Nanoengineering Ultra-High-Performance Concrete with Multiwalled Carbon Nanotubes

Abstract

Ultra-high-performance concretes (UHPCs) are characterized by extremely high packing densities. These densities can be achieved with optimization of grain size distribution by incorporating a homogeneous gradient of fine and coarse particles during mixing. Addition of steel fibers can increase the ductility under tensile loading tremendously. The packing density is a key parameter for the high bond strength between steel fibers and the UHPC matrix. The objective of this study is twofold: to enhance the behavior of the bond between the steel fibers and the matrix by increasing packing density with the inclusion of nanometer-sized particles while preserving the workability of the concrete mix. Multiwalled carbon nanotubes (MWNTs) were chosen for their unique physical and impressive mechanical properties (i.e., ultimate strength, stiffness, and ductility). However, because of the nanotubes’ inherent tendency to agglomerate, an extensive study was conducted to optimize their dispersion in solutions suitable for concrete mixing. An experimental validation study was then conducted to explore the effects of incorporating MWNTs in mixes with only 0.022% by cement weight and how they affect the bond behavior of two different high-strength steel fibers. Mechanical characterization studies revealed that low concentrations of MWNTs can significantly improve the bonding behavior of single pulled-out high-strength steel fibers. No conclusive evidence can be drawn with regard to MWNTs’ influence on the UHPCs’ compressive and bending strengths [i.e., 200 and 13 MPa (30 and 1.9 ksi), respectively].

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