Julius Fink, winter semester 2011/12
Artikel auf Deutsch
In civil engineering, it is necessary to characterise the setting and the hardening behaviour of fresh concrete. This is for instance, necessary in slip-form construction where the earliest possible ending time has to be determined to operate economically but also deliver high quality. A significant disadvantage of the currently applied methods for the assessment of setting and hardening behaviour, such as the Vicat needle method or the penetration method, is that these methods mostly give only a snapshot in time of the material’s properties and the hydration process cannot be continuously monitored with them. 
Additionally, information about the effect of concrete admixtures or precise information about concrete-mortar mixtures is often requested Here the ultrasound technique comes into play. As the propagation velocity, the damping and the frequency content of ultrasound waves depend on the medium and thus on the setting and hardening of the concrete, ultrasound measurement is especially suitable here. 
In ultrasonics there are several technical process possibilities to determine information about the hardening behaviour of fresh concrete. This article will describe the ’transmission method', that is the transmission of ultrasound waves through fresh concrete.
The transmission method takes advantage of the fact that the way an ultrasound signal wave propagates through a sample is subject to specific laws of physics. Different measurands of the wave field, such as the wave velocity, the amplitude of the signals and their frequency content, are changed by material characteristics and can be used for the evaluation of the object. 
Properties of materials that can influence the propagation of ultrasound waves are:
With fresh concrete analysis, it is of particular interest that there is a direct correlation between the concrete parameters and the ultrasound signal parameters, such as wave velocity, amplitude and frequency. In this case, the concrete parameter could be the water:cement ratio, the air pore content, the consistency, the final strength and the effect of admixtures. The most important measurand for the assessment of hardening of fresh concrete is the wave propagation velocity. The latter is given by the correlation between the time , a signal needs to travel a certain distance through a medium ().
In addition to the material, the ultrasound pulse is also altered by every device participating in the measurement. To clearly distinguish material effects from measurement effects, it has to be enured that these influences are minimal or at least known.  Measurement using transmission to test fresh concrete requires several measurement devices such as ultrasonic sensors or signal amplifiers that influence the signal evaluation. It is therefore necessary to know how the individually involved measurement devices influence the measurement and how these influences can possibly be taken into consideration arithmetically.
Using transmission for the evaluation of fresh concrete is an active testing method in which an ultrasound signal is generated by a signal source, is then transmitted through a body and detected by a sensor at a different location. As a result, the path that the signal is transmitted along is constant and known. The time the signal needs to travel from the signal source through the medium to the receiver is measured to calculate the velocity.
Now these travel times are continuously measured during the hardening of fresh concrete. This results in different wave velocities. If these are plotted in a graph relative to the concrete’s age, this results in characteristic and often s-shaped curves which visually capture the hardening and setting and more or less define a kind of fingerprint of the tested material.  Figure 2 shows anexample of the development of the velocities of concrete with addition of different admixtures in relation to the ongoing hardening process. Comparable curves can be created and compared for all hardening materials e.g. mortar or concrete.
|Fig 1: Schematic diagramof ultrasound measurements using transmission||Fig 2: Example of the effect of different concrete admixtures on the velocity profile of the ultrasound signal during the hardening of fresh concrete |
The fundamental method is based on travel-time measurement of the compression wave and this is suitable for the determination of the operational function of different concrete admixtures. One enhancement of the method is the use of shear waves. With the additional information provided by the shear wave velocity, thefollowing correlations are formed:
Material parameters such as dynamic Young’s modulus and the dynamic shear modulus can be directly determined. In contrast to the above mentioned methods, sensors are used here that are sensitive to compressor waves as well as to shear waves. This dynamic method thus offers a precise solution for the determination of dynamic material parameters.
The sensor-detected signal does not directly provide information about time. This information still has to be determined. Therefore, the signal has to be observed in detail and a point in time at which the onset time of the signal is recognisable must be determined. Here a distinction can be made between the compression wave (P wave) and the shear wave (S wave), that is, the P wave is always faster than the S wave. In this context it is called onset time picking. In figure 3, a typical signal is shown, which demonstrates the onset time of the P wave or the S wave. In practice, these signals are contaminated by reflections and noise, thus precise picking of the onset time presents a huge problem. However, as precise a determination of the onset time as possible is the basic pre-condition for calculation of the exact sound velocities, and this is why the main focus is placed on it. In practice, software has been specially developed to take on this work focused on signal form analysis and frequency information, for which specific input parameters have to be set and checked by the person measuring.
Figure 4 shows the example of the result of a series of measurements with shear waves on concrete during hardening: Here, three different areas can be identified. 
|Fig. 3: Ultrasound signal ||Fig. 4: Intensity graph with ultrasound signals created during the hardening time of a concrete during setting and hardening |
This article explains the FreshCon system in more detail. This method was developed by the University of Stuttgart and was eventually patented. The FreshCon system basically consists of one container made of polymethyl methacrylate (Plexiglas), two broadband piezo-sensors, one electric pulse-emitter and one signal amplifier. A computer equipped with special software is used for controlling and evaluating the measurement data (Fig.5).
At user-defined intervals, the pulse emitter transfers computer-controlled electrical impulses to the sensor, which due to piezoelectric behaviour emits an ultrasonic pulse at the sample. This pulse is transmitted by the medium, received by the receiver sensor and passed on as an electrical signal to the amplifier. This amplifies the electrical signal and forwards it to the computer. With the aid of software, the time that every signal requires to be transmitted through the medium can now be determined. As the sensors are firmly mounted on the container, the signal is also received by the container and thus may distort the electric signal at the receiver sensor. Therefore, an attempt is made to ensure/guarantee that as concentrated a signal as possible is delivered to the medium through use of component geometry and powerful damping materials such as foam. In the example of FreshCon systems, the container consists of two elongated panes of Plexiglas. These are connected together with four screws. In the centre of the component, a foam profile is now clamped between the two panes into which cement mortar can later be poured. As a result, the panes of Plexiglas are elongated as this ensures the longest possible signal path over the clamping screws to the receiver.
|Fig. 5: Example of the object model||Fig. 6: Container of the FreshCon-Systems for ultrasound measurements of transmission in fresh concrete|
Information about the setting and hardening process of concrete and mortar can be gathered non-destructively and continuously using the ultrasound measurement of transmission. For practical application on construction sites, this method has significant disadvantages compared with currently used methods such as the Vicat needle method, the slump cone test or the penetration method, as it is relatively complicated. Further development towards a construction site viable ultrasound testing device would be worthwhile due to the much more precise information it provides about the setting and hardening process of concrete. By enhancing the understanding of setting and hardening it is possible to directly determine elastic material parameter through combined P and S waves measurements. The ultrasound method can be efficiently applied in the field of building material development and optimisation or quality control.