Ongoing efforts for improved fracture toughness of engineered cementitious materials address the inherent brittleness of the binding matrix at several different levels of the material\'s geometric scale through the addition of various types of reinforcing fibers. Crack control is required for crack widths that cover the entire range of the grain size spectrum of the material, and this dictates the requirement of hybrid mixes combining fibers of different size (nano, micro, macro). Use of Carbon Nano-Tubes (CNT) and Carbon Nano-Fibers (CNFs) as additives is meant to extend the crack-control function down to the nano-scale where cracking is believed to initiate. In this paper the implications of enhanced toughness thus attained at the material nanostructure are explored, with reference to the global smeared constitutive properties of the material, through consistent interpretation of the reported experimental evidence regarding the behavior of engineered cementitious products to direct and indirect tension.
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