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Softening Reversal and Other Effects of a Change in Loading Rate on Fracture of Concrete

The time dependence of concrete fracture, and particularly the effect of loading rate, has so far been studied mainly in the dynamic range. The present study extends a preceding investigation of the rate effect in the static range that covered times to peak from 1 to 300,000 sec. Geometrically similar three-point-bend specimens of three different sizes are subjected to either a sudden 1000-fold increase of the loading rate or a 10-fold sudden decrease of the loading rate. It is found that the post-peak softening can be reversed to hardening, followed by a second load peak that can be either higher or lower than the previous load peak. The rise to the second peak depends on the previous post-peak load drop from the first peak load. A sudden decrease in the loading rate causes initially a steeper softening slope. The source of these time-dependent effects appears to be not only the thermally activated nature of the process of bond ruptures in the fracture process zone but also the effect of creep, both a nonlinear creep in the fracture process zone and a linear creep in the bulk of the specimen. The results of this study and a previous study suggest that there is a significant difference in fracture behavior for short-time and long-time loads. The phenomena observed are of interest, for example, for the analysis of concrete dams with cracks that evolve over many years. Mathematical modeling of the present test results is left for a subsequent study.