Ultrasonic Phased Array and Synthetic Aperture Imaging in Concrete

For ultrasound in the hundred kilohertz regime concrete is a very inhomogeneous propagation material. The prescribed definition of aggregates with regard to the respective percentage, the size distribution and the material composition together with the percentage of air inclusions can be realized as a computer model to serve as a platform for elastic wave propagation simulations. As a simulation tool we have developed the Elastodynamic Finite Integration Technique (EFIT) in 2D and 3D, thus being able to visualize the radiation field of either conventional transducers, ultrasonic phased arrays and arrays of point contact transducers. Through the measurement of the displacement on the opposite side of a specimen's excitation surface with a laser vibrometer the simulated data can be related to experiments thus clearly predicting the performance of these devices in the concrete environment. A particular NDT problem in concrete consists in the identification of grouting defects in tendon ducts via the application of imaging algorithms like SAFT (Synthetic Aperture Focusing Technique) and its diffraction tomographic Fourier transform version FT-SAFT. EFIT is able to produce synthetic data for a given situation thus providing a test bed for SAFT in concrete to reveal its potential for these applications. Various examples will be given and compared with pertinent experiments. Furthermore, EFIT combined with SAFT serves as a tool to predict the behavior of specimens to be fabricated before they are actually realized. Introduction: For all applications of remote sensing the phased array concept is very prominent because of its fast or real-time applicability and the high capabilities in respect to lateral and axial resolution. Up to now the application of this method for ultrasonic testing in concrete is not widely used because of the extraordinary expense and the rough environment in which the equipment has to be used. With the knowledge about the problems in nondestructive testing of concrete by ultrasonic waves it may be worth to think about it. The wave propagation in concrete material is dominated by a high degree of multiple scattering and high attenuation because of the heterogeneous structure of the "background" material which consists of three components: aggregate, cement, and pores distributed in a more or less statistical manner, roughly characterized by a grading curve for the size of the aggregate which itself consists of sand and gravel stones. The size of the material inhomogeneities recommends the use of ultrasonic waves in the 50 kHz to 200 kHz regime. Together with the problem of coupling this yields a size of our array with 10 elements in a line consisting of 34 mm commercial broadband transducers of about 34 cm. Phased arrays consist of a collection of similar transducers which are fed by a set of pulsers which can fire an impulse or a predefined signal with an appropriate delay and amplitude to reach a desired beam direction and focusing. Different constellations have been discussed in [1]. Because of the really strong behavior of waves in concrete the outcome of investigations with sophisticated techniques is more or less unpredictable. Therefore, empirical investigations in connection with modeling techniques have to be performed. Considering the use of measured data for further processing like Diffraction Tomography or the Synthetic Aperture Focusing Technique (SAFT) the whole cycle of analysis has to be repeated for any new detection problem. In this paper the methods are presented to apply phased array techniques for detecting voids in tendon ducts, which are partially filled with steel cables and mortar. Results: 1. Modeling The calculation of the wave propagation in the given complex structure is only possible by numerical methods. Based on the governing equations of linear elastodynamics a numerical modeling tool was developed and denoted as Elastodynamic Finite Integration Technique (EFIT) [2,3]. It has been validated against many analytical and experimental methods. Even the application of EFIT to strongly inhomogeneous structures like concrete has been investigated successfully [4, 5]. As a model for the medium serves an accumulation of ellipses with different orientation, size and material property, which is generated by a computer code according to statistical specifications and grading curves. This is possible for two and three dimensions. The pores developing during drying of the concrete in the range of 0 to about 0.1 mm are taken into account by grid cells with material properties of air in the interspace of the ellipses which is filled with mortar otherwise. Figure 1 shows a picture Figure 1: Modeling of a concrete sample: real (left), 2D modeling (center), 3D modeling (right). of a concrete specimen and the 2D and 3D presentation by the grid generator. Figure 2 shows the wave field (a snapshot at a certain time) which is transmitted by a transducer at the top of a 2D test geometry. Beside the propagation in a homogeneous isotropic background material (mortar) the wave in a concrete composition is shown. The numerical effort of EFIT is predetermined by the discretisation of the area or the volume, the number of time steps which is given by the way the wave has to cover. Additionally, the kind of the measurement which should be modeled by the simulation determines the effort. For monostatic measurements – this means that the position of the transmitting and receiving transducer is common and is changed for every pulse – a complete simulation is necessary for every transducer position (A-scan). The numerical effort is distributed by an extra MPI implementation for distributed computing on a compute cluster of 16 single nodes in our department. Figure 3 shows the used segmentation model and an example of the propagation of a plane wave in a cube of concrete with the edge length of 51 mm. Figure 2: Wave propagation by EFIT in a homogeneous background (left) and in concrete (right). Domain Decomposition 1 2