scan). The limits of the range of projection is indicated byhorizontal lines. The lower part of the figure is a side view (B-scan) showing the depthof the scattering centres. The projection range of the x-axis is also indicated. As a prin-cipal sketch Figure 2 shows the reconstructed volume and the position of the planes ofprojection.

The long wavelength of about 50 mm and the lacking sharpness of the image indi-cates why these projections should be studied instead of sections. In addition, it shouldbe noted that the tendon ducts are in general bent and are usually not embedded parallelto the surface.

As mentioned above the results described in this paper deal with the localization ofgrouting defects in tendon ducts and compaction defects around the ducts.

The EFIT-code (Elastodynamic Finite Integration Technique) has been used to mod-el the ultrasonic wave propagation in concrete with tendon ducts of good quality and inthe case of injection faults. A description of this method is given in [ 111.

In this example a 500 x 680 mm concrete specimen has been investigated. The basematerial is cement and the additives used are granitite, basalt, and plaster with a maximalaggregate size of 16 mm and a grading curve Al 6. The additives have been modelled by260,000 ellipses varying statistically in size and orientation. A normal pressure probewith a centre frequency of 80 kHz has been used. The time history of the pulse is mod-elled by a raised cosine with two cycles. The probe has a diameter of 50 mm. On the oneside the tendon duct has been modelled as a circle (cross section) and on the other sideas a rectangle (longitudinal section). The thickness of the duct is 1 mm, the cross sectionof the circle is 90 nun, and the size of the rectangle is 200 mm x 90 mm.

The depth of the embedded duct is 300 mm. To identify the echo signals of the ductproperly, the concrete has been simulated without any air inclusion. It has beenshown [8] that there is a strong dependency between the damping of the elastic wave andthe percentage of air inclusions. A two-dimensional modelling setup yields the upperlimit of about 6% of air for the detectability of a reflection signal.

fi,4

80

0

-800.0 41.6 83.2 124.8 166.4 208.0 249.6 291.2 332.8 374.4 416.0

Figure 3: EFIT-simulation of an air filled duct (longitudinalsection). The A-scan (above) clearly shows the multiplereflections RI and R2 between the duct and the surface of the p-wave. The wavefronts are shown in the snapshot (left) at a timeof 83.19 IM.

80

0.0 41.6 83.2 124.8 166.4 208.0 249.6 291.2 332.8 374.4 416.0t in ps *

Figure 4: EFIT-simulation of a cement filled duct (longitudinalsection). The A-scan (above) shows the front and back side p-wave echo (Rlf, Rib) of the duct and the backwall echo E of thespecimen. The snapshot of 83.19 ps gives an impression of thewavefiront penetrating the duct.

The perfectly filled duct is modelled as filled with cement, the non-perfectly filledduct is modelled as completely filled with air. Figure 3 shows a snapshot of the wavefieldsimulated with EFIT and the expected A-scan for a longitudinal section. The transduceris shown in all figures at the top of the wavefield. Figure 4 shows the same situation foran air filled duct. Both figures clearly show the pressure (cp) and the shear wave fronts(cs). The air filled duct shows a reflection of the duct surface (Rl) only. For the cementfilled duct the wavefronts travel through the duct. The respective A-scans clearly iden-tify this fact.

The amplitude of the reflected signal Rl f is significantly higher in the case of an emp-ty duct. But the filled duct only gives an indication of the back side of the duct (Rib).The scattered signals may be explained by the wave impedance of the media. Togetherwith the size and the shape of the duct the scattered signals depend upon the differenceof the wave impedance between duct and filling. A bigger impedance difference gives abigger reflected signal. The transition from steel to air gives such a big step in the im-pedance profile that the incident wave is totally reflected. This is the reason why the backside of the duct cannot be detected.

Figures 5 and 6 reflect the same behaviour for the circular cross section. But thecurved surface of the duct reflects less energy in direction of the receiver, and thereforethe amplitude of the signal is smaller.

In conclusion the following principles may be deduced from the simulation in orderto evaluate the 3D-SAFT reconstructions and to identify defects:1. The interfaces between concrete/steel/air (empty duct) lead to higher amplitudes of

reflected ultrasonic waves than the interfaces between concrete/steel/grout (filledduct).

0.0 41.6 83.2 124.8 166.4 208.0 249.6 291.2 332.8 374.4 416.0

Figure 5: EFIT-simulation of a circular cross section of an airfilled duct. In comparison to figure 3 the echo of the duct is verylow and the backwall echo is shielded by the duct itself.

0.0 41.6 83.2 124.8 166.4 208.0 249.6 291.2 332.8 374.4 416. 0

Figure 6: EFIT-simulation of a cement filled circular crosssection. The reflection of the duct hardly differs from the noiseof the scattering concrete additives.

