Optimising in-situ testing for historic masonry structures: a case study

Masonry structures were built by means of various techniques, using materials whose properties exhibit a considerable scatter depending on the period of construction, regionspecific soil resources and manufacturing procedures. Significant spatial variability within the structural system is often observed due to diverse deterioration mechanisms. Therefore, it is important to obtain case-specific information on the mechanical properties of existing masonry structures. The case study of a historic building illustrates how the number of destructive tests (DTs) used to calibrate non-destructive tests (NDTs) can be optimised for the reliability assessment. Calibrated NDTs are used to establish the probabilistic model for compressive masonry strength. The specific focus is on compressive strength of masonry units; tests of mortar strength are only briefly discussed. The preliminary assessment is based on non-calibrated NDTs. The number of DTs for calibration is then optimised by means of probabilistic cost optimisation, considering also possible subsequent actions – ‘do nothing’ or ‘strengthen the structure’. It appears that the estimates of masonry unit strength based on NDTs are associated with large dispersion and may be significantly biased. Consequently, it is often beneficial to conduct at least one DT. A practical tool is provided to determine optimum number of DTs for different outcomes of a NDT survey and different failure consequences.