INFLUENCE OF BOUNDARY-CONDITIONS ON STRAIN-SOFTENING IN CONCRETE COMPRESSION TEST

How do boundary conditions influence strain localization in concrete l oaded in compression? To answer this question, compressive tests were performed on prismatic specimens of mortar. The blocking or nonblockin g of rotation of the top platen of the press and the use or absence of an antifriction system have led us to consider four different boundar y conditions. The onset of strain localization and its development in the course of loading are determined by the stereophotogrammetric meth od. The results obtained show that rotation (when it is free) always s tarts after the peak load. The postpeak portion of the load-displaceme nt curve depends to a very large extent on the boundary conditions: th e use of an antifriction system increases the mean slope of this part. But, on the other hand, the onset of strain localization occurs at th e peak, whatever the boundary conditions.