Non-Destructive Assessment of Fire-Damaged RC Columns by Experimental Modal Analysis and Sonic Testing: Development, Validation and Application to Real Cases

Post-fire evaluation of reinforced concrete (RC) columns in existing buildings demands reliable non-destructive methods capable of quantifying stiffness loss, identifying damage patterns, and supporting decisions on repair or replacement. This study develops and validates a dynamic NDT methodology that integrates Experimental Modal Analysis (EMA) and sonic testing (ST) based on elastic wave propagation to characterise the mechanical condition of fire-damaged RC columns. The procedure combines global dynamic indicators (natural frequencies and mode shapes) with local measurements of the dynamic Young’s modulus along the column height. A numerical finite element (FE) model is employed to validate the sensitivity and consistency of the proposed dynamic indicators under controlled degradation scenarios. After validation, the methodology is applied to two real fire events affecting basement columns in residential buildings. In the first case, several columns exhibit significant stiffness reductions, with pronounced modulus losses in the most exposed regions, leading to the recommendation of comprehensive strengthening measures. In the second case, results show minimal variations in most elements, allowing targeted intervention on a single moderately affected column. The study demonstrates that the combined EMA–ST approach provides a robust and cost-efficient basis for diagnosing fire-exposed RC columns and for guiding post-fire structural decision-making in practice.