International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Impact-Echo: New Developments Regarding Hardware and SoftwareMarkus Motz, Patrick Haller
develogic GmbH, Dieselstr. 28, 70839 Gerlingen, Germany, http://www.develogic.de
Markus Krüger, Christian U. Grosse, Ralf Beutel
Institute of Construction Materials, University of Stuttgart, Pfaffenwaldring 4, 70569 Stuttgart, Germany; http://www.iwb.uni-stuttgart.de
Existing instruments designed for measurements involving the Impact-Echo technique are usually of limited use in field tests when investigating large concrete structures like buildings or tunnels. In 2001 demand for such instruments increased dramatically particularly in Germany, as German authorities made quality control tests of state road tunnel constructions mandatory. For this task, it is essential that the equipment used for Impact-Echo measurements along profiles uses the scanning technique and is easy to work with.
Together with the Institute of Construction Materials of the University of Stuttgart, develogic GmbH has developed a new concept for Impact-Echo testing systems. On the hardware side, an impactor generates very short transient waves. Single manual "shots" can be generated as well as repetitive impacts. A handheld (one hand control) and a stand-alone version are available. There is an external computer control via TTL (Transistor-Transistor-Logic). The device is small and easy to handle, robust and mobile. It is designed for rough environments - a water-proof version for underwater testing is also available on request. A TTL output triggers at the exact moment of impact onto the structure to be tested. Apart from the basic version of the impactor, an electronic version utilizing programmable impact intervals, energy levels and repetitive impacts was also developed.
As far as the software side is concerned, state-of-the-art requirements are fulfilled for scientific applications as well as field tests. Options for research purposes include different ways of processing and visualizing the data, giving the operator a maximum of control. Demands for industrial applications as, e.g., secure data storing or intuitive measurement control as well as fast and easy handling of software and equipment were also taken into consideration. Furthermore, a reliable software-based documentation system for quality management environments is included.
The possibility of controlling the Impact-Echo equipment with only one hand as well as the easy-to-learn analysis procedures of this new system will increase the reliability of this measurement technique and decrease the cost factor significantly. Some first results of in-situ measurements are shown here.
Due to the demands for quality control and sustainability of structures in civil engineering, a growing market for non-destructive testing devices has evolved. For concrete structures, several methods are known to be "classic", i.e., well-introduced, methods for defect characterisation. These are ultrasound, radar, thermography, electro-potential-field methods and others are currently being used to detect voids, cracks, corrosion, etc. - with varying success. Several years ago, the Impact-Echo (IE) method, that considerably improved the detection of voids and honeycombing, was introduced by Carino and Sansalone . The strength of this method is its ability to detect voids in structures and to measure the thickness of concrete parts with good accuracy. For that reason, Impact-Echo was chosen to be the standard technology for quality control of tunnels in Germany . It seems that IE is the first non-destructive technology to be part of a regulating standard for quality control in civil engineering in Germany. However, this technology is still not widely accepted due to the poor handling and limited functionality of commercially available equipment based on this approach. Moreover it was shown  that single-point measurements are somehow more difficult to be interpreted compared to measurements using a scanning technique. The so-called scanning IE technique was developed from measurements that were carried out by Weiler  and later on by Grosse and Weiler  as well as Köhler . A more sophisticated approach using a static scanning frame was described by Colla et al.  and Lausch et al. . It is obvious that the potential of this technique is currently not being used to its full extent regarding handling as well as analyzing techniques, therefore reducing the economic value of this method.
The Impact-Echo method uses transient stress waves generated on the surface of concrete or masonry structures by an elastic, low energy impact (Fig. 1). As the stress waves propagate through the material being tested, they are reflected by internal interfaces (discontinuities in the material) and external boundaries of the structure. Examples of such interfaces are delaminations, voids, honeycombing and cracks, as well as rising mains or large steel bars, etc. In order to detect such interfaces, the emitted waves are recorded by a displacement or acceleration transducer which is placed near the impact point on the surface of the structure.
The depth of any internal flaws or external interfaces can be determined by analyzing the recorded signal and its characteristic frequency spectrum (FFT) using the following simple equation
where d is the depth of the interface or void, vP is the measured compressional wave velocity and fR is the corresponding resonance frequency in the spectrum. A typical single point IE measurement is shown in Fig. 2. The transient wave recorded by the sensor is displayed in the left hand diagram, the corresponding Fourier spectrum in the right hand diagram. In this example the resonance frequency is the dominant frequency in the spectrum. Together with the compressional wave velocity of this structure (see next section), the depth of the void can be evaluated from equation (1). As the measured p-wave velocity is 4350 m/s, the depth is
However, practical application of Impact-Echo measurement is often circuitous and time-consuming thus reducing its benefits. Therefore, modern Impact-Echo testing techniques require better operability of the test equipment for quickly gaining repeatable and reproducible results leading to cost-effective measurements. Additionally, new techniques like the use of adaptive wavelet filtering can further improve signal analysis.
