Detection of free chloride in concrete by NMR

Laboratory experiments to detect chloride in a cement matrix using pulse nuclear magnetic resonance (NMR) were conducted. The coils were in the centimeter scale and the magnetic field was 2.35 T. NMR signals were obtained from both aqueous chloride solution and samples of both regular and white Portland cement (WPC). A concrete sample from a sidewalk that had been in the field for 20 years was also tested. The experiments demonstrated that the signal-to-noise ratio (SNR) for a centimeter-scale cement sample volume is so small, even after averaging, that sample volumes much lower than that are unlikely to produce measurable signals at fields of 1 T or below. The consequence is that the potential for realizing an embedded NMR-based sensor including the magnet is low. Parametric studies identify feasible alternative coil diameters and magnetic field strengths for detecting chloride ion concentrations in hardened concrete.