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Today, the desired intelligent civil engineering structures urge the development of high-performance smart structural materials. Self-sensing ultra-high performance concrete (UHPC) emerged in 2014 based on the observation that the concrete structures rendered with randomly distributed conductive additives exhibit electromechanical effects under external loading, and the demand of UHPC from high-strength to sustainable, intelligent, and multifunctional. This smart structural material has since grown greatly and has been considered as one of the potential alternatives to ordinary self-sensing concrete. This paper presents a state-of-the-art review of self-sensing UHPC in terms of its raw materials, mixture design, manufacturing approaches, microstructural characteristics, mechanical properties, electrical resistivity, electromechanical properties, and applications, with a particular focus on stress-deformation/damage behaviour and changes in electrical conduction under external compression, tension, and bending. The novel uncertainties regarding electromechanical effects are thoroughly discussed from the perspective of the continuity of conductive network. Compared with ordinary self-sensing concrete, self-sensing UHPC exhibits significantly reduced porosity, exceptional strength, marginally lower stress/strain sensitivity, heightened damage sensitivity, insensitivity to humidity, and delayed cracking behaviour. In the end, this paper identifies research gaps, challenges, and recommendations in three aspects: (1) scale fabrication and cost control, (2) satisfactory and reliable properties, and (3) from smart structural materials to integrated components to civil engineering structures. [ABSTRACT FROM AUTHOR]

Self-sensing ultra-high performance concrete (UHPC) features superior mechanical capacity, excellent erosion resistance, long life cycle, and broad range of stress sensing, and is expected to be a pathway towards next-generation smart cementitious composites. This study presents a novel strategy to achieve electrical percolation and elevate the piezoresistive sensing capability of UHPC through the incorporation of multiphase and multiscale functional additives including graphene (G) and carbon nanotube (CNT). The workability, compressive strength, microstructural characteristics, and alternating current (AC) impedance response are investigated. The effect of electrical percolation on the piezoresistive behaviour is studied and discussed using various ratios of G/CNT. Results indicate that as the G/CNT ratio decreases from 4:0 to 0:4, the compressive strength is reduced from 194.7 MPa to 166.3 MPa due to the certain increase in the plastic viscosity of fresh mixture and the proportion of harmful pores, although the basic requirement of 150 MPa is met. Moreover, the AC impedance response significantly moves left and the radius of high-frequency arc reduces from 72910 Ω to 1295 Ω, suggesting electrical percolation. Bounded by percolation threshold, the fractional change of resistance curves can be divided into a two-stage pattern (i.e., linear and nonlinear stages) and a three-stage mode (i.e., linear decrease, balance, and abrupt increase stages). An underlying mechanism is proposed to explain the tremendous change in piezoresistive behaviour of self-sensing UHPC, considering the tunneling-percolation theory and electromechanical coupling behaviour. Additionally, the gauge factors range from 11 to 28, which is higher than most of the existing reported values, demonstrating the great potential of using hybrid functional nano-additives in self-sensing UHPC. [ABSTRACT FROM AUTHOR]

Achieving continuous in-situ monitoring of repaired concrete structures has always been a great challenge. Ultra-high performance fibre-reinforced concrete (UHPFRC), featuring superior mechanical capacity, excellent erosion resistance, and long-life cycle, is expected to realize durable, cost-effective, and aesthetically pleasing repairs and possesses tremendous potential for intrinsic self-sensing. This paper presents the multifunctionality of UHPFRC with integrated self-sensing and repair capabilities using multi-walled carbon nanotube (MWCNT). An experimental evaluation on the bond performance and self-sensing properties, including the bond strength and failure patterns by the three-point bending, splitting tensile, and slant shear tests, electrical properties via alternating current impedance spectroscopy test, and electromechanical properties under three-point bending, was conducted through the designed composite layer between concrete substrate and UHPFRC. The microstructure of the interfacial zone was analysed to reveal the bonding mechanism. Results indicate that the addition of MWCNT improved the bond strength between concrete substrate and UHPFRC as MWCNT can enhance the adhesion and cohesion of the interfacial zone through inactive filling effect, as well as the frictional and mechanical interlocks through increased mechanical properties of repair UHPFRC. Moreover, the repair UHPFRC showed remarkable sensing capabilities for the initial cracking, deflection hardening or softening, and fibre pull-out and debonding processes of the composite layers. The optimal content of steel fibre and MWCNT in multifunctional UHPFRC should be 2 vol.% and 0.2 wt.%, respectively, considering the synergistic damage sensing and bonding properties. [ABSTRACT FROM AUTHOR]

