A Modified Virtual Fields Approach to Identify the Mechanical Properties of Mortar in Existing Masonry Structures Without Loading Information

Identifying the mechanical properties of mortar in existing masonry structures is challenging. While units can be extracted and tested in the laboratory for identification purposes, sampling of mortar is often unfeasible. This paper develops a new in-situ identification procedure for mortar, using surface strain measurements and knowledge of the mechanical properties of the unit. It makes use of the Virtual Fields Method (VFM) algorithm, which enables the direct identification of material characteristics from strain and loading information. In this new application, the VFM is modified to identify the mechanical properties of mortar using unit properties only, without recourse to loading information. This treatment helps eliminate the significant loading uncertainties encountered during flat jack tests. To explore the feasibility of this approach, a three-dimensional finite element model with a microscale representation of a brick masonry wall is constructed. Bricks and mortar are assigned isotropic and linear elastic material properties. Overburden and in-situ flat jack loads are applied. The resulting surface strains due to jack loading are obtained; these simulate full-field measurements that can be obtained using Digital Image Correlation. The modified VFM approach is then applied, where an excellent agreement was obtained between the estimated and true Young's modulus of the mortar. The influence of errors in unit properties and measurement noise on identification results was quantified and indicate the robustness of the identification procedure.