Keywords: Concrete, Impact-Echo, Radar, Ultrasound
This paper was presented at the International Symposium Non-DestructiveTesting in Civil Engineering (NDT-CE) 26.-28.09.1995 in Berlin. NDT-CE, Full Program or the Ultrasound Part

Abstract

Table of contents
1 Introduction
2 Test Specimens, Objectives, Methods
3 Results
3.1 Short Presentation of Special Results
3.1.1Radar
3.1.2 Impact-Echo
3.1.3 Ultrasonic Pulse Echo, A-scan
3.1.4 Ultrasonic Pulse Array
3.1.5 Ultrasonic Pulse Echo, B-scan
3.1.6 Ultrasonic Pulse Echo, B-scan, LSAFT
3.1.7 /8 Ultrasonic Pulse Echo: 2D-Synthetic Aperture and 3D-SAFT
3.1.9 EFIT Simulation
3.2 Comparison of Results
4 Conclusion
5 Acknowledgements
6 References

1 Introduction

One of the principal objectives of the development of NDT-CE techniques is a reliable assessment of the integrity or detection of defects of concrete members even when they are accessible only from a single surface. Especially on reinforced concrete structures such inspections could in the past only be solved by means of radiography (for concrete thickness less than 0.6 m) or by using more or less destructive methods. Some of the current research aims are outlined below:

• location of tendons
  
• detection of voids in tendon ducts
  
• detection of compression faults or honeycombing
  
• information on geometrical dimensions

  With the aim to solve these and similar problems research efforts have in the past years focused on the development of the pulse-echo technique (The term pulse-echo is used in a broader sense here ). In this context the pulse-echo technique includes the following methods:
  

• Radar: propagation, reflection and scattering of electromagnetic pulses
  
• Impact echo: propagation, reflection and scattering of elastic waves after mechanical impact
  
• Ultrasonic pulse echo: propagation and dispersion of sound waves after indication with ultrasonic transducers
  
• Synthetic aperture: reconstruction of backscatter and reflection from the interior of an object by means of SAFI' programs (SAFT: Synthetic Aperture Focusing Technique)
  
• Simulation: Calculation of wave propagation under specified test conditions.

  When some of the established methods used in other areas of materials testing are applied to concrete testing, certain difficulties are encountered as concrete is an inhomogeneous, porous building material. However, in the recent years great efforts have been made to adapt such methods to testing concrete and some of them are being applied successfully.

  Numerous companies and institutes are working on such projects employing, however, different types of test specimens and different test objects. Hence no comparable data are available on the specific advantages and limitations and also on the accuracy of these methods. What makes it even more complicated is the fact that concrete is a highly variable product, even when the main production parameters, such as maximum Aggregate size and cement mix are identical. There are quite a few more parameters, such as moisture content and pore size, which greatly influence the propagation of microwaves and elastic waves.

  Especially with regard to the ultrasonic pulse-echo methods we examined different test setups and evaluation techniques for testing concrete, i.e. direct evaluation of the echo signals, signal filtering and algorithms using the principle of synthetic aperture.

  In order to be able to make reliable statements on the advantages and limitations of these methods, we decided to carry out comparative tests on identical test specimens with the methods which we have partly modified and improved. Additionally, the propagation, especially of acoustic waves in concrete were simulated to substantiate the test results and solve specific problems.

  As the test specimens described in the literature often involve too many problems which render the interpretation of the results difficult, simple test specimens were designed for the first test series described here.

  Below the objectives of the investigations and the test specimens are described, the methods are outlined (chapter 2) and the results are compiled (chapter 3.1). In chapter 3.2 the results are compared, partly using simulation techniques. This is only a short presentation; for further information the reader is referred to publications which are partly included in the symposium proceedings.

2 Test Specimens, Objectives, Methods

Test specimens (2.0 x 1.5 x 0.5 m3) with metal ducts were produced which varied only with regard to the maximum Aggregate size (8 and 32 mm) (Figure 1). The two halves of one test specimen varied also in so far as a mesh reinforcement (mesh size 180 mm) was applied only in one half of the specimen. When cement was injected into the duct, intentional voids were filled with rigid foam. Tendons were not incorporated in this test. One side of the test specimens had a smooth formwork surface (A) and the other side a more uneven surface (B) to compare the effects of different probe-to-specimen contacts.

Figure 1: Plan of the concrete specimens. For the results discussed in the text note, that the x-axis leads to the top and the y-axis to the right, so that the z-axis is positive under the surface. (30k)

On these specimens, the following test series involved the


• thickness measurement of two 0.5 m thick concrete slabs (specimen 1: maximum Aggregate size 8 mm; specimen 2: maximum Aggregate size 32 mm); for the thickness measurement the wave velocity had to be determined.
  