• A self-sensing ultra-high-performance fiber-reinforced concrete (UHPFRC) is fabricated by incorporating few-layer graphene (FLG) • In the composite with 0.5 wt% FLG, the enhancement on compressive strength can reach to more than 25%. • The possible polarization and conduction mechanisms of the FLG/UHPFRC composites are proposed according to the dielectric polarization theory. • The FLG/UHPFRC composites exhibit distinct piezoresistive behavior from the ordinary cement-based sensors. Graphene nanomaterials have demonstrated tremendous prospects in the preparation of smart ultra-high-performance fiber-reinforced concrete (UHPFRC). However, few relevant researches are available on the piezoresistive behavior of the UHPFRC reinforced by few-layer graphene (FLG). In this research, the mechanical, electrical and mechanical–electrical behavior of the FLG/UHPFRC composites were investigated, and the possible polarization and conduction mechanisms of the FLG/UHPFRC composites were proposed. The findings showed that FLG was equably dispersed in the matrix, which enhanced the mechanical properties of the composite materials through the crack arresting effect, pinning effect and seeding effect, etc. FLG improved the conductivity and relative permittivity of the composites, but did not change the topology of the Nyquist plot. Moreover, the FLG/UHPFRC composites exhibited distinct piezoresistive behavior from ordinary cement-based sensors due to the coupling effect of the interfaces and microcracks. [ABSTRACT FROM AUTHOR]

• Limestone calcined clay cement (LC3) is adopted in UHPC. • D-optimal design is used to optimize the recipes of LC3-based UHPC. • LC3 can provide a high substitution ratio for ordinary Portland cement. • LC3 can promote the engineering properties of UHPC. Limestone calcined clay cement (LC3) is believed to be able to substitute Portland cement and promote low-carbon development in the concrete industry. Ultra-high performance concrete (UHPC) features high proportions of cement. Herein, an LC3-based UHPC with high cement substitution was designed and optimized based on the D-optimal method, and its engineering properties, including workability, mechanical properties, hydration process, microstructural evolution, and carbon emission were evaluated. Results show that with an increase in LC3, the dynamic yield stress increases and the water film thickness decreases, which indicates that the workability of LC3-based UHPC reduces. When the substitution ratio is about 55 percent, the optimized LC3-based UHPC achieves the maximum compressive strength, about 146.05 MPa. XRD and TG results show that LC3 consumes portlandite by the pozzolanic reaction, which generates additional gel products. However, as the substitution ratio exceeds 55 percent, the degree of pozzolanic reaction drops, resulting in a decrease in the count of hydration products and compressive strength. The pore structure of LC3-based UHPC is refined, with fewer air voids and a larger percentage of gel pores compared to the normal UHPC. The optimized LC3-based UHPC with a substitution ratio higher than 50 percent in this study achieves high integrated performance with engineering applications and low carbon environmental value. [ABSTRACT FROM AUTHOR]

