A practical method for robust elastic characterisation of mortar in flat jack tests

Abstract

Standard interpretations of in-situ flat jack tests rely on uniform stress states and only provide information on the mechanical properties of masonry as a composite and not those of its individual constituents. This paper develops a practical approach to identify the elastic characteristics of mortar using surface strain measurements (e.g. from Digital Image Correlation). It aims to provide robust identification under uncertain jack loads and stochastic material properties. A setup composed of three flat jacks inserted in masonry joints is explored in a micro-scale numerical model to develop the approach. The resulting surface strains are processed using the Virtual Fields Method to simultaneously and non-iteratively identify the mortar’s Young’s and shear modulus. Instead of the uncertain jack loads, mechanical properties of bricks, which are easy to obtain from laboratory testing of extracted samples, are used for the identification. The identified Young’s and shear moduli are robust against variations in jack penetration depth, measurement noise, stochastic material variability, and inaccuracies in brick properties. Poisson’s ratios are also identified, but less robustly, due to their limited influence on the measured strains. Finally, the use of VFM to reconstruct the uncertain jack forces is illustrated to deliver new insights on the applied loads during complex in-situ tests.