This study develops a novel class of 3D-printed auxetic lattice reinforced foamed cementitious composites, aimed at overcoming the brittleness and low strength of conventional foamed cement while maintaining lightweight characteristic. Polymeric auxetic lattices (mechanical metamaterials with negative Poisson's ratio) were 3D printed and embedded in foamed cement matrix. Static and cyclic compression tests were conducted to evaluate load-bearing capacity, energy absorption, and failure mechanisms. X-ray computed tomography (CT) analysis was performed to examine interfacial behavior between the lattice and cement matrix. Results indicate that 3D auxetic lattices significantly enhance strength and ductility through multidirectional lateral confinement, where the energy absorption increased by up to 2.8 times compared to unreinforced foamed cement at a density of 550 kg/m³. Specifically, the 3D auxetic lattices reinforced composites showed pronounced resilience under cyclic loading, exhibiting gradual and ductile damage evolution while sustaining performance beyond 700 cycles. In comparison, 2D auxetic lattices which provide negative Poisson's ratio only in-plane are less effective in reinforcing foamed cement matrix. Additionally, although non-auxetic lattice increased load-carrying capacity to some degree, the corresponding composites structure showed localized shear failure and premature structural degradation under cyclic loading. Overall, the active reinforcement effect of auxetic lattices enables the development of advanced foamed cementitious composites for impact mitigation, blast protection, and buoyant components requiring energy absorption and repeated-load resilience.

Anisotropy in 3D-printed concrete structures has persistently raised concerns regarding structural integrity and safety. In this study, an architected 3D printing strategy, “stitching”, was proposed to mitigate anisotropy in 3D-printed Engineered Cementitious Composites (ECC). This approach integrates the direction-dependent tensile resistance of extruded ECC, the mechanical interlocking between three-dimensional layers, and a deliberately engineered interwoven interface system. As a result, the out-of-plane direction of the printed structure can be self-reinforced without external reinforcements. Four-point bending tests demonstrated that the “stitching” pattern induced multi-cracking and flexural-hardening behavior in the out-of-plane direction, boosting its energy dissipation to 343 % of the reference “parallel” printing and achieving 48.6 % of cast ECC. Additionally, micro-CT scanning and acoustic emission tests further validated the controlled crack propagation enabled by the engineered interface architecture. The proposed strategy has been proven to substantially alleviate anisotropy and enhance structural integrity.

Auxetic cementitious cellular composites (ACCCs) possess advantageous mechanical properties in static tests, such as high fracture resistance and efficient energy dissipation. However, little attention has been given to understanding the impact resistance of ACCCs. In this study, two typical elliptical-shaped ACCC specimens, P25 and P50, were designed with major axis lengths increased by 25 % and 50 %, respectively, compared to the reference P0 with circular holes. The specimens were architected through additive manufacturing (AM) assisted casting, and subjected to low-velocity impacts from Schmidt hammer with a consistent initial impact energy. Their impact resistance was assessed based on impact responses, including rebound value, absorption energy, localized damage in the impact zone, crack propagation, and peak reaction force during impact. Besides single impact tests, multiple impact tests were conducted until specimens failed. Their impact results were compared with those of the reference (P0). A high-speed camera was further used for Digital Image Correlation (DIC) to analyze strain distribution of the specimens during the brief impact period. Furthermore, a numerical model considering strain rate effects was developed to simulate the impact behavior of ACCCs, demonstrating good agreement with experimental data. On this basis, a parametric analysis was performed to evaluate the effects of impact energy, relative density, specimen size, and RVE size on impact resistance. Both experimental and numerical results indicate that ACCCs demonstrate superior impact resistance compared to the reference (P0). They exhibit mitigated localized damage in the impact zone and increased contact stiffness. Moreover, ACCCs show greater endurance under multiple impacts and higher accumulated energy absorption until failure. This enhanced performance is attributed to auxetic behavior, which draws more material into the impact zone for dispersing energy and reducing localized damage, thereby maintaining overall structural integrity. Specifically, P50 exhibits higher impact resistance than P25 due to the enhanced auxetic behavior resulting from its greater aspect ratio. This creates a greater bending moment to enable more ligaments to dissipate energy through rotation-induced plastic deformation, thereby reducing localized damage. Considering the widespread availability of cementitious materials, this study highlights the potential of ACCCs for lightweight, high-performance protective structural materials for impact mitigation in infrastructure.