Background and objective: Calcaneal Sanders type II or III fractures are highly disabling with significant burden. Surgical treatment modalities include open reduction and internal fixation (ORIF) techniques and a variety of minimally invasive surgical (MIS) approaches. ORIF techniques are associated with complications and traditional MIS techniques need extensive intraoperative fluoroscopic procedures. The present study aims to investigate the effects of three different minimally invasive internal fixation (MIIF) techniques used to treat Sanders type II intra-articular calcaneal fractures using finite element analyses. Methods: A 64-row spiral computed tomography scan was used to observe the calcaneus of a healthy adult. The scanning data were imported into Mimics in a DICOM format. Using a new model of a Sanders type II-B intra-articular calcaneal fracture, three minimally invasive techniques were simulated. Technique A involved fixation using an isolated minimally invasive locking plate; Technique B used a minimally invasive locking plate with one medial support screw; and Technique C simulated a screw fixation technique using four 4.0-mm screws. After simulating a 640-N load on the subtalar facet, the maximum displacement and von Mises stress of fragments and implants were recorded to evaluate the biomechanical stability of different fixation techniques using finite element analyses. Results: After stress loading, the maximum displacements of the fragments and implants were located at the sustentaculum tali and the tip of sustentaculum tali screw, respectively, in the three techniques; however, among the three techniques, Technique B had better results for displacement of both. The maximum von Mises stress on the fragments was < 56 Mpa, and stress on the implants using the three techniques was less than the yield strength, with Technique C having the least stress. Conclusion: All three techniques were successful in providing a stable fixation for Sanders type II intra-articular calcaneal fractures, while the minimally invasive calcaneal locking plate with medial support screw fixation approach exhibited greater stability, leading to improved enhancement for the facet fragment; however, screw fixation dispersed the stress more effectively than the other two techniques. [ABSTRACT FROM AUTHOR]

Background and objective: Calcaneal Sanders type II or III fractures are highly disabling with significant burden. Surgical treatment modalities include open reduction and internal fixation (ORIF) techniques and a variety of minimally invasive surgical (MIS) approaches. ORIF techniques are associated with complications and traditional MIS techniques need extensive intraoperative fluoroscopic procedures. The present study aims to investigate the effects of three different minimally invasive internal fixation (MIIF) techniques used to treat Sanders type II intra-articular calcaneal fractures using finite element analyses. Methods: A 64-row spiral computed tomography scan was used to observe the calcaneus of a healthy adult. The scanning data were imported into Mimics in a DICOM format. Using a new model of a Sanders type II-B intra-articular calcaneal fracture, three minimally invasive techniques were simulated. Technique A involved fixation using an isolated minimally invasive locking plate; Technique B used a minimally invasive locking plate with one medial support screw; and Technique C simulated a screw fixation technique using four 4.0-mm screws. After simulating a 640-N load on the subtalar facet, the maximum displacement and von Mises stress of fragments and implants were recorded to evaluate the biomechanical stability of different fixation techniques using finite element analyses. Results: After stress loading, the maximum displacements of the fragments and implants were located at the sustentaculum tali and the tip of sustentaculum tali screw, respectively, in the three techniques; however, among the three techniques, Technique B had better results for displacement of both. The maximum von Mises stress on the fragments was < 56 Mpa, and stress on the implants using the three techniques was less than the yield strength, with Technique C having the least stress. Conclusion: All three techniques were successful in providing a stable fixation for Sanders type II intra-articular calcaneal fractures, while the minimally invasive calcaneal locking plate with medial support screw fixation approach exhibited greater stability, leading to improved enhancement for the facet fragment; however, screw fixation dispersed the stress more effectively than the other two techniques. [ABSTRACT FROM AUTHOR]

Secondary hydration is the main driving force of autogenous self-healing and directly impacts the closure of cracks in damaged cementitious materials. In fiber-reinforced cement composites, secondary hydration also dominates the regeneration of the fiber-matrix interface during the self-healing processes. As an important factor influencing the cement hydration, silica fume (SF) addition can be crucial to the self-healing performance of cementitious materials and composites. In this paper, the effect of SF on the autogenous self-healing of ultra-high-performance fiber-reinforced concrete was investigated. With a ≥20% SF addition, the secondary hydration the cement matrix adjacent to the cracks was accelerated. The dissolution of amorphous silica and the formation of Si-containing healing phases was observed. These effects predominated the closure of the mouth and the inner part of the cracks. SF addition also enhanced the regeneration of fiber-matrix interface by promoting the growth of self-healing products in these areas. The closure of cracks and the regeneration of fiber-matrix interface are both responsible for the restoration of flexural properties. • The secondary hydration acceleration of the cement matrix after cracking was found with a ≥20% SF addition, which promoted the self-healing of the cracks in UHPFRC. • The effect of SF on the structural regeneration of the cracks in UHPFRC was elucidated with a self-healing mechanism of the amorphous Si dissolution. • The self-healing of the deep matrix crack and fiber-matrix interface together induced the flexural property restoration of the SF-containing UHPFRC. [ABSTRACT FROM AUTHOR]