A combination of ESPI, microscopic ESPI and Digital Speckle Correlation to study salt-induced deterioration in porous materials

Although still poorly understood crystal growth of salts in porous materials is known to be a major cause of deterioration. In order to provide a better understanding of the underlying damaging processes the detection of material deformations due to the crystallization pressure is of elementary interest. Hitherto, mainly measurements were carried out to determine the integral expansion of porous samples under significant conditions. In contrast the combined usage of Electronic Speckle Pattern Interferometry (ESPI) and Digital Speckle Correlation (DSC) can yield much more relevant information. Since deformations and in-plane displacements can be measured locally resolved areas can thus be identified where damaging starts during changing ambient climatic conditions. Moreover, the local analysis of de-correlation in speckled images of the material’s surface displays regions, where phase transition of the salts at certain ambient conditions occurs. Imaging at high magnification enables such investigation even in the environment of single pores. Combined results of ESPI, microscopic ESPI and correlation measurements will be discussed on the basis of thermodynamical considerations.