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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.

This paper reports on some experimental investigations on the fracture energy and tension softening behavior of high strength concrete. Fibers are known to enhance the ductility and other performance requirements of concrete. With a small fraction of steel fibers, 0.62% by volume, the mode of failure changes from catastrophic to gradual. Further, with the addition of steel fibers, the energy absorption capacity of concrete increases very significantly. The compressive strength of fiber reinforced concrete is 70MPa and the tensile strength is 4.0 MPa. In the present case, the energy absorption in fiber reinforced concrete (FRC) is very high in comparison with the plain concrete. Though the experimental errors are inevitable, but they are negligible in comparison with the large energy absorption in FRC. The fracture energy in FRC decreases as the size of specimen increases. This indicates that the energy absorption decreases with increase in the size of specimen. It should lead to an understanding that the large size structures exhibit elastic behavior up to failure. It is further observed that the brittleness of concrete increases as the size of specimen increases. The compressive strength of plain concrete varies from 55 to 75 MPa, in which the increasing trend has been noticed with maximum size of coarse aggregate. As the heterogeneity in the matrix increases, the fracture energy also increases. In the specimens, one major crack is formed, which implies that the fictitious crack model can be used to FRC with small volume fraction of fibers.

This paper reports on fracture properties of the interface in high strength concrete. The fracture properties included in this study are: strength, fracture toughness and ductility factors. Experiments were conducted on composite specimens in Mode-I and Mode-II loading effects. Two types of composite specimens were designed. In the first type, the mortar was cast against the hardened concrete on only one face, and in the second type, the mortar was cast on both the faces of the aggregate material. The latter type has been designated as sandwiched type. For Mode-I case, both the types of composite specimens were used, while for the Mode-II loading only sandwiched type of composite specimens were adopted. Two types of aggregate materials have been adopted: granite rock and hardened HSC. In the case of granite rock aggregates, rough and smooth surfaces were used, while in the case of HSC concrete, rough, smooth and casting surfaces have been adopted. Three types of mortar mixes were used at 0.40 water-binder ratio with 1:3, 1:2 and 1:4 cement-sand ratios. Silica fume was used 10% by weight of cement. Strong interface has been noticed with rough aggregate surface. The fracture toughness of the interface seems to be very high with rougher aggregate surface in Mode-I. The type of mortar matrix significantly alters the type of interface. The influence of size of interface area on the fracture properties has also been reported. Size effect seems to be clearly observed in Mode-II loading. The fracture toughness increases as the roughness and the inclination of the aggregate surface increases. Interestingly, the sandwiched composite specimens exhibited decreasing fracture toughness with increase in the interface area. Ductility factor, defined as the fracture energy per unit of peak load decreases as the surface area of the interface increases in Series II and III specimens. The CMODc seems to be slightly higher with rough aggregate surface. The interface exhibits significantly brittle behavior showing catastrophic failure, which indicates a possible application of LEFM to the interface.