Youssef Mortada

This study investigates the feasibility of additively constructed concrete (ACC) under sustained subfreezing conditions (−20°C), an environment that has been largely unaddressed in the existing literature. A gantry-based ACC system was developed within a temperature-controlled enclosure, alongside a cold-weather cementitious formulation (LC-CW) using conventional materials and chemical admixtures to promote early hydration. A brief early-age heating regimen was applied after an initial 6-h subfreezing period to counter the strength suppression observed under sustained −20°C curing, after which curing proceeded at −20°C without further thermal intervention. Fresh-state evaluation confirmed that LC-CW retained approximately 84% of its initial slump flow after 60 min while achieving sufficient early-age structural buildup for continuous layer stacking. The heating regimen increased the 1-day compressive strength to 34.6 MPa and enabled the material to reach the 3.5-MPa frost-protection threshold at approximately 12 h. Laboratory-cast specimens exceeded long-term strength targets, and field-extracted cores reached higher strengths (79.6 MPa at 28 days) owing to increased thermal mass. Interlayer bond strength averaged 2.3 MPa, with a substantial portion of specimens failing in the bulk material rather than at the interface, and no mass loss or surface deterioration occurred after 300 freeze–thaw cycles. Full-scale validation was completed by printing a 1.8-m-tall wall under continuous exposure at −20°C. These results demonstrate the feasibility of ACC using conventional construction materials in extreme cold environments, broadening its applicability for cold-region and remote construction.

This study investigates the impact of varying steel fiber (SF) content (0%, 0.8%, 1.0%, and 1.2% by volume) on the mechanical and durability properties of 3D-printed Ca(OH)₂-activated geopolymer concrete (GPC). The addition of 1.2% SF improved flexural strength by 69% at 7 days and 16% at 28 days, while tensile strength more than doubled to 3.75 MPa at 28 days. Although compressive strength remained unaffected at 43 MPa, SF enhanced interlayer bond strength by 20%, which is crucial for layer cohesion in 3D-printed structures. Additionally, the elastic modulus increased by 7%, contributing to improved stiffness. Durability assessments, including autogenous shrinkage and self-induced stress, indicated a slight reduction in shrinkage of SF-reinforced samples, with no significant effect on self-induced stress. Microstructural analysis using scanning electron microscopy (SEM) and X-ray micro-computed tomography (µCT) demonstrated the crack-bridging behavior of steel fibers, enhancing ductility and fracture resistance. There was a slight increase in porosity (5.34%) of SF-reinforced samples without negatively affecting their mechanical properties. Notably, SF improved early-age toughness and controlled crack propagation across printed layers, addressing a critical challenge in 3D-printed concrete. The novelty of this work lies in successfully reinforcing 3D-printed Ca(OH)₂-activated GPC with recycled steel fibers, enhancing mechanical properties, interlayer bonding, and durability without compromising printability. This study offers a sustainable reinforcement strategy for 3D printing in construction.

This study proposes test methods for assessing the printability of concrete materials for Additive Manufacturing. The printability of concrete is divided into three main aspects: flowability, setting time, and buildability. These properties are considered to monitor the critical quality of 3DCP and to ensure a successful print. Flowability is evaluated through a rheometer test, where the evolution of shear yield strength is monitored at a constant rate (rpm), similar to the printer setup. Flowability limits were set based on the user-defined maximum thickness of a printed layer and the onset of gaps/cracks during printing. Setting time is evaluated through an ultrasonic wave pulse velocity test (UPV), where the first inflection point of the evolution of the UPV graph corresponds to the setting time of the concrete specimen. The results from this continuous non-destructive test were found to correlate with the results from the discrete destructive ASTM C-191 test for measuring setting time with a maximum difference of 5% between both sets of values. Lastly, buildability was evaluated through the measurement of the early-age compressive strength of concrete, and a correlation with the UPV results obtained a predictive model that can be used in real-time to non-destructively assess the material buildability. This predictive model had a maximum percentage difference of 13% with the measured values. The outcome of this study is a set of tests to evaluate the properties of 3D printable concrete (3DP) material and provide a basis for a framework to benchmark and design materials for additive manufacturing.