Influence of high-reactivity metakaolin and silica fume on the flexural toughness of high-performance steel fiber-reinforced concrete

For steel fiber-reinforced concrete with practical fiber volume fractions, the major post-peak energy dissipation mechanism is the pull-out of fibers across a crack. With undeformed, smooth fibers, post-peak energy dissipation or "toughness" is mainly a function of fiber-matrix adhesional bond whereas for the highly stressed deformed fibers, properties of the bulk matrix also become important. High-performance matrices tend to be brittle, and addition of pozzolanic admixtures, particularly silica fume, further increases the brittleness. An increased matrix brittleness can cause crushing and splitting of the matrix and irt turn, curtail the ability of fibers to transfer stresses during pull-out, thus reducing the overall toughness. This paper examines the influence of two pozzolanic materials-high-reactivity metakaolin (HRM) and silica fume-on the toughness characteristics of high-performance fiber-reinforced concrete. It is concluded that HRM is particularly effective in improving the post-peak energy absorption capacity of concrete with fibers, and unlike silica firme, no particular post-peak brittleness is seen to occur.