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As they degrade and age, concrete structures are susceptible to the development of multiple defects, such as alkali-silica reaction (ASR). ASR forms a gel that occurs over time between alkaline cement paste and reactive, noncrystalline silica in aggregates [1]. With the continuing extension of the life span of nuclear power plants, it is important to monitor the structural integrity of concrete structures after 40+ years. Compression strength, modulus of elasticity, flexural stiffness, shear strength, and tensile strength are performance characteristics susceptible to the development of ASR. Current methods for monitoring for defects and degradation within thick concrete structures require damaging the concrete. Oak Ridge National Laboratory, in collaboration with the University of Pittsburgh, has developed a nondestructive evaluation (NDE) and reconstruction technique that enables deep-image reconstruction of damaged concrete. The frequency-banded synthetic aperture focusing technique (FB-SAFT) is an augmented SAFT reconstruction that segments the time-series data into different frequency bands using wavelets immediately before performing SAFT reconstructions [2]. This paper presents the NDE instrumentation and reconstruction techniques applied to thick-reinforced concrete structures and concludes with preliminary results. [ABSTRACT FROM AUTHOR]

Oak Ridge National Laboratory (ORNL) comparatively evaluated the ability of a number of NDE techniques to generate an image of the volume of 6.5' X 5.0' X 10" concrete specimens fabricated at the Florida Department of Transportation (FDOT) NDE Validation Facility in Gainesville, Florida. These test blocks were fabricated to test the ability of various NDE methods to characterize various placements and sizes of rebar as well as simulated cracking and non-consolidation flaws. The first version of the ultrasonic linear array device, MIRA [version 1], was one of 7 different NDE equipment used to characterize the specimens. This paper deals with the ability of this equipment to determine subsurface characterizations such as reinforcing steel relative size, concrete thickness, irregularities, and inclusions using Kirchhoff-based migration techniques. The ability of individual synthetic aperture focusing technique (SAFT) B-scan cross sections resulting from self-contained scans are compared with various processing, analysis, and interpretation methods using the various features fabricated in the specimens for validation. The performance is detailed, especially with respect to the limitations and implications for evaluation of a thicker, more heavily reinforced concrete structures. [ABSTRACT FROM AUTHOR]