Mechanical and Electrical Properties of Cementitious Composites Reinforced with Multi-Scale Carbon Fibers.

Carbon fibers, with high modulus of elasticity, tensile strength, and electrical conductivity, can modify the mechanical and electrical properties of cementitious composites, facilitating their practical application in smart infrastructure. This study investigates the effects of carbon nanofibers (including carbon nanotubes, a special type of carbon nanofibers) and micron carbon fibers with different aspect ratios and surface treatments on the uniaxial tensile and electrical properties of cementitious composites. The results demonstrate that appropriate carbon fiber doping markedly improves the uniaxial tensile strength of cementitious composites, with enhancement effects following a gradient trend based on a geometric scale: carbon nanotubes (CNTs) < carbon nanofibers (CNFs) < short-cut carbon fibers (CFs). Hydroxyl-functionalized multi-walled carbon nanotubes (MWCNTs) form continuous conductive networks due to surface active groups (-OH content: 5.58 wt.%), increasing the composite's electrical conductivity by two orders of magnitude (from 3.56 × 10 to 2.74 × 10 Ω·cm), with conductivity enhancement becoming more pronounced at higher doping levels. Short-cut CFs also improve conductivity, with longer fibers (6 mm) exhibiting a 12.4% greater reduction in resistivity. However, exceeding the percolation threshold (0.5-1.0 vol.%) leads to limited conductivity improvement (<5%) and mechanical degradation (8.7% tensile strength reduction) due to fiber agglomeration-induced interfacial defects. This study is a vital reference for material design and lays the groundwork for self-sensing cementitious composites.