Prediction of compression strength of ancient mortars through in situ drilling resistance technique

The paper presents the results of a research project carried out to determine the mechanical characteristics of mortar in historical masonry buildings by drilling resistance technique (DRMS). A set of mortar mixtures have been produced in experimental campaigns covering different compressive strengths typical of historic mortars. The mortars have been obtained with different classes of binder and river sand with different grading curves. Specimens obtained were subjected both to flexural, compression and DRMS laboratory tests. Considering the results of the drilling measurement distributions, both the mode and the average value, the latter obtained after a statistical treatment in which extreme outliers are deleted, have been taken as reference values for each test performed. Effective correlations between the mode and average values of DRMS tests and the compressive strengths of the mortars are provided. Finally, a stepwise process to be followed for in situ use of DRMS is defined and applied to mortar joints of some existing masonry buildings in Tuscany (Italy).