Laboratory and GPR-Based Assessment of Grouting Performance in Historic Masonry: The Galata Tower Case Study

Historic masonry structures frequently develop internal voids, cracks and loss of bonding due to ageing, environmental actions and previous incompatible repairs. Grout injection is widely used to restore structural continuity; however, the in-situ performance of commercial hydraulic lime-based grouts remains insufficiently documented. This study evaluates a commercial natural hydraulic lime grout applied during the conservation of the Galata Tower, a 14th-century masonry monument in İstanbul, through an integrated laboratory-, Ground Penetrating Radar (GPR)- and Masonry Quality Index (MQI)-based assessment. Pre-injection GPR investigations identified internal voids, discontinuities and heterogeneous zones within the masonry walls. The grout was characterised through rheology, flowability, injectability, water retention and hardened-state tests, and the results were evaluated against the characteristics of the original mortars. Post-injection GPR monitoring was performed to evaluate filling efficiency and changes in internal continuity, while MQI assessment supported the structural interpretation of the diagnostic results. The grout exhibited stable rheological behaviour, adequate injectability and satisfactory resistance to segregation, enabling penetration into complex internal void networks. Post-injection GPR results indicated improved internal continuity after grouting. The study highlights the importance of combining laboratory material characterisation, MQI-based masonry quality assessment and non-destructive field validation for grout injection in historic masonry.