Failure mechanisms of outdoor applied large sized fully-vitrified tiles

R. Zurbriggen, K. Pass, H. Waser, Nouryon Chemicals AG, Switzerland;
A. Wetzel, M. Herwegh, Institute of Geosciences, University of Bern, Switzerland;
S. Harzer, J. Kaufmann, F. Winnefeld, EMPA, Dübendorf, Switzerland

There is an increasing number of damages related to the exterior application of large format porcelain tiles. The ongoing trend towards such large-sized fully-vitrified ceramic tiles produces two technical problems. The less porous and the smoother the porcelain tile, the more difficult it is for the mortar to provide adhesion. Secondly, the larger the tile format, the longer the distance from center to grout along which the strain increments sum up to become largest at the tile edges. This paper describes first results from a research program on the mechanisms of shrinkage and adhesion of large-sized porcelain tiles. Simple see-through experiments allow for macroscopic observation of micron-sized
shrinkage cracks and their propagation along the tile-mortar interface. Due to an interference colour effect these micro-cracks can be seen by the naked eye in their early stage of formation when they are 0.5 to 5 microns in width. Continuous field observations
of outdoor applied glass tiles (ordinary float glass as used for windows) indicate that the percolation rate for meteoric water drastically increases along these micro-cracks and that this is an important factor in the continued failure history of the future damage.