Conventional cement-based concrete is widely used as a construction material due to its ability to flow before hardening and to adopt the shape of the formwork as it is placed. Contrarily, in layered extrusion additive manufacturing, commonly known as three-dimensional (3D) printing, concrete is shaped without formwork. This imposes stringent time-dependent rheological requirements of materials used for 3D printing. Polymer concrete (PC) is a material extensively used in the precast industry. This paper reports on the potential use of PC for 3D printing applications. The influence of mixture design parameters—specifically rheology modifier content, filler-polymer ratio, and aggregate-polymer ratio—on the rheological properties of a 3D-printable PC are investigated. The rheological properties of seven PC mixtures are tested and characterized. PC can be described as a Bingham pseudoplastic material, and a Herschel-Bulkley model can accurately describe its rheological behavior (dynamic shear stress) over time. The evolution of static yield stress over time was found to follow an exponential trend. The use of these models to predict the dynamic and static yield stress of PC shall enable the design of efficient and stable 3D printing. Finally, 3D-printed PC shows good mechanical performance with compressive strength above 30 MPa (4351 psi) at 7 days of age. Automation of the PC precast industry using 3D printing will create new opportunities for the use of PC in civil infrastructure.

Digital fabrication and automated manufacturing technologies have been explored for civil engineering applications in the recent past and have rapidly gained momentum. Research and industrial development activities have been primarily focused on three-dimensional (3D) printing of concrete using the basic principle of extrusion along a predefined, automatically guided path. While the automated placement and shaping of concrete has advanced and has been refined significantly, the installation of reinforcement in the concrete is still largely done using traditional methods by manual placement of conventional steel reinforcing bar in a cavity between 3D-printed walls of formwork, which is subsequently filled by conventional cast-in-place concrete or grout. The concept for the construction of a structure in an entirely automated, digitally controlled process using alternative methods of structural reinforcement is currently still to be developed. Structural reinforcement is a key requirement in any efficient and economical concrete structure, and it is a challenge to invent a process for placing this reinforcement using an automated process in line with the printing process of concrete.

Three-dimensional (3D) printing, also known as additive manufacturing, is a revolutionary technique, which recently has gained a growing interest in the field of civil engineering and the construction industry. Despite being in its infancy, 3D concrete printing is believed to reshape the future of the construction industry because it has the potential to significantly reduce both the cost and time of construction. For example, savings between 35 and 60% of the overall cost of construction can be achieved by using this technique due to the possibility of relinquishing the formwork. Moreover, this innovation would free up the architectural gesture by offering a wider possibility of shapes. However, key challenges should be addressed to make this technique commercially viable. The effect of mixture composition on the rheological properties of the printed concrete/mortar is vital and should be thoroughly investigated. This paper investigates the effect of using red mud, nanoclay, and natural fibers on the fresh and rheological properties of 3D-printed mortar. The rheological properties were evaluated using the penetrometer test, flow table test, and cylindrical slump test. The estimated yield stress values were then calculated based on the cylindrical slump test. Further, relationships between the tested parameters were established. The main findings of this study indicate that the use of an optimum dosage of a nanoclay was beneficial to attain the required cohesion, stability, and constructability of the printed mortar. The use of natural fibers reduced pulp flow by improving cohesion with a denser fiber network and reducing the cracks. With respect to red mud, it may be appropriate for printable mortar, but more testing is still required to optimize its use in a printable mixture. A printability box to define the suitability of mixtures for 3D printing was also established for these mixtures.

To ensure a successful outcome when using cementitious materials during three-dimensional (3D) printing operations, the effects of chemical admixtures on rheological and setting behavior must be carefully adjusted to accommodate the needs for pumping, extrusion, deposition, and self-sustainability without the support of formwork. This paper highlights potential solutions offered by chemical admixtures, while discussing various testing methods and important influencing parameters. The impact of commercial polymers on viscosity, initial yield stress, thixotropy, and their variations over time are reported. Influencing factors, such as mixing energy and material interactions, are discussed. Accelerating and strength-enhancing admixtures are used to illustrate the adjustment of setting and early strength development of concrete. Understanding the possibilities of modifying fresh concrete properties will help to improve the robotic construction process as well as the design or adaptation of the printing equipment.

Ultra-high-performance concrete (UHPC) has progressively gained interest because of its favorable strength and durability properties. Considering applications of heat treatment and mass concrete, understanding the direct relationship between curing temperature and time is informative for construction decisions (such as formwork type and time of removal) to maximize performance per unit cost of UHPCs, as they can differ from conventional concrete. Limited datasets are currently available to ascertain the degree of change related to UHPC mechanical properties as a function of curing temperature and conditions. This study presents a systematic experimental program to investigate the effect of isothermal and submerged conditions on the rate and extent of compressive strength and elastic modulus development for UHPC, followed by development of numerical models that capture these effects with reasonable accuracy. Although the final elastic modulus appears to be unaffected by temperature, much higher compressive strength was achieved with higher curing temperatures compared to ambient conditions, and both properties were successfully modeled.