A non-invasive methodology for microstructural and chemical assessment of degradation in seawater-exposed concrete using correlative micro-CT and SEM-EDS

Submerged concrete infrastructure in semi-enclosed marine basins is exposed to a complex set of degradation factors, including chloride and sulphate attack, biofouling and, in the case of the Baltic Sea, the suspected influence of historically dumped chemical munitions. Conventional condition assessment relies on core drilling, which is costly, logistically demanding and unacceptable in areas where unexploded ordnance or chemical contamination cannot be ruled out. However, no validated, minimally invasive analytical protocol has been established for those environments. This study validates that “samples of opportunity” - loose concrete fragments retrieved by remotely operated vehicles from three Southern Baltic Sea harbours (Puck, Ustka, Kołobrzeg) - when inspected through correlative X-ray micro-computed tomography (µCT) and scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS), provide quantitative and site-specific evidence of the underlying degradation mechanisms. µCT analysis revealed pore volume fractions ranging from 0.78% to 6.37% and identified a near-surface degradation front confined to the topmost 2–5 mm. SEM–EDS quantification disclosed locally elevated chloride contents up to 7.28 wt% at Kołobrzeg and sulphate concentrations reaching 27.27 wt% as SO₃, while no heavy metal markers (As, Cu, Pb) potentially associated with chemical warfare agents were detected in any specimen. Loss on ignition values between 1.26% and 6.64% indicated a variable contribution of biomass-derived organic matter, in agreement with biofouling observations. The proposed protocol enables the differentiation between environmental chloride attack, sulphate-driven alteration and bioactivity, providing a transferable diagnostic tool for the condition assessment of submerged concrete infrastructure in complex marine environments.