Analysis of Alkali-Silica Reaction in Concrete with Microwave Backscattering and Impedance Spectroscopy
Abstract: Alkali-silica reaction (ASR) in concrete is a chemical reaction between alkaline ions in Portland cement pore solution and non-crystalline silica found in many aggregates. The product of reaction is highly hydrophilic ASR gel, which absorbs water and expands. The result of ASR is volumetric expansion of concrete structure, which over time may cause concrete cracking and tensioning of attached slender members. Prevention of ASR in future constructions is based on accelerated tests of concrete mix designs. There is no remedy for existing concrete structures diagnosed with ASR.
Typical mitigation approaches consist of structure confinement to re-direct expansion. While ASR in concrete has been known for decades, recent concerns have arisen due to the challenges that concrete degradation poses to sustainability of nuclear energy. A large number of commercial nuclear reactors is nearing the end of their current Nuclear Regulatory Commission (NRC)-approved licensing terms. Several instances of ASR occurrence in nuclear reactor concrete have been reported, and re-licensing of operations requires a plan for evaluating and monitoring conditions of the concrete structures. Current industry practice for ASR evaluation is based on destructive tests, which consist of core extrusions from structures and laboratory microscopy of thin sections. There is no capability for quantitative nondestructive evaluation (QNDE) of ASR in concrete. The only reliable non-destructive indicator of ASR time progression is concrete length expansion, which provides qualitative information.
We have explored applications of microwave backscattering and impedance spectroscopy to QNDE of ASR. Both of these methods are highly sensitive to free water in pore networks. The goal of using different NDE modalities is to increases confidence in detection. The study is based on a set accelerated ASR specimens, and a companion set of age-matched controls. Using the data from microwave backscattering and impedance spectroscopy measurements, we have developed an analysis procedure to identify signatures in the data which correlate with time evolution of standard length expansion indicator. This opens the possibility to obtain information about the content of ASR gel at the sub-surface layer of concrete specimens from inexpensive measurements which could be performed in-situ.