This study systematically investigates the role of lattice–matrix stiffness contrast in governing confinement and mechanical performance of cementitious composites reinforced by 3D-printed auxetic (negative Poisson's ratio) lattices. Through combined compression testing, numerical simulations, and representative volume element (RVE) analysis, the mechanistic link between stiffness ratio and macroscopic response is established. The results demonstrate that sufficient stiffness contrast is a prerequisite for activating auxetic-induced confinement, enabling the translation of lattice lateral contraction into effective confinement on the cementitious matrix. Auxetic cementitious composites with 3D-printed steel lattice achieved a compressive strength exceeding 80 MPa (nearly 300% higher than plain mortar and polymer lattice reinforced composites). The specific energy absorption was 90% greater than the theoretical sum of the steel lattice and matrix, owing to the strong confinement and synergy enabled by the stiffness contrast. In contrast, polymer lattice reinforced composites, despite possessing the same geometry and similar negative Poisson's ratios, exhibited limited confinement efficiency as the low stiffness suppressed the transfer of auxetic deformation to matrix. RVE analyses revealed that the stiffness contrast between the lattice and matrix governs the mesoscale confinement behavior, which in turn influences the macroscopic strength, ductility, and energy dissipation capacity of auxetic cementitious composites. These findings establish stiffness contrast as the governing design parameter for auxetic cementitious composites and provide a basis for tailoring architected cementitious composites.

Auxetic cementitious cellular composites (ACCCs) offer high deformability that is attractive for mechanical energy harvesting when integrated with flexible piezoelectric materials. However, the intrinsic brittleness of cement-based materials and the complex coupling between auxetic geometry and damage evolution hinder the efficient design of ACCC energy harvesters. This study proposes a novel learning-driven design framework that, for the first time, integrates a physics-based energy harvesting model with Bayesian Optimization (BO) to directly optimize the recoverable hinge-like strain capacity of ACCCs for enhanced electrical output. The optimization maximizes the voltage generated by piezoelectric materials bonded at hinge regions, while using constraints to prevent splitting failure and non-auxetic behavior under compression. The energy harvesting model combines the concrete damage plasticity (CDP) model for pre-compression damage with a secondary elastic model for cyclic loading, enabling prediction of recoverable strain in generalized ACCC geometries. The learning-driven approach proved far more efficient than random generation in identifying optimal ACCC configurations. Experimental validation of the optimized design achieved a peak-to-peak voltage of nearly 15.0 V per cycle, about 2.7 times higher than a reference design. This study provides a learning-driven approach to designing enhanced compliant auxetic cementitious energy harvesters for smart infrastructure applications.

The use of Additive Manufacturing (AM) to create reinforcements for cementitious composites has become a popular research topic in recent years. One illustrative example is the integration of 3D-printed auxetic reinforcements into cementitious matrices, which exhibit superior energy absorption due to their negative Poisson's ratio. This presents the opportunity to tailor the Poisson's ratio of reinforcements to align with local stress distributions and enhance structural efficiency. In this study, Tailored Poisson's Ratio-reinforcements (TPR) were proposed, characterized by a linear gradient of Poisson's ratios along the height of the reinforcement to accommodate varying stress profiles within beams. Specifically, the top chords of TPR exhibit negative Poisson's ratios (auxetic), undergoing lateral contraction under compression and providing confinement to the surrounded matrix. Conversely, the bottom chords possess positive Poisson's ratios, contributing to lateral contraction under tension. These lateral deformations cause a shift in the principal stress state of the confined matrix, extending the loading path in stress space and actively delaying failure. Three novel Tailored Poisson's Ratio-reinforced Cementitious Composite (TPRCC) designs are developed and tested under four-point bending in this study. Experimental recordings indicate increases in load capacity and toughness of up to 191% and 6900% with respect to plain mortar, respectively.

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.

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.

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.