Enhancing the initial cracking fracture toughness of steel-polyvinyl alcohol hybrid fibers ultra high toughness cementitious composites by incorporating multi-walled carbon nanotubes

In this study, the use of dispersed multi-walled carbon nanotubes (MWCNTs) to enhance the mechanical properties, especially the initial cracking fracture toughness, of the steel-polyvinyl alcohol hybrid fibers ultra high toughness cementitious composite (UHTCC) was reported. The influences of the MWCNTs contents (0.00%, 0.05%, 0.10%, 0.20% and 0.40% by the cement weight) were investigated by the fluidity test, compression test and uniaxial tensile test. An optimal concentration of 0.10 wt% was obtained, at which the compressive strength (to 60.8 MPa), tensile strength (to 6.8 MPa), elastic modulus (to 15.2 GPa), ultimate tensile strain (to 0.960%) were enhanced by about 18%, 42%, 12% and 16%, respectively. The pre-notched three-point bending beam tests were then conducted to study the improvements of the fracture properties at the optimal MWCNTs content, based on the double-K fracture model and the standard toughness index method. The initial cracking fracture toughness KIcini (to 1.8 MPa·m1/2) was improved by about 21%. The defined effective fracture toughness KIcun− (to 8.1 MPa·m1/2) and KIcun+ (to 20.5 MPa·m1/2) were improved by about 14% and 58%, respectively. The residual tensile strengths fR,1-CMOD (to 23.2 MPa), fR,2−CMOD (to 26.1 MPa), fR,3−CMOD (to 24.6 MPa) and fR,4−CMOD (to 20.7 MPa) were increased by about 24%, 27%, 47% and 59%, respectively. The toughening effect at the peak load state may imply that the bonding strengths between the hybrid fibers and the cement matrix were enhanced by MWCNTs addition. The scanning electron micrograph was used to observe the microstructures of the MWCNTs-enhanced hybrid fibers UHTCCs. Observations showed that the MWCNTs can act as nucleation spots to accelerate the hydration reaction, be embedded in the hydrates to form a more compact 3D cross network, and arrest the nano-cracks due to their bridging and load-transfer abilities.