Conventional Impact-Echo measurement techniques
Up to now the number of commercially available Impact-Echo systems is limited. The data acquisition and analysis capabilities of these systems are very similar. Prior to depth measurements, the wave velocity of the compressional wave (or P-wave) of the tested material has to be determined. The wave velocity information is essential for a correct localization of voids, delaminations, etc., and is usually measured with two separate sensors being placed in a certain distance to each other on the surface of the test object (Fig. 3). The impact is usually generated by steel balls with different diameters according to the desired impact energy (small ball-shaped hammers are also in use). With the selected ball the impact is excited manually by dropping it onto the surface in line with the two sensors. The velocity of the P-wave is measured by determining the travel time between the two. The second sensor is necessary because the pulse excitation of commercially available devices is not exactly measurable. For the depth measurements the test set-up is changed slightly using only one sensor together with the impactor (Fig. 1). With an equipment configuration as described, repetitive velocity measurements are neither practical nor cost-effective although a constant velocity cannot be expected to be present in large structures. Please note that simply assuming constant velocity values for large structures may significantly reduce the accuracy.
In general, Impact-Echo testing with currently available equipment takes up to two minutes per measurement point for data acquisition and verification of results. Considering the poor ergonomics of such devices, two operators are often necessary to handle the equipment. As personnel costs are crucial for in-situ measurements, reducing the complexity of the testing process is essential for further acceptance of this technique. Otherwise, only measurements at selected single points or with relatively wide grids are feasible. There is also great demand for better control of impact energy.
Modern Impact-Echo testing techniques require ergonomic test equipment for rapid, repeatable and reproducible measurements. For that purpose a new flexible test system has been developed. On the hardware side, it consists of a transducer and an automatic impactor as well as a data acquisition PC-Card. The equipment is light, mobile and controlled by a ruggedized sub-notebook or a tablet-PC. The device is optimized for rough environments and a fast and easy data acquisition.
An essential part of the device is the impactor. For the detection of voids according to the described theory, the impact should generate a short relatively high energy pulse with a broad frequency content. This requirement not only applies to Impact-Echo analysis but also to several other non-destructive testing methods. High impact energy is necessary to detect defects and boundary surfaces in greater depth.
The developed range of impactors operates on the basis of high speed tubular solenoids. Fig. 4 shows the automatic DAI-1 impactor developed by develogic. The impactor is equipped with a sophisticated electronic control unit interfacing to external devices that allows the operator to fully control the impact generation and also gives feedback on impact time and duration.
As the unit is able to deliver the exact time of impact, the second transducer so far required for velocity measurements (Fig. 3) is now obsolete. Furthermore, additional electronic features can be implemented. If necessary, it is now possible to adapt the impactor to rough environmental conditions (up to protection class IP67).
The described impactor can be used as a stand-alone pulse generator or, controlled by a PC providing the user with scanning techniques including repetitive measurements of many IE data. As described, there is great demand for lightweight, robust and mobile Impact-Echo testing equipment. This applies especially to the control unit. Fig. 5 gives an impression of a controlling unit based on a very small mobile PC equipped with a data acquisition board and a wireless display. The impactor is plugged to the PC-board inside the PC. The overall weight of this system is less than 3 kg.
As another important component of this IE system, powerful measurement and analysis software was developed. The features of this software are designed to fulfil scientific requirements in laboratory testing as well as requirements of daily application of testing on large structures in the field.
Much attention was given to the development of automation options in combination with the hardware with regards to measurements on large constructions like tunnels, bridge bodies, walls, etc. New features will be implemented continuously in the future. A screenshot of the current version can be seen in Fig. 6, showing a depth section along a profile.
The algorithms were designed specifically to meet the requirements of quality control testing and in-situ documentation of inspections. The specifications include a graphical user interface for flexible generation of measurement grids (Fig. 7).
Future releases of the software will include 3D visualization of structure data and additional advanced filtering algorithms for reducing undesired secondary echo signals.
Comparison of different test results
One of the advantages of using the described testing system is the reproducibility of testing results as the automatic impactor generates controlled transient waves of adjustable energy. On the left hand side of Fig. 8, five testing results of repeated measurements at the same position on a thin concrete wall (Fig. 9) are shown using standard equipment (sensor and impact ball). On the right hand side of Fig. 8, five repeated measurements at the same point using the described impactor are shown. It can be seen, that the results using the electronic impactor are more reliable and reproducible.
In this example, a concrete wall of 250mm thickness was tested using this new Impact-Echo analysis system. A horizontal wall section of 1000mm was tested with a measurement grid distance of 50mm (Fig. 9). With the new equipment it takes about 5 minutes to test all 21 measurement positions and to verify the results. The generation of the cross-section diagram (Fig. 9, bottom) takes less than a minute.
In field as well as in scientific application, an increasing demand for advanced Impact-Echo testing techniques that are easy to use for fast, repeatable and reproducible measurement can be observed. To meet these demands, a new flexible testing system has been developed. The system utilizes advanced impact generation, measurement, data acquisition, filtering and visualization techniques. This system will provide increased acceptance of Impact-Echo testing in civil engineering and is therefore another step towards making IE a standard testing method.