Combined Effects of Silica Fume, Aggregate Type, and Size on Post-Peak Response of Concrete in Bending

The influences of silica fume, type, and size of aggregate on the prepeak and postpeak response of high-strength concretes in bending were investigated by measuring the fracture energy, the characteristic length, and brittleness index. Degradation of stiffness and strength were also measured, and a unique focal point was determined using unloading-reloading cycles during the tests. The degradation of stiffness was correlated to the local fracture energy, strength degradation, permanent crack mouth opening displacement (CMOD), and permanent displacement at midspan. It was shown that relations between normalized stiffness, load, local energy, CMOD, and permanent displacement at midspan were independent of the partial replacement of cement by silica fume and of the type and size of aggregate. Based on the fracture tests and microscopic studies at the matrix-aggregate interface, it was concluded that, in both limestone and gravel concretes without silica fume, the cement-aggregate interface had a large amount of calcium hydroxide and also much less dense calcium silicate hydrate; however, in concretes with silica fume, the interfacial zone became stronger, more homogeneous, and dense. In the latter concretes, the fracture energy decreased dramatically, especially when they contained 20-mm maximum size aggregate, and in these concretes, the brittleness index was substantially high. In gravel aggregate concretes with and without silica fume, cracks developed around the aggregates and generally did not traverse them, because of the particle shape and smooth surface; however, in concretes with silica fume, crack surfaces were less tortuous and fracture was in a more brittle manner. In limestone concretes with silica fume, the cracks usually traversed the aggregates and a transgranular type of fracture was observed.