Expansion of mortar joints in direct shear tests of masonry samples: implications on shear strength and experimental characterization of dilatancy

Abstract

The expansion of masonry specimens during direct shear tests has been reported in several research studies. This phenomenon, known as dilatancy, is caused by the formation of cracking surfaces in mortar joints. In particular, when the cracking surface is not perfectly flat, the shear displacements tend to increase the volume of the sample. Experimental investigations focused on the characterization of this phenomenon are rather limited for masonry and the effects on shear strength have received little attention, with consequent issues for a correct interpretation of the results. The present article reports the results of an ongoing research on brick masonry aimed to characterize experimentally the dilatancy and to evaluate the role of this phenomenon in the interpretation of the direct shear test. If the expansion of the specimen is significantly restrained, the standard approaches used for the characterization of the mechanical parameters (as per EN 1052-3 and ASTM C1531) tend to overestimate the initial shear strength (fvo) and underestimate friction. Moreover, no indications are generally given to characterize dilatancy with experimental data. This aspect is particularly important for the micro-modelling of masonry because the constitutive models commonly used for mortar joints require this information. One of the objectives of the present article is to propose a simple model for a sound interpretation of the direct shear test of masonry samples taking into account the dilatancy. Several masonry samples composed of calcium silicate units and cement mortar joints have been subjected to triplet tests (EN 1052-3) and laboratory-simulated shove tests. First, a repeatable and objective methodology to measure and characterize the dilatancy is provided. Then, an extension of the standard methodology of the EN 1052-3 and ASTM C1531 that includes the contribution of this phenomenon is proposed. The novel formulation offers the possibility to characterize dilatancy with experimental data and the definition of mechanical parameters that are not biased by the presence of this phenomenon. The model presented in this article has proven to be consistent with the experimental data and it has been validated numerically in another recent research study.