• location of a metal duct (diameter 85 mm) in concrete with 8 mm maximum Aggregate size
  
• location of non-compressed areas (voids from 80 to 160 mm) inside this duct. The most important parameters of the concrete mix are: strength category B45; portland blastfurnace cement H0Z35L; grading curve AB8 (AB32); total water/cement ratio 0.48 (0.44) (data for maximum Aggregate size of 32 mm in brackets). Nine different working groups performed the experiments and calculations with the pulse-echo techniques and evaluated the results obtained.
  1. Pulse radar (commercial system GSSI SIR 10 A, modified and improved, BAM Berlin): georadar system using a 900 MHz antenna [ 1 ].
  2. Impact-echo technique (commercial system Germann Instr. DOC'ter, EADQ Dresden): frequency analysis of multiple reflections of sound waves after short mechanical pulse generation [2].
     Ultrasonic methods:
  3. Pulse-echo technique with analysis of A-scans in a frequency range of 100 to 500 kHz (Krautkrämer GmbH & Co.): Pulse echo with single element and separate transmitter and receiver probe at same side.
  4. Pulse-echo analysis with separate transmitter and receiver positions (TH Darmstadt 2): Utilization of an array of 7 probes in a frequency range of 80 to 250 kHz, coupling with quicksetting cement, evaluation by correlation analysis with broad-band transmitter signal [3].
  5. Analysis of B-scans (commercial system Ing. Büro W. Hillger NFUS 2300, University of Dortmund): transmit-receive operation (frequency 50 to 250 kHz) with low-pass filtering and signal amplification especially developed for concrete [4].
  6. Recording of pulse-echo A-scans along a linear aperture (commercial system: Ing. Büro W. Hillger NFUS 2700), MFPA Weimar, self developed analysis software: A-/ B- Scans on line, off line analysis by LSAFT reconstruction (LSAFT: Linear Synthetic Aperture Focusing Technique)[5].
  7. Recording of pulse-echo A-scans (separate transmitter and receiver) along a two dimensional synthetic aperture (BAM Berlin): manual taken sets of data (ca. 100 points) and automatic analysis of large sets of data (ca. 1000 points) with a scanning laser vibrometer used as receiver[6]. Thickness measurement according to the principle of phase correlated superposition [l].
  8. 3D-SAFT (lzfP-Saarbrücken): evaluation of the laser vibrometer data (no. 7) by means of 3D-SAFT.
  9. Simulation of the propagation of elastic waves with EFIT (University Gh Kassel): simulation of the sound propagation (scattering and reflection) in concrete taking the elastic constants of cement matrix and Aggregates separately into account. Studies of intentionally produced defects.

  2: Part of research project from Deutscher Ausschuß für Stahlbeton and Deutscher Betonverein (Authors: Prof. Dr. Ing. P. Grübel, Dr. rer. nat. O. Kroggel, Dipl.-Ing. R. Jansohn, Dipl.-Ing. M. Ratmann)

3 Results

In chapter 3.1 we describe some few examples, whereas in chapter 3.2 the results are summarized and compared. The subheadings in 3.1 correspond to the method number in chapter 2 and table 1.

3.1 Short Presentation of Special Results
3.1.1 Radar

The radar investigation have been performed with the nominal 900 MHz antenna (center frequency: 1 GHz). First the transmission velocity of the electromagnetic waves was determined at different positions of both specimens. From these velocities, average dielectric constants of (8.7 +/- 0.2) and (9.4 +/- 0.3) were determined for specimens 1 and 2, respectively.

For locating the metallic duct and for thickness determination, the antenna has been moved in parallel tracks over the surface of the specimen. Figure 2 shows the radar scans of specimen 1 measured at surface B (far from the duct). The amplitudes illustrate the strength of the reflections. From the dielectric constant and the transmission time, a concrete thickness of (508 +/- 10) mm is calculated. The concrete cover on the duct is (340 +/- 10) mm. The lateral position of the duct can be determined with an accuracy of +/- 10 mm. The localization of the duct at different points is shown in Figure 10.

Figure 2: Radar B-scan for specimen 1, measured from surface B. Note, that the y-axis is turned in comparison to figure 1. In the region of intense duct reflections the backwall echo is sup- pressed. (13k)

The measurement results of both types of specimens with different Aggregate size do not differ significantly. Because of electromagnetic shielding the Radar measurements are not suitable to detect voids in the metallic ducts. For more details sec [7].

