Portland cement mortar nanocomposites at low carbon nanotube and carbon nanofiber content: A fracture mechanics experimental study

A thorough fracture mechanics characterization of Portland cement mortars reinforced with multi wall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) took place. The critical values of stress intensity factor, K-IC(S); strain energy release rate, G(IC)(S) crack tip opening displacement, CTODc; and critical crack length, ac of 3, 7, and 28 days Portland cement mortars, reinforced with well dispersed carbon nanotubes and carbon nanofibers were experimentally determined. Prismatic notched specimens of neat mortars and mortars reinforced with 0.1 wt.% CNFs, and 0.1 and 0.2 wt.% MWCNTs were subjected to a three point closed loop bending test, using the crack mouth opening displacement, CMOD, as the feedback signal. The fracture parameters of the nanoreinforced mortars were then determined using the two parameter fracture model. The excellent reinforcing and toughening efficiency of MWCNTs and CNFs is demonstrated by a significant improvement in the critical stress intensity factor/fracture toughness (128.6%), critical strain energy release rate (154.9%), and critical crack tip opening displacement (39%). These results allow us to conclude that the MWCNTs and CNFs beneficially alter the nanostructure of the mortar matrix, resulting to a significant enhancement of all fracture and mechanical properties and provide the material, with the ability of performing multiple structural functions. (C) 2016 Elsevier Ltd. All rights reserved.