Recycled carbon fiber reinforced ultra-high performance concrete: mechanical, electrical, and self-sensing properties

The next-generation intelligent infrastructure requires cementitious composites that are not only mechanically robust but also electrically functional. This study explores the potential of recycled carbon fiber (RCF) as a multi-functional additive for ultra-high performance concrete (UHPC). Two types of RCF, denoted RCF-I and RCF-II, were used. The workability, mechanical properties, electrical conductivity, and self-sensing behavior of RCF-UHPC were systematically evaluated, with a focus on comparing its performance to that of virgin carbon fiber (VCF). The results show that using RCF-I to prepare UHPC can achieve comparable mechanical and electrical properties as using VCF, suggesting potential economic and environmental benefits owing to the recycled nature and lower reported cost of RCF. Although RCF reduces workability, incorporating RCF substantially improves flexural strength (up to 31.1%), whereas its effect on compressive strength is limited. Increasing the RCF length benefits the flexural strength of UHPC but reduces its compressive strength. Electrical resistivity decreases with increasing fiber content, with percolation thresholds of approximately 0.99 and 1.39 vol.% for RCF-I and RCF-II, respectively, and resistivity reductions of 98.1% and 97.6% at fiber contents near these thresholds. Below the percolation threshold, resistivity is sensitive to moisture and temperature, while above it, this sensitivity diminishes. Under cyclic compression, RCF-I reinforced UHPC shows a stress sensitivity coefficient (2.21%/MPa at load amplitude of10 MPa) occurring near the percolation threshold. Self-sensing performance varies non-monotonically with temperature (0–60 °C) and increases with moisture only below the percolation threshold. Microstructural observations suggest that RCF forms local fiber connectivity within the matrix and exhibits crack-bridging features, consistent with the observed improvements in both electrical conductivity and mechanical performance. Overall, RCF is a promising substitute for VCF in smart UHPC applications for intelligent infrastructure.