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This paper investigates the Near‐nozzle Mixing (NNM) technique for 3D printing cementitious mortar, focusing on achieving continuous and homogeneous material properties. The NNM's ability to print Natural Sand Mortar (NSM), Recycled Aggregate Mortar (RAM), and Lightweight Mortar (LWM) was assessed. Fresh and hardened properties of each mortar were examined, emphasizing standard deviation (STD) and coefficient of variation (CV) to measure material homogeneity. Fresh properties included slump flow, yield stress, air void volume, density, and water‐to‐cement ratio, while hardened properties covered compressive strength, flexural strength, hardened density, and thermal conductivity. Results showed low CV values for all mortar types, indicating high consistency and homogeneity. NNM showed is capability to print of various materials with customized properties. NSM showed the highest compressive strength, RAM balanced strength and sustainability, and LWM provided the best thermal insulation. The study highlights NNM's versatility and potential for construction applications requiring tailored properties, proving it a promising additive manufacturing solution. The research serves as a foundation for future studies on graded multi‐material printing. [ABSTRACT FROM AUTHOR]

3D concrete printing is an avant-garde building process with the potential to revolutionise the construction industry by automating construction procedures, reducing material consumption, lowering form-work costs, and optimising structural elements. The current study proposes a methodology for developing plain and fibered 3D printable concrete mixes using locally available binders (cement, silica fume, fly ash, and limestone powder), fine aggregate, water, admixtures, polypropylene fibers, and steel fibers. The study presents the results of 24 trials conducted to develop mixes for concrete 3D printing. According to the present studies, the recommended flow for the mix to be extrudable, printable, and buildable is between 19–21 cm and the open time is 45–60 min. The printed specimens under bending failed monolithically with a single dominant crack. The plain and fibered 3D printed specimens possessed greater flexural strength than the in-situ cast specimens. The increase in flexure capacity is found to be in the range of 07–22%. [ABSTRACT FROM AUTHOR]

Estimating the early-age strength of 3D printed concrete is more challenging than that of conventional concrete due to the absence of formwork and increased variability in curing conditions. The common maturity method is ineffective for 3D printed structures since it fails to account for moisture content variations. This paper introduces a new approach using embedded sensors to continuously collect data on the electrical properties and temperature of 3D printed concrete, enabling accurate strength estimation under varying curing conditions. Empirical models based on electrical resistivity, internal temperature, and relative permittivity are developed and evaluated. The permittivity-based model can estimate the flexural strength of 3D printed specimens with at least 83% accuracy and a maximum root mean square error of 0.27 MPa under different curing conditions across three concrete grades. Additionally, an innovative curing technique involving the automated application of curing compounds is proposed and proven effective for 3D printed concrete. • An automated strength monitoring approach for 3D printed concrete is proposed. • The effect of different curing conditions on strength development is investigated. • The permittivity-based model is able to estimate the strength with high accuracy. • An automated curing system is proposed, tested, and proven effective. [ABSTRACT FROM AUTHOR]

• W1.5 shows superior interlayer bonding and flexural strength. • Optimal stand-off distance is 13.5 mm to 15 mm. • Lattice beam achieved a failure load of 49.01 kN with flexural failure. This paper presents the optimal selection of cable reinforcement for concrete 3D-printed lattice beam. It also addresses key challenges associated with interlayer bonding and flexural strength of 3D printed elements. Three different types of reinforcement including 1.6 mm and 2 mm diameter rebar, (R1.6 & R2) and 1.5 mm wire (W1.5) with seven strings were evaluated with different stand-off distances of 10–20 mm. The interlayer bonding and flexural strength were investigated to determine the most effective reinforcement configuration. Results show that W1.5 reinforcement exhibited higher interlayer bonding and flexural strength as compared with R1.6 and R2. Subsequently, the optimal reinforcement configuration (W1.5) was embedded in a concrete 3D-printed lattice beam. The lattice beam was structurally evaluated using four-point testing, demonstrating an ultimate load-carrying capacity of 49.01 kN with a mid-span displacement of 2.99 mm. Moreover, it exhibited flexural failure with no premature failures such as inter-layer delamination. [ABSTRACT FROM AUTHOR]

In the current study, we examine the effect of using natural and recycled coarse aggregates in designing 3D printable concrete. We assessed the rheological behaviourusing a dynamic shear rheometer, and it was observed that the addition of coarse aggregates significantly decreased the yield stress and marginally lowered the plastic viscosity. This was attributed to the increase in the paste and water film thickness with the addition of larger aggregates. Therefore, a reduction in the superplasticizer dosage is required to obtain coarse aggregate mixtures with similar yield stress and buildability to the control mixture. The mechanical properties were evaluated by using beam and cube samples cut out from printed wall elements. A marginal decrease in compression and flexural strength was observed for both the mixtures with natural and recycled coarse aggregates. The total and autogenous shrinkage assessment was performed using mould cast prismatic specimens, while the shrinkage cracking potential was evaluated using the restrained ring test. The coarse aggregate mixtures showed lower total and autogenous shrinkage. Notably, the addition of the saturated recycled aggregates significantly lowered the autogenous shrinkage, possibly due to internal curing. As a result, a relatively lower strain rate factor and slower development of tensile stresses occurred in the restrained shrinkage test, increasing the cracking age for the coarse aggregate mixtures. The current study, therefore, shows good potential for using natural and recycled coarse aggregates in 3D printable concrete. [ABSTRACT FROM AUTHOR]

This paper investigates an approach of 3D printing lightweight concrete elements using extrusion of hollow-core filaments. Two different filament shapes of rectangular hollow shape and U-shape were printed by using customised nozzles. The primary challenge in hollow-core 3D printing is the extrudability and the stability of the overhang of the filament at fresh state. The stability of the overhang was assessed by determining the viscosity recovery rate and yield strength growth rate of the mix with nanoclay as a thixotropic additive. The mix containing 0.2 wt% of nanoclay has shown good extrudability and enhanced stability of the overhang. The hollow-core 3D printed elements have shown a reduced density of 1369.6 kg/m3 and 1701.7 kg/m3, compared to 2114.1 kg/m3 obtained for solid-core printing. The compressive strength and the Modulus of Rupture (MoR) of hollow-core filaments evaluated at different directions reveal a significant strength loss in perpendicular and lateral directions due to the effective bond area reduction between filaments. The thermal performance of 3D printed panels assessed using simulated test room experiments reports that the hollow-core panels have high thermal inertia due to increased insulation capacity. However, the lack of thermal energy storage has resulted in high indoor temperature in these test rooms. [ABSTRACT FROM AUTHOR]

This paper provides a state-of-the-art report on the up-to-date research on the emerging 3D concrete printing technology from the concrete materials perspective. It reviews the recent research focused on understanding and characterizing the rheological necessities of the concrete printing process and discusses how the researchers are tailoring compatible mix proportions for the 3D concrete printing process by using eco-friendly binders, waste aggregates, chemical admixtures, and nano-additives. This paper systematically evaluates anisotropic behavior in the mechanical properties of printed concrete and establishes an order for anisotropic behavior in the compressive, flexural, and tensile strengths along three different axes (X, Y, and Z axes) of printed concrete. It evaluates the ratio of flexural strength to the compressive strength of printed concrete along the above three axes. This article explains the influence of variation of printing process parameters on the mechanical properties and discusses reinforcement approaches used for increasing structural performance. The microstructure at the interface of adjacent layers and also at the interface of the reinforcement-cement matrix is discussed. The recent research on the durability performance of printed concrete is critically discussed and future research needs for 3D concrete printing are identified in this paper. [ABSTRACT FROM AUTHOR]