Robotic-based technologies such as automated spraying or extrusion-based 3-dimensional (3D) concrete printing can be used to build tunnel linings, aiming at reducing labor and mitigating the associated safety issues, especially in the high-geothermal environment. Extrusion-based 3D concrete printing (3DCP) has additional advantages over automated sprayings, such as improved surface quality and no rebound. However, the effect of different temperatures on the adhesion performance of 3D-printed materials for tunnel linings has not been investigated. This study developed several alkali-activated slag mixtures with different activator modulus ratios to avoid the excessive use of Portland cement and enhance sustainability of 3D printable materials. The thermal responses of the mixtures at different temperatures of 20 and 40 °C were studied. The adhesion strength of the alkali-activated material was evaluated for both early and later ages. Furthermore, the structural evolution of the material exposed to different temperatures was measured. This was followed by microstructure characterization. Results indicate that elevated temperatures accelerate material reactions, resulting in improved early-age adhesion performance. Moreover, higher temperatures contribute to the development of a denser microstructure and enhanced mechanical strength in the hardened stage, particularly in mixtures with higher silicate content.

The first exothermic peak of cement-based material occurs a few minutes after mixing, and the properties of three dimensional (3D) printed concrete, such as setting time, are very sensitive to this. Against this background, based on the classical Park cement exothermic model of hydration, we propose and construct a numerical model of the first exothermic peak, taking into account the proportions of C₃S, C₃A and quicklime in particular. The calculated parameters are calibrated by means of relevant published exothermic test data. It is found that this developed model offers a good simulation of the first exothermic peak of hydration for C₃S and C₃A proportions from 0 to 100% of cement clinker and reflects the effect of quicklime content at 8%–10%. The unique value of this research is provision of an important computational tool for applications that are sensitive to the first exothermic peak of hydration, such as 3D printing.

3D concrete printing has the potential to replace shotcrete for construction of linings of tunnels in hard rock. The shear strength of the interface between rock and printed concrete is vital, especially at super-early ages. However, traditional methods for testing the shear strength of the interface, e.g., the direct shear test, are time-consuming and result in a high variability for fast-hardening printed concrete. In this paper, a new fast bond shear test is proposed. Each test can be completed in 1 min, with another 2 min for preparing the next test. The influence of the matrix composition, the age of the printed matrices, and the interface roughness of the artificial rock substrate on the shear strength of the interface was experimentally studied. The tests were conducted at the age of the matrices at the 1st, the 4th, the 8th, the 16th, the 32nd, and the 64th min after its final setting. A dimensionless formula was established to calculate the shear strength, accounting for the age of the printed matrices, the interface roughness, and the shear failure modes. It was validated by comparing the calculated results and the experimental results of one group of samples.

Extrudability and constructability are two important, yet contradictory issues pertaining to the construction of three-dimensional (3D) printing concrete. Extrudability is easily achieved when 3D printing cement mortar has a high water content and low cohesion, but the printed structure is easily collapsible. However, a 3D printing cement mortar with a low water content and high cohesion has a relatively stable printed structure although the cement mortar might not be extrudable. This study proposes a particle-based method to simulate 3D printing mortar extrusion and construction as an overall planning tool for building design. First, a discrete element model with time-varying liquid bridge forces is developed to investigate the microscopic effects of these forces on global rheological properties. Next, a series of numerical simulations relevant to 3D printable mortar extrudability and constructability are carried out. The study demonstrates that the effects of time-varying liquid bridge forces on rheological properties and the resulting extrudability and constructability of 3D printing mortar are considerable. Furthermore, an optimized region that satisfies both the extrusion and construction requirements is provided for 3D printing industry as a reference.

Using an in situ lunar regolith as a construction material in combination with 3D printing not only reduces the weight of materials carried from the Earth but also improves the automation of lunar infrastructure construction. This study aims to improve the printability of a geopolymer based on a BH-1 lunar regolith simulant, including the extrudability, open time, and buildability, by controlling the temperature and adding admixtures. Rheological parameters were used to represent printability with different water-to-binder ratios, printing temperatures, and contents of additives. The mechanical properties of the hardening geopolymer with different filling paths and loading directions were tested. The results show that heating the printed filaments with a water-to-binder ratio of 0.32 at 80 °C can adjust the printability without adding any additive, which can reduce the construction cost of lunar infrastructure. The printability of the BH-1 geopolymer can also be improved by adding 0.3% Attagel-50 and 0.5% polypropylene fiber by mass at a temperature of 20 °C to cope with the changeable environmental conditions on the Moon. After curing under a simulated lunar environment, the 72-h flexural and compressive strengths of the geopolymer specimens reach 4.1 and 48.1 MPa, respectively, which are promising considering that the acceleration of gravity on the Moon is 1/6 of that on the Earth.

