International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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New developments in quality control of concrete using ultrasoundChristian U. Grosse, Hans-Wolf Reinhardt
University of Stuttgart, Institute of Construction Materials Pfaffenwaldring 4, 70550 Stuttgart, Germany
Phone: +49 . 711.6856786, Fax: +49 . 711.6856797 email: firstname.lastname@example.org
A testing device based on ultrasonic techniques was developed to analyze the setting and hardening of cement-based materials. This method is able to document and analyze the setting and hardening process continuously in a way that could not be achieved by conventional techniques such as the vicat-needle test, the penetrometer test, the slump test, or rheologic testing methods.
In this paper experiments with several newly developed or modified testing devices are presented. The focus of this development was set to the elimination of some disadvantages of typical other concrete ultrasound devices as well as an improvement of the handling and cost efficiency. A progress was made regarding the ultrasound emitter, where two different concepts were tested: one is basing on an impact producing short transient ultrasound pulses of broad frequency content. The second approach uses broadband ultrasound transducers as an emitter applied in combination with power-amplifiers.
The second part of this paper addresses tests performed during round robin tests. This is in the frame of the RILEM (Réunion Internationale des Laboratoires d'Essais et de Recherches sur les Matériaux et les Constructions) Technical committee 185-ATC, working in the field of advanced testing of cement-based materials during setting and hardening.
Experiments have been carried out in Lyon (France), Evanston (USA), and Brunswick (Germany). Data are represented along with a brief introduction to the TC's work.
Nowadays the characterization of cement-based materials during the stiffening process by ultrasound measurement techniques is well established. This paper deals with the ultrasound technique used in through transmission in opposite to ultrasound reflexion techniques [e. g. Rapoport et al. 2000]. In numerous publications [e. g. GROSSE & REINHARDT 1994, GROSSE ET AL . 1999, REINHARDT ET AL. 1999a] the patented test method [REINHARDT ET AL. 1999b] developed at the University of Stuttgart was described earlier. Methods based on ultrasound are better suited for the characterization of the setting and hardening of cement based materials than traditional test methods like the vicat-needle test, the penetrometer test or the flow test, because the travel time, the attenuation and the frequency content of ultrasound waves sent through the material are closely correlated with the elastic properties of concrete or mortar. These parameters can be continuously monitored during the stiffening giving a comprehensive picture instead of snapshots of workability for example.
A sophisticated device based on pulse excitation by an impactor was developed and numerous experiments have been conducted in the past, investigating the influence of water-to-cement ratio, the type of cement, the use of additives and admixtures, the air bubble content and so on, for the setting and hardening of concrete. Physical parameters measured and analysed during the hardening process are the pulse velocity, the energy and frequency content of the transient signals obtained in through-transmission. Features interesting for the characterisation of the material are the extraction of the initial and final setting time out of the signals [GROSSE & REINHARDT 2000] and the parallel registration of the state of hydration [GROSSE 2002]. The device developed by BEUTEL , STEGMAIER  and Herb used for concrete measurements is a re-design of the device described in REINHARDT ET AL. , GROSSE , WINDISCH , HERB  and REINHARDT ET AL. [1999a] for concrete measurements and illustrated in Fig. 1. Industrial needs to reduce the amount of waste and improving the reliability and handiness led to a separate device for mortar materials. The final container [GROSSE ET AL. 1999] had two walls of PMMA and a U-shaped rubber foam with an inner volume of 40 cm3 for the mortar (Fig. 2).
Regarding the concrete device many difficulties were eliminated, but the system still shows up unresolved problems. The wave is generated using a steel ball exciter, referred to as Ultrasound Impactor (USIP), hitting a small plate fixed on the PMMA casing. The resulting excitation can be seen as broad banded, having a relatively wide frequency bandwidth of up to 100 kHz. Specifically, a wave travelling through the container wall which onset is detected before the irradiating primary wave can be observed. Further on, the energy evolution during the hardening of concrete is still difficult to analyze since the steel ball transmitter USIP, as a mechanical system, provides unreliable energy data and the plate where the steel ball is shot on easily disbond so that the coupling of the excited energy into the PMMA container changes during tests. These factors influence the obtained results and the reproducibility of tests.
