THREE DIMENSIONAL IMAGING OF CONCRETE STRUCTURES USING ULTRASONIC SHEAR WAVES

Unlike the medical field, where significant advances in the area of three dimensional (3D) imaging of the human body has advanced considerably, reliable 3D imaging of concrete structures has not progressed much until recently. Among the nondestructive testing methods, Ground-penetrating radar (GPR) has been one of the most successful techniques in which 3D imagery can be produced. Although 3D GPR Images have been very successful in showing the orientation and location of the embedded targets, such as reinforcing steel, utility lines, etc., less successful results have been obtained when trying to produce a 3D image of internal voids and cracks. In addition, the use of GPR to locate voids in post-tensioned metal tendon ducts systems has yielded unsatisfactory results. Scanning Impact-echo (SIE) systems have also been recently developed to provide 2D and 3D images of concrete structures. The SIE systems have developed a more automated approach, thus relying less on the interpretation of the operator. Although test results to located internal voiding in concrete have shown some success, further research and testing is necessary to fully understand the capabilities and shortcomings of these SIE systems. Some of these SIE systems produced images that are based on computer algorithms that only compare the spectral response to that of a theoretical back wall or the full thickness of the section. Scanning impact-echo systems have not been very successful in assessing the grout condition in plastic ducts or embedded elements that have much lower acoustic impedance than concrete. Similarly, because of limitations intrinsic to the equipment, assessment of voids in thick concrete elements (thicker than 1 m) has been very difficult, and in some cases impossible to do. Conventional ultrasonic pulse velocity (UPV) testing has been used to painstakingly create 2D and 3D images of internal voiding. Such testing requires access to both sides of the structure and many readings at multiple angles or offset positions between the transmitter and receiver. The collected data is then analyzed in terms of propagation velocities and the arrival of the transmitted ultrasonic pulse. Finally, signal reconstruction computer algorithms are used to produce 2D or 3D images. 3D UPV testing of concrete is commonly used these days to assess deep foundations by means of the Cross-hole Sonic Logging (CSL) test method. This paper discusses the use of a recently developed instrument known as MIRA, which utilizes a patented phased array of dry point contact shear wave transducers to produce 2D and 3D tomography images of concrete structures. This new technology has allowed the operator to obtain in-situ, real time test results. The MIRA system is capable of creating cross-sectional 2D scans and 3D images of the concrete elements in less than 4 seconds after data collection. Similarly, data at individual inspection points are collected at a rate of about 3 seconds per location. The MIRA test method and recent equipment and analysis innovations are described; case studies from two real-life structures are presented and discussed. Similarly, the advantages and disadvantages of the system are discussed and conclusions regarding the overall feasibility of the system to accurately and efficiently detect internal voids, honeycombing, delaminations, and air voids in grouted post-tensioned cable duct systems will be presented. INTRODUCTION The MIRA equipment is not based on any new test concept, rather a merging of well-known stress wave propagation principles and the advent and application of advanced hardware and analysis software packages. The use of ultrasonic stress wave propagation dates back more than 60 years. In fact, the end of the Second World War propelled research into stress wave propagation in construction materials, particularly steel and concrete [1]. Laboratory equipment development, known as soniscope [2] and the ultrasonic tester [3], paved the way to the development of a widely used nondestructive test method for concrete known as the ultrasonic pulse velocity (UPV) method. The UPV test method traditionally employed the use of two transducers; a transmitting transducer and receiving transducer. To allow the transfer of the mechanically induced ultrasound waves, both transducers (transmitting and receiving) typically required a coupling agent to make the proper contact between the transducers and the surface of the concrete structure. Furthermore, a fairly smooth concrete surface is required. Similarly, development of other stress wave propagation techniques such as, the pulse-echo test method, which employs the use of only one transducer, has seen tremendous continuing improvement in recent years. However, despite recent hardware and software innovations, the use of a coupling agent to provide the proper transfer of the ultrasound signal continues to be a challenge for both methods (through-transmission (UPV) and pulse-echo). LONGITUDINAL vs. TRANSVERSE WAVES When a stress wave is induced into a concrete structure, three types of propagating mechanical waves are introduced; compression waves, also known as longitudinal or P-waves; Shear waves, also known as transverse or S-waves; and surface waves, also known as Rayleigh or R-waves. Each of these waves travel at a discretely different velocity. P-waves travel at a much faster speed than Swaves and R-waves. In concrete structures, Shear waves and Rayleigh waves travel at velocities with about 60 and 55 percent disparity from the longitudinal waves, respectively [1]. Commonly used UPV and pulse-echo equipment utilize transducers that generate longitudinal waves inducing particle motion, due to the stress, that propagates parallel to the direction of the wave front. Alternately, transverse waves have particle motion that propagates perpendicular to the direction of the wave front, that is, parallel to the concrete surface. The velocity of the propagating wave is directly proportional to the frequency f and wavelength λ of the wave motion by: