Nondestructive assessment of compressive strength in historical solid clay bricks using the rebound hammer test: influence of density

This study investigates the compressive strength of historical solid clay bricks using the N-type rebound hammer, a widely used non-destructive testing method. Although commonly applied in concrete assessment, its use in masonry is limited by the lack of well-established calibration curves and standardized testing protocols. Existing models, primarily developed for concrete, often overestimate brick strength due to the material's heterogeneity. Prior studies suggest that acceptable accuracy can be achieved for bricks with compressive strengths below 25 MPa using L-type hammers and up to 40 MPa with N-type hammers, but these thresholds are impractical without prior knowledge of the actual strength. To address this, 100 clay bricks—sourced from two existing historical buildings aged 125–140 years—were tested. The bricks were categorized by color (dark red, brown, yellow) and further grouped based on laboratory results. Rebound hammer tests were performed alongside standard compressive tests to develop empirical models using single- and multivariable regression. Results showed that the N-type hammer provides reasonable strength estimates for historical solid clay bricks, but caution is required when rebound values exceed 40, as prediction errors may surpass ±25%. Incorporating density as an additional variable in multivariable models significantly improved estimation accuracy. However, in heritage structures where removing a sufficient number of bricks for density measurement is generally restricted, single-variable models offer a more practical alternative. These findings highlight the potential of the N-type rebound hammer for non-destructive assessment of historical masonry, with recommendations for model selection based on the accessibility of supporting test data.