Flexural Behavior of 3D Printed High-Strength Ductile Concrete Beams with Optimized Topology

The technology of 3D concrete printing (3DCP) has the potential of revolutionizing the construction industry through an automated, rapid and economical method to build structures or infrastructure with minimal needs for formworks and labor. In order to realize the rational and wide adoption of 3DCP a series of fundamental challenges need to be addressed, ranging from new concrete materials science and engineering to enable the 3D printing process, to understanding the mechanical behavior of 3D printed concrete structural elements. Conventional cast concrete is known to have a quasi-brittle nature and possess significantly lower strength under tension than under compression. Therefore, steel reinforcement is commonly used in flexural members made of concrete such as beams. While steel reinforcements can be well pre-aligned or installed in formworks before casting concrete, they are difficult to be integrated into the concrete 3D printing process when formworks are absent. Therefore, this thesis proposes and investigates a new approach to 3D printing concrete beam members with novel concrete featuring a ductile (rather than quasi-brittle) behavior and an optimized topology. It is hypothesized that if a ductile strain-hardening tensile behavior of the 3D printed concrete can be achieved, it would lead to substantially improved flexural behavior of 3D printed beam members, in terms of flexural strength and ductility, thus reducing the need for steel reinforcement. It is further hypothesized that the 3D printed concrete beam with optimized topology and ductility would result in higher mechanical capacity but with less use of concrete materials. To test these hypotheses, this thesis has two objectives: first, it aims to generate the understanding on the flexural behavior of beam members made of the high-strength ductile concrete; second, it aims to reveal how the optimized topology enabled by 3D printing will affect the flexural behavior of beam members made of high-strength ductile concrete. To achieve these, this thesis performs both experimental and numerical studies on the flexural behavior of small-scale beam specimens either casted or 3D printed with newly developed high-strength ductile concrete materials, either with or without optimized topology. It first performs topology optimization of a concrete beam geometry using finite element method (ABAQUS). Then, four types of concrete beam specimens are manufactured with novel high-strength ductile concrete: (a) 3D printed beams with optimized topology, (b) casted beams with optimized topology, (c) casted prismatic beams without topology optimization, which has the same total volume as the beams in (a) and (b), and (d) casted prismatic beams without topology optimization, which has the same effective volume between supports as the beams in (a) and (b). These beams are subjected to 4-point bending test. In addition, two types of boundary conditions are considered: (1) pin–roller support condition without horizontal constraint, and (2) with horizontal constraints at both supports. The force-displacement relation, strain field and damage pattern are experimentally measured. In addition to the experimental study, finite element based modeling is performed using Abaqus to numerically analyze the behavior of the different beam specimens. The model considers the constitutive relation of 3D printed high-strength ductile concrete that is fundamentally different from that of conventional concrete, it also considers the given beam geometries and different boundary conditions same as the experimental setup. This study elucidates how concrete material ductility, 3D printing method, topology optimization, and beam boundary conditions affect the flexural behavior of concrete beams. The novel ultra-high-strength ductile concrete transforms the concrete beam behavior from brittle and localized fracture to a displacement-hardening behavior with spatial microcracking damage and a highly ductile failure mode. In addition, the study shows ultra-high-strength ductile concrete can be successfully 3D printed, into the optimized topology of the beam element. The 3D printing process tends to align the fibers into the direction along the printing filament, leading to increased resistance to stress and strain long the beam direction under bending, but less resistance in the direction perpendicular to the printing filaments. Furthermore, the study shows topology optimization can improve beam flexural behavior, depending on the boundary conditions. Without additional horizontal constraint in the four-point bending test setup, the beams with optimized topology exhibit larger displacement capacity before failure and higher load-carrying capacity, than the prismatic beams without topology optimization. The failure mode is flexure governed. However, with additional horizontal constraint, the beams with optimized topology still exhibits larger displacement capacity before failure and higher load-carrying capacity than the prismatic beams without topology optimization; but the displacement capacity is significantly lower and the load-carrying capacity is significantly higher their topology-optimized counterparts without horizontal constraint at the supports. This is because the failure mode becomes more dominated by shear and compression in the topology-optimized beams