Mechanical response of small-scale 3D-printed steel-mortar composite beams

This paper presents the mechanical properties of composite beams additively manufactured by 3D-printing of layers of mortar on top of custom-designed (also 3D-printed) steel plates, which served as external flexural reinforcement. The mechanical performance of the composite beams was evaluated using three-point bending test. The results were compared with the data obtained from two types of reference specimens: 3D-printed beams with no external reinforcing plate, and composite beams with mortar cast (rather than 3D printed) on the top of the steel plate. Four different architectures (also referred to as configurations) were created by either 3D-printing (using two different filament orientations), casting, or a combination of both processes (referred to as a hybrid configuration). This configuration consisted of 3D-printed external walls (outline) of the beam and cast interior (infill). The effects of these architectures were investigated using both unreinforced and composite elements, in order to identify the contributions of the orientation of filaments and associated interfaces on the initiation and propagation of the cracks. Analysis of the initiation and propagation of the cracks was based on the data obtained from the digital image correlation (DIC) technique. Mechanical performance parameters that were investigated included the following: load-displacement responses, flexural stress-flexural strain responses, shear stress-shear strain responses, and the work of failure. The results of the test revealed that the incorporation of steel plates as an external reinforcement resulted in the transition from flexural to shear mode failure. In terms of flexural strength, all composite configurations demonstrated comparable performance independently of the architecture used while hybrid configuration outperformed cast composite counterparts by attaining significantly higher values of the work of failure. This indicates that hybrid elements developed enhanced energy dissipation characteristics compared to the other configurations. The results of specific modulus of rupture and shear strength obtained from unreinforced and reinforced hybrid elements, respectively, were comparable to those obtained from cast counterparts, indicating that the hybrid configuration may offer a viable alternative for the construction of structural concrete elements. Finally, the values of modulus of rupture and shear stress obtained for different configurations used in the study were compared to the design expressions in the ACI 318–19 code. It was found that hybrid elements as well as cast elements used in this study satisfy the minimum requirements and further studies at larger scales could confirm their use in structural applications.