Auxetic cementitious cellular composites (ACCCs) exhibit hinge-type recoverable deformation during auxetic behavior phase, a rare pseudo-elastic property in cementitious materials. However, their low load-bearing capacity during this phase restricts their use in high-load applications. This study developed ACCCs using strain-hardening cementitious composites (SHCCs) with short (SHCC-SS) and long (SHCC-LS) softening tails, fabricated by additive manufacturing-assisted casting. Uniaxial compression tests employing Digital Image Correlation (DIC) evaluated their compressive behavior, peak strength, Poisson's ratio variation, and energy dissipation. Cyclic tests after pre-compression assessed their recoverable deformation resilience, with fiber bridging at joint cracks examined using digital optical microscope. Results were compared to a reference using fiber-reinforced cementitious materials with strain softening (SS). Compared to the reference (SS), ACCCs using SHCC mixtures exhibit superior load-bearing capacity and stable auxetic behavior under compression. After self-contact, they maintain a negative Poisson's ratio up to a considerably high compressive strain, preventing splitting failure and preserving structural integrity. This is because incorporating SHCC enables greater joint rotation by promoting multiple cracks with strain hardening, which delays primary crack formation and reduces its opening. During cyclic tests, P1-shaped ACCCs with SHCC-LS and SHCC-SS enhance the elasticity modulus of recoverable deformation by 4.8 and 3.0 times, respectively, compared to SS. SHCC-LS outperforms SHCC-SS in compressive resilience due to its prolonged softening tail, which improves fiber bridging in primary cracks and increases rotational stiffness in hinge joints. SHCC mixtures with initial strain hardening and extended softening enable scalable design of advanced auxetic cementitious materials across various load levels.

This study investigates the mechanical properties of cementitious composites with 3D-printed auxetic lattices, featuring negative Poisson's ratios (auxetic behavior) in multiple directions. These lattices were fabricated using vat photopolymerization 3D printing, and three base materials with varying stiffness and deformation capacities were analyzed to determine their impact on the composites’ mechanical behavior. To unravel the reinforcing mechanisms of multidirectional auxetic lattices, which exhibit auxetic behavior in both planar and out-of-plane directions, X-ray computed tomography (X-ray CT) was utilized to analyze composite damage evolutions under different strain levels. The micro-CT characterization reveals that auxetic lattices more effectively constrain crack growth and dissipate energy by distributing stress evenly within the cement matrix. In contrast, due to lack of lateral confinement, the non-auxetic lattice reinforced composites primarily dissipate energy through extensive crack propagation and interfacial damage, leading to lower peak strength. When strain exceeding 5%, although the confinement from the auxetic behavior diminished with crack propagation, the lattice can still maintain the composite's structural integrity, resulting in 1.7 times higher densification energy than conventional cement-based materials. These findings provide valuable insights for designing auxetic lattice-reinforced cementitious composites with enhanced load-bearing capacity and improved dissipation capabilities.

Auxetic cementitious cellular composites (ACCCs) exhibit desirable mechanical properties (e.g., high fracture resistance and energy dissipation), due to their unique deformation characteristics. In this study, a new type of cementitious auxetic material, referred to as peanut shaped ACCC, has been designed and subsequently architected using additive manufacturing techniques. Two peanut shaped ACCCs specimens with different pseudo-minor axes have been tested under uniaxial compression with Digital Image Correlation (DIC) to assess their compressive behavior, peak strength, Poisson's ratio, and energy dissipation capacity. Additionally, cyclic tests were conducted to investigate their compressive resilience properties, further elucidated through microstructural analysis using a digital optical microscope. The mechanical test results were also compared with those of previously developed elliptical-shaped ACCCs. Furthermore, a numerical model was used to simulate the mechanical behavior of peanut shaped ACCCs under uniaxial compression, and showed a good agreement with the experimental data. The auxetic behavior observed in peanut shaped ACCCs arises from the rotation of sections facilitated by fiber bridging at the ligament of adjacent holes within the cementitious unit cell. In comparison to elliptical-shaped ACCCs, peanut shaped ACCCs can exhibit a slightly more negative Poisson's ratio and mitigate stress concentration. The reduction of stress concentration enables peanut shaped ACCCs to dissipate substantial energy, showcasing enhanced ductility and toughness. In cyclic tests, peanut shaped ACCCs exhibit superior recoverable deformation elasticity, attributed to robust fiber bridging capacity. The exceptional mechanical properties exhibited by peanut shaped ACCCs offer a scalable solution for developing energy-absorbent and multifunctional cementitious materials for smart infrastructure.