Three-dimensional concrete printing (3DCP) can proliferate the industrialization of the construction sector, which is notoriously conservative and indolent toward changes. However, the mechanical behavior of 3DCP should be characterized and modeled considering the interfaces when its performance is thoroughly compared to that of the existing concrete construction methods. This study presents an experimental and numerical investigation of uniaxial compression and three-point bending (TPB) tests on extruded 3DCP beams in different loading directions. The orientation of translational and depositional interfaces with respect to the direction of loading influenced the strength. Both the elastic and post-damage behavior of the 3DCP specimens were compared with those of the conventionally cast specimen under quasi-static loading conditions. Despite the higher compressive strength of the casted specimen, the flexural strength of the 3DCP specimens was higher. This study employed the finite element and cohesive zone models of the appropriate calibrated traction-separation law to model fracture in the notched TPB specimens. Furthermore, the real-time acoustic emission test revealed the nature of failure phenomenon of three-dimensional-printed specimens under flexion, and accordingly, the cohesive law was chosen. The predicted load−displacement responses are in good agreement with the experimental results. Finally, the effects of cohesive thickness and notch shape on the performance under bending were explored through parametric studies.

Extrudability is one of the most critical factors when designing three-dimensional printable foam concrete. The extrusion process likely affects the foam stability which necessitates the investigation into surfactant properties particularly for concrete mixes with high foam contents. Although many studies have been conducted on traditional foam concrete in this context, studies on three-dimensional printed foam concrete are scarce. To address this research gap, the effects of surfactant characteristics on the stability, extrudability, and buildability of three-dimensional printed foam concrete mixes with two design densities (1000 and 1300 kg/m³) using two different surfactants and stabilizers (synthetic-based sodium lauryl sulfate stabilized with carboxymethyl cellulose sodium salt, and natural-based hingot surfactant stabilized with xanthan gum) were investigated in this study. Fresh density tests were conducted before and after the extrusion to determine stability of the foam concrete. The results were then correlated with surfactant qualities, such as viscosity and surface tension, to understand the importance of key parameters in three-dimensional printing of foam concrete. Based on the experimental results, surfactant solu1tion with viscosity exceeding 5 mPa·s and surface tension lower than 31 mN/m was recommended to yield stable three-dimensional printable foam concrete mixes. Nevertheless, the volume of foam in the mix significantly affected the printability characteristics. Unlike traditional foam concrete, the variation in the stabilizer concentration and density of concrete were found to have insignificant effect on the fresh-state-characteristics (slump, slump flow, and static yield stress) and air void microstructure of the stable mixes.

Three-dimensional printable concrete requires further development owing to the challenges encountered, including its brittle behavior, high cement requirement for the buildability of layers, and anisotropic behavior in different directions. The aim of this study is to overcome these challenges. First, three-dimensional printable concrete mixtures were prepared using silica fume, ground blast furnace slag, and metakaolin, instead of cement, to reduce the amount of cement. Subsequently, the rheological and mechanical behaviors of these concretes were investigated. Second, three-dimensional printable concrete mixtures were prepared using 6-mm-long steel and synthetic fibers to eliminate brittleness and determine the effect of those fibers on the anisotropic behavior of the concrete. As a result of this study, it is understood that printable concretes with extremely low permeability and high buildability can be achieved using mineral additives. In addition, results showed that three-dimensional concrete samples containing short steel fibers achieve fracture energies up to 36 times greater than that of plain concrete. Meanwhile, its characteristic length values, as indicators of ductility, are 22 times higher than those of plain concrete. The weakest strength was recorded at the interfaces between layers. The bending and splitting tensile strengths of three-dimensional printed plain concrete samples were 15% and 19% lower than those of casted samples, respectively. However, the addition of fibers improved the mechanical strength of the interfaces significantly.