Long-term tensile and bending strength of natural building stones

The results of experimental long-term tension and bending tests carried out on four natural stones formely widely used in Italian traditional building (a white-veined Carrara marble, the "pietra Serena" and "Pietraforte" sandstones, and the Piobbico-Cesana limestone) are presented. The tests consisted of applying continually different fractions of the "instantaneous" fracture loads and monitoring the strain evolution of the specimens until a time-delayed fracture was reached. A typical three-stage creep response was obtained in all the tests. At the same time, a 60% reduction in tensile and bending strength was observed in relation to testing periods not much longer than one year. A phenomenological constitutive model is proposed for analytically reproducing the experimental viscoplastic behaviour. Furthermore, a creep time-to-rupture function giving direct estimates of the strength decay effects is provided, with a view to technical applications. The results of the identification analyses carried out to calibrate the characteristic parameters of the above laws are also reported.