Creep in building stones under tensile conditions

Careful structural analyses of ancient stone constructions methodically showed crack openings under tensile stresses notably lower than the corresponding strength limits. Based on this observation, an experimental research was undertaken aimed at determining the mechanical effects related to permanent loading in time. The results of a first series of long-term tension and bending tests conducted on four natural building stones are presented in this paper. A time-delayed response analogous to the creep behavior of several geomaterials as well as of metals and ceramics was found; the response was then suitably described by the same analytical models usually applied to these last material classes. However, the stone building materials are characterized by a notably higher strength decay taking place in short times which confirmed the trend already shown by the structural studies. A thorough damage analysis developed with reference to the tertiary creep phase revealed both the physical and phenomenological correspondence of the adopted Kachanov-Rabotnov rule to the actual mechanical degradation time-progress.