2. When the ultrasonic waves travel through the interface concrete/steel/grout, the ultra-sonic energy passes and partly it is reflected from the lower surface of the tendonduct. In case of grouting defects in the tendon duct no ultrasonic energy passes intothe duct and therefore no image of the lower surface will be obtained.

Additionally it is assumed, that a compaction defect around the tendon duct means thatthere is no clear interface between the concrete and the loose aggregate. Such an area islikely to scatter the ultrasonic energy diffusely and to have a smaller amplitude than thereflection of the duct, the echo of the duct will vanish.

4 Results

We present the results obtained by a round robin test which was carried out in coop-eration with the Federal Highway Research Institute (BASt). Radar and six different ul-trasonic test methods, impact-echo and simulation calculations were used to compareand assess the present state of technology of these methods [5] [8]. The test series werecarried out as blind tests, i.e. the construction plans were revealed after the tests. In thepresent paper only the results are presented which were obtained by the ultrasonic meth-od described.

The construction plan and the results are presented in Figure 7. The tendon ducts tobe identified were partly embedded behind grid rebars (spaced 150 mm, diameter 12

123456789

IO111213

- w i t h o u t r e b a r s with rebars -I

- L - - ‘- - J - - -ti: I

-- L--‘--Interpretation of 3D-SAFT Results i j

- _I- -I ’I

Compaction fault around duct- - c - -i---I’I, ’Injection fault I----L--.IIII I I’ 1 ‘I’ I’ I ’ IU T S R Q P O N M L K J I H G F E D C B A

Figure 7: Construction plan for the specimen for the blind test series at BASt (dimensionsin mm). Compaction faults (K) and injection faults (H) are intentionally placed around andinside the two ducts.The grey fields indicate the location of the defects at the lower duct interpreted fromultrasonic testing. V: Compaction defect, which was not intended. The results presented inFigure 1 were measured from the back side.

mm), additionally stirrups were inserted. The lower tendon duct was scanned with thelaser vibrometer at 12 different transducer positions and the defects were interpretedfrom the 3DSAFT reconstruction. One void (H5) could clearly be identified and a com-paction fault around the tendon duct (K2) could be detected quite accurately. However,a part of the compaction fault had been misinterpreted as a grouting defect and one notintended compaction defect was identified.

An example of the 3D-SAFT reconstruction of six measured partly overlapping areasin the part without reinforcing rebars of the test specimen is shown in Figure 1, as men-tioned above (for further information refer to the references [5][S]). In the upper part ofthe figure the bent tendon duct can be identified (C-scan) while in the lower part (verti-cally to the surface, B-scan) its near surface and about 90 mm below its lower surfacemay be recognised.

In the upper tendon duct of the specimen the defects were measured after the con-struction plan was revealed and the grouting defects were additionally localized by de-structive testing. Here the lower surface of the duct is not clearly detectable in thereconstruction. So the intensity of reflection was taken into account for the interpreta-tion. In Figure 8 the C-scan obtained with the 3DSAFT reconstruction is superimposedto the design sheet and the defects measured by destructive testing. The points of highintensity of reflected ultrasonic waves fit most of the intentionally and unintentionallyplaced defects.

1

D E F G H I J K L M N 0 P Q

O/OO :::.::,.:. .: ::‘:y:::~:;,; s:j:j:::j:c ;, . . : ::::.. .. A : . : :.:i ::: . . . . ..I.. /.. A........ .o..:.‘::::..‘:‘.‘~:~..:,~:~:~::~:~~~~~~~,:~. ., y.: . . . . > . . . :::::::::.:::.:r:.. .,.,.,.,...,. :w .:.:.~.:,:,:,: ::::: , : , : ;::,.v . . . . . . . . . . . . ..~.~.‘...‘~......., : :.:,. :,>,.. , . . .,, .., ,.,.~.,,,/,,/,,. A . . . . . . . . ..C.~.T . . . . . . . . . . . . . . . . ,,,,.,.,__,.,, I , , , . . ,,//x.:..:: ,.: ,.~,.,.,.,.A... ._.. _.. .L . . . . . . . . ..t ..~ ::r_, _, ,, ,_ ‘_ ::‘::,‘,:.:c.:.:.:::,:::.:~::.:::::.:.::::::::::::::::::.:.:.:.:.,.:.:.:.:.:.:.~.:.:.:.:.:.:.:.:.:.:.~,~..:.,:.:.:.:.:.,.:.:

Figure 8: Result of the 3D-SAFT reconstruction at the upper duct of the test specimen ofBASt, measured area from axis E to P (see figure 7). This view is shown from the oppositeside compared to figure 7. The C-scan is compared with the result of destructive testing ofthe specimen. High intensity of reflected ultrasonic waves fit with intentionally andunintentionally placed defects.

