Self-sensing capability of ultra-high-performance concrete containing steel fibers and carbon nanotubes under tension

The feasibility of achieving self strain and damage sensing in an ultra-high-performance concrete (UHPC) mixture by incorporating micro steel fibers and multi-walled carbon nanotubes (CNTs) was investigated. Based on a preliminary study, the volume content of the CNTs was determined to be 0.5%, and 2% (by volume) micro steel fibers were included in the mixture to fabricate ultra-high-performance fiber-reinforced concrete (UHPFRC) that is similar to a commercially available product. Dog-bone specimens were fabricated using UHPC and UHPFRC with CNTs to evaluate the tensile performance and their self-sensing capability. Digital image correlation (DIC) and scanning electron microscopy (SEM) were also adopted to precisely analyze their mechanical and electrical properties. Test results indicated that the hybrid use of steel fibers and CNTs provided a significant improvement in tensile performance, including strength and post-peak ductility, compared to the use of CNTs alone. Crack bridging by CNTs was not achieved in the UHPC mixture, resulting in brittle tensile failure. Severe signal noise in the fractional change in resistance (FCR) and very high electrical resistance were observed in UHPC with CNTs, whereas very smooth FCR data with minor noise and much smaller resistance were obtained in the UHPFRC with CNTs. Furthermore, both pre- and post-peak tensile performance of UHPFRC with CNTs were well simulated based on the measured FCR with a high coefficient of determination (greater than 0.9). Consequently, the use of both steel fibers and CNTs in a UHPC mixture was recommended to improve post-cracking tensile performance and self strain and damage sensing capabilities.