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The Bond Mechanism in Stone‐ or Brick‐to‐Grout Interfaces

Abstract:  An experimental research on the bond between masonry units and grout has been carried out at the National Technical University of Athens. In this study, tension and shear tests on composite substrate/grout specimens were performed. The composite specimens were consisted of two pieces of substrate connected with one grout joint. The substrate was either stone or brick, whereas hydraulic lime or tripartite (lime–pozzolan cement) or cement grouts were used. The experimental results have demonstrated that the developed tensile and shear bond strength of the studied hydraulic lime and tripartite grouts is comparable with those of cement‐based grouts. Moreover, the value of the reached bond strength (tensile or shear) is governed mainly by the substrate characteristics and the binding properties of the grouts. It was also found that interfaces subjected to shear exhibited similar behaviour with interfaces within concrete; thus the beneficial effect of normal compressive stress on the interface was confirmed. Finally, the experimental results are used for the formulation of a Mohr–Coulomb criterion for grout to travertine joints in shear–compression and shear–tension. The results of this project clearly confirm the efficiency of grouts with medium or low mechanical properties (tripartite and hydraulic lime grouts) for retrofitting historic masonries.