Experimental Testing and Finite Element Modeling of 3D Printed Reinforced Concrete Beams (2020-03)¶

Additive construction through digital design and 3D printing of concrete elements is expected to transform the construction industry in the near future and is rapidly gaining acceptance. The advantages include the fact that it eliminates the cost of formwork and associated labor and enhances the ability to create more complex shapes, surface conditions, new details, and making new spatial and architectural expressions possible. The structural behavior of these 3D printed concrete components is different from conventional cast elements due to the nature of 3D printing by laying filaments side by side to make a layer and layer over layer to make a 3D shape. This may result in concrete with lower strength compared to cast concrete due to potentially less than perfect bond between filament (potential for cold joint) and tiny gaps between filaments due to the round shape of the filaments. There is limited research with many unknowns regarding the mechanical properties of printed hardened concrete components, which has hindered their practical application. One area of significance is how to reinforce these elements and how to model them numerically-as printed concrete exhibits low tensile strength. Embedding reinforcing rebars in a printed beam would be an option; however, the response of the printed component would be essentially different from reinforced cast beams. Using a pump to extrude filaments and the occasional presence of voids between these filaments make the printed material anisotropic with directionally dependent properties and with different mechanical behavior compared to conventional concrete. In this study, two different rebar types, FRP and Steel, were used to reinforce 100×150×1200 mm beams, and their strengths were measured by conducting three-point bending flexural tests. Structural analysis was then performed on finite element models of the beams to determine the response of printed beams. In this modeling, the printed concrete was assumed to be isotropic just as cast concrete, thus not accounting for the potentially weaker bond between filaments. The experimental results indicated low bond strength between the printed material and embedded rebars, causing the beams to fail under relatively low bending stress due to the separation of the reinforcement from the printed material (filaments) when concrete cracked. This means that the reinforcement did not reach the state of yielding as it is expected in reinforced concrete design. Autodesk Nastran that is embedded in Inventor 2019 by Autocad was used to carry out Finite Element Analysis using the tetrahedron element for volumetric mesh generation. The finite element modeling validation was done by comparing the numerical results to the analytical solution of simple reinforced concrete beams. The mechanical properties of the 3D printed concrete are estimated through a comparison of the analytical and numerical responses of the beam to the actual testing results.