A novel highly compressible auxetic cementitious composite (ACC) is developed in this work. Contrary to conventional cementitious materials, such as plain concrete and fiber reinforced concrete, the ACC shows strain-hardening behavior under uniaxial compression: the stress continuously increases with strain up to approximately 40 % strain. On one hand, in the early compression stage, the ACC exhibit highly recoverable deformability of 10 % strain under cyclic loading (20 times higher than the constituent cementitious material). In addition, the ACC shows fatigue damage until the stiffness/strength and energy dissipation plateau values are reached after 500 cycles. At 2.5 % strain amplitude, the plateau stiffness/strength is approximately 120 MPa/3 MPa, while these values are only 25 MPa/1.2 MPa at 5 % strain amplitude. In contrast, the energy dissipation plateau of the ACC is independent from the amplitude and remains at 0.05 J/cm3. On the other hand, due to the strain-hardening behavior, the ACC exhibits significantly improved energy dissipation capacity compared to both the conventional cementitious materials and the auxetic frame. This behavior is achieved by a tailored composite action: integrating cementitious mortar with 3D printed thermoplastic polyurethane (TPU) auxetic frame. A rotating-square auxetic mechanism was designed for the TPU frame for the ACC to achieve the tailored cracking behavior. The horizontal ACC cells enable large deformability by enlarging the crack width under the confinement of the auxetic frame, while the vertical cells work as stiffening phase to ensure load resistance. Owing to the outstanding mechanical properties, the ACC shows great potential to be applied in engineering practice where high compressive deformability is required, for instance yielding elements for squeezing tunnel linings.

Pore structure characteristics of cementitious materials play a critical role in the transport properties of concrete structures. This paper develops a novel framework for modeling chloride penetration in concrete, considering the pore structure-dependent model parameters. In the framework, a multi-scale transport model was derived by linking the chloride diffusivities with pore size distributions (PSDs). Based on the three-dimensional (3D) microstructure generated by “porous growth” and “hard core-soft shell” methods, two sub-models were computationally developed for determining the multi-modal PSDs and pore size-related chloride diffusivities. The predicted results by these series of models were compared with corresponding experimental data. The results indicated that by adopting pore size-related diffusivities, even if the total porosities were the same, the proposed multi-scale chloride transport model could better capture the effect of different PSDs on chloride penetration profiles, while the model without pore structure-depended parameters would ignore the differences. Compared with the reference transport models, which adopt averaged chloride diffusivities, the chloride penetration depths predicted by the proposed multi-scale model are in better agreement with experimental data, with 10%–25% reduced prediction error. This multi-scale transport model is hoped to provide a novel computational approach on 3D microstructure generation and better reveal the underlying mechanism of the chloride penetration process in concrete from a microscopic perspective.

Cementitious materials exposed to marine and saline environments are commonly threatened by a combined attack of sulfate and chloride ions. This study developed a numerical framework to investigate two combined coupling mechanisms of 1) coupled solid-liquid chemical reactions for competitive chloride-sulfate attack and 2) electrostatic multi-ion coupling effect on reactive-transport mechanisms. Various chemical reactions including sulfate attack with anhydrous calcium aluminates, secondary precipitation of expansive minerals, competitive binding, and calcium leaching have been quantified. The electrostatic potential caused by multi-ions coupling was solved according to constitutive electrochemical laws. After model validation, the chemical coupling mechanisms for solid-liquid reactions during competitive chloride-sulfate binding were investigated. On this foundation, the influence of electrostatic multi-ionic coupling effects on ionic transport and its interaction with chemical coupling were disclosed. It was found that neglecting multi-ions coupling effect would result in an underestimated chemical coupling strength in competitive chloride-sulfate binding.

Understanding the transport mechanisms within unsaturated porous media is essential to the durability problems associated with cement-based materials. However, the involvement of multi-ions electrochemical coupling effect, especially under unsaturated condition makes the transport mechanisms even more complex. In this study, the moisture and multi-ionic transport in unsaturated concrete have been modeled in three-dimensional cases. The contribution from both water vapor and liquid has been considered in moisture transport. By adopting the constitutive electrochemical law, the electrostatic potential induced by inherent charge imbalance was calculated. With parameter calibration, the numerical results agreed well with the experimental data, proving the validity of the presented model. Results from a parametric analysis showed that neglecting multi-ions coupling effect will lead to an underestimated chloride concentration, and saturated degree has an obvious impact on the coupling strength among different ions. In addition, the existence of coarse aggregates will not only block mass transport but also make the discrepancies between two-dimensional model and three-dimensional model results more obvious. Other findings which have not been reported in existing studies are also highlighted.