Surface and volumetric elasticity characterisation of hydraulic lime mortar prisms under compression

Masonry structures predominantly work under compression; elasticity in constituent materials is therefore typically characterised during uniaxial compression tests via load and deformation measurements. In most practical applications, deformation measurements can only be conducted at the surface of the sample. To understand the influence of this restriction on characterisation results, this study conducts in-situ load tests on hydraulic lime mortar samples with both surface and volumetric imaging, using stereo cameras and synchrotron X-rays. The imaging data are processed with Digital Image and Volume Correlation algorithms (local and global) to obtain full-field displacements and strains. Deformation measurements are then used to conduct surface characterisation with the Virtual Fields Method (VFM) and volumetric characterisation with Finite Element Model Updating (FEMU). Since the inverse models indicate limited identifiability for Poisson’s ratio, only the Young’s moduli are compared. Results indicate that accurate Young’s moduli can be obtained from surface measurements, provided that through-thickness load gradients are considered by averaging VFM results from front and back faces of the sample. In addition, it is shown that a surface characterisation domain away from contacts needs to be chosen to ensure that surface strains are representative of bulk behaviour. Finally, a systematic reduction of elastic moduli with increasing loads is observed, suggesting damage development at relatively low stress levels in hydraulic lime mortars.