To summarize the pros and cons of the concrete device the following statements can be given:
2 Details of used analysis methods
Using ultrasound methods the degree of hardening is characterized by the change of significant parameters. Not only the travel time of the ultrasonic pulse through the testing device, consequently the velocity of compressional waves but also the frequency content and the relative energy are recorded. In detail the velocity is determined by measuring the onset time of the signals knowing the travelpath of the wave, the energy by calculating the integral sum of the wave amplitudes and the frequency content by using Fast-Fourier-Transform techniques. On the basis of suitable parameters, e.g. the frequency content of the signal over the time, additionally a wavelet transformation (WT) is carried out in order to gather as much information as possible from the raw signal to evaluate concrete and mortar, respectively. The program AutoCWT, able to apply the WT was implemented by MANOCCHIO , where the calculation kernel is taken from the program IWB-CWT, coded by BAHR [2001a]. More information about the application of wavelets in the characterization of the setting and hardening of cementitious materials can be obtained from GROSSE , GROSSE & REINHARDT  or MANOCCHIO .
The software developed at the University of Stuttgart called FreshCon2 is able to do an online data analysis during the experiment in that way, that the operator has some control about test results. A screen-shot showing the software at the end of experiment RE5 is shown in Fig.3. Continuously the ultrasound signals with their Fourier transform (using an FFT algorithm) are recorded and plotted in the window at the upper left and lower left respectively. The shown graphs are representing the last transmitted signal and its frequency content. The whole experiment was finished after 720 minutes (12 hours) and started approximately five minutes after adding water to the mix. A recording interval can be chosen to collect the data. In the upper right window the compressional wave velocity as well as the energy or the temperature, alternatively, are plotted, and calculated continuously using the onset time of the signal and the signal amplitudes respectively. All three parameters are changing significantly during the hardening of the material.
Recently, a new feature of the FreshCon system, the ability to record the temperature evolution over the time, was introduced as well as the determination of the associated hydration heat, following DIN-EN 196 part 9 [KÖBLE 1999, GROSSE 2002]. The set-up to measure all these parameters is represented in Fig. 4.
3 Development of a new concrete FreshCon device
Comparing the two devices for mortar and concrete measurements, the advantages of the mortar set-up are evident (good reproducibility, easy onset time determination, no pressure air equipment necessary, full automatic measurement and storing of waveforms). To enhance the handling of concrete experiments accordingly, a new design of the device shown in Fig. 1 was suggested. For the new device the impactor was replaced by an US transmitter in combination with a wideband power amplifier and a function generator. No pressure air is need for this device; a control sensor next to the impactor recording the emitting pulse is no longer required. In several preliminary experiments the new set-up was tested by MANOCCHIO  to compare the results of measurements by the impact generated signals and by piezo-electric emitters in parallel (Fig. 5). The two curves at the bottom of the right side in Fig. 5, recorded at the same time using the same material, represent the velocity evaluation of gypsum.
Gypsum was used as a test material due to its fast hydration evolution. The two curves at the top position in Fig. 5 (right) represent the relative energy. A decrease of the velocity and energies values is caused by shrinkage effects. Both curve pairs look very similar in respect to differently used pulse generation methods.
This successful first test triggered the re-design of the concrete device(as well as of the mortar device). The electronic pulse is generated by a frequency generator and amplified by a power amplifier. Broadband piezo-electric transducers generate the ultrasound signal to be transmitted through the material. A transducer of the same type is used as a receiver and the signal is passed through a pre-amplifier to the PC-board A/D-converter. Special attention is given to the correct trigger time of the signal, what is essential for velocity measurements. A power amplifier of the company DEVELOGIC GMBH is used along with sensors of the company VALLEN INSTRUMENTS.
The new container/sensor design for concrete as well as for mortar experiments is shown in Fig. 6, demonstrating the similarity of these two. The U-shaped rubber in the middle of the container is essential. Regarding the concrete device, a special "long wall" container was produced for very "slow" materials to avoid waves propagating along the walls to be faster than the direct waves. The distance of the screw joints can be adjusted to the material properties.
First experiments in the frame of a master thesis [KALCKBRENNER 2002] and during round robin test of a RILEM technical committee showed very promising results.
4 Round robin tests-status
The International Union of Testing and Research Laboratories for Materials and Structures, RILEM, as a non profit-making, non-governmental technical association is structured in groups of international experts, the so called Technical Committees (TC). In the framework of advanced testing of cement-based materials during setting and hardening the TC 185 - ATC organized a round robin test series. The purpose of these tests is to assess the capability of existing test methods based on non-destructive techniques in terms of suitability, sensitivity and accuracy. Results will be summarized in a state of the art report andatest recommendation is planned to be released. In the context of providing a direct comparability, experiments are carried out by different members at the same place using the same charge of materials/mixtures.
