On the 3D Printing of Reinforced Concrete
Authors: Allameh Seyed M. , Allameh Hadi , Miller Roger , Lenihan Avery , Kota Dhruv 
Published: February 2024

Abstract
Automation of construction industry has been long due given the rapid advances in construction technology. This is particularly true about the fabrication of structures such as houses, office buildings, bridges and other life-sustaining structures. From the contour crafting experiments of two decades ago to the recent building of modern homes in US and abroad, the technology has seen growth in quality of the concrete and the processing methodologies utilized. However, these structures are not currently made with reinforced concrete that sustains dynamic shear forces imposed on multiple-story structures, tensile forces in bridges and concrete beams and roofs. To develop the construction automation one step further and pave the way to do 3D printing of reinforced concrete, several issues have to be addressed. The first one is the feasibility of incorporation of rebar into concrete by co-printing of rebar and its ceramic matrix. While this has been demonstrated to be possible, the strength of the 3D printed rebar and its interfacial strength has to be validated. A number of tests of both monotonic and cyclic tensile types have been conducted, showing that the mechanical properties of printed rebar are comparable to those of conventional rebar at microscale. These include yield strength and fatigue limit. The results of these studies clearly show the microscale ultimate tensile strength of the 3D printed rebar using mild steel MIG wire stands at about 630MPa which is nearly identical to that of mild steel conventional rebar. Micro-fatigue test results show that both mild steel rebar and its 3D welded counterpart have similar fatigue limits nearing 430 MPa. The results of microtensile and microfatigue tests, both show higher UTS and fatigue limits compared to macroscale properties of conventional rebar. The results of prior work cited above suggest the reliability of 3D printing of reinforced concrete. However, it is necessary to examine other factors that make reinforced concrete such a valuable life-sustaining composite. One such factor is the interfacial integrity of the interface between the printed rebar with its concrete matrix which is the focus of this study. To examine the interfacial strength of 3D printed rebar and its role in strengthening reinforced concrete, several tests are designed and carried out. These include fabrication of a beam of reinforced concrete with conventional construction technology to be used as the control specimen. This method consists of placing conventional rebar in a mold and pouring concrete on the beam. A similar beam is then made by fabricating rebar by 3D printing using a MIG welder torch. The bottom rebars of the second concrete beam are then chosen from the 3D printed rebars, completing the task by pouring concrete in the mold over the rebars. After curing for 7 days, both concrete reinforced beams are tested in 4-point bend configuration. The results of the tests have shown lesser tensile strength for the 3D printed rebar compared to the conventional counterpart. Furthermore, flexural strength of the concrete beam with 3D printed rebar at as the bottom cords was found to be lesser than the control beam that contained only conventional rebar. However, even the lower tensile and flexural load capacities of 3D printed rebar and the resulting reinforced concrete are still high enough to be of interest to construction industry.