Effects of initial creep conditions and temporary unloading on the long-term response of stones

A special part of a research programme concerning the tensile and bending creep behaviour of natural building stones is presented in this paper. The new experimental studies were aimed at inquiring the effects of temporary viscoplastic conditions on the time-delayed response produced by subsequent variations of the applied stress. Long-term tests were performed which consisted of unloading the specimens from different stages of the secondary creep branch and reloading them with the same, or decreased, shares of the ultimate-stress values. Further appreciable reductions in the delayed strengths were observed compared to the results of the continuous tests developed in the preceding phases of this research. At the same time, a series of instantaneous collapse reloading-tests highlighted a little influence of the initial deformation time-history on the relevant rupture-loads. Within the context of structural-diagnostic problems, these last values cannot be regarded as exhaustive references for estimating the safety limits of stone members, unless coherent interpretation criteria of the results deriving from in-situ extracted specimens are adopted. A simple methodology is herein suggested to this aim. The delayed elasticity effects following load removal were also analysed, and reproduced by means of a proper phenomenological law that allowed to complete the global creep model proposed in previous works.