The technical realization of the experiments is in the responsibility of the TC secretary (C. Grosse) and the local organizers. The ongoing test series started in 2001 with experiments in Vaulx-en-Velin(France) and was continued in Evanston/Chicago (USA) in spring 2002 and Brunswick (Germany) in summer 2002. The next round robin test is scheduled for 2003 in Delft (The Netherlands). In detail the following groups have been involved so far:
An experimental test program was compiled to be the basis for all experiments [GROSSE & REINHARDT 2002]. Six different mixtures are recommended to be tested . five other mixtures are testedadditionally. Some of the results obtained by the Institute of Constr uction Materials (IWB) at the University of Stuttgart are published by KALCKBRENNER and correlated to the results of other groups. A comprehensive report will follow.
To give an example of the data obtained during one test series Fig. 7 demonstrate the variation of the velocities over the age of the material. Concerning these velocities an S-shaped curve is typical for cementitious materials. After a certain time at the beginning, while the velocity variation is small, the gradient is increasing significantly. Regarding the data RE5 from a mix with added retarder this increase occurs relatively late. To make the basic statementsmore evident the curves are smoothed and bad data points are removed. It is obvious that concrete mixes are "faster" than mortar mixes in respect to hardening, while the RE5 mix with retarder is the "slowest" material.
It should be stressed that only material properties related to the elastic behavior canb e analyzed with ultrasound techniques.As far as the chemical properties are not related to the elastic properties, other measurement techniques have to be used in combination with ultrasound to get more data. The results of the round robin tests should indicate the value of the described ultrasound through-transmission technique in comparison to other techniques like ultrasound reflection, nuclear magnetic resonance, electric and maturity methods.
5 Summary and outlook
The measuring device developed at the University of Stuttgart is able to analyze the setting and hardening of cementitious materials in a comprehensive way. The method is based on ultrasound and can be used for numerous applications, where reliable and reproducible data are required, what addresses material parameters like the water-to-cement-ratio, the type of cement or the effect of additives as retarders or accelerators. At the concreting site, where efficiency and a low budget are boundary conditions, the application of this new technique can help to enhance the stability during construction or the progress of the construction work saving both: time and money. Some examples are the development of admixtures, the in-situ quality control, the slip form concreting or the precasting. Certainly, the applications are not restricted to cementitious materials.
Further improvements are concerning the velocity evaluation. Since the device consist of an analogue-to-digital converter of 5 MHz only, the resolution of the velocity calculations varies over age. Actually, the resolution decreases with increasing velocities. Using the offline version of the FreshCon picking algorithm the data can be re-evaluated after the test concerning the onset times of the signals only. Surprisingly, curves re-picked by the operator are usually very similar to the automatically processed data so that a time consuming manually picking is not improving the results anymore.
Formerly, the comparison of energy evaluation results was sophisticated due to the application of two different devices. Energy values as measured by the FreshCon software are basing on the squared amplitudes of the signal beginning at the signals onset of compressional waves. These values strongly depend on the energy released by the impact to the container. The reproducibility of the transmitter energy is low of impactor devices compared to devices using an ultrasound emitter. Changing the set-up as described made the interpretations regarding energies more reliable. There is still the disadvantage of energies emitted by piezo- driven devices to be of several magnitudes lower than impactor pulses. A new impactor device without pressure-air giving broad-band pulses of reproducible magnitude is under development. A further improvement can be achieved using container materials like two- component polyurethane elastomers (PUR) along with modified transducers. A new test series was conducted with this modified set-up giving promising results, which will be reported elsewhere.
Talking about the scientific aspects of the ultrasound technique, the method developed at the University of Stuttgart is under further progress. This is especially true concerning wavelet algorithms. The degree of automatization is enhanced and additional analysis techniques will be implemented in future.
With regard to the international activities of the RILEM technical committee more information can be obtained from the author or at the TC.s homepage: http://www.rilem.org/tc_atc.php. Colleagues working in this scientific field are offered to collaborate in this initiative.
The described design and re-design of ultrasound devices are the result of many years of scientific work. It is difficult to address the thanks to everybody who was involved. However, some colleagues should be mentioned in no particular order: Dr. B. Weiler, Dipl.-Ing. J. Fischer, Dipl.-Ing. I. Kolb, Dipl.-Ing. N. Windisch, Dipl.-Ing. A. Herb, Dipl.-Ing. S. Köble, Dipl.-Ing. R. Beutel, Dipl.-Ing. C. Manocchio, Dipl.-Ing. A. Kalckbrenner (M.Sc.), and Mr. G. Schmidt. A special acknowledgement is going to Mr. G. Bahr who's programming knowledge was very helpful during the years.
The results shown regarding measurements in the frame of the RILEM TC 185-ATC were obtained during a collaboration with the research group of Dr. Laurent Arnaud, Laboratoire Géomatériaux, Département Génie Civil et Bâtiment, of the Ecole Nationale des Travaux Publics de l'Etat (ENTPE) in Vaulx-en-Velin near Lyon, France.