Influence of environmental factors in the assessment of mechanical properties of masonry in historic construction

The mechanical characterisation of historic masonry is essential for assessing structural integrity and planning conservation interventions. This study investigates how environmental humidity influences measured parameters, such as wave propagation velocity and Schmidt hardness, which were acquired through non-destructive testing across multiple walls of the Monastery of Batalha, a UNESCO World Heritage Site in Portugal. Correlation analysis, humidity-group stratification and partial-dependence visualisations were employed to explore how humidity affects these mechanical indicators nonlinearly and alters the relationships among them. The results highlight humidity as a key variable, capable of significantly modifying both velocity and hardness measurements, and acting as a confounding factor in their interdependence. To address these findings, predictive models were developed to estimate P-wave velocity from indirect measurements, with humidity explicitly incorporated as a moderating parameter. The comparison of modelling strategies demonstrated that humidity-aware and non-linear approaches markedly improve predictive accuracy, offering a more reliable pathway for estimating elastic properties when direct testing is not feasible. Overall, this study establishes a framework for integrating environmental effects into the interpretation of non-destructive testing, thereby enhancing the reliability of mechanical characterisation in heritage masonry and supporting informed conservation practices.