Characterization of concrete failure from load-CMOD curves and tension softening modulus

This paper reports on softening and determination of softening modulus on high-strength concrete (HSC) tested on three-point bend (TPB) specimens. Softening functions have been established for concretes of varying strength. Various parameters such as mix proportions, water-binder ratio, mineral admixtures such as silica fume or fly ash and size of coarse aggregate have been considered. The concretes with high strength produce steep tension softening response with strong interfacial bond between mortar and aggregate. A long descending softening response has been observed in low strength concretes, which is further increased with larger size coarse aggregate. In order to understand the variation of softening modulus, the slope of Ioad-CMOD (p-w) curves have been estimated at different crack mouth opening displacements (CMOD) namely at CMOD (w) = 0,0.25 and 0.50 mm. It has been observed that rapid softening response could be possible immediately after the peak load. The slope of bilateral curve as a measure of the softening modulus is about 80 with the horizontal at the peak load. A bi-linear softening curve seems to fit well in HSC and has been verified with the experimental observations. The softening modulus increases as the strength of concrete and also as the size of coarse aggregate increase.