3.1.2 Impact-Echo

The impact-echo testing was performed using an impactor with a spring driven mass and piezo-electric displacement transducer, the frequency analysis was effected using a laptop. The multiple reflections of the longitudinal waves are analysed to locate the duct and to measure the thickness. This gives clear peaks when only backwall reflections are apparent. When the duct position is measured from the near surface, the maximum corresponding to the position of voids in the duct (17.3 kHz) is clearly detectable together with the backwall echo at 3.9 kHz. The impact response from the filled duct (figure 3) shows the dominant peak of the thickness frequency. The frequency of the reflections from the duct is characterized by multiple peaks with lower amplitude (interface section) and differs from the signals from an unfilled duct. The result shows that it is feasible to use the impact-echo method to detect the ducts, but has to be improved by field studies [8].

Figure 3: Impact-echo response of an unfilled duct. (9k)

3.1.3 Ultrasonic Pulse Echo, A-scan

In order to overcome the specific problems of concrete, a specially developed low frequency flaw detector for construction site testing of strongly attenuating (sound scattering) materials was applied in combination with low frequency probes especially optimized for this field of application. Its A-scan display enables, even with concrete, the evaluation of the signal amplitudes and times of flight as with common ultrasonic material testing [9][10]. Most of the test tasks set here could be solved for the specimen 1 (8 mm maximum Aggregate size) with satisfactory results. Figure 4 shows the first and second backwall echo from the 500 mm wall (digital reading T = 498.1 mm) measured with a single element probe from the surface A. With this equipment and with the application of the usual pulse echo method it looks possible to carry out tests on concrete with admixtures larger than 8 mm; for the specimen 2 (32 mm maximum Aggregate size) the echo signal is measurable, but the signal to noise ratio is not yet satisfying.

Figure 4: Wall thickness measurement for specimen 1 using the pulse echo method and A-scan evaluation.

3.1.4 Ultrasonic Pulse Array

The tests were performed with an array consisting of seven transducers. The fast setting mortar used as coupling agent is well matched to the acoustic properties of concrete. Another advantage is that the transducer array is held in the desired position without any additional device. The equipment used is described in detail in these proceedings [3]. Measurements were made at all transmitter/receiver-combination of adjacent transducers. This means a total of 24 single shots which cover an area of approximately 150 mm in diameter. The signals are processed by spatial averaging to suppress the effects of scattering at Aggregate particles. Signal processing was done by matched filtering and calculation of the envelope by utilizing the complex analytic signal. Figure 5 shows two A-scans at two locations of the specimen 1, surface A. The solid line shows the result over the void 2 of the tendon duct. The peak at 104 mm indicates the depth of the duct surface. An explanation for the slight deviation of the depth measurement is the fact that the array covers an area so that the geometrical situations are not exactly the same for all single measurements. The backwall echo at 505 mm (grey line) is taken at a point without duct and mesh reinforcement and is in good agreement with the true thickness of the specimen.

Figure 5: Specimen 1 Face A measured by means of ultrasonic pulse array technique.

3.1.5 Ultrasonic Pulse Echo, B-scan

By manually moving the US-IE transducer along the concrete surface (specimen 1, surface A) B-scans were generated on-line. This permits a lateral localisation of the duct within ñ 5mm. In figure 6 a B-scan measured along a vertical axis is shown. This was repeated along 16 axes of the surface, so that the localisation of the duct could be measured precisely, these points are included in figure 10. The position in the depth was interpreted from the simultaneous measured A-scans in the range from 102 to 78 mm depending on the site. The measurements were also possible from the reinforced half of the specimen, but yet no difference between void and filled duct could be detected. Further details are described in [ 11].

Figure 6: (8k) B-scan, measured at specimen 1 surface A along a vertical axis. The reflections from the duct begin at 50 µs, the hyperbolic form can be noted, resulting from the scan.

3.1.6 Ultrasonic Pulse Echo, B-scan, LSAFT

Ultrasonic data is recorded in single transducer pulse echo technique along a linear aperture at regular spacing. A SAFT-imaging algorithm [12] is employed to reconstruct a 2-dimensional section of the material below the aperture. The resulting images, usually B-scans and demodulated SAFT-images, are plotted by means of the analysis software. Figure 7 shows the reconstruction of an unfilled post-tensioning duct. The bright spot indicates the part of the ducts surface which is illuminated by the acoustic waves. The duct is easily distinguished from background noise originating from Aggregate scattering and reconstruction artefacts. In contrast, the image of a filled duct exhibits a smaller amplitude relative to back- ground noise (not shown here). Aggregate size, sound path, and transducers contact to surface are most important to influence the quality of the images. The accuracy of the localisation of an object depends on the knowledge of the exact pulse velocity which in turn depends on the length of the transit path due to dispersion. The values in Table 1 are found without dispersion correction. For more detailed information on this imaging method see [6] in these proceedings.

Figure 7: (13k) LSAF-R image of the metal duct calculated from a B-scan at specimen 1 (surface A). The B-scan was measured along the x-axis between 1 and K (horizontal axis in this figure, compare figure 1). 0 means point I/K-5/6, negative values show to the direction to the top of the specimen.

3.1.7 /8 Ultrasonic Pulse Echo: 2D-Synthetic Aperture and 3D-SAFT

When the two dimensional synthetic aperture technique is utilised with separate transmitter and receiver probes, the echoes from different directions will overlay. The negative influence of statistical materials inhomogeneities can better be minimised than with a linear aperture. Additionally the measurement accuracy will be improved when shading appears. When the method of phase corrected superposition is applied [l] (algorithm for thickness measurement on the basis of 100 A-scans), the thickness of a specimen without and with mesh reinforcement can be very accurately determined (figure 8a).

A large number of data is required for applying the 3D-SAFT algorithms. They are produced in our investigations by using a scanning laser Doppler vibrometer as an ultrasonic detector (typical number of dots 1000, screen width 10 mm). Another advantage of this technique is that apart from applying a reflective coating no special preparations are required even for very rough surfaces. As an example we demonstrate the location of void 2 measured from face B of specimen 1 (figure 8). The diameter of the void is about 20% of the aperture diameter so that the void and the tendon duct can easily be identified when the thickness-sensitive algorithm is applied.

The three-dimensional image is produced according to the principle of the three-dimensional SAFT reconstruction [14]. The intensity maximum correlates with the void position and indicates, that it is shifted to the right (figure 8b). No signals are received from the filled tendon duct so that the unfilled area can easily be identified.

Figure 8a and 8 b:
Evaluation of a set of 975 A-scans (point width 10 mm) over void 2 (measured from side B, see figure 1). A fixed emitter position (ultra- sound centre frequency 80 kHz) and a scanning laser Doppler interferometer as receiver were used. Left: Evaluation by means of thickness sensitive algorithm, the duct and backwall echo appear. Right: Reconstruction by means of 3D-SAFT, sectional view at z = 330 mm. The void is clearly separate from the filled duct and appears to be shifted to the right.

Fig. 8a

Fig 8b

3.1.9 EFIT Simulation

The Elastodynamic Finite Integration Technique (EFIT) [14] is an efficient numerical code to model the propagation and scattering of elastic waves in inhomogeneous solids. For applications to concrete a special two-dimensional version has been developed, which allows to model statistical inhomogeneities in terms of arbitrarily oriented ellipses of random distributed sizes and material properties. The code produces time frames of spatially propagating wavefronts and time histories of received signals on the specimen surface, so-called A-scans, A set of A- scans along a one-dimensional scan line can be postprocessed either with the conventional SAFT-algorithm or with a more elaborated EL-FT-SAFT scheme, which accounts for the elastodynamic nature of ultrasound and which includes mode conversion between pressure and shear waves [ 15].

Figure 9 compares EL-FT-SAFI' images in the pressure wave mode along a vertical line, which have been computed from EFIT simulated data for various scattering objects: solid metal cylinder, hollow metal tube, metal tube filled with cement, metal tube filled with cement and exhibiting delaminations. The solid metal cylinder is identified by strong reflections from the top surface and the backwall, the backwall signal is missing for the hollow tube; it appears again for the cement filled tube, but the top surface is lower in amplitude. The delamination causes the backwall to disappear, the result is nearly compatible to the hollow cylinder.


Figure 9: Comparison of EL-FT-SAFT images in the pressure wave mode along a vertical line.

3.2 Comparison of Results

All methods applied (with one exception from both sides) could be used successfully for the thickness measurement and the location of the tendon duct in the test specimens with 8 mm Aggregate size. Although the approximate position of the tendon duct was known from the drawing of the concrete slab, all groups identified the duct with slight deviations (figure 10). The results indicate, that the duct position has slightly changed during manufacture. An overview of the results is presented in Table 1.

Figure 10:
Results for the location of the duct from surface A (specimen 1). The numbers correspond to the group number used in the text. The result shows that the duct has not exactly the same position as it was given by construction (see figure 1).

