Research Progress on Intrinsic Self-Sensing Ultra-High Performance Concretes

Ultra-high performance concrete (UHPC) is fabricated via optimizing the fineness and reactivity of its constituents and employing thermal curing to modify the composition and structure of hydration products. These techniques minimize internal defects, resulting in a highly homogeneous and dense matrix with strong matrix-aggregate interfacial bonding. Also, the use of steel fibers effectively addresses the inherent brittleness, low tensile, and poor impact resistance of conventional cementitious materials. Consequently, UHPC exhibits exceptional mechanical properties, durability, and workability, while possessing the intrinsic material “gene” necessary for the development of self-sensing capabilities. Self-sensing ultra-high performance concrete (SSUHPC) extends the functionality of conventional UHPC via enabling in-situ self-monitoring, thereby providing a guarantee for enhancing safety and service life of UHPC structures. SSUHPC can be also implemented as an integrated structural element, surface coating, prefabricated component, or embedded sensors within infrastructure, providing a solution for the advancement of digital-intelligent infrastructure.

This review comprehensively represents the material composition, self-sensing properties and mechanisms, engineering applications, current challenges, and future strategies for SSUHPC. The existing research status of SSUHPC is summarized. The composition, fabrication, microstructure and performance characteristics of UHPC are analyzed, highlighting its intrinsic material “gene” that underpins the development of SSUHPC. The primary and most crucial material “gene” is steel fibers, which provide both excellent mechanical properties and electrical conductivity. Their one-dimensional morphology allows a uniform dispersion within the matrix, forming and strengthening the conductive network. The same important material “gene” is the sole use of fine aggregates without coarse aggregates, which avoids a disruption of the conductive network and enhances its integrity. In addition, highly reactive fine components, such as silica fume and mineral powder, can be also regarded as the third material “gene” due to the compact microstructure that supports the long-term stability of self-sensing properties. The review further discusses the role of functional fillers, particularly carbon-based fillers like carbon black and graphene, which complement steel fibers to establish robust conductive networks. These fillers introduce a large number of free electrons, forming the fourth conductive pathway, i.e., intrinsic conductive pathway of functional fillers, which synergistically improves both mechanical properties and self-sensing capabilities of SSUHPC. The sensing properties (i.e., sensing properties under compression, tension, and bending) of SSUHPC are reviewed, along with the underlying mechanisms and influencing factors. The sensing properties under compression arise from a reduction in filler spacing, which enhances contact conductivity, quantum tunneling, and field emission effects, thereby decreasing electrical resistivity. Sensing properties under tension are primarily governed by the disruption of conductive pathways due to microcracking and filler-matrix interface debonding, resulting in an increased electrical resistivity. The dense microstructure and high elastic modulus of SSUHPC limit its compressive deformation, resulting in a less pronounced sensing performance under compression, compared to under tension, which is better captured due to the material’s inherent brittleness. This is because the fractional resistivity change can accurately characterize the entire tensile behavior from elastic deformation, plastic deformation to failure of SSUHPC, and even allow for quantitative characterization. Sensing properties under flexure represent a combination of the characteristics observed in both the compressive and tensile zones. Regardless of load type, the material “gene”, i.e., functional filler composition and content, fundamentally governs the sensing properties, followed by environment factors (i.e., temperature and humility) and measurement methods (i.e., electrode materials and configurations). The resistance measurements can reveal percolation threshold and dominant conductive mechanism (i.e., ionic or electronic conduction) of SSUHPC, providing a theoretical foundation for optimizing the material design and measurement protocols. For instance, if ionic conduction is dominant, appropriate testing methods (i.e., measurement after drying) can improve the repeatability of sensing performance and reduce noise, or improving a functional filler content to shift ionic conduction to electronic conduction. Finally, the review summarizes the challenges of SSUHPC in the existing scientific research and engineering applications, proposes the corresponding strategies, and draws future development roadmaps. Some key areas include large-scale and low-cost manufacturing, in-depth understanding of sensing mechanisms and modeling, long-term sensing capacity and evolution models, design/optimization/software/hardware integration of intelligent structural system, development of “digital-intelligence” and resilient infrastructure for the future information environment, and development and establishment of relevant regulations/specifications/standards.

Functional fillers, such as steel fibers, graphene, and carbon nanotubes, introduce a large number of free electrons into UHPC, becoming a conductive material “gene” of UHPC for the establishment of the intrinsic conductive path. The material “gene” simultaneously endows UHPC with excellent mechanical, durability, and sensing capabilities, facilitating in-situ monitoring of the impacts of loads and environment on infrastructure. This provides a crucial data support for infrastructure operation and maintenance, paving a way for infrastructure “digital-intelligence”. SSUHPC as a high-performance/intelligent/multifunctional material with digital-intelligent characteristics is expected to transcend the boundaries between functional and structural materials, achieving structural-functional integration and functional diversification, thereby driving a paradigm shift in the field of civil engineering.