Study of cracking due to drying in coating mortars by digital image correlation

Drying shrinkage of coating mortars may induce cracks which could result in debonding and reduce the durability of the ‘mortar/substrate’ system. In order to study this phenomenon, a new device based on digital image correlation (DIC) was developed so as to measure 2D displacement fields on mortars and substrates at early age in drying conditions. Compared to intrusive methods (e.g. SEM observation, embedded rigid sensor) or impregnation techniques, the proposed device does not induce parasite cracks and specimen can be monitored continuously and automatically. Moreover, representative geometries and restraint conditions can be tested. A post-processing tool is proposed to determine the evolution of the cracking patterns by computing an equivalent strain. Besides, this enables the quantification of the widths and the depths of cracks inside the mortar and at the mortar/substrate interface. The device was validated by comparison with measurements of drying shrinkage using LVDT and investigations with an optical microscope. It was used successfully to analyze drying shrinkage cracking of coating mortars due to restraint by a rigid substrate.