CHARACTERISATION OF CRACKS IN CONCRETE
BY SHEARWAVE REFLECTION TECHNIQUE
Otto Kroggel, Jan Unger, Peter Grübl
SUMMARY
The characterisation of cracks has stepped forward from a laborious investigation of small areas or lengths of a crack found by visual inspection to a technique which even can make use of automated scanning methods and which produces images of cracked and deteriorated areas in the volume of the structure under investigation. This progress is based on the application of novel transducers and novel interpretation strategies. Nevertheless a good understanding of the material, the construction and the relevant loading mechanisms is needed for the proper interpretation of results and the planning of eventual repair work.
1. INTRODUCTION
A technique is needed which is able to scan areas of interest defined by a rough visual inspection and to deliver reliable results with respect to tightness and the need for repair work. The occurrence of unwanted cracks in concrete strongly affects the functioning of structures which ought to fulfil the requirement of tightness against liquids. External cellar walls or concrete tubes are well known examples. There is a need to examine the nature of cracks in a structure especially to estimate it relevance with respect to tightness and in the next step to define a strategy for repair work if necessary. Destructive investigations of cracked structures are in most cases useless because the damage done by the investigations is intolerable and the only punctual result is not a real help for the future repair work.
The recently developed application of shear waves for concrete inspection and the possibility to transmit and receive these shear waves without using a coupling agent offer the chance to collect a large number of data, which can be put together to construct a “picture”, covering the inspection area. The pictures allow a look into the concrete but a
careful interpretation is needed. Simple hints for the interpretations of resulting pictures are displayed in the article.
The propagation of cracks in the three dimensional space depends on the behaviour of concrete, the individual arrangement of reinforcement and the distribution of acting forces resp. the type of load like: bending load, impact load, etc. The crack pattern visible on the accessible surface of a concrete structure usually does not indicate clearly the three dimensional structure of the cracked area, especially in cases, where the nature of the actual loading having produced the crack pattern is unknown.
2. BASIC IDEA
Ultrasonic waves propagating in a heterogeneous material are scattered at the interfaces of the materials grains. The term scattering means the removal of a part of the energy of the incident wave and its omnidirectional reradiation [1]. The total wave field in a heterogeneous medium is composed of scattered waves with directions of propagation deviating from the original direction of excitation. The waveform at a receiver point therefore contains energy travelled along various paths in the material [2]. Figure 1 illustrates the nature of scattering. The transmitted primary wave travels along its sound path shown as a thick arrow in figure 1. During propagation the wave looses energy due to scattering at aggregates. The propagation paths of the scattered secondary waves are symbolised by thin arrows. It can be seen that there are various sound paths equal to the length of the path from the transmitter to the back-wall and to the receiver, which yield into a signal corrupted by so called grain noise. Usually grain noise makes it difficult to recognise those echoes which are of importance for the inspection personal, such as echoes generated at the back-wall, defects like voids, delaminations, cracks or honeycombs or build in parts like tendon ducts. In contrast to other test methods presented in [3, 4, 5, 6] grain noise can be used to estimate the depth of cracks in concrete.
As mentioned in [7, 8, 9] cracks which do not transfer forces between their crack borders behave like surfaces, so ultrasonic waves do not or only in very small fraction travel from one side of the crack to the over. The secondary waves generated at the non-homogeneities are nearly totally reflected at the crack border. If the transmitter and the receiver are separated by a crack, the shortest path l for secondary waves from the transmitter to the
receiver is given by the sum of distances from the transmitter to the tip of the crack and from the tip of the crack to the receiver. This is shown in figure 2. If the sound velocity of the material amounts c, the first time of the arrival of secondary waves is ta = l / c. Assuming that the crack pattern is perpendicular to the surface and the sound velocity c is known, the crack depth a can be determined with the measured quantities first time of arrival of secondary waves ta and distance between the transmitter and the receiver position d:
. 2
1 t 2c 2 d 2
a = a −
It is obvious that the determination of the ta can only be a rough estimate, but the measurements described here deliver in principle a pessimistic crack depth estimation. The remaining tight cross section cannot be thinner as estimated.
Fig. 2: Illustration of the shortest sound path for secondary waves 3. STATE OF THE ART
Research concentrated in the beginning on the investigation of simple cracks perpendicular to the surface of the concrete. Receiving and transmitting transducers had been positioned on both sides of the crack, which was found by visual inspection. The aim of the investigation was mainly to distinguish between penetrating and non-penetrating cracks which is an extremely important fact for the tightness of concrete slabs. Results of these investigations are reported in detail in [10]. An example of such a result is given in figure 4.
Fig. 4: Normalised results showing transferred energy from one crack border to the other; without crack - dotted line, 10 cm crack depth - dash dotted line, 20 cm crack depth - solid line. Backwall echo at 30 cm.
energy
depth [cm]
It should be mentioned that the measurement with a pair of transducers gives information only for a individual point and that a large number of measurements must be performed along the crack.
4. IMPROVEMENT IN TRANSDUCER TECHNOLOGY AND COUPLING
Compression waves had been the first choice in concrete testing. Main reasons for this are - Availability of transducers
- Speed of sound
Transducers for compression wave excitation produce a deflection of the concrete surface perpendicular to that surface therefore a coupling agent transferring the corresponding forces within a certain area is needed. The area should be as large as the piezoelectric material generating these forces and the transfer should be as homogeneous as possible to optimise the energy input. The speed of sound of compression waves is the highest compared to the other possible waveforms. The leading edge of a received signal in transmission of reflection will be formed by a wave front of compressional waves, even if
there is a multitude of mode conversion in the volume under investigation. This fact eases interpretation.
