A micromechanical model to describe thermal fatigue and bowing of marble

Marble slabs are frequently used as faade panels to externally cover buildings. In some cases a bowing of such faade panels after a certain time of environmental exposure is experienced. The bowing is generally accompanied by a reduction of strength which increases with increasing degree of bowing. In the present paper, a theoretical model to calculate the progressive bowing and the thermal fatigue of marble slabs submitted to temperature cycles is presented. The model, developed within the framework of fracture mechanics, takes into account the mechanical microstructural characteristics of the marble as well as the actual cyclic temperature field in the material. The slabs are subjected to a thermal gradient along their thickness (due to different values of temperature between the outer and inner sides of the slab) as well as to thermal fluctuation on the two sides of the slab due to daily and seasonal temperature excursions. This thermal action causes a stress field which can locally determine microcracks due to decohesion of calcite grains. Stress intensification near the cracks occurs and leads to crack propagation in the slab. Such crack propagation under thermal actions is evaluated and the corresponding deflection (bowing) is calculated. Some examples are presented which show the strong influence of material microstructure on the degree of bowing.