Partial replacement of carbon fiber by carbon black in multifunctional cement-matrix composites

Cement reinforced with discontinuous carbon fiber is known for its piezoresistivity-based strain sensing ability, its electrical conductivity and the consequent multifunctionality. The high cost of carbon fiber is disadvantageous. Both carbon fiber and carbon black (used with silica fume in the amount of 15% by mass of cement) increase the DC conductivity and the EMI shielding effectiveness of cement, but carbon fiber is more effective than carbon black. Partial (50%) replacement of carbon fiber by carbon black lowers the cost, in addition to increasing the workability, while the electrical conductivity and the electromagnetic interference shielding effectiveness are maintained. However, the partial replacement reduces the strain sensing effectiveness. Total replacement of carbon fiber by carbon black diminishes both the conductivity and the shielding effectiveness, further reduces the strain sensing effectiveness, decreases the compressive modulus and increases the compressive strain at failure, while the compressive strength is maintained. The increased workability due to the partial replacement enables a higher total conductive admixture content to be attained. The maximum total conductive admixture content is 3.5% by mass of cement. In contrast to fiber replacement, the addition of carbon fiber to cement with carbon black decreases the compressive strength, strain at failure and density.