This paper focuses on quantifying the micro-scale creep compliance of the C-S-H phase in well-hydrated, 1.5-year-old cement paste, measured using grid nanoindentation with a Berkovich tip. Four different load levels were tested to induce varying levels of deformation and stress. The results showed that the measured creep compliance is independent of the maximum vertical force within the range of 1 mN to 3 mN, standardly used in nanoindentation of cement pastes. The same trends and conclusions were observed for a reference polymeric material, PMMA, chosen for its homogeneity. In both cases, the creep falls in the linear viscoelastic regime. The linearity was further checked by calculation of the stress developed beneath the indentation tip. The stress was estimated using a finite element numerical model simulating the contact problem. The calculated stress reached 61.4% of the compressive strength measured by micro-pillar testing, 17.1%–46.1% of the strength measured by micro-beam testing, and 29.0% of the strenght estimated by the multiscale model.

Additive manufacturing (AM) of cementitious materials, also known as 3D printing, offers transformative potential for the construction sector. Given the inherently quasi-brittle behavior of cementitious materials and their vulnerability to fracture, it is essential to understand how the layer-by-layer extrusion process affects their fracture resistance. This study explored the interplay between early-age rheology, pore structure, and fracture toughness of additively manufactured cementitious materials. Using three-dimensional micro-computed tomography, the impact of early-age thixotropic behavior and varying printing time intervals on the pore structure of the interlayers was examined. Fracture tests were conducted on printed specimens, incorporating a digital image correlation system for precise monitoring and visualization of crack evolution. The findings demonstrated that fracture toughness of additively manufactured cementitious materials is strongly influenced by their early-age rheological behavior and printing conditions.

Cementitious materials are limited by its brittle nature, leading to the adoption of steel bars or fibers as reinforcement to improve ductility. With advances in additive manufacturing, 3D-printed lattice structures have emerged as a promising alternative for reinforcing cementitious composites, enabling enhanced mechanical properties This study explores the incorporation of a three-dimensional lattice structure with negative Poisson's ratios (auxetic behavior) into cementitious composites. Uniaxial compression test showed that the densification energy could reach 170% times of reference cement mortar. Because of the lateral contraction tendency of auxetic lattices which would constrain the expansion of cementitious matrix, the peak strength for auxetic lattice reinforced cementitious composites was 1.4 times of their non-auxetic counterparts. To further disclose the interaction mechanisms between the 3D-printed lattice and the cementitious matrix, X-Ray computed tomography (X-ray CT) was utilized to analyze the internal damage under varying strain levels. Micro-CT characterization revealed distinct failure mechanisms for auxetic and non-auxetic lattice reinforced cementitious composites. Due to a larger lateral expansion tendency of the non-auxetic lattice structure, interfacial shear cracking was observed between the lattice reinforcement and cementitious matrix. In contrast, the opposing deformation pattern of auxetic lattices resulted in fewer cracks in the core area, more even stress distribution, and prevention of large crack formation, thus enhancing the composite’s energy absorption capacity. Moreover, quantitative analysis from CT scans showed that the crack volume in the core of the auxetic lattice-reinforced composites was almost 60% lower than that of the non-auxetic samples at 2.5% strain. At 5% strain, the auxetic lattice continued to limit crack merging, but at 7.5% strain, although the total crack volume remained 20% lower, the ability to prevent crack coalescence diminished. These insights from micro-crack analysis provide valuable guidance for designing cementitious composites reinforced with auxetic lattice structures.

The layer-atop-layer construction process in 3D concrete printing (3DCP) has incurred two limitations: (1) the weak interlayer bond strength and (2) the difficulty of integrating steel reinforcement. Strain-hardening cementitious composites (SHCC) with high tensile properties have the potential to achieve self-reinforced structures in 3DCP. This study proposes a multi-material printing strategy by combining ductile (SHCC) and brittle (printable mortar) cementitious materials to address these two limitations simultaneously. SHCC are used as the bonding agents introduced at the interfaces of printed mortar. A dual-nozzle system is designed to achieve synchronized SHCC deposition and mortar printing. The splitting tensile test was adopted to evaluate the interlayer bond strength and four-point bending test was performed to study the flexural performance. The results show that the SHCC bonding agents can enhance the interlayer bond strength by 80% and reduce the interfacial porosity by 35%. In the SHCC-concrete beams, the flexural strength, deflection, and energy absorption capacity increase by 26%, 182%, and 800%, respectively, compared to the reference group without SHCC. The findings reveal that the proposed multi-material printing strategy has great potential to address the weak interlayer bond strength and reinforcement integration problems in 3DCP.