5 Conclusions

A modification of the ultrasonic pulse echo method using the Synthetic Aperture Fo-cusing Technique provides an imaging system for the detection of internal structuralchanges in reinforced concrete components. The data obtained were reconstructed bymeans of 3DSAFT revealing the image of tendon ducts in concrete. The exact positionof the tendon ducts and the depth of the concrete cover could be clearly detected. Themethod has been applied successfully on a concrete specimen of common characteristics

(maximum aggregate size 16 mm, pore content 5 %, depth of the tendon duct 300 mm)and was able to work in presence of grid rebars (spaced 150 mm, rod diameter 12 mm).

From the modelling of the ultrasonic wave propagation in concrete the principles ofthe interpretation of the results could be deduced. The modelling was performed for theconcrete admixture and the dimension of the specimen under investigation.

The good performance of this method was demonstrated in a blind test where severaldefects in and around tendon ducts were detected.

The high pore content of the concrete complicates the interpretation of the results, asit was also demonstrated by the modelling. It becomes evident that the localization ofmost of the defects is only possible using scanning systems including reconstruction cal-culations.When the lower surface of a duct is not visible in a reconstruction at all, an evaluationof the intensity of the reflected ultrasonic signals is necessary. Then not all defects can,however, unambiguously be identified because the intensity differences of the signalsoften are not distinct enough. Work on improving the measurement technology andreconstruction calculation, as well as simulation, must be continued at this point to sup-port the interpretation of defects.

The described results at test specimens presented in this paper show, that ultrasonicecho methods can identify important defects in post-tensioned concrete structures. It isa very useful task to develop those methods to be used for the condition assessment ofengineering structures in order to assure the durability and to obtain data for the decision,whether an construction is still in a serviceable condition or not.

6 Acknowledgments

We gratefully acknowledge financial support from Federal Highway Research Insti-tute (BASt) for the round robin tests.

7 References

1 .

2 .

3 .

4 .

5 .

6 .

Kroggel, O., Jansohn, R. and Ratmann, M. (1995) Novel ultrasound sysyem to detectvoided ducts in post-tensioned bridges. Proceedings of the 6th International Conference onStructural Faults and Repair, Vol. 1, M. C. Forde (ed.) , Engineering Technics Press, Lon-don, pp. 203-208.Schickert, M. (1995) Towards SAFT-Imaging in Ultrasonic Inspection of Concrete. Proc.Int. Symp. Non-Destructive Testing in Civil Engineering Sept. 26-28 1995 G. Schickert, H.Wiggenhauser (eds.), DGZfP, Berlin, pp. 4 11-4 18.Wollbold, F. and Neisecke, J. (1995) Ultrasonic-impulse-echo-technique - advantages of anonline-imaging technique for the inspection of concrete. Proceedings of the InternationalSymposium Non-Destructive Testing in Civil Engineering QVDT-CE) G. Schickert and H.Wiggenhauser (eds.), DGZfP, Berlin, pp.1 135-1143.Krause, M., Barmann, R., Frielinghaus, R., Kretzschmar, F., Kroggel, O., Langenberg, K.,Maierhofer, C., Miiller, W., Neisecke, J., Schickert, M., Schmitz, V, Wiggenhauser, H. andWollbold, F. (1997) Comparison of pulse-echo mehods for testing concrete. NDT&E Inter-national 30, pp. 195-203.Krause, M., Miiller, W. and Wiggenhauser, H. (1998) Ultrasonic Inspection of TendonDucts in Concrete Slabs using 3DSAFT. Acoustical Imaging 23, in press.Krause, M. and Wiggenhauser, H. (1997) Ultrasonic Pulse Echo Technique for Concreteusing synthetic Aperture. NDT in Civil Engineering 8-11 April 1997, Conference Proceed-ings, J. H. Bungey (ed.), The British institute of NDT, Northampton, pp. 135-142.

7 .

8 .

9 .

Funk, T., Maierhofer, C., Leipold, S. and Borchardt, K. (1997) Non-destructive Location ofTendon Ducts in Concrete Bridges for the Installation of Noise Insulation Walls UsingImpulse Radar. Proceedings of the Seventh International Conference on Structural Faultsand Repair, Vol. 1, M. C. Forde (ed.), Engineering Technics Press, London, pp. 323-340.Krieger, J., Krause, M. and Wiggenhauser, H. (1998) Erprobung und Bewertung vonzerstorungsfreien Ptifmethoden fur Betonbriicken, BASt, Bergisch Gladbach, in press.Schmitz, V., Kroning, M. and Langenberg, K. J. (1996) Quantitative NDT by 3D ImageReconstruction. Acoustical Imaging 22, pp 735-744.

10. Schmitz, V. and Miiller, W. (1998) Evaluation and interpretation of SAFT images. Acousti-cal Imaging 23, in press.

11. Fellinger, P. (199 1) Ein Verfahren zur numerischen Losung elastischer Wellenausbreitung-sprobleme im Zeitbereich durch direkte Diskretisierung der elastodynamischen Grund-gleichungen, Dissertation, Lehrstuhl fur Theoretische Elektrotechnik, UniversitatGesamthochschule Kassel.