Evolution of the crack-pattern as a visible effect of long term damage

The collapse of some monumental buildings, which have occurred during the last fifteen years, indicate that not only towers, but also particularly slender or heavily loaded elements like columns, pillars, etc., turn out to be greatly influenced by creep deformations, due to their geometry and to the heavy persistent compressive stress which are subjected to. Moreover, the stress distribution within the load-bearing area of these structural elements and buildings is generally non-uniform, due to their non-homogeneous cross section, often made by multiple leaf masonry. A combination of these factors, together with the fatigue effect due to cyclic actions induced by temperature variation, wind and earthquakes action, can well be responsible of very serious structural damage and in some cases even of failure. Although the collapse of massive buildings (towers, cathedral pillars) happens apparently suddenly, nevertheless crack propagation proved to develop in a relatively long time. This aspect is strictly related to the experimental observation that the volumetric and deviatoric secondary creep strain rate, which is the strain rate during the phase of stable damage growth, proved to be connected with the residual life of the material. This point has important implications on the safety assessment and the reduction of the risks of failure of ancient structures. In fact, the rate of propagation of vertical cracks and the rate of dilation (thickness increase) of load-bearing walls of ancient buildings can be interpreted as significant indicators of the structural conditions and a precocious individuation of their exceeding some limit values is an important index of damage