Ultrasonic Tomography Technique for Evaluation Concrete Pavements

Ultrasonic tomography is an emerging technology that shows promise for quality assurance/quality control (QA/QC) during construction, or for making rehabilitation decisions in concrete pavements. However, the benefits of this emerging technology have not yet been fully captured for widespread use in highway infrastructure management. Verification of a state-of-the-art ultrasonic tomography device (MIRA) is presented through multiple field trials involving typical pavement constructability and rehabilitation issues. Field trials indicate that, while the device is a useful portable pavement diagnostic alternative capable of consistent thickness measurement, reinforcement location, and distress evaluation, significant efforts and user expertise are required for measurement and data interpretation of large scale application. Thus, software was developed for a more productive, objective signal interpretation method with automated analysis of reinforcement location in continuously reinforced concrete pavement. This type of automation for multiple applications shows promise for the use of ultrasonic tomography to improve large scale pavement QA/QC and rehabilitation projects in the future. Nevertheless, the research in the paper shows ultrasonic tomography to be an accurate, reliable, and convenient alternative or supplement to traditional techniques that can be utilized for a wide variety of small scale pavement diagnostics applications. INTRODUCTION Non Destructive Testing (NDT) techniques have been used to evaluate concrete pavements for many years. Some commonly used devices include the Ground Penetrating Radar (GPR), MIT SCAN 2 based on magnetic tomography for accurate dowel location, chaining, and seismic methods This paper explores a state of the art ultrasonic tomography device (MIRA) for use in evaluating concrete pavements. Ultrasonic testing uses high frequency (greater than 20,000 Hz) sound waves to characterize the properties of materials or detect their defects. Sound waves are generated by transducers, travel through the material, and are received at the surface. Analysis of the signals by the receiving transducer provides information about the media through which the signal has propagated. Ultrasonic techniques have been successfully used in medical applications and flaw analysis in metals and composite materials for many years (Qi. et al, 2009; Sauvik et al. 2007). However, earlier applications of these types of ultrasonic technology for evaluation of Portland cement concrete (PCC) and asphalt structures have experienced difficulties. Traditional ultrasonic methods relied on time consuming liquid coupling and could not achieve the necessary penetration depths in pavement materials due to the heterogeneity causing excessive attenuation of the wavefront (Schubert and Köhler, 2001). These difficulties led to the use of rudimentary Hoegh, Khazanovich, and Yu 4 acoustic methods in pavement applications such as chain dragging or impact echo (IE). Conventional IE has experienced inconsistent results when applied to heterogeneous mediums such as concrete with only one mechanical impact that is highly dependent on variable duration (Schubert and Köhler, 2001; Carino, 2001). On the other hand, the dry point contact (DPC) transducers used in ultrasonic tomography have been developed with the capability of transmitting of relatively low frequency (55 khz) elastic waves that penetrate greater depths (Nesvijski, 1997; Mayer et al., 2008). Use of these transducers has been advanced and successfully applied for years in dealing with detailed evaluation of civil structures in Germany (Schubert and Köhler, 2001; Mayer et al., 2008; Khazanovich et al. 2005; Langenberg, K.J. et al., 2001; Marklein, R. et al. 2002). These developments lead to the development of MIRA, the state-of-the-art ultrasonic tomography device for diagnostics of concrete structures, which utilizes the same principles that have been successful in medical and metal applications (ACSYS, 2008). MIRA utilizes 45 transmitting and receiving transducer pair measurements (see figure 1) in a less than 3 second scan resulting in a 2D depth profile (b-scan). The DPC transducers provide the necessary consistency of impact and wavefront penetration for diagnositcs up to 3 ft. deep. The multiple sensor pairs in each scan allows for the required redundancy of information to evaluate heterogeneous mediums such as PCC or asphalt. Figure 1 shows a manual MIRA measurement on the left in which the transducers are placed flush to the surface for a b-scan measurement. A b-scan is a 2 dimensional reconstruction of the concrete directly below where the scan was taken with high intensity of reflection areas indicating changes in acoustic impedance. On the right side of figure 1 the increased redundancy of information of MIRA (bottom) over conventional IE (top) can be observed, where the multi-static array of transmitting and receiving transducers creates 45 measurement angle pairs compared to one measurement pair in traditional IE. MIRA 45 pairs per measurement 1 pair per measurement Impact Echo Hoegh, Khazanovich, and Yu 5 Figure 1. MIRA ultrasonic pitch-catch device and comparison with traditional impact echo method (Carino, 2001; ACSYS 2008). MEASUREMENT PROCESS AND SIGNAL INTERPRETATION The ultrasonic tomography device (MIRA), manufactured by Acoustic Control Systems, contains 40 “touch and go” transmitting and receiving DPC transducers (see Figure 1). The probes are firmly fixed in an array with 10 channels of 4 transducers resulting in a scanning aperture of 400 mm by 50 mm. Each of the probes can act as either receiver or transmitter with a default operation ultrasonic frequency of the 55 kHz (ACSYS, 2008). The device measures time of signal propagation between the transducers at fixed distances for material velocity calibration, and uses the synthetic aperture focusing technique (SAFT) for analysis in scan mode to reconstruct the medium below the measurement based on the shear wave reflections. SAFT has been found to be a feasible algorithm for use with the ultrasonic pitch-catch technology as well as other applications. The basis of the SAFT algorithm is given in equation 1 (Langenberg et al., 2001):