With regard to the thickness measurement the axial resolution determined by the different methods is given in parentheses (3). The additional error introduced when variation of velocities occur, can be disregarded here as the velocities were determined separately for each method at the point of interest. The data of the concrete cover thickness over the tendon duct in the area without mesh reinforcement are compared in Table 1 tot void 2. They differ from the nominal value (100 mm from surface A, 3 10 mm from surface B) only when the impact-echo method (no. 21) was used and the SAFT reconstruction technique (nos. 8, 9) was applied and when the thickness measurements were carried out from surface A. Depth measurements of the areas with mesh reinforcement could not be determined by all groups. The radar measurements of surface A produced interfering signals from reinforcing bars and the tendon duct. Although the measurement of the depth is affected by the reinforcing bars the ultrasonic B-scan reconstructions (no. 5), however, permit an accurate determination of the depth. Out results indicate that the determination of the geometry of the tendon duct involves principally no problems. Further investigations are, however, needed to optimise the techniques in relation to the effects of different mesh sizes and number of reinforcing mats. The location of unfilled areas in a tendon duct could so far only be determined adequately by means of the 3D-SAFT technique no. 8) or by the L-SAFT (no. 7) technique by comparing the signal intensity. The difficulties encountered can be explained by the EFIT-simulation technique:

It is inevitable that delaminations occur in the filled areas of the tendon duct as the cement shrinks. The acoustic waves are reflected from these delaminations as they are from other voids. When the stress wave patterns reflected from an unfilled area of a tendon duct and a tendon duct with delaminations are reconstructed by EFIT, it becomes evident that the ultrasonic intensity of reflections from an unfilled tendon duct are slightly higher (figure 9) than those of from a duct with delaminations, in our case calculated here the dominated areas of the tendon duct reach 50%. The results reveal clearly that only the image reconstructions offer the possibility to differentiate these areas clearly. In how far the different backscatter signals of spot measurements supply useful results - as was the case with the impact-echo technique - must further be investigated.
( 3. The axial resolution is determined here as follows: Half width at half maximum (FWHM/2) for gaussian like distributions of intensity vs depth and the slope time for systems who work with the pulse edge.)

Table 1: Over-view for the main results. The no. corresponds to the method-number in the text.

no.	Method		Data Represen- tation	Thickness (axial resolution [mm]b)		Detection of the Duct (Depth [mm])				Distinction void / filled duct
						near (Surface A)		far (Surface B)
				AS 8 c	AS 32	no reb. d	rebars	no reb.	rebars
1	Radar		B-scan	508 (20)	504 (20)	90	90 e	340	340	impossiblef
2	Impact Echo		Frequency Domain	500(30)	500(47)	130	83	310	331	not evident
3	UT	single element	A-scan	498(24)	uncertain	N. R.	N. R.	N. R.	N. R.	N. R.
		sep. transm. and receiver	A-scan	505 (24)	uncertain	105	105	290	305	N. R.
4		Array	A-scan	504 (21)	N. R.	115	96	341	---	N. R.
5		single element	B-scan	508 (15)	N. R.	97	90 - 87	---	---	N. R.
6		single element	B-scan, LSAFT	500 (8.3g)	uncertain	89	89	329	---	from surface A
7		2D-Aperture	Phase shifted (Scanning Laservibrometer) superposition	500 (40)	N. R.	140	---	330	---	N. R.
8			3D-SAFT	500(50)	---	120	---	330	---	from surface B

a. Corresponds to method number in the text
b. FWNY2 or slope time (see text)
c. Maximum Aggregate size
d. for most groups measured near void 2
e. detectable not at all sites
f. Electromagnetic shielding
g. Average from 20 A-scans
N. R.: No results, the problem could not yet be solved
--- : No results because of lack of time

4 Conclusion

The measurements presented here illustrate the latest state of the art of concrete testing in Germany. Significant improvements have been achieved in the past years in ultrasonic pulse testing, in the application of radar and the impact echo technique, in data analysis and in the interpretation and simulation of test results. All research teams who have cooperated in the experiments will continue to work on the further development, improvement and practical application of their techniques. The experiments were quite useful to determine the latest state of the art and to select the most suitable techniques for specific applications. It proved to be particularly suitable that we had developed a large dimension test specimen. The expectations of all research teams were confirmed and partly surpassed. It was, however, also revealed that a lot of research work has still to be done to identify compression faults in tendon ducts unmistakably.

The cooperation will be continued as the results obtained so far have shown that such comparative investigations do not only strengthen the confidence in one's own findings but are a valuable encouragement for further developments. Theoretical simulations proved to be very useful to understand better the processes involved and will definitely promote future investigations.

5 Acknowledgments

The authors acknowledge the assistance of BAM VII.2 (Dr.-Ing. Vielhaber and his colleagues) in manufacturing the specimens. Dipl.-Ing. K. Borchardt and Dipl.-Ing. D. Schaurich have assisted in drawing the figures.

6 References

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