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.

In this study, the effects of titanium dioxide (TiO₂) nanoparticles on the sulfate attack resistance of ordinary Portland cement (OPC) and slag-blended mortars were investigated. OPC and slag-blended mortars (OPC:Slag = 50:50) were made with water to binder ratio of 0.4 and a binder to sand ratio of 1:3. TiO₂ was added as an admixture as 0%, 3%, 6%, 9% and 12% of the binder weight. Mortar specimens were exposed to an accelerated sulfate attack environment. Expansion, changes in mass and surface microhardness were measured. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), Thermogravimetry Analysis (TGA) and Differential Scanning Calorimetry (DSC) tests were conducted. The formation of ettringite and gypsum crystals after the sulfate attack were detected. Both these products had caused crystallization pressure in the microstructure of mortars and deteriorated the mortars. Our results show that the addition of nano-TiO₂ accelerated expansion, variation in mass, loss of surface microhardness and widened cracks in OPC and slag-blended mortars. Nano-TiO₂ containing slag-blended mortars were more resistant to sulfate attack than nano-TiO₂ containing OPC mortars. Because nano-TiO₂ reduced the size of coarse pores, so it increased crystallization pressure due to the formation of ettringite and gypsum thus led to more damage under sulfate attack.

Extrusion-based 3D concrete printing (3DCP) is an additive manufacturing (AM) technology in the construction field and is gaining significant attention as an emerging construction method for the future. The variability in concrete composite material properties in this field poses a challenge in maintaining consistent printing quality. Furthermore, the economic and environmental costs of discarding large structures in case of printing failures are significantly higher compared to other AM technologies. These inherent characteristics highlight the critical need for real-time, in-situ quality monitoring of printed filaments during the 3DCP process. Existing studies on geometric quality monitoring techniques provide only partial information on the extruded concrete filaments, such as width or height, and some studies are limited to linear paths. Moreover, the requirements of 3DCP, such as cost, weight, installation space, minimum object distance (MOD), field of view (FOV), and maintenance, make it challenging to find suitable commercial 2D laser profile sensors. Therefore, this paper presents the design of a 2D profile sensing system tailored to meet the requirements of 3DCP and proposes an algorithm that can compute comprehensive cross-sectional information, including the width, thickness, and area of the extruded concrete filament during 3D printing. Notably, the proposed sensing system is integrated with an infinitely rotatable nozzle device on the end effector of a 3D printer robot arm, enabling continuous monitoring of extrusion in all directions along the printing path. Comprehensive experimental results from static, motion, and concrete 3D printing tests validate that the developed 2D profile sensing system is capable of performing real-time, in-situ quality monitoring in 3DCP by accurately computing filament cross-sectional shape information. Furthermore, this paper provides a comprehensive solution that includes practical calibration method along with design, offering a crucial technology for detecting defects and failures, ultimately contributing to the high-quality and safe construction of structures in 3DCP.