Shear waves in concrete had not been regarded earlier in detail because they were found in the received (A-picture) more or less as a disturbing effect between the relevant reflected compression waves. The generation and the reception of shear waves are suppressed by the special design of transducers applied for compression wave testing.
Recently it was found, that the coupling problem can be solved in an elegant way by the application of shear waves, introduced into the concrete volume by point-contacts. This means the coupling technique takes advantage of the roughness of the concrete surface, which is an obstacle for the ideal coupling of compression waves. A coupling agent is not needed.
Fig. 5: Transducer array with an arrangement of 12 receivers and 12 transmitters for shear wave application and point contact [11]
5. THE IMAGE OF AN “IDEAL” CRACK
In the following investigations quite similar to those shown above will be discussed. A single crack is produced by dowels at each end of the future crack. The crack width is observed by gauges and the unreinforced concrete block is kept together by an “external variable reinforcement” as it can be seen in figure 6 a and 6 b. The area indicated in the x/y-plane as shown in figure 6 a was scanned in steps of 2 cm in both directions. With the help of an appropriate software images of that area in different depths z are produced.
Of special interest are images near the accessible surface (small values of z) and images near the not accessible backwall (values of z nearly equal to the thickness of the specimen).
Fig. 6 a and b: Specimen with continuous, approximately linear crack
z = 7 cm z = 35 cm
Fig. 7a and b: Resulting “image” based on shear wave reflection with one-sided accessibility
Conclusion: The crack appears near the surface and near the backwall. That means there is a separating crack.
6. REALISTIC CRACK STRUCTURE
The ideal crack is characterised by a set of information namely position in the x/y-plane and depth. A realistic crack in general needs more sophisticated description which is position in the x/y-plane, depth, angle with respect to the surface and information about the accompanying deterioration zone. In many practical cases a number of cracks are combined and they might not be linear. The strategy of putting a transducer on each side of the crack surely will not lead to a interpretable result. A scanning system producing images as described above is the only chance to understand the real nature of the crack system. As an example for such a complex crack pattern the investigations on a concrete slab are reported. One surface of the slab is accessible and it was loaded by an impact from the other side. The visible crack is shown in figure 8 a and the crack with its inclination with respect to the surface can be recognised in figure 8 b. The corresponding line of measurement is indicated in figure 8 a by the dotted horizontal red line.
Fig. 8 a and b: Accessible surface of a concrete slab and resulting image of the y/z-plane (see figure 6 a)
Fig. 9: Image of the x/y-plane at a depth of 13 cm. (2/3 of the total thickness)
The spatial distribution of the crack system can be seen from 8 b and figure 9. The structure under observation is strongly reinforced and the crack distributing properties of this reinforcement lead to a system of primary and secondary cracks. The definition of the depth of a single crack obviously makes no sense.
7. CONCLUSION
The strategy as described in chapter 3 using single transducers on each side of the crack will fail in many cases of practical relevance. A scanning system is needed which is able to inspect large areas base on the information of a rough visual inspection. An automated system for mechanical transducer positioning and data interpretation will be available in the near future.
Statements concerning the tightness of the system with respect to liquid penetration and eventual future repair work will be possible.
REFERENCES
[1] Hecht, E.: Optik. Addison-Wesley, Bonn München 1989
[2] Wiberg, U.: Material Characterization and Defect Detection in Concrete by Quantitative Ultrasonics. Akademisk avhandling, Stockholm 1994
[3] Grübl, P.; Kroggel, O.; Jansohn, R.; Ratmann, M.: Die zerstörungsfreie Prüfung von Betonbauteilen mit dem Ultraschall-Impuls-Echo-Verfahren. Beton- und Stahlbetonbau, Heft 12, Berlin 1996
[4] Krause, M. et. al.: Comparison of Pulse-Echo-Methods for Testing Concrete. International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), Berlin 1995
[5] Jansohn, R.; Kroggel, O.; Ratmann, M.: Detection of Thickness, Voids, Honeycombs and Tendon Ducts Utilising Ultrasonic Impulse-Echo-Technique. International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), Berlin 1995
[6] Schickert, M.: Towards SAFT-Imaging in Ultrasonic Inspection of Concrete. International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), Berlin 1995
[7] Kroggel, O.; Jansohn, R., Ratmann M: Trennrißerkennung. Beton- und Stahlbetonbau, Heft 7, Juli 1994
[8] Kroggel, O.; Jansohn, R.: Ultrasonic Examination of Cracks in Concrete. Darmstadt Concrete Vol. 7, Darmstadt 1992
[9] Kroggel, O.; Jansohn, R., Ratmann M: Crack Characterisation trough Ultrasound Reflection. Structural Faults and Repair, 1997
[10] Jansohn, R.; Kroggel, O.; Ratmann, M.; Wiese, S.: Progress In The Estimation Of Crack Depth Utilising Ultrasonic Impulse-Echo-Technique. Darmstadt
Concrete Vol. 12, Darmstadt 1997
[11] MSIA Spectrum, 119048, 35, Usacheva st. Moskow, Russia, Tel. (095) 245-56-56, (095) 246-88-88, e-mail: [email protected]
Contact to the author(s): [email protected] Homepage of Darmstadt Concrete: www.darmstadt-concrete.de
