Application of Infrared Thermography based on Temperature Change to Surface Weathering Evaluation of Blue Brick from Pagodas in China

Maintenance programs for ancient buildings are of strategic importance today, and the evaluation of material weathering degree is an essential component of protection and restoration. In this study, two temperature change indices, the cooling rate index (CRI) and the temperature variation (Tv), were introduced into infrared thermography (IRT) to assess the weathering degree of ancient blue brick specimens collected from six pagodas in Northwest China. After controlled heating under two initial conditions, 90 and 50°C, IRT was used to monitor the cooling of the specimens, and CRI and Tv were calculated to describe their thermal dissipation behavior. Surface hardness measurements and P-wave velocity tests were conducted to verify the accuracy and significance of the indices. The results indicate that the cooling tendency during the first five minutes is the most sensitive indicator of surface degradation. CRI5, derived from the initial cooling stage at 90 °C, demonstrates the strongest correlation with hardness values (R² = 0.899), proving its diagnostic accuracy and stability across specimens with different degrees of weathering. Tv5 also shows potential as a supplementary index, although its predictive strength is lower compared to CRI5. In contrast, both CRI and Tv exhibit weak correlations with P-wave velocity, highlighting their particular sensitivity to surface conditions rather than internal structures. The findings confirm that IRT provides a dependable, non-invasive method for quantifying surface deterioration of ancient bricks. A preliminary grading framework based on CRI5 values is proposed, offering a practical and efficient diagnostic tool for preventive conservation.