Title:
Combined Effects of Silica Fume, Aggregate Type, and Size on Post-Peak Response of Concrete in Bending
Author(s):
Canan Tasdemir, Mehmet A. Tasdemir, Nicholas Mills, Ben I. G. Barr, and Frank D. Lydon
Publication:
Materials Journal
Volume:
96
Issue:
1
Appears on pages(s):
74-83
Keywords:
aggregate size; concretes; cyclic loads; gravel agregates; limestone aggregates; silica fume
DOI:
10.14359/431
Date:
1/1/1999
Abstract:
The influences of silica fume, type, and size of aggregate on the pre- and post-peak responses of high-strength concretes in bending were investigated by measuring the fracture energy Gf, the characteristic length Ich, and brittleness index B. 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 (d). It was shown that the relations between normalized stiffness, load, local energy, CMOD, and d were independent of the partial displacement 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 homogenous, and dense. In the latter concretes, the fracture energy decreased dramatically, especially when they contained 20 mm maximum size aggregate. In these concretes, the brittleness index was substantially high. In gravel aggregates with and without silica fume, cracks developed around the aggregates and generally did not traverse them due to 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; a transgranular type of fracture was observed.